Strengthening of CoCrNi Medium Entropy Alloy with Ti Additions

A series of (CoCrNi)100-xTix (x = 0, 1.0, 2.0, 3.0 and 4.0) medium entropy alloys (MEAs) was manufactured by high-vacuum arc melting in the present study to examine the effects of Ti additions on the microstructures and mechanical properties of the MEAs. For x = 0, 1.0 and 2.0, the alloys remained a single-phase FCC solid solution. The Ti(Ni, Co)3 precipitates of HCP phase formed for x = 3.0 and 4.0. The presence of Ti, which had the larger atomic radius than the other constituent elements, in the solid solution enhanced the lattice distortion and resulted in grain refinement. The addition of Ti led to the enhanced mechanical properties with the ultimate tensile strength and Vickers hardness increased from 866 MPa and 212 HV at x = 0 to 1490 MPa and 409 HV, respectively, at x = 4.0. The strengthening mechanisms of solid solution strengthening, grain refinement and precipitation strengthening were discussed, and the predicted strengthening was compared with measurements.