Massive transformation in the Fe–Ni system

Abstract The critical limit for the massive γ → α transformation in the Fe–Ni system has been measured by isothermal heat treatment of diffusion couples. The position of the α / γ interface at the end of the treatment could be identified but some growth occurred during the quench. Growth is probably hindered at the beginning of the quench by a redistribution of Fe and Ni at the interface during the isothermal treatment. During the quench it often happens that the α / γ interfaces develop jagged shapes and even very fine plates although there should usually be no orientation relationship. The critical limit agrees fairly well with classical results for the formation of equiaxed ferrite from continuous cooling of homogeneous specimens. At 1023 K it coincides with the α / α + γ phase boundary. At lower temperatures it moves inside the α + γ two-phase field. The driving force for the massive transformation increases at decreasing temperature and the limiting composition never approaches the T 0 line.