with T, the actual

with T, the actual temperature and Tm the melting point and γlv in N/m. The effect of oxygen on the γlv of liquid aluminium was evaluated by Gourmi and Joud (10) who found that the reduction of the surface tension was proportional to the oxide coverage on the surface. A mean value of 1.050 N/m was found for the surface tension of atomically clean aluminium at 700°C. In contrast, a mean value of 0.871 N/m corresponded to saturation coverage by a homogenous layer (19,20) . The higher values resulted from chemically pure aluminium and the lower values are determined using contaminated conditions which led to oxygen coverage as shown in Figure 2.3. It could be shown that the surface coverage of one monolayer led to a reduction in surface tension of more than 11%. Surface Surface tension tension / N/m(N/m) 1.10 1.05 1.00 0.95 0.90 0.85 0.0 0.5 1.0 1.5 X (Monolayer units) X / monolayer units Figure 2.3 Variation of surface tension of molten Al with level of oxide coverage (20) . Rocher et al. (21) achieved metallic surfaces on Al melts even at relatively high oxygen partial pressures. The thin Al2O3 layer on the liquid melt was removed by additions of K2ZrF6, a well known fluxing agent used in Al melts. The predominant effect to improve cleanliness of the melt is the dissolution of the thin alumina layer coating the liquid metal by the fluorides. The surface tension of a pure liquid melt is significantly affected by binary additions (18) . In general the γlv of Al is observed to decrease with increasing levels of Si, Mg and Sr. Solutes 9

that are most effective in decreasing the surface tension of the solvent typically exhibit surface tensions lower than the solvent (22) . Si, Mg and Sr exhibit lower surface energies as compared to Al and hence also decrease the surface tension (23) . The linear rule of mixture provides a first approximation for the surface tension of the resulting solution in terms of the surface tensions γlv1 and γlv2 of the pure components 1 and 2 respectively (18) . Thus γ N γ + N γ = Equation 2 lv12 1 lv1 2 lv2 where N1 and N2 are the solvent mole fractions of the components 1 and 2 respectively. Experimentally, the surface tension almost always deviates negatively from that predicted by Equation 2. Moreover, the latter is inconsistent with thermodynamics since the liquid surface is found to be enriched with the component with the lower γlv (24) . The addition of Si to the Al melt results in minor reductions in γlv as shown by Koerber and Loehberg (25) . The addition of 12 wt.% Si to pure Al reduces it from 0.84 N/m to 0.83 N/m which is in the range of γlv scatter of contaminated pure Al melts. In presumably more accurate measurements, the latter value was confirmed with an A356 alloy where γlv ranged between 0.801 and 0.889 N/m (19) . Koerber and Loehberg evaluated the effect of other alloying elements on the surface tension of Al melts. It is interesting to note that very low additions of Na in the range of 0.1 wt.% reduce γlv by more than 35%. The Si in Al alloys plays an important role in the production of Al/SiC MMCs. As an alloying element it retards the formation of the unwanted intermetallics Al4C3 and Al4SiC4 (26) which account for the brittle character of the composite materials. Compared to pure Al in oxide ceramic systems, no significant influence of Si additions on composite ductility could be observed. There is a eutectic in the Al-Si system at a Si mass fraction of 11.7 % (Figure 2.4). 10