Study Of Formation And Transformation Of TiC In Al Melt

The preparation of TiC by Aluminium melting reaction method and its transformation in different melting conditions were systematically studied using Optical micrograph (OM), Eletron-probe micro-analyzer (EPMA), Differential scanning calorimeter (DSC), X-ray diffraction (XRD), Transmission electron micrograph (TEM), and Field emission scanning electron microscope (FESEM), etc. By analyzing the morphologies, distribution and size of TiC prepared by different methods, the main influencing factors on the microstructures of TiC were discovered and a new method to prepare Al-Ti-C master alloys was also invented. The rule of TiC transformation in different melting conditions and the effect of carbon vacancies on those transformations were also studied. In addition, some measures were taken to mitigate and apply the transformation of TiC. The major research efforts of the present study are as follows:(1) The principle and technology of TiC synthesized by aluminium melt reaction methodThe morphology, distribution and size of TiC are important for its application. But it is found that microstructure of TiC prepared in the melt reaction method are often influenced by the synthesis conditions. It is considered that the different reaction processes account for the distribution and morphologies of TiC. Two main possible ways for formation of TiC in melt are: Ti and C represent dissolved Ti and C in the melt. If the formation is by the way (Ⅱ), the morphologies of TiC are big plate or block and most of TiC particles connect with each other to agglomerate seriously. In addition, the reaction can not occur easily and Al4C3 or metastable phase TiAlxCy will be remained in the final products. But if the formation is by (Ⅰ), the TiC particles are often dispersed in the Al matrix and the size is even. So in order to obtain the dispersive TiC particles, it is essential to avoid the formation way (Ⅱ) in the melt and promote the direct reaction between Ti and C.A new method was used to prepare the Al-Ti-C master alloy. Some measures are taken to decrease the size of graphite and improve the wettabiity between graphite and the melt in order to promote the reaction (Ⅰ) and restrain the formation of Al4C3. It is confirmed that TiC particles in Al-Ti-C master alloys prepared by the new method distributed quite well and their grain refining efficiency on Al alloys is excellent.(2) The transformation of TiC in Al meltsIn the commercial applications, TiC paticles must be stable in the alloys either as reinforcement phase or nucleation substrates. But TiC is often transformed in the Al melt. It is found that Zr will influence the refining efficiency of Al-Ti-C master alloys, but can hardly influence the transformation of TiC, while Si and B have serious effects on the transformation of TiC. The existence of Si in melt at lower holding temperature will accelerate the transformation and Si actually acts as the catalyst in this process; when the meting temperature is higher, Si will react directly with TiC to form Ti3SiC2. The influence of B is opposite at lower and higher melting temperature. When the temperature is lower than 900℃, the existence of B will prevent TiC from transformation and improve its stability. But it will react with TiC to form TiB2 and Al4C3 at higher melting temperature.It is considered that the transformation of TiC is closely relevant to its carbon vacancies. Some elements, such as Si and B, can diffuse into TiCx through carbon vacancies. Si will cause bigger lattice distortion, while B will cause smaller one. At higher temperature, the diffusion of the elements into TiC is not dominant at all, and the transformation mechanism is changed into the direct reaction between TiCx and different elements.In addition, the influence of carbon vacancies on the nucleation ability of TiCx is calculated by computer simulation in this part. The TiC particles without or a little carbon vacancies are good nucleation substrate for a-Al. Al atoms can be adsorbed around the TiC and form an order zone which is coherent with TiCx crystal lattice. The formation of this order zone is essential for the following nucleation. With the number of carbon vacancies increasing, the order zone around TiCx will be undermined and will be changed into part coherency and then absolutely none coherency due to different vacancies number. So it is crucial to reduce the number of carbon vacancies in order to improve the stability and refining efficiency of TiCx.(3) The methods to control the transformation of TiCx and its applicationWhen there is B or N element during the preparation of TiC, these elements will dope with TiCx to form TiCxBy or TiCxNy particles which possess less carbon vacancies. The grain refining efficiecy of TiCx and its stability can be improved by this method.A new kind of Al4C3-containing grain refiner Al-Ti-C-Si-B master alloy was prepared utilizing the transformation of TiCx and can effectively refine the AZ31 alloy. It is found that the Al4C3 can be easily formed in the Al-Ti-C-Si alloy, but Ti formed at the same time will poison the refining efficiency. So B is added to transfer the Ti into TiB2 particles. In this case, the poisoning effect of Ti can be avoided and the formed Al4C3 can be fully worked as the substrates for a-Mg. In addition, the prepared Al-Ti-C-B which contains TiB2 and Al4C3 is used to modify the Fe-containing phase in Al-Si alloys. It is found that the addition of Al-Ti-C-B master alloy can transfer the needle-like phase into the block-like or Chinese character-like phase.