Characterization of in situ α-Mo/Mo5SiB2 nanocomposite produced by mechanical alloying

The alpha-Mo/Mo5SiB2 nanocomposite was synthesized by mechanical alloying of the elemental powders. The structural evolutions of powder particles during mechanical alloying were studied by X-ray diffractometry, scanning electron microscopy and microhardness measurements. After 10 h of milling, a supersaturated solid solution of Si and B in Mo was formed and a composite microstructure consisting of fine boron and silicon particles distributed into molybdenum solid solution matrix was observed. In addition, the crystallite size of Mo reached to less than 23 nm. After 20 h of milling, new peaks related to Mo5SiB2 appeared in X-ray diffractograms. The microstructure of 20 h milled samples showed a composite structure in which intermetallic phases were embedded in a continuous alpha-Mo matrix. At this time, molybdenum crystallite size was 14 nm and a considerable increase in hardness value of powder particles was yielded due to the formation of Mo5SiB2. By further increase of milling time to 40 h, the molybdenum content of Mo5SiB2 increased whereas the crystallite size decreased. The 40 h milled powders had microhardness up to 1200 HV0.1 due to refinement of crystallite size. (C) 2010 Elsevier B.V. All rights reserved.