Appearance of concentration oscillations during martensitic alpha - > gamma transformation and control of the thermal expansion coefficient in Fe-Ni invars

We studied structural and concentration changes, which accompany the reverse martensitic alpha - > gamma transformation in metastable austenitic Fe-Ni alloys of the invar type subject to slow heating (0.2-0.4 deg min exp -1 ). The Mossbauer and electron microscopy studies showed that the alpha - > gamma transformation included several stages: (1) at the preliminary stage (523 K to A sub s ) 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 gamma -phase were formed by shear in martensitic crystals; (3) during the third stage (673-723 K) the thin-plate gamma -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 alpha + gamma 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) mult 10 exp -6 deg exp -1 . Alloy studied: Fe-32Ni.