Martensitic Transformation in Fe-29 wt%Ni Alloy Ultrafine Particles

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Abstract
The ultrafine-particle samples of Fe-29 wt% Ni alloy with different average particle sizes (12 to 100 nm) are prepared by the evaporation condensation method in N2 atmosphere. They exhibit a mixture of martensite α′ (bcc) and austenite γ (fcc) phases. Each individual particle is a single crystal (bcc or fcc). The volume fraction of martensite increases markedly with the decrease of the average particle size. During heating, as the average particle size decreases, the temperature (A s and A f) for α′→γ reverse martensitic transformation decreases, and the range (A s-A f) of reverse transformation temperature becomes narrower, whereas during cooling (down to 77 K), we have not observed γ→α′ martensitic transformation. The stabilization of austenite and the growth mechanism of martensite and austenite are preliminarily discussed in this paper.