Non-uniform solute drag in rapid alloy solidification

Rapid solidification of alloys raises a long-standing question: how the driving force is divided between structural ordering and solute redistribution at the crystal-melt interface (CMI). Atomistic simulations of Ni-Cu alloys uncover a previously unrecognized “preordering-partial remelting” cycle that governs solute redistribution before complete solute trapping (CST). This mechanism produces an undercooling-dependent, non-uniform solute-drag parameter λ, overturning the uniform-λ assumption of classical theories. Incorporating this non-uniform λ into a partial-drag framework yields a quantitative agreement with simulations across CST, establishing a unified kinetic-constraint framework and predictive input for phase-field modeling.