One of our early successes was in developing microscale methods for mapping stress using the ERC-funded laser cutting tool. Mechanical behaviour and degradation of materials were investigated via in situ microtesting methods. Notably, the initiation and growth of stress corrosion cracks in Al-alloys were investigated using in situ stress corrosion cells . Synchrotron experiments complemented high-resolution imaging from VoIs that were FIB extracted from the samples. Taken together these confirmed that corrosion sites are associated with intermetallic particle clusters. Semi-automatic imaging and standardised data output processes were coded and put in place to improve the data collection efficiency of X-ray CT measurements.
We also demonstrated the feasibility of using Xenon plasma FIB methods to investigate the time-lapse ductile fracture of steel . Submerged 100m Volumes of Interest (VoIs) were identified by X-ray and the VoI extracted by plasma-FIB for higher resolution imaging.
These projects paved the way for the development of our laser tribeam system which allows the user to extract a millimetric volumes within the microscope. This is the missing link between microCT on 100-1mm sized samples and 100-1nm resolution imaging electron microscopy on 1mm samples as summarised in a recently paper entitle ‘The frontiers of large volume electron microscopy with fs-laser tri-beam’ under review at Ultramicroscopy journal.
We have also developed lab-based diffraction computed tomography (DCT), capable of providing 3D maps of crystallographic orientations in grains of at least 15 μm. This was extended to mapping the grain boundaries & grain shapes for the sintering of Cu particles, and the results validated by destructive 3D plasm FIB-SEM . Further, we have developed digital image correlation techniques to map the plastic deformation in the AZ31 Mg alloy at the grain scale for the first time, shedding light on the changes in grain scale deformation that occur on moving from the low ductility room temperature regime to 200C where superplastic creep occurs.
Another significant package of work centred on developing simultaneous imaging and diffraction in collaboration with the European Synchrotron Radiation Facility (ESRF). The beamline was used in a study of the evolution of damage accumulation in steel specimens under in situ tensile loading. In addition, the effectiveness of SiC vs. Al2O3 reinforcement in bridging cracks in Al-based metal matrix composites was investigated highlighting the viability of correlating imaging (for damage) and diffraction methods (for stress mapping) at synchrotron facilities . This work has led to an International Centre-to-Centre collaboration with two research fellows permanently located in Grenoble, through which we intend to increase the volume of manufactured materials that can be X-ray imaged to identify defects.
Correlative in situ microscopy methods have been used in this project to characterise several other materials, including biomimetic Ti-aerogels ; synchrotron X-ray CT data allowed single domains to be tracked under constrained and unconstrained conditions, revealing insights into microstructure realignment during compression for supercapacitor electrodes. By quantitatively characterising the microstructural evolution of these composites during compression, we have been able to optimise their structure for low voltage, electrothermal heating .
Given the huge increase in interest in additive manufacturing we expanded our research in this area, which led to the development of in situ microCT techniques to understand the collapse of additive manufactured lattices with varying poisons ratio, and the impact of additive manufacture and investment casting on lattice structure. MicroCT was used to characterise the defect population for laser powder bed fusion processed Al alloy and machine learning techniques were employed to explore the influence of defect location, size, and morphology on the fatigue life of the alloy .
A final work package which yielded ground-breaking results is our hyperspectral “colour bay”, which enables us to obtain colour CT images. These provide information on the elemental composition of a sample, not just the density. On interacting with X-ray photons, every chemical element emits a unique, identifying spectral signal corresponding to a specific energy that can be used as an element’s ‘fingerprint’. We use these markers as fingerprints to identify the presence of an element within the sample. We developed novel algorithms to optimise the spatial and spectral image quality. We have used the technique on biological samples to map fluorescence of different elements in a single scan, and for in situ analysis of chemical reactions in catalytic materials and batteries .
