Environmentally-assisted grain boundary attack as a mechanism of embrittlement in a nickel-based superalloy

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Abstract

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.

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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

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