Microstructure and formation mechanisms of nanotwins in the shear band in a FeCoNiCrMo0.2 high-entropy alloy

Abstract

High-entropy alloys (HEAs) containing nanotwins exhibit simultaneous high strength and great plasticity. The instantaneous formation mechanism of nanotwins in a shear band in the hot-extruded FeCoNiCrMo0.2 HEA has been investigated. A series of material characterization and dynamic-loading experiments were carried out to study microstructures and dynamic-mechanical behavior of this alloy. The FeCoNiCrMo0.2 HEA is a single face-centered cubic (FCC) crystallographic phase and has coarse twins with sizes of about 20 μm. A shear band with a width about 25 μm appeared in the FeCoNiCrMo0.2 HEA at the nominal strain of 19.60. The shear band consists of elongated sub-grains, nanotwins, and equiaxed grains. Some elongated sub-grains are composed of nanotwins lamellae. The shear band containing nanotwins has a high strength of about 1500 MPa and good plasticity, and the true strain can reach 3.6. Moreover, nanotwins mainly exist in a lamellar form, and the thickness of these lamellae ranges from 1 nm to 7 nm. Nanotwins in the shear band are of a single FCC phase with Mo-poor contents at twin boundaries, and the twin plane is (111). Twins in the shear band and the matrix have the same orientation but different lattice constants. Severe shear deformation plays a key role in the instantaneous formation of deformation nanotwinning in the FeCoNiCrMo0.2 HEA.