摘要
The influence of grain boundary serration treatments on the microstructure and creep behavior of Udimet-720Li superalloy was investigated. The cooling rate of super γ′ solvus heat treatment has a profound impact on γ′ precipitation and its interactions with grain boundaries. While air cooling yielded straight grain boundaries, slower cooling processes altered the morphologies of grain boundaries into Type-I with moderate undulation due to continuous γ′ precipitation and Type-II with pronounced undulation due to discontinuous cellular γ/γ′ formation. Creep tests at 700 °C/700 MPa were conducted on samples with straight boundaries (STB), Type-I serration (SRB-1), and Type-II serration (SRB-2). Compared to the STB sample, the SRB-1 sample showed an improved creep resistance, exhibiting a minimum creep rate of 2.66 × 10−7 s−1 and a 17 % extended rupture life. This improvement is attributed to the higher fractions of secondary and tertiary γ′, which effectively hinder dislocation motion, and the presence of Type-I grain boundary serration, which deflects crack propagation. By contrast, the SRB-2 sample exhibited a higher minimum creep rate of 1.74 × 10−6 s−1 and a 42 % shorter rupture life than the STB sample. Although Type-II serration exhibited more pronounced boundary undulation than Type-I serration, the incoherent flowery γ′ particles and cellular structure reduced its impedance to dislocation motion, leading to a higher creep strain rate. The less cohesive grain boundaries caused by cellular γ/γ′ precipitates also facilitated void nucleation and microcrack formation, accelerating the onset of tertiary creep. This study demonstrates that Type-I grain boundary serration treatment can significantly improve the creep resistance of Udimet-720Li superalloy.