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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 effects of local atomic and magnetic configurations on the phase stability and elastic property of the face-centered cubic (fcc) and two body-centered tetragonal [face-centered tetragonal (fctI) and fctII, with 0.9 < c / a < 1 and 0.71 < c / a < 0.9 , respectively, in the fct unit cell] phases of Fe 1 − x Pd x (0.28 ≤ x ≤ 0.34) shape memory alloys are systematically investigated by using the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation. It is shown that, considering four types of atomic configurations in a fcc unit cell, the two with one random sublattice are both preferable in each x below 300 K. When T = 300 K, the one with three random sublattices also changes to be stabilized for x ≤ 0.30 , whereas that with four random sublattices becomes stable in most of these alloys until T ≥ 600 K. Upon tetragonal distortions, in these fully disordered alloys, both the fctI and fctII phases are unstable. The fctI phase is found for 0.29 ≤ x ≤ 0.33 , having only the configuration with one random sublattice on the same layer with the Pd site in the unit cell, whereas the fctII phase is obtained for x ≤ 0.30 , possessing all the configurations with one, two, and three random sublattices. These results representing the phase diagram of these alloys, their determined equilibrium lattice parameters, and elastic constants of the three phases at 0 K are in line with the experimental and theoretical data, and their estimated structural (T M) and magnetic (T C) transition temperatures are also close to the experimental data. Adding 4% magnetic disorder in Fe 0.70 Pd 0.30 , the fctII structure is effectively prevented, whereas the thermoelastic martensitic transformation of fcc–fctI can still be retained at 0 K. [ABSTRACT FROM AUTHOR]

The phase stability and elastic property of ϵ-Pu at 800 K are predicted through systematically theoretical calculations, by taking the temperature-dependent phonon smearing, spin fluctuation, phonon vibration, and volume expansion effects into account. Dominated by the phonon smearing at 800 K, ϵ-Pu is thermodynamically stabilized in the paramagnetic state with the absolute value of the local magnetic moment (μPu) about 3.8 μB, being smaller than the 0 K value (about 5.0 μB). The resulted thermal spin fluctuation, which prefers the δ−ϵ phase transition, greatly decreases the volume (V) and the shear elastic constants (C′ and C44) of the ϵ phase. However, the phonon smearing itself prefers the expansion of V and also the increase of C′ and C44. Besides the influence of the spin fluctuation and phonon smearing at 800 K, the equilibrium V of ϵ-Pu seems to be dominated by the phonon vibration, and the lattice thermal expansion also further increases C′ whereas decreases C44. The determined V as well as C′ and C44 of ϵ-Pu at 800 K is reasonable in comparison with the previous studies. This insight provides a good understanding of the physical mechanisms driving the thermodynamical stability of ϵ-Pu at high temperature. [ABSTRACT FROM AUTHOR]

• The alloy with the magnetic disordering degree larger than 0.2 can occur the MT. • Co 2 V( Ga 1 - x Z x ) (Z = Co, V, Ge, Si, Sb) can occur the MT with the different x values. • Their volume decreases upon the MT, C′ and C44 decrease but A increases with e/a. • Greater magnetocaloric and magnetostrain effects are expected in the Z = V alloys. • The origins can come from the minority Co d and V d states around the Fermi level. Using the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation, the alloying and magnetic disordering effects on the phase stability and elastic property of L 2 1 - and D 0 22 -Co 2 VGa Heusler alloy are systematically investigated. It is shown that at the ground ferromagnetic (FM) state, Co 2 VGa alloy possesses the L 2 1 structure and correspondingly, its shear modulus, Young modulus, and Debye temperature of the phase are all larger than those of the D 0 22 phase. When the FM state transiting to the paramagnetic (PM) one, both C ′ (= (C 11 - C 12) / 2) and C 44 of the L 2 1 phase decrease whereas the elastic anisotropy increases, and they do so as well with increasing the number of valence electrons per atom (e/a) in the off-stoichiometric ternary and doped quarternary alloys with the FM ordering. As a result, at 0 K the magnetic disordered Co 2 VGa alloy with the disordering degree (y) larger than 0.2 can show the martensitic transformation (MT) from L 2 1 to D 0 22 , and the MT is also obtained in the FM Co 2 V( Ga 1 - x Z x ) alloys with Z = Co (x ⩾ 0.2), V and Ge (x ⩾ 0.4), Si (x ⩾ 0.5), and Sb (x ⩾ 0.3). Accompanying the MT, their volume tends to decrease, and these L 2 1 alloys possessing a bigger volume than those with the same e/a ratios are also relatively more prone to the MT. In the Z = V alloys, both greater magnetocaloric and magnetostrain effects may be expected during the MT as well. Both the minority Co d and V d states around the Fermi level should be responsible for the composition and magnetic ordering dependence of the phase stability of Co 2 VGa alloy. [ABSTRACT FROM AUTHOR]

We conducted first-principles calculations to evaluate the elasticity, minimum thermal conductivity, and anisotropy of four orthorhombic superhard materials of bc-B x C y N and compared them with diamond and cubic boron nitride. All four bc-B x C y N materials are mechanically stable based on the basic evaluation of the mechanical stability of orthorhombic crystal. Our calculation results show that the bc-BC 4 N ( x = 1, y = 4) material is the hardest among the bc-B x C y N models and has the highest values of elastic modulus and ideal strength. The bc-BC 2 N material ranks second and has a slightly lower hardness compared with bc-BC 4 N, and the bc-B 2 CN possesses the lowest hardness and weakest ideal strength. The elastic modulus and slowness of all four bc-B x C y N are anisotropic. In addition, the calculated minimum thermal conductivities at high temperatures show that bc-BC 4 N exhibits the highest thermal conductivity and bc-B 2 CN exhibits the lowest thermal conductivity. [ABSTRACT FROM AUTHOR]

Highlights: [•] We calculated the hardness and elastic constants of four hexagonal compounds. [•] A shape of ‘8’ was observed for the 3D Young’s modulus. [•] We explained why the anisotropic graphics of Young’s modulus is in the shape of ‘8’. [•] The minimum thermal conductivities at high temperature were calculated. [•] We found that the conditions for the preparation of r3m-B2CN are most difficult. [Copyright &y& Elsevier]

Highlights: [•] We analyze mechanical deformation modes along the [100], [110], and [111] directions. [•] We study the anisotropy of the elasticity in detail. [•] We calculate the minimum value and anisotropy of the thermal conductivity. [Copyright &y& Elsevier]

Elasticity, hardness, and thermal conductivity of ZrBn ( n =1, 2, 12) were studied in this paper by using the first-principles calculations of plane wave ultra-soft pseudo-potential technology based on the density functional theory (DFT). Deep analysis of elasticity and anisotropy was carried out, indicating that ZrB and ZrB 2 are soft (the most anisotropic) and superhard (the least anisotropic) respectively. The transverse wave speed ( V t ), longitudinal wave speed ( V l ) and the minimum thermal conductivity κ min (both Clark and Cahill model) were calculated. Our investigations show that ZrB is a good heat insulating material and a good lubricant, ZrB 12 can be served as a good high-temperature-resistant material and superconducting material, and ZrB 2 is a good wear-resistant material. [ABSTRACT FROM AUTHOR]