The structure and properties of Fe–Ni alloys with a nanocrystalline austenite formed under different conditions of γ–α–γ transformations

It is shown that direct and reverse martensitic transformations in Fe–Ni-based metastable austenitic alloys during fast cooling and heating lead to the preferable recovery of the orientation of the initial austenite. The γ→α→γ transformation considerably increases the density of dislocations in the austenite (up to ∼5×1010 cm−2) thanks to a partial inheritance of dislocations from the α-phase, generation of new dislocations during the α→γ transformation, and the formation of a plate subgrain structure. Isothermal holding in the two-phase α+γ region or a slow heating (0.2–0.4 K min−1) of Fe–Ni-based alloys during the α→γ transformation provides the multiplication of γ-orientations and forms nanocrystalline plate γ phase ∼10–50 nm thick. A nanocrystalline austenite formed during a partial α→γ transformation increases considerably the strength of the martensite, enhances the coercive force and improves the square shape of the hysteresis loop of maraging steels. The thermal expansion coefficient of the austenitic alloy type 32Ni can be adjusted over broad limits (from 2.5 to 10×10−6 K−1) thanks to different α→γ transformation conditions.