Annealing effect on the phase stability and mechanical properties of (FeNiCrMn)_(100−x)Co_x high entropy alloys

The effect of annealing on the phase stability and mechanical properties of (FeNiCrMn)_(100−x)Co_x (x = 5, 10 and 20, denoted as Co_x hereafter) multi-component alloys referred to as high entropy alloys (HEAs) were investigated through annealing at 850 °C for different durations. The phase stability analyses were performed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Mechanical properties were characterized using hardness and tensile tests. All the investigated alloys exhibit a single face-centered cubic (FCC) phase in the as-cast state. Cr-rich sigma phase precipitated in the Co₅ and Co₁₀ alloys after being annealed for 48 h, while the Co₂₀ alloy remains a single FCC phase. With precipitation of the hard yet brittle sigma phase, the hardness and yield strength of Co₅ and Co₁₀ alloys increase while the tensile elongation decreases. Compared with the Co₅ alloy, the variation of hardness, yield strength and tensile elongation is smaller for the Co₁₀ alloy due to a smaller volume fraction of sigma phase. The hardness and yield strength of Co₂₀ alloy show a subtle decrease, however the tensile elongation remains almost unchanged. The present result could help to understand the phase stability and mechanical properties of this typical Co-Cr-Fe-Ni-Mn HEA system from compositional perspective.