Martensitic transformation in nanometer-sized particles of Fe-Ni alloys

We studied the martensitic transformation in nanometer(nm)-sized particles of Fe-Ni alloys with the composition range from 15 to 30 at.% Ni by means of transmission electron microscopy and electron diffraction. Here are reported results obtained mainly for nm-sized particles of an Fe-25.0 at.% Ni alloy. The forward transformation temperatures were depressed to a fair extent, compared to those in bulk, while the reverse transformation temperatures were less affected by size. It appeared that both the austenite and martensite particles were faceted by certain crystallographic planes. The temperature parameter in the Debye-Waller factor measured for the austenite particles was found to be about one order larger than those for Fe and Ni in bulk. These results suggested that the observed size-dependence of transformation temperatures is accounted for in terms of the surface free energy difference between the fee and bcc nm-sized particles. (C) 1999 Elsevier Science S.A. All rights reserved.