2>|V. 7^^

Report Nos. FAA-RD-79-51

NBSIR 79-1616

TITANIUM COMBUSTION IN TURBINE ENGINES

Thomas R. Strobridge

John C. Moulder

Alan F. Clark

Thermophysical Pro perties Division

National Engineering Laboratory

National Bureaujf Standards

Boulder, Colorado 80303

July 1979

Final Report

Document is available to the U.S. public through

the National Technical Information Service,

Springfield, Virginia 22161.

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U.S. DEPARTMENT OF TRANSPORTATION

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1. PUBLICATION OR REPORT NO. We. FAA-RD-79-51

NBSIR79-1616

3. Accession^.

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4. TITLE AND SUBTITLE

Titanium Combustion in Turbine Engines

5. Publication Date July 1979

7. AUTHOR(S)

Thomas R. Strobridge, John C. Moulder, Alan F. Clark

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

l#

" T3W439

/"°,k U”"

11. Contract/Grant No.

D0T-FA8WAI-831

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State, ZIP)

U. S. Department of Transportation

Federal Aviation Administration

Research and Development

Washington, D C 20591 — -

13. Type of Report & Period Covered

Final

ip. lumnrtm

; ARD-700 .

15. SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or teas factual summary of moat aignificant information . If document includes a aignificant bibliography or

* literature aurvey, mention it here.)

Pure and alloyed titanium components are routinely used in aircraft turbine

engines because of their uniquely high strength-to-weight ratios among structural

metals, combined with excellent fatigue and corrosion resistance. Like most

other rnetals, titanium is combustible under certain conditions of temperature,

pressure and oxygen concentration and in fact there have been several instances

of titanium combustion in service engines. Contemporary titanium use in aircraft

turbine engines and the limits of that use are explored. Combustion incidents

and their causes are discussed as well as the typical extent of damage. Current

preventative measures are outlined. For better understanding, the -fundamentals

of metal combustion and the experimental research related to titanium combustion

are thoroughly treated and analytical combustion models are presented. Finally,

the relevant experimental combustion dataare correlated as a function of static

temperature and Reymolds number and compared to the model predictions.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

aeparated by aemicotona

Aircraft turbine engine; analytic models; combustion; compressor, correlation of

experimettts ; dynamic combustion; ignition; jet engine; metal combustion; metal

oxidation; oxidation; static combustion; thermodynamics; titanium alloys; titanium com18. AVAILABILITY bUStion. £] Unlimited

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TABLE OF CONTENTS

Page

Abstract iii

List of Tables iv

List of Figures v

1. INTRODUCTION 1

1.1 Reasons for Using Titanium 2

1.2 Basic Metallurgy 3

1.3 Report Format 7

2. CURRENT TITANIUM USE 7

2.1 Titanium Applications in Turbine Engines 7

2.2 Limits to Titanium Use 8

3. TITANIUM COMBUSTION EXPERIENCE 10

3.1 Combustion Sources 10

3.2 Combustion Occurrences 10

3.3 Extent of Combustion Damage 13

4. PRECAUTIONS AND PREVENTIVE MEASURES 20

4.1 Design Philosophy 20

4.2 Design Measures 20

4.3 Extinguishment 24

5. FUNDAMENTALS OF METAL COMBUSTION 26

5.1 Characteristics of Metal Combustion 27

5.2 Thermodynamics of Metal-Oxygen Reactions 32

6. EXPERIMENTAL RESEARCH ON TITANIUM COMBUSTION 35

6.1 Oxidation 36

6.2 Ignition 41

6.3 Static Combustion 47

6.4 Dynamic Combustion 61

6.5 Analytical Models of Titanium Combustion 70

7. CORRELATION OF COMBUSTION EXPERIMENTS 75

7.1 Prediction of Self-Sustained Combustion 75

7.2 Analysis of Experimental Data 77

8. DISCUSSION AND CONCLUSIONS 87

9. ACKNOWLEDGMENTS 93

10. REFERENCES 95

APPENDICES

A. Glossary of Metal Combustion Terminology A-l

B. Excerpt from British Civil Airworthiness B-l

Requirements

C. Metallurgical Examination of Combustion Specimens. C-l

ii
LIST OF TABLES

1. Titanium Components in Four Modern Turbofan Engines.

2. Titanium Combustion Occurrences.

3. Circumstances Preceding or Event Leading to Titanium

Combustion.

4. Factors Affecting Metal Combustion.

5. Adiabatic Combustion Temperatures in Oxygen at One

Atmosphere Pressure.

6. Titanium Alloy Combustion Rates.

IV
FIGURE CAPTIONS

1. Estimated engine mass reduction through the use of titanium.

2. Schematic relationships: Titanium alloying effects on

structure and selected alloy characteristics.

3. Schematic of modern fan engine.

f4. Artist's rendition showing blade trailing edge damage.

5. Artist's rendition showing progressive damage to blades.

6. Artist's rendition showing combustion products on burned

blades

7. Weight gain versus time for oxidation of titanium.

8. Titanium-oxygen phase diagram.

9. Ignition temperature of titanium as a function of oxygen

pressure

10. Ignition and propagation limits for titanium in heliumoxygen and steam-oxygen mixtures.

11. Experimental arrangement for determining temperature and

oxygen uptake during combustion of titanium in oxygen.

12. Three frames from high-speed movies of titanium burning in

1-atmosphere oxygen.

13. The effect of flow on the rate of oxygen uptake for

Ti- 6A1-4V.

14. Composite time sequences from high speed movies showing

propagation of combustion.

15. Effect of angle-of-attack on combustion propagation in

wind tunnel experiments.

16. Physical model for the combustion of titanium airfoils.

17. Model discrimination lines and CAA recommendation as functions of static temperature and Reynolds member.

18. Experimental titanium combustion data from references 10

and 46.

19. Experimental titanium combustion data from reference 10.

20. Experimental titanium combustion data from reference 10.

v
21 . Experimental titanium combustion data from reference 46.

22. Experimental titanium combustion data from reference 7.

23. Experimental titanium combustion data from reference 13.

C-l Photomicrographs of the unburned and burned sides of a specimen cut from the compressor case of an engine.

C-2 Unburned and burned portions of a specimen from the compressor case.

C-3 Recast layer near a penetration in the bleed air manifold.

C-4 Microhardness as a function of depth below rub strip for

an unburned specimen.

C-5 Microhardness as a function of depth below burned surface

of compressor case.

C-6 Microhardness as a function of depth below burned surface

of compressor case. Different symbols denote independent

traverses of specimen.

C-7 Microhardness as a function of distance from penetration

site in bleed air manifold. Different symbols denote independent traverses of specimen.

v i
Titanium Combustion in Turbine Engines

T. R. Strobridge, J. C. Moulder and A. F. Clark

Thermophysical Properties Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

1. INTRODUCTION

A major portion of the titanium and its alloys produced each

year is used in turbine engines and airframes, taking advantage of

the metal's low density, high specific strength and corrosion resistance. In the evolution of jet engines, the addition of a two

stage titanium fan to a basic straight jet helped produce 42 percent more take-off thrust while reducing the fuel consumption and

specific weight by 13 and 18 percent . ^ ^ The current generation

of large, high-bypass-ratio engines have substantial amounts of

titanum alloys in the fan and compressor sections as well as in

other structures. These engines could not have been built without

titanium alloys ^1, although numerous research programs have been

and are being conducted to develop alternate materials such as

glass-epoxy, graphite-polyimide and boron-aluminum composites and

nickel superalloys. The potential of reduced weight and cost to

improve aerodynamic and economic performance is the primary driving force in the search for improved materials.

Titanium's ability to support combustion is not unusual; in

the proper environment all metals except gold, silver and platinum

will burn. Titanium is combustible above certain combinations of

air pressure, temperature and velocity. While the boundary between combustible and non-combustible regions on the pressure- temperature-velocity surface is not well defined at this time, there

1
are locations in modern turbine engines in which the values of

these parameters are high enough to support sustained or even propagating combustion if titanium were present and were somehow

ignited. Instances of sustained or propagating titanium combustion have occurred

^

in turbine engines both on test stands

and in aircraft and naval ship service. This study, specifically

excluding military engines, concentrated on the experience of engines operated in subsonic commercial air transport service.

We define titanium combustion as any rapid oxidation; self- sustained combustion as the continued or non-quenched rapid oxidation of a structure, for example a stator vane; and, propagating

combustion as the spreading of rapid oxidation to structures other

than those initially ignited.*

If there is an instance of titanium combustion wherein the

engine case remains intact but power production is reduced severely, the effect will be considered the same as an unscheduled engine shut-down caused for any other reason. Should the case burn

through or be mechanically penetrated, then molten titanium drops

can be entrained by the high pressure air flowing through the case

wall into the nacelle area where fuel and oil lines and reservoirs

are located; the potential for further damage is obvious. We cannot give the rate of occurrence of titanium combustion incidents

relative to flight or engine operating hours but the rate is apparently small compared to other causes of engine damage or unscheduled shut-down.

1.1 Reasons for Using Titanium

Considerable leverage exists in the relationship between airframe and engine mass. Jahnke has stated

^

that every unit

of mass removed from an engine translates to four to eight units

*See glossary of combustion related terminology in Appendix A.

2
that may be removed from the airframe and shows, figure 1, engine

mass reductions made possible by using titanium and its alloys

rather than other materials available in 1968. For a rotating

element, the stresses produced by centrifugal loads are proportional to the material density. We quote from a review

article 1 J , "The use of titanium alloys instead of steels for

compressor blades permits use of titanium disks of reduced section

compared to that of steel and so an all-titanium blade-disk assembly shows a greater weight saving (about 52 percent) than would be

expected (about 42 percent) from the difference in the densities

of the respective materials." Taking advantage of titanium's unique strength-to-weight properties is clearly beneficial and is reflected in one engine which is about 35 percent titanium by mass,

concentrated primarily in the fan and compressor. Many factors

are traded-off when selecting a material for a particular temperature-stress regime. Today, titanium is not used at steady state

above about 700K (800F) primarily because of lower creep strength

at elevated temperatures.

1.2 Basic Metallurgy

The first jet engine powered flight was in 1939 while the

first production engine for commercial service using titanium went

on the market in about 1954 ^. In the evolution of the engines, the amount of titanium in the structures increased; concurrently, efforts were devoted to improving strength-to-weight ratio, fatigue and corrosion resistance and tolerance for mechanical

and thermal shock. At room temperature, the equilibrium crystal

structure of titanium is close packed hexagonal (cph); this structure is denoted a . At high temperatures, the equilibrium crystal

structure is body centered cubic (bcc), called 3. For pure titanium, the phase transformation between alpha and beta crystal

3
25

FLIGHT MACH NUMBER

Figure 1. Estimated engine mass reduction through the use of titanium. (Reprinted from The Science , Technology and Applica - tion of Titanium by L.P. Jahnke by permission of Pergamon

Press. Year of first publication 1970).

4
structures, or beta transus, occurs at about 1160K (1625F). Most

close packed hexagonal metals are more ductile than body centered

cubic metals which have higher moduli and lower toughness.

1.2.1 Effects of alloying elements.

Alloying elements not only modify such properties as strength, moduli, hardness and toughness, but also alter the beta transus temperature which affects the heat treating procedures for the

different alloys. Aluminum is an alpha stabilizer (raises the

transus temperature) along with carbon, oxygen and nitrogen while

copper, chromium, columbium, iron, manganese, molybdenum, tantalum

and, importantly, vanadium act as beta stabilizers, lowering the

transus temperature

^ ^ . We quote from "Alpha alloys

are weldable, non-heat treatable, stable up to about 1000F, strong

and tough at cryogenic temperatures, more oxidation resistant than

beta or alpha-beta alloys and relatively difficult to form. Beta

alloys are generally heat treatable, weldable, stable up to about

600F, strong at higher temperatures for short time periods, relatively strong below -100F and quite formable at room temperature.

"Alpha-beta alloys are somewhat of a compromise between the

single phase alpha and beta alloys. They are heat treatable,

stable to 800F (in some cases 1000F), strong and more formable than

alpha alloys. However, they are less tough than alpha alloys and,

for the most part, difficult to weld."

The effects of some of the alloying elements on the crystal

structure and properties are shown in figure 2 which makes a further distinction in structure of near alpha and near beta.

As materials properties research progressed, emphasis was

also placed on processing and fabrication development. For

5
Alpha-Stabilizing

Elements

For example:

Aluminum

Oxygen

Nitrogen

Beta-Stabilizing

Elements

For example:

Molybdenum

Iron

Vanadium

Chromium

Manganese

Increasing Quantities of Alpha Stabilizers Promote Alpha Phase

Increasing Quantities of Beta Stabilizers Promote Beta Phase

Alpha

Structure

Near Mixed Near

Beta

Structure

Alpha Alpha-Beta Beta

(some beta) Structure (some alpha)

Unalloyed Ti- Ti- Ti- Ti- Ti- Ti- Ti- Ti- Ti- Ti- Till 5AI- 5AI- 8AI- 6AI- 6AI- 6AI- 6AI- 8Mn 8Mo ll.5Mo- 13V2.5Sn 6Sn- IMo- 2Sn- 4V 6V- 2Sn- 8V- 6Zr- 11Cr2Zr- IV 4Zr- 2Sn 4Zr- 2Fe- 4.5Sn 3AI

IMo- 2Mo 6Mo 3AI

0.2Si

Higher density * 1

Increasing heat treatment response

Higher short-time strength —

Higher creep strength

Increasing strain rate sensitivity -

Improved weldability

Improved fabricability

Figure 2. Schematic relationships: Titanium alloying effects on

structure and selected alloy characteristics. (Reprinted

from The Titanium Industry in the mid-1970's by R.H. Wood

by permission of The Metals and Ceramics Information

Center, Battelle Columbus Laboratories. Year of first

publication, 1975).

6
example, double and even triple vacuum arc, consumable electrode

melting was found necessary to reduce the amount of hydrogen to

below embrittlement levels, eliminate tungsten from fixed electrodes and prevent alloying element segregation ^ 2 ^

improved

techniques for heat treatment, welding, machining, casting, forging and diffusion bonding were developed, all aimed at lower cost,

lower weight or improved performance.

1.3 Report Format

The report that follows discusses where titanium is used in

turbine engines, what the experience has been with combustion of

these parts, including a breakdown of probable causes and the extent of possible damage, and what preventive measures have been

taken to limit this damage. Further, the fundamentals of titanium combustion are outlined with a discussion of both static and

dynamic laboratory experiments. An analysis of some of these experiments shows a potential correlation which is followed by a general discussion of the whole survey. Conclusions appear on

page 87.

2. CURRENT TITANIUM USE

2.1 Titanium Applications in Turbine Engines

The list of titanium components in Table 1 was compiled from

the information available to us on four current commercial engines.

Obviously, not all of the parts are in each of the engines because

of design differences, rather the lists in the table are composites. However, all four engines have titanum fan blades, fan

disks or hubs, low-pressure compressor blades and disks or drums,

and high pressure compressor blades and disks or impellers. The

titanium components in a modern turbofan engine are shown in

figure 3.

7
Table 1

Titanium Components in Four Modern Turbofan Engines*

Fan Low Pressure High Pressure

Compressor Compressor

Parts of spinner

Blades

Hub or disk

Case

Inlet guide vanes

1/4 stage cone

Bearing support

Mid ring

Bypass stator vanes

Splitter lip

Main mount frame

Exit case

*Alloys

Blades

Disks

Inlet guide vanes

Stator vanes

Stator vane spacers

Case

Blades

Disks

Case

Impeller

Bleed Air Manifolds

Ti-6Al-4V

Ti- 5A1-2 5Sn

Ti-6Al-2Sn-4Zr-2Mo

Ti-8Al-lV-lMo

Limits to Titanium Use

The alloys selected for similar components vary among the

manufacturers but none use titanium above about 700K (800F). This

steady state temperature limit is imposed by insufficient creep

strength, not by combustion considerations. At this time, there

is no clear evidence that any of the currently used alloys, including pure titanium, are more or less easily ignited or prone to

sustained or propagating combustion. Differences in combustion

f 7

1

characteristics have been noted 1 J but the effects are not

8
Bypass

Strator

Vanes

9
fully understood. The question of whether or not alloy substitution would prevent combustion propagation under the same initiating circumstances has not been answered and alloy substitution may

not be possible for certain components for many reasons including

strength, brittleness, manufacturing problems and room.

3. TITANIUM COMBUSTION EXPERIENCE

3.1 Sources

The open literature rarely contains mention of titanium combustion in engines^' ^5], [8], [9], [10], [11] an(j never,

to our knowledge, any detail of the event(s). The information for

this section comes from four engine manufacturers, the Federal

Avaition Administration ( FAA ) , National Transportation Safety

Board ( NTSB ) and one airline. The tabulations are incomplete

since all possible sources could not be contacted and all events

may not be reported to central offices. Certain events may go unnoticed; for example, minor blade tip combustion may be attributed

to erosion. Finally, we are not aware of any requirements or avenues for reporting and compiling information relative to titanium

combustion in aircraft engines within the FAA, NTSB or the military, and the manufacturers and operators are generally hesitant

to reveal such information although there are exceptions.

3.2 Combustion Occurrences

Table 2 gives the total number of occurrences known to us of

titanium combustion in aircraft gas turbine engines since 1957 and

the number of contained and non-contained cases. In only two cases

r

"3 1 were engine damage details available 1 J .

10
Table 2

Titanium Combustion Occurrences*

Total known 144

Contained+ 85

Non-contained + 59

*These data accumulated since 1957 from both U. S. and foreign

sources are incomplete for reasons given above and therefore the

division between contained and non-contained cases may be biased.

We believe that each occurrence resulted in engine shutdown.

+Contained is defined as titanium combustion which did not penetrate the case.

