Robust Melting Depression in Cu-Ag Despite Endothermic Mixing

Cu-Ag exhibits a melting depression despite endothermic mixing and limited solid solubility. We combine density functional theory (DFT), machine-learning interatomic potentials, and large-scale two-phase coexistence simulations to resolve its composition-dependent melting, energetics, density, and transport. The lowest sampled fixed-composition coexistence temperature is 970 ± 3 K at 45 at. % Cu, while the maximum end point-referenced depression is 328.9 ± 2.5 K at 55 at. % Cu. The depression persists across ordered solids and prescribed solid-liquid composition partitions, with partitioning-induced shifts remaining modest relative to the depression. Intermediate composition solids incur a larger energetic penalty than the endothermically mixed liquid, providing an energetic basis for the melting depression without requiring favorable liquid mixing. PBE and PBEsol alter absolute melting temperatures, densities, and diffusivities but yield nearly identical maximum end point-referenced depressions. The depression is therefore robust to chemical ordering and functional choice, while its equilibrium depth depends on explicit solid-liquid compositional partitioning.