| Mechanical alloying (MA) method has been considered as a powerful process and been widely used in fabrication of some advanced materials with unique properties, in particular, for those materials are difficult to obtain by traditional methods. By using a high-energy ball mill, large numbers of materials including carbides have been synthesized through mechanochemical reaction at a lower temperature. All the while, the research interest was focused on the formation of metal carbide powders and cemented carbides, and the analysis of ball-milling process had been neglected comparatively.In this work, using elemental powders as starting reactant materials several carbide powders such as WC, SiC, TiC, ZrC and corresponding alloys had been fabricated by ball-milling technique. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and other measurement techniques had been used to monitor the structural changes of powders after some ball milling time and the properties of the cemented tungsten carbides. And the mechanism for the formation of matal carbides by MA has also been discussed.On the basis of analyzing and studying the ball milling process, some new phenomena and encouraging results have been found.(1) A new technics for the formation of WC powders was developed. The mixture of tungsten and graphite powders would tranform into solid solution by ball milling for little time, and WC with high purity was obtained by treatment at a lower temperature. The milling time, producing energy and the disadvantage of contamination was reduced, which is favorable for the future application in the manufacture of tungsten carbide. The temperature for the carbonization of tungsten in conventional methods is about from 1673 K to 1873 K, and the direct formation of WC by ball milling need milling for 100 hours or even longer time. In compare, the powder of W and C was milled for 10~28 hours and then annealing at a lower temperature (973 K~~ 1173 K), WC with high purity was obtained.(2) A composite of CNTs-WC-Co was successfully synthesized, which led to a 28% increase of hardness compared with that of WC-wt 10% Co cemented carbide. The cementing metal and carbon nanotubes (CNTs) were added into the milled solid solution of W-C. By ball milling not only the additives were well dispersed in the the powder but also the size of cementing metal crystal was reduced, which was in favor of the reinforcement of alloys.(3) The stage of ball milling graphite and metals with a bigger atom radius was studied, which was available for the synthesis of nanacomposite of multi-carbide. During milling the systems of WsoC5o> TisoCso> ZrsoCso, the carbon atoms would entered into the interstices of metal lattice and formed interstitial solid solution. And in the SisoCso system, the carbon atoms replace part of Si atoms in the Si matrix, which coused the formation of substitutional solid solution. The solid solutions all were supersaturated and metastable, and would dissolve each other with further milling, which was available for the achievement of multi-carbide composite.(4) The transformation of solid solution upon further milling was unlike in different systems. The unstable solid solution would disintegrate and transform into carbide phases in these systems of SisoCso, TisoCso and ZrsoCso, based on the increase of free energy introduced by the effect mechanical alloying. For the system of WsoCso, the solid solution had not transformed into tungsten carbides but an amorphous phase, which was different from those systems. The exist of Fe in these experiments, which came from the abrasion of ball-milling tool because of the higher hardness of W and W-C solid solution, would induce the crystalline-to-amorphous phase transformation during milling.(5) The structures of samples obtained by ball milling were also related with the milling intensity (power), which indicated that the lower milling power was in favor of the formation of amorphous phase. In this work, the activ... |
PDF Full Text Request