+Non-con tained is defined as a case penetration caused by titanium

combustion.

Since the limiting service temperature for titanium and it's

alloys of about 700K (800F) is well below the ignition temperature

of titanium in air of about 1900K (2900F), some upset of the normal

operating conditions must occur to either mechanically or aerodynamically raise the temperature of a titanium structure to the ignition point before combustion can occur. The upset mechanisms can

be catagorized as:

11
° Mechanical rubbing

Foreign or domestic objects

Imbalance

Rotor displacement

Case bending

Stall induced mechanical displacement

° Aerodynamic heating

Stall

Mechanical rubbing or jamming can arise from several causes. Loose

particles of solid material, either ingested from outside or articles broken within the engine can interfere between rotating and

static structures or bend blades so that they strike the vanes.

Very rapid temperature rise from frictional heating is possible.

Failure of a rotating part (disk, spool or blade) can result in

rotor unbalance sufficiently large to cause heavy rubbing. A failed bearing can allow radial or axial displacement of the rotor with

subsequent rubbing and in some instances rubbing has occurred because of case flexing. Reverse flow through the compressor resulting from surge or compressor stall can deflect the rotating blades

into the static structure and flow from the aft combustion section

of the engine can be at high temperature. (Combustor flames ternperature is typically 1900 K (2900 F)).

In so far as possible, Table 3 lists the circumstances preceeding or the primary events leading to titanium combustion. Many

of the test cell events occurred while the engine was deliberately

being operated outside of the design envelope. The large number of

cases in which the primary cause is not known points out the inadequacy of the data base with which we must deal at this time.

Disregarding the test cell events and those for which the

cause is unknown, blade failure contributed to 22.5 percent of the

incidents followed by turbine failures at 20 percent.

12
Table 3

Circumstances Preceeding or Event Leading to Titanium Combustion

Unknown 97

Blade failure 9

Turbine failure 8

Test cell 7

Bird strike 4

Disk or spool failure 4

Vane failure 3

Bearing failure 3

Stall 3

Other 3

FOD 2

Air Seal 1

TOTAL 144

From these data it is not obvious that there is one major cause

of titanium combustion in aircraft engines. In some of the cases,

it is evidently straightforward to determine the cause and the following sequence of events. For other cases, it may be impossible to

determine the cause and to trace the mechanical and chemical damage

progression in the four to twenty seconds that have been estimated

for the duration of a typical event.

3.3 Extent of Combustion Damage

Combustion damage has ranged from light tip and trailing edge

combustion of airfoils to massive metal involvement ^ ^ ^ , sometimes with 360° case penetration^. Once started in an engine, titanium combustion continues until either the titanium fuel

is depleted, the air pressure falls below some critical value, the

combustion zone progresses to a heavy section where enough heat

13
is conducted away for quenching or the ignition energy source is

removed. Energy from the exothermic reaction is transferred away

from the combustion site by solid conduction, radiation, convection

and in the products of combustion and molten metal that may be

blown or thrown away. As long as the heat of combustion is greater than or equal to the heat transfer away from the site and there

is sufficient oxygen, the reaction will continue. The thin leading and trailing edges of an airfoil appear to be preferentially

damaged with several possible contributing factors. Conduction

away from the combustion site is inhibited by the smaller cross

sectional area so the temperature of the base metal will rise.

Additionally, the reaction may take place on both sides of the

thin section. Finally, the nature of the airflow all around the

blade and the flow of molten metal toward the trailing edge can

explain some of the effects. Minor trailing edge damage is shown

in figure 4, while the tendency to consume the thin sections is

evident from the rounded shape of the blades in figures 5 and 6.

In figure 5, the rough deposits are titanium oxides which are

brittle and shaded from grey at the tip of the stub to black

inboard on the blade. If the deposits are on one side only, heat

effects are see on the undisturbed side progressing from neutral

grey at the tip to light brown, to neutral grey to blue, a hint of

the brown and finally parent metal color.

Below is a description of an engine intended to illustrate the

nature and extent of damage possible in a severe event. The description is a composite of several damage reports. Typical heavy damage

with the first evidence of sustained titanium combustion at midstage

of the high pressure compressor, could be as follows:

1. High pressure compressor rotor

1.1 Forward of midstage, evidence of heavy tip rub

several stages.

1.2 First few stages, very light blade damage from

rub.

14
15

Figure

4.

Artist’s

rendition

showing

blade

trailing

edge

damage.
16

Artist’s

rendition

showing

progressive

damage

to

blades
4-1 •HT3C

CU T3 M 03

CO 43

17

burned
1.3 Next few stages, increasing evidence of combustion products on aft side of airfoils. Witness

marks show some blades had touched neighbor.

1.4 Next few stages, evidence of non-sustained airfoil melting or combustion on tips and trailing

edges. Accumulation of combustion products or

slag on aft side of airfoils.

1.5 Midstage, substantial blade combustion.

1.6 Aft of midstage, heavy combustion, all airfoils

gone

.

Combustors

2.1 Deposits of combustion products, especially at

fuel nozzles.

2.2 Liner burning, cracking and distortion due to

flow irregularities.

Turbine section

3.1 Distortion of inlet guide vanes with heavy metal

deposition.

3.2 Deposits of combustion products decreasing in aft

direction. Heat distortion under some deposits.

High pressure compressor case

4.1 Forward of midstage, evidence of heavy rub several stages.

4.2 First few stages, evidence of light stator vane

shroud rub from forward deflection.

4.3 Next few stages, unharmed, increasing evidence

of combustion products on aft side of vanes.

4.4 Next few stages, evidence of non-sustained vane

melting or combustion at trailing edge. Accumulations of combustion products or slag, both

sides

.

4.5 Midstage substantial vane combustion.

18
4.6 Aft of midstage, heavy combustion, all vanes

gone.

4.7 Just forward of midstage, case overheated 360°,

light combustion

4.8 Midstage, case overheated, evidence of molten

titanium.

4.9 All bleed air manifolds penetrated by combustion.

4.10 Aft manifold penetration directed airflow carrying molten titanium toward forward manifold,

causing melt through.

5. Exterior to case

5.1 Several air and oil lines penetrated by combustion. Fuel lines intact but shroud penetrated.

5.2 Fire warning system wires parted by melting.

5.3 Overpressure damage to cowling.

19
4 PRECAUTIONS AND PREVENTATIVE MEASURES

4.1 Design Philosophy

Millions of hours of service experience coupled with research

to reduce the chance of titanium combustion and limit damage have

induced modifications of design philosophy in several areas over

the years. Design goals sought by the manufacturers can be summarized as follows:

° Reduce possibility of ignition.

° Minimize possibility of propagating combustion.

° Eliminate case penetration.

Several precautionary measures can be incorporated to try to

achieve these design goals. After a brief discussion of which

titanium parts are susceptible, some of these measures are discussed below, several of which also have other purposes (e.g. rub

strips for increased efficiency). Rub strips, coatings, and increased clearances help reduce ignition possibilities and the

alternate materials and linings help inhibit propagation.

4.2 Design Measures

The trend [3] is toward less aggressive use of titanium

in modern designs, especially for static structures in the higher

pressure section of the compressor. We are not aware of any instance of titanium combustion in the fan or lower pressure compression regions where the oxygen concentration and temperature are

low. While rotor blades and structures have been involved in combustion events and indeed are frequently the ignition source, the

stator vanes and casing are susceptible to extended damage since

molten metal tends to sling off the rotating blades and collect on

the static parts. A drop of molten titanium on a blade tip may be

20
subjected to radial acceleration of 10^ to 10 5 times the acceleration of gravity. The centripetal force holding a growing molten

drop on a blade tip is coupled to the drop through surface tension.

At some drop diameter, the surface tension force is no longer sufficient to provide the centripetal acceleration and the drop breaks

away, traveling tangentially. A first order calculation, neglecting

aerodynamic drag forces, shows that the drops leaving a 76 cm

(30 in) diameter rotor revolving at 10,000 RPM will be about 0.1 mm

(0.004 in) in diameter, a fine spray with a high surface to volume

ratio. This high surface to volume ratio, both while traveling to

the static structure and after impinging more or less evenly, promotes high rates of oxygen solution.

Molten, reacting metal leaving the blades and arriving on the

case and vanes is a highly effective heat transfer mechanism that

tends to quench the blades and heat the static structures. Ignition of the static structures is entirely possible and one instance

of a steel case being melted or burned through by a pool of molten

titanium has been recorded

Rub Strips

Aside from stall or aerodynamic heating, rubbing, can cause

ignition. Rubs may result from foreign object damage (FOD), domestic object damage (DOD) , stall, bearing failure or case deflection.

During a titanium-on-titanium rub, both low thermal conductivity

titanium structures may rapidly rise to the ignition temperature

which is below the melting point. Therefore, designs incorporate

rub strips or coatings on lands and seals that prevent for example,

the rotating titanum blades from touching the titanium case. There

is a limit to the amount of violence these rub materials can tolerate since they have finite thickness. A deep incursion may penetrate, wear away or melt the rub strip and bond layer exposing

underlying titanium. Nonetheless, the rub strips appear to

21
significantly reduce ignition. Note that it is improbable that

rubs will be eliminated and that some light rubbing is considered

normal

.

Several different materials are used for rub strips ranging

from filled elastomers in the low temperature regions to sprayed

metallic coatings, felt metals and steel at higher temperatures.

To be successful, the rub strip must be able to limit the energy

deposition and hence the temperature in the underlying titanium.

Some of the materials will deform or abrade away easily. The elastomers are physically cut out or abraded away by interferring blade

tips or labyrinth knife edges with relatively low force which

translates to low energy. Felt metals, used at higher temperatures

are deformed by interference also with low energy. Others such as aluminum and steel may melt and provide large enough clearance so

there is no longer interference. Aluminum's relatively high thermal conductivity tends to spread the friction energy over a larger

volume with resulting lower temperatures.

Another purpose served by these sacrificial rub materials is

that they can, in a sense, absorb molten titanium emanating from

the rotating structure. The temperature of a molten reacting titanium drop will be at least 2000K (3200F). Upon impinging on aluminum, for example, heat transfer will immediately start to cool the

drop and heat the aluminum to the melting point thus tending to

quench the reaction. If the drop temperature goes below about

1850K (2900F) the combustion reaction will stop.

Coatings

Coatings and surface treatments to prevent ignition and combustion of titanium structures offer some promise. Some proprietary experimental work has been done, the military have conducted

programs directed to harden structures against laser damage and

22
NASA is conducting protective coatings evaluations. The coatings

are intended to prevent ignition by conducting heat away from the

potential ignition site or to quench combustion, also by conduction. The degree to which molten titanium will wet the coating can

also be important or some of the materials could break down before

ignition temperature is reached. At this time, a few candidates

have been found to suppress ignition and combustion but lack required mechanical properties. The effects of shot peening have not

been reported.

Clearances

Wider radial and axial clearances have been suggested (and in

some cases employed) to reduce the occurrence of tip rub and rubbing trapped debris. Larger axial clearance reduces the chance of

stator vane-rotor blade clashing caused by stall; however the stall

margin will be decreased. The effects of wider clearances on efficiency are not within the scope of this report; however, efficiency

is usually degraded by opening up clearances.

Other

Some more flexible engines have been found susceptible to rubbing caused by deflections from thrust and maneuvering as well as

unbalance loads. Offset machining has been used to inhibit ignition from deflection rubs and stiffer case designs have been proposed [12] # Offset machining of the case provides greater radial

clearance where deflection rubs may occur.

Bleed air manifold entrance edges, found to be susceptible to

ignition by molten titanium spray have been protected by steel

grommets H2 ] f in a sense coating those areas. All-steel linings

for the manifolds have been suggested in some cases.

23
Increased tolerance to foreign objects is de

reasons other than just a reduction in the chance

bustion. However, there is a practical limit to

of ingested material that will not cause damage,

(arising from a mechanical failure in the engine)

never be eliminated although blade, disk and vane

and inspection techniques are continuously being

crease toughness and reliability.

sirable for many

for titanium comthe type or size

Both FOD and DOD

will probably

design, materials

improved to inFinally, substitution of alternate materials for titanium in

those regions of the engine most susceptible to combustion is an

avenue taken in some designs and suggested by the British Civil

Aeronautics Authority ( CAA ) (see Appendix B). The CAA document

requires that thin titanium sections not be employed as static

structures in the engine where the pressure exceeds 2 atm (29.4

psia) and the velocity is above 50 m/s (164 ft/s). This proposal

would rule out titanium stator vanes in all but about the first

four stages of compression (assuming a compression ratio of 1.2 per

stage). As mentioned, the weight penalty incurred by substituting

other materials for titanium in static structures is lower than for

rotating parts. A CAA paper referenced in Appendix B shows that

there have been titanium fires in all three of the example engines

with titanium stator vanes while seven out of the eight engines

without titanium stator vanes have not had any known combustion

events. However, complete compliance with the suggested guidelines

might not assure complete elimination of titanium combustion.

4.3 Extinguishment

From start to finish, the time for propagating titanium com- bustion in an engine has been estimated at 4 to 20 seconds. We

are not sure of the titanium extinguishing effectiveness of on

board agents such as carbon fluoride or carbon bromide driven by

24
fluorocarbons, but Fox [13] h as shown that CO 2 supports titanium combustion better than air. At 23 percent CO 2 by volume,

the combustion rate increased 50 percent and at nearly 100 percent

CC> 2 / the increase in burn rate was 300 percent. A 60 percent by

volume mixture of argon in air effectively extinguished titanium

fires. It is suspected that helium would also be effective. In

either case, the short duration of the event inside the engine,

coupled with the need for large amounts of agent make detecting,

reacting and extinguishing seem impractical at this time. No

studies have been done as part of this effort to evaluate the

aerodynamic and mechanical effects of suddenly introducing enough

argon at molecular weight 39.9 into the airstream to raise the

concentration to 60 percent.

25
5. FUNDAMENTALS OF METAL COMBUSTION

Experiments on metal combustion date back to at least

1782, when von Ingenn-Hausz plunged heated spirals of iron and

steel into oxygen and observed that they ignited and burned,

producing a great deal of heat and light Although

nearly two hundred years have passed, metal combustion remains

a little studied, poorly understood phenomenon, as previous

reviewers have noted t 1 5 , 16

]

# it i s only during the past

twenty years that significant progress has been made toward

understanding the complex processes that occur when a metal

burns. Yet, when our understanding of metal combustion is compared to the detailed knowledge we have of metal oxidation or

the combustion of ordinary fuels, it is clear that the field of

metal combustion is still in its infancy.

There are a number of reasons for the present lack of understanding. Foremost among them is the small amount of research on metal combustion, although it must be realized that

until relatively recently there were few technologically important applications. Another factor contributing to our ignorance is the limited nature of the majority of research that has

been published. There are few fundamental studies; most research has been appl icat ions-or iented and has focused on one or

another aspect of the problem. Contributing to the confusion,

there is no single forum for presentation of results: data

pertinent to metal combustion is published in a wide range of

journals crossing many disciplines. Finally, perhaps overriding all other considerations is the enormous complexity of

metal combustion, consisting, as it does, of a large number of

complex, interrelated chemical and physical processes. Indeed,

to characterize completely the combustion of a single metal

would require the services of a solid state physicist, a surface scientist, a physical chemist, an analytical chemist, a

26
chemical engineer, a mechanical engineer, a thermodynamic ist , a physical metallurgist, a pyrometallurg ist , and a specialist in

fluid dynamics. To date, no such interdisciplinary team has

been assembled, although recent efforts of several independent

groups funded by the USAF to study different aspects of titanium combustion are a step in this direction.

Because of the great complexity of metal combustion processes and the large number of factors that affect them, this

section of the report begins with a general introduction to the

distinctive characteristics of metal combustion and a discussion of fundamental concepts. These include the thermodynamics

of metal-oxygen reactions, their kinetics, and the various mechanisms of metal oxidation, ignition and combustion. The remainder of the report reviews research on the oxidation, ignition and combustion of titanium and titanium alloys, with particular emphasis on research since 1972 on the problem of titanium fires in aircraft gas turbine engines. For the reader's

convenience, there is a glossary of metal combustion terminology included as Appendix A.

5.1 Characteristics of Metal Combustion

Some insight into the difficulties facing the scientist

who wishes to study metal combustion may be gained by examining

Table 4, which lists the factors affecting metal combustion.

While the list is fairly inclusive, it is by no means exhaustive. In addition to the usual environmental factors that

affect a chemical reaction (temperature, pressure, concentration of oxygen) , there are a number of factors that depend on

the particular configuration of the metal (size, shape, history, etc.) and a large number of material properties that

affect the combustion process. Note that in most cases, the

relevant material properties are needed for both the metal

27
Table

4.

