THE KINETICS AND MECHANISM OF INTERFACIAL REACTION IN SIGMA-FIBER-REINFORCED TI MMC

Interfacial reaction between titanium matrix and reinforcement plays a crucial role in determining the mechanical properties of titanium met al matrix composite materials. In order to improve the mechanical prop erties of composite materials, it is essential to understand the therm odynamics and kinetics of such interfacial reactions. Ti-6Al-4V foils and C/TiBx-coated SiC fbres were used to fabricate composite materials by diffusion bonding. The interface formed after annealing at differe nt temperatures has been characterized mainly by scanning and transmis sion electron microscopies to establish the reaction kinetics between the TiBx coating and Ti matrix. It is found that the major reaction pr oduct is TiB needles, although a TiB2 layer is also present as a trans ition phase during the initial stage of the reaction. Experimental res ults indicate that, at a temperature between 870 and 970 degrees C, th e growth rate of TiB needles along the needle direction is more than s ix times of that of the TiB2 layer. After a detailed analysis of the c rystal structures and the growth morphologies of both TiB and TiB2, th e diffusion mechanisms for B atoms in TiB and TiB2 have been identifie d as vacancy diffusion. However, the low activation energy path for B diffusion in TiB is in the [010](TiB) direction, effectively one-dimen sional, while that in TiB2 is along (1 (1) over bar 00)(TiB2) directio ns, which form a two-dimensional network. In addition, it is found tha t the estimated diffusion coefficient for B in TiB along the needle di rection is about 45 times larger than that in TiB2, although the activ ation energies for B diffusion in both TiB and TiB2 are effectively th e same, being 187-190 kJ mol(-1). Copyright (C) 1996 Elsevier Science Limited.