Mechanism of notch sensitivity elimination via precipitating Laves phases at grain boundaries in a low expansion superalloy

The effect of Laves phase on the notch sensitivity of Thermo-Span alloy has been systematically investigated in the study via scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). The results show that amounts of secondary Laves phase increase and γ′ phase decrease with the dissolution of primary Laves phase. The γ′ phase depletion zones are formed at the vicinity of secondary Laves phases, which are the critical factor eliminating the notch sensitivity. The γ′ phase depletion zones around secondary Laves phases at grain boundaries (GBs) resulted in a decrease in GBs strength along with improvement in plasticity. Therefore, GBs can accommodate high strain localization and produce Geometrically Necessary Dislocation (GND) to coordinate incompatible deformation among neighboring grains. Thus, the resistance to crack initiation and propagation at GBs is improved. Subsequently, the accumulated strain at GBs activates multiple slip systems to further coordinate deformation between GBs and intragranular regions.