The Martensitic Transformation in Small Pure Iron Particles

When Thin foil Specimens of Copper-iron alloys Containing metastable fcc r-iron are bombarded with argon ions the precipitates are induced to transform martensitically to bcc iron. Using stereo microscopy technigues 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 when the irradiation damage does not extend so far. The transformation is therefore thought to be initiated by elastic Surface (Rayleigh) waves Set up by the ion impacts.
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 Conventiond 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 partides that have a substantial amount of free Surface, while Partial transformation is probably Characteristic of particles that are fully constrained by the copper matrix.