Structural mechanism of reverse α → γ transformation and strengthening of Fe-Ni alloys

Fe-32% Ni alloy subjected to slow heating to a temperature below As at a rate of 0.01 K/min demonstrates the untwinning and appearance of an intermediate ɛ phase with an hcp lattice and lattice parameters a = 2.535, c = 4.132 A, and c/a = 1.63. Slow heating to 430–490°C leads to the formation of nanocrystalline austenite enriched in nickel, which substantially increases the hardness of martensite. The formation of austenite in the Fe-32% Ni alloy, which is a mixture of martensite with 20–30% nanocrystalline austenite, during its rapid heating to 600°C occurs via the bulk mechanism with short-range atomic diffusion. In this case, the diffusion does not eliminate the concentration micro-inhomogeneity of the alloy in nickel but leads to the reorientation of γ-phase nanocrystals, almost eliminates the dislocation structure, and removes the strengthening by phase hardening.