Effect Of Fe Addition On The Reaction Path And Behavior Of Combustion Synthesis In Ti-Si System

The refractory intermetallic compound, Ti5Si3, processes many excellent physical and chemical properties, such as high melting point (2403K), low density (4.32g/cm3), high hardness (11.3GPa), good high temperature stability, tensile strength, creep resistance and oxidation resistance. Thus, Ti5Si3 has been regarded as one of the most promising high-temperature-structure materials for aerospace and national defense industries, which distinguishes it from a large amount of silicides to become another focus of numerous researches following MoSi2.It has been found that the application on Ti5Si3 is hindered due to its extremely low room-temperature brittleness and fracture toughness, which essentially results from its complex hexagonal crystal structure. Hence, some investigations have been conducted recently to improve the natural brittleness of Ti5Si3, by adding alloying elements into Ti-Si system to improve its over-all properties.Among the alloying elements, it is of great significance to study the effect of metal element addition on the improvement of over-all properties in Ti5Si3. Since the metal element may replace Ti or Si in Ti5Si3 lattices, which can effectively reduce the CTE (Coefficient of Thermal Expansion) anisotropy of Ti5Si3 and hence lower the microcracks in amounts during the cooling process from high temperature to room temperature.Furthermore, an amount of alloying element distributes as bonding agents and plays an important role in the grain boundary joining. Nevertheless, up to now, the study on Me-Ti-Si (Me=Al, Cu, Cr, Nb) has been mainly focused on various technologies of sintering synthesis and testing property, only minor investigation has been focused on the formation mechanism and reaction path of Ti5Si3.Consequently, in the present study, the Fe was selected as an alloying element to study the effect of Fe addition on the reaction path and behavior of combustion synthesis in Ti-Si system. Besides, the kinetic parameters have been measured and calculated by using Kissinger, Merzhanov and Bondington modes. It is expected that the present study could lay some theoretical foundation on the further investigation of the kinetic analysis of combustion synthesis Ti5Si3 by a third metal element addition.The results are mainly concluded as below:(1) According to thermodynamic calculations ofΔG0 andΔH0 in Fe-Ti-Si system, reactions to synthesis Ti5Si3, TiSi2, TiSi, FeSi, FeSi2, FeTi and Fe2Ti are feasible in thermodynamics. Besides, Ti5Si3 is the most stable phase in all the reaction products and the reaction between Ti and Si to synthesis Ti5Si3 is the most exothermic among all reactions in the calculated temperature ranges. The calculation of adiabatic combustion temperature indicates that Tad decreases with the Fe content increasing, only except for the isothermal steps on the melting point of Ti5Si3 (2403K) and Fe (1809K), as well as the phase transformation point ofγFe→δFe (1665K) andαFe→γFe (1184K). Furthermore, the Tad calculation also indicates that the Fe content, without preheating, corresponds to 39.16wt.% when the adiabatic combustion temperature is 1800K. Besides, the Ti5Si3 diluent content is 37.84wt.% when the adiabatic combustion temperature is 1800K in Fe-Ti-Si system.(2) The reaction path of Fe-Ti-Si system under DTA condition can be described as: Fe and Si take reaction via solid-diffusion reaction and form the FexSiy phase at first. When the temperature exceeds theαTi→βTi phase transformation temperature,βTi and Si react via solid-diffusion reaction to form Ti5Si3 phase. As the temperature further increases, FexSiy melts via eutectic reaction, forming local binary melts of (Fe-Si) and subsequently spraying rapidly on the surface of powders and particles. Simultaneously, the unreacted Ti powder will dissolve into the melt and form local ternary melts of (Fe-Ti-Si). When the concentration of the [Ti] and [Si] in the ternary melts become saturated, the stable Ti5Si3 phase begins to precipitate out of the melts.(3) The reaction path of Fe-Ti-Si system under TE mode can be described as:αFe and Si as well asαTi and Si take reaction via solid-diffusion reaction, to form FexSiy and TixSiy, respectively. When the temperature exceeds theαTi→βTi phase transformation temperature,βTi and Si react via solid-diffusion reaction to form Ti5Si3 phase, which ignites the TE reaction among reactants Fe, Ti and Si to form the preliminary phase Ti5Si3. Simultaneously, heat generating from TE reaction makes the unreacted powders, FexSiy and TixSiy phases melt and form ternary melts of (Fe-Ti-Si). When the concentration of the [Ti] and [Si] in the ternary melts become saturated, the stable Ti5Si3 phase begins to precipitate out of the melts, and the preliminary Ti5Si3 continues to grow up in the ternary melts. Therefore, the ignition reaction of TE was unvaried by Fe addition, which keeps the solid-diffusionβTi+Si→Ti5Si3.(4) The reaction activation energy Ea of 10wt.% Fe-Ti-Si system with different reactant powder size has been measured. With Ti powder size decreasing from 150μm, 38μm to 25μm, Ea values increase from 220kJ/mol, 282kJ/mol to 351kJ/mol. When the Si powder size decreases from 150μm to 48μm, the Ea is 270kJ/mol and 282kJ/mol respectively. Moreover, with the Fe powder size increasing from 10μm to 48μm, the Ea values increase from 282kJ/mol to 348kJ/mol. Besides, the SHS reaction appearent activation energy measured in combustion wave mode in Ti-Si and 10wt.% Fe-Ti-Si systems are both 133kJ/mol, which further confirms that Fe addition has no great effect on the reaction path of Ti-Si system.(5) Analysis of reaction behavior shows that powder size of reactants has minor influence on the morphology, particulate size and phase composition of TE and SHS products in 10wt.% Fe-Ti-Si system. It can be attributed to that the addition of Fe provides more high-temperature liquids, which makes the substance in combustion zone melt. The final products all come from the precipitation and/or the growth from the ternary melts. Furthermore, 10wt.% Fe addition increases the thermal conductivity of the reactants, which improves the maximum of rising rate of temperature (dT/dt). Nevertheless, Fe addition plays a role of dilution, which leads to the decrease of reaction diffusion coefficientα, the increase of maximum of reaction rate (dη/dt), and reductionof combustion wave.