Kazuhisa Miyoshi, Bradley A. Lerch, and Susan L. Draper

Glenn Research Center, Cleveland, Ohio

Preliminary Study on Fatigue Strengths

of Fretted Ti-48Al-2Cr-2Nb

NASA/TM—2002-211718

August 2002
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Kazuhisa Miyoshi, Bradley A. Lerch, and Susan L. Draper

Glenn Research Center, Cleveland, Ohio

Preliminary Study on Fatigue Strengths

of Fretted Ti-48Al-2Cr-2Nb

NASA/TM—2002-211718

August 2002

National Aeronautics and

Space Administration

Glenn Research Center
L 

The Aerospace Propulsion and Power Program at 

NASA Glenn Research Center sponsored this work. 

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National Technical Information Service 

5285 Port Royal Road 

Hanover, MD 21076 Springfield, VA 22100 

Available electronically at http: / /gltrs.grc.nasa.go\' / GLTRS 
NASA/TM—2002-211718 1

Preliminary Study on Fatigue Strengths of Fretted Ti-48Al-2Cr-2Nb

Kazuhisa Miyoshi, Bradley A. Lerch, and Susan L. Draper

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Abstract

The fatigue behavior (stress-life curve) of gamma titanium aluminide (Ti-48Al-2Cr-2Nb, atomic

percent) was examined by conducting two tests: first, a fretting wear test with a fatigue specimen in

contact with a typical nickel-based superalloy contact pad in air at temperatures of 296 and 823 K and

second, a high-cycle fatigue test of the prefretted Ti-48Al-2Cr-2Nb fatigue specimen at 923 K. Reference

high-cycle fatigue tests were also conducted with unfretted Ti-48Al-2Cr-2Nb specimens at 923 K. All

Ti-48Al-2Cr-2Nb fatigue specimens were machined from cast slabs. The results indicate that the stresslife results for the fretted Ti-48Al-2Cr-2Nb specimens exhibited a behavior similar to those of the

unfretted Ti-48Al-2Cr-2Nb specimens. The values of maximum stress and life for the fretted specimens

were almost the same as those for the unfretted specimens. The resultant stress-life curve for the unfretted

fatigue specimens was very flat. The flat appearance in the stress-life curve of the unfretted specimens is

attributed to the presence of a high density of casting pores. The fatigue strengths of both the fretted and

unfretted specimens can be significantly affected by the presence of this porosity, which can decrease

the fatigue life of Ti-48Al-2Cr-2Nb. The presence of the porosity made discerning the effect of fretting

damage on fatigue strength and life of the specimens difficult.

Introduction

Fretting failure can occur in a variety of engine components (refs. 1 and 2). The most damaging effect

of fretting is the significant reduction in fatigue-resistance of the fretted component even with mild wear.

This form of damage arises when the contacting surfaces undergo oscillatory displacement of small

amplitude. As a result of the displacement, adhering metal particles are produced (adhesive wear), which

can oxidize. These oxidized particles are abrasive, subsequently causing abrasive wear (a severe form of

wear) of the surfaces. The failure probability of engine components abruptly increases at this stage. Progressive fretting produces galling, which in turn can cause premature fatigue crack initiation. Propagation

of such cracks under cyclic loads may result in the failure of the blade or disk and convey fragments

into the engine with catastrophic results. Even a small tribological failure can clearly lead to catastrophic

results (ref. 3).

The material of interest in this study is gamma titanium aluminide (γ-TiAl). This material has potential applications for low-pressure turbine blades. A concern for the γ-TiAl blades is fretting at the dovetail

caused by alternating centrifugal force and a natural high-frequency blade vibration (fig. 1). For example,

observations of service-exposed Ti-based alloy fan blade-disk couples in fan engine propulsion systems

revealed the presence of severe fretting fatigue damage on the contacting surfaces of blade dovetails and

disk slots (ref. 3).

