The effect of coarsening of γ′ precipitate on creep properties of Ni-based single crystal superalloys during long-term aging

The correlation between the coarsening of gamma' precipitate and creep properties during long-term aging at 1000 degrees C for 100-1000 h of a fourth-generation nickel-based single crystal superalloy was investigated. The average size of gamma' precipitate increased with the extension of aging time and the morphology of gamma' precipitate kept cuboidal shape after 1000 h. Experimental observation on the coarsening of gamma' phase demonstrated that the relationship between r(t)(3) - r(0)(3) and t was in accordance with the LSW theory and the coarsening rate k was about 5954 nm(3)/h. The creep life of aged samples at 1140 degrees C/137 MPa increased firstly and then decreased with the exposure time prolonging. The casting pores and creep pores were observed on the fracture surface of samples after creep rupture. Compared with the creep pores, the microcracks formed at the edge of casting pores had greater effect on the creep life. The creep properties of samples during aging were mostly affected by the size of gamma' phase. The promotion of creep life of aged sample was rationalized by the slightly grown gamma' phase, the diffusion of the refractory elements and the accumulation of dislocations in the gamma channel. The decrease of creep life was predominantly affected by the seriously coarsened gamma' phase because the significant increase of the size of gamma' phase leaded to a decrease in the resistance of dislocation movement.