The Influence of Both Major and Minor Element Composition on Void Swelling in Austenitic Steels

Data on the neutron-induced swelling of simple Fe-Ni-Cr austenitic alloys are presented to show that there are three compositional regimes of swelling as a function of nickel content. The intermediate transition regime can be shifted to higher or lower nickel levels as a function of chromium content, various solutes, displacement rate, temperature, and helium level. Such shifts can preclude the observation of either the lower or upper nickel regime. The lower nickel regime is also obscured in ion bombardment studies because of factors that are atypical of the neutron environment. Data are also presented to show the influence of silicon and phosphorus on the neutron-induced swelling of Fe-25Ni-15Cr and to demonstrate that the separate influence of each of these elements is more complex than previously observed. Similar complex trends have also been seen in ion bombardment experiments, but there are additional distortions relative to the neutron-induced behavior that arise from the interaction of these elements with point defect gradients. A model is presented to show that the major effect of composition on swelling lies in its influence on void nucleation, operating primarily on the intermediate transition regime. Compositional variations thus appear as a temperature shift analogous to that associated with changes in displacement rate. This shift is due to the effect of each element on the vacancy supersaturation via their effect on either the equilibrium vacancy concentration or the effective vacancy diffusion coefficient