Effects of austenite grain size on ε martensitic transformation in Fe-15mass%Mn alloy

The effect of austenite (γ) grain size on the morphology of ε martensite (ε) and the transformation from γ to ε has been investigated by means of optical microscopy, transmission electron microscopy and X-ray analysis in Fe-15 mass% Mn alloy, whose γ grain size was controlled between 1 and 130 μm by the reversion treatment of deformation induced bcc martensite to γ. With refining γ grain size, the formation of ε tends to be suppressed and the starting temperature of γ-ε transformation is also lowered. In the grain size range below 30 μm, the transformation is markedly suppressed. In small γ grains below 30 μm, one variant of ε plates go through a γ grain from one grain boundary to the other of the opposite side. When the size of γ grains is sufficiently larger than 30 μm, however, the microstructure changes to a different one in which a lot of ε plates intersect each other within a large γ grain. In this case, ε plates were formed on four {111}_γ planes in the Shoji-Nishiyama relation. The growing process of ε plates in a large γ grain is as follows: When one ε plate grows to a certain critical size, branching takes place at the tip of pre-formed ε plate in order to reduce the elastic coherent stress at γ/ε interface, so that austenite can smoothly transform to ε by the chain reaction through branching of ε. When the γ grain size was not large enough to the critical size of ε plate, the growth of ε should be stopped by grain boundaries before the branching of ε takes place. This leads to the suppressive effect in γ-ε transformation by γ grain refining.