The Impact of Grain Boundary Serration Treatments on the Creep Deformation of Udimet-720li Superalloy

Abstract

This study investigates the impact of grain boundary serration treatments on the high-temperature creep behavior of the advanced Udimet-720Li superalloy. Serrated grain boundaries were achieved through controlled cooling during heat treatment, affecting the size distribution and precipitation behavior of the γ′ phase. Two distinct types of boundaries were identified: Type-I with minimal undulation due to continuous γ′ precipitation and Type-II exhibiting significant undulation due to discontinuous cellular γ/γ′ formation. This paper examines the interplay between grain boundary morphology and the size and morphology of γ′ precipitates in influencing the creep properties.Samples with straight boundaries (STB), Type-I serration (SRB-1), and Type-II serration (SRB-2) were subjected to creep tests at 700 °C/700 MPa. The results revealed a clear relationship between grain boundary morphology, γ′ precipitation characteristics, and creep resistance. The SRB-1 sample, characterized by Type-I serration, demonstrated superior creep performance, exhibiting a lower minimum creep rate of ~2.66 × 10-7 s-1 and a 17% extended rupture lifetime compared to the STB sample. This improvement is attributed to the combined effect of a higher volume fraction of fine γ′ precipitates, which hinders dislocation movement, and the presence of Type-I serration, which deflects crack propagation. Conversely, the SRB-2 alloy with Type-II serration displayed a detrimental effect on creep resistance, exhibiting a higher minimum creep rate of ~1.74 × 10-6 s-1 and a 42% shorter rupture life compared to the STB alloy. This behavior is attributed to the increased grain boundary area associated with Type-II serration, promoting grain boundary diffusional creep, and the formation of incoherent flowery and cellular γ′ that accelerate void nucleation and lead to increased creep damage. This study emphasizes the critical role of grain boundary engineering and γ′ precipitation morphology in optimizing the creep performance of Udimet-720Li superalloys for high-temperature applications.

Keywords: superalloys, Heat treatment, γ′ precipitates, serrated grain boundary, Creep properties