P phase precipitation and strengthening behavior of a novel polycrystalline Ni3Al-based intermetallic alloy at 1100 °C

The microstructural evolution and strengthening behavior of a polycrystalline Ni3Al-based intermetallic alloy were investigated during varying durations of thermal exposure at 1100 °C. The study focused on the topologically closed-packed orthorhombic P phase precipitate, which was found to form in the interdendritic γ channel and grow in the γ'-matrix after thermal exposure. The morphology of the P phase evolved from needle-like to lath-like and then to blocky-like. The crystal structure and composition of the precipitates were analyzed using scanning transmission electron microscopy (STEM) and electron probe X-ray microanalysis (EPMA). The chemical composition of the P phase changed continuously during thermal exposure and the P phase was related to the γ'-matrix by three sets of orientation relationships (OR). The alloy's tensile properties and creep fracture time were enhanced during thermal exposure at 1100 °C for 2000 h due to P phase precipitation at grain boundaries or within the grain. Based on the experimental evidence of this study and theoretical strengthening behavior reported in the literature, a novel mechanism for in-situ precipitation strengthening was proposed.