Effect of overtemperature exposure on the microstructure and mechanical properties of a DD5 single crystal superalloy

To simulate the overtemperature service environment of aeroengine turbine stator vanes, thin plate specimens of second-generation single crystal (SX) superalloy DD5 after complete heat treatment were exposed for 1 h, 10 h, 50 h, and 100 h without load at 1100 °C and 1150 °C and then subjected to air cooling treatment. The microstructure (γ′ precipitates, γ matrix, carbides, topological dense phases (TCP)) of the samples under different conditions was analysed by scanning electron microscopy (SEM), and the mechanical properties of the processed samples were studied. The results indicate that the microstructure DD5 alloy was generally stable after thermal exposure at 1100 °C and 1150 °C, and no TCP phase was found after 100 h of exposure. After exposure to 1100 °C, the average size of the γ′ precipitates reached 0.90 µm after 100 h, and some of of the the γ/γ′ interfaces became serrated. At 1150 °C, the average size of the γ′ precipitates reached 1.05 µm after 100 h, and the third γ′ precipitated substantially in the alloy. For both the 1100 °C and 1150 °C exposures, the 870 °C tensile strength increased after 1 h and subsequently decreased and was 0.5% and 5.0% greater after 100 h than that of the standard heat treatment (SHT) samples, respectively. After 100 h of exposure, the rupture life at 980 °C and 250 MPa was 5.9% lower and 1.0% greater than that at 1100 °C and 1150 °C, respectively; the rupture life at 1070 °C and 137 MPa was 1.6% greater and 20.1% less than that at 1100 °C and 1150 °C, respectively; and the rupture life at 1093 °C and 158 MPa was 42.7% and 51.2% less than that at 1100 °C and 1150 °C, respectively.