The challenge of sintering ultrafine-grained tungsten to >99% density and towards nanocrystallinity is hereby addressed by optimized pressureless two-step sintering. It successfully produced tungsten samples with 99.3% theoretical density and 290 nm average grain size, with a uniform grain structure, good grain boundary cohesion, and 7.8 GPa hardness that is the highest in all pressureless sintered tungsten. The critical role of initial powders and the resultant green bodies was noted, which greatly affects later-on sintering kinetics and microstructural uniformity. Several key questions concerning two-step sintering of metallic tungsten were addressed, including the selection of the first- and second-step sintering temperature, the thermodynamically required critical density to start the second-step sintering, and grain growth kinetics during sintering. The lessons learned here should be directly transferable to other refractory metals and their alloys and would help address the ultimate task of pressureless sintering of bulk nanocrystalline refractory metals/alloys.

通过优化的无压两步烧结解决了将超细晶粒钨烧结至 > 99% 的密度和纳米结晶度的挑战。成功生产出理论密度为99.3%、平均晶粒尺寸为290nm的钨样品，晶粒结构均匀，晶界内聚力好，硬度为7.8GPa，是所有无压烧结钨中最高的。注意到初始粉末和所得生坯的关键作用，这极大地影响了后期的烧结动力学和微观结构的均匀性。解决了有关金属钨两步烧结的几个关键问题，包括第一步和第二步烧结温度的选择，开始第二步烧结所需的热力学临界密度，和烧结过程中的晶粒生长动力学。这里学到的经验教训应该可以直接转移到其他难熔金属及其合金上，并有助于解决大块纳米晶难熔金属/合金无压烧结的最终任务。