Abstract The challenge of producing dense ultrafine-grained (UFG) tungsten is hereby addressed by a simple pressureless two-step sintering method. It provides a uniform microstructure with ~99% theoretical density and ~700 nm grain size, which is among the best sintering practice of pure tungsten reported in the literature. Benefitting from the finer and more uniform microstructures, two-step sintered samples show better mechanical properties in bending strength and hardness. While parabolic grain growth kinetics is verified, a transition in the nominal grain boundary mobility was observed at 1400 °C, above which the effective activation enthalpy is ~6.1 eV, below which grain boundary motion is rapidly frozen with unusually large activation enthalpy of ~12.9 eV. Such highly nonlinear behavior in activation parameters with respect to temperature suggests that activation entropy and maybe collective behavior play a role in grain growth. We believe the as-reported two-step sintering method should also be applicable to other refractory metals and alloys, and may be generalized to multi-step or continuous-cooling sintering design using machine learning.

中文翻译：

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超细晶粒钨的无压两步烧结

摘要 生产致密超细晶粒 (UFG) 钨的挑战在此通过简单的无压两步烧结方法解决。它提供了均匀的微观结构，具有 ~99% 的理论密度和 ~700 nm 的晶粒尺寸，这是文献中报道的纯钨的最佳烧结实践之一。受益于更精细和更均匀的微观结构，两步烧结样品在弯曲强度和硬度方面表现出更好的机械性能。虽然验证了抛物线晶粒生长动力学，但在 1400 °C 时观察到名义晶界迁移率的转变，高于此值时，有效活化焓为 ~6.1 eV，低于此值时晶界运动迅速冻结，活化焓异常大，为 ~12.9 EV。与温度相关的激活参数的这种高度非线性行为表明激活熵和可能的集体行为在晶粒生长中起作用。我们相信所报告的两步烧结方法也应该适用于其他难熔金属和合金，并且可以使用机器学习推广到多步或连续冷却烧结设计。