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，特别是在各向异性金属中，如锆、铀和镁。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。例如，使用晶体塑性模型，对将具有更好的机械性能的合金进行微结构信息设计，需要关于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方法来获得应力和应变场的定量数据，几乎没有可靠的测量。牛津大学以前在全场弹性应变分析方面的工作表明，使用同步辐射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来分析以完全量化裂纹的弹性应力集中(Huber，J.E.，Hofmann，F.，Barhli，S.，Marrow，T.J.&Hildersley，C.(2017))。用同步辐射X射线衍射法观察多晶铁电体中的裂纹扩展。斯克里塔材料140，23-26。Barhli，S.M.，Saucedo-Mora，L.，Simpson，C.，Becker，T.，Mostafavi，M.，Withers，P.J.&Marrow，T.J.(2016)。通过基于绕射的裂纹场应变映射得到J积分。Procedia结构完整性2,2519-2526)。类似地，通过DIC(数字图像相关)的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.，Saucedo-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范围内进行的(厘米大小的试件和裂缝)。挑战是开发能够在微观尺度上研究应变集中特征的新方法。我们的初步研究，应用于高分辨电子背散射衍射数据(高分辨电子背散射衍射)获得的滑移带(郭勇，Britton T.B.和Wilkinson A.J.(2014)，滑移带-晶界相互作用的工业纯钛，学报，材料)。76，1-12)已经表明这是可能做到的。目前的挑战是验证这种高分辨率方法的可靠性，然后将其应用于定量研究工程材料微观结构中应力集中的相互作用。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的应力和应变场，如滑移带、孪晶和裂纹。这将使人们能够充分了解导致这些材料损坏的关键相互作用的强度，并将有助于设计具有更高抗力的新合金。因此，我们的目标是开发和验证一种高分辨率的有限元分析方法，使用HR-EBSD数据测量弹性应变场，使用DIC测量总应变场(弹性和非弹性应变)。将对模型材料(例如硅、时效硬化双相不锈钢和镁单晶)中受阻滑移带、孪晶和引发解理裂纹的应变场进行初步研究。之所以选择这些材料，是因为它们适合EBSD(易于制备样品)和具有良好特性的滑移/孪生行为，这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，在这些材料中，辐射损伤可以影响变形行为。本项目属于EPSRC工程研究主题，属于材料工程领域-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.