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

最近，一类新型材料——高熵和中熵合金（HEAs/MEAs）在科学界掀起了一股风暴，这种材料的原子部分大致相等，含有多种主元素。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，测试单晶是至关重要的，这样就可以评估激活滑移系统上的摩擦应力作为成分的函数，并与潜在的位错过程相关。由于大块单晶的生长是繁琐的，并不总是可行的，我们计划对由多晶材料的单个（单晶）颗粒通过聚焦离子束（FIB）铣削制备的微柱和微拉伸样品进行微力学测试。这将使我们能够使用常规工艺（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，将研究基于Cr， Mn, Fe， Co和Ni的各种组合的面心立方HEAs/ mea。元素取代的影响（Pd, Cu, Al, Cu+Al）故意选择揭示特定的机制，然后将被探索以及偏离等原子组成。换句话说，合金元素的数量、浓度和类型将会改变，以评估原子尺寸/质量、弹性模量和层错能等基本参数对微观结构和力学性能的影响。一些微力学测试将在扫描电子显微镜下进行，以实时将载荷-位移曲线上的屈服点现象与试样表面的物理滑移事件联系起来。将确定临界分解剪应力作为取向和组成的函数，并检验Schmids定律的有效性。将进行微拉伸试验以检查拉压不对称。由于显微组织分析对这些结果的解释至关重要，因此将在中断力学测试（不同应变量）之前和之后提取TEM箔并进行检查，以表征合金的单相性质，变形显微组织（孪晶，层错，位错等），以及成分复杂性如何影响显微组织，滑移特性和塑性。滑移行为将与热处理和化学成分相关，以实现可能的短期有序效应。为了检查位错核的结构是否对更高的摩擦应力负责，我们将研究位错核作为成分复杂性的函数。从这个项目中获得的知识将有助于更好地理解合金对集中的、大量合金化的固溶体的力学性能的影响，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.