Hierarchical nanotwins in Fe27Co24Ni23Cr26 high-entropy alloy subjected to high strain-rate Hopkinson bar deformation

After 70% cold-rolling at 25 degrees C and annealing at 1100 degrees C for 1 h, an Fe27Co24Ni23Cr26 high-entropy alloy had a bimodal-grained structure with two peak size distributions of 43.3 and 105.2 mu m. Both annealing microtwins and annealing nanotwins can be found in the small and large grains. The thickness of the annealing microtwins was in the range of 10-30 mu m, while that of annealing nanotwins was 5-150 nm. The samples were separately deformed under a Hopkinson bar compression system at strain rates of 2500 and 7000 s(-1) at 25 degrees C. The higher strain-rate caused an increase in the flow stress of up to 48%, from 992 to 1470 MPa, when the strain was 0.4. In 7000 s(-1) deformed-samples, original strip-like annealing-nanotwins of 10-30 nm thickness were refined by profuse single-variant deformation-nanotwins of 1-2 nm thickness. Moreover, the ultra-fine annealing-nanotwin of 5.2 nm thickness was revealed to be further partitioned by two variants of deformation-nanotwins of 0.6 nm thickness. The hierarchical structures, composed of micrometer, nanometer and sub-nanometer twins, are herewith reported.