NATIONAL BUREAU OF STANDARDS REPORT

9946

STRESS-CORROSION BEHAVIOR OF TI-6AL-4V

ALLOY IN A SYNTHETIC SEA WATER ENVIRONMENT

To

Naval Ship Research and Development Center

Department of the N avy

C a rderock , Maryland

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS

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NATIONAL

ms PROJECT

3120047

BUREAU OF STANDARDS REPORT

NOV 2 6 1968 NBS REPORT

9946

• STRESS-CORROSION BEHAVIOR OF TI-6AL-4V

ALLOY IN A SYNTHETIC SEA WATER ENVIRONMENT

By

W. F„ Gerhold

and

B . 2 . Weiss

Naval Ship Research and Development Center

Department of the Navy

Carderock, Maryland

IMPORTANT NOTICE NATIONAL BUREAU Of ST,

(or use within the Government, and review, for this reason, the whole or m part, is not authon

Bureau ol Standards, Washingtoi

the Report has been specifically

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015

ss accounting documents intended

subjected to additional evaluation

listing ot this Report, either in

9 Office ol the Director, National

iy the Government agency (or which

opies tor its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU Of STANDARDS

Stress-Corrosion Behavior of Ti-6A1-4V

Alloy in a Synthetic Sea Water Environment

Reference : (a) Department of the Navy, Naval Ship Research and

Development Center, Carderock, Maryland.

1 nt roduct i on : It has been known for several years that, given the proper

set of conditions, some titanium alloys are susceptible to stress-corrosion

cracking. Stress-corrosion cracking may occur in sea water if there is a flaw of sufficient size and acuity and if the stress reaches a certain

critical value. The mechanism of environmental cracking is not completely

understood, but St has been postulated that the following may play a dec i s i ve role.

1. Corrosion by chemical attack by one or more chemical species

at particular structural sites that are sensitive to stress,

such as at stacking faults, grain boundaries or dislocation

p i 1 e-ups

2. Embrittlement by hydrogen whereby in the presence of free

hydrogen there are formed brittle phase products or where

there is an impedance of plastic deformation due to the

pinning of dislocations by hydrogen in solution in the

material .

3. The adsorption of certain ions along particular crystallographic planes which results in a reduction of the

surface energy due to a weakening effect on the cohesive

bond at these areas.

4. Rupture of the thin protective surface film on the metal

which may trigger any or all three of the previous

mechan i sms

Material : Eighty-eight (8b) specimens that had been machined from 1/2 in.

and 2 in. thick Ti-6A1-4V alloy plate were submitted by Reference (a).

The specimens included samples from base-metal areas, weld areas and areas from welded material in the heat affected zone.

Specimens machined from the 2 in. thick plate had been obtained from areas on the plate at the surface and at areas in the center of the

thickness of the plate.

Some specimens of tne base-metal material from the 2 in. thick plate were oriented with their long axis in a direction longitudinal to the

direction of rolling and others with their long axis transverse to the

direction of rolling. Information relative to the orientation of specimens

machined from the 1/2 in. thick plate was not available.

2

Details of the distribution and identification of specimens

machined from plate of either thickness are given in Table i.

Specimen Preparation : As noted above, all specimens in the form of

rectangular bars approximately in. wide x 'J/]6 in. thick had been

machined to the test configuration prior to receipt by NBS. A sharp

"V11 notch (stress raiser) had been machined at the mid-length across one

face of each specimen. Side grooves in the form of "V" notches had been

machined across the two adjacent sides of the specimen. These latter

notches were provided to reduce the size of shear lips that may form

during test at the time of fracture. In order to increase the acuity

of the stress raiser, a fatigue crack was introduced at the base of the

"V" notch.

The apparatus used in this investigation to fatigue crack the

specimens is shown in Figures I and 2. The crank and lever system from

a Krause fatigue testing machine was used to apply a bending load on the

specimen, with the specimen acting as a cantilever beam. The specimen

was clamped, notched face down, rigidly at one end. A ball shaped anvil

located toward the other end of the specimen was fastened to the lever

which in turn was connected to the crank. A cyclic bending stress was applied to the specimen at the notched area by the anvil acting through

the crank and lever system. Repeated cycling resulted in the initiation

of a fatigue crack at the notch. The depth of the fatigue crack on both

sides of the specimen was measured with the aid of a microscope.

