Non-equilibrium solid solution of molybdenum and sodium: Atomic scale experimental and first principles studies

We report a combined experimental and first principles study of an extremely immiscible alloy of Mo with 1 and 2 at.% Na, which was produced by high-energy ball milling. The microstructure of the as-milled and annealed state were examined by various methods, including atom-probe tomography (APT), transmission electron microscopy, and energy-dispersive (EDX) analysis. Despite the complete immiscibility of the Mo-Na system in the solid and even in the liquid state, APT measurements clearly evidence the formation of a true nanocrystalline solid-solution microstructure with insignificant Na clustering for samples with 1 at.% Na. In agreement with our x-ray diffraction experiments, first principles calculations expose that the Na atoms do not expand the Mo lattice, which is in contrast to predictions using Vegard's rule. Heating at 700 °C induces only slight grain growth while the solid solution remains remarkably stable without any decomposition. On the contrary, after annealing at 900 °C first Na segregations at triple junctions and significant grain growth are observable, although the solid solution still retains most of the dissolved Na.