| The in situ Ti B2 reinforced Al or Al alloys composites exhibit excellent mechanical properties due to micro-nano particles reinforcement and the super interfacial bonding. However, the serious aggregation and the failure of the morphology control of the TiB2 particles in in situ process can degrade the properties of the composites significantly. In order to improve the mechanical properties of the TiB2/Al composites, the systematic study about the control of the particle distribution, the control of the particle morphology and the influence of Ti B2 on the Al-Si microstructure was conducted.Wedge-shaped copper casting experiment has been conducted to study TiB2 particle behavior and the interaction between particle or cluster and the solid/liquid front in commercial pure aluminum matrix. The experimental results show that the particle size distribution obeys to two separate systems in the whole wedge-cast sample. Furthermore, it is found that the big clusters were pushed to the center of the wedge shaped sample and the single particle or small clusters consisted of few particles were engulfed into the α-Al in the area of the sample edge. Subsequently, the influence of the alloy elements Si, Cu, Nb, Sc on the distribution of particle was studied and found that alloy element can improve the particle distribution, especially, in which the Sc element addition can decrease the cluster factor form 0.8 in commercial Al matrix about 0.1.To realize the morphology control in TiB2 reinforced aluminum matrix composites, a series of TiB2 particles with various morphology were obtained via controlling synthesis process. Si, Cu, V, Nb, Sc element was added separately to reveal the effect of alloy element on the morphology of in situ TiB2 particles. The results indicated that Si preferentially adsorbed on(10 1—1),(11 2—0) and(1 2—1 2—) planes resulting in the polyhedron morphology of TiB2; Cu adsorbed on(101—0) preferentially leading to the minimum aspect(the ratio of the diameter and thickness); V can promote the 2-dimension nucleation of the TiB2 particles; NbB2 can act as the effective nucleation sites of TiB2; in Al-Sc system, in situ Ti B2 particle with near-spherical morphology reinforced Al composites were prepared. The morphology and size of TiB2 particles can be adjusted by controlling the in situ reaction process and surface adsorption elements addition. The growth mechanism of TiB2 in Al includes 2-dimension nucleation and 3-dimension growth. Then the synergistic effect of various alloy elements on TiB2 morphology was studied.In TiB2/Al-Si composite, the primary silicon was found to be refined significantly by TiB2 particles, with the size being reduced from about 25μm to 5μm. According to the experimental results, it was supposed that a few atomic layer TiSi2 film between the TiB2 and Si would provide good nucleation conditions. The possibility of nucleation is determined by the conventional plane-on-plane model. Furthermore, the edge-to-edge model is used to predict the matching combinations among Si, C54-TiSi2 and TiB2 in detail. Meanwhile, it was found that eutectic Si can also be modified by TiB2 particles and the modification mechanism was discussed. Finally, the elevated-temperature mechanical properties of monolithic ZL109, TiB2/ZL109 and TiB2/ZL109 after P modified were tested indicating that the modification of Ti B2 on Si can improve the mechanical properties of composite significantly. |
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