Influence of cyclic deformation on surface microstructure and hardness of ion-implanted nickel
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I.
INTRODUCTION
R E C E N T studies 11] have shown that ion implantation can have a pronounced effect on the accumulation of surface fatigue damage. The implantation process produces a thin surface region of lattice defects that consist of interstitials and vacancies which may coalesce and form complex dislocation loop structures, t21 These implantationinduced defects can significantly increase the strength of the implanted region. If the implanted specie is different from the substrate, a "microalloyed" surface region is formed, and solid solution or precipitate strengthening may occur, t~J In addition, ion implantation can cause changes in the stacking fault energy I31 and residual stress state tal of the near-surface region. One of the primary ways in which an ion-implanted surface affects the fatigue response of a material is by inhibiting the emergence of persistent slip bands (PSBs) at the surface. ~3'5-121 This phenomenon has been attributed to increased strain homogeneity and reversibility caused by a reduction of the stacking fault energy, 111,12j the introduction of surface compressive residual stresses, [6,71 and a reduction of surface plastic strain caused by an increased surface yield strength. I3,7'81 Since PSB intersections with the surface have been shown to be preferential sites for the initiation of fatigue cracks, 113-16~the suppression of surface PSB features generally causes an increase in fatigue life. However, some cases of reduced fatigue lives in ion-implanted specimens have been reported. [7.11,~7.18]
D.J. MORRISON, formerly with The University of Michigan-Ann Arbor, is Assistant Professor, Department of Mechanical and Aeronautical Engineering, Clarkson University, Potsdam, NY 13699. D.E. ALEXANDER, formerly with The University of Michigan-Ann Arbor, is Postdoctoral Fellow, Materials Science Division, Argonne National Laboratory, Argonne, IL 60439. J.W. JONES, Associate Professor, Department of Materials Science and Engineering, and G.S. WAS, Professor, Department of Nuclear Engineering, are with The University of Michigan-Ann Arbor, Ann Arbor, MI 48109. Manuscript submitted May 21, 1990. METALLURGICAL TRANSACTIONS A
While numerous studies have addressed the effects of ion implantation on lattice defect structure and surface hardening, very little work has been accomplished on the effects of cyclic deformation on these implantationinduced properties. The ability of an ion-implanted surface to suppress the emergence of PSBs at the surface will depend on the cyclic stability of the implantationinduced properties. Therefore, the objectives of the present work were to determine the effects of cyclic deformation on microstructural and mechanical properties produced by ion implantation and to relate these property changes to observations of the evolution of surface fatigue damage. In order to separate the effects of implantation-induced lattice defects from the effects of ion microalloying, two types of ion-implanted specimens were examined. The effects of implantation-induced lattice defects
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