Effect of austenite grain size on the deformation induced γ→ε martensitic transformation and mechanical properties in an Fe-27 mass%Mn alloy

In an Fe-27 mass%Mn alloy, which has a fully austenitic structure at room temperature and undergoes deformation-induced transformation from austenite (γ) to epsilon martensite (ε), the effects of γ grain size on the deformation-induced ε transformation and mechanical properties have been investigated by means of optical microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffraction analysis. The mean γ grain size was controlled to be between 10 and 100 μm by a recrystallization method. With grain refining, the formation of ε is suppressed and mechanical properties are improved. When the size of γ grains is as large as 100 μm, very thin ε plates form in the early stage of tensile deformation. But when refined to around 10 μm, they do not form during the deformation up to the true strain of 0.1 but only the dislocation density increases. For the formation of deformation induced ε, a lot of partial dislocations have to move on {111}_γ planes closely. In the alloy with fine γ grains, however, the number of dislocations which can be piled up on each {111}_γ plane is decreased by grain-refining, so that a secondary slip system acts prior to the formation of ε plates. This is the reason why the deformation induced ε transformation is suppressed by grain-refining. Moreover, in the alloy with large γ grains, a significant stress concentration takes place at grain boundaries on which deformation induced ε plates impinge, and this results in the onset of a quasi-cleavage fracture attributed in grain boundary exfoliation. Austenite grain refining gives the favorable effect as follows: 1) reducing the stress concentration at grain boundaries, 2) uniform dispersion of deformation strain, thus this quasi-cleavage fracture is completely suppressed by the grain-refining to 10 μm.