| In this work, the reaction thermodynamics, three-dimensional morphology evolution and growth mechanism of TiB2and LaB6in Al melts were studied. Based on the results, the growth control and structural modification of these two borides were carried out. This work is conducive to develop a high-efficiency grain refiner and metal matrix composite. By using the high scope video microscope (HSVM), electron probe micro-analyzer (EPMA), field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), differential scanning calorimeter (DSC), transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) etc., the following research results are obtained.(1) The orientated stacking mechanism of TiB2in Al meltsThe correlation between particle morphology and reaction path of synthesizing of TiB2was systematically studied. It is found that TiB2particles mainly exhibit a hexagonal platelet-like morphology in the Al-Ti-B alloy prepared by "fluoride salt" method, which evolves into hierarchical tower-like or dendritic morphologies with the increase of reaction temperature, due to the reaction of [B] and [Ti]. However, TiB2particles obtained through the reaction of AlB2and [Ti] tend to agglomerate severely with each other.According to the derived thermodynamic data, TiB2is typically faceted on{0001} planes while non-faceted on{1011} and{1010} planes, indicating that it’s easier for solute atoms to deposit on{1011} and{1010} planes than on{0001} planes. As a result, TiB2tends to form its equilibrium morphology (hexagonal platelet) with the minimized total surface free energy. As{0001} planes have the highest growth rate and the preferential growth directions are along<0001>, TiB2particles undergo the following morphology evolution process:sphereâ†'polyhedronâ†'hexagonal platelet. With the increase of the reaction temperature, primary TiB2nanocrystals become unstable, and will grow up based on three models:parallel intergrowth, two dimensional nucleation and directional intergrowth.(2) The structural modification of TiB2and its applicationsAl-Ti-B-C master alloy with a uniform microstructure was prepared by Al melt reaction method in this study. It is found that TiB2particles in the alloy are energetically favorable with doping trace amount of C. Furthermore, the doping of C on the surface of TiB2is much more preferential. As a result, the orientated stacking tendency of TiB2particles is weakened and their distribution is improved.The TiB2and TiC formed simultaneously when the Al-Ti-B-C master alloy was prepared using Al3BC, which promotes the doping as well as the modification of TiB2by C. Al-Ti-B-C mater alloy shows a much better grain refining performance on Al-6Mg and A356alloys than that of Al-Ti-B. By the addition of1.0%Al-5Ti-0.8B-0.2C master alloy, the average grain size of α-Al in Al-6Mg alloy is refined to about25μm. Both macrohardness and microhardness of the alloy are improved obviously after grain refinement. By the addition of0.2%Al-5Ti-0.8B-0.2C master alloy, the average grain size of a-Al in A356alloy is around167μm. Moreover, the effect does not fade within60min. The tensile and yield strengths of A356alloy are increased by5.9%and11.7%, respectively, while the elongation is increased by124.4%.Besides, TiB2particles were in-situ synthesized in A390melt and TiB2/A390composite was thus prepared. TiB2particles distribute in and around the primary Si in the composite. From the edge-to-edge model, it is found that there exists a good lattice matching coherence between TiB2and Si. Specifically, the matching planes and directions are as follows,<112>Si/<1010>TiB2,{111}Si/{0001}TiB2. Compared with the A390alloy, the ultimate tensile strength and elongation at300℃of the composite are increased by8.8%and33.3%, respectively, and the hardness and wear resistance are also improved a lot.(3) The in-situ synthesis and strengthening behavior of LaB6in Al alloyLaB6particles were in-situ synthesized by Al melt reaction method and Al-3LaB6alloy was prepared. LaB6particles exhibit a cubic morphology and distribute uniformly in the alloy. The morphology evolution undergoes the following process: sphereâ†'octa-armed crystalâ†'dice-like crystalâ†'perfect cube. The final morphology of LaB6is decided both by its crystal structure and the external growth condition.LaB6tends to form its equilibrium morphology (cube) with the minimized total surface free energy. As {111} planes have the highest growth rate and the preferential growth directions are along<111>, perfect cubic LaB6is terminated by {100} surfaces, whereas {111} and {110} planes are concealed to corners and edges, respectively. However, growth condition variations will change the relative growth rates along<111> and<100> directions during solidification, which leads to polyhedral morphologies of LaB6. In addition, by increasing the relative content of the reactants (6%), the heaves on the surface of LaB6crystal nucleus extend rapidly to the solute concentration area. Then LaB6cube with whisker-like dendrites was obtained. Furthermore, LaB6develops to pyramidal dendrite with the further increase of the reactant content (9%).According to Turnbull-Vonnegut equation, the calculated misfit between (200) planes of LaB6and Al is only2.64%. Crystal lattice correspondence indicates that LaB6can act as the nucleation core of a-Al. By the addition of3%LaB6, the hardness and wear resistance of A390alloy are obviously improved. Besides, compared with polyhedral and cubic LaB6, the whisker-like cube shows a much better performance on the improvement of the high temperature tensile strength as well as the reduction of the linear expansion coefficient of A390alloy. |
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