The structure of burst and isothermal martensites in an Fe-24 wt% Ni-0.5 wt%C alloy

Abstract

It has been found that an Fe-24 wt% Ni-0.5wt% C alloy transforms to martensite on cooling to −80° C by two separate processes to give burst and isothermal plates. Previous work has only indicated burst transformation at −196° C. The burst plates have a characteristic lenticular shape with midribs and internal twinning whereas the isothermal plates are chevron-shaped, each having two arms meeting at an obtuse angle on a common crystallographic plane. The isothermal plates have dislocation as their substructure with no evidence of twinning or midribs. Trace analysis shows that the isothermal martensite has a {2 5 9}γ habit plane which is the same as that of burst martnesite as found by earlier workers. This suggests that the habit plane is controlled by composition rather than by the substructure of martensite or kinetics of the process.