Chemical free energy profiles for martensitic transformation of CuAlNi at finite temperatures

The free energy profile for atomistic configurations of a given deformation process is the key to understand behaviors of materials at all scales. The martensitic transformation of CuAlNi at finite temperatures features a twinning deformation process. The chemical free energy profiles for intermediate twinning configurations at finite temperatures are constructed in two steps incorporating an internal variable describing the twinning microstructure development in the martensitic transformation process. The first takes place at 0 K where one unique lattice parameter is constrained to obtain the twinned configurations with different levels of twinning shear via first-principles density functional theory (DFT) optimizations. Resultant free energy profiles from a hybrid-atom model and three localized lattice models show that the lattice inhomogeneity has a strong connection with the twinning growth in the MT process. The second covers a range from 0 K to a finite temperature with lattice sizes fixed. The chemical free energy profiles of martensitic microstructures at finite temperatures obtained via the corresponding thermodynamic analysis differ the standard Landau double-well potential significantly on symmetry and curvature, which are responsible for the dynamics of twinning development. The method proposed can be applied to other martensitic material systems.