Cryo-rolling and annealing-mediated nano/ultrafine structure, texture, and properties of extremely low stacking-fault energy high entropy alloys: Comparative perspectives

Abstract

The effect of heavy (∼90% thickness reduction) cryo-rolling (CryoR) on microstructure, texture, and tensile properties of an extremely low stacking fault energy (SFE) Co20Cr26Fe20Mn20Ni14 high entropy alloy (HEA) was investigated. Comparisons with other heavily cold- (CR)/cryo-rolled materials were also presented to highlight the effect of CryoR. CryoR resulted in the formation of heterogeneities, shear bands, deformation twins, stacking faults, and deformation-induced nano-lamellar structure with a spacing of 35 ± 8 nm. Remarkably, the nano-grain size after CryoR was smaller than other low SFE alloys and achieved at a much lower strain level. These outcomes indicated the combined effect of CryoR, massive solid-solution formation, and extremely low SFE towards extreme grain refinement. CryoR also resulted in the development of a strong brass texture ({110}<112 >), similar to other low SFE alloys. Annealing resulted in recrystallization, precipitation, and dissolution of Cr-rich tetragonal σ phase precipitates in the CryoR material at lower temperatures due to finer nanostructure and greater stored energy for transformation. Grain growth was inhibited up to 950 °C by the presence of the σ precipitates but was triggered beyond that by the dissolution of the precipitates. Annealing textures retained weak α-fiber (normal direction (ND)//<110>) components and a high fraction of random components, which resembled other low SFE HEAs. Meanwhile, ultrafine microstructures and multistage strain-hardening resulted in a remarkable ambient temperature strength-ductility (1025 MPa-20%) combination in the CryoR and annealed HEA.