The loss of ductility in the high strength polycrystalline superalloy 720Li is studied in air between room temperature and 1000 °C. Tensile ductility is influenced profoundly by the environment, leading to a pronounced minimum at 750 °C. A relationship between tensile ductility and oxidation kinetics is identified. The physical factors responsible for the ductility dip are established using energy-dispersive X-ray spectroscopy, nanoscale secondary ion mass spectrometry and the analysis of electron backscatter diffraction patterns. Embrittlement results from internal intergranular oxidation along the γ-grain boundaries, and in particular, at incoherent interfaces of the primary γ ′ precipitates with the matrix phase. These fail under local microstresses arising from the accumulation of dislocations during slip-assisted grain boundary sliding. Above 850 °C, ductility is restored because the accumulation of dislocations at grain boundaries is no longer prevalent.
Németh, A. A. N., Crudden, D. J., Armstrong, D. E. J., Collins, D. M., Li, K., Wilkinson, A. J., … Reed, R. C. (2017). Environmentally-assisted grain boundary attack as a mechanism of embrittlement in a nickel-based superalloy. Acta Materialia, 126, 361–371. https://doi.org/10.1016/j.actamat.2016.12.039