Thermal expansion coefficient of WRe alloys from first principles

We calculate the coefficient of thermal expansion (CTE) in tungsten-rhenium random alloys for Re concentrations between 0% and 50% and for temperatures up to 2400 K by employing the quasiharmonic approximation within the ab initio framework of density functional theory. We treat chemical disorder by the virtual crystal approximation and compute the phonon density of states at two levels of sophistication. While the traditional Debye-Gr\"uneisen (DG) model fails to account for the experimentally observed increase in CTE upon Re addition for concentrations above 10% Re, explicit phonon calculations within density functional perturbation theory lead to an overall good agreement with experiment. Thereby we identify the pronounced phonon softening and anisotropy between transversal and longitudinal modes in W-Re to be responsible for the breakdown of the DG model.