The influence of ion beam mixed NiAl surface layers on fatigue in polycrystalline nickel

Scr£pta METALLURGICA

Vol. 20, pp. 311-316, 1986 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

THE INFLUENCE OF ION BEAM MIXED Ni-A1 SURFACE LAYERS ON FATIGUE IN POLYCRYSTALLINE NICKEL

D. S. Grummon and J. W. Jones Department of Materials and Metallurgical Engineering University of Michigan Ann Arbor, MI 48109 USA

(Received July 18, 1985) (Revised December 23, 1985) Introduction Though fatigue crack propagation is generally used as the basis for engineering design for fatigue, it is well established that up to 90% of fatigue life may be spent in the crack initiation phase. The phenomenon of fatigue crack initiation is known to be a highly surface sensitive process: cracks nucleate there, environmental attack is mediated there, and the egress of mobile dislocations is influenced by surface properties. It is natural, then, to look to surface treatments for means to improve the fatigue life of metals. Many previous approaches have employed modifications to the surface stress state or structure using mechanical methods like shot peening (1,2) or by some metallurgical treatment, such as plating ( 3 ) . Recently, the technique of ion implantation, now established as a commercial process for wear control, and which shows promise for use to impart corrosion resistance, is beginning to be evaluated for its potential application to fatigue problems (4-9). When annealed FCC single crystals are cyclically loaded in plastic strain control, rapid dislocation multiplication gives rise to very high initial hardening rates (10), which eventually decline to zero at accumulated strains of ten or so, producing a plateau in the cyclic hardening curve at a stress level commonly called the saturation stress (II). The behavior of polycrystals in this respect is now thought to be generally analogous to the case for single crystals, though it is naturally more complex (12). Cyclic plastic straining is also known to produce intense strain localization in narrow lamellar bands of "soft" material, called persistent slip bands (PSB's), which invariably intersect the surface, producing intrusions and extrusions that are intimately associated with crack initiation processes (1315). The suppression of PSB formation, or the alteration of PSB action at the free surface, could thus be expected to have beneficial effects on fatigue crack initiation lifetimes. In the present study, the effect of an ion beam modified surface layer on fatigue crack initiation phenomena and cyclic plasticity is examined using pure polycrystalline nickel samples which have been implanted with aluminum using ion beam mixing. The results described here show that an ion beam mixed surface layer increases the saturation stress and tends to suppress the emergence of PSB's at the free surface of nickel samples cycled under plastic strain control. These effects are seen to be strongest for strain amplitudes in which PSB's are known to carry the bulk of the plastic strain during cycling.

Experimental Procedures Fatigue s p e c i m e n s w e r e m a c h i n e d from nickel alloy 270 to t h e g e o m e t r y shown in Fig. 1. Flat s u r f a c e s a p p r o x i m a t e l y 4mm wide were m a c h i n e d on opposite sides of t h e gage s e c t i o n to f a c i l i t a t e optical micrographic o b s e r v a t i o n of s u r f a c e morphological changes during f a t i g u e and to p r e s e n t a uniform incidence a n g l e for t h e ion b e a m . A f t e r machining, the f l a t s u r f a c e of e a c h s p e c i m e n was polished by hand using silicon carbide papers and 1 m i c r o n diamond paste. All of t h e specimens were t h e n a n n e a l e d in an a t m o s p h e r e of dry hydrogen for t h r e e hours a t 800 C. The r e s u l t i n g g r a m size was O

311 0036-9748/86 $3.00 + .00 Copyright (c) 1986 Pergamon Press Ltd.

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IN NICKEL

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The specimens were then electropolished using 20% perchloric acid in

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FIG. 1 Axial fatigue specimen geometry. A f l a t surface was machined onto opposite sides of the guage section to f a c i l i t a t e surface study.

