| Continuous SiC fiber reinforced titanium matrix composites hold immense potential for advanced aircraft and aerospace structural applications. However, due to the high reactivity of titanium, brittle reactive products at fiber/matrix interface are easy to form and may lead to a degradation of the mechanical properties of SiC/Ti composites. In contrast, very little is reported on theory study of interfacial reaction in SiC/Ti composites. Therefore, in this paper, the interfacial reactions of SiC/Ti composites were studied by means of quantum chemistry, thermodynamics, kinetics, diffusion dynamics and experiment.It is the first time to study the mechanism of interfacial reaction in SiC/Ti composites by quantum chemistry computation methods. A suitable method to calculate titanium carbide and silicide was found and the thermodynamic and dynamic data involved in interfacial reaction of SiC/Ti composites have been obtained.Based on Kohler's ternary solution model and Miedema's model for calculating the heat of formation of binary solution, the integral equation has been established for calculating activity coefficients in ternary alloys and intermetallics. The constant, a, in the formula was determined. The activity coefficients for components in alloys Ti-5Al-2.5Sn, Ti-6Al-4V and intermetallics TiAl, TiaAl have been calculated with the equation. The calculated data coincide well with the experimental ones found in literatures, which are more accuracy than ones calculated by dissymmetry formula. After getting thermo-chemical data of some compounds by using Gaussian 98 software of quantum chemistry and obtaining activities of elements in Ti matrix, the Gibbs function increments, rG, for 16 chemical reactions at the interface of 12 SCS-6 SiC/ Ti composites were calculated. Results predicted by thermodynamic calculation indicate that 16 interfacial reactions may take place spontaneously from the left to the right, namely reaction products TiSi, TiSi2, Ti5Si3, Ti3Si, TiC, Ti5Si4, TiaAlC, Ti2AlC, Ti3SiC2 and TisSiC may form. It is shown that the interfacial reactions of SCS-6 SiC/TiAl, SCS-6 SiC/Ti2AlNb and SCS-6 SiGTi40 are not so severe as in other SCS-6 SiC/Ti composites. The result is useful for us to choose the Ti matrices.Based on quantum chemistry, a two-step dynamic model of interfacial reaction in SCS-6 SiC/ Ti composites was built up. Combining quantum chemistry calculation with experimental evaluation, the activation energies of possible interfacial reaction were calculated. It is shown that the first step in which the atomic Ti, C and Si were decomposed from Ti matrices and fiber, respectively, is a rate-controlling step because the activation energy of the step is larger than second one. It is indicated by quantum chemistry calculation that dissociation energy of Ti, C and Si is 108KJ/mol, 499.7 KJ/mol and 626.1 KJ/mol, respectively, coincided well with the experimental ones found in literatures. It was shown from both dissociation energy and rate constant that dissociation of Ti is much easier than others. It was confirmed that the first reaction layer of TiC with very fine crystal grains is due to the reaction Ti+SiC-TiC+Si. This reaction is a reaction-controlled one. However, the other reactions in the second step are controlled by elements diffusion.Based on the thermodynamics for getting activity coefficient, a symmetry equation wasestablished for calculating inter-diffusion coefficient. The calculated data coincide well with the experimental ones found in literatures. It means that the data from the symmetry equation are more accurate than the ones from dissymmetry formula By calculation, the distribution of C and Si in the interfacial layers of SCS-6 SiC/Ti2AlNb composite were gained, which is consistent well with the experimental ones. It is obviously that the interfacial reaction is diffusion controlled. Therefore, the form of interfacial reaction products should follow the phase rule in phase diagram, So interfacial reaction products existed in SCS-6 SiC/Ti2AlNb composite sh...
|
PDF Full Text Request