Grain-Size-Dependent Hall-Petch Relationship in CoCrFeTiNi_2.1 High-Entropy Alloys

Molecular dynamics simulations were performed to investigate the size-dependent mechanical behavior of CoCrFeTiNi_2.1 high-entropy alloys (6-18 nm) across varying temperatures (300-1200 K) and strain rates (4 × 10¹⁰-10 × 10¹⁰ s^-1). A transition from inverse Hall-Petch (IHP) to classical Hall-Petch (HP) behavior was identified at a critical grain size of 12 nm, where the tensile strength and flow stress reach their maximum. Elevating the temperature from 300 to 1200 K induces a 15.67% reduction in strength due to hardening phase depletion and dislocation density reduction. Conversely, increasing the strain rate to 10 × 10¹⁰ s^-1 enhances strength by 18.12%, driven by strain-induced BCC/HCP transformations and dislocation proliferation. This study reveals the "structure-temperature/strain rate-property" linkage in the HEA, providing an atomistic mechanistic reference for understanding the grain size effect in nanocrystalline high-entropy alloys under thermal and kinetic loading conditions.