Influence of Co content on the microstructures and mechanical properties of a Ni–Co base superalloy made by specific additive manufacturing process

Microstructures and tensile properties of a Ni–Co base superalloy containing different Co contents made by additive manufacturing (AM) after standard heat treatment have been investigated. Microstructure observation by scanning electron microscopy (SEM) reveals that a great deal of η precipitates form in the interdendritic region and distribute linearly along the columnar microstructure in 5Co alloy. In addition, transmission electron microscopy (TEM) results reveal that spherical γ′ phase can be found in 5Co alloy and nearly cuboidal γ′ phase can been observed in 23Co alloy. The different morphologies of the γ′ precipitate can be attributed to the different γ′ compositions and the differences in γ/γ′ lattice misfit in the two alloys. The results of tensile tests reveal that, when tested below 500 °C, the ultimate strength and the elongation of the alloys made by AM are lower than that of the cast & wrought (C&W) alloys. When tested at 750 °C and 800 °C, alloys made by AM exhibit higher yield strength and superior plasticity. With the increment of the testing temperature, deformation mechanism of the alloys transforms from dislocation glide to stacking fault shearing and deformation twinning. Increasing of Co content can decrease the stacking fault energy (SFE) and facilitate the initiation of twinning, therefore, deformation mechanisms between the 5Co alloy and 23Co alloy made by AM are of notable difference.