Twinning and precipitation strengthening in low stacking fault energy CoCr1.3FeNi0.7MnNbx (x=0.3, 0.367, 0.45) eutectic high entropy alloys upon cold forging and annealing

We report the strategy to enhance the mechanical properties of eutectic CoCr1.3FeNi0.7MnNbx (x = 0.3, 0.367, 0.45) high entropy alloy (EHEA) comprising of ultrafine lamellae of low SFE face-centered cubic (FCC) and hard Laves phase. The impact of forging up to 20 % plastic strain at room temperature followed by annealing at 500 degrees C and 850 degrees C on the evolution of microstructure and mechanical properties of EHEAs were investigated. Such cold forging induces high dislocation density (1.5 x 1015/m2) and twin density (7.8 x 106/m) in the low SFE FCC phase causing lamellae bending and their fragmentation. Furthermore, nano-sized sigma-phase precipitated in the FCC phase of cold forged (CF) and annealed EHEA. The refinement of microstructure, formation of dislocation substructure, thermally stable deformation twins, and controlled precipitation of sigma-phase (3 vol%) reduce the mean free path (Lambda) for dislocation glide and provide a good combination of high strength of 1389-1537 MPa and large fracture strain of 11%-13 %. The preexisting twin lamellae in CF specimens act as a site for dislocation nucleation and storage providing good plastic strain under compression. Nanoindentation and TEM studies have revealed the high strain rate sensitivity (m = 0.0105) and low activation volume (V*=13b3) in CF and annealed EHEAs, pointing the dislocation glide, dislocations interactions with lamellae interface, sigma-phase precipitates, and twin boundaries are the dominant deformation mechanisms leading to strong hardening. The synergistic effect and the individual contributions of precipitation strengthening (sigma orowan) and Hall-Petch strengthening (sigma int) on the superior strength in CF and annealed EHEAs are explored.