Characteristics and formation mechanism of Hf inclusions in directionally casted Ni-based superalloy

Scanning electron microscopy, transmission electron microscopy and energy disperse spectrum were used to analyze the morphologies, microstructures and compositions of Hf-rich inclusions in DZ125 and DZ22 Ni-based superalloys. The solidification microstructures of Ni-based alloys with different Hf additions were investigated by simulation experiments, and the formation mechanism of Hf inclusions was clarified. The results indicate that there are irregular inclusions on the edges of DZ125 and DZ22 alloys. The white inclusions are HfO2 phase, and the accompanying gray inclusions mainly comprise Al2O3. The Hf has a strong tendency to accumulate in the alloy, and large size Hf-rich phase of above 100 mu m can form in the alloy when the Hf content is high. With the decrease of Hf content, the Hf-rich phases become smaller and distributed more evenly. At high temperature, Hf has a high strong driving force of spontaneous reaction with oxygen in the alloy melt and ceramic shell/core oxides, leading to the formation of HfO2 inclusions on the casting surface. When the dispersion of Hf is insufficient, Hf tends to segregate at the interface of oxide ceramic phases and melt, reacting with Al2O3 in the ceramic and SiO2 in the silica sol binder at high temperature, resulting in the formation of HfO2 inclusions on the casting surface.