Cooperative strain accommodation over grains in martensitic transformation from Fe-Ni nanocrystalline austenite

This paper reports an investigation of martensitic transformation behaviour from austenite with various grain sizes ranging from 290 nm to 34 m in an Fe-Ni alloy fabricated by electrodeposition and subsequent heat treatment. We confirmed that martensite morphology changed from lath to thin plate with decreasing the austenite grain size. Crystallographic orientation analysis revealed that the variants of thin plate martensite formed in the austenite with relatively coarse grains achieved self-accommodation of the transformation strain inside one austenite grain. In contrast, the transformation strain accompanying martensitic transformation from the ultrafine-grained or nanocrystalline austenite was not accommodated by the martensite variants formed in one austenite grain but accommodated cooperatively by those formed in the several adjacent austenite grains.