Multiscale quantitative characterization of microstructure and its influence on toughness of GH4169 nickel-based alloy for measurement while drilling instruments

Commercially produced GH4169 alloys of similar composition can exhibit significant variations in impact toughness, posing challenges for their reliability in critical applications. This study investigates the mechanism responsible for such a toughness difference by comparing two specific alloys for Measurement While Drilling (MWD), designated as the high-toughness alloy (Alloy-HT) and the low-toughness alloy (Alloy-LT) based on their impact performance. A multiscale characterization approach, including optical microscopy, scanning electron microscopy(SEM), electron backscatter diffraction(EBSD), transmission electron microscope (TEM), Aspex automated quantitative inclusion analysis, and mechanical testing, was employed. The results show that although the Alloy-LT possesses finer austenite grains and a higher density of high-angle grain boundaries, its impact toughness (40 ± 7 J) is merely 50% of that of the Alloy-HT (83 ± 11 J). Further analysis reveals a substantial population of brittle, chain-like (Nb,Ti)(C,N) particles in the Alloy-LT, with a density approximately ten times higher than in the Alloy-HT. These particles tend to fracture under impact loading, promoting void formation and crack initiation, thereby severely degrading toughness. The findings indicate that the variation in the amount and morphology of these second-phase particles is the primary reason for the toughness difference between the two alloys. Consequently, controlling the formation of such detrimental particles during melting and processing is critical for improving the metallurgical quality and mechanical performance of GH4169 alloys.