Effect of Co on microstructure and properties of iron-rich AlCoxCrFe2.5Ni (x = 0, 0.2, 0.5, 1.0) high entropy alloys

Abstract

To develop cost-effective and thermally stable iron-rich AlCoCrFeNi system high entropy alloys, this study systematically investigated the microstructure, mechanical properties, and phase stability of AlCo_xCrFe_2.5Ni (x = 0, 0.2, 0.5, 1.0) high entropy alloys. For x = 0, 0.2, and 0.5, the alloy exhibits a dendritic morphology. In the dendrite region, the matrix is the A2 (disordered BCC) phase, and the B2 (ordered BCC) phase exists as particles. In the interdendritic region, the A2 and B2 phases are arranged in a weave-like pattern. Notably, increasing the Co content reduces the dendritic area, and the dendritic morphology disappears entirely at x = 1.0. After annealing at 600 °C, 800 °C, 1000 °C, and 1200 °C for 5 h, it was found that lowering the Co content enhances phase stability, suppresses FCC phase precipitation, and mitigates high temperature softening. CALPHAD phase diagram calculations further confirm that when the alloy is Co-poor, the A2/B2 structure can be retained over a wide temperature range. When x = 0, the alloy achieves a high yield strength (1205 MPa) and compressive ductility (>50 %) at room temperature, while retaining good strength below 600 °C. The B2 particles have a remarkable strengthening effect. As the Co content decreases, the area of the high-strength dendritic regions containing B2 particles expands, which is the primary mechanism for the alloy's strength retention.