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 revers e martensitic alpha-->gamma transformation in metastable austenitic Fe-Ni a lloys of the invar type subject to slow heating (0.2-0.4 degrees min(-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(s)) nickel was redistributed between martensite and the retained aus tenite; (2) at the second stage (673-723 K) dispersed plates (10-50 nm thic k) of the gamma-phase were formed by shear in martensitic crystals; (3) dur ing 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-N i equilibrium diagram); (4) at temperatures higher than 723 K (the fourth s tage) globular austenite was formed. The globular austenite inherited the c oncentration inhomogeneity of the alpha + gamma mixture as nickel concentra tion oscillations. Varying the degree of the nickel redistribution in auste nite by special thermal treatment, it was possible to control the thermal e xpansion coefficient in a broad range: (2-10) x 10(-6) deg(-1). (C) 1999 El sevier Science S.A. All rights reserved.