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 the phase transformation upon heating have been investigated by means of 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 carbon or alumina films at room temperature. As-deposited specimens were island-like, and composed of fine grains smaller than 5 nm in size, which were supposed to consist of the bcc martensite and possibly of an oxide, NiFe2O4. Upon heating at 773 K, the reflections characteristic of these products disappeared, and a single set of reflections 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 alloy, of which the austenite start temperature, AS, is still roughly 50 K higher than the annealing temperature in the bulk state. Concurrently with this transformation, the initial fine grains coalesced to form isolated particles with average sizes of several nm in diameter. The austenitic particles thus formed were found to be entirely stable even when cooled to room temperature. This should also be noted, considering that the martensite start temperature, MS, of the Fe-22.1 at.%Ni alloy is about 230 K higher than room temperature in the bulk state. Then, it is considered that the austenite of the Fe-Ni alloys is stabilized by more than 200 K, when the size is reduced down to a nanometer scale. The origin of the remarkable stabilization of austenite remains unclear at present.