Development of refractory metal-based CCAs with improved mechanical properties

本提案的目的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。在第一个资助期内，对Ta-Nb-Mo-Ti-Cr-Al合金系中的单相难熔高熵合金（RHEA）和多相难熔成分复杂合金（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。通过结合热力学建模和详细的实验表征进行相关属性的探索。Ta-Mo-Ti-Cr-Al系统中的合金具有出色的前景：某些合金的固相线温度高达2100°C，同时在高达1500°C的温度下显示出良好的氧化保护。更具体地，来自Ta-Mo-Ti子系统的合金表现出单相A2微观结构、在升高的温度下足够的强度和在室温下大的（压缩）变形性。添加Al和Cr的Ta-Mo-Ti合金，如果是单相B2，则在高温下具有非常高的强度，但在中等温度下具有低的延展性。Ta-Mo-Ti-Cr-Al系统中的一些合金甚至具有双相A2-B2显微组织，这被认为有利于高温应用，类似于Ni基超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系统的合金，该系统具有A2基体和大体积分数的有序B2沉淀物。追求室温延展性、高温抗蠕变性以及氧化保护性的平衡组合。为了实现这些目标，将采取两种主要办法。从（已知的）韧性三元系Ta-Mo-Ti开始，其在宽的温度和组成范围内具有单相A2显微组织，其高温机械性能将通过以下方式调整：（i）单相A2 RHEA的固溶强化和（ii）在韧性A2基体中形成均匀分布的B2沉淀物。通过系统地改变Ta、Mo和Ti的含量以及在Ta-Mo-Ti三元系中添加低浓度的Cr和Al来研究固溶强化。由热力学计算的指导下，B2的A2矩阵内的析出物将实现通过增加Al和Cr的浓度与适当的热处理的应用相结合。除了热力学建模，实验工作包括热和微观结构分析，机械性能（压缩，拉伸和蠕变测试）的表征和随后的氧化行为的检查。

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.