Pseudospin-phonon coupling model for martensitic transformation in bcc-based alloys

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Abstract
We propose a new prescription to describe a general mechanism of the martensitic transformation in bcc-based alloys. We define a ‘‘three-state spin’’ variable which specifies local creation and annihilation of the ‘‘pseudospin’’ of the low-temperature phase and take into account the coupling of the spins to the lateral displacements of pseudospins thereby greatly reducing the degrees of freedom of the whole system. We obtain an effective Hamiltonian of the system given in terms of the pseudospin-creation energy and the pseudospin-pseudospin interaction energy via phonons. The phase-transition scheme is investigated based upon the model Hamiltonian. Overall characteristics of martensitic transformations including the premartensitic phenomena are successfully explained. Applications to the so-called 7R martensite in NiAl as well as premartensitic phenomena in AuCd are discussed.