The precipitation behavior of γ′ precipitates in typical section dimensions of DD6 single crystal superalloy turbine blade was investigated experimentally during directional solidification process. The phase transformation temperatures in the single crystal Ni-based DD6 superalloy from DSC analysis and JmatPro simulation were basically in consistent with the isothermal solidification experiments. The solidification route of DD6 single crystal superalloy could be described as follows: L1 → γ + L2; L2 → (γ + γ′)eutectic + MC; γ → γ′/γ. With increasing continuous cooling rates, the primary γ′ precipitates tended to be refined, and the size distributions of the primary γ′ precipitates at every temperature measuring position followed the normal distribution. In comparison to the interdendritic regions, nearly a 60% reduction in the average sizes of the primary γ′ precipitates was measured in the dendritic core regions. The result of the primary γ′ size difference was strongly affected by the multi-component segregations between the interdendritic and dendritic regions, where the γ′ forming elements of Al and Ta segregated towards the interdendritic regions. Furthermore, the secondary γ′ precipitation was found to occur within a relatively wide corridor of γ matrix for low cooling rates (12.6, 23.3 and 29.7 °C/min) during the directional solidification process. The occurrence of the secondary γ′ precipitation resulted from the complex interaction of multiple thermodynamic and kinetic factors in the γ′ nucleation and the diffusion rate of γ′ forming elements.

The microstructural evolution of DD9 single crystal superalloy turbine blade was studied after heat treatment. In comparison to the as-cast microstructures where the sizes of the γ′ precipitates have an obvious difference between the dendritic core and interdendritic regions, the γ′ sizes of the heat-treated microstructures tend to be uniform and more cubic. And in the heat-treated microstructures, the γ′ sizes and the related size dispersion degrees of the dendritic cores are slightly increased, while those of the interdendritic regions are obviously decreased. After all, all the γ′ sizes follow the normal distribution law. With the raise of section thickness, the γ′ sizes tend to increase and the related size dispersion degrees are enhanced during the cooling process after heat treatment, and the γ-γ′ eutectics are dissolved, left little residual eutectics and pores.