Nanostructured tungsten through cryogenic attrition

• Pure nanocrystalline tungsten powder was fabricated by cryomilling.
• Tungsten grain size decreased to ~ 15 nm after 4 h of cryomilling.
• Tungsten grain size decreased down to ~ 5 nm after 12 h of cryomilling.
• Nitrogen content increased with increasing cryomilling time.
• Tungsten carbide and FeNiCr-rich phase was observed as contamination phases.

Nanostructured pure tungsten (W) powders have been fabricated through cryogenic attrition (i.e., cryomilling) in a liquid nitrogen medium for the first time. The microstructure and chemistry of W powders before and after 4 and 12 h of cryomilling were thoroughly examined by gas fusion chemical analysis, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy (TEM). Cryomilling in liquid nitrogen protected the tungsten from oxygen and hydrogen contamination while introducing nitrogen. Results showed that the W grain size decreased with cryomilling time, and reached approximately 5 nm after 12 h of cryomilling. High resolution TEM suggested that nitrogen reacted with W to form tungsten nitride (WN). Additionally, amorphous W was identified in the 12 h cryomilled W powder. Tungsten carbide (WC) contamination from the milling media and minor Fe–Cr–Ni-containing impurities from the stainless steel vessel were also documented. The WC had grain size ranging from 20 nm to 150 nm, and was homogeneously dispersed in W matrix.