Tailored precipitation (B2, L21) strengthened, compositionally complex FeAlCr (Mn, Co, Ni, Ti) alloys for high temperature applications

高熵合金(HEAs)或成分复杂合金(CCA)是具有五种以上合金元素的多元体系。大的成分空间提供了吸引人的合金设计策略，最初的假设是通过最大化混合的熵，可以建立真正的单相固溶体。然而，在过去10年的HEA发展中，成功的单相合金寥寥无几，在大多数情况下，形成了包括金属间化合物相在内的多相组织。然而，这种多相合金系统可以在合金密度和应用温度方面弥合钛合金和镍基合金之间的巨大差距，但这一应用领域很少被开发。本工作的目的是开发基于B2和/或L21相的体心立方(BCC)CCA，其密度为&lt；7g/cm3，用于高温应用，最高温度可达900℃。第一步，通过基于薄膜沉积技术的高通量筛选和表征，探索在相当未被探索的FeAlCr(Mn，Co，Ni，Ti)的成分空间中的相形成。确定了满足具有体心立方晶体结构和显示B2、L21或这两种类型的析出相的条件的有前途的候选合金。在进一步的步骤中，通过常规铸造技术获得这些候选合金，以研究铸态合金的组织和析出物结构，并评估其在高达900℃的热暴露后的高温稳定性。合金设计方法基于从析出物形态、与过饱和的体心立方基质的一致性及其体积分数方面的微观结构优化。然后对定制的微结构进行从室温到900°C的机械测试，以建立微结构-性能关系，以便进行进一步的优化步骤。此外，首次高温蠕变实验深入了解了高温加载下的组织稳定性和工作变形机制。整个合金设计过程中的组织表征和力学性能评估是本研究的基石。机械测试与先进的表征技术相结合，如X射线衍射、透射电子显微镜和原子探针断层扫描，正在指导这些新型合金系统从介观长度尺度到原子尺度的发展。

High entropy alloys (HEAs) or compositionally complex alloys (CCAs) are multicomponent systems with typically more than five alloying elements. The large compositional space offers attractive alloy design strategies and initially it was assumed that by maximizing the entropy of mixing, true single phase solid solutions can be established. However, only very few successful single phase alloys could be obtained in the past 10 years of HEA development and in the majority of cases multi-phase microstructures, including intermetallic phases, form. However, such multi-phase alloy systems could yield alloys bridging the enormous gap between Titanium and Nickel-based alloys in terms of alloy density and application temperature, but this application field is rarely explored.The aim of the present work is to develop body-centered cubic (BCC) CCAs with tailored precipitates based on B2- and/or L21-phases with densities of < 7 g/cm3 for high temperature applications up to 900ºC. In a first step, the phase formation in the rather unexplored composition space of FeAlCr (Mn, Co, Ni, Ti) is explored by high throughput screening and characterization based on thin film deposition techniques. Promising alloy candidates are identified fulfilling the conditions to have a BCC crystal structure and showing either B2, L21 or both types of precipitate phases. In further steps, these alloy candidates are then obtained by conventional casting techniques to investigate the microstructure and precipitate structure in as-cast alloys and to evaluate their high temperature stability after thermal exposure of up to 900ºC. The alloy design approach is based on a microstructural optimization in terms of precipitate morphology, coherency with the supersaturated BCC matrix and their volume fraction. Tailored microstructures are then mechanically tested from room up to temperatures of 900ºC to establish the microstructure-property relationship for further optimization steps. In addition, first high temperature creep experiments give insights into the microstructural stability at elevated temperatures under load and the operating deformation mechanisms.The microstructural characterization and mechanical property evaluation throughout the alloy design process are cornerstones of the presented research. The combination of mechanical testing with advanced characterization techniques such as X-ray diffraction, transmission electron microscopy and atom probe tomography is guiding the development of these novel alloy systems from a mesoscopic length scale down to the atomic scale.