Composition dependent elastic modulus and phase stability of Ni2MnGa based ferromagnetic shape memory alloys

Abstract

Ni 2 MnGa based ferromagnetic alloys are ideal candidates for applications such as actuators, magnetic refrigerators or magnetostrictive transducers due to their attractive properties such as magnetic field induced shape memory effect and large magnetocaloric effect. The properties of these alloys (e.g., the martensitic transformation temperature T M ) sensitively depend on the composition. Understanding the composition dependence of these properties so as to design the alloy as desired is one of the main research topics in this area. In recent years, we have investigated the composition dependent elastic modulus and phase stability of Ni 2 MnGa-based alloys by using a first-principles method, in hope of clarifying their connection to the properties of these alloys. In this article, we review the main results of our investigations. We show that the tetragonal shear modulus C' is a better predictor of the composition dependent T M than the number of valence electrons per atom (e/a) since the general T M~C ' correlation works for some of the alloys for which the T M~e /a correlation fails, although there exist several cases for which both the general T M~C ' and T M~e /a correlations break down. Employing the experimentally determined modulation function, the complex 5-layer modulated (5M) structure of the martensite of Ni 2 MnGa and the Al-doping effect on it are studied. We find that the shuffle and shear of the 5M structure are linearly coupled. The relative stability of the austenite and the martensites is examined by comparing their total energies. The non-modulated martensite   ′′′ with the tetragonality of the unit cell c/a>1 is shown to be globally stable whereas the 5M martensite with c/a<1 is metastable. The critical Al atomic fraction over which the martensitic transformation between the 5M martensite and austenite cannot occur is predicted to be 0.26, in reasonable agreement with experimental findings. Ni 2 MnGa, martensitic transformation, elastic modulus, phase stability, density functional theory Citation: Hu Q M, Luo H B, Li C M, et al. Composition dependent elastic modulus and phase stability of Ni 2 MnGa based ferromagnetic shape memory alloys.The ferromagnetic Ni 2 MnGa-based alloys have drawn much attention in recent years due to their attractive properties such as magnetic field induced shape memory effect and large magnetocaloric effect, resulting from the coupling between the magnetic and reversible martensitic transitions (e.g., refs. [1-4]). These properties make the alloys ideal candidates for applications such as actuators, magnetic refrigerators or magnetostrictive transducers, and so on. The shape memory effect results mainly from the reversible martensitic transformation (MT) between the high temperature austenite structure and the low temperature martensite. The austenite has cubic L2 1 structure as shown in Figure 1(a).

First-principles study of Co- and Cu-dopedNi2MnGaalong the tetragonal deformation path

“…In order to obtain the best agreement with experiments for a (c/a) NM , we introduced an additional optimization of the muffin-tin potential on the Ni sublattice according to Refs. [16,42] by choosing the atomic radius R Ni ws = 1.10R ws and overlapping potential spheres R Ni mt = 0.95R ws , where R ws is the average Wigner-Seitz radius. For the other sublattices, the usual setup R mt = R ws was used.…”

“…The effect of compositional changes in Ni-Mn-Ga alloys and doping by different elements have been studied intensively in the recent decade by experimental and theoretical techniques and have been summarized in several reviews [14][15][16][17]. One of the important reasons for the alloy doping and composition adjustments is the increase of the operating temperature of the MSM material by shifting T M and T C to high temperatures.…”

Effect of Magnetic Ordering on the Stability of Ni–Mn–Ga(–Co–Cu) Alloys Along the Tetragonal Deformation Path

“…The Green's function was calculated for 32 complex energy points distributed exponentially on a semicircular contour. In order to obtain the best agreement with experiments for a (c/a) NM , we introduced an additional optimization of the muffin-tin potential on the Ni sublattice as in [46] and [47] by choosing the atomic radius R ws Ni = 1.10R ws and overlapping potential spheres R mt Ni = 0.95R ws , where R ws is the average Wigner-Seitz radius. For the other sublattices, the usual setup R mt = R ws was used.…”

First-principles study of Zn-doping effects on phase stability and magnetic anisotropy of Ni-Mn-Ga alloys

“…In the EMTO basis set s, p, d and f orbitals were included and Ni 3d 8 4s 2 , Mn 3d 5 4s 2 , Ga 3d 10 4s 2 4p 1 and Zn 3d 10 4s 2 were considered as the valence orbitals. In order to get a better agreement with experiment for non-modulated martensitic structure, the muffin-tin potential on the Ni sublattice was optimized by choosing the atomic radius = R R 1.10 ws Ni ws [42] and the overlapping potential sphere radius = R R 0.95 mt Ni ws , [43] where R ws is the average Wigner-Seitz radius. In the one-center expansion of the full charge density, the number of components was truncated to eight.…”