Transformation Paths from Cubic to Low-Symmetry Structures in Heusler Ni2MnGa Compound

Abstract

In order to explain the formation of low-temperature phases in stoichiometric Ni2MnGa magnetic shape memory alloy, we investigate the phase transformation paths from cubic austenite with Heusler structure to low-symmetry martensitic structures. We used ab initio calculations combined with the generalized solid state nudged elastic band method to determine the minimum energy path and corresponding changes in crystal lattice. The four-, five-, and seven-layered modulated phases of martensite (4O, 10M, and 14M) are built as the relaxed nanotwinned non-modulated (NM) phase. Despite having a total energy larger than the other martensitic phases, the 10M phase will spontaneously form at 0 K, because there is no energy barrier on the path and the energy decreases with a large negative slope. Moreover, a similar negative slope in the beginning of path is found also for the transformation to the 6M premartensite, which appears as a local minimum on the path leading further to 10M martensite. Transformation paths to other structures exhibit more or less significant barriers in the beginning hindering such a transformation from austenite. These findings correspond to experiment and demonstrates that the kinetics of the transformation is decisive for the selection of the particular low-symmetry structure.

The Characteristic Properties of Magnetostriction and Magneto-Volume Effects of Ni2MnGa-Type Ferromagnetic Heusler Alloys

“…It has been insisted that the transition proceeds as A→6M→10M (RC = 0.29, RC’ = 0), which differs with the experimental X-ray study of Ni 2 MnGa [30]. Zelenyý et al mentioned that, for the A→14M transition, there is an energy barrier of a certain degree in the transition at RC ≈ 0.15 [39]. Thus, it can be concluded that the A→6M→10M transition is easier process than the A→14M transition.…”

“…Zelený et al performed a theoretical study on the phase transition from the cubic austenitic Heusler structure to low-symmetry martensitic structures [39]. They used ab initio calculations combined with the generalized solid state nudged elastic band method (G-SSNEB) to determine the minimum energy pathway in the crystal lattice.…”

“…In order to classify with different lattices, a referential co-ordinate, which is independent of the characteristic lattice geometry or arrangement of atoms along the transition pathway, has been introduced [39]. The reaction co-ordinate, RC , defines a referential co-ordinate which universally defines the advance of transition between austenite ( RC = 0) and fully transformed martensite ( RC = 1), or the transition between 6 M premartensite ( RC’ = 0) and fully transformed martensite ( RC’ = 1).…”

“…The numerically computed minimum energy pathways of Ni 2 MnGa along RC and RC’ can be seen in Figure 2a,b in Ref. [39]. As for the austenite–martensite transition, RC is applied as the parameter of the phase transition.…”

Superlattice in austenitic Ni-Mn-Ga shape memory microwires

“…The phase transformation process illuminated in Fig. 4(a) depending on the composition [46,47]. A hierarchy emerged in the microstructure of Ni-Mn-Ga film exhibits the morphology martensite with micro-twins → nano-twins → modulated stacking [48].…”