The ball to powder ratio (BPR) dependent morphology and microstructure of tungsten powder refined by ball milling

The nanocrystalline tungsten powder with the minimum grain size of around 5 nm was fabricated by mechanical alloying. The powder refinement process and the influence of ball to powder ratio (BPR) on the morphology and microstructure of tungsten powder were investigated. The morphology and microstructure were characterized by X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). Results revealed that the high energy ball milling process has four stages. Compared with the samples with the BPR of 10:1 and 4:1 under the same ball milling conditions, the samples with the BPR of 15:1 experience the whole four stages after 60 h ball milling. One key factor in determining the final morphology and microstructure of tungsten powder by ball milling was revealed. The model to illustrate the fundamental mechanism for refining the particle size was discussed.