Transmission electron microscopy studies of thin foil martensite in Fe-Ni and Fe-Ni-C alloys

Abstract

To clarify certain characteristics of the martensite transformation in thin foils, both foil and bulk martensites were studied by transmission electron microscopy of Fe-Ni-(C) alloys with subzero M s temperatures. Thin-foil martensite often exhibits a gradual growth during cooling of the specimen and the growing martensite stops short of the foil edge, leaving a rim of retained austenite. The M-A interface is not governed by a particular invariant crystallographic plane (habit plane). Unlike bulk martensite. foil martensite does not contain midribs. As with bulk martensite. (112) transformation twins and dislocations are observed in the foil martensite. The transformation twins are more segmented in foil martensite than in bulk martensite. Furthermore, in thin foil martensite. the distribution of transformation twins and the twin density are variable, differing from martensite to martensite. Therefore, the “typical” morphology and substructure of thin-foil martensite are rather difficult to characterize. The orientation relationship between austenite and martensite in foils is essentially the K-S relationship. No unusual orientation relationships peculiar to thin foils were found.