An ultrafine grain structure formed as a result of cyclic γ→α→γ transformations

Abstract

A considerable refinement of grains (by several orders of magnitude) can be realized if the structural inheritance, which causes restoration of the initial austenite grain, is eliminated in the γ→α→γ cycle. After the direct and reverse martensitic transformations every initial γ grain will have 24 2 = 576 orientations of the γ phase. This situation was realized in metastable austenitic Fe-Ni alloys subject to the direct martensitic transformation during cooling in liquid nitrogen and the reverse α→γ transformation during a very slow heating (0.2–2 deg./min.) up to 773–873 K. Given these conditions of heating, a nickel-depleted “buffer” layer is formed in accordance with the Fe-Ni equilibrium diagram at the boundaries of a martensite crystals. The layer prevents nucleation of the “restored” γ phase at the boundary with the retained (initial) austenite. The strength properties are improved. The disperse γ phase can be enriched with nickel and the thermal expansion coefficient can be varied over a broad interval.