Synthesis Of Solid Solutions And Intermetallic Compounds By Mechanical Alloying

Mechanical alloying (MA) has been widely used as a non-equilibrium processing technique to synthesize materials such as extended solid solution, nanocrystalline, amorphous and quasicrystalline materials, via severe plastic deformation, fracture and cold-welding induced by ball collision. Alloys composed of refractory and fusible metals are hardly produced by conventional metallurgical methods because of the conspicuous differences in melting temperatures of the components. MA is much superior to conventional metallurgical methods in synthesizing refractory-fusible systems and fusible metals systems.The microstructure evolution of W-16.7at.%In mixture during MA was characterized by means of X-ray diffractometry, differential scanning calorimetry (DSC), scanning electron microscopy (SEM) as well as computer simulation. Experimental results indicated that W_83.3In_16.7 solid solution was synthesized by MA. W_83.3In_16.7 solid solution is stable until it decomposes into W and In at about 600℃.The effects of externally driven mixing favor the alloying of equilibrium immiscible elements, overriding the thermodynamic driving force (positive heat of mix) that would dictate phase separation in equilibrium. Based on the interfacial roughening model, we discussed the formation of the W-In solid solution at a moderate shearing rate under ball milling even though W and In are immiscible element under conventional conditions. Several factors which contribute to the high stability of W-In solid solution are discussed.The solid-state reaction and microstructure evolution of In₂O₃+ Al, SnO₂ + Al and In₂O₃/SnO₂+Al systems were studied by XRD and DSC. It was confirmed that there were no alloys to be formed between equilibrium-immiscible elements Al-Sn and Al-In even under our ball milling conditions. Intermetallic compound InSn₄ was synthesized in the In₂O₃/SnO₂+Al system after milling for 12h. InSn₄ decomposed, incorporating with Fe from the milling medium, and transformed into FeSn₂-type phase during further milling. We attributed the transformation to the fact that FeSn₂ has a lower Gipps free energy than that of InSn₄.Nanocrystalline WC was synthesized via a continuous W+C→WC reaction by ball milling elements of W and C. The progress of the reaction has been monitored by X-ray diffraction method. The W powders used in the present experiment was firstly annealed in the atmosphere of N₂/H₂ blend to improve the reactivity of W particles, a single bcc phase was obtained after 80h of milling. Several factors which influence the one-step mechanochemistrysynthesis of WC were discussed. The WC produced by MA was further mixed with 8wt.% Co , 0.53wt.%VC and different concentrations of CNTs. The as-prepared mixtures were consolidated and sintered under high vacuum at a temperature of 1450℃ for 1.5h. SEM technique was used to study the microstructures of the cross-section of the sintered samples, especially the morphology of the CNTs embedded in the WC-CNTs compounds. The results indicated that the mechanical properties of WC, such as hardness and transverse rupture strength, have not been improved by adding CNTs, the reasons have been discussed in the article. Although the mechanical properties of WC-CNTs compounds have not been improved in present stage, some useful information and potential improvement methods are obtained.