Characterization of Surface Mechanical Properties and Residual Stresses in Ion Implanted Nickel
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CHARACTERIZATION OF SURFACE MECHANICAL PROPERTIES AND RESIDUAL STRESSES IN ION IMPLANTED NICKEL DJ. MORRISON,* J.W. JONES,* G.S. WAS,** A. MASHAYEKHI,** AND D.W. HOFFMAN*** * Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, MI 48109 ** Department of Nuclear Engineering, The University of Michigan, Ann Arbor, MI 48109 *** Research Staff, Ford Motor Company, Dearborn, MI 48124 ABSTRACT In this study, we have measured changes in hardness and residual stress state in polycrystalline nickel samples which have undergone a variety of ion beam surface treatments including self-implantation, amorphization by ion beam mixing Ni-Al multilayers, and the formation of a two-phase microstructure by elevated temperature aluminum implantation. Hardness-depth profiles of the the near-surface regions were determined using an ultra-low load microindentation system, and residual stress states induced by the surface modifications were analyzed using optical interferometry. These property changes were correlated with the accumulation of surface fatigue damage. INTRODUCTION Ion implantation techniques are frequently used to produce microstructures that result in surface properties that are significantly different from the substrate. Nastasi et al.[l] found increases in the surface hardness of nickel specimens implanted with carbon; and Oliver, McHargue, and Zinkle [2] reported surface hardening in self-implanted copper. In addition to producing changes in surface hardness, ion implantation can induce residual stresses in the surface region [3]. Previous research [4] has shown that the formation of persistent slip band (PSB) features at the surface was significantly inhibited by using ion beam processes to microalloy the surface of nickel specimens with aluminum. In this study, the effects of several Ni-Al system ion beam modifications on near-surface hardness and residual stress were analyzed
and compared to the ability of the ion beam modified region to suppress surface fatigue damage in polycrystalline nickel. Previous studies on the effects of thin surface layers on fatigue damage accumulation primarily addressed elastic modulus [5] and stacking fault energy [6] effects. EXPERIMENTAL PROCEDURES Experiments were conducted on three types of nickel specimens. Specimens used for the ultra-low load microindentation tests were cut from a Ni-270 (99.98% Ni) rod. These specimens were annealed in a flowing argon-3% hydrogen atmosphere at 500°C for three hours resulting in a grain size (average linear intercept) of 24 gm. After annealing, the disks were electropolished in a 20% perchloric acid and ethanol solution at -50C. Cantilever beam fatigue specimens (design similar to ASTM D-671) were fabricated from Ni-200 (99.0% Ni) sheet. These specimens were annealed in the argon-hydrogen atmosphere at 800'C for four hours which produced a grain size of 70 gIm. Following heat treating, the fatigue specimens were electropolished in the same manner as previously discussed. Residual stress specimens were made from
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