Creep Mechanism and Microstructure Evolution of a Directionally Solidified Ni-Based Superalloy with Different Orientations at 850 °C

The creep properties of directionally solidified superalloys are largely influenced by the degradation rate of the γ/γ' microstructure and the dislocation motion, which exhibit distinct mechanisms under varying temperature and stress conditions. In this study, the creep deformation mechanisms and microstructural evolution of a directionally solidified nickel-based superalloy in the longitudinal (L) and transverse (T) orientations at 850 °C are comprehensively investigated. Creep testing and characterization of the dislocation structure revealed superior creep properties in the L direction compared to the T direction. The creep mechanism in the L direction involves the activation of multiple {111}<110> slip systems, shearing the γ' precipitates through antiphase boundaries (APBs). Conversely, the creep mechanism in the T direction involves the activation of {111}<112> slip systems, shearing the γ' precipitates through a superlattice intrinsic stacking fault (SISF) and forming slip bands inclined to the stress axis. Aluminum was identified as the controlling element for the γ' rafting. The longitudinal specimens exhibited P-type rafting due to the activation of multiple slip systems and sufficient plastic strain flow from the dislocation motion. In contrast, the transverse specimens show little rafting due to limited slip system activation. These findings can serve as a reference for better understanding the anisotropy of directionally solidified superalloys and provide a basis for their broader application.