Composition-Dependent of 6 M Martensite Structure and Magnetism in Cu-Alloyed Ni-Mn-In-Co by First-Principles Calculations

The composition dependence of the crystal structure and magnetism of the 6 M martensite for the Cu-doped Ni_(43.75)Mn_(37.5)In_(12.5)Co_(6.25) alloy at different site occupations(Cu substitution for Ni, Mn, In, and Co, respectively) is investigated in detail with the first-principles calculations. Results show that the austenite(A) phase exhibits a ferromagnetic(FM) state in all occupation manners, the 6 M martensite possesses an FM state except for the case of Cu substitution at the normal Mn(Mn1) site, and the non-modulated(NM) martensite displays a ferrimagnetic(FIM) state apart from the Cu substitution at the Ni, Mn1, or In sites. The Cu atom destabilizes the A, 6 M, and NM phases regardless of the occupation manner. The one-step martensitic transformation from the A to NM phase occurs in the case of Cu substituting for Mn1, excess Mn(Mn2), or Co;for Cu substituting Ni, a martensitic transformation including 6 M martensite happens, i.e., A → 6 M → NM;however, the martensitic transformation disappears when Cu replaces In site. From the equilibrium lattice constants, it can be speculated that the substitution of Cu for Ni can effectively reduce the thermal hysteresis( ΔT_(Hys)). The magnetic properties are found to be greatly reduced by the substitution of the non-magnetic element Cu for the ferromagnetic Mn atom, whereas the effect is fewer in the remaining cases. It is predicted that the alloy has more favorable properties when Cu replaces Ni. The present results can lay a theoretical foundation for further development of multielement magnetic shape memory alloys.