A unified physical mechanism for martensitic phase transition and ductility in Ni-Mn-based ferromagnetic shape memory alloys: the case of Cu-doped Ni2MnGa

Taking Cu-doped Ni2MnGa alloys as an example, the unified physical mechanism for martensitic phase transition and ductility in Ni-Mn-based ferromagnetic shape memory alloys was investigated theoretically. It was found that Cu doping at the Mn and Ga sites in Ni2MnGa led to a decrease in the interatomic covalent hybridization level, which softened the elastic moduli and enhanced the shear deformation ability. Consequently, the martensitic phase transition temperature increased. Moreover, the simultaneous increase in metallicity was beneficial for improving ductility. In contrast, the substitution of Cu for Ni in Ni2MnGa increased the interatomic covalent hybridization strength, resulting in the reduction of ductility. The hardened elastic moduli due to strengthened covalent hybridization suppressed the martensitic phase transition and consequently led to a decrease in the martensitic phase transition temperature. The style and strength of interatomic orbital hybridization also aid in the determination of atomic occupation styles and lattice sizes. Magnetism is neither the necessary condition for martensitic phase transition nor the inherent parameter to describe martensitic phase transition and ductility in Ni-Mn-based ferromagnetic shape memory alloys. Current theoretical results agree well with recent experimental observations. Since the unified mechanism was derived from interatomic orbital hybridization, it was the most inherent parameter to describe the structure and properties of materials at any scale. The universality of the current unified mechanism was further justified in novel all-d-metal Heusler alloys Ni-Mn-Ta theoretically. It indicates that interatomic orbital hybridization can serve as an inherent tuning parameter to design and explore multifunctional materials belonging to the Heusler family.