The Effect of Thermomechanical Processing on the Microstructural Evolution of Nickel-Based Alloy 625

Abstract

Nickel-based superalloys, such as Alloy 625, have excellent strength and corrosion resistance at elevated temperatures. They can find use in demanding nuclear environments such as those found in Pressurised Water Reactors (PWRs). However, processing nickel-based superalloys is far from trivial. This is because the resulting microstructural evolution can be very difficult to control, which leads to large monolithically forged pieces having highly variable mechanical properties. Such variability is not acceptable for nuclear scenarios where materials need to be predictable and reliable during long service. The unpredictable microstructural evolution can arise due to recrystallization that occurs during deformation, i.e. partial Dynamic Recrystallization (DRX), which easily happens as a result of low Stacking Fault Energies (SFEs) in nickel superalloys. The final microstructures can also be affected by recrystallization that occurs after deformation, i.e. Post-Dynamic Recrystallization (PDRX), which consists of two phenomena known as Meta-Dynamic Recrystallization (MDRX) and Static Recrystallization (SRX). When left uncontrolled, these recrystallization phenomena often do not finish during Thermomechanical Processing (TMP), leading to partially recrystallized microstructures that contain both coarse and fine grains. Consequently, this results in large forgings having variable mechanical properties. Understanding the kinetics of PDRX after limiting the amount of DRX could therefore allow partially recrystallized microstructures to be better controlled, or even avoided altogether. As a result, this project has microstructurally assessed the PDRX kinetics of Alloy 625 (with SEM, TEM and EBSD), after performing deformation experiments that limited DRX at three different temperatures (950 °C, 1050 °C and 1150 °C). A strain rate of 1 s-1 was used and post-deformation anneals up to 100 s were performed. Kinetics were analysed using Johnson-Mehl-Avrami-Kolmogorov (JMAK) models and compared to methodologies used in steel literature as TMP of nickel has yet to be thoroughly explored. It was found that no PDRX took place at 950 °C, yet microstructures were fully recrystallized at 1150 °C. JMAK exponents were always lower than predicted (i.e. < 0.7) due to recrystallization initiating heterogeneously at High Angle Grain Boundaries (HAGBs) or via Particle Stimulated Nucleation (PSN). Additionally, it has been argued that thorough recrystallization modelling cannot be performed until MDRX and SRX are distinguished. As these phenomena compete in post-deformation regimes, the modelling will require in situ or 3D electron microscopy.

Date of Award	31 Dec 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Grace Burke (Supervisor)