On the role of μ phase during high temperature creep of a second generation directionally solidified superalloy

The role of μ phase in a second generation directionally solidified Ni-base superalloy during high temperature creep has been characterized in the present work. A large amount of blocky μ phase enveloped by thick γ′-film is found at the grain boundaries and in the vicinity of primary MC carbides after heat treatment. However, the creep-rupture properties of the alloy are not affected by the presence of the blocky μ phase at 760–1070 °C under a stress of 120–850 MPa. By the investigation of creep microstructure at 975 °C/255 MPa, it is found that the thick γ′-film plays a pivotal role in toughening the grain boundaries and the interfaces between the μ phase (or carbides) particles and the matrix, and preventing the formation of cracks. Meanwhile, during high temperature creep, M23C6 carbides are continuously coarsening with the dissolution of the μ phase particles and MC carbides, especially at the grain boundaries. The poor interface cohesion strength between such large M23C6 carbides and the γ matrix at the vertical segments of the directionally solidified grain boundaries leads to the final creep failure of the present alloy.