Effect of heat treatment on the microstructural evolution and mechanical properties of GH4099 additive-manufactured by directed energy deposition

For additive-manufactured nickel-based superalloy, heat treatment is an effective method to modify the microstructure and mechanical properties. The strength and ductility of as-deposited GH4099 have been improved via heat treatment. The results indicate that the microstructure of as-deposited sample was mainly composed of epitaxial growth columnar grains. And the (γ+γ^') eutectic phase formed at the terminal solidification stage because of the segregation of Al and Ti elements. After solution treatment at 1120 °C for 2 h, the columnar dendrites were replaced by smaller equiaxed grains with twin boundaries due to the occurrence of recrystallization. The γ^' phase precipitated in the matrix after the aging treatment at 800 °C for 8 h. By comparing the tensile properties at room temperature of three states samples, it could be found that the solution treated samples had the lowest strength with the highest plasticity, while the solution-aging treated samples had the best performance with high strength and ductility. After aging treatment, though the dislocation density decreased after solution treatment, the precipitated γ^' phase could block dislocation movement to improve the strength of GH4099. Additionally, the formation of refine equiaxed grains and twin boundaries promoted ductility to some degree during the tensile process.