In-Situ Synthesis Of Al₂O_3(p,w)/Ti-Al Composites

Ti-Al composites, reinforced with Al₂O₃ particles and whiskers, were synthesized by in-situ reaction hot-pressing method and embedding sintering method respectively. The microstructures of the composites, having introduced reinforcing phase and in-situ growing reinforcing phase, were compared with each other. The dispersion of reinforcing particles was improved by doping Nb₂O₅ and the whisker evolution was controlled by doping different metal oxides to participate in the exothermic reactions. So the dispersion of the whisker was boosted and the nanometer whisker with controlled morphology was obtained.The thermodynamic and kinetic courses of the particle and whisker reinforcing Ti-Al composites synthesized by in-situ reaction were investigated with XRD, SEM and DSC. The influence of additives on the dispersion and morphology of whisker was analyzed. The results showed that the additive influenced the microstructures of the composites by changing the thermodynamic course, then leading to the variation of surface tension, wetting ability and saturation of the solute in liquid and gas phase. Due to the combination of all the thermodynamic and kinetic parameters, the dispersion of particles and the relative density of the composites were improved. The diameter, the ratio of length and diameter and the dispersion were also changed with the different additives.The study also showed that due to the addition of Nb₂O₅, a lot of heat was released during the aluminum exothermic reaction, so the reaction in the system happened ahead of time, and the Al₂O₃p/TiAl composites were sintered at relatively low temperatures, andγ+(α₂/γ) double phase matrix was obtained at 1200℃. The main phases of the composite were TiAl, Ti₃Al and Al₂O₃.Microstructure analysis indicated that Nb₂O₅ additives refined the grain of the composites, enhanced the dispersion of Al₂O₃ particles and the density of the composite. The corresponding Vickers hardness, density were also increased with the increase in Nb₂O₅ amount. The bending strength and fracture toughness reached their peak values as the Nb₂O₅ content was 6wt%, which were about 1.65 and 1.92 times of that without addition of Nb₂O₅. It was the refined grains, adjusted ratio of the double phase of alloy (by doping Nb₂O₅), uniformed dispersion of Al₂O₃, and the perfect density of the composite that endowed the composite with excellent strength.The catalyst drop on the top of the whisker revealed that for the aluminum-rich matrix, the growing mechanism of whisker was gas-liquid-solid one. As aluminum content of the matrix decreased, the growing mechanism was gas-solid one. The whiskers were formed at relatively low temperature by doped TiO₂ due to the presence of TiO_x-1 gas phase. Contrary, for the specimen doped Nb₂O₅, the reaction happened early but the whisker only occurred at high temperature because Nb₂O₅ can not turn to its gas phase at lower temperature.The microstructure analysis showed that smoothα-Al₂O₃ whisker with high length-diameter ratio could be synthesized in Ti-Al matrix by doping proper additives using embedding sintering method.The oxidation resistance analysis showed that the fairly good oxidation resistance of Al₂O₃p/TiAl₃ composites was attributed to the double effect of Nb ions and Al₂O₃. During the oxidation at high temperatures, very small Al₂O₃ grain in the composites favored the initial nucleating and growing of Al₂O₃, which facilitated the formation of dense Al₂O₃ film from the viewpoint of thermodynamics, decreased the concentration of Ti ions, increased the barriers of Ti ions diffusing from M/MO to O/MO interface and decreased the TiO₂ forming rate. Through substituting Nb⁵⁺ for Ti⁴⁺ of TiO₂ the existence of Nb⁵⁺ favored the decreasing of the anion vacuum concentration of TiO₂, which inhibited the growth of TiO₂ in hybrid oxide film and accelerated the growth of Al₂O₃. In addition, the density of the composites was improved by doping Nb₂O₅. All the effects made the oxidation resistance enhanced obviously.The oxidation resistance of the Al₂O₃w/TiAl₃ composites varied with the species of additives and the Al contents of the matrix. However it was still not as good as the oxidation resistance of Al₂O₃p/TiAl composite for its relatively low density.