Theory of aging phenomena in shape-memory alloys

We formulate a theory for the aging phenomena in AuCd-type shape-memory alloys. The kinetics of the martensitic transformation is represented in terms of the local strain as the order-parameter field. We introduce a secondary slow variable coupled with the order parameter. This theory is an extension of the previous paper by one of the authors, where the secondary variable was assumed to be a nonconserved quantity. In the present paper, we explore the consequence that the secondary variable is conserved. It is shown that the conserved slow variable is more favorable to account for martensite stabilization after aging observed experimentally. In order to study the rubberlike elasticity, the stress-strain relation is also derived both by the direct simulations of the set of kinetic equations and by the interface (twin boundary) dynamics. We propose experiments to elucidate whether the slow variable is conserved or not.