Micromechanics of oxides - From complex scales to single crystals
Doctoral thesis, 2019
In this work, a modified microcantilever geometry with partially removed substrate is proposed for testing of oxide scales. Room temperature microcantilever bending of thermally grown superalloy oxide (complex oxide with an upper layer of spinel and lower layer of Cr2O3) revealed the presence of plasticity, which is attributed to the deformation of the upper cubic spinel layer and low defect density of the volume being probed. Due to difficulties in isolating Cr2O3 from the complex oxide layer, dedicated oxidation exposures are performed on pure chromium to generate Cr2O3 which is tested using the same cantilever geometry at room temperature and 600 °C. Results show lower fracture strain at 600 °C in comparison to room temperature and presence of cleavage type of transgranular fracture in both cases, pointing to a need for studying cleavage fracture of Cr2O3. This was analysed using microcantilever bending of single crystal Cr2O3 to identify the preferential cleavage planes. Finally, fracture toughness was also measured through microcantilever bending and micropillar splitting.
Thus, it is shown that micromechanical testing is an effective tool for measuring fracture properties of oxide scales. The fracture study of Cr2O3 scales show that it is a complex process in which the crystallographic texture also plays a role. Surface energy and fracture toughness criterion was unable to explain the fracture behaviour of single crystal Cr2O3 observed from experiments. Such a comprehensive analysis can contribute towards the development of reliable models for oxidation assisted failure.
Anand Harihara Subramonia Iyer
Chalmers, Physics, Microstructure Physics
Room temperature plasticity in thermally grown sub-micron oxide scales revealed by micro-cantilever bending
Scripta Materialia,; Vol. 144(2018)p. 9-12
Microscale fracture of chromia scales
Materialia,; Vol. 8(2019)
Fracture of chromia single crystals on the microscale - Anand H.S. Iyer, Krystyna Stiller, Magnus Hörnqvist Colliander
On the cleavage fracture toughness of Cr2O3 single crystals - Anand H.S. Iyer, Krystyna Stiller, Magnus Hörnqvist Colliander
However, their size is in micrometre range, i.e. they are about 1/100th the width of human hair. Therefore, it is difficult to design experiments to study their mechanical properties. Scientists over the last few decades have done such fracture studies on protective oxides, but most data available is valid for oxide layers that are thicker than 10 microns. Also, the mechanical properties measured are influenced by internal stresses and the underlying metal. Micromechanical testing is a field which, as the name suggests, enables mechanical testing at the microscale. Specialised instruments can be used to make specimens that are in the same size range as the oxide scales we need to test. In this work, we have developed a sample geometry based on a micro sized cantilever, which can be bent using high precision instruments and record how the oxide scale behaves under those conditions. This geometry has the advantage that the oxide layer is isolated from the underlying metal, so the internal stresses do not affect the properties being measured.
This method has been used successfully to demonstrate the microcantilever bending of oxide layers grown on a high temperature material used in jet engines, where plasticity was seen at room temperature, even though oxides are generally brittle. It is also possible to calculate under what conditions the scales fail, and also how they fail, which has been demonstrated with the help of a commonly used protective oxide scale, Cr2O3. The methods developed in this work can contribute information critical to developing reliable life estimation models for such materials used in high temperature applications and make them safer.
Oxidation assisted crack growth in gas turbine materials
Competence Centre for High Temperature Corrosion, 2016-03-18 -- 2017-12-31.
In-situ micromechanical testing of interfaces for multiscale modeling of fracture
Swedish Research Council (VR), 2016-01-01 -- 2019-12-31.
Other Materials Engineering
Metallurgy and Metallic Materials
Chalmers Materials Analysis Laboratory
Areas of Advance
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4697
Chalmers University of Technology
Kollektorn, MC2, Kemivägen 9
Opponent: Dr. Finn Giuliani, Imperial College London, UK