Ultrafine lamellar microstructures for enhancing strength-ductility synergy in high-entropy alloys via severe cold rolling process

In the present study, the effect of severe cold rolling (SCR) on the microstructure and mechanical characteristics of X0.25FeCoNiMnV (X: Al, Ti) high-entropy alloys (HEAs) specimens was investigated. It was observed that the as-cast and homogenized specimens exhibited a two-phase microstructure consisting of face-centered cubic (FCC) and a minor body-centered cubic (BCC) structure. Electron backscattered diffraction (EBSD) analysis indicated that SCR could effectively induce a strong rolling texture with stretched grains along the rolling direction and lamellar deformation bands as the major microstructural features for all the HEAs specimens. Moreover, a large number of nano-scale grains are formed within the coarse-stretched microstructure of Al0.25FeCoNiMnV and Ti0.25FeCoNiMnV HEAs specimens due to local fragmentation, resulting in a bimodal grain structure. After 85 % cold rolling, the fraction of high angle grain boundaries and mean misorientation angle of the boundaries are 54 % and 22.15 degrees in the FeCoNiMnV specimen, while they are 61 % and 26.42 degrees for Al0.25FeCoNiMnV, and 57 % and 28.53 degrees for Ti0.25FeCoNiMnV. Ti0.25FeCoNiMnV specimen exhibited the best combination of strength-ductility (tensile strength of 1225 MPa and elongation of 26 %) compared to FeCoNiMnV (720 MPa and 15 %) and Al0.25FeCoNiMnV (970 MPa and 18 %). Observations revealed that the failure mode of HEAs specimens was a ductile type fracture with a combination of deep and shallow dimples. The findings revealed that SCR can effectively improve the strength and microhardness of HEAs, while it highly retains their elongation owing to the formation of a bimodal structure consisting of new grain boundaries and dislocation substructures.