Stress-induced martensitic transformation in metastable austenite grains during nanoindentation investigation

ABSTRACT In general, the formation of martensite in the metastable austenite grains in steels during nanoindentation investigations is believed to be induced by strain as the plastic deformation of austenite takes place prior to the martensitic transformation. However, it is not clear whether the formation of martensite occurs without the prior initiation of plasticity (stress-induced martensitic transformation) during nanoindentation measurement. The present work demonstrates that the martensitic transformation can be triggered during elastic deformation of austenite under an ultralow load when the indenter is close to the annealing twin boundaries. The indentation pressure interacts with the martensite transformation strain, providing the mechanical interaction energy to overcome the nucleation barrier of the martensite embryo. The present work suggests that the annealing twin boundaries can also serve as the nucleation sites of martensite and the stress-induced martensitic transformation is possible during nanoindentation investigation.