In Situ Full-Field Characterisation of Strain Concentrations (Slip Bands and Twins)

Lead Research Organisation: University of Oxford
Department Name: Materials

Abstract

The intense localised deformation of twins and slip bands can be sufficient to initiate fracture, particularly in anisotropic metals such as zirconium, uranium and magnesium. In other engineering alloys, slip localisation can be induced by the effects of neutron irradiation or through the cyclic deformation of fatigue. Microstructure-informed design of alloys that will have improved mechanical performance, using crystal plasticity modelling for instance, requires data on these interactions. However, there are few reliable measurements due to the complexity of the problem and the lack of high-resolution methods to obtain quantitative data for the stress and strain fields.

Previous work at Oxford on the full-field analysis of elastic strains has shown that low resolution strain maps, obtained using synchrotron X-ray diffraction, can be analysed to fully quantify the elastic stress concentrations of cracks via a novel finite-element based method (Huber, J. E., Hofmann, F., Barhli, S., Marrow, T. J. & Hildersley, C. (2017). Observation of crack growth in a polycrystalline ferroelectric by synchrotron X-ray diffraction. Scripta Materialia 140, 23-26. Barhli, S. M., Saucedo-Mora, L., Simpson, C., Becker, T., Mostafavi, M., Withers, P. J. & Marrow, T. J. (2016). Obtaining the J-integral by diffraction-based crack-field strain mapping. Procedia Structural Integrity 2, 2519-2526). Similarly, the full field displacement data, measured by the image analysis technique of DIC (digital image correlation), can be used together with knowledge of the inelastic strain/stress relationship, to extract the stress field and hence the stress concentration factor (Marrow, T. J., Liu, D., Barhli, S. M., Saucedo-Mora, L., Vertyagina, Y., Collins, D. M., Reinhard, C., Kabra, S., Flewitt, P. E. J. & Smith, D. J. (2016). In situ measurement of the strains within a mechanically loaded polygranular graphite. Carbon 96, 285-302). These studies were done at a relatively large scale (cm-size specimens and cracks). The challenge is to develop new methods that can study strain concentrating features at the microscale. Our preliminary studies, applied to HR-EBSD data (High-resolution Electron BackScatter Diffraction) that were obtained for slip bands (Guo Y., Britton T. B. & Wilkinson A. J. (2014), Slip band-grain boundary interactions in commercial-purity titanium, Acta Mater. 76, 1-12) have shown this may be done. The challenge now is to verify the reliability of this high-resolution method, and then apply it in a quantitative study of the interactions of stress concentrations in the microstructure of engineering materials.

This project aims to develop a novel method to characterise the stress and strain fields of strain concentrating features, such as slip bands, twins and cracks, in the microstructure of engineering alloys. This will allow a full understanding of the intensity of the critical interactions that lead to damage in these materials and will aid the design of new alloys with improved resistance. The objective is therefore to develop and validate a high-resolution finite element analysis method, employing HR-EBSD data to measure the elastic strain field and DIC to measure the total strain field (elastic plus inelastic strains).

The initial studies will be conducted on the strain fields at blocked slip bands, twins and initiated cleavage cracks in model materials (e.g. silicon, age-hardened duplex stainless steel and magnesium single crystals). These materials have been chosen due to their suitability for EBSD (ease of sample preparation) and well characterised slip/twinning behaviour, which represent different modes of loading. The study will continue into further materials, particularly those relevant to nuclear energy in which irradiation damage can affect the deformation behaviour.

This project falls within the EPSRC Engineering research theme, in the area of Materials engineering - metals and alloys.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509711/1 01/10/2016 30/09/2021
2113627 Studentship EP/N509711/1 01/10/2018 31/03/2022 Abdalrhaman Mohamed
EP/R513295/1 01/10/2018 30/09/2023
2113627 Studentship EP/R513295/1 01/10/2018 31/03/2022 Abdalrhaman Mohamed