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Surface modification by recoil ion implantation was carried out at the Tandem Accelerator Facility at Argonne National Laboratory. First~ alternate layers of nickel and aluminum were deposited on the surface by vacuum evaporation at 2x10-~ Pa. Seven layers were applied as follows: I0.I nm Ni, 9.6 nm AI, I0.0 nm Ni, 5.9 nm A1, 11.4 nm Ni, 5.4 nm A1, and finishing with I0 nm of nickel. After vacuum deposition, the specimens were promptly transferred to the accelerator target chamber and exposed to 0.5 MeV krypton ions to a dose of ixl0 la cm -2 at 5x10-5 Pa, to ballistically mix these surface layers. A 5mmxl2mm rectangular aperture confined the beam so that it irradiated the flat portion of the gage section, and each specimen was irradiated on both sides, such that about 30% of the total gage section area was treated. Plastic strain controlled fatigue tests were carried out on a servo-hydraulic load frame using cast metal grips and a high sensitivity extensometer. A digital system was used which was capable of operation at I Hz while maintaining the plastic strain amplitude to within 1 or 2 percent of the target strain at plastic strain amplitudes as low as 3x10- s or as high as 7x10-3, and which provided high quality cyclic hardening curves for saturation stress determination. Detailed hysteresis loops were acquired at lower frequencies (.05-oi Hz.) and stored for subsequent analysis. In order to determine the effect of the ion beam mixed Ni-AI surface layer on cyclic stress-strain behavior, several samples of both implanted and unmodified nickel were cycled at 1 Hz to total accumulated strains of I0 to 20 at plastic strain ranges of 3xi0 -s, 6.6xi0 -5, 3x10-4, lxi0 -3, 3xi0 -3, and 7x10-2. Saturation stresses were determined and the cyclic stress strain curve was established for both modified and unmodified material. In some cases, several saturation stress values were determined from a single specimen using step tests. In addition to m e c h a n i c a l property m e a s u r e m e n t s , d e t a i l e d o b s e r v a t i o n s were made of t h e evolution of s u r f a c e slip and t h e d e v e l o p m e n t of s u r f a c e morphology as a f u n c t i o n of plastic s t r a i n accumulation. To t r a c k changes in s u r f a c e f e a t u r e s , single s t a g e r e p l i c a s were made of t h e flat s u r f a c e s of t h e specimens a t f r e q u e n t i n t e r v a l s during the f a t i g u e t e s t . In this way, t h e first e m e r g e n c e of p e r s i s t e n t slip bands a t t h e s u r f a c e of t h e s p e c i m e n could be d e t e r m i n e d as a function of a c c u m u l a t e d s t r a i n . The d e g r e e of s u r f a c e roughening and t h e density of slip bands was also c o m p a r e d b e t w e e n s u r f a c e modified and unmodified specimens. Specimens w e r e cycled to s a t u r a t i o n and r e m o v e d for d e t a i l e d SEM o b s e r v a t i o n of t h e surfaces. Results and Discussion Typical cyclic hardening curves d e t e r m i n e d in this study a r e shown in Figure 2, where stress a m p l i t u d e is p l o t t e d as a f u n c t i o n of a c c u m u l a t e d plastic s t r a i n for modified and unmodified m a t e r i a l s a t two levels of p l a s t i c s t r a i n a m p l i t u d e . This d a t a shows t h a t t h e s u r f a c e modified m a t e r i a l displays a higher s t r e s s a m p l i t u d e beginning in t h e first few cycles, and c o n t i n u e s to do so throughout the tests. An i n c r e a s e is o b s e r v e d of about 22 MPa in s a t u r a t i o n s t r e s s for t h e t e s t a t 6.6x10 - s plastic strain, and 16 MPa a t 3xl0"~plastic s t r a i n a m p l i t u d e . Similar b e h a v i o r was o b s e r v e d for all but t h e lowest and t h e highest s t r a i n r a n g e s examined. No d i f f e r e n c e s were n o t e d in t h e a m o u n t s of a c c u m u l a t e d s t r a i n required to a c h i e v e s a t u r a t i o n .

