On the mechanism for martensitic transformation from fcc to bcc

The mechanisms for martensitic transformation from fee to bcc, those by Bai n, Kurdjumow and Sachs (K-S), Frank or Bogers and Burgers, are reexamined o n the basis of the comparison of the cohesive energy. The cohesive energy o f the transient lattices formed by those mechanisms is estimated by use of the embedded atom method potential for Fe. In the process of the calculatio n, it has been realized that the shear deformation involved in all of the l atter three mechanisms must be carried out in a reinterpreted sense slightl y different from the strict sense of the word. In an ideal situation, where the formation of the martensite without any constraint from the surroundin g austenite is allowed, the saddle point energy for the latter three mechan isms is equal to the maximum energy for the Bain deformation process betwee n fee and bcc. However, if the constraint that the distance between the clo se-packed planes is kept constant during the martensitic transformation is imposed, only the mechanism by K-S can bring fee lattice to bcc lattice amo ng the latter three. (C) 2001 Elsevier Science B.V. All rights reserved.