Reaction Mechanism And Products Of Al-Ti-Si System By Combustion Synthesis

Structural intermetallic compounds, due to their high specific strength at elevated temperature, have been the subjects of numerous research works. In recent years, intermetallics of Ti–Si system have been the focus of significant research and development efforts. Among them, titanium silicide (Ti5Si3), has been considered as one of the most potential materials for high-temperature structure applications, in view of its high melting temperature (2130 oC), low density (4.32 g/cm3), capacity to retain high strength up to 1200 oC, and good oxidation and creep resistance at and below 850oC. However, Ti5Si3 has rather low toughness (~2.5MPam1/2) below the ductile-brittle transition temperature, and it often demonstrates brittle failure before plastic deformation, which severely limits its engineering application. Overall, the major challenge in applying Ti5Si3-based materials, like ceramic materials, is to reduce its brittleness or improve the room-temperature fracture toughness.Despite a lot of efforts have been devoted and many routes are introduced to overcome the deficiencies of Ti5Si3, results are not as good as expected. During recent years, researchers have conducted a series of work which involves adding the third metal element to improve the combined properties of Ti5Si3. Theoretical calculation suggests that some alloying elements can reduce the CTE (Coefficients of Thermal Expansion) anisotropy of Ti5Si3 significantly, and hence reduce the microcracks in Ti5Si3. Experiment results show that the added alloying elements can serve as a diluent which decreases the reaction temperature in synthesizing Ti5Si3, and therefore may refine the Ti5Si3 grains. Furthermore, proper alloying elements can act as bonding agents after the reaction, which distributes in boundaries of grains, and thus improve the properties, such as strength and toughness, of Ti5Si3. In summery, the information above indicates that the addition of metal elements can enhance the fracture toughness of Ti5Si3, which inspired us to do more research on Me-Ti-Si system.Since Al atoms might take place of either Ti or Si sites in Ti5Si3 lattices, and the remaining Al can distribute between the grains, it is expected that the addition of Al might improve the properties, especially the brittleness at room temperature or fracture toughness, of intermetallics Ti5Si3, and therefore establish considerable significance of the present study. Nevertheless, recent studies of Al-Ti-Si system mainly focused on certain aspects, such as technologies of fabrication and characterization of properties, while little research concentrated on reaction mechanisms, phase formation and evolution process of combustion synthesis in Al-Ti-Si system. As a consequence, in the present paper, we chose Al as an alloying element to fabricate Ti5Si3-basesd material via combustion synthesis, aimed to investigate the reaction mechanism of Al-Ti-Si system under DTA and TE modes. Particular attention was also paid to the effect of some dynamic factors, such as Al content and reactants size, on phase compositions and microstructures of reaction products of Al-Ti-Si system. It is expected that the present work could lay some theoretical and experimental foundation in the course of alloying of Ti5Si3. Besides, the present work also involves some research on Ti5Si3-TiAl3 and Ti5Si3-Al2O3 composites, mainly investigating the influence of designed volume fraction of TiAl3 and Al2O3 on the type and microstructures of the end products, respectively. Results of the present study are:(1) The addition of Al changes the reaction path of Ti-Si system thoroughly under DTA condition. The raction path of Al-Ti-Si system can be described as following: Al(l)+Ti(s)+Si(s)→(Al-Si)(l)+Ti(s)+Si(s)→Ti(Al,Si)3(s)+Si(s)→Ti5(Si,Al)3(s)+Al(l). Moreover, the increase of Al addition increases the amount of Ti(Al,Si)3 considerably while decreases that of Ti5(Si,Al)3 remarkably.(2) Compared with Ti-Si system, the reaction mechanism changes considerably under TE condition: first, Al and Si took reaction to form Al-Si liquid phase (pre-combustion reaction), and then Al-Si liquid reacted with Ti to form Ti(Al,Si)3 (ignition reaction), releasing some amount of heat, which triggered the reaction of Ti(Al,Si)3 and Si to form Ti5(Si,Al)3 (initial stage of combustion reaction), which released great amounts of heat, and thus, the prior intermediate phase Ti(Al,Si)3, remnant Si and Ti particles were likely to melt to form Al-Ti-Si ternary liquids (middle stage of combustion reaction). Once Ti and Si atoms in melts become sufficiently supersaturated, the precipitation of Ti5(Si,Al)3 grains occurred (end stage of combustion reaction). This process can be distinguished as a solution and precipitation mechanism. Moreover, with the Al addition increasing, the combustion temperature decrease significantly, and the dominated reaction in the initial stage of combustion changes from that between Ti(Al,Si)3 and Si to that between Al-Si liquid and Ti.(3) With Al content increasing from 0 to 50 wt.%, the maximum combustion temperature of Al-Ti-Si system decreased, and the microstructure evolution process can be concluded through a transition chain as follows: Ti5Si3→Ti5(Si,Al)3→Ti(Al,Si)3. Furthermore, the solubility of Si in TiAl3 increased significantly with increasing Al addition, while that of Al in Ti5Si3 mainly depended on the Al content. As Al addition increased from 0 to 25wt.%, the size of Ti5Si3 particle decreased from ~20μm to ~8μm, also the morphology changed from irregular polygon to cobblestone-like shape with smooth surface. Furthermore, the size of reactants didn't cause any significant influence on the type and size of reaction products, but the size of Ti and Si particles impacted the morphology of the products to some extent, eg. Ti5Si3 grains gradually changes from tuber or plates -shape to cobblestone shape.(4) Compared with the the microstructure of single-phase Ti5Si3 or TiAl3, that of Ti5Si3-TiAl3 dual phases refined apparently; and the microcracks reduced gradually as the designed TiAl3 volume fraction increasesd. When the designed Al2O3 volume fraction increased over the range of 10-20 vol.%, the actual Al2O3 content in Ti5Si3-Al2O3 composites increased, and the size of Ti5Si3 particles decreased significantly while its morphologies changed remarkably. With the designed Al2O3 volume fraction increased from 10 to 20 vol.%, the hardness values, of Ti5Si3-Al2O3 composites syntheised by TE combined with pressure, decreased gradually.