Effect of grain boundary engineering on grain boundary character distribution and deformation behavior of a TRIP-assisted high-entropy alloy

Abstract

This study carried out two-step thermo-mechanical processing (TMP) in grain boundary engineering (GBE) to optimize the grain boundary character distribution (GBCD) and tensile properties of a transformation-induced plasticity-assisted high-entropy alloy (TRIP-HEA). The evolution and mechanism of GBCD in the HEA were characterized and discussed using electron backscatter diffraction (EBSD). The tensile properties of the HEA after undergoing different TMPs were evaluated through room-temperature uniaxial tensile tests, and the influence of GBE on the deformation mechanisms was investigated using techniques such as EBSD and transmission electron microscopy (TEM). The results indicated that the fraction of low Σ coincident site lattice (CSL, Σ ≤ 29) boundaries exhibited a decreasing trend followed by an increasing trend with an increase of second-step cold rolling strain during TMPs, and the overall fraction of Ʃ9 + Ʃ27 boundaries showed an increasing trend. When the reduction ratio was 10%, the low-ƩCSL boundaries fraction reached ~66.35%. In terms of tensile properties, all GBE specimens showed improvement in strength and ductility. When the reduction ratio of second-step cold rolling was 20%, the yield strength was 506.7 MPa, which was ~59% higher than base material (BM) sample. The ultimate tensile strength was 885.6 MPa, ~26% greater than BM sample. In addition, when the reduction ratio of second-step cold rolling was 3%, the total elongation was 52.6%. The main deformation mechanisms of the HEA were dislocation slip, stacking fault (SF), nano-twining, and phase transformation. The optimization of GBCD had multiple effects on the deformation mechanisms. It manifests in several ways, such as the enhancement of strain uniformity through ΣCSL grain boundaries, improving the ability of dislocation multiplication and stress-induced cracking resistance, as well as the strengthening due to the combined effects of phase transformation and grain refinement.