Environmental Effects on the Creep Response of Thin-Walled Ni-Based Single Crystal Superalloys

Environmental effects on 980 °C high temperature creep response of thin-walled Ni-based single crystal superalloys were isolated by testing in air and vacuum, respectively. For each gaseous environment, different stress levels and specimen thicknesses were considered. Results showed that the creep rupture life reduced at 275 MPa in both air and vacuum, when the specimen thickness decreased from 1.3 to 0.5 mm. The reduction in life was more obvious in air than that in vacuum. At 330 MPa, the variation tendency of creep life with the specimen thickness was not evident in both gaseous environments. In the air environment, metallographic analyses indicated that oxidation effect, voids and cracks growth could affect the creep response of thin-walled specimens. Cracks propagation in air exhibited a go-stop mode, which was the competition result of air strengthening and air weakening. The former was dominant under the low stress, while the latter was dominant under the high stress. Phase-field analyses pointed out that the existence of multilayer structure caused by the oxidation effect led to larger plastic strain in the normal γ/γ′ phase region. And this was more serious in the thin specimen with relatively wider oxidation layers. In the vacuum environment, only the crack growth contributed to different creep lives in thin-walled specimens. Cracks in vacuum continued to grow in the whole propagation process. Once their length reached the critical value, cracks would grow dramatically in the thin specimen and finally led to the fracture of specimens.