Effects of Ti on the ageing microstructure and mechanical properties of Ni-Co-Cr-Ti multi-principal element alloy

We investigated the impact of Ti addition on the microstructure, tensile properties, and deformation mechanisms of the Co50Cr30Ni20 multi-principal element alloy. Our findings show that different heat treatments yield varied microstructures, including combinations of incoherent precipitates at grain boundaries with coherent precipitates within grains, mixtures of lamellae in discontinuous precipitation zones and coherent precipitates in continuous zones, and configurations of a Widmansta<spacing diaeresis>tten microstructure with cuboidal precipitates. Among all aged specimens, the strongest alloy exhibits a yield strength of 1193 MPa, ultimate tensile strength of 1535 MPa, and notable elongation of 17%. Detailed microstructural characterization reveals that Ti addition effectively suppresses the formation of the brittle hexagonal close-packed phase, while the uniform distribution of nanoscale L12 phase particles within grains contributes to grain refinement and dislocation pinning, enhancing the alloy's strength. Furthermore, the development of stacking fault networks introduces an additional strengthening mechanism via the dynamic Hall-Petch effect. Together, these mechanisms are essential for improving strainhardening capacity and ultimately enhancing the alloy's ductility.