Aluminium depletion layer

The striking features after oxidation of Ti35A15Nb for 4h at 900°C are the slight enrichment of aluminium in the metal subsurface zone instead of aluminium depletion, the preferred formation of AlON in wide parts of the metal/oxide interface and the development of a rather dense, coarse-grained partial layer consisting of titania in the oxide scale. Several reasons for the beneficial effect of niobium addition on the oxidation behaviour are discussed in the literature [5,10,11]. Beside the influence of niobium on the aTl/aAI ratio and expansion of the 7-TiAl phase field the effect of doping of titania by niobium is often discussed. By doping of titania with niobium the concentra-... 

In the niobium containing alloy which shows a better oxidation resistance the doping of titania with niobium may reduce the dissolution of AlON. By this means a thin layer AlON is formed at the interface leading to a reduced oxidation rate. Thus it is assumed that the oxidation behaviour of titanium aluminides could be improved by stabilizing the aluminium oxide at the metal/oxide interface either by prevention of aluminium depletion of the metal subsurface zone or by reduction of A1203 dissolution in Ti02. 

Examples of this procedure for dilute solutions of copper, silicon and aluminium shows the widely different behaviour of these elements. The vapour pressures of the pure metals are 1.14 x 10, 8.63 x 10 and 1.51 x 10 amios at 1873 K, and the activity coefficients in solution in liquid iron are 8.0, 7 X 10 and 3 X 10 respectively. There are therefore two elements of relatively high and similar vapour pressures, Cu and Al, and two elements of approximately equal activity coefficients but widely differing vapour pressures. Si and Al. The right-hand side of the depletion equation has the values 1.89, 1.88 X 10- , and 1.44 X 10 respectively, and we may conclude that there will be depletion of copper only, widr insignificant evaporation of silicon and aluminium. The data for the boundaty layer were taken as 5 x lO cm s for the diffusion coefficient, and 10 cm for the boundary layer thickness in liquid iron. 

In a study of dental silicate cements, Kent, Fletcher Wilson (1970) used electron probe analysis to study the fully set material. Their method of sample preparation varied slightly from the general one described above, in that they embedded their set cement in epoxy resin, polished the surface to flatness, and then coated it with a 2-nm carbon layer to provide electrical conductivity. They analysed the various areas of the cement for calcium, silicon, aluminium and phosphorus, and found that the cement comprised a matrix containing phosphorus, aluminium and calcium, but not silicon. The aluminosilicate glass was assumed to develop into a gel which was relatively depleted in calcium. 

The passage of chloride ion through a column of an anion-exchange resin leads to an enrichment of C1 in the first fractions and a depletion in the last fractions (Figure 1). This behaviour can be accounted for if the lighter isotope has the larger ionic radius. The separation of, and detection-limits for, the halide ions on a thin layer of activated silica gel (bonded on aluminium foil) have been studied... 

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