First-principles study of the structural and elastic properties of rhenium-based transition-metal alloys

Abstract

Structural, energetic, and elastic properties of hexagonal-close-packed rhenium-based transition-metal alloys are computed by density-functional theory. The practical interest in these materials stems from the attractive combination of mechanical properties displayed by rhenium for structural applications requiring the combination of high melting temperature and low-temperature ductility. Single-crystal elastic constants, atomic volumes, axial $c/a$ ratios, and dilute heats of solution for Re-$X$ alloys are computed, considering all possible transition-metal solute species $X$. Calculated elastic constants are used to compute values of a commonly considered intrinsic-ductility parameter $K/G$, where $K$ is the bulk modulus and $G$ denotes the Voigt average of the shear modulus, as well as the anisotropies in the Young's modulus and shear modulus. The calculated properties show clear trends as a function of $d$-band filling, which can be rationalized through tight-binding theory. The results indicate that solutes to the left of rhenium in the periodic table show a tendency to increase the intrinsic ductility parameter, a trend that correlates with an increase of the $c/a$ ratio towards the ideal value associated optimal close packing. The Young's modulus shows a trend towards increasing isotropy with alloying of solutes $X$ to the left of Re, while the shear modulus shows the opposite trend but with an overall weaker dependence on solute additions.