Transmission electron microscopy and electron diffraction study on structure and phase transformation of nanometre-sized Fe-15-30 at.% Ni alloy particles

Structure and phase transformations of nanometre-sized particles of Fe-Ni a lloys in the composition range from 15 to 30 at.% Ni were studied by means of transmission electron microscopy and electron diffraction. The nanometre -sized particles were prepared by vacuum evaporation of master alloys onto amorphous alumina films at room temperature. The deposited films of all the alloys were composed of islands of a few nm sizes with amorphous-like stru ctures. The islands were changed upon heating at around 673 It into crystal line particles with several nanometre diameter sizes consisting of a mixtur e of fee and bce phases or an fee single phase, depending on composition of the master alloys. When cooled to room temperature after austenitization a t higher temperatures, the particles of the alloy with 30 at.% Ni remained fee, and even those of alloys with compositions less than 25 at.% Ni only p artly underwent the martensitic transformation. The austenite and martensit e particles exhibited polyhedral shapes with facets. The stabilization of a ustenite in the nanometre-sized particles was discussed in terms of the sur face free energy.