Cyclic strain localization in ion beam microalloyed nickel

Abstract

During cyclic deformation, many metals exhibit localization of plastic strain in persistent slip bands (psbs) (1). Fatigue cracks may initiate where the psbs intersect the surface. Numerous studies have established that the emergence of psb features at the surface can be significantly suppressed by the use of surface modification techniques such as ion beam microalloying (2,3). The modifications may alter the strength, elastic modulus, stacking fault energy, and residual stress state of the surface (4). As a result, the properties of the surface region can differ significantly from the properties of the subsurface. Even though the evolution of surface psb features can be significantly affected by a surface modification, the question remains as to the effect of a surface modification on bulk cyclic deformation mechanisms. Prior studies have concentrated on the effects of the modifications on cyclic hardening. The results of these studies have been inconclusive. Studies on copper single crystals indicated in one case (5) that ion beam surface modifications increased cyclic flow stress, while in another case (6) no change in cyclic flow stress was observed. Similar work on polycrystals has been somewhat mixed with one study (7) indicating that surface modifications decrease cyclic flow stress and another study (8) reporting that surface modifications had no effect on cyclic flow stress.