Interphase boundary structures of lath martensite and bainite in steel

Interphase boundary structures of lath martensite and bainite formed in austenite were examined by means of conventional and high-resolution transmission electron microscopy. Orientation relationships of the isothermal lath martensite in Fe-20Ni-5.5Mn and of the bainitic ferrite in Fe-2Si-1Mn-0.6C are both slightly scattered in the vicinity of K-S, N and G-T relationships with respect to austenite. The broad face of martensite lath contains transformation dislocations (monoatomic steps with the (111)(fcc)//(011)(bcc) terrace). Two kinds of accommodation dislocations exist alternately on every three layers of the parallel close packed planes, in a glissile arrangement. Such dislocations can fully accommodate the shear component of transformation strain. It is found that the atomic structure of bainitic ferrite/austenite interphase boundary is similar to that of lath martensite.