| Facing to the rigorous working environments, composites with comprehensive properties have become an important direction for the development of engineering structural ceramics. The toughness and strength of ceramics can be improved with addition of metal elements, ceramics particles, whiskers and fibers, however, shortcomings in oxidation resistance, wetting and application at high temperature limit the fraction of the additives. Meanwhile, the intermetallics exhibit some excellent properties expected. And it is inevitable for materials experts to find a compatible combination of both merits. But up to now, the intermetallics reinforced composites mainly focus on Fe-Al system and Ni-Al system, and information about intermetallics o f Ti-Al system as reinforcing phases was little documented. S o this subject wants to prepare the Ti-Al intermetallics reinforced multi-composites with a matrix of TiC+Al2O3 by hot-pressing reaction sintering matrix powders and MAed Ti/Al powder mixture. And the main jobs and results are simply generalized as follows:First, the Ti/Al powder mixture was mechanical alloyed (MAed) by high-energy ball milling. After serious mechanical treatment, the structures and properties of Ti and Al elements undergo larger changes, and the grains can be refined to nanometer in very short time. Due to the introduction of defects and inner stresses under the collision and wear stresses, supersaturated solid solution, amorphous phases and intermetallics formed in consequence during ball milling. The relative stability of solid solution in mechanical alloying was studied according to regular solution model, and a new method using coordinate form of (mole fraction of Al, temperature) was proposed to evaluate the relative stability. The model suggests that in this experiment the solid solution of aluminum in titanium elements is dominant, according with the experimental results.With proper sintering processes, Ti3Al/TiC+Al2O3 multi-composites were prepared successfully by hot-pressing reaction sintering. The matrix was reinforced by intermetallics remarkably, and the bending strength and fracture toughness ofmulti-composites was improved by 150% and 220% respectively. The samples sintered at 1550℃ have better properties with the hardness over 91.5HRA, bending strength 623.33MPa, fracture toughness 8.02MPa.m1/2 and relative density 96.8%. But abnormal grain growth makes properties of samples sintered at 1650℃ decrease. The composites were heat treated at 700 ℃ and 900 ℃ in air atmosphere for 1 hour. Short-term heat treatment has a neglectable effect on hardness and toughness, but makes an remarkable decreasing in the strength of composites sintered at 1650 ℃. And the continuous oxidation behavior at 200, 400, 600, 800, 900, 1000℃ was also studied, the results show that serious mass increasing occurs at 900 ℃, which agrees with the data in the former documents.On the basis of composites mentioned above, the reinforcing mechanisms and effects of the weight fraction of nano TiC particles on the properties of the composites were discussed by adding 5wt%, 8wt%, 10wt%nano particles into the composites. Nano TiC particles facilitate the formation of uniform and refined structures. And not only the intergranular and intragranular nano-structures, but also intragranular structure of Al2O3 in Ti3Al and elongated TiC grains were found in nano particle reinforced composites. The results also show that the mechanical properties including relative density, strength and toughness have been greatly improved by different reinforcing mechanisms of nano particles such as grain-refining, residual stresses and drawing effect of elongated grains and so on. However, the properties of composites as the function of weight fraction of nano TiC particles were characterized as increasing firstly and then decreasing. In this experiment, the sample with 8wt% nano TiC particles owns the most excellent mechanical properties: the relative density over 99.5 %, bending strength 819MPa and fracture toughness 14.06MPa·m1/2.According to the experimental results, there are some new viewpoints in the present study:1) The relative stability of solid solutions in MAed Ti/Al powder mixture is studied according to regular solution model. And on this basis, a new method using coordinate form of (mole fraction of aluminum, temperature) is proposed to evaluate the relative stability of solid solutions.2) Ti3Al/TiC+Al2O3 multi-composites were prepared successfully by hot-pressing reaction sintering. And the composites possess excellent mechanical properties.3) The effects of nano TiC particles on the structures and properties of composites are investigated. Besides the intragranular structure of nano TiC particles in matrix, intragranular structure of alumina in Ti3Al and elongated TiC grains are also observed, which contribute to the great increasing of mechanical properties. |
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