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When cyclic stress strain curves are plotted for FCC single crystals, a plateau in the stress level is generally observed between plastic strains of 5x10-s and 5x10-3, which is the range in which PSB's are thought to carry most of the plastic strain in constant plastic strain amplitude fatigue. Polycrystals are known to behave in a similar manner, with the plateau replaced by a region of positive but diminished i

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FIG. 2 Cyclic hardening curves for surfac) modified and unmodified specimens, a: plastic strain range 6.6xI0"~, b: plastic strain range 3xlO-q

slope over a similar i n t e r v a l of plastic s t r a i n r a n g e s (16). The cyclic s t r e s s - s t r a i n (CSS) curves d e t e r m i n e d in the present study a r e shown in Fig. 3. An i n c r e a s e in s a t u r a t i o n s t r e s s is a p p a r e n t in t h ~ modified m a t e r i a l and occurs over a broad r a n g e of plastic s t r a i n a m p l i t u d e s b e t w e e n 5x10 ~ and 5x10-. Within this i n t e r v a l , the i n c r e a s e in s t r e s s is roughly c o n s t a n t a t a b o u t 16 MPa. Both c u r v e s also display a s h i f t to lower slope in this i n t e r v a l , and s t e e p e r slopes a t both ends of e a c h curve, b e h a v i o r which is c o n s i s t e n t with the o p e r a t i o n of PSB's a s . t h e main c a r r i e r of plastic s t r a i n in this regime.

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FIG.3 Cyclic stress strain curves f o r implanted'and un-implanted specimens showing saturation stresses as a function of the plastic strain range used in each t e s t . The upper curve plots data f o r the surface modified material, and shows an increase in the saturation stress compared wtth the unmN)dlfted material ~t strain ranges between 5x10-5 and 5x10-

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In a d d i t i o n to t h e m e c h a n i c a l b e h a v i o r changes observed, we have seen s t r i k i n g changes in the evolution of s u r f a c e morphology and in the r a t e and i n t e n s i t y of slip band e m e r g e n c e resulting from the ion b e a m s u r f a c e modification. Figures 4a and 4b show SEM m i c r o g r a p h s of s p e c i m e n s u r f a c e s t a k e n from modified and unmodified samples which had i d e n t i c a l s t r a i n histories. The lower density of persistent slip bands and the decrease in surface rumpling are readily apparent in the surface alloyed sample. Similar observations were obtained from other specimens, even those at the higher plastic strain ranges in which an increase in saturation stress was not clearly demonstrated.

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Figure 5 shows scanning electron micrographs taken from replicas of modified and unmodified specimens, both cycled at 3x10 -4 plastic strain. The replicas were made at total accumulated plastic strains of 2.0, 4.0 and 8.0 (the latter representing the material condition near saturation). From these and other observations, it is clear that the ion beam modified specimen shows a delay in the emergence of PSB's at the surface. In addition9 the density of slip bands was found to be generally lower at the surface of the modified material when compared to the surfaces of the unmodified specimens.

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FIG. 4 Low magnification SEM mtcrographs of specimen surfaces, at saturation; a: surface of ion beam modified nickel, b: surface of unmodified material. Specimens have identical stratn histories: the plastic strain range ts 3x10 "4

