The authors have previously found that the morphology of martensite in Fe–Ni-0.25%C alloys changes at about −150°C from a lenticular to a thin plate. In this investigation, the relation between the transition temperature from the lenticular to the thin plate and the carbon content of Fe–Ni–C alloys was examined by adjusting the carbon content up to 0.9% and the nickel content from 25 to 35%. Furthermore, some features in the progress of transformation to the thin plate martensite from austenite were observed as compared with those of lenticular martensite. The main results obtained are as follows.
(1) The transition temperature at which the morphology of martensite in Fe–Ni–C alloys changes from the lenticular to the thin plate rises with increase in carbon content. For example, in carbon-free Fe–Ni alloys, the transition temperature is lower than −196°C, and in Fe–Ni–C alloy containing about 0.8%C, the transition temperature is about −100°C.
(2) The following features in the progress of transformation to the thin plate martensite were observed. (a) During the subsequent cooling, the martensite plate itself thickens in addition to the formation of a new martensite plate in the other part of austenite. (b) The burst phenomenon is not observed. (c) Accommodation slip caused by martensitic transformation is hardly observed in the austenite adjacent to the martensite plate. These phenomena are entirely different from the usual lenticular.