Factors

Affecting

Metal

Combustion

CO 44c

0) a; 44 -H

*H

u -—- £ H

03 34 44 \ 03 44

<N -C 03 E 44 O o w CO O •—i

34 44 c 03 o C 34 0 03 4-3 03 03 •H in 03 03 c C 44

+J co 34 1—1 E 0 0) -H CO

03 03 03 03 •H

H £ CO c

34 c 4-1 34 4-3 JZ 03 03 03 3 i—1

03 ,X 03 03 03 03 •H 44 o 34 co 03 O o o E JZ E CO C 34 44 •H 4-> 03 •H (0 4-> 34 CO O O o

44 <13 44 JZ 44 44

O

O

O C3 44 03 CO c E 34 4-3 34 0 44 a 44 CO 0 3 3 44 44 »H 44 03 CJ 44 to E W CO O O C 0 03 03 E 0 rH in •rH 0 O'

44 •H

O 03 0) 44 •iH <D 3 44 03

>, 0 44 -H E o O 44 o 44 0 C

34 M-J c 03

O

C •rH 03 0 03 0) -H rH 03 4-1 03 0) 34 03 44 4-> T5 O 03 in 3 0) co 44 rH 4-> CO 03 •H 0) 4-3 03 03 34 03 c CO 03 •H 4-1 x a o 34 — 34 0) c 0) 43 in o co 4-3 a a o 03 -C cO 4-3 •H C

rH 4-3 0) 4-3 m 03 3 c

44 34 i—1

0) 34 c 4-3 -H 3 03

cuC XI <—1

XI e 03 •n4 >1 E c 03 U 44 XI 4-3 3 o U C 0 •rH C

34 3 0 44 o

•H 4-> 03

O 0)

O 0 CO > 03 34 o rH XI c 34 3 c c 0) 0> rH

Cd 03 to 0) 03 34 > O

Oi CO O' in 3 C E 03 >1 03 4-1 to >i

03 34

X 34 03 <0 03

4-3

a o a c co 03 OE+E—

.—X 0 — CO 0 E o c E * + E O +

CO E + H E E

03 ~ E 4-3

—• - —

*H

—- -

•H 0 O

4-> 0 -*1 CO E c £ —

03

34 E 4-3 >1

O c + O -r 0 0 34

03 •r4 4-1 E 03 E •rH E E 3 a > -H + 34 —

4-3

C + + 4-> 0 E 4-3 H > E 4J 03 O E E 03 p JZ 4-3 -H • . cn N -H — - 34 C4 Co o co 03 c H 44 03

4-1 •rH 3 3 >i

m •rH 34 03 44 44

a i—1

JZ 03 T3 44 4-3 03 4-3 o £ c c E

03 CP S C 44 •rH JZ rH

a 34 •rH •rH 0) •H •rH O -p o a 03 03 O 0 0 4-» 34 03 34 U X 03 E > 44 a a

0) 03 0,44 c

4-> <—1

r—1 rH 03

O 44 44 44 CO CP o

03

O 3 03 03U

O

O 0 c c •rH S •rH O E E CO •H •rH 44 E 0) 34 34 4-> 44 44 4-3 4-> 4-1 rH •rH O -i 0) 03 03 03 03 (0 «H •H c

4-3 0 -C £ 0) 0) 03 0) 03 03 0 CO

03 E 4-3 4-3 JZ JZ JZ JZ JZ E XI •H o .—. E E E

03 rH rH XI

03 r— 03 •rH 44 O — 44 X

03 E O 03 0 E + E E E + 44 C — E E 34 0

•rH ' — 0 —. O c 44 0) o E C 03 rH rH 34 E + 03 03 03 03 CO 3 + E CO >1 44 44 CO E — >1 X 03 .—

0) 03 CO 03

—

.—

X

O E 0 C rH 03

X o c 0 E 34 •rH 0) E o 44 44 + 44 0) a X

34 + H 44

O

O E 0 44 0 3 E CO O - 03 c

CO

— c >1 >1 c 34 0 rH rH

CO 01 >1 44 44 >i

O *H 03 03

03 >1 44 44 -H -H 44 *HC 44 44 44

34 44

H > > •H 44 0 03 03 03 a-H 03 rH -H *H > •H •H •H E E

CO

u •H (fl CO •rH CO 44

u

34

O 03 XI 3 3 co 0 03 o II II 0 O 44 3 44 44 CO ax CO a co 34 rH 44 44 •H E •rH CO 0

03 •rH 3 O *H *H E 0 X *H E E > > CO CO X X 0) C3 0 X *

28
and its oxides. Where these properties vary with temperature, the

variation must be known. In some cases, e.g. viscosity and surface tension, the properties change not only with temperature, but

with the concentration of oxygen as well, and this dependence must

be known too.

Many of the distinctive features of metal combustion are a direct result of the stability exhibited by most of the metal oxides. Their stability implies that the heats of formatiQn for the

compounds will be large and so a great deal of energy may be released during metal combustion. But the oxides' stability also

means that the products of combustion, as well as the fuel, will

be present in the reaction zone as condensed (solid or liquid)

phases. Several important implications that follow directly from

this fact are discussed below.

In the first place, very high temperatures will be attained

during metal combustion. In the combustion of an ordinary hydrocarbon, the flame temperature is limited by dissociation of CC^.

This begins to become appreciable at temperatures of about 1700K

(1427°C) and effectively limit hydrocarbon flame temperatures to

less than 2500K (2227°C). Although most oxides decompose partially at elevated temperatures, the dissociation pressure of the base

metal oxides remains very low at quite high temperatures. Thus,

the upper limits on the temperature of a metal flame will be the

boiling point of the metal oxide. Because metal oxides evaporate

nonstoichiometr ically , the boiling point must be defined in terms

of the equilibrium of the condensed oxide with its gas phase decomposition products at the ambient pressure. The adiabatic combustion temperatures (see glossary) for several metals burning in

oxygen at 1 atm are listed in Table 5^ 4 1. It is obvious that

temperatures in excess of 3000K (2727°C) are quite common in metal

combustion

29
Table 5. Adiabatic Combustion

Temperatures in Oxygen at 1-Atmosphere Pressure

Metal Adiabatic Combustion

Temperature (Kelvins ) Al 3800

B 2900

Ba 3000

Be 4300

Ca 3800

Cr 3300

Fe 3000

Hf 4800

Mg 3350

Mn 3400

Mo 3000

Pb 1800

Sn 2700

Sr 3500

Th 4700

Ti 3300

W 3200

Zn 2200

Zr 4800

The presence of metal oxides in the reaction zone also means

that at t.hey will play a role in determining the rates and mechanisms

of the combustion process. This is perhaps best illustrated in the

criterion proposed by Glassman to predict the combustion mode for

metals^ 7 ]. If the boiling point of the metal oxide is higher

than that of the metal, then the flame temperature can exceed the

metal's boiling point, the metal will evaporate and combustion will

occur in the vapor phase. On the other hand, if the metal oxide's

boiling point is lower than that of the metal, the flame temperature

cannot reach the metal boiling point and surface combustion will result. Because vapor phase combustion can be suppressed, either by

the formation of a protective oxide layer or by reduction of the

flame temperature below the metal boiling point by radiation losses,

this criterion must be modified slightly. Thus, the metal oxide's

boiling point exceeding that of the metal is a necessary but not sufficient condition for vapor phase combustion, while the opposite case

of a metal's boiling point exceeding that of the oxide is a sufficient but not necessary condition for surface combustion.

30
On the basis of Glassman's criterion, titanium, iron, steels,

nickel, zirconium, and hafnium are predicted to burn on the surface, while magnesium, calcium, strontium, barium, and aluminum,

are predicted to burn in the vapor phase.

The presence of metal oxides in the combustion zone can also

have a strong influence on the propagation of metal fires. Due to

the refractory nature of metal oxides, most of the heat of reaction will reside in the enthalpy of the products. In a surface

burning metal, the molten oxide products will collect on the surface where their heat content aids in melting more metal. This

molten mixture of metal and oxide with a high heat content can

then be spread to other parts of a structure by centrifugal action

or by aerodynamic forces and thus aid in spreading a fire rapidly.

Two other characteristics of metal combustion deserve mention: the very high radiant emission of burning metals and the

frequent occurrence of fragmentation and explosions during combustion. The highly luminous character of metal flames is, of

course, a direct result of their high temperatures; at temperatures in excess of 3000K (2727°C), a significant fraction of the

thermal radiation falls within visible wavelengths. Even metals

that burn in the vapor phase have highly luminous flames owing to

the presence of large numbers of condensed oxide particles which

give the flames a high emissivity. This characteristic feature of

metal flames is used to good advantage in one of the continuing

practical applications of metal combustion: the photographic

flashbulb.

The violent fragmentation and explosions which often accompany metal combustion are not as readily understood. Explosions

are most often observed during the combustion of small particles

(d <lmm) of reactive metals like zirconium, titanium, tantalum and

aluminum [10 ] . Small particles of these metals will explode

31
shortly after being ignited in an oxidizing atmosphere. Nelson

discovered that for zirconium droplets to burn explosively, nitrogen had to be present, either in the metal or in the oxidizing

atmosphere ^1 , 22 ] # Presumably similar considerations apply to

the other metals. When larger quantities of metal are burned

(> 100 mg), they no longer explode violently, but rather "boil"

vigorously, ejecting a continuous spray of small molten droplets

which continue to burn and may, themselves, explode^3] # a

photograph showing this behavior during the later stages of titanium combustion is included in figure 12 (see section 6).

It must be emphasized at the outset that there is no fundamen- tal difference between oxidation and combustion, only one of degree. Chemically and thermodynamically, metallic oxidation is

really just slow burning: slow enough that the metal can dissipate

the heat generated to its surroundings, thus preventing a temperature rise. Ignition is a thermal instability that occurs when the

heat generation rate from the chemical reaction exceeds the metal's

ability to dissipate it, thus leading to a thermal runaway. The

steady state combustion that follows ignition is a new equilibrium

state where, again, the heat generation rate, which is now governed

by a new set of rates and mechanisms, is just balanced by the heat

lost to the surroundings. The critical difference between oxidation and combustion, and it is an important one, is the rate at

which the chemical reaction proceeds. During the linear, non-protective oxidation of titanium at 1268K (995°C), the rate of oxidation is 2 x 10“ 2 mg 02/cm2 • s

18 J . The corresponding rate during

steady-state combustion at 3250K (2977°C) is 1 x 10 2 mg 02/cm2 *s^^.

The combustion rate is thus 50,000 times faster than the oxidation

rate

.

5.2 Thermodynamics of Metal-Oxygen Reactions

None of the metallic elements are chemically inert, although

their relative activities vary markedly, depending on position in

the periodic table. In particular, with the exception of gold, no

metal or alloy is stable in air at room temperature. All other

metals and alloys will form oxides; many form nitrides as well.

32
The force that drives metal oxidation is the free energy change

associated with the formation of a metal oxide. Since the metal

oxides are as a rule highly stable, they have a lower free energy

than the free metal; consequently, metal oxidation is a thermodynamically favored process.

The overall chemical equation for the reaction between a metal and oxygen to form the metal oxide may be written as

aMe +j 0 2 = Me a

O b (5-1)

This is a reversible reaction; the direction it proceeds depends

upon the concentration of oxygen present in the gas phase. If the

partial pressure of oxygen is greater than the dissociation pressure of the oxide at the ambient temperature, then the metal will

be oxidized. The metal will remain unaffected and any metal oxide

will decompose if the ambient oxygen partial pressure is lower

than the dissociation pressure. The noble metals have dissociation pressures that are greater than or equal to ambient oxygen

pressures at temperatures below their melting point. This is why

these metals (Au,Ag,Cu) can be heated in air without oxidizing excessively and why they may be found natively in their pure, unoxidized state. The base metals have much lower dissociation pressures and will oxidize under most conditions found terrestr iallly

.

Titanium, for example, has a dissociation pressure of lO

- -*-^ atm at 1873K (1600°C), which helped to explain its remarkable

affinity for oxygen.

Although the chemical driving force for metal oxidation is

the standard free energy of formation, AG° , the heat liberated

during the chemical reaction is often the determining factor in

sustaining and propagating a reaction. This important thermodynamic quantity, the standard heat of formation, A H°

,

may be calculated from the standard free energy and the entropy change, A S°, by

use of the well-known Gibbs-Helmhol t z equation,

AH° = AG° + T AS° (5-2)

33
or obtained from readily available tabulat ions t ' 25

]

# There

are also several excellent textbooks that discuss the chemical

thermodynamics of metal-oxygen reactions and give examples of

thermophysical calculations t 26-28

]

#

34
6 EXPERIMENTAL RESEARCH ON TITANIUM COMBUSTION

Due to the complexity of metal-oxygen reactions, as discussed

in the preceeding sections, experimenters usually concentrate on

one aspect of the overall process. For this reason it is convenient to divide the research into four categories: oxidation , ig- nition, static or steady-state combustion , and dynam ic or propa- gating combustion . In general, the degree of complexity of the

phenomena involved increases in the same order as presented; as

complexity increases, the experimenters' ability to control all

the variables decreases correspondingly.

In the following sections we discuss, for each of these categories, relevant research on titanium that is available in the

open scientific literature. Information on the basic rates and

mechanisms of titanium oxidation are required because the kinetics

of oxide film formation govern the ignition process. Ignition

studies are usually simple go/no go tests that seek to define the

temperature, oxygen pressure, and gas velocity necessary to initiate a combustion reaction. Static combustion experiments are

done in a static or slowly flowing gas environment to obtain the

maximum amount of experimental control and reproducible results.

These experiments give basic information on the rates and mechanisms governing steady-state titanium combustion. In the experiments on dynamic combustion , an effort is made to reproduce, to

some extent, the conditions prevailing in a gas turbine engine by

conducting the experiments in a wind tunnel. Such experiments give

a great deal of insight into the mechanisms by which a titanium

fire spreads, and have been instrumental in establishing the temperature-pressure-velocity limits for sustained titanium combustion in engine environments. They have also led to an analytical

model that has been able to predict with some success the conditions under which titanium will undergo self-sustained combustion.

To date, no experiments have tried to duplicate high rotational

speeds in concert with high velocity airflow.

35
Finally, in section 7, we present a correlation of all available experimental data on the limits for sustained titanium combustion with the limits predicted by existing models and guidelines. No such correlation for all of the available data has been

published before and the results are both interesting and encouraging. In general, there is good agreement between the limits

predicted on the basis of analytical models and the limits found

by dynamic combustion experiments.

It is worth noting that there has been a great deal of proprietary research on the titanium engine fire problem. One manufacturer^] estimated that over 100 man-years had been expended in research on the titanium fire problem in the last few years

alone. Of course, many of these tests are full scale runs on an

engine test stand, and require many personnel. Nonetheless, a great deal of work has been done by the manufacturers themselves

on rub mechanics, development of new rub strip materials, design

changes in engines, new alloy testing and development, and development of protective coatings. Very little of this information

was available to us because of its proprietary nature; no proprietary information has been included in this report.

6.1 Oxidation

The isothermal oxidation kinetics for titanium have been

studied by a number of investigators, and much of the data has

been summarized by Kof s tad t 1 8 ] . oxidation proceeds according

to various rate laws depending on temperature and time of exposure. This behavior is illustrated in figure 7 (taken from

Kofstad), which is a double logarithmic plot of weight gain vs.

exposure time for pure titanium oxidizing at different temperatures in 1 atmosphere oxygen. We quote directly from Kofstad'

s

Characterization of titanium's oxidation behavior f 18

]

; "Below

about 400°C the oxidation is logarithmic, whereas at 400-600°C a transition from logarithmic to parabolic or an approximately cubic

36
WEIGHT

GAIN,

mg/cm

2

10

TIME, min

Figure 7. Weight gain versus time for oxidation of titanium. Afterreference 1 18 ] .

37
oxidation is observed. Above 600-700°C the oxidation is parabolic

and, after extended reaction, transforms into an approximately

linear rate. Above 900-1000°C the linear oxidation is followed by

a decreasing rate of oxidation."

The mechanisms of oxidation during low temperature logarithmic and cubic oxidation have not been established definitely,

but parabolic oxidation is generally thought to involve simultaneous parabolic oxide film formation and oxygen dissolution in the

metal. The high solubility of oxygen in titanium (up to 30 atomic

percent at 1173K (900°C)) is obvious from the phase diagram shown

in figure 8. Oxygen dissolution probably becomes important at

673K ( 40 0 °C )

.

The change to non-protect ive linear oxidation kinetics at

1073-1273K (800-1000°C) is accompanied by an increase in the rate

of reaction. The reason for the transition is not clear, but it

has been suggested that the oxide loses its protective character

due to growth stresses in the oxide scale. An alternative proposal is that the breakdown in protective behavior occurs after an

outer layer of titanium reaches a critical oxygen concentration.

The latter mechanism is supported by X-ray studies which show that

the outer layer of metal tends toward a limiting oxygen concentration of approximately 35 atomic percent.

Following extended linear oxidation, titanium's oxidation

rate again begins to diminish with time, which appears to be related to sintering and grain growth in the outer portion of the

oxide layer. In oxidation above 1473K (1200°C) the linear oxidation stage is not observed at all.

38
ATOMIC

PER

CENT

l>1xo -

Cfl 4-> rH C <G. Q) 4J o <u

OJ

>, ir» X )j cr> 0 <U rH

1 4J e u-i D <c - *h r- C

• -P C ^ •H a; rrl

E-< aioo|

00 CO CM OO a>D

Cr> 3. '3UfUVH3dW31

39

WEIGHT

PER

CENT

OXYGEN
The kinetics of titanium oxidation under conditions of rapid

heating are of considerable importance to an understanding of titanium ignition, since in most practical cases ignition occurs

during rapid anisothermal heating caused by some external energy

source. Studies of this nature have been performed by Wolf, et

ai. [29-32] us ing an infrared furnace and a volumetric oxidation apparatus. The effect of subjecting thin plates of commercially pure titanium to heating rates of from 0.5°C/s to 100°C/s

was studied for 0.25 atm (3.67 psia) oxygen. A thermocouple located near the bottom of the specimen was used to record specimen

temperature during experiments while another thermocouple near the

specimen's top was used to provide feedback to the temperature

control loop. Oxygen consumption was monitored with a pressure

transducer

.

These experiments showed that at low heating, rates (_<0.5°C/s)

the oxidation could be described by parabolic reaction kinetics.

Intermediate heating rates ( 10°C/s) led to asymptotic kinetics

followed by parabolic kinetics after a time delay that depended on

temperature. Higher heating rates (_> 20°C/s) apparently led to an effective reaction area multiplication which permits an initially

rapid oxidation rate. To explain the oxidation kinetics at these

higher heating rates it was necessary to account for this expansion of the reaction zone and to include the effects of heat deposition within the oxide scale, near the metal-oxygen interface.

