On the mechanism for martensitic transformation from fcc to bcc

The mechanisms for martensitic transformation from fcc to bcc, those by Bain, Kurdjumow and Sachs (K–S), Frank or Bogers and Burgers, are reexamined on the basis of the comparison of the cohesive energy. The cohesive energy of the transient lattices formed by those mechanisms is estimated by use of the embedded atom method potential for Fe. In the process of the calculation, it has been realized that the shear deformation involved in all of the latter three mechanisms must be carried out in a reinterpreted sense slightly different from the strict sense of the word. In an ideal situation, where the formation of the martensite without any constraint from the surrounding austenite is allowed, the saddle point energy for the latter three mechanisms is equal to the maximum energy for the Bain deformation process between fcc and bcc. However, if the constraint that the distance between the close-packed planes is kept constant during the martensitic transformation is imposed, only the mechanism by K–S can bring fcc lattice to bcc lattice among the latter three.