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

A.A.N Németh,D.J Crudden,D.E.J Armstrong,D.M Collins,K Li,A.J Wilkinson,C.R.M Grovenor,R.C Reed

doi:10.1016/j.actamat.2016.12.039

A.A.N Németh, D.J Crudden + Show 6 more

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https://doi.org/10.1016/j.actamat.2016.12.039

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Journal: Acta Materialia	Publication Date: Jan 13, 2017
Citations: 114	License type: cc-by

Affiliation: University of Oxford

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.

Highlights

Polycrystalline g=g0-strengthened Ni-based superalloys are very strong, but their grain boundaries are a source of weakness at elevated temperature due to their interaction with gaseous environments containing oxygen
The layering was found to be in agreement with thermodynamics: thermodynamically unstable oxides consisting of Ni, Fe and Co were found at the beginning and centre of the crack while more stable oxides consisting of Cr, Ti and Al were found closer to the interface of the oxide and metal [6,8,11]
Three temperature regimes are seen with markedly different tensile ductility

Summary

Introduction

Polycrystalline g=g0-strengthened Ni-based superalloys are very strong, but their grain boundaries are a source of weakness at elevated temperature due to their interaction with gaseous environments containing oxygen. The distribution of oxygen at an intergranular crack tip has been characterised recently using techniques of high spatial resolution including transmission electron microscopy combined with energy dispersive X-ray spectroscopy (EDX) [9], nanoscale secondary ion mass spectrometry (NanoSIMS) [10], and atom probe tomography [8]. These studies revealed the formation of a layered oxide structure along the ggrain boundaries ahead of the cracks supporting a damage mechanism similar to the SAGBO concept. The proposed concepts so far fail to explain fully why a super-solvus heat treatment with only intragranular g0-precipitates is observed to be more resistant to environmentally assisted cracking than a sub-solvus heat treatment where both intra- and intergranular g0-precipitates are present [12]

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