Phase stability, point defects, and elastic properties of W-V and W-Ta alloys

Abstract

The structure and phase stability of binary tungsten-vanadium and tungsten-tantalum alloys are investigated over a broad range of alloy compositions using ab initio and cluster expansion methods. The alloys are characterized by the negative enthalpy of mixing across the entire composition range. Complex intermetallic compounds are predicted by ab initio calculations as the lowest energy structures for both alloys. The effect of atomic relaxation on the enthalpy of mixing is almost negligible in W-V, but is substantial in W-Ta alloys. Canonical Monte Carlo simulations are used for predicting the order-disorder transition temperatures for both alloys. Differences in the short-range order between the two alloys are explained by the opposite signs of the second nearest-neighbour cluster interaction coefficients for W-V and W-Ta. Using the predicted ground-state structures, we evaluate the monovacancy formation energies and show that in W-Ta alloys they are highly sensitive to the alloy composition and the local environment of a vacancy site, varying from 3 to 5 eV. In the dilute tungsten alloy limit, a 111 self-interstitial atom crowdion defect forms a configuration strongly bound to a vanadium solute atom, whereas interaction between the same defect and a tantalum solute atom is repulsive. Values of elastic constants computed for all the ground-state structures and several metastable cubic alloy structures are used for assessing the effect of alloying on mechanical properties. Values of the Young modulus and the Poisson ratio, as well as the empirical Rice-Thompson criterion, are applied to screening the alloys, to assess the effect of chemical composition on ductility.

Ab initioprediction of vacancy properties in concentrated alloys: The case of fcc Cu-Ni

“…(5) in Ref. [27]. It is now apparent also why an enthalpy formulation is preferable over an energy formulation because maintaining equal pressure is much easier than maintaining equal volume in the supercell with vacancy plus that of the i atom vis-a-vis the supercell without the vacancy.…”

“…We consider the problem of vacancy formation energies in disordered alloys, a problem that recently is receiving increased attention [27,28,31,33]. We limit ourselves initially to configurationally random alloys, i.e., the reference state is the configurationally random state without vacancies.…”

“…[20,21]), and from the theoretical side also, there have been rather few studies that deal with specific alloy systems. Initially using empirical potentials [22][23][24][25], and later through ab initio approaches, both through supercell calculations [26][27][28][29] and ab initio based cluster expansions [30][31][32][33], it has been established that the local atomic environment around a vacancy plays a significant role. While the influence of vacancies on phase stability [30,34] and kinetics [31][32][33] received some attention for Al-Ni [30], Sc-S [34], and Al-Li [31][32][33] alloys, the actual vacancy properties in specific alloys were mostly neglected with the exception of an empirical potential study of Cu-Ni alloys by Zhao et al [24].…”

Recent progress in research on tungsten materials for nuclear fusion applications in Europe

“…Ab-initio based simulations of the phase stability of tungsten-tantalum (W-Ta) and tungsten-vanadium (W-V) binary alloys were performed in [73,74]. In these references, the phase stability of W-V and W-Ta alloys was investigated using first-principles quantum-mechanical electronic structure based calculations, combined with cluster expansion Monte-Carlo simulations.…”

First-principles calculation of phase equilibrium of V-Nb, V-Ta, and Nb-Ta alloys

“…2 Recently, V, Nb, and Ta have been found to undergo a martensitic-like structural distortion [3][4][5] in which they transform from the high-temperature bcc structure to either a rhombohedral structure or to a tetragonal structure. Apart from this, vanadium-based alloys are known to have the desired combination of physicomechanical properties, compatibility with nuclear fuel, and high resistance to corrosion 6,7 making them potential structural materials for fast-neutron nuclear power plants which can be operated at temperatures exceeding 1000 K for increasing the thermodynamic efficiency of electric power production.…”