Development and Characterization of Novel Refractory High-Entropy Alloys for High Temperature Applications

目前，镍基高温合金是高温应用的主要材料类别。然而，由于它们的熔点在1400°C左右，它们的应用受到了严格的限制。在这个项目中，以难熔金属为基础的所谓的高熵合金(HEA)将被提出作为新型高温结构材料。HEA概念的显著区别在于，所有合金元素都呈现等摩尔或接近等摩尔浓度。因此，从简化的热力学观点来看，与金属间化合物相比，随机固溶体的形成是潜在的延性相，而金属间化合物往往是复杂和脆性的。结果表明，20Mo-20W-20Al-20Cr-20Ti合金具有体心立方结构的单相组织，具有良好的热稳定性和高温强度。虽然许多含有难熔金属的材料遭受严重氧化，但所研究的合金显示出令人惊讶的良好的高温抗氧化性。此外，计算的熔点比商用镍基高温合金的熔点高出约300℃。在该方案中，将开发基于Mo-W-Al-X-Y系统的新型HEA。选择了三种化学成分如下的参考合金：(I)20Mo-20W-20Al-20Cr-20Ti，(Ii)20Mo-20W-20Al-20Cr-20Mn和(III)20Mo-20W-20Al-20Mn-20Ti。由于HEA方法允许微合金化，因此将研究几种附加元素，如B、Zr、Si和Y的影响。从传统合金的使用经验来看，添加B和Zr可显著提高合金的力学性能，而添加Si和Y可显著提高合金的抗氧化性。这项建议主要是为了探索新型耐火材料HEA的潜力和开发物理冶金策略，以满足高温应用的要求。实验研究将集中在广泛的显微组织分析、蠕变行为和高温抗氧化性上。在此基础上，可以得出目标合金开发的冶金方法，这可能有助于在未来的高温应用方面改善性能。

Nowadays, Ni-based superalloys are the dominant material class for high temperature applications. However, their use is strictly limited by their melting point of around 1400°C. In this project, so called High Entropy Alloys (HEA), which are based on refractory metals, will be proposed as novel high temperature structural materials. The appealing difference of the concept of HEA is that all alloy elements exhibit equimolar or near-equimolar concentrations. Thus, from a simplified thermodynamic point of view, the formation of random solid solutions being potentially ductile phases is favored against intermetallic compounds, which are often complex and brittle in nature. As a further consequence, this simple microstructure may yield an attractive property profile such as high thermal stability and high temperature strength while being tough and even ductile at ambient temperatures.First results clearly show that the alloy 20Mo-20W-20Al-20Cr-20Ti possesses a single phase microstructure, which is body centered cubic. Although many materials containing refractory metals suffer from severe oxidation, the alloy studied reveals a surprisingly good high temperature oxidation resistance. Besides, the calculated melting point is approximately 300°C higher than that of commercial Ni-based superalloys. In this proposal, new HEA based on the system Mo-W-Al-X-Y will be developed. Three reference alloys with the following chemical compositions are chosen for further investigations: (i) 20Mo-20W-20Al-20Cr-20Ti, (ii) 20Mo-20W-20Al-20Cr-20Mn und (iii) 20Mo-20W-20Al-20Mn-20Ti. Since the HEA approach allows micro-alloying, the effect of several additional elements, such as B, Zr, Si, and Y will be studied. Well known from past experience with conventional alloys, the addition of B and Zr may lead to a significant improvement of mechanical properties, whereas alloying with Si and Y may cause a substantial increase of the oxidation protectiveness. This proposal mainly aims at the exploration of potential of novel refractory HEA and the development of physical metallurgy strategies to satisfy the requirements for high temperature applications. Experimental investigations will be focused on extensive microstructural analysis, creep behavior, and high temperature oxidation resistance. Based on this, metallurgical approaches on targeted alloy development can be derived which may facilitate property improvement with respect to future high temperature applications.