Low-temperature sintering of W-HfC core-shell powders for microstructure regulation

Carbides as typical secondary particles are effective in strengthening tungsten (W) alloys, particularly at elevated temperatures. However, the addition of carbides generally necessitates a higher sintering temperature for W in the powder metallurgy process, resulting in coarsened microstructures and degraded properties. In this study, W-HfC composite nanopowders with a core-shell structure are prepared by a freeze-drying method to overcome the densification barrier caused by HfC during subsequent sintering. The constant-heating-rate sintering experiments demonstrate that the sintering kinetics are independent of HfC content for W-HfC core-shell nanopowders. Utilizing a pressureless two-step sintering (TSS) method at low temperatures, we fabricated a dense W-0.5 wt% HfC alloy with an ultrafine-grained microstructure (average grain size: 420 nm). Benefitting from low-temperature sintering, HfC particles with sizes below 20 nm are uniformly dispersed within the W matrix and exert a significant strengthening effect on W. This study demonstrates that freeze-drying combined with TSS is a promising method for fabricating dispersion-strengthened W alloys with well-controlled microstructure and properties.