Electron backscatter diffraction (EBSD) is a powerful technique for microstructural analysis, capable of discriminating crystal phases and orientations. Many useful insights can be derived from EBSD data, in particular the nature of grain structures, orientation relationships, ion channelling fractions, and dislocation distributions. In this study, the technique was applied to explore the evolution of the austenite to ferrite/martensite phase change of AISI-304 stainless steel resulting from exposure to gallium ions. While this phase change has been studied before, the volume of published literature is relatively small, lacking consensus, and typically confined to the examination of certain special cases, such as crystal grains oriented with low-order zone axes parallel to the beam.

A method of reliable and rapid reimaging of regions of interest on the material surface was developed in order to image them before and after ion exposure, based on marking surfaces with trenches milled by focused ion beam followed by repolishing to restore the surface to a pristine state, enabling collection and analysis of data on the full range of austenite grain orientations. Selected regions were exposed to gallium ions multiple times to study the effects of orientation on phase change evolution, and imaged by EBSD before and after successive exposures, with the data aggregated to derive insights into the phase change process.

It was found that the phase change proceeds according to two distinct mechanisms, one diffusive and one displacive. The propensity and mechanism of phase change were observed to have a strong dependence on orientation, well correlated with predictions made by current Channelling Theory. At the cusp of phase change, observed to occur around a critical ion dose of roughly 3000 µC cm⁻² for a beam energy of 30 keV, austenite grains with orientations lying close to the [0 0 1] and [1 0 1] crystal axes were observed either to be amorphised or remain untransformed, those close to the [1 1 1] axis were observed to transform via the diffusive mechanism, and those not lying close to any of the three principal axes were observed to transform via the displacive mechanism.