| Abstract Scope |
This study investigates the coarsening of ã′ precipitates in a directionally solidified (DS) CM247LC Ni-based superalloy under cyclic thermal exposure at 850°C for up to 500 hours. Continuous ã′ coarsening was observed, accompanied by slight spheroidization and coalescence at prolonged exposures, while largely retaining its cuboidal morphology. The ã′ phase exhibited continuous coarsening, following the modified Lifshitz-Slyozov-Wagner (MLSW) model, with a coarsening rate constant (Kr) of 0.20766 × 10⁵ nm³/h. Concurrent with these microstructural changes, a triple-layered oxide scale (outer NiO, intermediate Cr2O3, and inner Al2O3) formed on the surface. High defect density within the inner alumina layer facilitated outward diffusion of Ni and Cr, accelerating oxide growth. Furthermore, a precipitate-free zone (PFZ) developed near the surface due to ã′ depletion and the formation of oxides and carbides, with decreasing lattice misfit confirming the ã′ coarsening. These findings offer critical insights into the thermal stability and phase evolution of DS Ni-based superalloys in high-temperature environments. |