Wolf and his colleagues also observed that many of the specimens

ignited at heating rates greater than 40°C/s. These observations

will be discussed in the following section on ignition (Section

6 . 2 ).

To summarize briefly, titanium is observed to follow protective oxidation kinetics under prolonged exposure to constant temperatures below about 1123K (850°C). Thus, titanium would not be

expected to ignite spontaneously during exposure to constant temperatures in this range, without an external energy input. At

temperatures in excess of 1123K (850°C), non-protec tive oxidation

kinetics are observed, which would lead to a greater likelihood of

ignition at temperatures exceeding this value.

40
6.2 Ignition

The ignition of titanium in air and various other oxygen-containing atmospheres has been studied frequently because of titanium's widespread application in aerospace programs. These studies

have attempted to establish a lower limit to the conditions for

ignition, especially where no supplemental energy is provided by

means such as mechanical impact or fracture. Schmidt and

Forney[33J have plotted all the spontaneous ignition data as a function of temperature and pressure, which is shown in figure 9.

The solid line indicates a probable lower limit to spontaneous

ignition of bulk titanium extending from about 1873K (1600°C) for

air at one atm. to as low as 1073K (800°C) in pure oxygen at about

20 atm (300 psia)

.

Several factors can affect this limit, including the size of

the specimens, the flow or makeup of the oxidizer, or any supplemental energy provided. Powders or very small section specimens

will ignite much more readily as the oxidizing surface to volume

ratio gets higher. Fine powders of titanium will spontaneously

ignite in air at room temperature and pressure. The data plotted

in figure 9 are thought to be representative of bulk titanium

specimens, although actual specimen dimensions were not reported

in every case. The effect of flow on the ignition temperature is

smallt34] but, as will be seen later, has a much larger effect

on propagation limits. The presence of moisture, nitrogen, or

carbon dioxide also has a small effect, raising the ignition temperature slightly for equivalent pressures ^5

]

# j n experiments

with a high velocity flow of oxygen through a small orifice of titanium, the metal did not ignite.

The addition of supplemental energy, however, is a different

matter. In all cases the added energy lowers the ignition temperature. This could come, for example, from two surfaces rubbing

together, impact by another object, or fracture and exposure of a fresh surface to oxidation. This latter experiment t 36 J is also

plotted as a dashed line on figure 9 to show the effect. Other

energy sources could alter the curve in other ways.

41
E

CO

CO

CO

CD

X

o

3o aanivaadi/m noiiinoi

-X c c

03 •H

CT' % P

Eh rH • X ro T)

0 • C to rH

T3 ~ rH 03 E VO

4-1 C CO 03 03 CT\ 0 <0 in c • 4-> rH

• OC >4 tX 03 —

»

c • rH 3 • Eh o <j\ EG — o £ CO * •H LO • *3 T3

4-» <y\ a rH C

O rH A » 03 03 c 3 • >1 o E

3 TJ CM 05 o •H E

4-1 — C P cq 4-> O

<N 03 in . 3 O

<U 00 rH in \s 03 rH 0 P • C 03

03 1 Ch Q 0 E

n3 Q 2 c • 34 03 V « O CO 0) 4-> e g

• 03 < 03 3 • a < a. Eh

•H r—

i

u e u fe. CO Cro < »• < o Q) o>

03 ro CO c 2 JG rH r- 03 —

’

< 0 Eh rH O • •

* H 0) 2 03 •H l 34 r—

»

1—

,

4-> u e • TJ z rH Or- CO c 03 U C CO Pro (3

4-t 03 • T3 03 1 i__i i 0 34 u < • • 05 34 03 03 <D • £ - Q E-* •H o O

03 4-1 • x—

s

• 1 < c c u 03 • * u rH »3 Q 03 03 3 34 a 03 V£> CO 34 34

4-> 03 • 4-» • <X> < c 03 0)

03 34 TD 4-> P rH C O 04 04

34 03 i—1 03 —

0 •H 03 03

03 4-3 O 03 - 03 • 03 34 34 a>w c rH 03 C * -X 2 •H E < Eh rH 03 03 r- o > 03 03

03 03 •H 34 01 rH 03 • •H 03 03 4J 05 as CQ Q ON •3 O-i Q 03 03 c 03 0 34 • • • • • • • • • • • • •H 3 < CQ o Q U Uj

4-1 03

03 • • • • • • c <13 4-4 4-1 44 4h 04 04

cn >-i 03 03 03 03 03 03

t-t 05 05 05 05 05

CTi

03u

3

cn •H

&4

42
Static tests will always give the highest ignition limit,

dynamic tests of ignition a lower limit, and the propagation of

combustion is possible at even lower temperatures and pressures.

This is clearly shown in figure 10 by the results of an experiment

determining the relation between total pressure and oxygen content

for limiting behavior under these three conditions, i.e., static

ignition, dynamic ignition (flow), and propagation. Thus, once

ignited, titanium combustion will readily propagate under the

same, or even less severe conditions. There have been several

other ignition studies under special conditions which are summarized by White and Ward^5] #

Dynamic heating to ignition at very rapid heating rates also

dramatically reduces ignition temperatures. In their studies of

anisothermal oxidation kinetics, Wolf and his colleagues observed

that many of the specimens ignited when subjected to heating rates

greater than 40°C/s . [29-32] # Furthermore, the apparent ignition temperature decreased as the heating rate was increased. For

0.010 cm- thick specimens which were heated in 0.25 atm oxygen from

room temperature at a constant rate, r (°C/s), the "sensible" ignition temperature could be fit to the equat ion ^ 32 ]

.

5

4 97x10

T. = 338 + -

-

' --- °C (6-1)

19

r

2

This relation extrapolates to the accepted value for the bulk ignition temperature of titanium, 1866K (1593°C) at a heating rate

of 20°C/s

.

When similar specimens were first slowly heated to 1073K

(800°C) and held at this temperature for t seconds, then heated to

ignition at 75°C/s, it was found that the ignition temperature

followed the expression

43
TOTAL

PRESSURE,

psig

Figure 10. Ignition and propagation limits for titanium in heliumoxygen and steam-oxygen mixtures. After ref erence l 36

J

#

44
( 6 - T. =(900 + 2.58 tV 2 ) 2 )°C ig

Further experiments showed that this inhibiting effect of preoxidation was apparently due to solution of oxygen in the metal and

not to the formation of a thicker oxide scale.

The observation of ignition temperatures as low as 648K

(375°C) is quite puzzling when compared to the bulk ignition temperatures measured with slower heating rates. Another discrepancy

with earlier investigations in the high-heating-rate experiments

was the absence of any strong effect of oxygen pressure,

0.13-0.53 atm (1.9-7. 6 psia) , on the ignition temperature

t

22 1

.

Although Wolf does not discuss the lack of pressure dependence in

any detail, he does suggest that the low sensible ignition temperatures imply that ignition is probably precipitated by localized

deposition of heat in small hot spots with an attendant rapid increase in the local oxidation rate and a subsequent thermal runaway. That is to say, the metal temperature at ignition as measured with a thermocouple near the bottom of the specimen may not

accurately reflect the local temperature in the hot spot where ignition begins. Another consideration that probably affects these

results is the high surface-to-volume ratio (S v ) of the specimens (200 cm”l for the 0.010 cm-thick specimens). As pointed

out above, the surface-to-volume ratio is important in metal ignition, and specimens with high values of Sv can ignite at much

lower temperatures than bulk specimens. Mellor^^J estimates

the limiting value of S v for titanium to behave as a bulk material as S v

- 25.6 cm

_ l, and cites several earlier studies that

found much-reduced ignition temperatures for titanium powders with

Sv

- 400 cm

- -*-.

Wolf also compared several titanium alloys for ease of ignition under rapid heating [32] i n a limited number of tests.

The results showed that TiAl and T^Al were most resistant to

ignition (did not ignite), Ti-13 Cu and Ti-8III alloys were less

45
resistant to ignition (T^g :s 1173K (900°C)), while commercially

pure titanium, Ti- 8 Mn, and Ti-6Al-4V were the least resistant to

ignition (T ig =1023-1073K ( 750-800 °C ))

.

In summary then, the ignition studies show several things relevant to the use of titanium in turbine engines. First, a probable lower limit to ignition can be established for given tempera - ture and pressure conditions (e.g. about 1866K (1593°C) in 1 atmosphere air)# but any supplemental energy will lower these limits.

Higher pressure also lowers the ignition temperature. Second, titanium will ignite and burn in a variety of atmospheres, including

nitrogen and carbon dioxide. Once ignited it will burn readily,

even in carbon dioxide. Third, bulk titanium, which will not spontaneously ignite at moderate temperatures and pressures, will ignite and burn readily if subject to supplemental energy such as impact, rubbing, or fracture leading to exposure of a fresh surface.

Fourth, when thin specimens (S v

= 200 cm“l) of titanium are

heated at rates greater than 40°C/s, they are observed to ignite

readily at temperatures below the bulk ignition temperature, 1866K

(1593°C). If heated at 100°C/s, these specimens can ignite when

their average (or "sensible") temperatures are as low as 573 to

673K (300 to 400°C). Conditions that lead to oxygen solution in

the base titanium prior to rapid heating result in an inhibition

of titanium ignition. For different titanium alloys, the relative

resistance to ignition decreased in the order: TiAl , T^A1>

Ti-1 3Cu , Ti- 3 II I> CP Ti , Ti- 8 Mn, Ti-6Al-4V.

46
6.3 Combustion in Static Environments

There has been surprisingly little fundamental research on

the basic rates and mechanisms of steady-state titanium combustion. The only detailed studies are the early wire-burning experiments of Harrison and Yof f ee ^ 38 , 39 ] and the more recent work

of the authors at NBS^36, 19, 23, 40—45] with small (0.2— 0.5g)

bulk specimens.

Between 1954 and 1961, Harr ison t 38 , 39 ] reported measurements of the burning rates of small wires of aluminum, iron, magnesium, molybdenum, titanium and zirconium. Vertical wires were

ignited at the lower end by a small titanium coil and photographed

while burning in various oxygen concentrations up to 100 percent

oxygen at 1 atmosphere. Harrison found that the burning velocity

of titanium wires was proportional to the square root of the pressure for pressures below one atmosphere. He also found that a critical oxygen concentration exists, below which combustion does

not propagate; this minimum concentration increased with increase

in wire diameter and varied for different materials. Photographic

images of the burning titanium wires were clear and distinct, with

no evidence of a diffuse vapor phase combustion zone, leading

Harrison to conclude that titanium burns at the surface of a molten mixture of the metal and its oxide. The color temperature of

the burning titanium varied from 2700 K (2430°C) at an oxygen

pressure of 0.05 atm to 3330 K (3060°C) at 1.0 atmosphere.

Some of Harrison's observations on the way combustion propagated along titanium wires are particularly germane to the present

discussion. The propagation of combustion into the unburned wire

was oscillatory in nature. A nearly spherical globule of molten

metal would form at the end of the wire, remaining stationary in

space as the melted zone moved up the wire. When the globule became large enough to overcome its surface tension, it would

47
separate from the wire and fall while a small amount of melt would

jump up to form a new globule further up the wire, starting a new

cycle. Harrison found that the droplets that fell from the wire

were only about 15 percent oxidized. From this observation

Harrison concluded that [38] "...no great significance can be

placed upon the absolute values of the propagation rates..." As

he recognized, however, propagation rates are valuable as a relative measure for comparing different materials or environmental

conditions

.

The distinction between burning velocity and the more fundamental combustion rate is an important one that will arise again

when we discuss propagating combustion in dynamic environments.

In this section we discuss measurements of the combustion rate , the actual rate at which oxygen is absorbed by molten titanium.

We will see that this rate is mass transport limited; under most

conditions oxygen is absorbed as fast as it can diffuse to the

surface. We will also see that the combustion rate does not vary

significantly for different alloys, since the factors that govern

the rate of oxygen diffusion to the surface will be essentially

the same for different alloys. Then, in section 6.4 we will discuss measurements of the burning velocity , the rate at which titanium is lost from a blade-like specimen as it burns in a simulated

compressor environment. It should be clear that the combustion

rate and the burning velocity are quite different parameters. Although the burning velocity will depend to some extent on the combustion rate, it will also be affected by a number of other parameters such as the thermal diffusivity, surface tension and viscosity of liquid titanium, the Reynolds number at the leading edge,

and the viscous drag of the airstream. Unlike the combustion

rate, which varies only slightly for different alloys, the burning

velocity shows significant variations for different alloys. This

does not mean that the results of the two experiments are at odds;

rather, it implies that factors other than the combustion rate

must be the cause of these variations in burning velocity.

48
The authors' research has extended over several years and has

been directed toward developing new quantitative techniques for

studying metal combustion and extending fundamental studies to

samples of larger dimensions. The use of larger, stationary specimens has considerably eased some of the experimental difficulties

associated with metal combustion research and more closely approximates the conditions prevailing in large-scale accidental metal

fires. The objectives of this research have been to better define

the heterogeneous combustion process for bulk titanium metal, identify the reacting species and products, determine the reaction

location, and discover the primary rate-limiting mechanisms.

Early stages of the NBS research program included literature

surveys t 6 * 40 ] , studies of the ignition and combustion of

various steels and alloys of aluminum, titanium, and copper^-U,

and preliminary experiments on laser-initiated combustion of pure

titanium using a new optical technique t 23

]

# Several novel

analytical techniques were developed and, together with more conventional techniques, applied to the combustion of bulk titanium

and titanium alloys. The results of experiments employing these

newly developed methods have yielded substantial data on the rates

and mechanisms of titanium combustion and considerable insight

into the very complex heterogeneous reactions involved in metal

combustion.

The experimental techniques used include: a new gravimetric

technique employing an electronic weight sensor to determine oxidation rates during self-sustained combust ion f

1

,

temperature

measurements during combustion using two-color py rometry 1

;

time-resolved spectroscopy to identify vapor-phase reactants and

products and their location and duration during combust ion ^42 , 43

]

novel laser technique for phase contrast visualization of flame

structure and temperature fields in vapor-phase combustion zones [23]

high-speed cinematography of burning metalsf23]. as Well as

x-ray and chemical analysis, metallography, and scanning electron

microscopy (SEM) of combustion products t

1

.

49
Most recently, a dramatic improvement in the method of timeresolved spectroscopy was achieved through the use of an electronically-scanned spectrometer, with periodic sampling and digital

storage of the spectral scans^^]. This technique was used to

study the role of condensed products in metal flames and to explore a newly discovered laser-flame interact ion [45 J

.

In the experimental arrangement a 100 watt cw CO 2 laser

provided a clean, remote and controllable ignition source for

130-500 mg specimens of commercially pure (99.2%) titanium and

various titanium alloys. Cleaned and etched specimens were placed

on a polished graphite block in one atmosphere of gently flowing

oxygen, permitting an unobstructed view for a high-speed camera,

photodiodes and the entrance optics of the spectrometer. A typical

experimental arrangement used to measure temperature and oxidation

rate during titanium combustion is shown in figure 11.

The controllability of the laser ignition source and constant

specimen sizes permitted readily reproducible experimental conditions. A characteristic total luminosity versus time curve, as

measured by a photodiode, was remarkably reproducible and provided

a convenient time scale as a reference for the other analytical

techniques. These were:

(a) High-speed color cinematography to study the general

behavior, approximate color temperature, and the relative

rates of reaction.

(b) High-speed Hilbert transform cinematography to visualize the general morphology of the molten metal surface, normally obscured by self luminosity, and to provide estimates of

the temperature gradients in the vapor zone surrounding the

specimen

.

(c) Time- and space-resolved visible spectroscopy to

identify vapor species and possible excitation mechanisms.

50
Focussing

Mirror

51

Figure

11.

Experimental

arrangement

for

determining

temperature

and

oxygen

uptake

during

combustion

of

titanium

in

oxygen.
(d) Two-color pyrometry to determine surface temperatures.

(e) Dynamic gravimetry to provide a direct measure of the

uptake of oxygen during combustion.

(f) Metallography and scanning electron microscopy of specimens quenched at various stages in the process to identify

reactants and products and their locations.

(g) X-ray and chemical analyses of products.

Visual observations and high-speeed movies showed relatively

few differences in the combustion of titanium and various titanium

alloys. Following the ignition (and melting) stage, which is

accompanied by a very steep rise in total luminosity, specimens

assume a characteristic sessile-drop shape and burn steadily and

quietly by surface reactions for a little more than half of their

combustion history (the total combustion event lasts only 3-4 s).

The end of this steady burning stage of combustion is marked by

the specimen erupting into violent boiling, with the ejection of a shower of particles that continues until self-extinguishment and

solidification of the specimen. Metallographic exmaination of the

residue has shown that combustion is always incomplete, with a core of titanium and oxygen in solid solution overlain by layers

of TiO and TiC^. Oscilloscope traces of the total luminosity

during the event show it to be quite reproducible. The various

stages of titanium combustion are shown in Figure 12.

To summarize our current state of knowledge of the ignition

and combustion of titanium and its alloys, the highlights of our

research are listed below for the three distinct stages of titanium combustion, which were identified very early in our studies.

STAGE 1 - IGNITION . The specimen is heated in oxygen with a CC>2 laser until a self-sustaining combustion reaction is obtained.

* Local ignition possible . High speed movies show small

regions can ignite and exhibit signs of advanced stages of

combustion, e.g. ejected particles. The heat of combustion

contributes to the rapid melting and ignition of the

remainder of the specimen.

