Composition dependent phase stability of Ni–Mn–Ga alloys studied by ab initio calculations

Composition-dependent structural and magnetic properties of Ni–Mn–Ga alloys studied by ab initio calculations

“…We substitute one Mn (or Ga) atom of the Ni 40 Mn 20 Ga 20 supercell with one extra-Ni (or extra-Mn) atom, which is corresponding to x = 0.05 for each series (Ni 2?x Mn 1-x Ga, Ni 2?x MnGa 1-x , and Ni 2 Mn 1?x Ga 1-x ). As reported in our calculated results [37,38], the formation energy of each off-stoichiometric structure is higher than that of the stoichiometric structure. It indicates the extra-Ni (or extra-Mn) atoms prefer to occupy sites that are not adjacent in the austenite of each series.…”

“…Furthermore, according to our previous calculation [37], Ni-doping (Ni substitution for Mn and/or Ga) relatively stabilizes the NM martensite, whereas proper Mn-doping Table 2 Lattice parameters and volume per motif of the NiMn-Ga alloys (Mn substitution for Ga) relatively stabilizes the 7 M martensite. It indicates that NM martensite is relatively more stable when the optimized volume of the offstoichiometric Ni-Mn-Ga alloy is smaller than that of the stoichiometric Ni 2 MnGa alloy; otherwise 7 M martensite is relatively more stable.…”

Experimental Observations versus First‐Principles Calculations for Ni–Mn–Ga Ferromagnetic Shape Memory Alloys: A Review

“…When a full structural optimization of the lattice parameters and atomic coordinates was considered, Xu et al reported a significant decrease in the total energy of the harmonic‐wave modulated structures, closer to the energy level of NM martensites. [ 99,100 ] Niemann et al [ 64 ] and Gruner et al [ 47 ] observed a similar relaxation‐related energy decrease in their nanotwinning‐based model but did not achieve a comparable match for the lattice parameters. This demonstrates that both harmonic and nanotwinning‐based approaches for describing modulation provide nearly equal energies for the modulated structures if full structural optimization is allowed and properly performed; thus, these approaches can be combined.…”

“…[61,157] Luo et al used the CPA method to calculate the total energy of a 10M structure with sinusoidal modulation in Al- [96] and Fe-doped alloys, [97] but they were not able to achieve properly relaxed structures with low energies. Dutta et al [100] constructed a full-phase diagram for off-stoichiometric Ni-Mn-Ga with excess Ni or Mn by combining smooth modulations and nanotwinning, as mentioned in the previous section. However, the comparison of the calculated phase diagram with the experimental data is questionable.…”

“…This could potentially result in mixed phases, particularly 10M and 14M or 14M and NM, which is not usually observed in single crystals. In addition, an ab initio‐based phase diagram by Dutta et al [ 100 ] predicted that a mixture of all three phases exists in a certain temperature range, which was not observed in single crystals. The transition between these phases is usually sharp, without any broad interval of coexistence.…”