Procedure ; The test method used in this investigation was similar to that

introduced by B. F. Brown.(') In this method a precracked specimen is

stressed in bending as a cantilever beam. Figure 3 shows the test

apparatus used. The specimen is enclosed at the notched area in a plastic

container (stress-corrosion cell). It is then clamped at one end, notched

face up, to a rigid post. The other end is clamped to a long beam at the

end of which is a container to which the load is applied. The corrodent

(synthetic sea-water solution ASTM D-llUl-52) is pumped from a reservoir

through plastic tubing into the stress-corrosion cell. A return system

is provided, at the bottom of the cell, through which the solution is

returned to the reservoir. When the solution has reached a level

sufficient to cover the specimen (this is controlled by a pet cock in

the return line), the load is applied by adding weights to the container

at the end of the canti lever beam.

The applied stress which is dependent upon the size of the flaw

(stress raiser) is expressed in terms of the fracture mechanics stress

intensity factor, K, . For a rectangular precracked notched bar, this

factor can be calculated from the equation from Kies, et al.( ; -)

K 23 4.12 m

y~~~5T

- O'

BD

3'2

where: a

a

D

BM

D Notch depth + depth of fatigue crack.

Vertical depth of the specimen.

Horizontal depth of the specimen.

Bending moment at the notch.

If measurements are in inches and if the moment J_s expressed in in. -lbs,

the stress intensity factor has the units Ksi /in".

For purposes of this investigation two quantities of stress intensity were determined. One designated Kj (the stress intensity required for

fracture in air) is a rough approximation of the toughness of the material.

The second designated ^I sccis the threshold stress intensity factor for

a specific corrosive environment, below which stress-corrosion cracking

will not occur

The pertinent dimensions of all specimens tested in this investigation were determined with a micrometer caliper. The depth of the "V" notch was measured with the aid of a toolmaker's microscope and the depth of the

fatigue crack was determined with the aid of the microscope previously

men t i oned

One specimen from each group of 4 was used for determining the K-, A precracked specimen was enclosed at the notched area by a plastic J‘ x container and placed in the stress apparatus. Anhydrous CaSO^ crystals were added to the container and the container was then sealed. The

CaSOij was used to obtain a relatively dry-air condition. A load was then

applied at the end of the beam. This load was increased until fracture

of the specimen occurred. After fracture, calculations were made, using

the above equation, to obtain the stress intensity factor required for

fracture in air, Ki . The quantity determined for each group of specimens

is g i ven in Table 2

Two specimens from each group of 4 were used to determine the

approximate K.^ see Each one of these specimens was precracked, enclosed

by the stress-corrosion cell and then stressed in the sea salt

envi roment at a known stress intensity level, . if failure did not occur within one hour, the specimen was removed from test and returned

to the fatigue testing machine. The depth of the fatigue crack was increased slightly in order to provide a fresh sharp notch at the base

of the crack. (Other investigators have reported that possibly due to

creep effects, some blunting of the crack occurs during the stress

corrosion tests.) The newly precracked specimen was then returned to
.
the stress apparatus and stressed again at a slightly higher stress

intensity. This was repeated until failure occurred or until the depth

of the fatigue crack had increased to a value which made the specimen

unsuitable for further tests. Figure k shows the uniformity of the

fatigue cracks initiated by the precracking process and typical fractured

surfaces for 3 failed stress-corrosion test specimens.

Generally, a stress intensity factor of 50 Ksi */ in . was used as the

initial level and this was increased in 10 Ksi /Tin. steps until the specimen

either fractured or was removed from test. By comparing the stress intensity

factor for both specimens at which fracture occurred an approximate K| was determined. if fracture of only one specimen was obtained, then scc

the stress intensity factor at which fracture occurred was assumed to

be the approximate Kt

^scc

After the approximate Kj was determined for each group of specimens,

see

a fourth specimen was used to determine the apparent *\f scc A maximum of

3 tests were performed on each specimen by alternately precracking and

stressing the specimen as noted above. The intensity level jthosen for

the first test on the fourth specimen was at least 10 Ksi / in. below the

approximate Kj scr . This was increased in 5 Ksi / in .steps until the stress

intensity level during test was 5 Ksi s/Tn „ a bove the approximate Kt

x scc

Resu 1 ts : The results obtained from an investigation of the scress-corros i on behavior of T i -6A 1 ~b\J alloy in a synthetic sea-water environment are sum- marized in Table 2. Data are given for individual specimens in terms of

the maximum stress intensity, K-r at which no failure occurred as well

as the maximum Kj and time at which the specimen did fail. In interpreting

the data obtained to establish a threshold stress i ntens i ty \I SCC / below

which failure will not occur, it is necessary to take into account both the

Kj and time to produce failure. The data, based solely upon Kj at failure,

could lead to the establishment of an erroneous KI S cc value, For this reason in some instances there are two different values given for the K

the apparent and tne probable

re 1 i ab 1 e

The latter is considered to be more

I

’ J see

The Ki

scc

for specimens obtained from base metal areas and heat

affected areas of weldments was found to be between 75 and 80 Ksi /"in.

