Towards pressureless sintering of nanocrystalline tungsten

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 pressurelessly 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 temperatures, 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.