Microstructure Evolution and Stress Rupture Properties of DD6 Single Crystal Superalloy after Overheating

The second generation single crystal superalloy DD6 after standard heat treatment was respectively overheated at 1100°C, 1150°C, 1200°C, 1250°C, 1300°C, 1320°C for 1h and air cooled. The effect of overheating on the microstructure and stress rupture properties at 980°C/250MPa of the alloy was investigated. The results showed that the size of γ′ phase was slightly increased overheating at 1100°C, 1150°C and 1200°C. The size of γ′ phase had a big increase and its size distribution was very uneven after overheating at 1250°C. The small part of γ′ phase has serrated γ′/γ phase surface as a result of un-completely solution and the irregular small γ′ phase was in the majority when overheated at 1300°C. While all the irregular small γ′ phase precipitated again after completely solution when overheated at 1320°C. There was no fine second γ′ phase in the γ matrix channel of the alloy after standard heat treatment and overheating at 1320°C. But the fine second γ′ phase precipitated in the γ matrix channel after overheating at every temperature of 1100 °C~1300°C. No obvious change of the stress rupture life was found after overheating at 1100°C, 1150°C, 1200°C and 1250°C. The stress rupture life considerably reduced after overheating at 1300°C, whereas slightly reduced after overheating at 1320°C. The appearance of the raft had almost no change after overheating at 1100°C. With increasing of overheating temperature from 1100°C to 1250°C, the length of raft became shorter and the width thickening. The γ phase formed the wavy raft after overheating at 1300°C and 1320°C and the thickness of latter was larger than that of the former. Finally, the relationship between the microstructural evolution and stress rupture properties of the alloy after overheating was discussed.