Investigation of nanotwins in the bimodal-structured Fe₂₂Co₂₂Ni₂₀Cr₂₂Mn₁₄ alloy subjected to high-strain-rate deformation at cryogenic temperatures

Cold-rolling Fe₂₂Co₂₂Ni₂₀Cr₂₂Mn₁₄ high entropy alloy (with a deformation of 70%), and annealing it at two temperatures (1100 and 800 °C) respectively created a bimodal grain size distribution in large-grain (LG) samples having an average grain size of 45.9 ± 20.0 μm and in small-grain (SG) samples having an average grain size of 4.3 ± 3.0 μm. Under three cryogenic temperatures (−50, −100, and −150 °C), high-speed deformation (~9 × 10³ s⁻¹) was conducted on a split Hopkinson pressure bar (SHPB) system to investigate the microstructural evolution of deformation nanotwins in the bimodal-structured samples. Subjected to high-speed deformation at decreasing cryogenic temperatures, the mechanical behaviors of LG samples were superior to those of SG samples. Notably, under high-speed deformation at −150 °C, LG structures achieved excellent mechanical strength of ~3.3 GPa with good ductility of ~31.9%. Profuse lamellar annealing nanotwins, which pre-existed in the coarse grains of LG samples, promoted efficient refinement strengthening. High-resolution transmission electron microscopy (HR-TEM) clearly revealed that the deformation nanotwins induced by high-speed deformation further refined the pre-existing annealing nanotwins in the coarse grains of LG samples, presumably providing advanced mechanical sustainability for high-speed deformation at cryogenic temperatures. It is suggested that the micrometer-scaled and nanometer-scaled annealing twins appear first in the matrices of coarse grains, enhancing the initial work-hardening; subsequently the deformation nanotwins form in pre-existing annealing nanotwins and narrow strips of the matrix, effectively providing the dynamic grain refinement and the work hardening capacity.