A First Principles Investigation of W1-x Irx Alloys: Structural, Electronic, Mechanical, and Thermal Properties

Tungsten (W) possess comprehensive physical and chemical properties that are suitable for aerospace and space nuclear power applications, including the highest melting temperature (3410 degrees C) among metals, high elastic modulus, thermal shock resistance, and high temperature strength. However, its poor ductility at room temperatures significantly hinders its fabricability and potential use in the above-mentioned fields. Accordingly, to improve the ductility of W, solid solution strengthening is the primary method considered besides grain refining and deformation strengthening. Experimental studies have shown that Ir is a brittle metal with an fcc structure, but it can greatly improve the ductility of W; however, the corresponding mechanism is still unclear. Thus, using the first principles method based on density functional theory together with phonon spectrum calculations, the effect of the addition of different contents of Ir on the structure, phase stability, mechanical properties, and thermodynamic properties of W were studied. The relation between the addition of different contents of Ir and above-mentioned properties of W-Ir alloys were theoretically investigated. It was found that Ir can induce instability in the W-Ir alloy in the ground state due to the occupation of its antibonding electrons below the Fermi level. When content of Ir added is less than 7.4%, the formation of the W-Ir alloy becomes stable in the ground state. With an increase in temperature and the content of Ir, the thermodynamic stability is improved, implying that Ir is suitable for incorporation with W for application at high temperature. The addition of Ir helps to improve the toughness of the W alloy, which is consistent with the experimental observation. Besides, Ir can simultaneously improve the planar shear resistance. Furthermore, the pCOHP analysis revealed that the inherent mechanism of the ductile effect of brittle Ir in W is attributed to their different modes of electron transition and overlapping. For Ir, electrons transfer from its higher energy orbital of d(x2-y2) to the lower energy dxz and dyz orbitals. In contrast, for W, the electrons transfer from its low energy orbital of d(z2) to the d(xz) and d(yz) orbitals. The d(xz) and d(yz) orbitals of Ir and W form a metallic bond, which is further enhanced with an increase in the content of Ir added. Therefore, Ir acts as a toughness-enhancing element in W-Ir alloys.