Effect of Cr/Al ratio on the microstructure and mechanical properties of CoFeNiCrxAl1−x high-entropy alloys

Achieving strength-ductility and functional integration is a key issue to promote the application of alloys in the field of functional materials. CoFeNiCrxAl1−x alloys exhibit excellent magnetic and corrosion resistance properties; however, their mechanical properties remain underexplored and insufficiently understood. In this work, the microstructure and mechanical properties of CoFeNiCrxAl1−x alloys were investigated, and their intrinsic deformation mechanisms were elucidated. The results indicate that as Cr is gradually replaced by Al, the phase structure transforms from a single-phase face-centered cubic (FCC) structure to a dual-phase FCC and body-centered cubic (BCC), and finally to a BCC/B2 structure. Mechanical tests demonstrated that alloy hardness rises with higher Al content, with the Cr0Al1 alloy exhibiting a hardness approximately 3.3 times greater than that of the Cr1Al0 alloy. Notably, the Cr0.5Al0.5 alloy exhibits an optimal strength-ductility balance, with a yield strength increase in about 60% to 248 MPa and tensile strength increase in about 36% to 610 MPa, while maintaining nearly the same ductility as the Cr1Al0 alloy. The deformation mechanisms were found to be driven by solid solution strengthening due to severe lattice distortion, a high dislocation density resulting from reduced dislocation formation energy, the second-phase strengthening and interface strengthening via the micrometer-scale BCC phase, and twin-induced plasticity induced by the reduced stack fault energy. This work broadens the potential applications of CoFeNiCrxAl1-x alloys as versatile engineering and magnetic functional materials.