Investigation on the ballistic induced nanotwinning in the Mn-free Fe27Co24Ni23Cr26 high entropy alloy plate

Mn-free Fe27Co24Ni23Cr26 high-entropy alloy (cold-rolled 40% and then annealed at 800 °C) was subjected to ballistic deformation (strain rate > 106 s−1) at room temperature. The microstructural examinations of the bulletproof plate were focused on three regions (labeled I, II and III) starting from the no-deformation region to the heavy deformation region adjacent to the bullet hole. In region I, lamellar annealing nanotwins were found in the nearly recrystallized grains, and the boundaries of annealing nanotwins had accumulated dislocations, presumably created by prior cold-rolling. The intermediate deformation region was full of complex nanotwin structures and a high density of dislocations. The intersection block structures created by the boundaries of annealing and deformation nanotwins significantly confined the dislocation movement. Alternatively, in the heavy deformation region, besides the typical intersection structures of deformation nanotwins, the boundaries of annealing and deformation nanotwins were found to be significantly deviated from the ideal twin orientation, presumably due to successive deformation during the course of ballistic deformation. Moreover, high-resolution transmission electron microscopy (HRTEM) clearly revealed that bundles of stacking faults were created at the intersections of two bundles of deformation nanotwins.