Effect of cobalt on the mechanical properties of 718 base alloy with 1.2 wt% Al

In this paper, the effect of Co (0–5 wt%) on the mechanical properties of 718 alloy and its strengthening mechanism were studied to clarify the function of Co on the mechanical properties of 718 base alloy. The experimental results showed that the addition of Co refined the grain. After the heat treatment, the addition of Co promoted the precipitation of Laves phase and γ′ phase, but inhibited the precipitation of γ″ phase. With the increasing of the Co content, the tensile strength was increased first and then decreased. It was found that the increase in the tensile strength of the alloy was caused by refining the grains and promoting the intergranular precipitation of the Laves phase. Nevertheless, Co increased the volume fraction of intragranular γ′ phase, while suppressed the precipitation of intragranular precipitation of γ″, thus the intragranular strength of the alloy was reduced. Accordingly, the alloy with 3 wt% Co shows the highest yield strength among the experimental alloys. Besides, the Co content extent 3 wt% will show a yield plateau under room temperature tension, but not in the case of 650 °C tension. It was revealed by analyzing the TEM morphology of the Laves phase in the room-temperature tensile samples and 650 °C tensile samples that the dislocation could shear through the intergranular Laves phase under the 650 °C tensile test, but it was not the case under room temperature tensile test. Therefore, the yield plateau in the stress-strain curves is due to the intergranular Laves phase impeding the dislocation motion. •The Co addition causes peak effect on the alloy tested.•Co additions affects tensile properties by refining grains and reducing γ″ precipitation.•Laves phase causes yield plateau on room temperature stress-strain curve.