News

Title: Correlating evolution of defects and microstructure with material
performance: A combined multi-scale model and multi-technique
experiments study
Speaker: Dr. Suchandrima Das
Department of Mechanical Engineering, University of Bristol, UK

Abstract: Probing individual grains in polycrystals and local structure in composites and understanding the behavior of defects in them (e.g. dislocations, micro-cracks, voids, vacancies) is of fundamental importance for the performance and longevity of structural and functional materials. For components in high-performance applications such as aerospace, medical implants, nuclear reactors etc., where materials are operated with narrow margins of error, a detailed understanding of mechanism of defect-induced changes, associated failure mechanism and their microscopic origins is important. Material damage and fatigue proceed by microscopic
weakest link type mechanisms such as persistent slip bands, micro/nano crack initiation and fatigue crack propagation. To accurately predict component performance and lifetime, characterisation across multiple contributing length-scales and across a range of deformation regimes, including quasi-static and creep loadings, fatigue, impact/high strain rate and shock loading is required. Today’s talk will demonstrate how such a complete picture of material behaviour can be obtained using multi-scale models, formulated and validated
using multi-technique experiments. The demonstration will be done using the case of characterizing irradiation damage in ion-implanted tungsten. Tungsten is the front-runner material for armour components in future fusion reactors. In service, irradiation with fusion neutrons will generate displacement cascades, leaving behind lattice defects.
Helium, injected from the plasma and produced by transmutation, strongly interacts with these lattice-defects, modifying their behaviour and retention. As the fusion reactor conditions cannot be recreated yet, alternative means are necessary to study irradiation damage in armour components. Self-ion-implantation and helium-ion implantation provides a convenient approach for mimicking this irradiation damage. A complementary combination of techniques such as nano-indentation and atomic force microscopy
(AFM), electron microscopy (e.g. SEM, EBSD, TEM), high-resolution EBSD (for probing local lattice distortions and dislocation densities), electron channel contrast imaging (ECCI) (for non-destructively probing
defect content) and synchrotron X-ray Laue diffraction (for in-situ 3D, non-invasive measurements of lattice distortions at sub-micron scale) are used to map the dynamics of property evolution of the ion-implanted tungsten as a function of defect nature, defect concentration, crystallographic orientation etc. The talk will illustrate the small-scale response information from the experiments and subsequently how they are then used to develop, tune and validate crystal-plasticity (CPFE) models of irradiated tungsten. With only one fitting parameter, the models, were able to make predictions in quantitative agreement with experimental measurements, capture the experimentally observed orientation-dependence in deformation behaviour in implanted tungsten and predict macroscopic stress-strain curves for similarly implanted polycrystalline components; a task challenging to achieve experimentally owing to limited ion penetration depth.
However, there are areas of improvement in the CPFE model as there remain some discrepancies between the experimental observations and the predictions from the simulations. The talk will underline these discrepancies and demonstrate how using simulations at lower length- and time-scales such as Molecular Dynamics can rectify incorrect assumptions made in the formulation of the CPFE model. This highlights the importance of using atomistic simulations to provide a mechanistic basis for larger scale simulations. Such integrated multi-scale models captures the physically based mechanisms underlying microstructural evolution. Virtual toolboxes built on such models can therefore simulate in-service loading, provide direct information about grain-scale behaviour that cannot be obtained in any other way. This will enable translation of the experimentally acquired information into the design of structural components and is vital for industrial application

Bio: Suchandrima is a Faculty member at the Department of Mechanical Engineering at University of
Bristol. She has a bachelor’s degree from National University of Singapore, focusing on electrical
engineering and specifically on optoelectronic materials. After working on product engineering at
Advanced Micro Devices for a year, she went on to do a master’s degree at ETH Zurich on biomedical
engineering, centring specifically on biomechanics. Subsequently she spent a year as a research
associate in Singapore General Hospital, working on developing high-resolution optical coherence
tomography imaging tools for biological tissues. In 2016, she joined the Department of Engineering
Science, at University of Oxford, as a DPhil student of Solid Mechanics, associated with Mansfield
College. In her PhD she used new synchrotron X-Ray diffraction techniques along with numerical modelling to study the changes in material properties due to irradiation damage, with the aim of developing armour components for future fusion reactors. Subsequently she pursued a Career Development Fellowship at Jesus College, at Oxford before moving to Bristol in 2022.
Her current research combines the fields of electrical engineering, biomedical engineering and solid mechanics under the realm of material engineering. It focusses on development of physically-based multi-scale computational material models with the help of ab-initio calculations and multi-technique experiments, such as micro-mechanical tests, synchrotron X-ray diffraction and electron microscopy. The combination of multi-technique experiments and multi-scale modelling is targeted to provide a complete understanding of the microstructural evolution in materials that spans across hierarchically organised chain of multiple length- and time-scales. This is vital for designing pathways for a desired microstructure and thereby desired material properties and is thus the fundamental key for design and characterisation of novel materials for high-performance applications such as medical implants, nuclear reactor components, high-frequency transistors etc.

Thanks and Regards,
Sangeeta and Dibyajyoti