Micromechanical and microstructural investigations of monocrystalline face-centred cubic high- and medium-entropy alloys

最近，一类新的材料，高熵和中熵合金（HEAs/MEAs），含有多个基本相等的原子部分的主要元素，已经席卷了科学界。在多晶HEAs/MEA中，强度取决于合金元素的类型和数量，但缺乏对基本机制的严格理解。为了加深我们对合金化效应的科学理解，测试单晶体至关重要，这样就可以在激活的滑移系上评估摩擦应力作为成分的函数，并与潜在的位错过程相关联。由于大块单晶体的生长是繁琐的，并不总是可行的，我们计划进行微柱和微拉伸试样通过聚焦离子束（FIB）铣削从单个（单晶）晶粒的多晶材料的微观力学测试。这将使我们能够使用常规加工（熔化，铸造，均质化，轧制，再结晶，晶粒生长）来制造任何所需的合金。首先，面心立方HEAs/MEA的基础上的各种组合的Cr，Mn，Fe，Co和Ni将进行研究。然后将探测故意选择以揭示特定机制的元素取代（Pd，Cu，Al，Cu+Al）的影响以及与等原子组成的偏差。换句话说，合金元素的数量、浓度和类型将变化以评估基本参数（例如原子尺寸/质量、弹性模量和堆垛层错能）对微观结构和机械性能的影响。将在扫描电子显微镜中原位进行一些微观力学测试，以将载荷-位移曲线上的屈服点现象与试样表面上的物理滑移事件在真实的时间内相关联。临界分解剪应力将被确定为取向和成分的函数，并将检查Schloss定律的有效性。将进行微拉伸试验，以检查拉伸-压缩不对称性。由于微观结构分析对于解释这些结果至关重要，因此在中断的机械试验（不同的应变量）之前和之后将提取和检查TEM箔，以表征合金的单相性质、变形微观结构（孪晶、堆垛层错、位错等），以及成分复杂性如何影响微观结构、滑移特征和塑性。滑移行为将与热处理和可能的短程有序效应的化学成分相关。要检查是否位错核心配置负责较高的摩擦应力，我们将调查位错核心作为组成复杂性的函数。从这个项目中获得的知识将有助于更好地理解合金化对浓缩的、大规模合金化的固溶体的机械性能的影响，这些固溶体的行为不能用教科书理论来解释。

Recently, a novel class of materials, high- and medium-entropy alloys (HEAs/MEAs) containing multiple principal elements in roughly equal atomic parts, has taken the scientific world by storm. In polycrystalline HEAs/MEAs, strength depends on the type and number of alloying elements, but a rigorous understanding of basic mechanisms is lacking. To deepen our scientific understanding of alloying effects, it is vital to test single crystals so that friction stresses can be evaluated on the activated slip systems as a function of composition and correlated with the underlying dislocation processes. Since growth of bulk single crystals is tedious and not always feasible, we plan to perform micromechanical tests on micropillar and microtensile specimens prepared by focused ion beam (FIB) milling from individual (single crystal) grains of polycrystalline materials. This will allow us to use conventional processing (melting, casting, homogenization, rolling, recrystallization, grain growth) to make any desired alloy. Initially, face-centered cubic HEAs/MEAs based on various combinations of Cr, Mn, Fe, Co and Ni will be investigated. Effects of elemental substitutions (Pd, Cu, Al, Cu+Al) deliberately chosen to uncover specific mechanisms will then be probed as well as deviations from equiatomic compositions. In other words, the number, concentration, and type of alloying elements, will be varied to evaluate the influence of fundamental parameters such as atomic size/mass, elastic modulus, and stacking fault energy, on microstructure and mechanical properties. Some of the micromechanical tests will be performed in situ in a scanning electron microscope to correlate yield point phenomena on the load-displacement curves with physical slip events on the specimen surfaces in real time. Critical resolved shear stress will be determined as a function of orientation and composition, and the validity of Schmids law will be checked. Microtensile tests will be conducted to check for tension-compression asymmetries. Since microstructural analysis is crucial to the interpretation of these results, TEM foils will be extracted and examined before and after interrupted mechanical tests (different amounts of strain) to characterize the single-phase nature of the alloys, deformation microstructures (twins, stacking faults, dislocations, etc.), and how compositional complexity affects microstructure, slip character and plasticity. Slip behavior will be correlated with heat treatment and chemical composition for possible short range ordering effects. To check whether dislocation core configurations are responsible for higher friction stresses, we will investigate dislocation cores as a function of compositional complexity. The knowledge gained from this project will contribute to a better understanding of alloying effects on the mechanical properties of concentrated, massively alloyed, solid solutions whose behavior cannot be explained by textbook theories.