TEM investigations of the fibre/matrix interface in SCS-6 SiC/Ti–25Al–10Nb–3V–1Mo composites

Abstract

Abstract Interfacial reactions in SCS-6 SiC/Ti–25Al–10Nb–3V–1Mo composites processed by fibre coating with matrix material, hot isostatic pressing and thermal treating for simulation of service conditions were studied by analytical transmission electron microscopy. In the as-processed specimen three reaction layers were observed. Adjacent to C coating of the SCS-6 fibre no mixture of TiC and Ti5Si3 was found reported in literature [Rhodes, C. G., Mat. Res. Soc. Symp. Proc., 1992, 273, 17; Baumann, S. F., Brindley, P. K. and Smith, S. D., Metall. Trans., 1990, 21A, 1559; Smith, P. R., Rhodes, C. G. and Revelos, W. C., in Interfaces in Metal–Ceramics Composites, eds R. Y. Lin, R. J. Arsenault, G. P. Martins and S. G. Fishman, TMS Press, New York, 1989, pp. 35–58; Badini, C., Terraris, M. and Marchetti, F., J. Mater. Sci., 1994, 2]. Instead, two sublayers were determined. Sublayer 1 close to C coating was identified as (Ti,V)C and sublayer 2 as (Ti,V,Nb)5Si3. The second layer is composed of larger equiaxed grains of (Ti,Nb)C and separated from matrix by a third layer of (Ti,Nb)5(Si,Al)3. Small amounts of (Ti,Nb)3(Si,Al) and (Ti,Nb)3(Al,Si)C were also identified. In the specimens heat treated at 700°C and 800° for up to 3000 h, three, four or five reaction layers were found. In all specimens the reaction products of the first two layers are the same. The additional layers consist of the (Ti,Nb)3(Al,Si)C, the (Ti,Nb)3(Si,Al) or the (Ti,Nb)5(Si,Al)3 phase. These phases are arranged in different sequences. At the treatment temperature of 700°C the thickening of the interfacial reaction zone is mainly due to the growth of (Ti,Nb)3(Al,Si)C and (Ti,Nb)3(Si,Al). The growth of these phases is probably responsible for the slow decrease of mechanical properties of the composites during heat treatment. A kinetic analysis indicates that the growth of the reaction zone is a diffusion controlled process. The activation energy was determined to be 368 kJ/mol for the total reaction zone, 302 kJ/mol for (Ti,Nb)3(Al,Si)C and 181 kJ/mol for (Ti,Nb)3(Si,Al) phase. The experimental results and a crystallographic analysis indicate that (Ti,Nb)3(Si,Al) is not a diffusion barrier for the carbon and titanium diffusion.