Effect of Re Content on Mircrostructure and Stress Rupture Properties of DD15 Single Crystal Superalloy

The fourth generation single crystal superalloy DD15 with Re content of 5wt%, 6wt%, and 7wt% was cast in the directionally solidified furnace, while other alloying elements were basically the same in content. The long term aging after full heat treatment was performed at 1100 degrees C for 1000 h. The stress rupture tests of the alloy were conducted under 1100 degrees C/137 MPa. The effect of Re content on the microstructure and stress rupture properties of the alloy was investigated. The results show that the size of gamma' phase decreases, the volume fraction and cubic degree of gamma' phase slightly increase with increase in Re content. After long term aging, the gamma' phase coarsens, and the drafting rate, precipitate rate and volume fraction of TCP phase are all improved with increase in Re content. With the increase in Re content, the microstructure stability and the stress rupture life of the alloy decline significantly. The three ruptured specimens all exhibit the presence of TCP phase. The amount of TCP phases increases greatly with increase in Re content. TCP phase can be the site of crack initiation. This is the main reason for the decrease in stress rupture life of the alloy with increase in Re content. The dislocation networks formed at the gamma/gamma' interface of ruptured specimen turn denser with increase in Re content. Re has a strong segregation tendency in gamma matrix. The partition ratio of Re rises significantly with increase in Re content. The lattice misfit of the alloy becomes larger toward negative with increase in Re content.