J. Bouix et al. Composites Science and Technology 61(2001)355-362 silicon to the aluminium matrix prior to infiltration such a situation will for example occur at 730@C with Al/Si alloys containing more than 4.5 at. of silicon. The phase diagram of the ternary system Al/C/Ti[18] is characterized by the existence at 812+15C of a quasi- peritectic invariant transformation which can be written on heating as: Al3Ti+Al,C3 2 Li+ TiC (3) where Li designates an aluminium base liquid containing 0.3 at. of titanium and TiCx a carbon-rich composi- tion in the homogeneity range of the titanium carbide content in the liquid is very low). As a consequence of this transformation, carbon-rich titanium carbide coating will be in thermodynamic equilibrium with aluminium base liquids at temperatures higher than or equal to 812C but will be decomposed by solid or liquid alumi nium into Al3Ti and Al4C3 at any temperature lower than 812C according to the equation 3TIC+13Al- 3Al3T1+ Al4C3 Fig 8. Fractured surface of BN-coated Hi-Nicalon fibre. As pressure infiltration of liquid aluminium is difficult e at temperatures higher than 650@C and up to at least to realize at temperatures higher than 812C, the TIC/Al 1000C, a three-phased equilibrium tends to be estab- couple has to be considered as a reactive couple in lished between SiC, Al4C3 and a Al/Si/C liquid L Under regard to composite manufacture. However, if the reac- a constant pressure, this three-phased equilibrium: tion corresponding to Eq.(4)cannot be thermo- dynamically stopped by adding titanium to the SiC+Al4C32L (2) aluminium matrix, this addition can modify the kinetics. Effectively, it has been shown that interaction is monovariant and the composition of the liquid L only between TiC and aluminium below 812C develops in depends on the temperature. At 650 C, this composition two successive stages: (i) a first stage which proceeds at a is that of Lo, i.e. 1.5 at. of silicon and about I at ppm fast rate and in which titanium produced in the convertion of carbon. Then, the silicon content of the liquid L reg- of TiC into AlC3 simply dissolves in aluminium;(i)a ularly increases with the temperature to attain 4.5 at second stage which proceeds at a much slower rate and at 730C and 13 at at 1000 C(the carbon content of in which Al3Ti crystallizes from a liquid saturated in L also increases but remains very low) titanium. Consequently, if liquid aluminium is saturated Existence of the quasi-peritectic transformation (1) in titanium prior to infiltration, the first fast-rate stage is implies that SiC is in thermodynamic equilibrium with avoided and one directly enters in the second slow-rate solid aluminium at any temperature lower than 650C. stage In this low temperature range, SiC coatings can then The Al/BC couple appears to be reactive at any protect ca on fibres very efficiently against aluminium temperature up to at least 1000 C. Decomposition of attack. At temperatures higher than 650C, Sic reacts boron carbide by aluminium below 868C gives only with pure aluminium in the solid(T<660C)or in the two solid compounds: the aluminium diboride alB 2 and liquid state(> 660C)to give solid Al4C3 and a liquid a carbide which is not Al4C3 as in the two preceding ISi alloy SiC coatings will then be damaged by reaction cases, but a ternary aluminium-boron carbide with the with pure aluminium. But on the one hand, this reaction chemical formula Al3 BC [19]. The reaction can be writ progresses at a slower rate than a direct C/Al interac- ten as tion owing to a smaller Gibbs free energy variation Further, interaction stops as soon as the silicon content 9Al+ 2B4C-3AIB2+2Al3BC in the al/si liquid phase has attained the proper com- position for the three-phased equilibrium(Eq 2)to be On the basis of these results, coating carbon fibres established. Decomposition of the SiC coatings can then with B,C does not a priori appear as a very interesting be completely avoided by adding the proper amount of solution for composite fabrication. In fact, B4C is. at temperatures higher than 650C and up to at least 1000C, a three-phased equilibrium tends to be estab￾lished between SiC, Al4C3 and a Al/Si/C liquid L. Under a constant pressure, this three-phased equilibrium: SiC ‡ Al4C3 ÿ!