The objective of this preliminary study was to evaluate the effects of fretting on the fatigue-resistance

of γ-TiAl (Ti-48Al-2Cr-2Nb, atomic percent). Ti-48Al-2Cr-2Nb fatigue specimens were machined from

cast slabs. Two tests, a fretting test and a fatigue test, were conducted with the Ti-48Al-2Cr-2Nb fatigue

specimens. First, baseline-fretting tests were conducted with a Ti-48Al-2Cr-2Nb fatigue specimen in contact with a typical, aged nickel-based superalloy contact pad in air at temperatures of 296 and 823 K using
NASA/TM—2002-211718 2

a fretting apparatus. Second, fatigue tests in air at 923 K using a high-cycle fatigue test setup were separately conducted with the prefretted fatigue specimens. Reference fatigue tests were also conducted with

unfretted Ti-48Al-2Cr-2Nb fatigue specimens in air at 923 K.

Materials

The Ti-48Al-2Cr-2Nb specimens were determined to be of the following composition (in atomic

percent): titanium, 47.9; aluminum, 48.0; niobium, 1.96; chromium, 1.94; carbon, 0.013; nitrogen, 0.014;

and oxygen, 0.167 (ref. 4).

Specimens and Tests

Specimens.—All Ti-48Al-2Cr-2Nb fatigue specimens were machined from cast slabs. Each fatigue

specimen (fig. 2) had a total length of 152.4 mm (6 inches) with rectangular (2.03 mm  12.70 mm) cross

section ends and a straight gage section, 25.9 mm long. The specimens had an elliptical cross section in

the gage. The average surface roughness, Ra, and Vickers hardness for the Ti-48Al-2Cr-2Nb fatigue

specimens are summarized in table 1. Figure 3(a) presents a three-dimensional, optical interferometry

image of the gage section of the fatigue specimen. The surface shows a relatively homogeneous surface

texture containing machined grooves along the length of the specimen in the direction of the fatigue loading. The mean value of centerline average roughness, Ra, measured parallel to the length of the specimen

is 0.57 μm with a standard deviation of 0.17 μm. The Ra measured perpendicular to the length of the

specimen is 0.75 μm with a standard deviation of 0.10 mm. However, the detailed optical interferometry

image shows a large casting pore with a length as large as 70 μm (fig. 3(b)). The Ti-48Al-2Cr-2Nb

fatigue specimen generally contained cavities with depths of 2 to 4 μm and cutting grooves with depths

of 3 μm. Therefore, the maximum height (peak-to-valley) of the surface, Rt, was relatively large, having

a value of 8.8 μm and a standard deviation of 2.2 μm. Note that Rt is the vertical distance between the

highest (Rp) and lowest (Rv) points as calculated over the entire measured surface. It is defined as:

Rt = Rp + Rv. The maximum profile peak height Rp is the distance between the highest point of the

surface and the mean surface for the entire measured surface. The maximum profile valley depth Rv is

the distance between the lowest point of the surface and the mean surface for the entire measured surface.

Figure 1.—Fan and compressor blade dovetail displacements.

Causes of Fretting

Alternating Centrifugal Force Natural Blade Vibration

Centrifugal

blade load

Blade 

vibration

Slip Slip Slip

Source: S. Chakravarty, R.G. Andrews, P.C. Patnaik, and A.K. Koul

JOM, April 1995, pp. 31–35

Slip

Rim slot

displacement
NASA/TM—2002-211718 3

Figure 2.—Ti-48Al-2Cr-2Nb fatigue specimen.

 LC

5.08 mm

9.91 mm

2.03 mm

0.31 mm-radius

Smooth transition from 

ellipse to rectangular 

grip end, 

all sides. Gage

25.9 mm

31.75 mm

12.7 mm

152.4 mm

368.3 mm-radius

 = 20°

Gage cross section

Gage width

9.91 mm

TABLE 1.—SURFACE ROUGHNESS AND VICKERS HARDNESS OF SPECIMENS

Centerline-average roughness,

Ra,

nm

Vickers hardness, Hv,

GPa

Specimen

Mean Standard

deviation

Mean Standard

deviation

Ti-48Al-2Cr-2Nb fatigue specimen 370 49 3.78 0.57

9.4-mm-diameter, hemispherical,

nickel-based superalloy pin

40 8.9 5.52 0.44

Nickel-based superalloy contact pad:

two-sided wedge with a

12-mm-long knife-edge

2.75 ¥ 103 65 4.85 0.28

Nickel-based superalloy contact pad:

wedgelike shape with rectangular flat

419 42 4.78 0.21
NASA/TM—2002-211718 4

Figure 3.—Three-dimensional, optical interferometry image of gage section of machined Ti-48Al-2Cr-2Nb fatigue

 specimen. (a) Overview showing a homegeneous surface texture with grooves. (b) Casting porosity.

