Evolution of TCP Phase During Long Term Thermal Exposure in Several Re-Containing Single Crystal Superalloys

The application and component designs of single crystal superalloys are restricted by the precipitation of topologically closed packed (TCP) phases, which can deteriorate the microstructural stability of the alloys severely. Limited researches concerning the type and morphology evolution of TCP phases under elevated temperature conditions have been reported previously. In the present work, three Re-containing single crystal alloys were designed to investigate TCP phase evolution via long term isothermal exposure tests at 1120 °C while the effects of Re on the microstructural characteristic and elements segregation were also clarified. The results showed that the addition of Re increased the instability of the alloys and the volume fraction of the TCP phases exceeded 5 vol% when the Re content reached 3 wt%. The increasing Re content had also raised the precipitation temperature of TCP phases but it did not change the type of them after long term aging; all the TCP particles were identified as μ phase in this study. Moreover, the elements segregation became considerably serious as Re addition increased constantly, which brought about various morphologies of the μ phase in the experimental alloys. In particular, the rod-like and needle-like μ phases demonstrated the typical orientation within γ matrix while the blocky μ phase was dispersedly distributed in the space. No specific orientation relationship could be observed in the μ phase when the addition of Re exceeded certain threshold value.