Effect of microstructural and processing parameters on the fatigue properties of severely deformed iron

在原材料和能源成本不断上升的时代，资源效率方面对产品的生产和应用都越来越重要。面对这一挑战的一个可能的方法是改善与其应用最相关的材料的关键特性。在金属结构材料领域，疲劳性能是其广泛应用的基础。提高金属静强度和循环强度的一种很有前途的方法是通过剧烈塑性变形（SPD）工艺进行晶粒细化。然而，对于这些工艺，人们对工艺参数（以及由此产生的显微组织）对疲劳性能，特别是循环载荷下的损伤机制的影响知之甚少。这种知识的缺乏是当前研究项目的动力，该项目将通过对纯铁作为模型材料的基础研究，有助于对这些相关性的基本理解。该项目不仅关注超细晶（UFG）微观组织，还关注应变在1至4范围内的严重变形状态。这种变形状态可以被视为UFG结构的前驱体，并且需要显着降低制造成本。该项目专注于识别10^4次循环以上的主要损伤机制及其与SPD加工引起的严重变形微观结构和过早损伤的某些特征的相关性。为此，将系统地改变工艺和微观结构参数，以研究它们对主要损伤机制以及循环强度的影响。通过预应变在1至4范围内的变化，将评估前驱体状态作为UFG材料的成本效益替代品的潜力。在应变率、静水应力等相关工艺参数变化的情况下，会产生预应变大于6的UFG微结构。这些将用于研究细长结构中有效滑动长度的影响，以及SPD加工对疲劳性能及其各向异性的过早损伤的相关性。

In times of constantly rising raw material and energy costs, the aspect of resource efficiency is of growing importance for both production and application of products. A possible approach to face this challenge is to improve the crucial properties of materials which are most relevant for their applications. In the field of metallic structural materials the fatigue properties are fundamental for a wide range of applications. A promising method to increase both static and cyclic strength of metal is the grain refinement by Severe Plastic Deformation (SPD) processes. However, for these processes there is only little knowledge on the implications of processing parameters (and the resulting microstructures) on the fatigue properties and especially the damage mechanisms under cyclic loading. This lack of knowledge is the impetus for the present research project which shall contribute to the basic understanding of these correlations by fundamental investigations on pure iron as a model material. The project not only focuses on ultrafine grained (UFG) microstructures but also on severely deformed states with strains in the range of 1 to 4. Such deformed states can be regarded as a precursor to UFG structures and require significantly lower manufacturing costs.The project concentrates on the Identification of the dominant damage mechanisms above 10^4 cycles and their correlation with certain features of the severely deformed microstructure and premature damage caused by the SPD processing. For this purpose processing and microstructural parameters will be systematically varied to investigate their effect on the dominating damage mechanism as well as on the cyclic strength. Through the variation of the prestrain in the range of 1 to 4, the potential of precursors states as a cost-efficient alternative to UFG materials will be evaluated. Furthermore, UFG microstructures with a prestrain above 6 will be generated with varying aspect ratio and varying damage relevant processing parameters such as strain rate and hydrostatic stress. These will be used to investigate the impact of the effective glide length in elongated structures and relevance of premature damage from the SPD processing on the fatigue properties and their anisotropy.