1.02

0.50

0.00

–0.50

–1.00

–1.50

–2.00

–2.50

–3.29

–3.00

(a)

(b)

300 μm

2.5 mm

2.5 mm

μm

604.2 μm

459.9 μm
NASA/TM—2002-211718 5

There were three types of nickel-based superalloy contact pads. One (fig. 4(a)) had a wedgelike shape

with two rectangular flats (1.00 mm  12.70 mm) and one of the rectangular-shaped flats was brought

into contact with the Ti-48Al-2Cr-2Nb fatigue specimen (fig. 5(a)). The second (fig. 4(b)) was a twosided wedge with 12.70 mm long knife-edges. The contained angle between the two sides was 35° and

the average radius of curvature for the knife-edge was less than 0.1 mm (fig. 4(b)). One of the knifeedges was brought into contact with the Ti-48Al-2Cr-2Nb fatigue specimen, as schematically shown in

fig. 5(b). The third was a 9.4-mm-diameter hemispherical pin (not shown in the figure). The spherical

surface was brought into contact with the Ti-48Al-2Cr-2Nb fatigue specimen. The average surface

roughness, Ra, and Vickers hardness for the nickel-based superalloy contact pads (including pins) are

also summarized in table 1.

Fretting Test. —The fretting apparatus used in this investigation was basically the same as that

shown in fig. 6. Fretting experiments were conducted with aged nickel-based superalloy pads in contact

with Ti-48Al-2Cr-2Nb fatigue specimens (fig. 5). The Ti-48Al-2Cr-2Nb fatigue specimen was placed

on a rectangular parallele piped, strip heater (127 mm 16 mm  6.4 mm) and was held by four clamps

(6.4 mm 12.7 mm  2.0 mm), as shown in figure 5.

All fretting wear experiments were conducted under the conditions shown in table 2(a) to (d). Both

nickel-based superalloy contact pads and Ti-48Al-2Cr-2Nb fatigue specimens were rinsed with 200-proof

ethyl alcohol before installation in the fretting apparatus.

Fatigue Test.—Both prefretted Ti-48Al-2Cr-2Nb fatigue specimens and unfretted Ti-48Al-2Cr-2Nb

fatigue specimens were tested in high-cycle fatigue at 923 K and at a frequency of 80 Hz. Based on

previous experience, the fatigue-crack growth rates are highest at 923 K (650 °C) for Ti-48Al-2Cr-2Nb

(ref. 5), and therefore this temperature was chosen for testing to simulate the worst-case scenario. Fatigue

tests were all run with a load ratio R of 0.05 (R = σmin/σmax). Due to the flat nature of the S-N (stress vs.

cycles to failure) curve for γ-TiAl, step fatigue tests (refs. 6 and 7) were used to determine the maximum

fatigue strength for prefretted Ti-48Al-2Cr-2Nb fatigue specimens. Based on the previous experience,

Figure 4.—Nickel-based superalloy contact pads. (a) Wedge-like pad with flat. (b) Wedge with knife-edge.

12.7 mm

Flat 1.0 mm

4.8 mm

22 mm

11°

(a)

Knife-edge with

0.1 mm-radius 

of curvature

12.7 mm

4.1 

mm

21 mm

55°

(b)
NASA/TM—2002-211718 6

Figure 5.—Fatigue specimen and contact configuration. (a) Wedge-like 

 pad with flat on fatigue specimen. (b) Wedge with knife-edge on 

 fatigue specimen.

