Micromechanics of oxides - From complex scales to single crystals
Doktorsavhandling, 2019

Protective oxide scales shield high temperature materials from corrosion, thus ensuring safety and long material life under adverse operating conditions. Cracking and spallation of such scales can lead to fatigue crack initiation and expose the material to further oxidation. It is therefore imperative to measure the fracture properties of oxides so that they can be incorporated in the life estimation models of high temperature materials. Existing models require inputs on oxide properties such as fracture strain and elastic modulus. The established measurement methods are mainly applied for thick (several microns) scales, but for many materials such as superalloys the oxides are thinner (< 1 µm), and the results would be affected by the influence of substrate and residual stresses. Focused ion beam machining (FIB) enables the preparation of micro sized specimens in the size range of these scales.

 

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.

fracture

Cr2O3

oxide scales

micromechanical testing

electron microscopy

Kollektorn, MC2, Kemivägen 9
Opponent: Dr. Finn Giuliani, Imperial College London, UK

Författare

Anand Harihara Subramonia Iyer

Chalmers, Fysik, Materialens mikrostruktur

Room temperature plasticity in thermally grown sub-micron oxide scales revealed by micro-cantilever bending

Scripta Materialia,; Vol. 144(2018)p. 9-12

Artikel i vetenskaplig tidskrift

Microscale fracture of chromia scales

Materialia,; Vol. 8(2019)

Artikel i vetenskaplig tidskrift

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

At some point in your life, you might have heard of a plane crash occurring due to jet engine failure or a power plant shutting down due to turbine failure. Why do these happen in these critical applications, even after careful design and taking numerous safety measures? Advanced materials have been developed that can withstand high temperatures while maintaining their mechanical strength. Still such unexpected failure can occur which is because of the conditions these materials experience during operation. The temperatures vary, so does the mechanical loads, and all this happens in a corrosive environment. Oxygen might be a lifesaver for us, but it reacts with most metals and corrodes them. This is prevented by adding elements such a chromium and aluminium, which react with oxygen to form a continuous and dense oxide layer and prevent further reaction of oxygen, the same way as a controlled burn prevents spreading of wildfire. These layers also experience the same conditions as the high temperature materials do, which can lead to their cracking and removal from the surface, since oxides are brittle. This can initiate fracture in the material as well, and thus their life is significantly reduced. Therefore, it is very important that we study the mechanical properties of such layers so that they can be included in the life estimation of these advanced materials.

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.

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Vetenskapsrådet (VR), 2016-01-01 -- 2019-12-31.

Oxidationsbefrämjad spricktillväxt i gasturbinmaterial

Kompetenscentrum i högtemperaturkorrosion, 2016-03-18 -- 2017-12-31.

Ämneskategorier

Teknisk mekanik

Annan materialteknik

Metallurgi och metalliska material

Infrastruktur

Chalmers materialanalyslaboratorium

Styrkeområden

Materialvetenskap

ISBN

978-91-7905-230-0

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4697

Utgivare

Chalmers tekniska högskola

Kollektorn, MC2, Kemivägen 9

Opponent: Dr. Finn Giuliani, Imperial College London, UK

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Senast uppdaterat

2020-01-08