52
Figure 12. Three frames from high-speed movies of titanium burning in

one atmosphere oxygen. Top: after ignition, titanium

burns steadily and quietly by surface reactions, with

occasional ejection of particles. Surface is obscured by

intense continuum radiation from vapor region. Middle:

about halfway through combustion process, specimen erupts

into violent boiling, ejecting a shower of molten droplets. Some of these particles explode violently. Bottom:

towards end of combustion, shower forms "linear spray"

with no further explosions. After reference [23] #

53
* Temperature spikes observed . Two-color pyrometry indicates

temperature peaks as high as 4000 K (3727°C) correlated with

ignition flares, whereas steady burning temperatures are consistently about 3200 K (2930°C), near the boiling point of

Ti0 2.

* Influence of vapor phase combustion on ignition possible . Spectroscopy and phase contrast cinematography show an

initial surge of titanium vapor during ignition, which may

contribute to the temperature spikes. Time-resolved spectra

reveal the prompt appearance of alloying elements in this

vapor cloud, suggesting that vapor phase processes could

enhance ignition for alloys containing highly volatile and

combustible components. Phase contrast movies show that the

vapor cloud collapses to a narrow boundary layer l-2mm thick

after ignition.

STAGE 2 - STEADY BURNING. The specimen appears to burn steadily

and quiescently on the surface following ignition and melting.

*Two rate-controlling processes determined . Mass uptake

measurements reveal two distinct rates for this stage of

the combustion process, differing by a factor of about

three. The change from an initially fast, linear rate of

oxygen uptake to a slower rate correlates with the disappearance of the vapor zone and its associated spectra, a decrease in temperature and the appearance of accumulating oxides which hinder the flow of oxygen to the metal

f uel

.

* Combustion rate shown to be mass-transport limited.

Studies of the effect of oxygen velocity on titanium's

combustion rate show that gas-side diffusion is rate-limiting during the initial part of the reaction. An exchange of

control to liquid-side diffusion occurs when oxides begin to

accumulate

.

*Alloying shown to have little influence on combustion . The measured rates of oxygen uptake, the color temperatures and visual observations of the steady burning stage

show remarkably little variation for the alloys studied.

54
STAGE 3 - SHOWER . The steady burning stage ends abruptly with an

extensive and violent shower of particles.

*Subsurface combustion reaction discovered . The high

speed cinematography shows violent oxide motion on the

surface during this stage. Metallurgical and SEM observations of quenched specimens indicate a subsurface

boundary between the outer layer of TiC>2 and the core,

consisting of a Ti-0 solid solution. The subsurface combustion appears to create voids, segregate alloy components, and provide the necessary driving force to blow off

molten droplets of the burning specimen.

*Products of the shower are hollow spheres of TiQ ?

.

Early in the shower when the ejected droplets are incompletely burned, the same subsurface reactions that cause

the violent boiling lead to explosive fragmentation of

ejected particles. Later in the shower the driving force

is insufficient to cause explosive fragmentations and the

collected products of the shower are hollow spheres of

nearly stoichiometric TiC^/ as revealed by scanning

electron microscopy and chemical analysis.

*Accumulated oxides shut off reaction . The decreased

rate of oxygen uptake towards the end of combustion and

the metallurgical observation of a thicker and thicker

oxide layer lead to the conclusion that limitation of

oxygen diffusing to the subsurface combustion zone soon

quenches the reaction. Increased oxygen flow across the

surface stirs the molten oxide permitting a faster combustion rate.

55
As discussed in the preceeding synopsis, the steady burning

phase of titanium combustion is kinetically governed by two sequential rate-controlling processes. The initial rate-limiting

step, which is the faster of the two, is the diffusion of oyxgen

to the specimen's surface through a boundary layer of vapor products. The second rate-limiting step, which takes control when

oxides begin to accumulate on the surface, is the diffusion of

oxygen through a growing liquid oxide layer. Preliminary experiments with commercially pure titanium showed that the flow of oxidizer can markedly increase combustion rates during both stages of

the reaction.

More recently, measurements were extended to study the effects of flow on the combustion rates for Ti-6Al-4V over a greater

range of flow velocities. The results of this experiment are

\ shown in figure 13. The rate of oxygen uptake during both stages

of combustion increases with increasing flow of oxygen to a maximum, beyond which the average rate decreases. Although there is

as yet no detailed theoretical model with which to compare this

data, it seems plausible that increasing the flow of oxygen increases the combustion rate during the first part of the reaction

by blowing away products and reducing the thickness of the vapor

diffusion barrier. The effect of flow on combustion rate during

the second part of the reaction is more difficult to explain, but

high speed movies of burning titanium suggest that stirring of the

oxide layer by the flowing gas may be the cause. There is a maximum for both curves at a flow velocity which is near the limit for

laminar flow from the oxygen supply tube. Turbulence disturbs the

oxygen flow pattern, causing a decrease in the average combustion

rate and an increase in the scatter of the data. Clearly, however, much work remains to fully explain these effects.

56
OXYGEN

UPTAKE,

g

em

OXYGEN VELOCITY, cm s

1

Figure 13. The effect of flow on the rate of oxygen uptake during

combustion of Ti-6A1-4V. and J 2 are the rates of

oxygen uptake during the first and second parts of the

steady-burning stage of combustion.

57
One goal of the studies of titanium combustion by dynamic

gravimetry was to measure the combustion rates for a number of

different alloys to see if the addition of certain alloying elements could reduce the rate of oxygen uptake. It is known, for example, that the addition of small amounts of silicon to iron or

copper systems can markedly reduce their rates of oxygen absorption during combustion by forming a thin slag layer on the surface. The combustion rates for commercially pure titanium and a series of 23 titanium alloys were measured in a gentle flow of oxygen ( ~ 6 5 cm^/s) at atmospheric pressure. The combustion rates

for these alloys during the first (J^) and second (J 2 ) parts

of the combustion reaction are presented in Table 6. These results are the average from several specimens of each alloy. With

only two exceptions, little variation in combustion rate was observed among the alloys studied; the rate during the first part

of the reaction varied from a high of 0.12 grams C^/cm^ s for

Ti-5Mo to a low of 0.07 grams C^/cm^ s for Ti-5.8

Al- 2Mo- 2 . ISn-l . 9Cr-l . 8Zr . The two exceptions, Ti-6Sn and Ti-9Sn,

exhibited a curious bimodal behavior. Specimens of these two alloys either burned in a "fast" mode comparable to the combustion

rates of other alloys, or in a "slow" mode that was lower by a factor of 30-50 percent. No explanation for this behavior has yet

been found.

In general, the addition of tin or tin in combination with

aluminum seemed to result in the lowest observed rates. But the

rates for alloys containing these elements were only about 20 percent lower than average. This difference is not much greater than

the estimated experimental uncertainty of + 10 percent. One factor that may help to explain the lower rates observed for tin- and

aluminum-bearing alloys is the high vapor pressure of these elements at 3200 K (2930°C). Loss of mass from the specimen by vaporization of tin or aluminum during combustion would result in a lower apparent combustion rate as determined by dynamic gravimetry. Spectroscopic observations of the prompt appearance of

aluminum vapor during ignition^4] tend to support this hypothesis .

58
Table

6.

Titanium

Alloy

Combustion

Rates

i CO

CN CM d i g

o

ro in ro co no ^ i n* in co m n* m m r- co r- co in n m <n m ooooo ooooo ooooo ooooo oooooo

ooooo ooooo ooooo ooooo oooooo

C7>

CO

CN

I g

o

cn

o o oo o

I—I r—) O O O

o o r—i o

>H O O i—1 i—

I

HOM^C\ r—I i—I O O O

OOOOO ooooo ooooo

CN pH O O O

H i—I H H H

OOOOO

H O LD O '3* O H H O H O H

OOOOOO

-U £ -U £

cn o cn o

fQ rH fO rH

4-1 0) 4-1 CO

<J Vi •i

H

H CN U CO H 00 CN N’ H • CTi CN <n rH • •

•

-

l CO 1 u 1 U

u 0 IN

Cu ac £ OC r- • CN • o i—1 • 1 1 • l •N1 1

l c 1 0

> C > CO U £

0 0 CO CO CO 1—

1

CJ ac £ £ • m • • <Ti • CN CO CN • 00 CN • i—

1

>1 1 1 1 •n< 1 1 LO l 0 S-i u H l 0 0 1 G

rH > c CN CN < y> £ £ > CO rH CHM N* CO IS) CN o i—

1

O

< CO 1 CN 1 1 c c • CO • • • • — m o 1 C C CO CO ro 1 CN 00 LO

df> > • £ > CO CO CN N* | 0 1 1 1 i CN •tf CN H VO CN CN H 1 <

V

£ pH H rH

• 1 1 1 1 1 1 1 0 in CJ < < < O H H i—1 H H H i—

1

£ -N* • ac 00 co ac

<Ti < < < < < < < CN CN rH • • • • —* vo m oo co Bill co VO ^R 1 rH •

1 II rH

D LO

1 inn CO1

1 i 1 1 1 •H -H *H *H -H

i •H

II 1 •H *HI 0 •H *H1 •H

1 •H

1 •H

1 •rH

1 •rH Eh Eh Eh Eh Eh Eh Eh Eh Eh Eh Eh Eh Eh Eh Eh

O O

0 £ £

O 0 £ £ C C C

c

CO £ H o o o CO CO CO CM m H CN

1 1 1 CO H*1 1 CN co1 0

<7\

1 rHI 1 1 u •rH *rH *rH

1 1 •rH *rH

1 1 •rH *rHI •rH

i •rH Eh Eh Eh Eh Eh Eh Eh Eh Eh

59
These static experiments and the experiments at low flow

rates have helped to elucidate the basic phenomena of titanium

combustion. The experimental conditions, necessarily simplified

in order to permit analytical observations, are quite different

from those found in turbine engines. However, for thb purposes of

this review we can list several aspects of titanium combustion

that are pertinent to further discussion of the dynamic experiments and the use of titanium in turbine engines.

° Under the proper conditions, titanium is readily

ignitable and, as long as sufficient oxygen and

fresh metal are available, it will continue to

burn.

These conditions are well above room temperature

and pressure and ambient oxygen concentrations.

As will be seen, the dynamic experiments will help

quantify these conditions.

° When titanium does burn, it does so violently with

large amounts of heat and molten material released

which can contribute to the propagation of combustion.

° The combustion reaction occurs on the surface of

the molten metal. Initially the reaction rate is

limited by transport of oxygen to the metal surface . In later stages of the reaction, the accumulation

of molten oxides drives the reaction zone beneath

the surface, slowing the reaction rate. This behavior would not be expected to occur in an engine

fire where aerodynamic forces constantly remove

the molten metal, exposing fresh metal.

Increasing the flow of oxidizer increases the combustion rate by increasing the availability of oxygen at the surface.

Alloying titanium, at least for the alloys studied

in these experiments, has little effect on its

combustion rate but may affect its igni tabil ity

.

The fact that the combustion rate for different

alloys is constant does not necessarily imply that

their burning velocities in a compressor environment will be the same.

Further relevant aspects of titanium combustion can be

obtained from a summary of recent dynamic experiments.

60
6.4 Titanium Combustion in Dynamic Environments

Compared to the quiescent environments used to study steadystate titanium combustion, the inside of a jet engine is a veritable maelstrom. Centripetal acceleration can exceed lO^g's;

typical modern compressors achieve pressures and temperatures in

excess of 30 atm (450 psia) and 870 K (597°C), at airstream velocities of 300 m/s (1000 ft/s). It is obvious that the progress of

a metal fire in this environment will depend to a great extent on

the effect of these extreme conditions on the propagating molten

mass of burning metal. Metal fires involve propagating combustion, and the propagation of burning must be understood as well as

fundamental reaction mechanisms in order to eventually prevent

fires

.

In an engine, the high airstream velocity and pressure will

act to increase the combustion rate by increasing the availability of oxygen and by removing any gas phase products that might

impede the flow of oxygen to the surface. It is quite likely that

the actual combustion rate (see glossary for distinctions between

combustion rate, burning velocity and propagation velocity) will

be limited by mass transfer of oxygen to the surface by diffusion

through the aerodynamic boundary layer. While combustion rate,

and therefore the heat generation rate, will be controlled by mass

transport, the burning velocity and propagation characteristics

will probably be controlled by the balance between the aerodynamic

and centrifugal forces tending to remove the molten metal and the

surface tension and viscosity effects that make it adhere to the

surface. The heat and oxygen content of the molten material

sloughed from the burning titanium, as well as the size of the

droplets, will affect the propagation of a fire from one stage of

the engine to another.

61
In order to understand these complex phenomena, techniques

have been developed to study titanium combustion in small experimental wind tunnels. This, at least, can simulate the pressure,

temperature, and velocity of the airstream in a compressor environment for static components (stators, guide vanes, etc.) and

give some insight into the behavior of rotating components, which

will tend to sling molten metal off tangentially. All the dynamic

combustion experiments reported in the open literature were performed at two facilities: the Air Force Electric Aerodynamic

Laser Test Facility at Wr ight-Patterson AFB 1 13 , 47 , 48 ] ancj i n the laboratories of an engine manuf acturer

• 46 , 50 , 51 ] . This

work has also led to an analytical model for predicting self- sustained combustion in titanium airfoils [ 10 '46

]

. MUC h of the

work done by these two groups is complementary and has been summarized in an excellent joint paper HO], Other manufacturers

have similar experimental capabilities and have performed tests

similar to those discussed here, however, the reports of these

studies have not been published in the open literature.

In the Air Force experiments, flat, plane-parallel Ti-6Al-4V

specimens with overall dimensions similar to compressor blades

were clamped at one end in the wind tunnel and ignited at the

upper leading edge corner with a high power CO 2 laser. Combustion was mQnitored with closed-circuit TV and high speed movies.

In the experiments of the engine manufacturer, uncambered, airfoil-shaped specimens of Ti-8Al-lMo-lV were clamped at both ends

in the experimental chamber and ignited with a CO 2 laser focused

on the middle of the leading edge. TV monitoring and high speed

movies were also used to observe behavior in these tests. Some

early tests used different ignition sources, including electrical

arc [46] an<3 impingement of molten titaniumfl^]

62
Both groups of experimenters found it necessary to coat the

laser irradiated areas of the specimens with black coatings to improve laser coupling and insure ignition. Because of the difference in experimental configurations, the nature of combustion propagation was different. The Air Force tests usually resulted in

rapid propagation of combustion along the chordwise direction of

the top edge (parallel to flow) , followed by more-or-less uniform

spanwise burning down the specimen (perpendicular to flow). In

the manufacturer's tests, combustion following ignition spread in

the streamwise direction across the chord at midspan. When the

blade had been burned completely through in the chordwise direction, burning progressed spanwise, perpendicular to the exposed

edges. Composite time sequences from high speed movies showing

the propagation of combustion in these two different types of test

shown in figure 14.

Specimens were ignited over a wide range of pressure, temperature and velocity of the airstream and the results reported in

terms of percent damage as a function of the environmental conditions. Since the Air Force specimens had a uniform cross section,

the rate of mass removal (burning velocity) could be measured from

the high speed films.

The results showed that self-sustained combustion of

Ti-6Al-4V and Ti-8Al- IMo-lV depends strongly on pressure, with

lesser effects being noted for temperature and velocity. The

spanwise burning velocity was essentially constant in time for

given conditions, but varied depending on pressure and velocity of

the airstream. At low pressures (near atmospheric) there was

almost no dependence of burning velocity on airstream velocity,

while at higher pressures the rate of burning was influenced by

both pressure and velocity.

63
2.54

mo ee g

iuo ig g

OS 15

t=> CO a.

a3ue;pejj| Jai^v auijj.

u

as

V)

CO

64

Figure

14.

Composite

time

sequences

from

high

speed

movies

showing

propagation

of

combustion

in

(a)

an

engine

manufacturer's

Air

Force

wind

tunnel

experiments.

Test

conditions

were:

(a)

6.8

atm,

700

K,

219

m/s;

(b)

2.5

atm,

540

K,

222

m/s

After

reference

1

.
The complex dependence of degree of damage on environmental

conditions is summarized in reference 10:

"The degree of damage from combustion is observed to be

significantly pressure dependent over the entire

temperature-velocity region investigated. As velocity

increases, temperature-pressure regions of no combustion

decrease in area until, at approximately 600 ft/sec, no

region of non-sustained combustion is observed.

At velocities above 600 ft/sec, areas of massive burning

(virtually total combustion) are observed at higher

temperatures and damage is temperature dependent.

While damage is observed to increase with increasing velocity

at most pressures, this trend is reversed above 900 ft/sec.

The early extinguishment observed at these higher velocities

is the result of a more rapid melt removal and greater convective cooling."

The importance of metal removal by aerodynamic forces was

readily apparent in high speed films. At low airstream velocities

considerable amounts of liquid metal flowed over the surface of

the unburned portion of the specimen, aiding in the spread of combustion. As airstream velocity was increased, more rapid removal

of molten metal occurred due to the greater aerodynamic forces.

As the airstream velocity increased, the droplet size of metal removed from the surface decreased from large globules to a finer

and finer spray. This behavior has obvious implications for the

rapid propagation of a titanium fire to downstream components and

structures

.

65
Another interesting result was the effect of angle-of-attack

on degree of damage sustained. In one test, a specimen inclined

at a 10° angle-of-attack suffered 80 percent damage (essentially

complete combustion) while an identical specimen burned in the

same environmental conditions but with 0° angle-of-attack sustain

ed only 30 percent damage^^l. This behavior is illustrated

in figure 15. The difference in damage appears to be due to

boundary layer separation on one side of the airfoil, which

results in more molten metal adhering to the surface. However,

recent tests at the Air Force Aero Propulsion Laboratory have so

far failed to confirm this behavior^].

