On the abnormally large tetragonality of martensite in FeNiC alloys

Abstract

In order to elucidate the origin of abnormally large tetragonality ( c/ a) of martensite in the FeNiC alloy system, a large number of alloys of which the Ni content is varied for a constant carbon content have been examined, using low temperature X-ray diffraction, optical and electron microscopy. The results show that the tetragonality of martensite in this alloy system is strongly dependent on martensite morphology. Namely, it is very large for a plate martensite, while it is normal or not so large for a lenticular martensite. In more physical words, the martensite tetragonality is dependent on the mode of the lattice invariant deformation in martensitic transformation. If the lattice invariant deformation is twinning, the resulting c/ a is large, while in the case of slip it is small.