Some new aspects of first-order displacive phase transformations: Martensites

Over recent years new experimental information from X-ray and neutron scattering, electron diffraction and imaging, computer simulation, and first-principles electronic structure calculations have greatly increased the amount of information we have on the microscopic mechanisms of structural transformations. In this paper, a particular class of martensitic transformation which exhibits both “soft modes” (out in the Brillouin zone, q ≠ 0) and elastic instabilities are discussed. The intent is to provide a heuristic review of some background in phonon theory, symmetry constrained elastic distortions and a pedagogical statistical mechanical model which is appropriate to NiAl and FePd and similar alloy transformations.