Effects of Al addition on structural evolution and mechanical properties of the CrCoNi medium-entropy alloy

In this study, we synthesized a series of (CoCrNi)100-xAlx (x = 0–30 at.%) medium-entropy alloys (MEAs), investigated alloying effects of Al element on microstructure and mechanical properties of CrCoNi MEA, then analyzed the strengthen mechanism caused by Al element. Microstructural evolutions were evaluated by means of X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The microstructures of (CoCrNi)100-xAlx MEAs evolved from single face-centered cubic (FCC) (x < 12 at.%), to duplex FCC plus body-centered cubic (BCC) (12 ≤ x < 22 at.%), then to double BCC (x ≥ 22 at.%) structures with Al content increase. When Al content increase to 16 at.%, BCC phase decomposed into plate-like intervened structure with ordered and disordered BCC phases (B2 and A2 structure) by spinodal decomposition mechanism. When Al content higher than 22 at.%, spinodal structure evolve from plate-like intervened structure to spherical particles dispersed in matrix structure. The hardness of Alx alloys increased from HV170 to the maximum of HV700 and the compressive yield strength rose form 204 MPa–1792 MPa. The solid-solution strengthening of Al element and formation of hard BCC phase were the two main factors contributing to the strengthening effect in this alloy system. Experimental results showed that the duplex FCC plus BCC structure can significant enhanced the strength-ductility synergy of CrCoNi MEA. It is especially noted that Al19 alloy which consist of 40% volume fraction FCC and 60% volume fraction BCC exhibits the most attractive properties accompany with compromise compressive yield strength, fracture strength and facture strain of 1226 MPa, 2542 MPa and 21.97%, respectively, and the corresponding hardness of 494 HV.