Search

Using the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation, the crystal structure and site preference, magnetic and elastic properties, and martensitic transformation (MT) are systematically investigated with the three groups of Heusler alloys: (Co 2 − x M x)VGa (M1 x), Co 2 (V 1 − x M x)Ga (M2 x), and Co 2 V(Ga 1 − x M x) (M3 x , M = Ni and Fe, 0 ≤ x ≤ 1.0). It is shown that instead of the L 2 1 and X A structures, the fcc one is energetically preferred in the cubic M3x (x ≥ 0.8) alloys. In L 2 1 -Ni2x (x ≤ 0.6) and fcc-Ni3x (x = 0.8), Ni atoms even prefer the Ga and Co anti-sites, respectively, and the replaced atoms move to the sublattices of the deficient ones. Their total magnetic moment is dominated by the magnetic exchange interactions corresponding to the pairs of two Co atoms on the different sublattices in M = Ni and Fe1x, Co and Fe in Fe2x and Fe3x (x < 0.8), and Fe and Fe atoms in Fe3x (x ≥ 0.8) alloys, respectively. These Ni1x, Ni2x, and Fe3x with x ≥ 0.4 as well as Ni3x with x ≥ 0.2 alloys are predicted having the MT behavior and also the better mechanical property relative to Co 2 VGa. A lower shear modulus (C ′ = (C 11 − C 12) / 2) generally corresponds to a higher MT temperature, and these alloys, which can undergo the MT are further evaluated with C ′ < 36.50 GPa. Both considerable magnetocaloric and magnetovolume effects can be also expected during the MT of these Fe3x alloys (x = 0.4 and 0.6). In the remaining Fe1x and Fe2x alloys, the Fe doping disfavors the MT and also improves their brittleness. The structural preference of these cubic alloys and also their stability relative to the tetragonal martensite can be mainly attributed to the number of their minority density of states at the Fermi level: the smaller they are, the more stable their system tends to be. [ABSTRACT FROM AUTHOR]

The structural, elastic, and basic thermodynamic properties of hexagonal ScxAl1−xN crystals are calculated and discussed over the whole range of possible random alloys, including the transition from wurtzite to the layered hexagonal structure. Based on a review of lattice and internal parameters in combination with complete datasets of stiffness coefficients published in the literature, differing in the considered alloying intervals and the predicted structural transitions, changes in the crystal lattices caused by the substitution of aluminum by scandium atoms are discussed and illustrated. Crystal properties like the mass densities, average bond angles, and bond lengths are calculated, and the compliance coefficients, Young's modulus, shear modulus, Poisson's ratio, compressibility, and sound velocities are determined depending on the alloy composition and in relation to the orientation of crystal planes and axes. Particular attention is paid to the occurring directional anisotropies and the changes in structural and elastic properties in the alloy region of the structural transition between wurtzite and layered hexagonal ScxAl1−xN crystals. The acoustic velocities determined are used to calculate basic thermodynamic properties such as the Debye temperature, heat capacity, and minimum heat conduction, as well as to evaluate both the influence of the alloying and the structural transition on these properties. [ABSTRACT FROM AUTHOR]