Insights Into the Evolution of Powder Metallurgy Ni-Base Superalloys During Consolidation and Thermomechanical Processing Using Microstructure Analytics

Abstract

A quantitative assessment of the changes occurring in the microstructure of an advanced polycrystalline, powder metallurgy Ni-base superalloy, RR1073, was conducted using a microstructure analytics approach. This study systematically tracked the microstructural features and physical metrics to understand the evolution of the microstructure starting from the hot isostatically pressed (HIP) condition through billet conversion. From the billet state, two isothermal forging conditions were compared based on the level of effective strain applied. Using advanced quantitative electron backscatter diffraction (EBSD) coupled with energy-dispersive X-ray spectroscopy (EDS), the microstructural state of the HIP, billet, forged unrecrystallized, and forged recrystallized material was evaluated to provide insights into the mechanisms that result in grain refinement and clustering of the intermetallic γ′ precipitates. The methodology used in this study allows for rapid extraction of physically based metrics that can provide quantitative information that can be compared across various thermomechanical processing (TMP) steps directly and help rationalize the changes in the various material states. Findings from this investigation show that the billet conversion process can substantially change microstructural features such as primary γ′ precipitates, grain subsets, and retained plastic strain in the microstructure. It is seen that concomitant mechanisms such as recrystallization, precipitate dissolution, and coarsening compete during TMP.