Martensitic transformation in ultrafine FeNi powders

Abstract

The austenite ↔ martensite transformation in ultrafine FeNi powders (25–35 wt.% Ni; particle size, about 10–200 nm) was investigated by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and differential scanning calorimetry. The ultrafine powders were prepared by levitation melting in liquid nitrogen. The as-prepared ultrafine powders were a mixture of martensite (b.c.c. α′) and austenite (f.c.c. γ). Each individual particle was a single crystal of either b.c.c. martensite or f.c.c. austenite. In contrast, bulk alloys of the same composition quenched into liquid nitrogen were found to be entirely martensitic. Austenite stabilization of some particles is probably related to the fabrication process and to the small size of the particles. It is suggested that austenite and martensite particles might be formed directly from the metallic vapours in the cold parts of the calefaction layer. On further heating, the reverse transformation of martensite to austenite occurs in the same temperature range for the ultrafine powders and for the bulk alloys. The martensite → austenite transformation was found to obey a shear mechanism.