19 mm

Wedge-like pad 

with flat

Strip heater

Fatigue 

specimen

Strip heater Specimen hold clamp 

(6.412.72 mm)

Fatigue 

specimen

Wedge with

knife edge

(a)

(b)
NASA/TM—2002-211718 7

Figure 6.—Fretting apparatus.

Detail

Pin

Test

specimen

Thermocouple

Sensor

CD-01-81402

Oscillation

Oscillation

Sensor

Load
NASA/TM—2002-211718 8

TABLE 2.—FRETTING TEST CONDITIONS

(a) Fretting with spherical superalloy pin specimens.

 (a.1) Fretting conditions on both front and back sides of fatigue specimen (I.D. No. 063-03-3).

9.4-mm-diameter spherical pin specimen Superalloy

Flat specimen g-TiAl, Specimen I.D. No. 063-03-3

Environment Air at 296 K

Load 1.5 N for first 600 000 cycles and subsequently 3.0 N for

7 million cycles

Frequency Hz: 81

Amplitude 50 mm

Total number of cycles 7 million

Fretting direction Parallel to the length of the fatigue specimen

(in the direction of the fatigue loading)

(b) Fretting at 823 K.

(b.1) Fretting on front side of fatigue specimen (I.D. No. 6-3-3).

Knife-edge-shaped contact pad specimen Superalloy

Fatigue specimen g-TiAl, Specimen I.D.: 6-3-3

Environment Air at 823 K

Load 1.5-3.0 N

Amplitude (mean value) 50 mm

Frequency 80 Hz and 160 Hz

Number of cycles 20 million cycles at 80 Hz and subsequent 80 million

cycles at 160 Hz

Total number of cycles 100 million cycles

Fretting direction Perpendicular to the length of the fatigue specimen

(perpendicular to the direction of the fatigue loading)

(b.2) Fretting on back side of fatigue specimen (I.D. No. 6-3-3).

Knife-edge-shaped contact pad specimen Superalloy

Fatigue specimen: g-TiAl, Specimen I.D.: 6-3-3

Environment Air at 823 K

Load 1.5-3.0 N

Amplitude (mean value) 50 mm

Frequency 80 Hz

Number of cycles 40 million cycles at 80 Hz

Total number of cycles 40 million cycles

Fretting direction Perpendicular to the length of the fatigue specimen

(perpendicular to the direction of the fatigue loading)
NASA/TM—2002-211718 9

TABLE 2.—(CONTINUED).

 (c) Fretting at different temperatures.

(c.1) Fretting on front side of fatigue specimen (I.D. No. 5-1-3).

Knife-edge-shaped contact pad specimen Superalloy

Fatigue specimen g-TiAl, Specimen I.D. No. 5-1-3

Environment Air at 823 K and at room temperature

Load 1.5-3.0 N

Amplitude (mean value) 50 mm

Frequency 160 Hz

Number of cycles 36 million cycles at 823 K and subsequently 73 million

cycles at 296 K

Total number of cycles 109 million cycles

Fretting direction Parallel to the length of the fatigue specimen

(in the direction of the fatigue loading)

(c.2) Fretting on back side of fatigue specimen (I.D. No. 5-1-3).

Knife-edge-shaped contact pad specimen Superalloy

Fatigue specimen: g-TiAl, Specimen I.D. No. 5-1-3

Environment Air at 823 K and subsequently at 296 K

Load 1.5-3.0 N

Amplitude (mean value) 50 mm

Frequency 160 Hz

Number of cycles 109 million cycles at 823 K and subsequently 42 million

cycles at 296 K

Total number of cycles 151 million cycles

Fretting direction Parallel to the length of the fatigue specimen

(in the direction the fatigue loading)
NASA/TM—2002-211718 10

a starting stress level was chosen. If the specimen survived 106 cycles, the stress was increased by

14 MPa and run to failure or 106 cycles. The stress level was increased until failure (two pieces) occurred.

The use of step fatigue testing avoided wasting a large number of specimens due to runouts as each specimen was forced to fail.

