Formation and deformation mechanisms in gradient nanostructured NiCoCrFe high entropy alloys upon supersonic impacts

Bulk large-gradient nanostructured NiCoCrFe high-entropy alloys with millimeter-scale gradient layer were prepared by supersonic Taylor impact. The microstructure evolution over a wide range of strain rates (10(-3)-10(5)/s) was characterized, and the formation mechanism of nanocrystals with well-defined boundaries at high strain rate (> 10(4)/s) was identified to be the emerging, thickening, and mutual intersection of lattice rotation bands, high-density dislocation bands, and deformation twins. This Letter not only reveals the deformation mechanisms of high entropy alloys under a wide range of strain rates but also provides an idea that could be applied to the preparation of bulk gradient nanostructured materials.