On the nucleation and growth of martensite in small iron particles in copper

Abstract: When thin foil specimens of copper-iron alloys containing metastable fcc - iron are bombarded with argon ions the precipitates are induced to transform martensitically to bcc iron. Using stereo microscopy techniques it was established that (1) transformation is limited to those particles near the surfaces of the specimen, (2) particles on the non-irradiated surface can transform even although the irradiation damage does not extend this deeply. After considering many different reasons for this behaviour it was eventually concluded that the transformation is initiated by elastic surface (Rayleigh) waves set up by the ion impacts. This implies that the transformation in this case is semi-classical rather than heterogeneous in nature. High resolution lattice fringe electron microscopy of bombarded specimens showed that both partially transformed and fully transformed precipitates exist. The fully transformed precipitates have the twin structure already seen with conventional TEM, but the partially transformed precipitates contain very thin laths or plates of martensite parallel to the close-packed planes of the untransformed fcc regions. Complete transformation is thought to be associated with particles that have a substantial amount of free surface, while partial transformation seems to be characteristic of particles that are fully constrained by the copper matrix.