Gamma Prime Precipitate Evolution During Hot Forging of a γ–γ′ Ni-Based Superalloy at Subsolvus Temperatures

Abstract

Nickel superalloys are used to manufacture high temperature rotating engine parts such as high pressure discs in gas turbine engines. Alloy 720Li (720Li) is a precipitation-hardened Ni-based superalloy commonly produced by cast and wrought processes. Conventional ingot-to-billet conversion is an expensive and very complex process, requiring multiple open die forging operations and reheating steps in order to achieve a homogeneous microstructure. The present work studies the microstructural evolution of 720Li billet material with the presence of large unrecrystallized structures. The interaction of γ′ precipitates with recrystallization during hot forging at subsolvus temperatures was investigated. Double truncated cones were forged at subsolvus temperatures following two forging approaches: single blow and double blow with an intermediate heat treatment. Combined EBSD-EDX analysis was employed to characterize the microstructural evolution of 720Li during hot forging operations. Primary γ′ precipitates promote hetero-epitaxial recrystallization during slow cooling, whereas secondary precipitates, formed during slow cooling are not dissolved during reheating, prior to the forging operations. The presence of undissolved secondary γ′ promotes strain accumulation and the occurrence of continuous dynamic recrystallization (CDRX). Intermediate heat treatment plays an instrumental role on the recrystallization behaviour for the alloy 720Li. Dissolution of the secondary γ′ precipitates results in a strong preconditioning of the 720Li microstructure prior to the second blow of deformation, promoting the formation of fully recrystallized structures and removing the undesired large unrecrystallized regions. The coalescence of intragranular γ′ precipitates into clusters of coalesced γ′ precipitates represents a clear transition from the apparent unrecrystallized regions to the fully recrystallized structures.