Nature of gallium focused ion beam induced phase transformation in 316L austenitic stainless steel

The microstructural evolution and chemistry of the ferrite phase (alpha), which transforms from the parent austenite phase (gamma) of 316L stainless steel during gallium (Ga) ion beam implantation in Focused Ion Beam (FIB) instrument was systematically studied as a function of Ga+ ion dose and,gamma grain orientations. The propensity for initiation of a phase transformation was observed to be strongly dependent on the orientation of the gamma grain with respect to the ion beam direction and correlates well with the ion channelling differences in the gamma orientations studied. Several a variants formed within a single,gamma orientation and the sputtering rate of the material, after the gamma -> alpha transformation, is governed by the orientation of a variants. With increased ion dose, there is an evolution of orientation of the a variants towards a variant of higher Ga+ channelling. Unique topographical features were observed within each specific gamma orientation that can be attributed to the orientation of defects formed during the ion implantation. In most cases, gamma and alpha were related by either Kurdjumov-Sachs (KS) or Nishiyama-Wassermann (NW) orientation relationship (OR) while in few, no known OR's were identified. While our results are consistent with gallium enrichment being the cause for the gamma -> alpha phase transformation, some observations also suggest that the strain associated with the presence of gallium atoms in the lattice has a far field stress effect that promotes the phase transformation ahead of gallium penetration. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. This is an open access article under the CC BY license.