Element-specific dynamical decoupling and local structural ordering in liquid NiCoCr medium-entropy alloy.

Abstract

The Stokes-Einstein relation (SER) links diffusion and viscosity, but its applicability is challenged in systems with complex structure and chemical properties. Medium-entropy alloys (MEAs), with their inherent chemical complexity and propensity for diverse local ordering, offer a compelling platform to study such coupling phenomena in the liquid state. However, how element-specific structural preferences influence SER breakdown remains unexplored. Here, we investigate the microscopic mechanisms underlying SER breakdown in liquid equiatomic NiCoCr MEA using high-energy x-ray scattering and molecular dynamics simulations. A dynamic transition is observed near 1700 K (melting point 1682.9 K). While the self-diffusion coefficients of all elements retain Arrhenius behavior down to the deeply supercooled regime, the viscosity and structural relaxation times exhibit a clear cross-over between two Arrhenius regimes below this temperature, leading to the breakdown of the inverse scaling between diffusion and viscosity. This decoupling is primarily governed by the anomalous temperature dependence of the viscosity. Fractional SER analysis reveals element-specific decoupling, with Cr showing a fundamentally distinct departure from classical scaling compared to Ni and Co. Concurrently, non-Gaussian parameters reveal growing dynamic heterogeneity upon cooling. Structurally, short-range order strengthens with significant increases in icosahedral-like and mixed clusters, especially those centered on Cr, accompanied by an enhancement of local five-fold symmetry. The formation of these rigid, Cr-centered ordered domains amplifies local geometric constraints, which severely hinder cooperative atomic rearrangements, while leaving single-atom diffusion less affected. These results connect element-specific ordering to the viscosity-driven breakdown of SER in NiCoCr, providing a structural perspective on diffusion-viscosity decoupling in MEA liquids.