Stabilizing internal stress as the thermodynamic factor of martensite aging effects

Abstract

The symmetry-conforming Landau theory has been developed for the description of interplay between the symmetry of the deformable crystal lattice and the configuration of crystallographic defects in martensitic alloys. For this purpose, the multi-component non-scalar order parameter describing the slow reconfiguration of defects after the deformation of crystal lattice has been introduced within the framework of the Landau theory of martensitic transformations (MTs). Due to the transformational equivalence of the configurational order parameter and strain/stress tensor components, the conception of stabilizing internal stress (SIS), which is linearly related to this parameter, has been formulated. The complete agreement between the developed theory and the symmetry-conforming short-range-order principle formulated by Ren and Otsuka has been established. The effect of stabilizing the product (martensitic) phase after aging has been described by considering the stress–temperature phase diagram, which was constructed by taking into account the time dependence of SIS. The applicability of the theory to the aging effects in the Au–Cd shape memory alloy has been demonstrated. The time dependencies of the experimentally observed slow changes in the MT temperatures, lattice parameters and yield stress values have been derived from the SIS conception.