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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - CORREL-CT (Correlative tomography)

Teaser

Summary

The vision is firstly, to develop correlative tomography to radically increase the nature and level of information (morphological, structural and chemical) that can be obtained for a 3D volume of interest (VoI) deep within a material or component by coupling non-destructive (3D+time) X-ray tomography with destructive (3D) electron tomography and, secondly to exploit this new approach to shed light on damage accumulation processes arising under demanding conditions. Successful completion of this project will provide new 3D & 4D insights across many areas and yield key experimental data for multiscale models.
Objective 1: To build the capability of correlative tomography
- To connect platforms across scales and modalities in order to track a VoI that may be located deep below the surface and to combine multiple techniques within a single platform.
- To add new facets to correlative tomography including
+ 3D chemical imaging
+ 3D crystal grain mapping
+ the local stress distribution
+ mechanical performance mapping at the VoI scale
Objective 2: To apply it to gain new insights into damage accumulation
Correlative tomography will provide a much richer multi-faceted hierarchical picture of materials behaviour from life science to food science from geology to cultural heritage. This project will focus specifically on identifying the nucleation, propagation and aggregation of damage processes in engineering materials.
- We will identify and track the mechanisms that control the progressive degradation of conventional bulk engineering materials operating under demanding conditions.
- We will examine the hierarchical strategies nature uses to control failure in natural materials through heterogeneous chemistry, morphology and properties. Alongside this we will examine the behaviour of man-made nano-structured analogues and whether we can exploit some of these strategies.

Work performed

Several advancements have already been realised relating to the exploitation and correlation of novel destructive and non-destructive 3D tomography methods. Through the project, the feasibility of using Xenon plasma FIB methods to extract submerged volumes of interest (VoIs) identified by X-ray CT has been explored through two case studies: the time-lapse ductile fracture of steel and the stress corrosion cracking of Al-alloys. In both cases, VoIs were successfully extracted and a recipe for plasma-FIB sample preparation was identified for these extractions.
Another significant package of work also centred on further developing and applying 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 (voiding) 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 also investigated highlighting the viability of correlating imaging (for damage) and diffraction methods (for stress maping) at synchrotron facilities.
We are also developing lab-based diffraction computed tomography (DCT), capable of providing 3D maps of crystallographic orientations in grains of at least 15 Î¼m. This has now been extended to mapping not just the grain orientations, but also the grain boundaries, grain shapes for the sintering of Cu particles, and the results validated by distructive 3D plasm FIB-SEM. Microscale methods for mapping stress have been developed using the ERC funded laser cutting tool. Mechanical behaviour and degradation of materials investigated via in situ microtesting methods, either within electron microscopes, XCT machines or synchrotron facilities, has formed the majority of the work carried out to date. Notably, studies on the stress corrosion cracking of Al-alloys used in situ microtesting to investigate the initiation and subsequent growth of cracks under different conditions. Synchrotron experiments were complemented with additional high-resolution imaging from VoIs that were FIB extracted from the samples, as well as post-mortem analysis, Figure 1. The results indicated that under constant strain conditions, no additional crack initiation sites were generated. Semi-automatic imaging and standardised data output processes were coded and put in place to improve the data collection efficiency of XCT measurements. In situ microtesting was also used in the study of Ti-aerogels under compression, Figure 2. XCT data coupled with digital image correlation allowed single domains to be tracked under constrained and unconstrained conditions. The use of correlative microscopy methods has been proven to be a powerful tool in the characterisation of cracking in several types of material, including bearing steels for which correlating electron back scattered diffraction (EBSD), electron probe micro analysis (EPMA) as well as nano-indentation, Figure 3, has provided insights inaccessible through any other characterisation method (Curd et al. 2019).

Final results

Our work on dual beam plasma FIB-SEM and coupled BIB_SEM have increased the volume that can be correlated between CT and electron microscopy from 50 Î¼m to hundreds of microns. We will be extending the volumes to mm through the testing of a new tribeam microscope exploiting a laser, a plasma FIC and scanning electron microscopy. Our hyperspectral â€œcolour bayâ€ enables us to obtain colour CT images providing info on elemental composition not just density. Case studies will focus on multiple staining of biological samples to distinguish and map fluorescence of different elements in a single scan, as well as in situ analysis of chemical reactions taking place in catalytic materials and batteries. WE are using nano and microCT to follow in 3D (+time) the deformation behaviours of natural hierarchical microstructures (e.g. beetle cuticle (Sykes et al. 2019), elephant dentin (Lu et al. 2018, Lu et al. 2019)), and synthesised ones (e.g. aerogels and additively manufactured lattice structures) to better optimise structural design and to develop new biomaterials and battery electron materials. In order to better understand the mechanisms of grain boundary creep in Mg alloys we will be using our labDCT system to map the grain shapes of Mg alloy prior to creep, following it during creep using in situ testing in 2D grain boundary digital image correlation and then correlating this with 3D plasma-FIB/EBSD to understand grain interactions and rearrangements during grain creep for the first time. Following the mid-term review of the project, future opportunities identified by the external panel of experts reviewing the progress of the project will also be pursued.