Substructures of lenticular martensites with different martensite start temperatures in ferrous alloys

This study investigated the substructures of lenticular martensites with different martensite start temperatures (Ms) by transmission electron microscopy. Observation of Fe–33Ni revealed a substructural change from fine transformation twins in the midrib and twinned region to several sets of screw dislocations in the untwinned region during growth. Tangled and curved dislocations also appeared near the martensite–austenite interface of the untwinned region, as the martensite inherited the dislocations in the surrounding austenite. In contrast, curved and tangled dislocations appeared in the entire untwinned region in Fe–31Ni and in the whole martensite plate in Fe–20.5Ni–35Co, as the higher Ms temperatures facilitated the plastic deformation of the surrounding austenite. Thermally transformed thin plate martensite in Fe–31Ni–10Co–3Ti grew into a lenticular shape accompanied by a substructure with dislocations after deformation at temperatures above the Ms temperature. The change in the substructure of lenticular martensite presumably resulted from the local temperature rise in the martensite plate.