Effects of Nitrogen and Helium Ion Implantation on Uniaxial Tensile Properties of 316 SS Foils
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EFFECTS OF NITROGEN AND HELIUM ION IfiPLANTATION ON UNIAXIAL TENSILE PROPERTIES OF 316 SS FOILS J. A. SPITZNAGEL,* B. 0. HALL,* N. J. DOYLE,* RAMAN JAYRAM,** R. W. WALLACE,** J. R. TOWNSEND,** AND M. MILLER*** *Westinghouse R&D Center, Pittsburgh, PA 15235; ** University of Pittsburgh, Pittsburgh, PA 15260; *** Oak Ridge National Laboratory, Oak Ridge, TN 37830 ABSTRACT Implantation of nitrogen into steels is known to affect surface sensitive mechanical properties. Tensile properties of thin foils implanted with either nitrogen or helium at 300 K hrve been measured. Fluences greater than 1 x 1016 ions/cm raise the yield stress and fracture stress and reduce the plastic strain to failure. Both nitrogen and helium give comparable stress-strain responses fkr equal average concentrations of implanted ions. The mechanical response is discussed in terms of plastic flow of laminated structures and hardening mechanisms. Initial results of atom probe field ion microscopy examinations of nitrogen implanted Fe-15 wt.% Cr-12 wt.% Ni alloy are described. INTRODUCTION Implantation of nitrogen into steels often results in altered mechanical and electrochemical properties. The effects have been attributed to the formation of various nitride phases, implantation induced residual stresses and decoration of mobile dislocations by the implanted solute atoms [1]. Auger spectroscopy and T.E.M. studies [2,31, for example, have indicated that CrN may precipitate in 304 and 316 type austenitic stainless steels during implantation at -300 K when implanted concentrations exceed %2-8 atomic percent. Cantilever beam deflection experiments, however, suggest that large residual stresses approaching the yield stress may be introduced at much lower fluences [4,5]. Theoretical treatments of surface hardening or softening presuppose a knowledge of the effective volume change per implanted ion, which determines the magnitude of the residual stresses and of the nature and distribution of barriers to dislocation motion, e.g., precipitate particles, small dislocation loops, etc.[6]. Such information has been very difficult to obtain experimentally because the microstructural changes occur on a very fine scale for room temperature implants. In this paper we examine the feasibility of using simple uniaxial tension tests and atom probe field ion microscopy to deduce the origin(s) of mechanical property changes arising from implantation of a chemically active species (nitrogen) oran inert gas ion (helium) in polycrystalline 316 SS. EXPERIMENTAL Thin foils of polycrystalline 316 SS, five microns thick, with grain sizes larger than the foil thickness and initial nitrogen concentrations "%1600 appm, were prepared by cold rolling bulk strip with intermediate anneals. This same material had been used in a previous extensive study of the stress-strain behavior of unimplanted micron thickness foils [7]. As in the earlier study, tensile specimens were stamped from the foil in hard Supported in part by NSF grant DMR-81-02968. Mat. Res. Soc. Symp.
Proc. Vol.
27 (19
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