Results and Discussion

Observations

Fretting Wear of Superalloy.—Surface and subsurface damage always occurred on the interacting

surfaces of the Ti-48Al-2Cr-2Nb and superalloy during fretting in air (ref. 4). The surface damage of the

Ti-48Al-2Cr-2Nb and superalloy consisted of material transfer and back transfer, pits, oxides and debris,

scratches, fretting craters and wear scars, plastic deformation, and cracks. In general the damage of

Ti-48Al-2Cr-2Nb is analogous to that of superalloy. Since the observation of surface and subsurface

damage of Ti-48Al-2Cr-2Nb was already reported (ref. 4), the damage of the superalloy will be primarily

discussed below.

Figures 7(a) to (b) show examples of surface damage produced on superalloy: metallic wear debris

of Ti-48Al-2Cr-2Nb and superalloy, oxides, scratches (grooves), small craters, plastically deformed

asperities, and cracks, all of which are similar to those observed on Ti-48Al-2Cr-2Nb. The scratches

(fig. 7(a)) can be caused by hard, oxidized wear particles of superalloy and Ti-48Al-2Cr-2Nb, which

are trapped between the interacting surfaces or are adhered to or embedded in the counterpart Ti-48Al2Cr-2Nb surface. In fig. 7(a) scratches and fracture pits (craters) were produced by abrasion, a severe

TABLE 2.—(CONCLUDED)

(d) High load fretting.

 (d.1) Fretting conditions on front side of fatigue specimen (I.D. No. 5-1-6).

Wedgelike contact pad specimen with

rectangular flat

Superalloy

Fatigue specimen g-TiAl, Specimen I.D.: 5-1-6

Environment Air at 823 K

Load 450 N

Amplitude 90 mm

Frequency 80 Hz

Total number of fretting cycles 16 million fretting cycles at 823 K

Fretting direction Parallel to the length of the fatigue specimen (in the

direction of the fatigue loading)

(d.2) Fretting on back side of fatigue specimen (I.D. No. 5-1-6).

Wedgelike contact pad specimen with

rectangular flat

Superalloy

Fatigue specimen g-TiAl, Specimen I.D.: 5-1-6

Environment Air at 296 K

Load 450 N

Amplitude (mean value) 90 mm

Frequency 80 Hz

Total number of cycles 16 million fretting cycles at 296 K

Fretting direction Parallel to the length of the fatigue specimen (in the

direction of the fatigue loading)
NASA/TM—2002-211718 11

(a)

2 μm

2 μm 10 μm

10 μm

(b)

(c) (d)

Figure 7.—Surface and subsurface damage in superalloy pin in contact with Ti-48Al-2Cr-2Nb flat. (a) Scratches.

 (b) Cracks in oxide layers. (c) Cracks in metal. (d) Fracture pits and plastic deformation. Fretting conditions: load,

 1 N; frequency, 80 Hz; slip amplitude; 50 μm; and total number of cycle, 1 million: environment, air: and temper-

 ature, 823 K.

form of wear. The trapped wear particles and the adhered (or embedded) wear particles plow or cut the

superalloy surface. The trapped wear particles have a scratching effect on both surfaces; and because they

carry part of the load, they cause concentrated pressure peaks on both surfaces (ref. 4). The pressure

peaks may well be the origin of crack nucleation in the oxide layers and the bulk alloys.

Oxide layers readily form on the superalloy surfaces at 823 K and are often a favorable solution to

wear problems. However, cracks occurred in the oxide layers both within and around the contact areas, as

shown in fig. 7(b).

Fractures in the protective oxide layers produced cracks in the bulk superalloy (fig. 7(c)) and also

produced wear debris, chemically active fresh surfaces, plastic deformation, craters, and fracture pits

(fig. 7(d)). This wear debris can cause third-body abrasive wear. Local, direct contacts between the fresh

surfaces of Ti-48Al-2Cr-2Nb and superalloy resulted in increased adhesion and local stresses, which may

cause plastic deformation, flakelike wear debris, and craters.

