Materials characterisation and XFEL science

Dr Brian Abbey’s research interests are in the area of X-ray imaging, diffraction and materials characterisation. He has been instrumental in the development of a number of new synchrotron techniques.

Among his many collaborators are researchers at:

• University of Melbourne – Professor Keith Nugent and Dr Harry Quiney
• LCLS – Dr Garth Williams
• APS – Dr David Vine and Dr Ross Harder
• Oxford University – Professor Alexander Korsunsky
• MIT – Dr Felix Hofmann.

Partially coherent diffractive imaging (PCDI)

High-resolution X-ray imaging techniques using optical elements such as zone plates are widely used for viewing the internal structure of samples in exquisite detail. The resolution attainable is ultimately limited by the manufacturing tolerances for the optics.

Combining ideas from crystallography and holography, this limit may be surpassed by the method of Coherent Diffractive Imaging (CDI). Although CDI shows particular promise in applications involving X-ray free-electron lasers (XFELs), it is also emerging as an important new technique for imaging at third-generation synchrotrons. The limited coherent output of these sources, however, is a significant barrier to obtaining shorter exposure times. A fundamental assumption of coherent diffractive imaging is that the incident light is fully temporally and spatially coherent. Research carried out at the Centre for Coherent X-ray Science (CXS) has recently shown that through modification of the ‘standard’ CDI algorithms Partially Coherent Diffractive Imaging (PCDI) is possible.

This project aims to explore the possible applications for these new techniques in terms of the increased flux provided and the additional information that can be gained from partially coherent diffraction.

Key References

1. B. Abbey et al. Nature Photonics, 5:420-424. 2011
2. L. W. Whitehead et al. Physical Review Letters.  103. 2009 

Collaborators

Materials characterisation via coherent diffractive imaging

This project aims to make use of the latest developments in techniques for non-destructively mapping materials samples in three-dimensions at the nanoscale, to provide the necessary information to test and develop new and existing theories in materials science and engineering.

These theories impact fundamentally on our understanding of how materials behave during deformation. Conventional techniques in this area are limited in two ways:

1. the resolution of the technique is determined by the detection system and is usually of the order of 2-3 µm
2. the absorption contrast is sometimes insufficient to resolve key structures.

Coherent diffractive imaging (CDI) meanwhile offers a solution to both these problems, since it provides a resolution – usually < 50 nm – that is significantly better than that achieved in standard absorption tomography. In addition it offers additional phase contrast that is expected to provide greater sensitivity than absorption contrast alone.

Key References 

Collaborators

• F. Hofmann, Massachusetts Institute of Technology
• A. M. Korsunsky, Oxford University

Bragg coherent diffractive imaging of crystalline defects 

Crystallographic defects and their interactions are fundamental in determining the properties of materials. Understanding their behaviour has direct consequences for a range of phenomena, from work hardening in metals to device pathology in semiconductor laser diodes.

For the last 50 years electron microscopy has been the dominant means for characterising individual dislocations. This project aims to expand our knowledge of defect behaviour by exploiting recent developments in X-ray bragg coherent diffractive imaging to obtain images of individual defects at the atomic scale in crystals using X-rays. The successful development of these new techniques will allow a much wider range of materials to be examined than is currently possible. 

Key references

1. M. Pfeiffer, Nature, 442, 63-66, 2006
2. M. C. Newton, Nature Materials, 9, 120–124, 2010

Collaborators 

• R. Harder, Advanced Photon Source
• F. Hofmann, Massachusetts Institute of Technology
• A. G. Peele, La Trobe University

Neutron strain tomography

Predicting the fatigue lifetime of components relies on knowledge of the residual elastic strain present throughout the bulk of the material. Non-destructively mapping the complete strain distribution throughout large volumes presents significant practical challenges.

Recently a technique known as Bragg-edge neutron transmission has been developed as a means of non-destructive bulk elastic strain evaluation. Whilst conventional radiography measures the integral absorption, Bragg-edge neutron transmission probes the average strain along the incident beam direction. A 'strain radiogram' is thus a two-dimensional average projection of the strain within the sample.

In collaboration with Oxford Engineering Science, this project aims to demonstrate how strain radiograms can be used for 'neutron strain tomography', allowing complete characterisation of spatially resolved elastic strains.

Key references

1. B. Abbey et al. Nuclear Instruments and Methods in Physics Research B, 270, 28–35, 2012 
2. B. Abbey et al. Procedia Engineering, 1, 1, 185–188, 2009
3. A. M. Korsunsky et al. Acta Materialia, 54, 8, 2101-2108. 2006

Collaborators

X-ray free electron (XFEL) diffractive imaging and nanocrystallography 

X-ray free-electron lasers (XFELs) deliver X-ray pulses with a coherent flux that is approximately eight orders of magnitude greater than that available from a modern third generation synchrotron source. The power density in an XFEL pulse may be so high that it can modify the electronic properties of a sample on a femtosecond timescale.

Exploration of the interaction of intense coherent X-ray pulses and matter is of both intrinsic scientific interest, and of critical importance to the interpretation of experiments that probe the structures of materials using high-brightness femtosecond XFEL pulses.

In collaboration with the University of Melbourne we have a number of projects that explore the behaviour of matter and, in particular, nanocrystals subjected to these extreme conditions.

Key references 

1. H. M. Quiney et al. Nature Physics, 7, 142–146, 2011
2. P. Emma et al. Nature Photonics, 4, 9, 641, 2010
3. R. Neutze, Nature, 406, 6797, 752, 2000 

Collaborators 

• H. M. Quiney, The University of Melbourne
• K. A. Nugent, The University of Melbourne
• R. A. Dilanian, The University of Melbourne
• C. Darmanin, CSIRO

Imaging a highly perfect crystal via the two-dimensional detection of X-rays diffracted from its constituent lattice planes (diffraction topography) has been employed since the 1940s. This technique has very different contrast mechanisms from those of conventional absorption imaging.

Diffraction contrast, which can be partly accounted for by the dynamical theory of diffraction, allows visualisation of individual crystallographic line defects (dislocations) and local lattice strains. Plastic deformation of metallic crystals is mediated by the formation and propagation of dislocations which self-organise into 'hard' dislocation-rich walls enclosing 'soft' regions of low dislocation density (cells).

This dislocation patterning has profound consequences for material hardening and failure. However, because of lattice rotation between cells, ray-tracing from the diffraction image to the sample required for topography is not possible. This intrinsic problem of real and reciprocal space convolution has previously prevented X-ray topography from being used to image deformation structure within (poly) crystals. The recent availability of hard X-ray nanofocusing permits the deconvolution of real and reciprocal space information by reducing the size of the incident beam to the point where the cumulative amount of lattice curvature within the sampling volume is sufficiently small.

This project aims to investigate inter- and intra-grain deformation structure via X-ray microdiffraction techniques and relate this information to theoretical models of defect behaviour at the nanoscale.

Key references

1. B. Abbey et al. Scripta Materialia.  64:884-887. 2011
2. F. Hofmann et al.  Journal of Synchrotron Radiation.  19:307-318. 2012

Collaborators

• A. M. Korsunsky, The University of Oxford
• F. Hofmann, Massachusetts Institute of Technology