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

孪晶和滑移带的强烈局部变形足以引发断裂，特别是在锆、铀和镁等各向异性金属中。在其他工程合金中，滑移局部化可由中子辐照效应或疲劳循环变形引起。合金的微观结构设计将提高机械性能，例如使用晶体塑性建模，需要这些相互作用的数据。然而，由于问题的复杂性和缺乏高分辨率的方法来获得应力和应变场的定量数据，很少有可靠的测量方法。牛津大学之前对弹性应变的全场分析工作表明，使用同步加速器x射线衍射获得的低分辨率应变图可以通过一种新的基于有限元的方法进行分析，以充分量化裂缝的弹性应力集中（Huber, j.e., Hofmann， F., Barhli， S., Marrow, t.j. & Hildersley， C.(2017)）。多晶铁电材料裂纹扩展的同步x射线衍射观察。材料圣经，140,23-26。Barhli， S. M, Saucedo-Mora， L., Simpson， C., Becker， T., Mostafavi， M., Withers， P. J. &骨髓，T. J.（2016）。基于衍射的裂纹场应变映射得到j积分。结构完整性学报，2,2519-2526)。同样，通过DIC（数字图像相关）图像分析技术测量的全场位移数据可以与非弹性应变/应力关系的知识一起使用，以提取应力场，从而提取应力集中系数（Marrow, t.j., Liu， D., Barhli, s.m., saucdo - mora， L., Vertyagina， Y., Collins, d.m., Reinhard， C., Kabra， S., Flewitt, p.e.j. & Smith, d.j.(2016)）。机械加载多颗粒石墨内应变的原位测量。碳96,285-302)。这些研究是在相对较大的尺度上进行的（厘米大小的标本和裂缝）。挑战在于开发新的方法来研究微观尺度下的应变集中特征。我们的初步研究，应用于从滑移带获得的HR-EBSD数据（高分辨率电子背散射衍射）（Guo Y., Britton T. B. & Wilkinson A. J.(2014)，商业纯度钛的滑移带-晶界相互作用，Acta Mater. 76, 1-12），表明这是可以做到的。现在的挑战是验证这种高分辨率方法的可靠性，然后将其应用于工程材料微观结构中应力集中相互作用的定量研究。本项目旨在开发一种新的方法来表征工程合金微观结构中应变集中特征（如滑移带、孪晶和裂纹）的应力场和应变场。这将使我们充分了解导致这些材料损坏的关键相互作用的强度，并有助于设计具有更高电阻的新合金。因此，目标是开发和验证一种高分辨率有限元分析方法，利用HR-EBSD数据测量弹性应变场，DIC测量总应变场（弹性和非弹性应变）。初步研究将对模型材料（如硅、时效硬化双相不锈钢和镁单晶）中堵塞滑移带、孪晶和初始解理裂纹的应变场进行研究。之所以选择这些材料，是因为它们适合于EBSD（易于制备样品）和表征良好的滑移/孪生行为，它们代表了不同的加载模式。这项研究将继续研究更多的材料，特别是那些与核能有关的材料，其中辐照损伤会影响变形行为。该项目属于EPSRC工程研究主题，在材料工程领域-金属和合金。

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.