Effect of roll speed ratio on the texture and microstructural evolution of an FCC high-entropy alloy during differential speed rolling

Abstract

A very coarse-grained (335 μm) Fe 41 Mn 25 Ni 24 Co 8 Cr 2 high-entropy alloy with a single FCC phase was cold rolling to a 80% reduction in thickness using the differential speed rolling technique with various speed ratios (SRs) ranging between 1 and 4. As the SR was increased, the volume fraction of the region of high-density micro-shear bands increased to accommodate the higher shear strain. At SR = 4, the entire thickness of the sheet was covered with micro-shear bands, and ultrafine (sub)grains with a size of 1.4 μm were uniformly formed along the shear bands. A continuous dynamic recrystallization (CDRX) mechanism occurred during rolling, and a higher SR accelerated the CDRX process. During conventional rolling (at SR=1), a brass { 110 } 〈 112 〉 orientation texture with minor components of S { 123 } 〈 634 〉 and Cu { 112 } 〈 111 〉 orientations developed. At higher SRs, shear texture developed as the main type, while the development of rolling texture was suppressed. The microstructure at SR=4 obtained after annealing at 973 K showed a fully recrystallized microstructure composed of a five times smaller grain size (4 μm) with a higher intensity of γ fiber texture compared with that prepared by conventional rolling. The samples processed with high SRs exhibited better tensile properties compared with the conventionally rolled sample in terms of strength and ductility after annealing. The current results demonstrate that by using differential speed rolling with a high SR, one can achieve a significantly finer and more homogeneous microstructure, stronger shear texture, and superior tensile mechanical properties for an FCC high-entropy alloy compared to that obtained by conventional rolling. The strength of the as-rolled and annealed samples was quantitatively explained by considering the contribution of grain size and dislocation density to strengthening.