| The morphological evolution and growth mechanism of TiC synthesized through the Al melt reaction method were systematically studied using Optical micrograph (OM), Eletron-probe micro-analyzer (EPMA), X-ray diffraction (XRD), and Field emission scanning electron microscope (FESEM), etc. By analyzing the morphological differences of TiC prepared through the different reaction routes and the morphological evolution of TiC synthesized through the same reaction route, the main influencing factors on the microstructures of TiC were discovered and the growth mechanism of TiC was explored. The main research efforts of the present study are as follows:1. The reaction routes account for the morphology of TiC synthesized through the Al melt reaction method. The morphologies of TiC particles, fabricated through different reaction routes, are varied; the size and distribution of carbides prepared via the same reaction route are mainly decided by the reaction temperature.As for the TiC particles synthesized through the reaction between [Ti] and [C], their initial morphology is spherical when they are prepared at 1100℃. With the temperature increasing, crystal planes will appear in order to reduce the surface energy and thus the carbides with truncated octahedral shape would arise. In addition, the distribution of TiC is also improved with the reaction temperature increasing.As for the TiC particles synthesized through the reaction between [Ti] and C(s), the octahedral morphology emerges and their size rises with the reaction temperature increasing.As for the TiC particles synthesized through the reaction between [Ti] and Al4C3, their morphology is plate-like with the width of 5-20μm and height of 2μm. The reaction temperature also plays an essential role in the distribution of carbides in the Al matrix. When the reaction temperature is 900℃, the TiC particles tend to be agglomerate, but when it rises to 1500℃, the Al-Ti-C alloy with dispersive carbides can be obtained. 2. As for the reason why different reaction routes cause varied TiC morphologies, it can be explained as follow:At the initial stage of the formation of TiC, synthesized via the reaction between [Ti] and [C], a higher density of defects would appear. In this case, the interface between TiC nuclei and Al melt can be considered as rough and there is no crucial nucleation barrier for TiC. Therefore, TiC nuclei can undergo a quick growth in all directions and then form the spherical grains. As the temperature increases, in order to reduce the surface energy of TiC particles, their grain faces would appear and grains covered by flat plains emerge.When the TiC particles are synthesized through the reaction between [Ti] and C(s), we speculate that the reaction mode accelerates the growth rate of{111} planes in Al-Ti-C system, thereby causing the ratio of{100}/{lll}close to 1.73. Hence the octahedral TiC particles appear.As for the reaction between [Ti] and Al4C3, besides the carbides, Al atoms are also the products of the reaction. Some of them will dope in the carbides. The presence of Al can induce high density of planar defects in the TiC crystal structures and change the octahedral coordination of the Ti atom in a perfect crystal to the trigonal prismatic coordination at a twin boundary. The trigonal prismatic substructure leads to the formation of plate-like TiC.3. Because flake-like TiAl3 particles in Al-Ti-C master alloys prepared in a melt reaction method dissolve slowly when they are added into Al melt at 720℃, Ti atoms cannot be released rapidly to play the assistant role of grain refinement, leading to a poor refinement efficiency of Al-Ti-C master alloys. A united refinement technology by Al-1OTi and Al-Ti-C master alloys was put forward in this paper. The rational combination of fine blocky TiAl3 particles in Al-lOTi and TiC particles in Al-Ti-C can improve the nucleation rate of a-Al. It not only improves the grain refinement efficiency of Al-Ti-C master alloys, but also reduces the consumption.
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