Microstructures and elementary deformation mechanisms of single-phase fcc and bcc high-entropy alloys

这项研究旨在帮助人们更好地理解一类新的金属材料，即所谓的高熵合金（HEAs）。这些是由四种、五种或更多种金属成分以接近等原子浓度组成的成分复杂的合金。对于特定的元素组合，已经表明可以生产由具有立方晶体结构的单相固溶体组成的HEAs，例如，面心立方或体心立方，并且具有表面上高度的热力学稳定性。单相HEAs的机械性能的第一次评估揭示了有趣的机械性能，如高机械强度和延展性。此外，详细的研究等摩尔，面心立方结构的HEAs组成的钴，铬，铁，锰和镍的特征机械性能表现出很强的温度依赖性。特别是强屈服应力随温度降低的增加是不寻常的合金与这种晶体结构。目前还不清楚所观察到的特性是否反映了HEAs的固有特性，或者它们是否仅仅与这种特定的合金组合有关。因此，本项目仅关注表现出单相微观结构的合金。除了HEA组合物的研究，这被证明产生单相微结构，新的bcc和fcc结构的HEA将产生其组合物将被改变的目标，直接控制本征性质，如堆垛层错能。重点将放在确定合适的热机械加工路线，允许建立化学和微观结构均匀的材料，这方面往往被忽视，在以前的研究。然而，均匀的微观结构是机械性能的有意义的表征的先决条件。后者是拟议工作的另一个主要目标。一方面，开放的问题，这是在以前的实验研究中提出的处理HEAs的机械性能将解决精心设计的实验。这样一个开放的问题的一个例子是强烈的温度依赖性的屈服应力在面心立方结构的HEAs的性质，这将是第一次准确的评估的激活参数进行调查。另一方面，计划对其他单相HEA进行第一次彻底表征，其机械行为尚未以系统的方式表征，即，大多数BCC结构的合金。结合起来，所获得的结果将大大有助于确定HEAs是否具有独特的固有特性，因为经常假设。此外，拟议的实验将显着扩展HEAs的机械性能数据库，这对于确定进一步研究工作甚至实际应用的潜在候选材料是必要的。

The proposed research aims at contributing to a better understanding of a new class of metallic materials, the so-called high-entropy alloys (HEAs). These are compositionally complex alloys consisting of four, five or more metallic components in near-equiatomic concentrations. For specific element combinations, it has been shown that HEAs can be produced that consist of a single-phase solid solution with a cubic crystal structure, e.g., face-centered cubic or body-centered cubic, and with a seemingly high degree of thermodynamic stability. First assessments of the mechanical properties of single-phase HEAs revealed interesting mechanical properties, such as high mechanical strength and ductility. Moreover, detailed studies on an equimolar, fcc-structured HEAs consisting of Co, Cr, Fe, Mn and Ni showed that the characteristic mechanical properties exhibit strong temperature dependencies. In particular the strong yield stress increase with decreasing temperature is unusual for alloys with this crystal structure. It is yet unclear whether the observed peculiarities reflect intrinsic properties of HEAs or whether they are merely related to this specific alloying combination.As a consequence, the present project focuses solely on alloys that exhibit single-phase microstructures. Besides the investigation of HEA compositions which were demonstrated to yield single-phase microstructures, new bcc- and fcc-structured HEAs will be produced whose compositions will be altered with the goal of directly controlling intrinsic properties, such as the stacking fault energy. Focus will be placed on the identification of suitable thermomechanical processing routes that allow for establishing chemically and microstructurally homogeneous materials, an aspect which has often been neglected in previous studies. Homogeneous microstructures are, however, a prerequisite for a meaningful characterization of the mechanical properties. The latter is another major goal of the proposed work. On the one hand, open questions which were raised in previous experimental studies dealing with mechanical properties of HEAs will be addressed by well-designed experiments. An example of such an open question is the nature of the strongly temperature dependent yield stress in fcc-structured HEAs which will be investigated by a first accurate assessment of the activation parameters. On the other hand, a first thorough characterization is planned for other single-phase HEAs whose mechanical behavior has not yet been characterized in a systematic fashion, i.e., most bcc-structured alloys. In combination, the obtained results will help greatly in determining whether HEAs possess unique intrinsic properties as is often hypothesized. In addition, the proposed experiments will significantly extend the mechanical property data base for HEAs, something that is necessary to identify potential candidate materials for further research efforts or even practical applications.