Damage tolerance of low angle grain boundary in a fourth generation Nickel-based single crystal superalloy at 1100 °C service conditions

The effect of different low angle grain boundaries (LAGBs) on the microstructure and mechanical properties in a fourth generation Nickel-based single crystal superalloy was studied. Double seed crystals techniques were used to obtain the specimens with LAGBs. The results showed that there were coarse γ′ phases at LAGBs with the grain boundary angle increasing, the shape of which changed from block to long strip at as-cast and the size of which increased gradually after heat treatment. After thermal exposure at 1100 °C for 100 h, the microstructure stability deteriorated when the angle exceeded 12.6°, where the discontinuous precipitation (DP) colonies region occurred along the grain boundary because of high interfacial energy and mobility at LAGBs. With the grain boundary angle increasing, the ultimate tensile strength of the sample rose slightly and then fell, while the elongation and creep rupture life decreased significantly exceeded 4.9°. Under 1100 °C service condition, the carbides and topological close-packed (TCP) phases were precipitated at LAGBs, which seriously impaired the grain boundary bonding strength. On the other hand, the misorientation at LAGBs led to the stress concentration, which increased the difficulty of grain boundary coordinated deformation. Finally, the damage tolerance of the LAGBs in this fourth generation Nickel-based single crystal superalloy was approximately estimated as about 7° at 1100 °C/150 MPa creep rupture condition based on a 70% creep rupture life standard.