Effect of thermal exposure on subsurface microstructure evolution of nickel-based single crystal superalloy DD5 after milling

Thermal exposure experiments were performed on the second-generation nickel-based single crystal superalloy DD5 under different conditions, and the evolution of microstructure and mechanical properties in the milling subsurface were systematically investigated. The results show that when the thermal exposure temperature is lower than 1000 °C, the thickness of the heat-affected layer is 5–20 μm, which does not affect the performance of DD5. When the thermal exposure temperature is 1100 °C, the milling subsurface of DD5 is composed of a surface recrystallization (SRX) layer and a cellular recrystallization (CRX) layer both with lower nano-hardness than the DD5 matrix. The thicknesses of both SRX and CRX layers increase monotonically with the extension of holding time, and the thickness of the heat-affected layer (composed of SRX and CRX layers) is 20–40 μm. As the thermal exposure temperature increases to 1200 °C, the milling subsurface of DD5 is composed of an SRX structure with large size topologically close-packed (TCP) precipitates and a transition layer with refined γ/γ′ structure and TCP precipitates. The transition layer with higher nano-hardness than other heat-affected layers is formed between the recrystallization region and DD5 matrix, and its thickness increases from 44.5 μm to 66.8 μm with the extension of holding time increasing from 6 h to 24 h. The results will provide an experimental basis and a theoretical reference for the determination of removal allowance in subsequent processing and the applicable service conditions of DD5.