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

Abstract

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 °C showed a simple face-centered cubic phase with an average grain size of 90 μm. 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 (ΔHmix) 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 °C and 600 °C, respectively, which were associated with the precipitation of profuse σ + η phases. For the three alloys, the σ and η phases completely dissolved into the matrix at 1000 °C and 1100 °C, respectively. After age treatment of the alloys at 1200 °C, the average grain diameter of the X-0 to X-1 alloys increased from 216 μm to 324 μm, 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 = H0 + kH d−1/2) revealed that with increases in x value, H0 and kH increased from HV36 and 2236 to HV62 and 2586, respectively. The more negative ΔHmix of Ni with other constitutional elements also explains this phenomenon.