More recent experiments by Anderson and Manty ^ ^ , 50 , 51

]

have employed dynamic test methods to investigate the effects of

alloying elements on the combustibility of titanium and to begin

studies on the efficacy of coatings for preventing ignition and

combustion of titanium alloys. For these experiments either

airfoil-shaped specimens or plane-parallel specimens (with the

leading edge machined to a knife edge to promote ignition) were

clamped in the test fixture as in figure 14a and ignited with a laser. The environmental variables of temperature, pressure and

airstream velocity varied somewhat from one experiment to another

but generally only two or three sets of test conditions were used

in any one experiment; typical values were^7] 700 K (427°C),

4.7 atm (70 psia) , and 240 m/s (789 ft/s) or 480 K (207°C), 6.0

atm (90 psia) and 115 m/s (376 ft/s).

The alloys tested to date include

:

Ti-6A1-4V

Ti- 8 AI-IM0-IV

Ti-6Al-6V0-2Sn

Ti- 6Al-2Sn-4Zr-2Mo

Ti-6Al-2Sn-4Zr-6Mo

Ti- 8 Mn

Ti-ll.5Mn-6Zr-4.5Zn ( III)

66
aui;x pasde|3

5©

u®

V)

auijj, pasde|3

LD

a;D

CF>

Cu

67

Effect

of

angle-of-attack

on

combustion

propagation;

(a)

0

angle-of

-attack

,

(b)

10°

angle-of-attack.

Test

conditions

for

both

tests

were

9.5

atm,

700

K,

and

40

m/

s.

After
Ti-13.5Al-21.5Nb

Ti-13V-llCr-3Al

Ti-5Ni

Ti-lONi

Ti-20Ni

Ti-1 3Cu

Ti 3AI

Most of these alloys sustained significant damage from combustion

under the conditions of the tests. Although alloys could be ranked by the severity of damage sustained, the. rankings were different for different sets of environmental conditions, making generalizations impractical. Of all the alloys tested, only three did

not burn under any of the test conditions: T^Al, Ti-20Ni, and

Ti-13Cu. A fourth alloy, Ti-13V-llCr-3Al, burned in only one of

four tests at different environmental conditions; ironically, it

was under the least severe test conditions that this alloy burned.

Ti 3 Al was found to be combustible in Air Force tests using a more powerful laser for ignition^ 3 ].

Unfortunately, with the possible exception of Ti-13Cu, none

of the combustion- resistant titanium alloys so far identified have

adequate mechanical properties for aircraft turbine engine service. Most are excessively brittle or suffer some other serious

degradation of mechanical properties. But this research has helped to identify the properties that contribute to an alloy's ability to resist combustion. One of these is a melting point substantially below the ignition temperature of titanium. A low

melting point alloy resists ignition because it can melt and flow

away from an ignition source before reaching the ignition temperature; Ti-20Ni is such an alloy. Other characteristics that enhance combustion resistance are high thermal conductivity, high

heat of fusion and low heat of combustion. In short, anything

that reduces the buildup of heat in an alloy subjected to an external energy input helps to prevent the material from reaching

its ignition temperature.

68
Manty and Anderson have also started preliminary studies on

the effectiveness of coatings in preventing titanium combustiont7'50] # Two diff erent types of coating were investigated:

a chromium-molybdenum coating plated on Ti-8Al-lMo-lV and

Ti-6Al-2Sn-4Zr-2Mo, and an aluminum coating applied to Ti-8-1-1 by

ion vapor deposition (IVD). Both coatings gave a significant degree of protection to the titanium alloys for the two environmental conditions studied; uncoated specimens of both alloys suffered

significant damage at both conditions. The IVD aluminum coating

totally prevented sustained combustion of Ti-8-1-1 at both test

conditions. The chromium-molybdenum coatings prevented combustion

in the less severe tests, but only reduced the degree of damage

sustained in tests at a higher temperature and airstream velocity.

The success of the IVD aluminum coating was attributed to aluminum's low melting temperature, high thermal conductivity and high

heat of fusion. Aluminum coatings are not considered viable for

aircraft turbine engines, however, because the aluminum forms

intermetallic compounds with titanium, degrading its mechanical

properties. But these encouraging preliminary results should help

stimulate further research on coating development. The influence

of the type and intensity of the ignition source on coating effectiveness should be addressed in any further studies, since recent

Air Force tests have succeeded in igniting coated alloy specimens

using a more powerful laser ignition source

^

.

We have seen in this section that as experimental conditions

come closer to the actual environment encountered in an aircraft

turbine engine, the combustion behavior of titanium becomes much

more complex. Changes in environmental conditions affect not only

the reaction kinetics, but such physical processes as melt flow

and retention as well. The importance of these physical processes

to the spread of combustion over a titanium blade also means that

specimen size, shape and orientation come into play. In order to

explain and predict this complex behavior, Glickstein HO '46

]

has developed an analytical model of the self-sustained combustion

of titanium airfoils. In the next section we discuss the evolution and capabilities of this model.

69
6.5 Analytical Models of Titanium Combustion

Because of the complexity of the titanium fire problem and

the large number of factors affecting combustion, it is most desirable to have an analytical model that, given the relevant material properties, environmental conditions, and ignition sources,

can predict the ignition and subsequent quench or self-sustained

combustion of titanium alloys in engine environments. The model

could be of great benefit to engine designers, enabling them to

evaluate a large number of candidate alloys without the need for

extensive experimental tests. Just such a model is currently

being developed by M. R. Gl ickstein HO *46 ] ancj substantiated

by dynamic combustion experiments of the type discussed in the

previous section. The model has been under development for several years now, with new capabilities being added as they are developed .

Glickstein's first approach to modeling involved several simplifying assumptions to allow the results to be expressed analytically in closed form.f46] These assumptions include: (1) ignition has already occurred, (2) the leading edge region can be

represented as a flat plate with a cylindrical edge, (3) combustion proceeds on the cylindrical surface at a steady rate limited

only by diffusion of oxygen to the surface, (4) thermal conduction

is one-dimensional (a function of the chordwise distance from the

leading edge), (5) boundary layer flow over the leading edge region is laminar, and (6) combustion occurs at the melting temperature of the metal, and liquid metal is blown away as soon as it is

formed

.

The analysis is based on the heat balance between the heat

generated at the leading edge due to combustion and the heat lost

by conduction, convection and radiation. The variables that are

70
considered are the blade thickness, the airstream conditions, the

heat of reaction, and the thermophysical properties of the metal.

The result is an expression for the burning velocity as a function

of airstream temperature and the leading edge Reynolds number,

based on edge thickness. A thermal stability analysis of this

functional relationship yields a definition of the limiting conditions for self-sustained combustion.

When the predictions of the model were compared to the results of a limited number of wind tunnel experiments t

J it

was found that the predicted limits for self-sustained combustion

agreed well with experiment. But the predicted burning velocity

was an order of magnitude lower than the rate observed in experiments performed at temperatures and pressures well above the sustained combustion limit. The low predicted burning rates were

attributed to two factors: disruption of laminar flow near the

leading edge by vigorous combustion leading to highly turbulent

flow and higher oxygen diffusion rates, and heating of the boundary layer air by the combustion zone at the leading edge. Empirical correlations were devised to modify the model so that the predicted burning rates agreed with the observed rates for all conditions .

The good agreement between the combustion limits predicted by

Glickstein's model and the experimental results is quite encouraging. It suggests that self-sustained titanium combustion may be

reliably predicted on the basis of such a model, and thus that

such an analytical model may be used to help establish guidelines

for using titanium alloys in turbine engines within the limitations of the model (i.e., thin static parts at 0° angle-ofattack) . Since the validation of the model's predictions is based

on a limited number of data published in reference 46, it is

71
important to correlate more of the available data with the predicted limits. As part of this review, we have undertaken this

task; the results are discussed in detail in Section 7.

Further evolution of Glickstein's analytical model has been

based upon the new knowledge about titanium combustion that has

been obtained since 1974. Of particular importance are modifications to the model to account for the flow of molten titanium

across the titanium surface, a phenomenon that was not included

in the first model. The need to account for the effects of molten

metal flow became apparent from observing high speed films of wind

tunnel combustion experiments. Another important difference in

the current model is the use of numerical finite-difference techniques to simultaneously solve the equations for heat and mass

transfer in the aerodynamic boundary layer, transient thermal conduction in the solid metal, melting and liquid metal flow from the

leading edge, and retention of the molten metal on the solid substrate with attendant thermal interaction. Instead of assuming

ignition to have occurred, the current model is capable of predicting ignition from a variety of ignition sources including radiation, mechanical friction, and exposure to high ambient temperatures .

The mathematical model itself is based on a physical model of

the combustion process. This physical model, shown in figure 16,

assumes that liquid metal is formed at the leading edge, with part

being blown away by the airstream while the remainder flows across

the surface driven by aerodynamic shear and drag forces. As the

molten metal flows across the surface, part of it resolidifies,

giving up its latent heat of fusion to the underlying solid metal.

Heat from the reacting molten metal together with any heat supplied from external sources constitute the driving force for

72
73

Figure

16

.

Physical

model

for

the

combustion

of

titanium

airfoils.

After

ref

erence

l

10J.
propagation of the process. Whether or not the process will be

self-sustaining and if so, the burning velocity, are determined by

the energy balance between these heat gains and the heat lost to

the environment by radiation and convection. The rate of internal

energy change is given by the relation: HO]

dE

dt ^conduct ion +c*convect ion +<^rad iat ion ^input ^reaction

^ ^

where the various q's refer to the heat lost or gained by different mechanisms. A change in internal energy may be manifested as

either sensible heating or phase change. A discussion of the derivation of the mathematical expressions relating the various heat

losses and gains to the environmental conditions and material properties would be much too complex to include here. The interested

reader is referred to the original papers.

The analytical model as it now stands is incomplete, but work

is continuing to add the capability to consider a number of other

factors. Most recently, the model has been extended to allow evaluation of the combustion of rotating blades, although there is

not yet any experimental data to validate the model. Still to be

added to the model are: flO]

"Effect of alloys and surface coatings on surface

properties related to liquid metal flow and melt

retention

.

Effects of aerodynamic angle-of-attack on combustion

propagation

.

Susceptibility to liquid metal impingement in airfoil

cascades .

Some of the above will help to refine the analysis, while others

will allow the modeling to more accurately simulate actual conditions in an operating engine.

74
7 CORRELATION OF COMBUSTION EXPERIMENTS

As discussed in the preceding section, several titanium combustion wind tunnel experiments have been reported [7,10,13,46,

47,48], wherein both airfoil-shaped and flat specimens of typical blade size have been tested at varying pressures, temperatures

and velocities. In some experiments, the specimens were ignited

with a CO2 laser. In others, a titanium bar was melted electrically so that liquid blown downstream to the specimen caused ignition. The tests have been performed in several wind tunnels located at Wright-Patterson Air Force Base and at an engine manufacturer's laboratory. All have pressure, temperature and flow instrumentation and high-speed photography equipment.

7.1 Prediction of Self-sustained Combustion

Glickstein's analytical model of the combustion process

takes into account heat and mass transfer through the boundary

layer, transient heat transfer and melt behavior. Among other

things, the model is intended to predict whether or not ignition

will occur for various conditions and energy inputs and whether or

not there will be subsequent self-sustained combustion. In addition the model generally predicts the lower limit for combustion.

Glickstein suggests that a discrimination line between

quenching and self-sustained combustion can be described in terms

of ambient air static temperature and blade or vane leading edge

Reynolds number, where the charactert is t ic dimension is taken as

the leading edge diameter or thickness, the velocity as the free

stream velocity and the density and viscosity are evaluated at the

mean of the air static temperature and the melting temperature of

the metal, 2050K (3200F). Glickstein's discrimination line is

shown in figure 17.

75
•X ^nivy3cM3-L DLLV1S

w • to V-i 0)

<Du

a

cn •H

Du

76

Model

discrimination

lines

and

CAA

recommendation

functions

of

static

temperature

and

Reynolds

memb
[ 52 ] Pyne ' 5*1 has independently made a simplified analysis of

the sustained combustion limit which, as seen also in figure 17,

agrees well with Glickstein's criteria. This analysis, considering

heat loss from the combustion site by convection and the oxygen

diffusion rate to the surface, leads to the expression

( m-n/2

)

Taa= 2000 - CN 0Re (7-1)

where m and n are coefficients related to diffusion and convection

respectively that have values of 0.5 for laminar and 0.8 for turbulent flow. Ta is the free stream static temperature, C is an

empirical constant and N Re is the leading edge Reynolds number. Pyne concludes that at the leading edge, the flow should be

laminar. Thus m = n = 0.5 and C is determined from the experimental data. Equation (7-1) becomes

Ta

= 2000 - 259.3 N^25 . (7-2)

For comparison with the CAA recommendation (no thin static titanium

structures above 2 atm and 50 m/s velocity) we assumed a leading

edge diameter of 0.05cm (0.025 in) and show the temperatureReynolds number relationship as a dashed line in figure 17. Good

agreement between Glickstein, Pyne and the CAA is seen above about

400K (260F). Also shown are temperatures and Reynolds numbers for

typical" rotor inlet conditions where the numbers indicate

compression stage.

7.2 Analysis of Experimental Data

Using this type of analysis we can now assess some of the experimental data available. The figures will be in pairs; the upper

figure in each pair will show the temperature and Reynolds numbers

for those specimens which exhibited self-sustained combustion. The

companion figure will show the same information for those samples

that quenched after the ignition source was removed or consumed.

77
In interpreting the data from all sources, we made the decision to

term an event self-sustained combustion if seven percent or more of

the sample was consumed or, if given, we accepted the experimenter's judgment of whether or not there was self-sustained combust ion

.

Where the amount of material consumed is given for a data

set, the quenching cases group between zero and our seven percent

cutoff, then there is a gap in the population with few cases recorded between seven and about 25 percent, followed by the remainder of the specimens which were more heavily damaged. This supports our assumption of a 7 percent cutoff. Seven percent is also

approximately equal to the mass consumed by the igniting source.

Figure 18a shows the self-sustained cases reported in reference^^ and^^l. The discrimination line is fronJ52]^

the most conservative of the three criteria. We see that seven of

the specimens suffered over seven percent damage when tested in the

predicted quench zone. Also it may be seen that at the lower temperatures, the CAA criteria would allow titanium use under conditions that are demonstrated to have supported self-stained combustion. Here we must note that none of the source documents give any

estimates of systematic or random errors; only one gives estimates

of inaccuracies in the measurements. We suspect that the leading

edge diameters given for airfoil shapes are subject to variation

from the test articles. Without estimates of errors and inaccuracies it is difficult to correctly compare the data sets directly.

Nevertheless, the discrimination line closely approximates the

lower limit of self-sustained combustion in terms of temperature

and Reynolds number. The experimental data shown in figure 18a

that support the prediction are for two currently popular titanium

alloys (and a few others) in thin static sections at 0° angle-ofattack.

78
000-1

700 -

300-

200 T T 1 1 V

~ TTT| 1 1 1 1 T ‘T ~n |

1 1 1

1 f T'l T'l

Figure 18. Experimental titanium combustion data from references 10

and 46.

79
Figure 18b, shows the temperature and Reynolds number values

for all of the specimens that quenched after ignition. It is

readily apparent that not every specimen exposed to conditions to

the right and above the discrimination line will exhibit self-sustained combustion if ignited. (We have removed all data for those

specimens which are known not to have ignited). The reasons for

quenching under these conditions are not understood but we must

point out that these combustion events involve, and may be controlled by, many phenomena. Thus it is difficult to experimentally regulate all of the necessary parameters to exactly the same

value for each test.

Examination of the data given in^ 0 ] shows that perhaps

differences in experimental methods have some effect on quenching

in the predicted self-sustaining region. The data in^Ol w ere

taken in two wind tunnels. Figures 19a and 19b show the data from

one and figures 20a and 20b, data from the other. We see in both

instances (figures 19a and 20a) the self-sustained cases are, with

very few exceptions, shown to the right of the discrimination

line. In figure 19b, only a few of the quenched cases are to the

right of the discrimination line, while in figure 20b, about half

of the quenched cases are to the right. In both experiments,

equilibrium pressure, temperature and flow are established and

then the specimens are laser ignited. In one case, the specimen

is held at one end and ignited on the leading edge tip; in the

other apparatus, the specimen is held at both ends and ignited on

the leading edge at mid-span. Which configuration more closely

approximates engine conditions is debatable and perhaps immaterial, but it appears that quenching at conditions to the right of

the discrimination line is more probable in the latter apparatus.

Data froirJ^], believed taken in the same apparatus as

the data given in figures 20a and 20b, do not show in figures 21a

and 21b any quenching in the predicted self-sustained region.

However, we believe that these data were taken to substantiate the

analytical model development and therefore lie only along the discrimination line, not far to the right.

80
STATIC

TEMPERATURE,

K.

STATIC

TEMPERATURE,

K.

000-1

700 -

600 -

500-

400 -

300-

200 -

LEADING EDGE REYNOLDS NLfvGER

Figure 19. Experimental titanium combustion data from reference 10.

81
STATIC

TEMPERATURE,

K.

STATIC

Figure 20. Experimental titanium combustion data from reference 10.

82
STATIC

TEMPERATURE,

K.

STATIC

7B/PERATURE,

K.

LEADING EDGE REYNOLDS NUMBER

Figure 21. Experimental titanium combustion data from reference 46.

83
Data given were not included in figures 18 because

the actual leading edge diameters were not given. These data,

figures 22a and 22b, would not significantly have changed the

character of figure 18 and the specimens were divided among several alloys and several coating materials which could have altered

the behavior significantly.

Some of the effects of angle-of-attack are seen in figure 23

which comes from^^J. All specimens, held at the bottom, were

set at a 40° angle-of-attack for these tests and were ignited with

melt at the tip. Two trends are apparent. First, a significant

number of self-sustained events appear to the left of the discrimination line. Second, a significant number of quenches appear to

the right of the discrimination line, an effect more pronounced

than in the other data. This effect is an indication that the

Reynolds number is not sufficient to describe the heat and mass

transfer along a steeply angled airfoil.

