Appearance of concentration oscillations during martensitic α→γ transformation and control of the thermal expansion coefficient in Fe–Ni invars

We studied structural and concentration changes, which accompany the reverse martensitic α→γ transformation in metastable austenitic Fe–Ni alloys of the invar type subject to slow heating (0.2–0.4° min−1). The Mössbauer and electron microscopy studies showed that the α→γ transformation included several stages: (1) at the preliminary stage (523 K to As) nickel was redistributed between martensite and the retained austenite; (2) at the second stage (673–723 K) dispersed plates (10–50 nm thick) of the γ-phase were formed by shear in martensitic crystals; (3) during the third stage (673–723 K) the thin-plate γ-phase was enriched in nickel and depleted in the retained martensite (in accordance with the Fe–Ni equilibrium diagram); (4) at temperatures higher than 723 K (the fourth stage) globular austenite was formed. The globular austenite inherited the concentration inhomogeneity of the α+γ mixture as nickel concentration oscillations. Varying the degree of the nickel redistribution in austenite by special thermal treatment, it was possible to control the thermal expansion coefficient in a broad range: (2–10)×10−6 deg−1.