Abstract An equiatomic, single-phase TiZrNbHfTa high-entropy alloy was subjected to high-pressure torsion, leading to a grain size below 100 nm. Introducing a nanocrystalline microstructure to the material should help to accelerate a possible phase decomposition of the material by having a high amount of fast diffusion pathways and possible nucleation sites in the form of grain boundaries. In order to test the materials thermodynamic stability the nanocrystalline high-entropy alloy. was subjected to various heat treatments for temperatures between 300 °C and 1100 °C. Isochronal heat treatments (1 h) resulted in a hardness increase from 420 HV1 for the as-processed state to 530 HV1 for an annealing temperature of 500 °C, while for temperatures of 700 °C and higher a softening compared to the as-processed state occurred. In order to clarify this unexpected annealing response, analysis of selected microstructural states was performed utilizing electron microscopy, x-ray diffraction as well as mechanical testing to gain further information on microstructure-property relationships. Complementary, thermodynamic simulations were performed via the Calphad approach and compared to the experimental results. A phase decomposition of the originally equimolar single-phase high-entropy alloy into a NbTa-rich body-centered cubic phase and ZrHf-rich phases, which occurred in two different crystal structures depending on the annealing temperature, was the main reason for the property changes. The obtained results not only give valuable new insights into the phase stability of the TiZrNbHfTa alloy, but also demonstrate the impact of the newly forming phases in regards to mechanical properties and its implication for a possible practical application of this alloy.

中文翻译：

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纳米晶单相 TiZrNbHfTa 合金的热力学不稳定性及其对力学性能的影响

摘要 对等原子单相 TiZrNbHfTa 高熵合金进行高压扭转，使其晶粒尺寸小于 100 nm。通过具有大量快速扩散路径和可能的晶界形式的成核位点，将纳米晶体微结构引入材料应有助于加速材料可能发生的相分解。为了测试纳米晶高熵合金材料的热力学稳定性。在 300°C 和 1100°C 之间的温度下进行各种热处理。等时热处理（1 小时）导致硬度从加工状态的 420 HV1 增加到 500 °C 退火温度的 530 HV1，而与加工状态相比，在 700 °C 和更高的温度下软化状态发生。为了阐明这种意外的退火响应，利用电子显微镜、X 射线衍射以及机械测试对选定的微观结构状态进行分析，以获得有关微观结构-性能关系的进一步信息。通过 Calphad 方法进行补充的热力学模拟，并与实验结果进行比较。最初等摩尔的单相高熵合金的相分解为富含 NbTa 的体心立方相和富含 ZrHf 相，这取决于退火温度发生在两种不同的晶体结构中，是导致该性能的主要原因变化。获得的结果不仅为 TiZrNbHfTa 合金的相稳定性提供了有价值的新见解，