Effect of boron on the microstructure and mechanical properties of as-cast and annealed CrFeNi medium-entropy alloys

A Co-free Bx(CrFeNi)100-x (x = 0, 0.25, 0.5, 1.0, 1.5, 2.0, 3.0, 4.5, and 6 at%, denoted as Bx alloys hereafter) medium-entropy alloys (MEAs) system was designed to investigate the effect of boron (B) content on the microstructure and mechanical properties of as-cast and annealed CrFeNi MEA. The as-cast Bx(CrFeNi)100-x MEAs exhibited an FCC + Cr2B dual-phase structure and dendritic morphology, whose volume fraction gradually increased with an increase in B content. The addition of B element promoted the improvement of yield strength due to the synergistic effect of solution strengthening and precipitation strengthening, but caused an obvious reduction in elongation. Then B0 and B0.25 alloys experienced the subsequent cold rolling and recrystallization annealing process. Compared with B0 alloy, B0.25 alloy in all states exhibited a considerable improvement in yield strength but only a slight decline in elongation. Thereinto, B0.25 alloy annealed at 800 °C presented a favorable strength-plasticity configuration with a yield strength of 745 MPa and an elongation of 24.32%, which profited from the refinement strengthening caused by the inhibition of B element on recrystallization and the precipitation strengthening caused by the formation of BCC-structured precipitates and Cr2B borides. These findings provide a deep insight into the design and optimization of MEAs as structural applications.