STRUCTURE AND PHASE-TRANSFORMATION OF NANO-SCALE PARTICLES OF FE-NI ALLOYS

Atomic structure of nano-scale particles of Fe-rich Fe-Ni alloys and t he phase transformation upon heating have been investigated by means o f high resolution electron microscopy and electron diffraction. Fe-Ni nano-scale particles were prepared by vacuum-depositing master alloys with compositions of Fe-22.1, 25.2 and 28.6 at.% Ni on amorphous carbo n ol alumina films at room temperature. As-deposited specimens were is land-like, and composed of fine grains smaller than 5 nm in size, whic h were supposed to consist of the bce martensite and possibly of an ox ide, NiFe2O4. Upon heating at 773 K, the reflections characteristic of these products disappeared, and a single set of reflect:ions appeared instead, which was identified to be of the fcc austenite. This phase transformation was observed even in specimens of the Fe-22.1 at.% Ni a lloy, of which the austenite start temperature, A(s), is still roughly 50 K higher than the annealing temperature in the bulk state. Concurr ently with this transformation, the initial fine grains coalesced to f orm isolated particles with average sizes of several nm in diameter. T he austenitic particles thus formed were found to be entirely stable e ven when cooled to room temperature. This should also be noted, consid ering that the martensite start temperature, M(s), of the Fe-22.1 at.% Ni alloy is about 230 K higher than room temperature ill the bulk sta te. Then, it is considered that the austenite of the Fe-Ni alloys is s tabilized by more than 200 K, when the size is reduced down to a nanom eter scale. The origin of the remarkable stabilization of austenite re mains unclear at present.