Development of refractory metal-based CCAs with improved mechanical properties

本提案的目标是开发新的难熔金属基高温材料，其热性能超过最先进的镍基高温合金。在第一期资助期内，对Ta-Nb-Mo-Ti-Cr-Al合金体系中的单相难熔高熵合金（RHEA）和多相难熔复合合金（RCCA）的晶体结构、显微组织、力学性能和高温氧化行为进行了深入研究。通过热力学建模和详细的实验表征相结合，对相关性质进行了探索。Ta-Mo-Ti-Cr-Al体系中的合金具有出色的前景：一些合金具有高达2100°C的固相温度，同时显示出良好的氧化保护，最高可达1500°C。更具体地说，Ta-Mo-Ti子系统的合金表现出单相A2组织，在高温下具有足够的强度，在室温下具有较大的（压缩）变形能力。添加Al和Cr的Ta-Mo-Ti合金，如果是单相B2，在高温下具有很高的强度，但在中等温度下具有很低的塑性。Ta-Mo-Ti-Cr-Al体系中的一些合金甚至具有有利于高温应用的双相A2-B2显微组织，类似于镍基高温合金的两相显微组织。第二个资助期的研究活动将侧重于开发基于Ta-Mo-Cr-Ti-Al体系的合金，该体系具有A2基体和大体积分数的有序B2沉淀。在室温下的延展性，在高温下的抗蠕变性以及抗氧化性的平衡组合是追求。为了实现这些目标，将采取两个主要办法。从（已知的）韧性三元体系Ta-Mo-Ti开始，它在宽温度和成分范围内具有单相A2微观结构，其高温力学性能将通过：(i)单相A2 RHEA的固溶强化和（ii）在韧性A2基体中均匀分布的B2析出相的形成来调整。通过Ta、Mo和Ti含量的系统变化以及在Ta-Mo-Ti三元体系中加入低浓度的Cr和Al来研究固溶强化。在热力学计算的指导下，通过增加Al和Cr浓度并进行适当的热处理，可以实现A2基体中B2的析出。除了热力学建模之外，实验工作还包括热学和微观结构分析、机械性能表征（压缩、拉伸和蠕变测试）以及随后的氧化行为检查。

The objective of the present proposal is to develop new refractory metal-based high temperature materials with a thermal capability beyond that of the state-of-the-art Ni-based superalloys. Single-phase refractory high entropy alloys (RHEA) as well as multi-phase refractory compositionally complex alloys (RCCA) within the alloy system Ta-Nb-Mo-Ti-Cr-Al have intensively been investigated in the first funding period in terms of their crystal structure, microstructure, mechanical properties and high temperature oxidation behavior. The exploration of relevant properties proceeded by combining thermodynamic modeling and detailed experimental characterization. Alloys within the Ta-Mo-Ti-Cr-Al system yield outstanding perspective: Some alloys possess solidus temperatures up to 2100°C whilst showing decent oxidation protection up to 1500°C. More specifically, alloys from the Ta-Mo-Ti sub-system exhibit a single-phase A2 microstructure, adequate strength at elevated temperatures and large (compressive) deformability at room temperature. Ta-Mo-Ti alloys with Al and Cr additions, if being single-phase B2, possess very high strength at elevated but low ductility at moderate temperatures. Some alloys within the system Ta-Mo-Ti-Cr-Al even possess a dual-phase A2-B2 microstructure, which is regarded to be favorable for high temperature applications, akin to two-phase microstructures of Ni-base superalloys. The research activities in the second funding period will focus on the development of alloys based on the system Ta-Mo-Cr-Ti-Al with A2 matrix and large volume fractions of ordered B2 precipitates. A balanced portfolio of ductility at room temperature, creep resistance at elevated temperatures as well as oxidation protectiveness is pursued. In order to achieve these objectives, two main approaches will be followed. Starting from the (known) ductile ternary system Ta-Mo-Ti, which possesses a single-phase A2 microstructure in a wide temperature and compositional range, its high temperature mechanical properties will be tuned by: (i) solid solution strengthening of single-phase A2 RHEA and (ii) formation of homogeneously distributed B2 precipitates in a ductile A2 matrix. The solid solution strengthening will be studied by systematic variation of Ta, Mo and Ti contents as well as addition of Cr and Al in low concentrations to the ternary system Ta-Mo-Ti. Guided by thermodynamic calculations, B2 precipitates within the A2 matrix will be realized by increasing the Al and Cr concentrations in combination with the application of appropriate heat treatments. In addition to thermodynamic modeling, the experimental efforts cover thermal and microstructural analyses, characterization of mechanical properties (compressive, tensile, and creep tests) and ensuing examination of oxidation behavior.