Effect of grain size on the ratcheting behavior of metastable interstitial high-entropy alloys

Abstract

High-entropy alloys (HEAs), renowned for exceptional strength, corrosion resistance, and thermal stability, hold promise for engineering applications. While ratcheting behavior under cyclic loading is critical for durability, it remains underexplored in HEAs. This study investigates the uniaxial tensile properties and ratcheting behavior of metastable interstitial high-entropy alloys (iHEAs) across various grain sizes, elucidating the profound influence of grain size, loading condition, and microstructure on the evolution of ratcheting strain. Through detailed microstructural characterization, the underlying deformation mechanisms are unveiled. The results demonstrate that as the recrystallization annealing temperature decreases from 1050 °C to 850 °C, the grain size of iHEAs undergoes significant refinement, decreasing from 36 μm to 7.0 μm, resulting in a substantial enhancement in yield strength, which rises from 286 MPa to 408 MPa, driven by the Hall-Petch effect. The ratcheting strain rate of iHEAs for all grain sizes under a range of stress amplitudes (e.g., 1.1σ_y, 1.2σ_y, where σ_y = 317 MPa) and mean stress (e.g., 0.3σ_y, 0.4σ_y), asymptotically approaches zero after 300 cycles, demonstrating the remarkable anti-ratcheting ability of iHEAs. In addition, the ratcheting strain saturation value decreases as the grain size decreases. This improvement is attributed to the suppression of martensitic transformation and the dominance of dislocation slipping in fine grains. In contrast, coarse-grained iHEAs exhibit significant strain-induced martensitic transformation and detwinning, with the martensite volume fraction increasing to 22 % after 300 cycles, leading to marked ratcheting behavior, characterized by a ratcheting strain of up to 10 %. This investigation unveils the synergistic interplay among grain refinement, martensitic transformation, and dislocation slipping during ratcheting deformation in iHEAs. These findings provide a theoretical basis for optimizing high-entropy alloys in engineering applications.