Effect of Grain Boundary Serrations on Creep Deformation of Udimet-720Li Superalloy

This article investigates grain boundaryGrain boundary serration and its effects on high-temperatureHigh temperature creepCreep behavior of Udimet-720Li. Grain boundaryGrain boundary serration is induced by controlled cooling during heat treatmentsHeat treatment, with continuous and discontinuous precipitationDiscontinuous precipitation of γ′ phase identified as the competing mechanisms affecting serration formation. Continuous precipitationPrecipitation of coarse γ′ particles pins grain boundariesGrain boundary and leads to a slight serration termed the continuous precipitationPrecipitation type (type-I) boundary, while discontinuous reaction forming cellular γ/γ′ behind the mobile grain boundaryGrain boundary causes larger serration known as the discontinuous precipitationDiscontinuous precipitation type (Type-II) boundary. Samples with straight (STB), Type-I (SRB-1), Type-II (SRB-2) grain boundariesGrain boundary were produced, and creepCreep behaviors under 700 °C/700 MPa were investigated. The SRB-1 sample exhibits a notably lower minimum creepCreep rate at about 2.66 × 10–7 s−1 and a prolonged 17% rupture lifetime compared to the STB sample. These improvements are primarily attributed to the different size distribution of γ′ precipitatesPrecipitates, which contributes to a higher hindrance to dislocation movements. The presence of Type-I serration also hinders intergranular crack propagationCrack propagation, thereby extending creepCreep life. In contrast, the SRB-2 sample exhibited a higher minimum creepCreep rate, around 1.74*10–6 s−1, and a considerable 42% reduction in rupture lifetime in contrast to the STB sample. This is attributed to the enhanced grain boundaryGrain boundary fractions through the formation of Type-II serration in the SRB-2 sample, promoting grain boundaryGrain boundary diffusionDiffusion creepCreep. Additionally, the presence of incoherent cellular γ/γ′ interface associated with Type-II serration facilitates void nucleationNucleation, leading to increased creep damageCreep damage and a shortened creepCreep lifetime for the SRB-2 sample.