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

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 273; pp. 453 - 456
• Main Authors: Goshchitskii, B.N, Sagaradze, V.V, Mukoseev, A.G, Shabashov, V.A
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.12.1999
• Subjects: Fe–Ni; Invar; Martensitic transformation; Mössbauer spectroscopy; Transmission electron microscope; Martensitic transformation; Invar; Mössbauer spectroscopy; Transmission electron microscope; Fe–Ni
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/S0921-5093(99)00326-3

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 A 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 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.