Effect of ausforming strain on martensite start temperature

Abstract

Ausforming is generally believed to decrease the martensite start (Ms) temperature. However, the present experiment found that Ms temperature firstly increased at small strain and then decreased at large strain. This interesting finding may be explained by the competing role of dislocation played in martensitic transformation. The austenite grain boundary is the potent nucleation site of martensite. The plastic deformation of polycrystalline material results in the distribution of geometrically necessary dislocation (GND) at the austenite grain boundaries. The large deformation can result in the formation of subgrain boundaries in austenite grain interior. The pile-up of GND at the austenite grain boundary may increase the potency of martensite embryo by facilitating its plastic initiation, resulting in an increase of Ms temperature after small deformation. The formation of subgrains in austenite grain interior inhibits the propagation of the martensite glissile interface, leading to a decrease of Ms temperature after large deformation.