These results a r e believed to be s i g n i f i c a n t , especially since t h e s u r f a c e m o d i f i c a t i o n responsible for t h e observed changes in m e c h a n i c a l b e h a v i o r is very shallow, probably not e x c e e d i n g 100 to 200 nm in depth, and was applied t o only 30% of t h e gage s e c t i o n s u r f a c e a r e a . If t h e i m p l a n t e d aluminum were to be distributed in t h e bulk, it would c o n s t i t u t e an i m p u r i t y on t h e o r d e r of only 30 ppm. Also, the e f f e c t of the s u r f a c e t r e a t m e n t continued to be observed in t e s t s a t m o d e r a t e l y high s t r a i n amplitudes, in which the s u r f a c e s u f f e r e d considerable a c c u m u l a t i o n o f damage. It is a p p a r e n t t h a t modification of a very shallow s u r f a c e l a y e r has produced changes in both bulk p r o p e r t i e s and in s u r f a c e d e f o r m a t i o n behavior. While t h e r e is, as y e t , no simple e x p l a n a t i o n for t h e s e results, some f a c t s may be considered. First, and most obvious, is t h a t t h e c h e m i s t r y a t the s u r f a c e has been a l t e r e d . Aluminum, a c t i n g as a solid solution s t r e n g t h e n e r , would i n c r e a s e the yield s t r e n g t h of t h e s u r f a c e layer. Also, t h e s t a c k i n g f a u l t energy, which is high in pure nickel, would be reduced by alloying of the s u r f a c e region, a change which could i n f l u e n c e the d e v e l o p m e n t of slip line morphology (17). Secondly, a s u r f a c e region exposed to energetic ion b e a m i r r a d i a t i o n wiU acquire a high d e f e c t c o n c e n t r a t i o n , which would be e x p e c t e d to f u r t h e r i n c r e a s e t h e yield s t r e s s t h e r e . TEM observation of t h e near s u r f a c e l a y e r of nickel disks p r e p a r e d by the s a m e process used in t h e p r e s e n t study were found to have a n e x t r e m e l y fine grain size (on t h e o r d e r of h u n d r e d s o f n a n o m e t e r s ) ( 1 8 ) . It is r e a s o n a b l e to assume, t h e n , t h a t the modified s u r f a c e layer possesses a much higher yield s t r e n g t h t h a n t h e bulk, pure, nickel. If t h e surface yield s t r e s s is g r e a t e r t h a n t h a t of t h e bulk, t h e plastic s t r a i n in t h e s u r f a c e would be lower t h a n in t h e bulk, and in f a c t , t h e s u r f a c e p l a s t i c s t r a i n m i g h t r e m a i n a t ~ero until a threshold s t r e s s a m p l i t u d e was a t t a i n e d . It should also be pointed out t h a t , unlike oxide layers, p l a t e d layers, or e v e n diffusion layers, t h e ion beam mixed s u r f a c e l a y e r is continuous with r e s p e c t to

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the substrate, and has no distinct interface, but rather a continuous gradient of composition and defect structure.

FIG. 5 .4 SEM Micrographs of replicas made at progressive levels of plastic strain accumulation in two tests at 3x10 plastic strain range. A-C: unmodified specimen; D-F: ion beam modified specimen surface. Accumulated strains are 2.0, 4.0 and 8.0 respectively from l e f t to right.

The experimental data show two distinct effects of the surface modification: the increase in saturation stress, and the inhibition of PSB extension to the surface. Changes in the yield strength and lowering of the stacking fault energy may help to account for the latter effect which may be associated mainly with changes in the properties in the near surface region. However, the observed increase in cyclic saturation stress, which is a change in behavior of the bulk material, suggests that the operation of surface dislocation sources has been influenced by the surface modification. Further study is under way, including the preparation of plated transverse section TEM foils, to determine what differences in near-surface and bulk dislocation substructure can be associated with the observed effects of ion beam surface modification in these specimens.

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Conclusions I. The application of an ion beam mixed surface layer containing approximately 25 at.% aluminum increases the cyclic saturation stress of pure.polycrystalline nickel by approximately 15 to 20 MPa when cycled at plastic strain ranges between 5xi0 5"and 5x10-3. At any given level of accumulated strain, the stress amplitude of the surface modified material is greater than for an unmodified material tested under the same conditions. 2. The emergence of persistent slip bands at the surface is delayed by the surface modification, and at saturation, the surface modified material shows a lower density of PSB's and a lesser degree of surface rumpling when compared to an unmodified material. Acknowledgements This authors Nuclear Tandem

research was funded by the National Science Foundation under grant #DMR8310032. The gratefully acknowledge the contribution of J. Eridon and G. S. Was of the Department of Engineering at the University of Michigan, and Lynn Rehn, of the Argonne National Laboratory Accelerator Facility. References

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

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