There was no apparent difference for plate of either thickness. The stress

relieving heat treatment had no noticeable effect on the Kt. value

obtained for specimens from either one of these areas. Specimen orientation

had no effect: upon the Kt of the base metal.

see
>o a ..ujdbti* J . ti : > o ><.-•-

. )\ 3 i' vitixcriqq 19 i a J 1
- 5

-

The stress relieving heat treatment did substantially decrease

the ^I

s (60 Ks.i. /Tn ») at the weld area of the specimens machined from

surface areas of the 2 in. thick plate. At similar areas on specimens

that had not been stress relieved there was a slight increase in the

^I S cc(^5 Ksi / irw) . There was also a slight decrease in the ^I scc

(70 Ksi /irT.

for specimens machined from weld areas of the 1/2 in. thick plate.

Cone I us i ons : It has been demonstrated that immunity to stress-corrosion

can be defined in terms of a threshold stress i ntens i ty , ^I £CC . The

threshold stress intensity is that quantity, expressed in units ofKsi/in.,

below which a material will not fail by stress-corrosion cracking, in a given corrosive medium.

Comparison of the ^I scc values obtained from stress-corrosion tests

conducted in this investigation revealed the following.

1. Base-metal material and materia) obtained from weldments

in the heat affected zone.

a. The K t was between 75 and 80 Ksi

i scc

b. Thickness of the plate (l/2-in. vs 2-in.) had no effect

on the Kt^-tsec

C» Similarly a stress-relieving heat treatment had no effect

on the Kt _ __

J-scc

2. Base-meta! material.

a. Orientation (longitudinal vs transverse) of specimens

obtained from 2 in. thick plate had no effect in either

increasing or decreasing the KT

-*-scc

3.

Weld-metal material.

a. The Kj

c at weld-metal areas of stress-relieved

material was lower. This decrease was substantially

greater for specimens machined fr om t he surface areas

of the 2-in. thick plate (60 Ksi / in . ) than for spec i mens machined from the 1/2-in. thick plate (70 Ksi /in . )

b. There was an increase in the Kj scc (85 Ksi / in. ) for

specimens machined from the surface areas of non-stress

relieved 2-in. thick plate.
.

amis j 'h i

t:. : q A ... I :
- 6 -

REFERENCES

(1) 8. F. Brown, "A New Stress-Corrosion Crac.king Test for High

Strength Alloys," Materials Research and S tandardsr, Vol. 6,

No. 3(1 960 )

(2) J „ A. Kies, et al., "Fracture Testing o’f Weldments," In,

"Fracture Toughness Testing and It’s Appl icat i ons," AST

Special Technical Publication 381 ., 19^5? PP- 328-356.

Table ! „ Oi st r i but i on and i deni; i fi cat i on of specimens

machined from 1/2 in. and 2 in. thick plate.

Coupon j Heat

r —

Specimen Plate Number of spec s mens number t reatment 1 orientation thi ckness. Base We 1 dments

(a) (b) in. metal We 1 d area

Heat affected

zone

5 SR - i/2 - 4 4

K SR - 1/2 4 - -

L None 1/2 4 4

V None - 1/2 4 - -

A SR L, surface 2 4 4 4

A SR L, surface 2 4 4 4

A SR I, surface 2 4 - ”

A SR T, center 2 4 ”

j a None L, surface 2 4 4 4

3 None L, center 2 4 4 4

B None T, surface 2 4 -

B

i

None T, center 2

J

4

J

-

(a) Heat treatment after fabrication -- SR » stress re 1 i eved. Pr i or heat

treatment, not reported.

(b) L - longitudinal, T - transverse, surface - machined from surface area

of plate and center - machined from areas at center of thickness of plate.
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Figure 3* Stress corrosion test apparatus

Figure 4. Stress-corrosion test specimens showing typical fractured surfaces.

The uniformity of the fatigue cracks (resulting from the precracking process) may be seen in the upper area of each specimen.

Each horizontal striation delineates the ba's'e of a fatigue crack.

The coarser, fibrous areas are those resulting from stresscorrosion and subsequent fast fracture.


V