The effect of ion beam surface modifications on fatigue crack initiation in polycrystalline nickel

Abstract

Flexural fatigue tests at surface strain amplitudes of +/-0.0012 were conducted on polycrystalline nickel to determine the effect of ion beam surface modifications of the Ni/Al system on fatigue crack initiation. An Ni-75at.%Al surface region 1000 A thick was produced by evaporating alternate layers of nickel and aluminum followed by ion beam mixing with 3 MeV Ni2+ ions to a fluence of 1 x 1016 ions cm-2. A two-phase [gamma]-[gamma]' surface structure was produced through implantation of 350 keV Al+ ions to a fluence of 5 x 1017 ions cm-2 into the nickel specimens at 723 K. Nickel specimens that were self-implanted with 350 keV Ni+ ions to a fluence of 1 x 1016 ions cm-2 were also analyzed.The addition of aluminum to the surface resulted in an increase in surface hardness and suppression of surface slip band features. The nickel self-implantation increased surface hardness but had very little effect on surface slip feature suppression. Transgranular crack initiation was seen in unmodified and self-implanted specimens, and intergranular crack initiation was prevalent in aluminum micro-alloyed specimens.