Phase stability, precipitation kinetics, nanoscale elemental distributions and their effect on tensile properties in refractory TiZrNbHfTa BCC high-entropy alloys

由高熔点元素组成的难熔高熵合金（RHEAs）是镍基高温合金和其他传统难熔合金之外的有希望的高温应用候选者。在RHEAs中，等原子体心立方（BCC）TiZrNbHfTa合金在室温下表现出优异的延展性和强度的组合。这种合金最初被认为是稳定的，因为它的高混合熵应该抑制第二相的形成。然而，在低于900 ° C下时效时，六方密堆积（HCP）相沉淀，导致延展性严重劣化。此外，局部聚集和/或短程有序（SRO）可能会影响拉伸性能。为了提高体心立方RHEAs的力学性能，必须知道如何控制这些微结构特征的形成，并了解它们在变形行为中的作用。因此，这项合作研究的目的是提供一个更好地了解这些基本方面，并建立微观结构和力学性能之间的关系。在这里，一个等原子的单相TiZrNbHfTa RHEA与再结晶微观结构将退火范围[300 - 1200 ° C]的不同时间，然后在多个长度尺度的微观结构和化学分析。将进行1000 h退火，然后进行X射线衍射、扫描电子显微镜和电子背散射衍射，以检查热力学平衡。更短时间的额外退火将使我们能够建立第一个时间-温度-转变图的TiZrNbHfTa RHEA。为了进一步揭示相应的速率控制机制，随着时间和温度的演变的微观结构以及浓度分布建立在基体/沉淀界面将进行系统的研究。通过原子探针层析成像（APT）和透射电子显微镜（TEM）详细研究了晶粒内早期HCP沉淀、偏析和高角度晶界处的分配，以揭示HCP沉淀物的成核机制。此外，纳米级组成调制和/或SRO将通过在BCC基质中的APT和TEM在各种温度和持续时间下进行评估。不同的微观结构（纳米级的成分调制，SRO，不同的体积分数和形态的HCP相）从各种退火拉伸性能的影响将进行研究，以获得更好地了解TiZrNbHfTa的变形和失效机制。总的来说，这个项目将推进我们对沉淀动力学和相稳定性相关的基本过程的理解，但也将揭示不寻常的微观结构特征，这些特征可能导致RHEAs中出色的机械性能。

Refractory high-entropy alloys (RHEAs), consisting of elements with high melting points, are promising candidates for high-temperature applications beyond Ni-based superalloys and other conventional refractory alloys. Among RHEAs, the equiatomic body-centered cubic (BCC) TiZrNbHfTa alloy exhibits an excellent combination of ductility and strength at room temperature. This alloy was initially thought to be stable since its high mixing entropy should inhibit the formation of secondary phases. However, upon aging below 900 °C, a hexagonal close-packed (HCP) phase precipitates, leading to a severe deterioration in ductility. Also, local clustering and/or short-range ordering (SRO) could affect tensile properties. To improve the mechanical performance of BCC RHEAs, one must know how to control the formation of these microstructural features and understand their roles in deformation behaviors. Therefore, the objective of this collaborative study is to provide a better understanding of these fundamental aspects and establish the relationships between microstructure and mechanical properties.Here, an equiatomic single-phase TiZrNbHfTa RHEA with a recrystallized microstructure will be annealed in the range [300 1200 °C] for various times followed by microstructural and chemical analyses at multiple length scales. 1000 h anneals followed by X-ray diffraction, scanning electron microscopy, and electron backscatter diffraction will be performed to examine thermodynamic equilibria. Additional anneals for shorter times will allow us to establish the first time-temperature-transformation diagram for the TiZrNbHfTa RHEA. To further reveal the corresponding rate-controlling mechanisms, the evolution with time and temperature of the microstructures as well as concentration profiles building up at matrix/precipitate interfaces will be systematically studied. Early-stage HCP precipitation within grains, segregation, and partitioning at high-angle grain boundaries will be investigated in detail by atom probe tomography (APT) and transmission electron microscopy (TEM) to unravel the nucleation mechanisms of HCP precipitates. Furthermore, nanoscale compositional modulations and/or SRO will be assessed by APT and TEM in the BCC matrix at various temperatures and durations. The effects on tensile properties of different microstructures (nanoscale compositional modulation, SRO, different volume fractions and morphologies of HCP phase) resulting from various anneals will be investigated to gain a better understanding of deformation and failure mechanisms in TiZrNbHfTa. Overall, this project will advance our understanding of the elementary processes associated with precipitation kinetics and phase stability but will also uncover unusual microstructural features that potentially lead to outstanding mechanical properties in RHEAs.