Precipitation strengthening is paramount in the development of high-performance medium/high entropy alloys (M/HEAs). In this work, we showcase a phase-selective precipitation design applied to a (Ni67.2V32.8)90Ti5Al5 MEA to enable enhanced strength-ductility synergy. Upon annealing at 950 °C, multiple precipitates form in this MEA, including L21, σ and hexagonal close packed (HCP) phases. However, an increase of 50 °C in annealing temperature removes most of the aforementioned precipitates except for the L21 phase. Density functional theory calculations are conducted to elucidate the formation mechanisms of phase-selective precipitation. Such selective approach to precipitation induces a brittle to ductile transition, increasing tensile elongation from 4 % to 43 % in our MEAs. Remarkably, the ultimate tensile strength of 1000 °C annealing MEA is maintained at ∼1.4 GPa, surpassing that of the precipitation-free Ni67.2V32.8 base alloy (∼1.1 GPa), but with a comparable tensile elongation. Analytical models suggest that the increase in strength is attributed to both precipitation strengthening and grain refinement strengthening due to the pinning effect of precipitates. In particular, we investigate the complex deformation response of the L21 phase, which includes the formation of slip steps and a phase transformation from body-centered cubic (BCC) to body-centered tetragonal (BCT) structures, with the underlying mechanisms revealed through experimental characterization and molecular dynamics simulations. This co-deformation of matrix and L21 precipitates alleviates stress concentration at phase boundaries during straining and further maintains the microband-induced plasticity in the matrix till later deformation stage. All these result in the excellent strain hardening and thus, markedly enhancing ductility. Our findings pave new ways to craft strong and ductile M/HEAs by selecting hard-yet-deformable intermetallic precipitates.

中文翻译：

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通过相选择性沉淀增强中等熵合金的强度-延展性协同作用


析出强化在高性能中/高熵合金 （M/HEA） 的开发中至关重要。在这项工作中，我们展示了应用于 （Ni67.2V32.8）90Ti5Al5 膜电极的相选择性沉淀设计，以实现增强的强度-延展性协同作用。在 950 °C 下退火后，该膜电极中会形成多种沉淀，包括 L21、σ 和六方密堆积 （HCP） 相。然而，退火温度升高 50 °C 可去除除 L21 相外的大部分上述沉淀物。进行密度泛函理论计算以阐明相选择性沉淀的形成机制。这种选择性的沉淀方法导致脆性到延展性转变，在我们的 MEA 中将拉伸伸长率从 4 % 增加到 43 %。值得注意的是，1000 °C 退火 MEA 的极限抗拉强度保持在 ∼1.4 GPa，超过了无析出的 Ni67.2V32.8 母合金 （∼1.1 GPa），但具有相当的拉伸伸长率。分析模型表明，强度的增加归因于沉淀物的固定效应引起的沉淀强化和晶粒细化强化。特别是，我们研究了 L21 相的复杂变形响应，其中包括滑移步骤的形成和从体心立方 （BCC） 到体心四方 （BCT） 结构的相变，并通过实验表征和分子动力学模拟揭示了潜在机制。基体和 L21 析出物的这种共变形减轻了应变过程中相边界处的应力集中，并进一步保持了基体中微带诱导的塑性，直到后期变形阶段。 所有这些都导致了出色的应变硬化，从而显着提高了延展性。我们的研究结果为通过选择坚硬但可变形的金属间化合物来制备坚固且具有延展性的 M/HEA 铺平了新方法。