This paper focuses on the characterisation of fatigue crack initiation mechanisms under fully reversed torsional loads for the porosity-containing cast AlSi7Mg0.3 aluminium alloy by synchrotron X-ray tomographic imaging and Diffraction Contrast Tomography (DCT). The aim is to analyse the relation between crack initiation and the microstructure, in the fatigue regime close to the fatigue limit of the material (fatigue lives of approximately 2×106 cycles). In-situ torsion fatigue tests have been conducted at the European Synchrotron Radiation Facility. A large number of cracks (approximately 80 cracks) were analysed, at the surface and in the bulk, highlighting the role of the surrounding microstructure on the crack initiation mechanisms. Contrary to uniaxial loads in which fatigue crack initiation from casting pores in an opening mode (mode I) is generally only observed for this material, it is shown in this work that multiple crack initiations mechanisms can appear simultaneously under torsional loads. This is very interesting because it is possible to analyse different mechanisms (and the effect of the surrounding microstructure) using the same specimen, that is under identical loading conditions. More precisely, two different crack initiation mechanisms were observed. The first mechanism involves intra-granular fatigue crack initiation from Persistent Slip Bands (PSBs) formed on the slip system with the highest Schmid factor. For this mechanism, the grain orientation is the key parameter. A shear stress threshold of approximately 80 MPa is observed to trigger this mechanism. The presence of a pore, in the crack initiation zone reduces this threshold. The second mechanism concerns crack initiation from pores, in mode I, on planes of maximum principal stress. No link to the local grain orientation was found in that case. The pore size is the key factor governing this crack initiation mechanism. Regarding the competition between these mechanisms in the formation of the final crack, it was observed that at high applied stress, the cracks initiated from PSBs are dominant due to their high density and high growth rates resulting from coalescence. At lower stress, cracks initiated from pores are dominant, mainly because of the little effect of grain boundaries and eutectic zones, while most of cracks initiated from PSBs are arrested within the grain where they have initiated.

中文翻译：

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同步加速器 X 射线 3D 表征原位扭转循环测试期间疲劳裂纹的萌生


本文重点介绍了同步辐射 X 射线断层扫描和衍射对比断层扫描 （DCT） 对含孔隙率的铸造 AlSi7Mg0.3 铝合金在完全反转扭转载荷下的疲劳裂纹萌生机理的表征。目的是在接近材料疲劳极限的疲劳状态下（疲劳寿命约为 2×106 次循环）分析裂纹萌生与微观结构之间的关系。欧洲同步辐射设施已进行了原位扭转疲劳测试。分析了表面和体中的大量裂纹（大约 80 个裂纹），突出了周围微观结构对裂纹萌生机制的作用。与单轴载荷相反，在单轴载荷中，通常只观察到这种材料在张开模式（模式 I）下从铸孔开始产生疲劳裂纹，而这项工作表明，在扭转载荷下可以同时出现多个裂纹萌生机制。这非常有趣，因为可以在相同的载荷条件下使用相同的试样来分析不同的机制（以及周围微观结构的影响）。更准确地说，观察到了两种不同的裂纹萌生机制。第一种机制涉及在具有最高 Schmid 因子的滑移系统上形成的持续滑移带 （PSB） 引发晶内疲劳裂纹。对于这种机制，颗粒取向是关键参数。观察到大约 80 MPa 的剪切应力阈值会触发这种机制。裂纹起始区中存在孔隙会降低此阈值。第二种机制涉及在模式 I 中，在最大主应力平面上从孔隙开始产生裂纹。 在这种情况下，没有找到指向局部颗粒方向的链接。孔径是控制这种裂纹萌生机制的关键因素。关于这些机制在最终裂纹形成过程中的竞争，据观察，在高施加应力下，由 PSB 引发的裂纹占主导地位，因为它们的高密度和聚结产生的高增长率。在较低应力下，由孔隙引发的裂纹占主导地位，这主要是因为晶界和共晶区的影响很小，而大多数由 PSB 引发的裂纹被阻止在它们开始的晶粒内。