On the influence of strain energy on order-disorder transformations

Abstract

The change in elastic free energy on ordering is examined for its influence on the nature of the phase transitions in CuAu. CuPt and CuZn. The elastic free energy is shown to result from two contributions: the size difference between atoms and the volume and shape change on ordering. For CuAu the size-effect free energy remains virtually unchanged on ordering but the volume change is found to make the transition strongly first-order in agreement with earlier results of Tachiki and Teramoto. For CuPt the size-effect contribution cannot be evaluated at present because the elastic modulus has not been measured through the transition. Inclusion of the shape change contribution alone makes the transition strongly first-order. For CuZn, the size-effect contribution causes the order parameter to rise much faster with decreasing temperature than the curve given by rigid lattice models. The transition, however, remains second-order.