Insight into the FCC→HCP Transformation in Co-Rich Co-Cr-Fe-Mn-Ni High-Entropy Alloys

The existence of an HCP phase in FCC-type high-entropy alloys can improve the alloy's mechanical properties. In many cases, an HCP phase is induced by deformation. In the present work, an FCC to HCP transition was detected during the cooling of Co1.5CrFeMnNi0.5 and Co1.75CrFeMnNi0.25 alloys. Therefore, arc-melted annealed CoxCrFeMnNi2-x (x = 0.25-1.75) alloys that were then subjected to long-term vacuuming were investigated using XRD, DSC, HT-XRD, thermodynamic calculation, and first-principle calculation. It was confirmed that the FCC to HCP transition occurred at similar to 450 degrees C during the cooling of the alloys with x >= 1.5. The volume fraction of the HCP phase increased with Co content. It was proven that the HCP phase was not stable above 600 degrees C. First-principle calculations further indicated that the HCP structure was more stable than the FCC structure for Co1.75CrFeMnNi0.25 alloy, and there was a likelihood of an FCC to HCP transition. Moreover, experimental tests confirmed that the microhardness of the Co1.75CrFeMnNi0.25 alloy reached 213 HV because it contained a substantial HCP phase. This value is much higher than those of other non-HCP-containing alloys, either in their as-cast states or after annealing. These results provide guidance for the design of FCC-type high-entropy alloys with desirable mechanical properties through HCP phase strengthening.

Automated summary

The presence of HCP phase in FCC-type high-entropy alloys can enhance their mechanical properties, which is usually induced by deformation. In this study, CoxCrFeMnNi2-x alloys were investigated after vacuum treatment, and it was observed that FCC to HCP transition occurred during cooling for alloys with x ≥ 1.5. The volume fraction of HCP phase increased with Co content, but HCP phase was not stable above 600°C. Experimental tests showed that the Co1.75CrFeMnNi0.25 alloy had a microhardness of 213 HV due to the substantial HCP phase present, significantly higher than other non-HCP-containing alloys. These findings offer guidance for designing FCC-type high-entropy alloys with desirable mechanical properties through HCP phase strengthening.