Mechanisms of Nanoparticles as Novel Grain Refiners for Thermo-mechanically Loaded Aluminum Cast Components

铝合金用于热机械负荷严重的区域，如气缸盖。这类由人工时效合金制成的部件的疲劳寿命急剧下降，因为在最关键的区域（例如阀桥），在高温（约200℃）下，组织会广泛地变粗。由于固溶硬化在这些体系中只起很小的作用，因此机械性能和寿命基本上由晶粒尺寸决定。在这种情况下，拟议项目的目的是通过使用新型晶粒细化剂来显著提高铸铝合金的循环承载能力。具体来说，纳米粒子将用于这一目的。原位热像显微镜和显微组织分析应使用，研究它们对凝固过程和相应晶粒组织的影响。本项目将使用AlSi7Mg合金，因为它通常用于气缸盖。此外，还研究了二元合金AlSi7和纯铝，以便对合金成分的影响有一个基本的了解。由于硼和钛在工业晶粒细化中的成功应用，基于这些元素的纳米颗粒显得特别有前途。在凝固过程中，纳米颗粒可以作为铝的成核位点，也可以与结晶前沿相互作用。因此，有必要详细了解纳米颗粒在这一过程中的转化机制。因此，本研究的目的是确定所使用的颗粒本身是否是有效的成核种子，或者它们是否与其中一种合金成分相互作用，而发生的反应产物充当活性成分。在这种情况下，确定有效的反应产物以及研究必要的反应条件（温度、时间和气氛）是很重要的。显微组织和凝固过程分析将伴随着力学测试，以研究颗粒对力学性能的影响，特别是对热机械疲劳行为的影响。从而更好地了解纳米颗粒在铝熔体中表现出晶粒细化效应的情况，以及导致微观组织细化的实际晶粒细化机制。

Aluminum cast alloys are used in thermo-mechanically severely loaded areas, such as the cylinder head. The fatigue life of such components made of artificially aged alloys decreases drastically due to an extensive coarsening of the microstructure at elevated temperature (> 200 °C) in the most critical areas (e.g. the valve bridge). The mechanical properties, and thus, the lifetime, are then essentially determined by the grain size as solid solution hardening plays only a minor role in these systems.Given this scenario, the aim of the proposed project is to significantly increase the cyclic load-bearing capacity of cast aluminum alloys through the use of novel grain refiners. Specifically, nanoparticles will be used for this purpose. In-situ thermographic microscopy and microstructural analysis shall be used to study their influence on the solidification process and the corresponding grain structure. The alloy AlSi7Mg will be used in this project as it is commonly used in cylinder heads. In addition, binary alloy AlSi7 and pure aluminum are also investigated in order to obtain a fundamental understanding of the influence of the alloying components. Due to the successful use of boron and titanium in industrial grain refiners, nanoparticles based on these elements appear particularly promising. During solidification, nanoparticles can act as nucleation sites for the aluminum or they can interact with the crystallization front. Thus, it is necessary to obtain detailed knowledge of the transformation mechanisms of the nanoparticles during this process. Therefore, the objective of this investigation is to determine whether the particles used are effective nucleation seeds themselves, or if they interact with one of the alloying components and the occurring reaction products act as active ingredients. In this case, it is important to identify the effective reaction products as well as to investigate the necessary reaction conditions (temperature, time, and atmosphere). The microstructure and the solidification process analyses will be accompanied by mechanical testing in order to investigate the impact of the particles on the mechanical properties, particularly on the thermo-mechanical fatigue behavior. Thus, a better understanding of the circumstances under which nanoparticles exhibit a grain-refining effect in aluminum melts and the actual grain refinement mechanisms leading to the refined microstructure shall be gained.