Effect of cobalt on the precipitation of Laves, γ′ and γ″ phases in a 718 base alloy with 1.2 wt.% Al

Cobalt is an important strengthening element in superalloys, but its effect on 718 alloy family has rarely been studied and reported, and it is difficult to understand the controversy policy for controlling the Co content in the alloys such as 718 and 718 Plus. Therefore, the function of Co on the precipitation of 718 family alloy has been investigated in a reference alloy, in which 1.2 wt.% Al was added into the 718 base composition for raising the sensitivity of precipitation to the composition variation. It was found that the Co additions favored the intergranular Laves precipitation and refined the grains. The thermodynamic calculation showed that the range of the precipitation temperature was enlarged by the Co additions, and the peak temperature of the Laves precipitation was located around 980 °C. Unexpectedly, the Co addition increased the volume fraction of γ′ while decreased the volume fraction of γ″ precipitations. It was revealed by analyzing the distribution of elements in the microstructures of the as cast experimental alloys that the Co addition increased the solubility of Nb while hardly changed the solubility of Al in the γ matrix. It is apparent that the Co addition influenced the precipitations of γ′-Ni3Al and γ″-Ni3Nb by affecting the solubility of Al and Nb in the γ matrix and the precipitates of Nb-rich and Al-poor Laves phase. Accordingly, the controlling policy of Co content in alloys 718 and 718 Plus is discussed in the text.