Fretting Scar on Fatigue Specimen and Contact Pad.—All of the Ti-48Al-2Cr-2Nb fatigue specimens and the superalloy contact pads showed fretting damage. Typical damage observed on a Ti-48Al2Cr-2Nb fatigue specimen at a low magnification is shown in figure 8. Because of the contact geometry,

a long wear scar was produced perpendicular to the length of the fatigue specimen in the center of gauge

section. In figure 8 the wear debris has been removed from the fatigue specimen surface to show the

overall geometry of wear scar.
NASA/TM—2002-211718 12

2 μm

2 mm

Figure 8.—Wear scar produced on Ti-48Al-2Cr-2Nb

 fatigue specimen in contact with superalloy contact

 pad. Table 2(b.2) presents the fretting conditions.

100 μm

Figure 9.—Wear scar with wear debris particles pro-

 duced on Ti-48Al-2Cr-2Nb fatigue specimen in 

 contact with superalloy contact pad. Table 2(b.2)

 presents the fretting conditions.

10 μm 10 μm

Figure 10.—Protective oxide layers produced on both (a) Ti-48Al-2Cr-2Nb fatigue specimen and (b) superalloy

 contact pad. Table 2(b.2) presents the fretting conditions.

(a) (b)

10 μm 10 μm

Figure 11.—Surface damages produced on both (a) Ti-48Al-2Cr-2Nb fatigue specimen and (b) superalloy contact

 pad. Table 2(b.2) presents the fretting conditions.

(a) (b)
NASA/TM—2002-211718 13

Figure 9 shows a typical example of a wear scar with wear debris from fretting produced on the

Ti-48Al-2Cr-2Nb fatigue specimen in contact with the superalloy contact pad. Because of the specimen

geometry, a large amount of wear debris is deposited just outside the line contact area. Pieces of the

Ti-48Al-2Cr-2Nb and its oxides are generally torn out or sheared off during fretting.

Protective oxide layers readily form on both Ti-48Al-2Cr-2Nb and superalloy at 823 K in air.

Although cracks occurred in the oxide layers formed on Ti-48Al-2Cr-2Nb and superalloy (figs. 10(a) and

(b), respectively), the alloy surfaces were protected by these oxide layers.

On the other hand, figures 11(a) and (b) show examples of the Ti-48Al-2Cr-2Nb and superalloy

surfaces that are torn out or sheared off during fretting. There are fractured pits, flakelike wear debris,

and oxide wear particles in the contact areas of both Ti-48Al-2Cr-2Nb and superalloy surfaces.

Baseline Fretting Fatigue Tests

Fatigue tests were conducted with both prefretted fatigue specimens and unfretted Ti-48Al-2Cr-2Nb.

These tests were conducted to determine the number of cycles to failure at each stress level, and in effect

create a preliminary stress-life curve. The fatigue results for prefretted Ti-48Al-2Cr-2Nb specimens from

high-cycle fatigue tests are included in table 3(a). The prefretted specimen shown in table 2(a) failed near

the grip section at a maximum stress of 282 MPa at 11,200 cycles. Fretting damage (wear scars)

produced by spherical superalloy pin specimens was not significant enough to cause failure. The fretted

specimen shown in table 2(b) failed near the clamp section at a maximum stress of 331 MPa at 9,040

cycles. Also, the fretted specimen shown in table 2(c) failed near the clamp section at a maximum stress

of 248 MPa at 6,560 cycles. Fretting damage produced by the clamps could be greater than that by the

contact pads. The fretted specimen shown in table 2(d) failed near the fretted area at a maximum stress

386 MPa at 12,400 cycles.

The fatigue results for unfretted Ti-48Al-2Cr-2Nb specimens from high-cycle fatigue tests are

included in table 3(b). The stress-life results for the prefretted specimens exhibited a behavior similar

to those of the unfretted specimens. The values of maximum stress for the prefretted specimens were

almost the same as those of the unfretted specimens.

 TABLE 3.—STRESS-LIFE RESULTS

 (a) Prefretted Ti-48Al-2Cr-2Nb.