Figure 18, which displays all of the experimental data, shows

that a rough boundary for the limits of self-sustained combustion

in these experiments exists and is predicted by the models. However, given a sufficient energy input, titanium structures normally operating in the predicted quench zone may burn until the energy source is removed. Also, it must be remembered that the boundary applies to the experiments, not to real engine conditions. We

conclude that such models may be applicable to present and future

engine designs, however the analysis and interpretation must be

made by engine specialists.

84
STATIC

TEMPERATURE,

K.

STATIC

TEMPERATURE,

K.

800 -i

Figure 22. Experimental titanium combustion data from reference 7.

85
000

700

600

500

400

300

300

000

700

600

500

400

300

300 -

1

23.

DATA SOURCE 13

O \0 O OOOO <3D

ooo oc

SELF-SUSTAINED

“I 1 1

1 1 1 T T

|

1 1 1 1 I I I T

| (a)

1 1 1 1 I T I I

|

f-ttttf + -H+ QUENCH

-h-f-t-

(b) —1

1 11)11 “I 1 1 1 I TTT*|

10

4 10

* LEADING EDGE REYNOLDS NJUBER

Experimental titanium combustion data from reference 13

86
8 DISCUSSION AND CONCLUSIONS

This section contains the conclu sions that we feel can be

drawn from the information gathered, summarized and analyzed in

the rest of the report. These conclusions are listed below along

with a short discussion of each. Many of the points are based on

limited information and/or have further qualifications which can

only be understood from the more complete discussion in the body

of the report. To assist the reader, each point has listed the

section where it is discussed in more detail; we urge that these

be read also.

USE OE TITANIUM IN TURBINE ENGINES

° Tit ani um is th e op t imum material for certain turbine

engine components . At this time there are no suitable

alte rn ate materials for large fans and other components

even though extensive materials development efforts have

been expended. For example, the high strength to weight

ratios exhibited by some composites are not accompanied

by other necessary properties such as impact strength,

stiffness, erosion resistance and so on (See Sec. 1,

1 . 1 ) .

° Originally, t itanium was used as far aft in engines as

it s mec h anical properti es would permit . The primary

limitation was reduced creep strength at elevated temperatures (See Sec. 1.1).

Recent combustion experience has led t o further limita- tions , i.e ., less 'agressive 1 use of titanium . Some

newer designs have accepted the weight penalty and use

steel or other materials instead of titanium for certain

components in the aft compressor section, particularly

for static components such as stator vanes and cases.

There are several reasons for this substitution: the

weight penalty for static parts is less than for rotating

parts, titanium rubbing on titanium has a high ignition

potential, and static parts can trap molten metal thrown

off by rotating parts, leading to rapid propagation of a

f ire .

( See Sec . 4.1).

87
ACCIDENTAL COMBUSTION OF TITANIUM IN TURBINE ENGINES

° The combustion incident data ba s e is very poor i n q u a

1

i ty

and quantity . Our tabulation (Table 3) shows 67 percent

of the incidents as cause "unknown". For others, the two

or three word cause-description does little to add to

scientific understanding of the incident. There is no

information available about titanium combustion incidents

short of complete engine shut down. Information about

such "partial burns" could help in understanding the conditions for non-propagating combustion (See Sec. 3.2).

° Forty percent of the titanium combustion i nc idents known to us have resulted in c ase penet rat ion . Penetrations

ranged from small holes in the case or air extraction

manifolds to 360° circumferential burn through. Essentially no information about subsequent, external damage

was available. This high percentage of case penetrations

may also reflect the lack of statistics on non-propagating combustion incidents (See Sec. 3.2).

° The cause of most combustion events (as we k now them now)

is a mechanical upset of some kind—FOP, POD , blade or vane failure, bearing failure, etc. The most common of

all known causes is failure of a blade or disc (42%).

However, it is not inconceivable that a stall could lead

to mechanical damage sufficient for ignition and yet the

incident be interpreted as DOD followed by a stall. (See

Sec . 3.2).

IGNITION AND COMBUSTION BEHAVIOR OF TITANIUM

Fundamental metal combustion mechanisms are not well

understood . Metal ignition and combustion phenomena are

affected by many factors, including the thermophysical

properties of the metal and its oxides, environmental

conditions, and the history and configuration of the

metal. Not enough basic research has been done to fully

characterize the influence of all these parameters. It

is known that under most practical conditions the titanium combustion reaction occurs on the surface of the

molten metal at a rate limited only by the availability

of oxygen. Very little is known about titanium reactions

with nitrogen or CC> 2

» (See Sec. 6.3)

88
o Spontaneous ignition limits can be established in terms

of temperature and pressure or oxygen concentration . These limits apply only to bulk specimens heated

relatively slowly to the ignition temperature in an

oxidizing atmosphere. It has been amply demonstrated

that any one of several factors can lower the ignition

limit; in particular, heating at rates greater than

20°C/s, surface- to-volume ratios in excess of 25

cm“l, exposure of a fresh surface, or the addition of

supplemental energy in any form (friction, impact or

fracture) all have been shown to lower the temperature

and/or pressure at which titanium ignites. (See Sec.

6 . 2 ) .

BUT, accidental ignition is nearly always associated with

a supplemental energy source so the spontaneous ignition

limit cannot be used in practice . Although ignition

limits can be established for well characterized supplemental energy sources, it is impractical to design around

these limits because there is no way to reliably predict

what the external energy inputs will be during a mechanical failure. Furthermore, it is apparent that sufficient

energy may be released during a failure to exceed even

the spontaneous ignition limit. (See Sec. 6.2).

The major mechanism for combustion propagation is molten

titanium and its oxides thrown or blown ag: inst other

titanium structures . Since rotating parts will tend to

slough off any molten material, static structures are

much more susceptible to impingement by molten metal and

rapid propagation of a fire. Further damage may be

caused downstream from a fire by molten titanium lodging

against nickel alloy structures and melting them by

formation of a low-melting Ti-Ni eutectic. (See Sec.

6.4)

. Alloying does not in a practical sense affect combus t ion

in engines for the alloys commonly used today . While

some alloys have been shown to resist ignition and combustion, those alloys do not have other properties suitable for engine environments; for example, they may be

too brittle or have low fatigue strength. (See Sec.

6.4) . Coat ings and surface treatments show some promise to inhibit ignition and combustion but practic a l systems are

not yet developed . Coatings may affect other properties

such as fatigue resistance or may not tolerate the engine

operating conditions. (See Sec. 6.4).

89
DESIGN PRECAUTIONS AND LIMITATIONS

Several design precautions are now in practice . There is

general agreement that titanium on titanium rubs are a predictable ignition source. Rub strips or coatings are

commonly used to prevent, insofar as practical, titanium

on titanium rubs while allowing closer operating clearances, and to absorb molten titanium and its oxides without igniting. Thin, static titanium structures which can

be susceptible to ignition have not been used above certain pressures and temperatures in some modern designs

with apparently good results, as evidenced by the lack of

titanium fires. Wider clearances reduce the possibility

and severity of rubs.

Increased tolerance to foreign objects and designs to reduce domestic object damage lower the probability of system upsets that can lead to ignition. (See Sec. 4).

Self-sustaining combustion limits can be established

analytically and experimentally in terms of temperature

a nd Reynolds number for thin static structures . These

limits are lower than the spontaneous ignition limits and

are very dependent on configuration and surrounding conditions. Most of the experimental work has been at 0°

angle-of-attack ; however there is evidence that angle-ofattack is an important parameter. Although the sustained

combustion limits have not been determined for rotating

parts, those for static parts are expected to be a lower

limit. ( See Sec . 7 )

.

Extinguishing titanium f ires in turbine e ngines is proshor t

t ingu

gon i rate

tec t i

( See

not prac t i cal at this t ime

.

The events are of

d ura t ion ( 4-20 s) and large volumes of speci al exis hing ag en ts would be required. Si x ty perce nt arn air is ef f ect i ve but CO 2 3C tual iy increases the

of t i tani urn combus t ion

.

Suita ble tec hn iq ues f or deng t i tan

i

urn fires would also have to be devel oped

.

Se c . 4.3) •

90
Several areas of concern and lack of understanding would benefit

from further work. We have listed below several areas for

continued effort that would provide the best return in terms of

alleviating present problems and increasing our understanding in

order to promote safer future designs.

Incident data . A thorough and uniform method of gathering and analyzing the data from accidental titanium combustion in turbine engines would provide a sound basis

for future design. These data should include probable

cause and source of ignition, environmental conditions at

the ignition point and in propagation zones, extent and

direction of propagation, analysis of, products, and the

engine's past history and operating conditions. Of particular help would be data from minor events which could

give some indication of conditions which have inhibited

propagation

.

Properties data for combustion modeling . Fundamental

properties data for liquid and solid titanium and its

alloys are needed for accurate modeling of the titanium

combustion process. These data include the surface tension of titanium and its alloys as a function of oxygen

concentration, the interfacial energy (wetting behavior)

under oxidizing conditions that lead to fire propagation,

and the viscosity of liquid titanium as a function of oxygen content.

Minimum ignition conditions . Methods must be devised to

determine the minimum ignition conditions for titanium in

a dynamic environment. These could use highly controllable, pulsed-laser techniques for accurate energy input,

rate and location for comparison to molten drop propagation studies.

Ignition by molten-titanium . Because propagation is

largely controlled by impingement of molten, burning

metal on static structures, the ignition of titanium by

burning droplets of the metal should be studied. These

experiments should try to define the minimum size and

temperature of a molten titanium droplet necessary to

ignite titanium plates of varying thickness.

Coatings and alloys . Research on ignition- and propagation-inhibiting coatings and alloys should continue because several have shown some promise. Those that do

should be included in the properties data and ignition

studies for further evaluation.

91
Dynamic experiments . Self-sustained titanium combustion

should be further studied in wind tunnel experiments to

provide a firm data base on the environmental conditions

which will or will not propagate a burn. Further tests

should concentrate on environmental conditions near the

self-sustained combustion limit and the experimental

uncertainties associated with this limit must be

evaluated. Structures other than air foils, such as

bleed air ducts and manifolds, should also be studied.

Rotation effects . The one

dynamic experiments is tha

centrifugal forces on the

rotating part should be de

of impact of molten metal

Simple calculations indica

fugal forces may be large

pagation of combustion on

untested parameter in the

t of rotation. The effect of

propagation of combustion on a termined as well as the effects

on adjacent static components,

te that the effect of centriand may well help curtail prothe rotating component.

Analytical modeling . Efforts to develop a comprehensive

analytical model to predict ignition and self-sustained

combustion of titanium alloys in engine environments

should be continued. The addition of the capability to

consider the effects of angle-of-attack and surface coatings would considerably enhance the power and utility of

the current model.

In summary, although titanium combustion damage has occurred

in several instances, the technology exists to confine this damage

to the engine. We therefore conclude that with proper application

of good design and maintenance practices titanium can be used

safely in present day turbine engines.

92
9 ACKNOWLEDGMENTS

The authors gratefully acknowledge the kind cooperation and

assistance of the following organizations and individuals. The

information and consultations provided by them contributed immeasurably to this report.

1. Airesearch Manufacturing Company

A. M. Walacavage

J. Pyne

W. Danesi

2. Air Force Aero Propulsion Laboratory

C. W. Elrod

3. Air Force Materials Laboratory

S. R. Lyon

4. Civil Aviation Authority

K. A. Burdon

5. Federal Aviation Administration

A. K. Forney

C. C. Troha

6. General Electric Company

B. L. Koff

J. Melink

P. Linko

7. National Transportation Safety Board

E. P. Wizniak

8. Pratt and Whitney Aircraft Company

A. W. Oberg

R. J. Makowski

W. F. Laverty

M. R. Glickstein

B. A. Manty

V. Anderson

9. National Aeronautics and Space Administration

L. Ludwig

S. Weiss

93
10 . Rolls-Royce Ltd.

K. R. Welling

J. B. Sheldon

R. Ebbs

P. A. Withers

P. B. Smith

B. Lewis

11. United Airlines

J. K. Goodwine

P. Sellers

J. Uhl

94
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98
APPENDIX A

A GLOSSARY OF METAL COMBUSTION TERMINOLOGY*

I. Oxidation

breakway oxidation: in the transition from protective oxidation

kinetics to nonprotect ive , if the linear rate is faster than the

instantaneous (tangential) rate before the transition, the oxidation is called breakaway. Titanium exhibits this type of behavior

at temperatures greater than about 1170K (900°C).

catastrophic oxidation: excessively fast oxidation leading to the

disintegration of an alloy (but not necessarily combustion) that

occurs when one component of the alloy forms a liquid oxide phase

during oxidation.

nonprotective oxidation : metallic oxidation that proceeds at a constant rate, independent of exposure time. This may be due to

the oxide's lacking sufficient volume to cover the metal surface

completely or to the presence of cracks and pores in an otherwise

coherent oxide film.

oxidation; the chemical reaction of any substance with oxygen or

oxygen-bearing compounds to form oxides. When applied to metals,

the term can also refer to the formation of an oxide film on

metals exposed to oxygen.

oxidation rate; in order to distinguish between film formation

and actual combustion, we limit use of the term "oxidation rate"

to descriptions of the rate at which a solid oxide film or scale

is formed on a metal at elevated temperatures. Oxidation rate may

be expressed alternatively as the amount of oxygen uptake per unit

surface per unit time, the amount of metal transformed to oxide

per unit time, or the change in thickness of the oxide film per

unit time (provided the film is uniform).

*This glossary is divided into four sections; I. Oxidation,

II. Ignition, III. Combustion and IV. Propagation. Entries are

arranged alphabetically within each section.

A-l
oxidation rate law : refers to the form of the analytical expression describing the oxidation of a metal as a function of time.

The most commonly observed rate laws are logarithmic, parabolic and

linear. Occasionally cubic and quartic rate laws are observed. A

logarithmic rate law applies when the amount of oxide formed per

unit area (or, alternatively, the thickness of the oxide film) is

proportional to the logarithm of the exposure time. The remaining

rate laws are described by the expression.

X

n = k n

t + c,

where x is the amount of oxide formed per unit area, kn is the

rate constant, t is exposure time at constant temperature and c an

integration constant. According to whether n = 1, 2, 3, or 4 , the

rate law is termed linear, parabolic, cubic or quartic. These rate

laws represent ideal behavior; often real metals exhibit deviations

from the ideal rate equations. Also, a single metal may obey different rate laws at different temperatures.

Pilling and Bedworth ratio: the oxide/metal volume ratio (the

ratio of the molar volume of the metal oxide to that of the metal).

Used as a qualitative indication of whether a metal will undergo

protective or non-protect ive oxidation. If the Pilling and

Bedworth ratio is greater than one, the oxide will occupy more

volume than the metal it displaces and oxidation will be protective; if it is less than one, then the oxide will not cover the

metal surface completely and nonprotect ive oxidation will be ob

served. However, this principle is of somewhat limited applicability since many factors affecting oxidation kinetics are not taken

into account.

A2
protective oxidation : metallic oxidation that proceeds at a rate

that diminishes with exposure time at constant temperature.

Caused by the formation of a compact, coherent oxide film on the

metal surface. The film acts as an increasingly effective barrier

to the diffusion of oxygen.

transition temperature: the temperature at which protective

oxidation kinetics give way to a nonprotect ive , linear rate law.
II. Ignition

autogenous ignition temperature: the lowest temperature from

which a metal can self-heat by oxidation to the spontaneous ignition temperature. The period of self-heating is termed the induction period or ignition delay.

bulk ignition temperature: experimentally, it is observed that

the ignition temperature varies with the state of subdivision of a metal; small particles ignite at lower temperatures than massive

specimens. The bulk ignition temperature is measured with a sufficiently large specimen that size effects play a negligible

role. It is generally hypothesized that the bulk ignition temperature is a property of the metal itself, susceptible to measurement and independent verification. But the experimental evidence

for this hypothesis is far from conclusive.

critical temperature: in the analytical treatment of ignition by

a heat balance approach, the temperature at which the rate of heat

generation by chemical reaction is equal to the rate of heat loss

by radiation, conduction, and convection combined. Above the critical temperature, the system may self-heat to the spontaneous,

ignition temperature; thus, the critical temperature corresponds

to the autogenous ignition temperature.

ignition : the initiation of combustion. More specifically, a thermal instability that occurs when the heat-release rate during

a chemical reaction exceeds the reactants' ability to dissipate

it, leading to a rapid rise in temperature and a corresponding

increase in the reaction rate until a new equilibrium state is

attained

.

ignition limit : minimum conditions of temperature and oxidizer

pressure (or concentration) necessary for a metal to ignite spontaneously; thus, the minimum ignition temperature as a function of

some other parameter such as pressure or airstream velocity or

Reynolds number. Any factor that adds supplementary energy to the

metal can lower the ignition limit.

A4
incendivity ; the ability to ignite a flammable or explosive

mixture; for example, the ability of grinding sparks from metal

tools to ignite coal dust dispersions or methane-air mixtures in

coal mines.

minimum ignition temperature: the lowest temperature at which a metal is experimentally observed to ignite.

pyrophoric : capable of igniting spontaneously in air at room temperature. This refers to the ability of extremely fine metal

powders to self heat rapidly to the spontaneous ignition temperature upon exposure to ambient air. It is caused by the very large

specific surface area (cm^/g) of small particles; as the particle diameter is decreased, the amount of surface area available

for oxidation per gram of metal increases, leading to a reduction

in the critical temperature.

spontaneous ignition temperature : the temperature at which instantaneous inflammation of a metal occurs. The spontaneous ignition temperature is always higher than the autogenous ignition

temperature. Analytically, it corresponds to a maximum in the

derivative of temperature with respect to time.

