Characterization of Substructure Evolution in Ferrous Lenticular Martensite

This study investigated the substructure evolution in lenticular martensite. The substructure of lenticular martensite changes from fine transformation twins in the midrib and twinned region to a high density of dislocations in the untwinned region during growth. On the basis of careful observation of the morphology and substructure of midrib and examination of the stress-induced growth behavior of thin plate martensite, we concluded that the midrib in lenticular martensite is thin plate martensite itself. Tangled and curved dislocations appeared near the martensite-austenite boundary of the untwinned region in Fe-33Ni and in the entire untwinned region in Fe-31Ni, because the martensite inherited the accommodation dislocations in the surrounding austenite. The difference of Ms temperature causes the difference in the substructure between Fe-33Ni and Fe-31Ni. The higher Ms temperature of Fe-31Ni induces the plastic deformation of the surrounding austenite at an earlier stage of transformation, resulting in the appearance of tangled and curved dislocations in the entire untwinned region.