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Microstructural evolution contributing to abnormal grain growth in Ni-base superalloys
McArthur, Byron William Jack
McArthur, Byron William Jack
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2021
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Ni-base superalloys exhibit excellent high temperature mechanical and corrosion resistant properties, and are utilized in turbine engine applications. Disk alloy microstructures contain an ordered L12 phase,’, as a strengthening phase that can also help control the grain size during thermomechanical processing and heat treatments. The matrix phase has the propensity to undergo abnormal grain growth (AGG) during specific conditions of isothermal forging and super-solvus heat treatment (SSHT). The present work focuses on the microstructural evolution during these steps as a precursor state to AGG and tracks the progression of AGG. Material heated to deformation temperatures widens the grain size distribution, leading to varying contributions from localized superplastic deformation mechanisms at elevated temperatures. This widened grain size distribution occurs as a result of varying dissolution rates for intragranular versus intergranular primary ’ rather than initial spatial clustering of primary ’. In conjunction with dynamic recrystallization during isothermal forging, a heterogeneous distribution of stored energy remains. The material exhibits high strain rate sensitivity during near-solvus isothermal forging, with higher strain rates creating more homogeneous stored energy post-deformation. Isothermal forging at near-solvus temperatures and low strain rates (e.g., 10-3/s) produces dynamic and post-dynamic AGG with sufficient strain, and static AGG when subjected to a low heating rate (e.g., 0.12 °C/s) SSHT to between the isothermal forging temperature and the super-solvus temperature. This work has shown conclusively that, as primary ’ dissolves with increasing temperature, it forms strain free, co-oriented that acts as a recrystallization nuclei for AGG, similar to heteroepitaxial recrystallization (HERX). The sensitive temperature dependence of ’ phase fraction at near-solvus temperatures controls the site availability for HERX. Deformation effects on primary ’ appear to enhance co-oriented formation through disruption of -’ interfaces. Dislocation content in primary ’ as a result of sub-solvus deformation appears to influence primary ’ recovery and stability, but more work is necessary to understand the development/evolution of the primary ’ during processing.
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