INHOMOGENEOUS GRAIN COARSENING BEHAVIOR IN SUPERSOLVUS HEAT TREATED NICKEL-BASED SUPERALLOY RR1000

摘要

RR1000 billet material has been isothermally forged using various nominal strain rate and temperature conditions. A supersolvus heat treatment has then been applied in order to assess the effect of local strain, strain rate and forging temperature on the extent of grain coarsening. This work has found that a range of forging strain exists where inhomogeneous grain coarsening is promoted during slow heating through the γ' solvus temperature and results in excessively coarse final grain microstructures. Interrupted supersolvus heat treatments have shown that this process is caused by the inhibited and/or premature growth of certain grains relative to their neighbors. It has been found that excessive grain coarsening can be limited by increasing the applied forging strain rate, reducing the forging temperature or increasing the heating rate to the supersolvus heat treatment temperature. This coarsening mechanism has been described as abnormal grain growth (AGG), primarily occurring at low applied forging strain rates in the fully superplastic forging regime. It is thought to be distinct from critical grain growth (CGG) coarsening behavior which is typically related to higher applied forging strain rates where superplastic deformation is limited.