| Critical Grain Growth (CGG) in powder metallurgy nickel-based superalloys is defined as abnormally coarse grains growing in a region of critical strain after heat treatment above the gamma prime solvus temperature. As these large grains are detrimental to the fatigue life of an aircraft engine component, a better understanding of the mechanism of this phenomenon is desired. The goal of the present research is to examine the evolution of the dislocation structure in the alloy René 88 DT for isothermal heat treatments in order to understand why the microstructure coarsens abnormally at a critical strain. TEM, SEM, X-Ray Diffractometry, and optical microscopy were used to document the dislocation structure, crystallographic grain structure, and secondary phases in the microstructure. It was found that recrystallization, not recovery, occurs above (10%), at (6%), and below (3%) the critical strain for CGG after supersolvus heat treatment, and is complete before CGG begins. This result is at odds with current theory that explains CGG as either a recrystallization phenomenon, or driven by dislocation structure in boundaries after recovery. However, this work confirms the theory that recovery occurs for subsolvus temperatures, which has proven to be a method to reduce the severity of CGG. These two results show that recrystallization is required for CGG to occur.; Additionally, Abnormal Grain Growth (AGG) via crystallographic texture was investigated to see if the effects of recrystallization could set the stage for AGG locally, at a critical strain. It was found that a weak <111> fiber texture was present in the as-extruded material, but no strong texture evolves upon recrystallization, and no difference in texture occurs across the critical strain; thus, CGG cannot be due to a local AGG via texture. This means that CGG must be either a local AGG phenomenon due to unpinning of pinning particles, such as carbides, borides, and oxides that are still present at supersolvus temperatures, or due to a destabilization of the grain boundary structure through a variation in annealing twin structure after recrystallization. |
