Project Title: An investigation of Stress Accelerated Grain Boundary Oxidation (SAGBO).

Supervisors: A P Sutton FRSa (ICL), D S Balint b (ICL), A T Paxtonc  (KCL) 

Project description:

Stress Accelerated Grain Boundary Oxidation (SAGBO) is one of the most important mechanisms of time dependent intergranular cracking of nickel-based superalloys at high temperatures (e.g. 700 oC) in air. These superalloys, such as IN718, are used because of their combination of strength at high temperature and resistance to creep, most notably as components of aircraft engines, such as turbine rotor disc applications. 

SAGBO suggests that crack growth in nickel-based superalloys at high temperatures in air follows the local formation and rupture of brittle oxides, such as NiO and Cr2O3, ahead of the crack and along grain boundaries, which are known as oxide intrusions1,2. However, it is not well understood how the oxide intrusion breaks and, consequently, how the crack grows. For instance, does the oxide intrusion break near the crack-tip or at its furthest extent away from the crack, and is the stress even high enough in the intrusion for the latter to occur for a reasonable flaw size? And hence, does the fracture move backwards to the crack-tip? Although these questions have arisen quite recently, they need to be answered in order to understand and simulate the crack propagation and, consequently, to predict the rate of crack growth in Ni-based superalloys at high temperatures in air.

SAGBO will be investigated using both analytical3 and numerical models that capture the important physical mechanism of the process in order to predict of the rate of the crack growth under realistic conditions. The end goal of this study is to suggest ways to mitigate SAGBO, which will help to prevent the premature failure of aircraft engines, as well as, to make them more efficient, reliable and less polluting.

1. H.S. Kitaguchi, H.Y. Li, H.E. Evans, R.G. Ding, I.P. Jones, G. Baxter and P. Bowen, Acta Materialia, 2013, 61, 1968.

2. L. Viskari, S. Johansson and K. Stiller, Materials at High Temperatures, 2011, 28 (4), 336.

3. R.W.  Lardner, Mathematical theory of dislocations and fracture, 1974, Toronto and Buffalo, University of Toronto Press.