Maximum stress,

MPa

Number of cycles

to failure

Table showing

fretting conditions

Specimen

I.D. number

286 11,200 2 (a) 063-3-3

331 9,040 2 (b) 6-3-3

248 6,560 2 (c) 5-1-3

386 12,400 2 (d) 5-1-6

 (b) Unfretted Ti-48Al-2Cr-2Nb.

Maximum stress,

MPa

Number of cycles

to failure

288 5,100

266 681,400

314 7,100

346 132,000

245 947,040
NASA/TM—2002-211718 14

The data from tables 3(a) and (b) are plotted in figure 12. Figure 12 appeared that the resultant stresslife curve for the unfretted fatigue specimens was very flat and exhibited a behavior similar to that of

ceramic materials (refs. 8 –11). Similar fretting fatigue stress-life curves have also been reported for

γ-TiAl based materials (refs. 12 and 13). The flat appearance in the stress-life curve is attributed to the

presence of a high density of casting pores.

Thus, the fatigue strengths of both prefretted and unfretted Ti-48Al-2Cr-2Nb fatigue specimens

were significantly affected by the presence of casting pores. This porosity obscured the effect of fretting

damage on fatigue strength and life of Ti-48Al-2Cr-2Nb.

Concluding Remarks

The fatigue behavior (stress-life curve) of γ-TiAl (Ti-48Al-2Cr-2Nb) was examined by conducting

two tests: first, a fretting wear test with a Ti-48Al-2Cr-2Nb fatigue specimen in contact with a typical

nickel-based superalloy contact pad in air at 296 and 923 K and second, a fatigue test of the fretted

specimen in air at 923K. Reference fatigue tests were also conducted with as-received, unfretted

Ti-48Al-2Cr-2Nb specimens in air at 923 K. All Ti-48Al-2Cr-2Nb fatigue specimens were machined

from cast slabs. As a result, the following conclusions are drawn from the data presented:

1. For both fretted fatigue and unfretted fatigue specimens, fatigue strengths can be significantly

affected by the presence of casting porosity.

2. Casting porosity can obscure the effect of fretting damage on the fatigue strength of cast Ti-48Al2Cr-2Nb. The stress-life results for the fretted specimens exhibited a behavior similar to those of the

unfretted specimens. The values of maximum stress for the fretted Ti-48Al-2Cr-2Nb specimens were

almost the same as those of the unfretted Ti-48Al-2Cr-2Nb specimens.

3. The resultant stress-life curve for the unfretted Ti-48Al-2Cr-2Nb fatigue specimens was very flat.

The flat appearance in the stress-life curve is attributed to the presence of a high density of casting pores.

Figure 12.—Baseline fatigue test results for both fretted Ti-48Al-2Cr-2Nb 

 fatigue specimens and as-received, unfretted Ti-48Al-2Cr-2Nb fatigue

 specimens.

500

400

300

200

100

1000 10 000 100 000

Cycles to failure

1 000 000

Maximum stress, MPa

As-received 

Fretted
NASA/TM—2002-211718 15

References

1. D.W. Hoeppner, V. Chandrasekaran, and C.B. Elliott III (editors), Fretting Fatigue: Current

Technology and Practices, ASTM STP 1367, West Conshohocken, PA 19428–2959, 2000.

2. S.R. Brown (Symposium Chairman), Materials Evaluation Under Fretting Conditions, ASTM 780,

Philadelphia, PA 19103, 1982.

3. S. Chakravarty, R.G. Andrews, P.C. Patnaik, and A.K. Koul, The Effect of Surface Modification

on Fretting Fatigue in Ti Alloy Turbine Components, JOM, vol. 47, No. 4, 1995, pp. 31–35.

4. K. Miyoshi, B.A. Lerch, S.L. Draper, and S.V. Raj, Evaluation of Ti-48Al-2Cr-2Nb Under Fretting

Conditions, NASA TM-211205, Nov. 2001.

5. J.M. Larsen, B.D. Worth, S.J. Balsone and J.W. Jones: in Gamma Titanium Aluminides, Y.W.

Kim, R. Wagner, and M. Yamaguchi, eds., TMS, Warrendale, PA, 1995, pp. 821–834.

