Ultra-grain-refinement of face-centered cubic high/medium-entropy alloys: Approaching the limit of grain refinement through severe plastic deformation and recrystallization
Shuhei Yoshida, Nokeun Park, Kohei Shiotani, Yu Bai, Tomoaki Niiyama, Akinobu Shibata, Tomotsugu SHIMOKAWA, Nobuhiro Tsuji
Abstract
• Characteristics of recrystallization in FCC high entropy alloys (HEAs) were studied. • High-pressure torsion on HEAs resulted in nanocrystalline microstructures. • Significantly fine recrystallized microstructures were obtained after annealing. • Grain growth kinetics of HEAs were found to be much slower than pure FCC metals. • Slow grain growth can be due to solute drag enhanced by severe lattice distortion. Face-centered cubic (FCC) high- and medium-entropy alloys (HEAs/MEAs) develop fully recrystallized ultrafine-grained (FRex-UFG) microstructures after simple deformation and annealing processes. However, the mechanistic origins remain unclear. This study explains why FRex-UFG microstructures are readily formed in FCC HEAs/MEAs and clarifies their characteristics of microstructural evolution during deformation and recrystallization. CoCrFeMnNi HEA and CoCrNi MEA were subjected to high-pressure torsion (HPT) followed by short-term annealing. HPT-processed HEA and MEA showed finer nanocrystalline structures with higher dislocation densities than in conventional materials, indicating that dynamic recovery is more inhibited in HEAs/MEAs. Subsequent annealing produced FRex-UFG microstructures with grain sizes (with/without annealing twins) of 410/200 nm and 448/80 nm in the HEA and MEA, respectively, which are the smallest among single-phase materials reported to date. Grain-growth activation energies of the HEA and MEA were three times higher than that of pure Ni. Hybrid Monte‑Carlo/molecular‑dynamics simulations of grain boundary (GB) structures in model HEAs revealed that severe lattice distortion reduces GB free volume, intensifying solute drag and retarding GB migration. These results demonstrate that inhibited dynamic recovery and sluggish GB migration, both inherent to HEAs/MEAs, elevate the density of potential nuclei for recrystallization while suppressing microstructural coarsening, leading to the formation of FRex-UFG microstructures.