Electronic origin of the main-group element dependences of elastic moduli in the Ni2Mn-based magnetic shape memory alloys

Abstract

In this work, the main-group (sp) element dependences of anisotropic elastic moduli, including elastic constants Cij, Young's moduli En, shear moduli Gm, n, Poisson's ratioνm,n and Zener anisotropy index Az, and their correlated electronic origins in the Ni2MnZ (Z = Al, Ga, In, Si, Ge, and Sn) alloys were studied by first-principles calculations. Results show that all the studied alloys are strongly anisotropic. The ratio of E[111] and E[100], which owns the maximum and minimum tension resistance, respectively, is larger than 20 in Ni2MnGa. For the sp element dependence, the mechanical strength-related elastic moduli, such as E111, EH, and GH, show a similar tendency as that of C44. The lattice stability-related elastic moduli, such as Az and ν[110][-110], follow the sp element dependence of tetragonal shear modulus C’. Analyses show that the atomic radius and valence electron number of the sp element are the key parameters to tune C11 and C12 in the studied Ni2Mn-based alloys. One more valence electron of the sp element would lead to an increase of 0.035 Å for the equilibrium lattice parameter. For C44, it is closely related to the electronegativity of the sp element. As for C′, the strength of p-d orbital hybridization between Ni and the sp element plays a decisive role. This work is expected to lay a theoretical foundation for designing advanced Ni2Mn-based magnetic shape memory alloys.