Thermal Stability of Carbohydrides Obtained by the High-Energy Ball Milling of Titanium in a Toluene Medium

We obtain titanium carbohydrides by the high-energy ball milling (HEBM) of titanium in a planetary ball mill in toluene. Depending on the duration of HEBM, face-centered cubic (FCC) and hexagonal close packed (HCP) carbohydrides with different carbon and hydrogen contents are obtained. The structural-phase state and the thermal stability of the solid phase of the reaction mixture, during successive stages of mechanosynthesis, are investigated by means of X-ray diffraction, differential scanning calorimetry, thermal desorption, and scanning electron microscopy. At short milling times (50 min), the endothermic peak appears at a temperature of 430 degrees C due to the alpha -> beta phase transformation in Ti in the presence of hydrogen. Hydrogen stabilizes beta-Ti, reducing its formation temperature to 300 degrees C. The exothermic peak, at about 530 degrees C, corresponds to the deformation-energy release of the crushed Ti powder. Processing in the planetary mill for 5 h leads to the formation of an alpha-Ti (HCP) and Ti carbohydride (HCP and FCC) mixture. We state that for the HCP carbohydrides, a higher hydrogen desorption temperature is observed as compared to the FCC carbohydrides. The correlation of the thermal desorption peaks and the differential scanning calorimetry curves is discussed. We make assumptions concerning the mechanisms of the thermally stimulated structural-phase transformations of the carbohydrides.