Work hardening in the surface layer and in the bulk during fatigue

Scripta METALLURGICA

Vol. 12, pp. 129-131, 1978 Printed in the United States

Pergamon Press,

Inc

WORK HARDENING IN THE SURFACE LAYER AND IN THE BULK DURING FATIGUE R.N. Pangborn*, S. Weissmann* and I.R. Kramer** *Dept. Mechanics and Materials Science, Rutgers University Piscataway, N.J. 05854 and **David W. Taylor Naval Ship R&D Center, Annapolis, Maryland 21402 (Received October 24, 1977) (Revised December i, 1977)

It has been reported previously(1)that during uniaxial fatigue cycling, as in undirectional tensile test, (2,3) the surface layer work hardens more rapidly than that of the bulk material. A systematic investigation(4)on A1 2014-T6, Ti (6AI-4V) and a 4130 steel showed that a propagating fatigue crack was formed whenever the work hardening in the surface layer reached a critical value. This critical value was independent of the stress amplitude, prior fatigue history, and environment. It was proposed that the surface layer acted to oppose the motion of dislocations by providing a barrier to support a piled up array of dislocations of like sign. When the barrier becomes sufficiently strong, fracture occurs when the local stress field associated with an accumulation of excess dislocations exceeds the fracture strength. At lower surface layer strengths, slip occurs to allow relaxation of these local stress fields. More recently, using x-ray double crystal diffraction technique, we have studied the distribution of excess dislocations in fatigued A1 2024-T3 specimens as function of depth from the surface. On applying this method each reflecting grain of the specimen was considered to function independently as the second crystal of the diffractometer. CuK~ radiation was used and rocking curves were obtained by incremental specimen rotation and appropriate film shifts between settings. The AI(2024) fatigue specimens had a gage diameter of 3mm and a gage length of 6.35mm. They were solution treated at 495°C for 2 hrs in argon, quenched in ice water and naturally aged. Afterwards they were mechanically polished with 0.05 pm alumina and, to eliminate any remaining surface damage, were electropolished to remove 100 pm from the gage diameter with a 1-nitric acid 2-methanol solution at -10°C. This solution was also used to remove, incrementally, the surface layers to obtain the rocking curves as a function of depth. The specimens were fatigued in tension-compression with a constant stress amplitude and zero mean stress. In a preliminary investigation (7), the excess dislocation density of an A1 single crystal strained 10% at 0°C was determined by rocking curve technique (Fig. i). The excess dislocation density, D, was calculated from the relationship given by Hirsch(8): D:

BZ/9b_ 2

(l)

where B is the width of the rocking curve at half of the intensity maximum (half width), and ~ is the magnitude of the Burgers vector. On the basis of these data from the x-ray rocking curves, it is evident that a dislocation gradient with a negative slope exists in the surface layer. The value for D was about 3 times larger at the surface than in the bulk. Note, that the depth of the surface layer determined by the x-ray method was about 100 ~m, the same as that determined previously (9) by mechanical property changes.

129

130

WORK H A R D E N I N G DURING FATIGUE

Vol.

12, No.

