Study on the Mechanism of Enhanced Reactivity by Mechanical Alloying

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1.Reaction Systems : Reactivity at mechanical alloying has been related with the energy stored during milling for M (Zr, Ti, Nb, Ni, Al) -heptane and Ti-ceramics (Si_3N_4, SiC) systems. Correlation between the stored energy and the reacted quantity was revealed. Existence of the incubation period and threshold stored energy was observed.2.Reaction Process : The reaction products were examined by X-ray diffraction and DSC analyzes. For Ti-and Zr-heptane systems, at the early stage of reaction, the reaction phases are metastable that endure reversible change with temperature. The intermediate phases are likely the stacking faults stabilized by H and C atoms which are decomposition products of heptane. The lattice site of H atoms was investigated with thermal desorption spectroscopy and the difference between hcp Zr and bcc Nb was revealed.3.Sinterbility of the Milled Powders and Microstructures of the Sintered Materials : The particle size after milling was found to be refined about one order by the reaction resulting in improved sinterbility compared with conventional powders of ceramics or intermetallic compounds. Fine composites of TiAl-Ti_4Al_2C_2 and TiN-Ti silicides with the grain size of about 100nm were obtained.4.Mechanical Properties of the Sintered Materials : Tensile tests at elevated temperatures demonstrated elongation up to 400% at 1000C and 100% at 1500C for TiAl-Ti_4Al_2C_2 and TiN-Ti silicide composites, respectively. From the strain rate dependence of the flow stress, superplastic behaviors were confirmed.