6. A.F. Madayag, Metal Fatigue: Theory and Design, John Wiley & Sons, NY, 1969, pp. 144–148

7. J. Denk and S. Amhof, Fatigue ’96, Proc. 6th Int. Fatigue Congr., G. Lutjering and H. Nowack,

eds., Pergamon Press, 1996, vol. III, pp. 1967–1972.

8. F. Guiu, M.J. Reece, and D.A.J. Vaugham, Cyclic Fatigue in Some Structural Ceramics, Fatigue

of Advanced Materials, Eds. R.O. Ritchie et al., MCEP, Birmingham, 1991, pp. 193–210.

9. Y. Mutoh, M. Takahashi, T. Oikawa, H. Okamoto, Fatigue Crack Growth of Long and Short Cracks

in Silicon Nitride, Fatigue of Advanced Materials, Eds. R.O. Ritchie et al., MCEP, Birmingham,

1991, pp. 211–225.

10. M. Okane, T. Satoh, Y. Mutoh, and S. Suzuki, Fretting Fatigue Behavior of Silicon Nitride, Fretting

Fatigue, Eds. R.B. Waterhouse and T. C. Lindley, Mech. Engng. Publ., London, 1994, pp. 363–371.

11. M. Okane, Y. Mutoh, Y. Kishi, and S. Suzuki, Static and Cyclic Fretting Fatigue Behavior of

Silicon Nitride, Fatigue Fract. Engng. Mater. Struct., 19, (12), 1996, pp. 1493–1504.

12. E.J. Dollay, N.E. Ashbaugh, and B.D. Worth, Isothermal High-Cycle-Fatigue of a Ti-46.6Al3.0Nb-2.1Cr-0.2W (at. %) Gamma Titanium Aluminide, Proc. of Fatigue ’96, Berlin, Pergamon,

1996, vol. III, pp. 1755–1760.

13. T. Hansson, M. Kamaraj, Y. Mutoh, and B. Pettersson, High Temperature Fretting Fatigue Behavior in an XDTM γ-Base TiAl, Fretting Fatigue: Current Technology and Practices, ASTM STP

1367, D.W. Hoeppner, V. Chandrasekaran, and C.B. Elliot III, Eds., American Society for Testing

and Materials, West Conshohocken, PA, 2000, pp. 65–79.
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21

Preliminary Study on Fatigue Strengths of Fretted Ti-48Al-2Cr-2Nb

Kazuhisa Miyoshi, Bradley A. Lerch, and Susan L. Draper

Materials

Unclassified -Unlimited

Subject Category: 26 Distribution: Nonstandard

Responsible person, Kazuhisa Miyoshi, organization code 5160, 216–433–6078.

The fatigue behavior (stress-life curve) of gamma titanium aluminide (Ti-48Al-2Cr-2Nb, atomic percent) was examined by

conducting two tests: first, a fretting wear test with a fatigue specimen in contact with a typical nickel-based superalloy

contact pad in air at temperatures of 296 and 823 K and second, a high-cycle fatigue test of the prefretted Ti-48Al-2Cr-2Nb

fatigue specimen at 923 K. Reference high-cycle fatigue tests were also conducted with unfretted Ti-48Al-2Cr-2Nb

specimens at 923 K. All Ti-48Al-2Cr-2Nb fatigue specimens were machined from cast slabs. The results indicate that the

stress-life results for the fretted Ti-48Al-2Cr-2Nb specimens exhibited a behavior similar to those of the unfretted Ti-48Al2Cr-2Nb specimens. The values of maximum stress and life for the fretted specimens were almost the same as those for the

unfretted specimens. The resultant stress-life curve for the unfretted fatigue specimens was very flat. The flat appearance in

the stress-life curve of the unfretted specimens is attributed to the presence of a high density of casting pores. The fatigue

strengths of both the fretted and unfretted specimens can be significantly affected by the presence of this porosity, which

can decrease the fatigue life of Ti-48Al-2Cr-2Nb. The presence of the porosity made discerning the effect of fretting

damage on fatigue strength and life of the specimens difficult.