Investigation of the structural stability of Co2NiGa shape memory alloys via ab initio methods

Effects of Annealing on the Martensitic Transformation of Ni-Based Ferromagnetic Shape Memory Heusler Alloys and Nanoparticles

“…A similar phenomenon on the magnetization curve could be observed for Ni-Mn-Sn thin films [55]. The Curie temperatures of the austenite, T [35]) and a unit cell parameter of a = 2.8752(1) Å. As a minor phase with a content of 5.5(1.1) wt%, a fully-ordered γ phase with a slightly tetragonally distorted structure (P/4mmm, sg: 123, ICSD Code 157788 [34]) and unit cell parameters of a = 3.580(3) Å and c = 3.567(5) Å is detected (Table 1).…”

“…As a minor phase with a content of 5.5(1.1) wt%, a fully-ordered γ phase with a slightly tetragonally distorted structure (P/4mmm, sg: 123, ICSD Code 157788 [34]) and unit cell parameters of a = 3.580(3) Å and c = 3.567(5) Å is detected (Table 1). In the β Co 2 NiGa structure with B2-type ordering, the Ni and Ga atoms occupy the same position, whereas in the tetragonal γ-phase, different atoms are placed in different positions [34,35] and the unit cell volume is approximately two-times larger than that of the β phase. The B2-type ordering in the β phase is clearly indicated by the presence of the weak reflection, with Miller indices hkl = 100 at 2θ = 31.12…”

“…The formation of a rather large amount of the highly ordered Co 2 NiGa martensite phase (β') is observed (27.8(3.5) wt%) according to the results of the Rietveld quantitative analysis. In the β'-phase (sg: 139, I/4mmm, ICSD Code 169733 [35]), the Co, Ni and Ga atoms are placed in different atomic positions, similar to what occurs in the γ-phase, but with approximately twice the unit cell volume, owing to the two-times larger tetragonal unit cell parameter c. Recently, the structure of the Co 2 NiGa (β') phase was theoretically predicted [35] by means of density functional theory (DFT) calculations. According to Arróyave et al [35], the tetragonal Co 2 NiGa (β') phase exhibits the greatest structural stability among Co 2 NiGa modifications.…”

“…In the β'-phase (sg: 139, I/4mmm, ICSD Code 169733 [35]), the Co, Ni and Ga atoms are placed in different atomic positions, similar to what occurs in the γ-phase, but with approximately twice the unit cell volume, owing to the two-times larger tetragonal unit cell parameter c. Recently, the structure of the Co 2 NiGa (β') phase was theoretically predicted [35] by means of density functional theory (DFT) calculations. According to Arróyave et al [35], the tetragonal Co 2 NiGa (β') phase exhibits the greatest structural stability among Co 2 NiGa modifications. Thus, annealing results in a partial β −→ β' phase transformation from the partially ordered β phases (B2-type of ordering) to the highly ordered tetragonal β' that has a unit cell volume approximately four-times larger than that of the β phase.…”

“…Concomitantly, the Curie temperature of the austenite T In addition to these well-investigated Ni-and Mn-based compounds, other FSMAs have been studied, i.e., Ni-Fe-Ga [21][22][23], Co-Ni-Al [24,25] and Co-Ni-Ga [26][27][28][29]. The Co-Ni-Ga Heusler system was intensively studied as a promising alternative to Ni-Mn-Ga alloys, especially for high-temperature shape memory device applications [30][31][32][33][34][35][36][37]. Co-Ni-Ga compounds generally exhibit a dual-phase microstructure: a parent phase (β) and a non-transformable secondary phase (γ).…”