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

Area of Science:

• Materials Science
• Condensed Matter Physics
• Physical Chemistry

Background:

• The Stokes-Einstein relation (SER) connects diffusion and viscosity but is often inaccurate in complex systems.
• Medium-entropy alloys (MEAs) present unique chemical complexity and ordering, making them ideal for studying SER deviations.
• Element-specific structural influences on SER breakdown in liquid MEAs are not well understood.

Purpose of the Study:

• Investigate the microscopic mechanisms of SER breakdown in liquid equiatomic NiCoCr.
• Explore how element-specific structural preferences impact diffusion-viscosity coupling.
• Understand the role of local ordering in deviations from classical scaling.

Main Methods:

• High-energy x-ray scattering experiments.
• Molecular dynamics simulations.
• Analysis of self-diffusion coefficients, viscosity, and structural relaxation times.

Main Results:

• A dynamic transition observed near the melting point of NiCoCr.
• Viscosity and structural relaxation times show a crossover, breaking the inverse scaling with diffusion.
• Element-specific SER breakdown, particularly for Cr, linked to increased icosahedral-like and Cr-centered clusters.
• Growing dynamic heterogeneity and local geometric constraints observed upon cooling.

Conclusions:

• Element-specific ordering, especially Cr-centered domains, drives SER breakdown in NiCoCr.
• Rigid, ordered domains hinder cooperative atomic movement, affecting viscosity more than diffusion.
• Provides a structural explanation for diffusion-viscosity decoupling in MEA liquids.