KUMAR and KNOWLES: SIC REINFORCED ALUMINOSILICATESI matrix, but in addition the XRD pattern suggested that orthoenstatite existed 1500 observations showed the presence of a few twinned rains typical of clinoenstatite and orthoenstatite in P+L he matrix. Therefore, while it is possible that pre L toenstatite remains stable at room temperature fo the matrix composition used to fabricate comp 1400 B, further work is clearly required to be able to C(ss)+P+L understand the mechanism by which it can happen C(ss) Hot-pressing of composite A at high temperature ields a pore-free microstructure because of increased C(ss)+P+S viscous fow due to the formation of a liquid How- 1300 C+P+s increase markedly the diffusion of matrix elements 80 Cordierite in the back-scattered electron image [Fig. 3(a)] would (2: 2: 5) suggest that the averag omic number of the layer is higher than the average atomic number of the rest cortical compound"Mg- beryl"[15]. L, P, M, C, C(ss) elsewhere from SEM work on a SiC fibre-reinforced refer to liquid, protoenstatite, mullite, stoichiometric cordierite composite [26]. EDS analysis by SEM lierite, cordierite solid solution and silica respectively. revealed higher amounts of Al and Mg in the bright layer compared to the remaining fibre in composite stability of this phase at room temperature [6, 22-24]. A. further evidence for this local increase in concen It is interesting to note that the possibility of the tration of Al and Mg is obtained from the digital stabilisation of protoenstatite at room temperature compositional maps of these elements in Figs 6(a) by incorporation of a related silicate or a foreign and(b), respectively. A composition gradient is ob oxide into solid solution in the crystal lattice of served from the centre of the fibre to the matrix protoenstatite was suggested some time ago by Foster However, it should be noted that an apparent compo. [25] sitional gradient could also tocnstatite were the major crystalline phases in the interaction volume of X-rays at an interface between PE+SiO, +Liq. f Cordierite +Lig. a-b PE+Fo+Liq e Co+Mu+Liq. a Forsterite Mu+Sa+s Mullite Forsterite+ Spinel+Corund 0 2 Fig. 5. Isothermal(1460C)section of the AlO-Mgo- Sio, phase diagram [15]
2906 KUMAR and KNOWLES: Sic REINFORCED ALUMINOSILICATES-I L C(ss) + P + s C+P+S Mg’-beryl io $0 io Cokerite (2:2:5) Fig. 4. Pseudobinary section going through cordierite and the theoretical compound “Mg-beryl” [15]. L, P, M, C, C(ss) and S refer to liquid, protoenstatite. mullite, stoichiometric cordierite, cordierite solid solution and silica respectively. stability of this phase at room temperature [6,22-241. It is interesting to note that the possibility of the stabilisation of protoenstatite at room temperature by incorporation of a related silicate or a foreign oxide into solid solution in the crystal lattice of protoenstatite was suggested some time ago by Foster [25]. As we have already stated, cordierite and protoenstatite were the major crystalline phases in the matrix, but in addition the XRD pattern suggested that orthoenstatite existed in trace quantities. TEM observations showed the presence of a few twinned grains typical of clinoenstatite and orthoenstatite in the matrix. Therefore, while it is possible that protoenstatite remains stable at room temperature for the matrix composition used to fabricate composite B, further work is clearly required to be able to understand the mechanism by which it can happen. Hot-pressing of composite A at high temperature yields a pore-free microstructure because of increased viscous flow due to the formation of a liquid. However, hot-pressing at high temperature will also increase markedly the diffusion of matrix elements into the fibre. The bright layer around the fibres seen in the back-scattered electron image [Fig. 3(a)] would suggest that the average atomic number of the layer is higher than the average atomic number of the rest of the fibre. Similar layers have also been reported elsewhere from SEM work on a SIC fibre-reinforced cordierite composite [26]. EDS analysis by SEM revealed higher amounts of Al and Mg in the bright layer compared to the remaining fibre in composite A. Further evidence for this local increase in concentration of Al and Mg is obtained from the digital compositional maps of these elements in Figs 6(a) and (b), respectively. A composition gradient is observed from the centre of the fibre to the matrix. However, it should be noted that an apparent compositional gradient could also arise from overlapping of interaction volume of X-rays at an interface between SiO, Protoenstatite h- PE+Fo+Liq. e p/ Forsterite+ Periclase+Spinel A_ SiOp+Mu+Liq.g MgO 20 40 60 Spine1 80 A’203 Fig. 5. Isothermal (1460°C) section of the AlzO,-Mg&SiO, phase diagram [15]
KUMAR and KNOWLES: SiC REINFORCED ALUMINOSILICATES-I 2907 Al (c) Fig. 6.(a)X-ray Al map;(b)X-ray Mg map osite A; and (c)colour intensity scale: white and black correspond aximum and minimum x ray intensities respectively
KUMAR and KNOWLES: Sic REINFORCED ALUMINOSILICATES-I 2907 Fig. 6. (a) X-ray Al map; (b) X-ray Mg map in composite A; and (c) colour intensity scale: white al black correspond to maximum and minimum X-ray intensities respectively