Oxidation induced failure of superalloys: High temperature crack growth and oxide scale properties

Licentiate thesis, 2017

Gas turbine materials are designed to work in extreme environments in high temperature with an oxidising environment and variable mechanical loading. The study of high temperature fatigue properties of these materials is therefore important. Recent evidences show that oxidation plays an important role in the crack growth of superalloys at high temperatures. The formation and cracking of brittle oxides ahead of the crack tip leads to accelerated crack growth under dwell times. Protective surface oxide scales on superalloys prolong their life by preventing further oxidation. The cracking and spallation of such scales can lead to further oxidation of the material, thus reducing its strength or even lead to crack initiation at the surface. This work is aimed at two different aspects of damage in superalloys – high temperature crack growth and fracture properties of oxide scales. The long-term goal is to develop an oxidation based life assessment model for real microstructures using experimental data.   The initial part of the study focuses on the influence of dwell times in high temperature crack growth in superalloy welds. This work showed that the combination of oxidising atmosphere, high temperatures and sustained tensile loads led to accelerated crack growth, and that the interaction of the crack with the materials microstructure depends strongly on the combination of these parameters. In the second part, methods were developed to test the room temperature deformation properties of thermally grown oxides on a superalloy substrate. In-situ micro-cantilever bending tests in a scanning electron microscope showed the presence of plasticity in the oxides, which is mainly attributed to the size of the scale and lack of internal defects. These methods can be extended to high temperature as well, which can aid in giving an insight into high temperature properties of surface and grain boundary oxide scales, contributing to development of models for oxidation assisted crack growth.