Dislocation dissociation assisted formation mechanism of sigma phase and its impact on producing heterogeneous lamellar microstructure in CoCrV medium-entropy alloy

Abstract

Control of topologically close-packed sigma phase, meaning limiting its massive presence to avoid embrittlement but benefiting its refinement and strengthening effect, is of particular interest. In-depth knowledge of dislocation-associated formation mechanisms is needed but not well addressed. In this work, an FCC-phased Co_66.66Cr_16.67V_16.67 medium entropy alloy (MEA) with a propensity to form the sigma phase at non-equilibrium conditions was studied. The alloy was conventionally cold-rolled and heat-treated. The dislocation activity rooted formation mechanisms of the sigma phase were thoroughly characterized and evidenced by in-situ and ex-situ multi-scale diffraction techniques. It was revealed that nano-sized sigma particles enriched in Cr and V and depleted in Co were precipitated ultra-rapidly and uniquely during the heating process after the cold-rolling. The precipitation is spatially inhomogeneous, mainly in the severely deformed regions. The ultra-rapidity of the precipitation was achieved by the segregation of the Cr and V atoms via crystal defect-aided diffusion for composition change and by structure transformation via dislocation dissociation. The similarity of the atomic arrangement of the partial dislocations to that of the {001} sigma planes provides favorable structure transformation stimulus. In consequence, the orientations of the intensively activated dislocation slip planes dictated those of the sigma {001} planes via the FCC {111} to sigma {001} heredity, leading to the specific sigma texture. Owing to the spatially inhomogeneous precipitation, a heterogeneous lamellar microstructure was formed, composed of alternatively distributed fine dual-phased layers and coarse single-phased layers. This work provides comprehensive information on the dislocation-dissociation-assisted formation mechanism of sigma phase.