ÿL …2† is monovariant and the composition of the liquid L only depends on the temperature. At 650C, this composition is that of L0, i.e.1.5 at.% of silicon and about 1 at.ppm of carbon. Then, the silicon content of the liquid L reg￾ularly increases with the temperature to attain 4.5 at.% at 730C and 13 at.% at 1000C (the carbon content of L also increases but remains very low). Existence of the quasi-peritectic transformation (1) implies that SiC is in thermodynamic equilibrium with solid aluminium at any temperature lower than 650C. In this low temperature range, SiC coatings can then protect carbon ®bres very eciently against aluminium attack. At temperatures higher than 650C, SiC reacts with pure aluminium in the solid (T<660C) or in the liquid state (T>660C) to give solid Al4C3 and a liquid Al/Si alloy. SiC coatings will then be damaged by reaction with pure aluminium. But on the one hand, this reaction progresses at a slower rate than a direct C/Al interac￾tion owing to a smaller Gibbs free energy variation. Further, interaction stops as soon as the silicon content in the Al/Si liquid phase has attained the proper com￾position for the three-phased equilibrium (Eq. 2) to be established. Decomposition of the SiC coatings can then be completely avoided by adding the proper amount of silicon to the aluminium matrix prior to in®ltration: such a situation will for example occur at 730C with Al/Si alloys containing more than 4.5 at.% of silicon. The phase diagram of the ternary system Al/C/Ti [18] is characterized by the existence at 81215C of a quasi￾peritectic invariant transformation which can be written on heating as: Al3Ti ‡ Al4C3 ÿ!ÿ L1 ‡ TiCx …3† where L1 designates an aluminium base liquid containing 0.3 at.% of titanium and TiCx a carbon-rich composi￾tion in the homogeneity range of the titanium carbide phase with x>0.9 (as in the former case, the carbon content in the liquid is very low). As a consequence of this transformation, carbon-rich titanium carbide coatings will be in thermodynamic equilibrium with aluminium base liquids at temperatures higher than or equal to 812C but will be decomposed by solid or liquid alumi￾nium into Al3Ti and Al4C3 at any temperature lower than 812C according to the equation: 3TiC ‡ 13Al ! 3Al3Ti ‡ Al4C3 …4† As pressure in®ltration of liquid aluminium is dicult to realize at temperatures higher than 812C, the TiC/Al couple has to be considered as a reactive couple in regard to composite manufacture. However, if the reac￾tion corresponding to Eq. (4) cannot be thermo￾dynamically stopped by adding titanium to the aluminium matrix, this addition can modify the reaction kinetics. E€ectively, it has been shown that interaction between TiC and aluminium below 812C develops in two successive stages: (i) a ®rst stage which proceeds at a fast rate and in which titanium produced in the convertion of TiC into Al4C3 simply dissolves in aluminium; (ii) a second stage which proceeds at a much slower rate and in which Al3Ti crystallizes from a liquid saturated in titanium. Consequently, if liquid aluminium is saturated in titanium prior to in®ltration, the ®rst fast-rate stage is avoided and one directly enters in the second slow-rate stage. The Al/B4C couple appears to be reactive at any temperature up to at least 1000C. Decomposition of boron carbide by aluminium below 868C gives only two solid compounds: the aluminium diboride AlB2 and a carbide which is not Al4C3 as in the two preceding cases, but a ternary aluminium-boron carbide with the chemical formula Al3BC [19]. The reaction can be writ￾ten as: 9Al ‡ 2B4C ! 3AlB2 ‡ 2Al3BC …5† On the basis of these results, coating carbon ®bres with B4C does not a priori appear as a very interesting solution for composite fabrication. In fact, B4C is Fig. 8. Fractured surface of BN-coated Hi-Nicalon ®bre. 360 J. Bouix et al. / Composites Science and Technology 61 (2001) 355±362