The Morphology of Thin Plate-Like Martensite in Fe–Ni–C Alloys

Abstract

The authors have previously found that a thin plate-like martensite which is entirely different from lenticular one is formed at temperatures below about −150°C in Fe-Ni-0.25%C alloys. In this paper, the morphology of thin plate-like martensites (both thermally transformed and strain-induced martensites) which were formed at liquid nitrogen temperature in Fe-31%Ni-0.23%C (Ms = −190°C, Fe-31%Ni-0.28%C (Ms = −171°C and Fe-33.5%Ni-0.22%C (Ms < −269°C was precisely observed. These martensites are usually very thin and have a smooth straight boundary with austenite. Transformation twins run completely through the martensite plate and no mid-rib is observed. The habit plane of this martensite is near {259}γ. This plate-like martensite shows a peculiar nature in morphology which can hardly be observed in the usual lenticular martensite. The thin plate-like martensite can cross each other and make kinking or branching. The martensite plate is considered to be thickened by a combination of many parallel thin plate-like martensites with same variant. Consequently, the thick martensite contains many parallel faint boundaries which are boundaries of the original thin plate-like martensite. Furthermore, it seems that this martensite has the nature capable of gradually growing sidewise.