Mechanical Properties and Microstructure of the Shear Band Formed at Cryogenic Temperature in the NiCrFe Medium-Entropy Alloy

There are nanotwins in the shear band formed in a moment (about 10-5 s) in some NiCrFe-based medium-entropy alloys (MEAs), and these shear bands can be recognized as a special kind of materials due to their high strength and good plasticity. In this study, the single shear band of the NiCrFe MEA was prepared at 77 K. A series of characterizations were carried out to analyze the microstructures in the shear band. The strength of the shear band was investigated by the split Hopkinson pressure bar and in-situ compression. The micropillar in the shear band containing nanotwins exhibits excellent strength-plasticity synergy. The compressive yield strength of the shear band measured by in-situ compression is 175% higher than that of the matrix, reaching 1405 MPa, with the fracture strain exceeding 0.5. The strengthening mechanism of the shear band was revealed by the combination of the experimental results and molecular dynamics simulation. The synergistic effect of multiple strengthening mechanisms enhances the strength of the NiCrFe MEA containing nanotwins, in which the grain boundary strengthening of the ultrafine equiaxed grains and the dynamic Hall-Petch effect of the nanotwins dominate. In addition, the good plasticity of the shear band is ascribed to the stress concentration reduction of the twin boundaries of nanotwins and the activation of multiple slip systems due to the randomly oriented nanotwins. These findings provide theoretical guidance for the design of nanotwinned MEAs to realize excellent strength-plasticity synergy for structural materials.