First-Principles Study of Structural Stability and Mechanical Properties of Ta-W-Hf Alloys

In order to obtain the effect of W and Hf elements on the mechanical properties of Ta-W-Hf alloys, the structural, mechanical, and electronic properties of Ta-xW-6.25Hf (x = 6.25, 12.50, 18.75, 25.00, 31.25, 50.00) alloys were studied using first-principles calculation based on density functional theory, and the supercell method. The calculated formation enthalpy and elastic constants clarify that Ta-W-Hf alloys have structural and dynamical stability. The formation enthalpy and the cohesive energy decrease with the increase in W content, and the cohesive energy increases when Hf element is added to Ta-W alloy. In addition, bulk modulus (B), shear modulus (G), and Young's modulus (E) for each of the Ta-xW alloys increase gradually with the increase in W concentration. The B, G, and E of Ta-xW-6.25Hf alloys is lower than that of Ta-xW alloy under the same W content conditions, suggesting that Hf alloying with higher Ta-W concentration becomes softer than the Ta-W alloy. Based on the mechanical characteristic, the B/G and Poisson's ratio of Ta-W-Hf alloys are higher than those of Ta-W alloys with W content over 25%, the ductility of Ta-W-Hf alloys improves with the addition of Hf, and Hf can reduce the anisotropy of Ta-W-Hf alloys. Furthermore, the electronic density of states shows that alloying W and Hf improves the metallicity of Ta. The results in this work provide the underlying insights needed to guide the design of Ta-W-Hf alloys with excellent mechanical properties.