Synthesis, characterization and formation mechanism of Ta–W nanocrystalline composite powders fabricated by nano-in situ composite method
To overcome the challenges involved in fabricating Ta–W nano composite powders by using traditional methods, a novel synthesis process of preparing Ta–W nanocrystalline composite powders with a uniform particle size and low oxygen content is proposed in this paper. The process mainly included nano-in situ composite coprecipitation, hydrogen reduction, and a series of reactions with Na and Mg atmospheres. The constitutions, morphologies, and microstructures of the synthesized nanocrystalline powders were characterized by XRD, SEM, TEM, and SAED analyses. The results showed that the (Ta,O)-coated W core–shell structure resembles a chocolate-covered peach in an H2 atmosphere, and the surface of the (Ta,O) phase was changed into a nano-filamentous shell structure in Na and Mg atmospheres. The (Ta,O) phase was transformed into Ta2H in an Na atmosphere at 1193 K, and finally into Ta in an Mg atmosphere at 1213 K. Based on the experimental results, the possible reaction process and formation mechanism of Ta–W nanocrystalline composite powders were investigated. Ta(OH)5 and APT grains grew together by nucleation and became a nano-sized in situ composite through co-precipitation, and the composite powders containing (Ta,O) and W nanocrystallites reacted in an Na atmosphere by chemical vapor transport (CVT) and a crack-drop process.