Reaction Mechanism In The Al-(Ti,TiO₂,ZrO₂)-C Systems And Liquid In Situ Reaction Synthesis Of The Related Materials

In this dissertation, the reaction mechanism in the Al-(Ti,TiO₂,ZrO₂)-C systems was systematically studied by using differential scanning calorimeter (DSC), X-ray diffraction (XRD), electron-probe micro-analyzer (EPMA), optical microscope (OM), transmission electron microscope(TEM), and high-resolution transmission electron microscope (HRTEM). Based on the results, a novel method i.e. liquid in situ reaction synthesis, to prepare aluminum matrix composites and master alloy grain refiners relating to the above systems was developed.Due to its low density, low solubility in Al melt and poor wettability with Al melt, carbon is especially difficult to directly add into pure aluminum or aluminum alloy melt, and this has long been a problem hindering the progress of aluminum matrix composites and master alloy grain refiners based on Al-X-C systems (X=Ti, Zr etc.). To study the reaction and preparation method of materials in the multi system of Al-X-C, this dissertation first investigated the basic and simple Al-C binary system. Both DSC and thermodynamic analyses indicate that the reaction between Al and C occurs favorably before and after the melting of Al, if the two are fully mixed together. So, the most important thing in the preparation of materials based on Al-X-C systems is to find the way to add carbon into aluminum or aluminum alloy melts. Through dynamic analysis and heating theball milled Al-C powders or pressed Al-C pellets in DSC, it was found that the size of graphite particle and reaction temperature have great influences on the reaction process: smaller the particle size and higher the reaction temperature, greater the reaction rate and shorter the reaction time. During heating the fully ball milled Al-C powders, the reaction completely finished at about 725 °C and the highest reaction rate occurs at about 700°C; when heating the pellet of unmilled graphite (600 mesh) and Al powder, the reaction completes at about 1122°C and the highest reaction rate occurs at bout 907 °C. Through direct adding graphite powder (600 mesh) into pure Al or Al-Mg melt, Al-C or Al-Mg-C alloys was successfully synthesized.When the fully ball milled Al-Ti-C powders with excessive Al content are heated in DSC, the Al melts and then reacts simultaneously with Ti and C to form TiAl3 and AI4C3, which then react in the temperature range of 835-890°C, forming TiC. If the Al content in the ball milled Al-Ti-C powder is not enough, all the Al reacts with Ti to form TiA^, then the T1AI3 phase reacts with C at 1100°C, resulting in the formation of TiC. Further heating mixtures of AI-AI4C3 and Al-Ti alloys at different temperatures confirms that the reaction between AI4C3 and TiAl3 to form TiC in Al melt occurs between 830-890°C, while another ternary carbide Ti3AlCx (x is an unknown) was also found to form in this temperature range. It was shown that TiA^ particle decompose into solute Ti and Al, then the solute Ti diffuses in Al melt towards AI4C3 solid particle to form TiC or Ti3AlCx. Calculation of activation energy for the reaction between T1AI3 and AI4C3 shows that this reaction is controlled by the dissolution rate of TiAl3 particle and (or) the nucleation process of TiC from solute Ti and AI4C3 particle.When heating fully ball milled Al-TiO2-C powders, Al melts first and then reacts with TiO2 and C, forming (TiAl3+a-Al203) and AI4C3, respectively. Thenthe T1AI3 and AI4C3 phases react in the range of 835-890°C to produce TiC.Through direct addition of the mixture of sponge Ti and graphite powder into pure Al melt in the temperature range of 900-1300°C, Al-Ti-C alloys with different phase were successfully produced. When the Ti/C molar ratio in the Al-Ti-C system exceed, equate with or is less than the stoichiometric value 1:1 in the TiC compound, the following alloys were obtained: Al-(TiAl3+TiC), Al-TiC, and Al-(TiC+Al4C3,), respectively. Using the same liquid in situ reaction synthesis method, Al-(TiC+a-Al2O3) and Al-(TiAl3+TiC+a-Al203) alloys were also produced after the mixture of TiO2 and graphite powders with different ratios were added directly into pure Al melt between 900 and 1300°C. It was found that the presence of TiAl3 and AI4C3 phases, especially the latter one, in the Al matrix of the above alloys greatly decreased the tensile strength and plastic properties, while the Al-TiC and Al-(TiC+a-Al2O3) alloys were enhanced by more than 100% in the tensile strength and still possess good plastic property. When the above alloys were used to refine pure Al, it was found that the presence of TiAb in the alloy matrices is of great importance in obtaining high refinement efficiency. Observations of aluminum grains refined after the addition of the above Al-Ti-C-(O) alloys revealed that TiC particle clusters are located at the grain center and the excessive Ti segregates to the TiC particles, forming dendrites with the Ti content gradually decreasing from the centre to the outer part. It is clear that the high refinement efficiency of Al-Ti-C-(O) alloys in pure Al results from the combined action of TiC particle and excessive solute Ti, but the exact mechanism needs to be further studied.During heating the fully ball milled Al-ZrO2-C powders in DSC, the Al melts, then reacts with ZrO2 and C to form (ZrAb+AbC^) and AI4C3, respectively. Then at about 896°C, ZrA^ reacts with AI4C3, causing the formation of Z^A^Cs phase.At about 1140°C, Zr2Al3C5 reacts with ZrAl3, producing ZrAlC2 and ZrC compounds. Calculation of activation energy for the reaction between ZrAl3 and AI4C3 shows that this reaction is controlled by the dissolution rate of ZrAl3 particle and (or) the nucleation process of Zr2Al3Cs from solute Zr and solid AI4C3. Heating Al-ZrO2-C powders with different ZrO2/C molar ratios (r) to 1250°C results in different phases in the final aluminum matrix: Al-Al2O3-Zr2Al3C5-Al4C3 (r<2:5), Al-AhC^-Z^AbCs (r=2:5), Al-Al2O3-Zr2Al3C5-ZrC-ZrAlC2 (2:53:4). Zr2Al3C5 presents fine platelet morphology ¹.5um in width and ⁵jim in length, while ZrAlC2 and ZrC have polygonal shape 0.4⁴um in size.The liquid in situ reaction synthesis method to produce materials based on the Al-C, Al-Ti-C, and Al-TiO2-C systems was also successfully applied to the preparation of materials in the Al-ZrO2-C system. Through direct adding powder mixtures of ZrO2 and graphite into pure Al melt, Al-Zr2Al3Cs-Al203 and Al-ZrAl3-ZrAlC2-ZrC-Al2C>3 alloys were produced. The morphologies of Z^A^Cj s ZrAlC2 and ZrC compounds are similar to those obtained by heating the Al-ZrOi-C powders in DSC.