2

The curves in Fig 2 show the depth profile of the excess d i s l o c a t i o n d e n s i t y of specimens fatigued to 25 and 75% of their fatigue life. The excess d i s l o c a t i o n density can be computed from Eq. 1 using ~n, which is the a v e r a g e rocking curve width of the grains normalized with respect to the width value measured prior to fatigue. Similar to the profile in Fig i, for a specimen strained in tension ~ was highest at the surface up to a depth of about i00 ~m. Beyond this depth B increased and attained a c o n s t a n t value at about 250 ~ m into the bulk. The value for 8 in the interior increased with the number of fatigue cycles but never exceeded that at the surface. Prima facie, the o b s e r v a t i o n that 8 has increased in the bulk of the specimen after cycling might suggest that fatigue damage has o c c u r r e d in the bulk. It m i g h t also be expected that after removal of the surface layer, the excess d i s l o c a t i o n density as indicated by ~, would increase with additional cycling. In c o n t r a d i c t i o n to such expectations, however, it is well known that the fatigue life of specimens can be p r o l o n g e d i n d e f i n i t e l y if appropriate amounts of metal are removed from the surface at various fatigue intervals (107. Furthermore, it was shown that after prolonged fatiguing and surface removal treatmeht, the fatigue resistance of the remaining metal was the same as that of the virgin specimen (3). As a c o n s e q u e n c e of these considerations, the stability of the bulk excess d i s l o c a t i o n structure of fatigued specimens, in the absence of the surface layer, was investigated. Inspection of Fig. 3A shows that after cycling for 75% of the fatigue life and s u b s e q u e n t removal of a 400 ~m surface layer by e l e c t r o p o l i s h i n g , additional cycling of the specimen gave rise to a rapid decrease of the excess d i s l o c a t i o n s at the new surface. Thus, it can be seen that the 8 value d e c r e a s e d from about 44 to 17 m i n u t e s of arc after a p p r o x i m a t e l y 250 cycles ~/Nf =0.017. With further cycling 8 began to increase again. The cycling was stopped at N/Nf =0.05 and the d i s t r i b u t i o n of the density of excess d i s l o c a t i o n s with d i s t a n c e from the surface was reexamined. Comparison of the curve in Fig. 3B with that for N/Nf = 0.75 in Fig. 2 d e m o n s t r a t e s that 8 not only d e c r e a s e d at the surface, but also in the bulk of the specimen. The B - d e p t h profile in Fig. 3B is about the same as that of a virgin specimen cycled N/Nf=0.05. A c c o r d i n g to Fig. 3B, the value at the surface was 30', c o r r e s p o n d i n g to a D=10~°/cm 2 (Eq. i), and was 15'(D= 2,6x109/cm 2 7 at a depth of 100-150 ~m. There was an indication of a slight rise in the curve with depth between 150 and 250 ~m. Thereafter, the values remained constant at about 18'. Therefore, it may be c o n c l u d e d that the excess d i s l o c a t i o n structure or a r r a n g e m e n t in the interior formed during fatigue is very unstable without the p r e s e n c e of the hardened surface layer. It should also be clear that the d i s l o c a t i o n structure formed during the fatigue process in the bulk of the specimen fatigued to (N/Nf =.757 did not impair the s u b s e g u e n t fatigue life once the surface layer was removed. This c o n c l u s i o n was reached because the 8 profile with depth after fatiguing, removal of the surface layer, and recycling was the same as that of virgin specimens cycled to the same fraction of the fatigue life. Whether this decrease in the 8 value is associated with a r e a r r a n g e m e n t of d i s l o c a t i o n s from a high energy c o n f i g u r a t i o n to that of low energy b o u n d a r y subgrains or with an actual d e c r e a s e in the d i s l o c a t i o n d e n s i t y will not be d i s c u s s e d here, but will be made the subject of another paper. In a g r e e m e n t with the concept of Kramer(4)and data of Taira(llT, the rocking curve data showed that fatigue fracture o c c u r r e d at a constant ~ value equal to about 70 minutes of arc (D= 6xl01°/cm 2 ); this critical 8 value was independent of the cyclic stress amplitude. In summary, the following c o n c l u s i o n s can be made on the basis of x-ray rocking curve analysis: During the first half cycle of fatigue it was deter• ined from the 8 values that a surface layer is formed with a higher d e n s i t y of excess d i s l o c a t i o n s than the bulk material. On s u b s e q u e n t cycling, the 8 values increased in the surface layer, and after about 5% of the fatigue life, a m i n i m u m began to appear in the B -depth profile. Further cycling increased the values in the surface layer and in the bulk. The excess d i s l o c a t i o n configuration.or d e n s i t y in the bulk, however, is not stable in the absence of the

Vol,

12

No, 2

WORK HARDENING

DURING FATIGUE

131

surface layer. This latter observation explains why the fatigue life may be extended indefinitely when the surface layer is removed at appropriate intervals. Thus the fatigue damage is confined to the surface layer and fracture occurs when the excess dislocation density attains a critical value. References (i) (2) (3) (4) (5) (6) (7)

I.R. Kramer, Proc. Air Force Conference on Fatigue, AFFDL-TR-70-144(1969). I.R. Kramer and L.J. Demer, Trans TMS-AIME, 227, 1003 (1963). I.R. Kramer, Trans TMS-AIME 230, 991, (1964). I.R. Kramer, Met Trans ~, 1735 (1974). S. Weissmann and D.L. Evans, Acta Cryst. 7, 733 (1954). S. Weissmann, J. Appl. Phys., 27, 389 (19~6). R. Pangborn, A. Gysler and S. ~ i s s m a n n , Second International Conference on Mechanical Behavior of Materials, 16-20 Aug 1976, p. 1746. (8) P.B. Hirsch, "Mosaic Structure," Prog. Met. Phy., 6, 283, (1956) (9) I.R. Kramer, Trans. Met. Soc. AIME, 233, 1462 (196~). (i0) N.T. Thompson, N.J. Wadsworth and N. Louat, Phil. Mag., i, 113 (1956). (ii) S. Taira, K. Tanaka and T. Tanabe, Proc. 13th Japanese C~ngress on Materials Research, 1970, pp. 14-19.

I

I

I

I

I

I

I

>'5 D- z

I

I

0

50

i

I

I

I

100

150

2O0

Y 300

DISTANCE FROM SURFACE (pm) FIG. I DISLOC,ATION DENSITY GRADIENT FROM SURFACE TO BULK FOR ALt~INUM SINGLE CRYSTAL

I

I

I

I

I

A1 --2024--1"3 STRESS: _+ 200.8 MpPa (29.1 KSI

I-I

I

I

I

6O

I

I

I

i

A1, 2024-1"3 STRESS: + 200.6 MPa(29.1

KSll

N/Nf = .75

N/Nf =

.25

2C

A

I

I

I

i

I

100

2OO

3OO

40O

5OO

DISTANCE FORM SURFACE. {~m) FIQ. 2 EXCESS DISLOCATtON PROFtLE WITH DEPTH

I

I

B

I

I 0

8OO I

2 3 4 N/Nf (%)

I

I

100

2gO

I ~

400

DISTANCE FROM SURFACE, (/Jm)

FIG. 3 pVALUE$ AFTER FATIGUINq 7E% OF LIFE AND REMOVING SURFACE LAYER (A) PVALUI~E AT SURFACE. (s) B--OEPTH PROFILE AT N/NI =