FATIGUE OF THO2 DISPERSION STRENGTHENED NICKEL-ALLOY WITH SURFACE MODIFICATION BY AL+ ION-IMPLANTATION

Surface modification by ion implantation improves fatigue lives in systems which are similar in chemistry but which differ with respect to the rate of cyclic hardening, the homogeneity of cyclic slip, and the saturation stress level reached during low cycle fatigue. In pure nickel, the initiation process is surface sensitive, and the effects of the ion beam modified surface layer can be rationalized in terms of the inhibition of the penetration of PSBs to the free surface. Crack initiation in DS-nickel, may apparently proceed in the absence of apparent surface damage accumulation, though PSB involvement is also sometimes observed. PSB emergence at the surface is inhibited by the ion beam modified surface film on DS-nickel sheet specimens cycled at plastic strain ranges up to .001. Though this effect may account in part for the observed increase in fatigue life for implanted DS-nickel, it is also possible that the presence of the hardened surface film retards the growth of subsurface nucleated cracks during the early stages of fatigue.