Effect of nitrogen on generalized stacking fault energy and stacking fault widths in high nitrogen steels

We use a generalized Peierls-Nabarro model fitted to generalized stacking fault energies (GSFE) calculated from ab initio density functional theory to study the effect of interstitial nitrogen content on stacking faults (SF) in {111} plane of face-centered cubic (fcc) Fe-N alloys. These simplified systems are reliable representatives of fcc Fe-Mn-N steels, for example, as Mn acts to stabilize fee relative to body-center phase but hardly affects the GSFE. Here we study the dependence of stable SF widths of Fe-N alloys on GSFE versus percent nitrogen. In contrast to the classical Volterra solution, in which the SF width depends only on intrinsic SFE value, our results reveal a strong dependence of SF widths on intrinsic, unstable and maximally unstable SFE values. The model predicts finite SF widths for negative intrinsic SFE values, a result which cannot be explained by the Volterra model, but arises due to the finite (and positive) unstable SFE barrier that must be traversed even if the intrinsic SFE is negative, i.e. stable. This result has critical importance for the observed SF formation and maximum widths. Namely, the stacking fault width is found to depend non-monotonically on the percentage of nitrogen with a maximum critical shear stress at 1 wt.% (4 at.%) N, in agreement with experiment, and also suggests a non-monotonic critical shear stress dependence on nitrogen content. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.