Abstract: A new “mid-relief” plate martensite was observed together with lenticular plate and thin plate martensites in Fe-29.37Ni-0.34C alloy after twice stepped cooling:-100/-120℃, -120/-140℃,-155/-165℃ and -165/-196℃,by means of colour metallography and TEM.The morphology of martensites changes with decreasing M_s temperature.The “mid-relief” plate martensite is characterized by the intensive mid-relie and the substructures of symmetric double twins or half twins and half dislocations,the formation of this martensite can be interpreted by a stress-induced co-generated mechanism.

Key words: martensite, morphology, substructure