Strain ordering in BCC metals and the associated anelasticity

Abstract

The BCC to BCT transformation is thought to occur as a consequence of strain ordering due to the interaction between impurity interstitials. A Hamiltonian is given which involves the interaction energies between the strain fields of the interstitials belonging to three distinct sublattices. In the BCT phase, one of the sublattices is preferentially occupied. The free energy of the system is calculated in the mean field approximation. In this, the BCC to BCT transformation is found to be a first-order transition at a temperature T_p that is proportional to the concentration of the interstitials and certain basic interaction parameters. The anelastic behaviour of the interacting interstitials is then studied in the region T > T_p. From the anelastic strain, which is proportional to the order parameter associated with the phase transition, the static compliance is obtained. The latter obeys a Curie-Weiss type of law. The creep function, which determines the response to a constant applied stress, is found to exhibit viscous behaviour near T_p. From the creep function, the frequency dependent compliance and the internal friction are evaluated. The results predict a shift and a broadening of the internal friction peak as T_p is approached from above. The features show qualitative resemblance with the recent data on Ta-O.