The Morphology of Strain-Induced Martensite and Thermally Transformed Martensite in Fe–Ni–C Alloys

Abstract

The relation between formation temperature and morphology of strain-induced and thermally transformed martensites in various Fe–Ni–C alloys has been studied by means of optical and transmission electron microscopy. The morphology of the strain-induced martensite varied with the formation temperature (i.e., deformation temperature below the Md) even in the same alloy. Three types of strain-induced martensites were recognized in the temperature range between 0 and −196°C, namely, a butterfly-like dislocated martensite was formed at temperatures between 0 and −60°C, a lenticular partially or completely twinned martensite with mid-rib at −20∼−150°C, and a thin plate-like completely twinned martensite without mid-rib below −150°C. The Ms temperature was remarkably depressed in these alloys with decrease in austenitizing temperature. Therefore, the relation between formation temperature and morphology of thermally transformed martensite was clearly determined in the same alloy by utilizing this phenomenon (thermal stabilization of austenite). The morphology of thermally transformed martensite also varied with the formation temperature even in the same alloy. Three types of the martensite were also observed in the same morphologies, and they were similarly formed in the same temperature ranges as the strain-induced martensites, respectively. The main factor determining the morphology of martensite (both the strain-induced and thermally transformed martensites) in Fe–Ni–C alloys was considered to be the formation temperature.