Microstructure And Properties Of The Third Generation Low-cost Single Crystal Nickel-Base Superalloy

In this dissertation, the microstructure of both as-cast and heat treated condition, creep behavior at different temperatures and stresses, the microstructure evolution during the long-term thermal exposure were investigated in DD90 single crystal superalloy. The results show that:The morphology of the as-cast single crystal alloy shows a dendrite structure. Compositional segregation analysis of the branches and the dendrite in as-cast state show that: Al, Ta is rich in the interdendritic regions and W, Re rich on the branches of the dendrites. The morphologies of script and flower form have been formed for the eutectic structure.When solid-solution temperature increased to 1320℃, initial melting in the alloy occurred. Through observing the structure of different processing dissolution, Determine the optimal processing dissolution isl310℃/8h+1315℃/16h+1320℃/24h,AC. Microstructures and stress rupture properties are influenced by heat treatment obviously. The average size and squareness of γ' phase increases with primary aging temperature increasing. The suitable size and distribution of γ' phase was obtained under the condition of 1310°C/8h+1315℃/16h + 1320℃/24h,AC+1180℃/4h,AC+870℃/24h,ACthe morphology of γ' particle were influenced by the aging time, temperature and cooling rate. When the aging temperature is above 1200℃ and cooled at the air cooling condition, the secondary γ' phase can precipitate within the matrix channel. Increasing the aging temperature or prolonging the aging time, the number and size of secondary γ' phase are increased. But under furnace cooling the secondary γ' phase can not precipitate within the matrix channel. Under different temperature and stress test, the properties of DD90 and the Larson-Miller curve during the thermal exposure of DD90 at 900℃, the size of γ' particle increases with the prolongation time, γ' particle keep cubic shape until 1000h. The growth rate of γ' particles at1000°C is higher than that at 900 "C. After 500h, y' particles begin to link each other and rafts. No TCP phase is found during thermal exposure at 900 °C and 1000°C, which indicates that DD90alloy has good thermal stability.