Micromechanics of oxides - From complex scales to single crystals

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 \ub5m), 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. \ua0In 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 \ub0C. Results show lower fracture strain at 600 \ub0C 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. \ua0Thus, 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

Cyclic Deformation of Microcantilevers Using In-Situ Micromanipulation

Background: The trend in miniaturisation of structural components and continuous development of more advanced crystal plasticity models point towards the need for understanding cyclic properties of engineering materials at the microscale. Though the technology of focused ion beam milling enables the preparation of micron-sized samples for mechanical testing using nanoindenters, much of the focus has been on monotonic testing since the limited 1D motion of nanoindenters imposes restrictions on both sample preparation and cyclic testing.Objective/Methods: In this work, we present an approach for cyclic microcantilever bending using a micromanipulator setup having three degrees of freedom, thereby offering more flexibility.Results: The method has been demonstrated and validated by cyclic bending of Alloy 718plus microcantilevers prepared on a bulk specimen. The experiments reveal that this method is reliable and produces results that are comparable to a nanoindenter setup.Conclusions: Due to the flexibility of the method, it offers straightforward testing of cantilevers manufactured at arbitrary position on bulk samples with fully reversed plastic deformation. Specific microstructural features, e.g., selected orientations, grain boundaries, phase boundaries etc., can therefore be easily targeted

Fracture of Cr2O3 single crystals on the microscale

Studying cleavage properties of protective oxide scales is imperative to understand their fracture behaviour, since transgranular fracture is observed in many cases. The small thickness and polycrystalline structure of such scales makes it difficult to identify active cleavage planes directly from mechanical testing. To resolve this issue for Cr2O3, we present an approach to experimentally identify cleavage planes through micro-cantilever bending. Single crystal wafers are used to prepare micro-cantilevers of pentagonal cross-section in different orientations, targeting possible cleavage planes. Fracture surface imaging showed rhombohedral and pyramidal fracture, though surface energy studies predict rhombohedral as the dominant plane. There does exist a preference for rhombohedral fracture over pyramidal, which is also revealed from the experiments

Characterization of as-deposited cold sprayed Cr-coating on Optimized ZIRLO™ claddings

As-produced Cr-coated Optimized ZIRLO™\ua0cladding material\ua0fabricated with the cold-spray (CS)\ua0deposition process\ua0is studied. Cross-sectional\ua0electron microscopy, nano-hardness profiling,\ua0transmission electron microscopy, transmission Kikuchi diffraction, and\ua0atom probe tomography\ua0(APT) were performed to investigate the nature of the CS Cr-coating/Optimized ZIRLO™ interface, the microstructure of the coating, and the effects of the deposition on the Zr-substrate microstructure. The former surface of the Zr-substrate was found to have a highly deformed nano-crystalline microstructure, the formation of which was attributed to dynamic recrystallization occurring during\ua0coating deposition. This microstructural change, evaluated with\ua0electron backscattered\ua0diffraction and nano-hardness profiling, appeared to be confined to a depth of a few microns. Through APT analysis, a 10–20\ua0nm thick intermixed bonding region was observed at the interface between coating and substrate. The chemical composition of this region suggests that this layer originated from a highly localized shearing and heating of a thin volume of the outermost former surface of the substrate. The study of the intermixed bonding region\u27s crystalline structure was performed with\ua0high resolution transmission electron microscopy\ua0and revealed a distorted hexagonal close-packed structure

Effect of lithiation on the elastic moduli of carbon fibres

Carbon fibre electrodes can enable a solid-state battery to carry mechanical load as normal construction materials. The multifunctionality is promising for most lightweight applications. Like all electrode materials, both volume and elastic moduli of the carbon fibre electrodes change during battery cycling. Such changes jeopardize the mechanical integrity of the battery. Due to the challenging corrosion problem of the lithiated component in air, the effect of lithiation on the carbon fibre\u27s elastic moduli has yet to be explored. Also, robust data on the expansion of carbon fibres from lithiation are lacking. In the present work, we demonstrate a method and perform tests of corrosion protected carbon fibres in scanning electron microscope. The volume, and longitudinal and transverse moduli of a carbon fibre at three states of lithiation are determined and compared. The transverse modulus of the lithiated fibre is found to be more than double that of the pristine and delithiated fibres

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

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. \ua0The 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

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

We propose a new geometry for focused ion beam milled micro-cantilevers, which allows production of residual stress-free, isolated thin film specimens from film-substrate systems. This geometry was used to demonstrate the presence of permanent deformation in about 200 nm thick thermally grown oxide scales on a Ni-base superalloy, after applying large bending displacements in-situ in a scanning electron microscope. Stiffness measurements performed before and after the bending tests confirmed the absence of micro-cracks, leading to the conclusion that plastic deformation occurred in the oxide scale. The proposed method is extendable to other film-substrate systems and testing conditions, like non-ambient temperatures

Crack Growth Studies in a welded Ni-base superalloy

It is well known that the introduction of sustained tensile loads during high-temperaturefatigue (dwell-fatigue) significantly increases the crack propagation rates in many superalloys. Onesuch superalloy is the Ni-Fe based Alloy 718, which is a high-strength corrosion resistant alloy usedin gas turbines and jet engines. As the problem is typically more pronounced in fine-grainedmaterials, the main body of existing literature is devoted to the characterization of sheets or forgingsof Alloy 718. However, as welded components are being used in increasingly demandingapplications, there is a need to understand the behavior. The present study is focused on theinteraction of the propagating crack with the complex microstructure in Alloy 718 weld metalduring cyclic and dwell-fatigue loading at 550 \ub0C and 650 \ub0C

Influence of dwell time on fatigue crack propagation in Alloy 718 laser welds

The introduction of welded assemblies in aerospace components aid in weight reduction, but also lead to an increased risk of defects. It is therefore important to analyze the high temperature crack growth resistance of such welds. The results from high temperature cyclic and dwell-fatigue testing of surface flawed Alloy 718 welds are presented here. An increasing temperature and application of a dwell time accelerate the crack growth and increase interaction with secondary phases. During cyclic loading at 550 \ub0C, there is little interaction with the microstructure during transgranular propagation, but the application of dwell times results in a mixture of transgranular propagation and intergranular cracking of boundaries between different dendrites. At 650 \ub0C, mixed intergranular and transgranular mode of crack growth is seen under both cyclic and dwell conditions. However, during dwell-fatigue the interfaces between the secondary arms of the same dendrite are also weakened, leading to an interfacial type of crack growth also in the intergranular parts

Prediction of mechanical response for 5000-series Aluminum alloy coupling visco-plastic self-consistent approach with finite element method

In the present work, a hybrid micro macro-approach was adopted to investigate the material behavior of the A5XXX-O object of the Benchmark 3 of Numisheet2018. Starting from the provided uniaxial stress-strain curve and in house microstructure measurements, a mean eld approach, by using the VPSC7c code, was used to perform numerical experiments in order to derive the anisotropic macroscopic behavior of the aluminum alloy.\ua0Then, at the macroscale, a constitutive model was built on coupling a non-quadratic yield surface function with a damage model developed in the framework of the continuum damage mechanics. Finally, by using MSC.Marc2017.1, finite element simulations of uniaxial and Nakajima bulging tests were performed with the purpose of obtaining the Fracture Formin gLimit Curve for the aluminum alloy under investigation