Study Of The Thermodynamics And Kinetic Process On XD Reaction Synthesis Of Al₂O_3p-TiC_p/Al Composite

The aluminum matrix composite reinforced by in-situ Al₂O_3p and TiC_p was fabricated through XD process in Al-TiO₂-C system. The influences of reactant components, high-energy milling, pretreatment and sintering process parameters on XD reaction temperature characters and microstructures of the Al₂O_3p -TiC_p/Al composites were studied by using SEM, TEM, XRD, DTA and DSC. Some thermodynamics and kinetic problems of XD reaction synthesis of Al₂O_3p -TiC_p/Al composites were dealt.The formation free energy of reinforcement and adiabatic temperature of the Al-TiO₂-C reactants were calculated according to thermodynamic laws to predict the possible reactions and their products. Results showed that the XD reaction in the Al-TiO₂-C system was a strong exothermic reaction, which resulted in the appearance of metallurgical micro region with high temperature. When the diluted aluminum was 50 wt % and the initial temperature was 1173 K the adiabatic temperature of the reactions reached 2272 K. The adiabatic temperature decreased with increasing of the diluted Al content and decreasing of the initial temperature. There existed a critical temperature T_tr whether TiC or Al₃Ti was formed in the products. When the adiabatic temperature was higher than T_tr , formation free energy of TiC was lower than that of Al₃Ti, and thus it was more favorable to the TiC formation. Increasing of the diluted aluminum increased the critical temperature T_tr and decreased the adiabatic temperature and is not benefit to the TiC formation. Thus, an enough high initial temperature and suitable reactant component were needed to reaction synthesis of Al₂O_3p-TiC_p/Al composites.Kinetic process of the XD reaction was analyzed by using macro kinetics theory of multiphase reaction, and size of the reactants and high temperature metallurgical micro region on the kinetic process were mainly discussed. It showed that reaction time had a parabolic varation with the size. The finer is the particles, the shorter is the reaction time. Strong exothermic reaction resulted in the formation of high temperature micro region in the reactive zone and the great temperature gradient between the reacted zone and un-reacted zone and the melted aluminum. Great temperature gradient brought the thermal stress in un-reacted solid particles, and natural convection and thermal pulse in the melted aluminum. When the thermal stress was higher than its maximum strength of the reactant, the reactant would be broken and the reaction speed was increased. The thermal pulse to the melted aluminum helped the mass transport in the reactant and accelerated the XD reaction.Based on the above thermodynamic and kinetic analysis, it was indicated that the main process parameters that affected the XD reaction temperature character and microstructure of thecomposites were reactant composition, high energy milling process, pretreatment and sintering process. The XD reaction temperature character and microstructures of the composites at different above parameters were systematically studied by experiments. Parameters of a process to obtain high quality AhCh-TiCp/Al composites were determined as followings: C: TiO2 was 1 and the diluted aluminum was 50-70 wt % in the reactant;ratio of ball to powder 10:1, rotating speed 200 rpm and the milling time is larger than 4 hours in the high energy milling process;reactant pressure 130-280 MPa;sintering parameter: 670 °C/30 min+800°C-900 °C/30 min.Because of dramatic influences of high-energy ball milling process on the Al-TiCh-C XD reaction and the microstructure of A^Osp-TiCp/Al composites, the effects of milling process on the morphology, size and elemental distribution of the reactant powders were experimentally dealt with in details. Ratio of ball to powder, rotation speed and milling time were main parameters in the milling process. High ratio of ball to powder, high rotation speed and enough long milling time were favorable to the powders refinement and the composition uniform.Macrostructure of the A^Oip-TiCp/Al composites was lamellar;it was dense in the layer and porous at the layer boundary. There were aggregation of synthetic phases and pores at the layer boundaries. The XD reaction synthesis reinforcements were in situ AljC^pand TiCp, and size of the reinforcements was 0.1⁵um. Most of the reinforcements located at the layer boundaries, and few of them located in the layers.Friction coefficient of the composites increased but friction sensitivity to the load decreased with the milling time in the process. There was little influence of the diluted aluminum on the friction, but wear of the composites increased with the diluted aluminum content owing to the hard reinforcement reduction in the composites. Wear mechanism of the composites were abrasive wear and micro fracture.