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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

Even though air is

Even though air is forced out of the preform, oxygen was consumed during infiltration due to the high oxygen affinity of liquid Al and its alloys. When liquid Al is exposed to air, oxygen is consumed to a large extent before nitrogen (8) . Cappleman et al. (116) reported that due to the kinetics of aluminium oxide formation a reaction leading to an alumina monolayer can keep pace with a superficial velocity v0 up to 10 -2 to 10 ־1 m/s in Saffil fibre preform infiltration with Al melts. In the present infiltration experiments v0 varied between 2·10 -2 m/s and 1.44 m/s (Table 3.8) for DSQC and for the maximum velocity in ISQC, respectively. Therefore oxide film formation on a newly formed melt surface seems probable during DSQC. In contrast, at higher infiltration velocities, pure metal surfaces have to be assumed, leading to oxide-free reinforcement contact. Therefore, for high infiltration velocities, a separation layer between the reinforcement and the metal matrix is prevented, shown schematically in Figure 2.17. This seems favourable in terms of interfacial bonding and the resulting mechanical properties (31) . Nevertheless, the present work concentrated on the evaluation of mechanical properties of the MMCs produced in DSQC as the ISQC-infiltrated MMCs were not free of infiltration defects. The threshold velocities proposed by Cappleman (116) were based on Saffil fibre preforms. The reinforcement morphologies in the present investigations were different and therefore a simple calculation accounting for this was developed. Assuming that the die cavity was sealed from the outer atmosphere, oxygen for Al oxidation is only supplied from inside the preform. Assuming complete consumption of oxygen, coverage of at least a monolayer of aluminium oxide between the reinforcing phase and the metal is reached when the specific area per unit preform volume of newly formed aluminium oxide Sml is larger than the specific inner surface per preform volume SpHg (Table 4.2): S > Equation 47 ml S pHg 193

The area Sml was derived using data for the standard atmosphere at a given processing temperature of 800°C (ρT,air, vf,O2) the molar mass fractions of oxygen in alumina (3MO/MAl2O3), its density (ρAl2O3) and the nominal thickness of a monolayer of Al2O3 (dml) as: S ml M Al O Φtot ρT,airv 2 3 f,O2 = Equation 48 d ρ 3M ml Al O 2 3 O The O2 volume fraction of the standard atmosphere vf,O2 is reported as 0.2095 and the density of air ρT,air at 800°C as 348 g/m³ (160) . The mean pore volume fraction Φtot of the preforms was 0.65. Using the density of Al2O3 in Table 3.3, neglecting probable differences in Al2O3 allotropy which was reported (6) to consist of amorphous γ-alumina rather than of α-alumina immediately after reaction, and assuming a monolayer thickness of dml of 8·10 -8 m (116) , the monolayer would cover an area of 3.2·10 4 m² per m³ of preform. When comparing this value with the SpHg values of the preforms in the target volume content range presented in Table 4.2, which were between 1.05·10 6 m²/m³ (MOPC20) and 6.5·10 6 m²/m³ (FATOIS), it is obvious that these are more than two orders of magnitude larger. Based on these calculations, less than 3% of the newly formed liquid surface was oxidized with a monolayer of alumina and further kinetic calculations were discarded. The ratio gets even lower when the air is expelled from the preform during infiltration, as was obviously the case in the present preform infiltration experiments. Therefore the predominant melt fraction had a non-oxidized contact with the reinforcing phase. Nevertheless the residual air fraction inside the preform, consisting predominantly of nitrogen, may be consumed by the melt to form AlN. Zheng and Reddy (8) proposed that AlN formation does not start until the oxygen partial pressure is reduced to 10 -17 Pa. This low value may not be reached in relatively fast infiltration and it is proposed that nitride reactions are absent. 194

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