The effects of substitution of Co with Ni on microstructure, mechanical properties, and age hardening of Co1-xCrFeNi1+xTi0.3 high-entropy alloys

In this study, we investigated the effects of substitution of Co with Ni on the microstructure, mechanical properties, and age hardening of three Co1-xCoFeNi1+xTi0.3 high-entropy alloys (x values describe molar ratios; where x = 0, 0.5, 1, denoted as X-0, X-0.5, and X-1, respectively). All alloys after hot rolling at 1200 degrees C showed a simple face-centered cubic phase with an average grain size of 90 mu m. As the value of x increased, the hardness, yield stress, and ultimate tensile strength of the alloys also increased from HV263, 540 MPa, and 772 MPa to HV335, 672 MPa, and 982 MPa, respectively. However, the elongation decreased from 60% to 37% because of the more negative mixing enthalpy (Delta H-mix) of Ni with other constitutional elements, resulting in a simultaneous increase in the strength and brittleness of the grains. The aged alloys X-0 and X-0.5 to X-1 showed optimal age hardening at 700 degrees C and 600 degrees C, respectively, which were associated with the precipitation of profuse sigma + eta phases. For the three alloys, the sigma and eta phases completely dissolved into the matrix at 1000 degrees C and 1100 degrees C, respectively. After age treatment of the alloys at 1200 degrees C, the average grain diameter of the X-0 to X-1 alloys increased from 216 mu m to 324 mu m, indicating that alloys with larger mixing entropy suppressed grain growth at high temperatures because of the sluggish diffusion effect. Application of the Hall-Petch equation (H = H-0 + k(H) d(-1/2)) revealed that with increases in x value, H-0 and k(H) increased from HV36 and 2236 to HV62 and 2586, respectively. The more negative Delta H-mix of Ni with other constitutional elements also explains this phenomenon.