The mechanical properties of the refractory high-entropy alloy (RHEA) NbMoTaW with columnar and equiaxed microstructures and a nanoscale metal oxide layer on the grain boundaries are investigated at ambient temperatures (RT) to 1200 °C. Under compression, the alloy shows a yield strength, , of ⁓1390 ± 20 MPa at RT and retains a high yield strength, , of ⁓301.5 MPa at 1200 °C. However, in tension, the fracture strengths, , of both columnar and equiaxed microstructures are far lower - below 70 MPa - and the material fails in the elastic regime without showing any measurable ductility at all temperatures. The fracture mechanisms in tension transition from transgranular cleavage at RT to a mixture of transgranular and intergranular fracture at 800 °C to a complete intergranular fracture at 1200 °C due to selective weakening of the metal oxide layer on the grain boundaries driven by a structural change. The transition in the fracture mechanism in the equiaxed microstructure promotes extrinsic toughening at higher temperatures, resulting in a ⁓6.4 times higher fracture toughness ( ⁓10.3 MPa√m) at 1200 °C compared to that at RT. NbMoTaW is a well-known RHEA, but the poor tensile strength and fracture toughness measured in this study highlight the critical importance of investigating the mechanical properties of these high-temperature RHEAs in tension.

中文翻译：

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NbMoTaW 难熔高熵合金拉伸力学性能的退化


研究了具有柱状和等轴微观结构以及晶界上纳米级金属氧化物层的难熔高熵合金 (RHEA) NbMoTaW 在环境温度 (RT) 至 1200 °C 下的机械性能。在压缩状态下，该合金在室温下的屈服强度 为⁓1390 ± 20 MPa，并在 1200 °C 时保持高屈服强度 ⁓301.5 MPa。然而，在拉伸状态下，柱状和等轴微观结构的断裂强度 远低于 70 MPa，并且材料在弹性状态下失效，在所有温度下均未表现出任何可测量的延展性。由于结构变化驱动晶界上金属氧化物层的选择性弱化，拉伸转变中的断裂机制从室温下的穿晶解理到800℃下的穿晶和沿晶断裂的混合，再到1200℃下的完全沿晶断裂。等轴显微组织中断裂机制的转变促进了较高温度下的外在增韧，导致 1200 °C 时的断裂韧性比室温高 ⁓6.4 倍（⁓10.3 MPa√m）。 NbMoTaW 是一种众所周知的 RHEA，但本研究中测得的较差的拉伸强度和断裂韧性凸显了研究这些高温 RHEA 在拉伸状态下的机械性能的至关重要性。