A5
III. Combustion

adiabatic flame temperature : the equilibrium combustion temperature calculated from the heat of reaction, taking into account the

heat capacity of the products, the latent heat of fusion, and the

heats of any allotropic phase transitions, but assuming^ no heat

losses. Because metal oxides are stable and do not dissociate at

high temperatures, the boiling point of the metal oxide provides an upper limit to the flame temperature.

Arrhenius equation : Experimentally it is found that for most chemical reactions the temperature dependence of the reaction rate follows an expression of the form

k = A exp (-E/RT)

where k is an appropriate rate constant, A is a frequency factor

(accounting for the frequency and geometry of collisions between

reactants), R is the universal gas constant, T is the absolute temperature, and E is the activation energy of the reaction. Expressions of this form are called Arrhenius equations, after Svante

August Arrhenius, the Swedish chemist who first proposed this type

of temperature dependence for reaction rates in 1889.

chemically controlled reaction : a reaction in which the rate limiting step is an elementary chemical reaction rather than a transport process.

combustion : a rapid chemical reaction accompanied by heat and,

usually, light. Synonymous with burning.

combustion limit : minimum conditions of temperature and oxidizer

pressure (or concentration) necessary for a metal that has been

ignited to undergo self-sustained combustion. In almost every

case, the combustion limit for a particular metal will be lower

A6
than the ignition limit. This means that in order to extinguish a metal once ignited, it is necessary to reduce the temperature and/

or pressure to values below both the ignition and combustion limits. If, however, an external energy source causes ignition below

the combustion limit, the combustion reaction would not sustain itself .

combustion rate: the rate at which oxygen is consumed during sustained metal combustion. For a metal that burns on the surface

such as titanium, the combustion rate would be expressed as weight

of oxygen taken up per unit area per unit time. (g/cm 2 s or

mole/cm2 s). For a metal like magnesium that burns in the vapor

phase, the combustion rate would be expressed as volume of oxygen

consumed per mole of metal per second (1/mole s) . A distinction

should be maintained between the more fundamental combustion rate

and burning or propagation velocity.

diffus i on c ontrolled reaction : a chemical reaction in which the

rate limiting step is mass transport of one of the reactants or

products. Most metal-oxygen reactions are diffusio*. controlled

under conditions of practical interest.

Glassman criterion : thermophysical criteria for predicting whether

a metal will burn by the vapor phase or surface combustion mode. A

necessary but not sufficient condition for combustion to occur in

the vapor phase is that the metal oxide's boiling point exceed that

of the metal. Conversely, a sufficient but not necessary condition

for surface combustion is that the metal's boiling point exceed

that of the oxide. The qualifications of necessary and sufficient

imposed on these criteria are required because vapor phase combustion may be suppressed by the formation of a protective oxide layer

and/or by reduction of the flame temperature below the metal boiling point by radiation losses.

A7
homogeneous reaction ; pletely in one phase.

a chemical reaction that takes place comheterogeneous reaction : a chemical reaction with one or more condensed phases participating in the reaction.

mixed combustion : under certain conditions, the presence of condensed oxide phases in the combustion zone may lead to competition

between heterogeneous and homogeneous reaction paths as, for example, in the low pressure combustion of magnesium. Here, the heterogeneous reaction of Mg and O 2 occurs on MgO smoke particles

which were nucleated from MgO vapor originally formed by homogeneous gas phase reactions. This mode of combustion is called mixed

combustion, and is relatively rare. It is not unusual for there to

be a small amount of vapor phase combustion occuring during surface

combustion of a metal owing to the high temperatures evolved.

Likewise it is not uncommon for a metal that burns in the vapor

phase to collect some oxides on the surface due to back diffusion

and condensation of oxide vapor as well as some surface reaction.

But the significance of these competing reaction modes is generally

quite small and one mode of combustion will usually predominate.

mixed control ; control of a reaction is said to be

there is more than one rate limiting step; that is,

constants for the two slowest steps of the reaction

in magnitude. This situation ususally occurs during

control from one rate limiting step to another as, f

during the ignition of a metal.

mixed when

when the rate

are comparable

transfer of

or example,

A8
APPENDIX B

Excerpt from British Civil Airworthiness Requirements

Paragraph 3, Appendix to Chapter C3-2, Design and Construction,

Turbine Engine for Aeroplanes, British Civil Airworthiness Requirements .

3. TITANIUM FIRES (see C3-2, 1.3.3)

3.1 Experience has shown that most titanium alloys used for

manufacturing engine rotor and stator blades will ignite and sustain combustion if the conditions are favourable. In such cases,

the Engine casings may be penetrated allowing expulsion of molten

or incandescent material which could, depending on the installation, hazard the aircraft.

3.2 Unless the constructor can produce adequate evidence

that titanium items in the Engine are such that they will not ignite or will not support combustion, the Engine design should be

assessed for vulnerability to titanium fires. It will normally

be assumed that a titanium fire is possible if stationary titanium material exists in areas where:

(a) pressure will exceed 200 kN/m 2 (29.4 lbf/in 2

); and

(b) relative air velocities are in excess of approximately

50 m/sec. (150 ft/sec); and

(c) the geometry is such that relatively thin titanium sections exist which can be rubbed, directly or after shedding, by rotating parts. Stator blades of conventional

design, of up to 15 cm (5 in) of aerofoil height, are

regarded as falling into this category.

NOTE: Theoretical studies have attempted to describe the conditions for ignition and continued burning of titanium (using fundamental parameters) in more scientific terms but because many

of the parameters can adopt quite variable and unpredictable

values during a damage process the above arbitrary values chosen

from actual experience appear to provide the best guidance.

3.3

Where the assessment of 3.2 indicates that the possibility

of a titanium fire cannot be ruled out on the particular design,

it may be possible to show that a titanium fire will be confined

to areas within the Engine where it cannot present a hazard, if

necessary by the provision of additional protection. Otherwise

the Engine approval will be endorsed to the effect that:

"Some risk of non-contained titanium fires exists on this

Engine design, and the Engine is thus approved only for use

in aircraft where such fires are unlikely to be hazardous."

If the risk is dependent on specific circumstances (for example

the danger only exists at low altitude) such circumstances may

also be stated.

B—

1
3.4 Experience has shown that the following features can reduce the susceptibility of Engines to titanium fires and such

aspects will be taken into account by the Authority in making

judgements under 3.2

(a) The type of alloy, i.e. its constituents other than titanium.

(b) Blade coatings which inhibit ignition or subsequent combustion.

(c) Coatings, on casings and other parts, which resist fire

penetration but can if necessary accommodate rubbing.

(The possibility of eutectic reduction of the casing

melting point by reaction with coating materials should

be considered .

)

(d) The way in which the design minimizes potentitally dangerous rubs by such methods as:

(i) large interblade row clearances,

(ii) the provision of material in areas of potential rub

which melts safely or otherwise clears the rub before ignition temperatures are reached, and which

has sufficient depth to prevent direct rubbing between rotating and other parts of the Engine during

maximum predicted rotor or casing deflections including those likely to occur in fault conditions,

(iii) not using titanium for adjacent rotating and static

parts, i.e. avoiding titanium/titanium rubs,

( iv) attention to rotor movements under transient and

bearing failure conditions,

(v) ensuring that any titanium features at the front of

the Engine, e.g. entry guide vanes, are robust and

unlikely to shed thin, easily-ignited sections.

See also: British Civil Aviation Authority, Paper No. 678, 26

November 1976.

B-2
APPENDIX C

METALLURGICAL EXAMINATION OF COMBUSTION SPECIMENS

During the course of this study, several pieces of a gas turbine engine that had experienced an in-flight titanium fire were

obtained for metallurgical examination. Samples were taken from

areas of the engine casing and the bleed air manifold that had

been penetrated by combustion. The cause of this titanium fire

was ascribed to blade failure, with one or more broken titanium

blades lodging against downstream stator vanes where they were

ignited by friction from the adjacent rotating stage.

Small specimens were cut from the burned engine parts, generally in the vicinity of a penetration site. One specimen was cut

from an area far removed from the combustion zone for comparison

with the burned specimens. All of the specimens were mounted,

ground flat on a belt sander, sanded by hand on successively finer

grades of sandpaper, and then mechanically polished. They were

then examined metallographically and photographed at a magnification of 150 diameters. Selected regions of the specimens were

traversed with a microhardness tester in order to estimate the

depth of penetration of oxygen into the metal.

The limited time and resources available for this study necessarily limited its scope to these analytical techniques. Undoubtedly, more extensive analyses of specimens from actual titanium fires would yield much additional knowledge; chemical and

x-ray analyses and scanning electron micrography would be particularly useful.

The visual appearance of the specimens as received was quite

distinctive. The interior portions of the engine case showed

little accumulation of molten metal or oxides; the interior edges

of burned- through holes were relatively sharp, while the exterior

of the case showed much more extensive melting and burning. The

C-l
rub strips showed evidence of severe rubbing; very little abradable material was left on the rub strip land. The burned areas on

the outside of the engine case were dark brown in color and extended several inches beyond penetration sites. Penetrations were

generally elongated along the circumference. Splatters of bright,

shiny metal were evident near some of the penetrations and these

appeared to have been molten material from the rub strip; the

bright metallic appearance of this material indicates that it did

not participate in combustion. Subjectively , the rub strip seemed

to inhibit rather than enhance combustion. There was no evidence

that the rub strip coating burned and the pattern of propagation

of the fire would tend to support the conclusion that the rub

strip area was somewhat less combustible than adjacent areas. It

should be emphasized, however, that these are qualitative and subjective observations that must be checked against other evidence

before reaching any conclusions.

Photomicrographs of specimens cut from the combustion zone

are shown in figures C-l to C-3. The left half of figure C-l

shows a cross section of the unburned rub strip side of the first

specimen while the right half shows a typical burned area on the

opposite side of the same specimen (the exterior surface of the

engine case). Note that the rub strip layer is mostly gone, with

only deformed fragments present above the bond layer. The burned

surface of this specimen, shown in the right half of figure C-l,

illustrates several features that were common to all burned specimens examined. The outermost layer of metal (top of the photograph) is harder and more brittle than the underlying base metal,

as evidenced by the lack of polishing scratches and the microhardness determinations. The morphology of the outer layer and the

presence of numerous voids and bubbles suggest that this metal was

molten at some time during combustion and resolidified in its present location. This molten, recast layer is quite brittle and

often exhibits cracks; it occasionally chipped off during cutting

and grinding operations (figure C-2). The thickness of the recast

layer varied from 50-500ym. To the unaided eye this layer retained a metallic luster, although often imbued with a brownish tint

C-2
C- 3

Figure

C-l

.

Photomicrographs

of

the

unburned

(left)

and

burned

(right)

sides

of

specimen

cut

from

the

compressor

case

of

an

engine.
near the surface. Under the microscope, with a polarized light

source, the recast layer shows no optical activity, indicating

that oxides higher than TiO are not present. A tentative hypothesis is that this layer is composed of a mixture of alpha-Ti plus

TiO. The dark brown oxide layer observed visually on burned surfaces of the original specimens was not observed on the polished

specimens; it apparently chipped off during specimen preparation.

It is likely that this surface oxide layer was formed after the

molten layer of metal resolidified.

Underneath the recast layer is a heat-affected zone, 50-300ym

thick, where the temperature of the metal exceeded the beta transus ( 1270K (1830°F) for Ti-6Al-4V) but did not reach the melting

point. Upon cooling from the beta field, thin, needle-like grains

of alpha-Ti nucleate and grow; the grain size of the transformation product depends upon the rate of cooling. These needle-like

grains are termed acicular alpha. Often the acicular alpha will

form a basketweave structure, termed Widmanstatten, due to nucleation on preferred crystallographic planes of the parent beta

phase. This type of structure is clearly visible in the center of

the right half of figure C-l. Because the acicular alpha nucleates first on the beta grain boundaries, it leaves a definite outline of the prior beta grains. The prior beta grain size in figure C-l is on the order of 50ym. This is rather a large grain

size in view of the very short duration of a typical combustion

event, and suggests that the temperature in the heat-affected zone

could have been well over 1340K (1950°F).

Beneath the heat-affected zone, there is no visible difference in the photomicrographs of burned and unburned regions.

Another specimen, cut from a different area of the rub strip,

is shown in figure C-2. Here, again, the rub strip did not burn

as severely as the surface on the outside of the engine case. In

this instance the recast layer broke off while the specimen was

being cut. The heat-affected zone remains, but the acicular alpha

is not as clearly defined as that in figure C-l.

C-4
I I 100 urn

Figure C-2.

portions of

Unburned (top) and burned (bottom)

a specimen from the compressor case.

C-5
I I 100 ;im

Figure C-3. Recast layer near a penetration

in the bleed air manifold.

Ob
A very thick recast layer (~500ym) is shown in figure C-3.

This specimen was cut from the bleed air manifold; the photomicrograph in figure C-3 shows a cross-section of the outside edge of a burn-through . Note the extensive cracking in this layer.

Microhardness testing is a useful technique for sudying concentration gradients of oxygen dissolved in titanium. Hardness

increases with increasing oxygen content of the metal to a peak,

beyond which the hardness falls off as the concentration approaches the solubility limit (~ 35 atomic percent for pure titanium). Thus, by measuring hardness as a function of depth below

the burned surface, the oxygen penetration depth may be detrmined.

For the present investigations, a diamond pyramid indenter was employed with a load of 500 g. The results of microhardness traverses of four specimens, including one unburned specimen, are

presented in figures C-4 to C-7.

Hardness as a function of depth below the surface of the rub

strip for an unburned specimen is shown in figure C-4, which

serves to illustrate the relatively low hardness of the rub strip

material and an average diamond pyramid hardness number (DPHN) of

325 for the bulk titanium prior to combustion. Figures C-5 and

C-6 show hardness profiles for two specimens cut from the engine

case; figure C-7 is the hardness profile for a specimen cut from

the bleed air manifold.

The results reveal several interesting facts. The oxygen

penetration depth ranged from 500 to lOOOym, well beyond the heataffected zone in all specimens. The hardness of the outermost,

recast layer was difficult to determine because of its tendency to

crack under the indenter, and so only one value of the DPHN for

this layer was determined for each specimen. These values ranged

from 398-1345. The highest hardness values were found in the

heat-affected zone, but this is consistent with a decreasing oxygen content since the curve of hardness vs. oxygen concentration

goes through a maximum. Below the heat-affected zone the hardness

C-7
1500

o

LU

trQ

LUo

oc

LUo

c

t—ooQ

yaawnN ssbnqhvh aiwmd qnowviq

C-8

Figure

C-4.

Flicrohardness

as

a

function

of

depth

below

rub

strip

for

an

unburned

specimen
1500

E

a

UJ

<SQ

UJo

DCO

z

<

»—oo

—Io

dBaWON SS3NQdVH QIWVdAd 0N0WVIQ

C-9

Figure

C-5.

Microhardness

asa

function

of

depth

below

burned

surface

of

compressor

case.
1500

o

o o o

o in

CDO

o

o:

cj

z

<t

i—

in

i—

i

Q

M38WnN SS3NQtiVH OIUV^Ad QNOWVIQ

C-10

Figure

C-6.

Microhardness

as

a

function

of

depth

below

burned

surface

of

compressor

case.

Different

symbols

denote

independent

traverses

of

specimen.
1500

o

o o o O LO

E

a

UJ

CDQ

UJO

oc

UJ

t—ooQ

aaawnN ss^NoavH aiwvaAd qnowviq

C-ll

Figure

C-7.

Microhardness

asa

function

of

distance

from

penetration

site

in

bleed

air

manifold.

Different

symbols

denote

independent

traverses

of

specimen.
fell off rapidly with increasing depth; for most specimens the

hardness was down to normal levels within 750ym.

One question does arise from the results of the microhardness

testing: how can the high values of hardness measured in the

heat-affected zone be reconciled with the fact, noted earlier,

that this layer is transformed beta? At its highest temperature,

this layer would have had a beta structure, but the beta phase has

a much lower oxygen solubility than the alpha phase (8 atomic percent at 1970K (3086°F), and less at lower temperature). Yet the

oxygen concentration in this layer is thought to be on the order

of 20-30 atomic percent. If this quantity of oxygen diffused into

the heat-affected zone while it was at high temperature, the crystal structure would have transformed to an a - 8 mixture, or perhaps even pure a , without any cooling. If this had occurred it

would not have been possible to form acicular alpha, since the alpha would have had sufficient time to form a plate-like microstructure. Thus, we are drawn to the conclusion that oxygen diffused into the heat-affected zone after this layer had cooled below the beta transus.

One possible sequence of events that could rationalize this

conclusion would be if liquid, burning titanium flowed across the

solid titanium surface, rapidly resolidifying by giving up its

latent heat of fusion to the underlying solid metal and raising

the temperature of this material above the beta transus. Both the

recast layer and the heat-affected zone would then cool fairly

rapidly below the beta transus by conduction, convection and radiation. While these two layers cooled, oxygen would diffuse from

the oxygen-rich recast layer to. the heat-affected zone, but this

would not occur rapidly enough to exceed the solubility limit of

the beta phase. Of course this sequence of events is quite similar to the mechanism that has been proposed to explain propagating

C-12
titanium fires; the only difference is that if the molten, flowing

titanium has sufficient enthalpy, it may succeed in igniting more

metal rather than quenching itself. Thus, the metallurgical examination of specimens from an actual titanium fire helps to confirm the model of titanium combustion that is emerging from the

extensive experimental studies in industry and government laboratories .

* U S GOVERNMENT PRINTING OFFICE: 1979 - 6 30 - 84 2 / 268 3
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