3 It is possible that the volume changes associated with poly-

morphic transitions of the alumina f r o m a n d 0 to a in this inter- mediate temperature range will affect the compactness of the reaction layers formed. At higher temperatures when sintering takes place together with the formation of the a form of alumina, shrinkage occurs and cracks form allowing access of the aluminium or aluminium alloy to the silica interface.

The incidence of the cubic phase as detected is limited and although it may have some significance in affecting the surface area of the 'active' interface it cannot be evidenced to postulate the arrest of the reaction in the intermediate temperature range.

The considerable differences in the rate of reaction in this intermediate zone can only reasonably therefore be attributed to physical restraint of the counterflows of the aluminium required for the reaction to continue and the solution of silicon metal produced at the interface, the oxides being retained as the product layer.

In the absence of confirmatory data from X-ray studies it is somewhat speculative to propose the polymorphic change in the alumina as being reduction rate controlling in this range, but it must be considered as one of the most likely explanations. The theoretical evaluation of mullite formation (or preliminary defect spinel) although feasible, and likely evidence for which has been found (Fig. 4.3.1.54 b and c), cannot be definitive as high concentrations,.would be necessary at the reaction interface to influence the kinetics of the process. No evidence of the high concentrations which would be required to suppress the reaction has been observed. It must also be remembered that the reaction front 'is advancing with respect to time and a stepped reaction rate would be likely

for the reaction to continue. The resultant barrier layers which would have had to be breached to allow more ingress of liquid aluminium should have been a prominent feature of the microstructure but were in fact totally absent. No evidence of such a mechanism has therefore, been found at the temperatures where the anomalous behaviour occurred.

The mechanism in this range of temperatures 860-1060°C requires further investigation to test the hypothesis presented here.

5*2 Practical implications

At the outset of the work it was envisaged that the invest- igation would make a contribution to knowledge of the potential for

further reaction in steels of residual aluminium with silicate inclusions. This work has shown and, in a certain way, confirmed that if aluminium as deoxidant is added to liquid steel, the reaction potential of alum­ inium is such that it will reduce the pre-existing inclusions whenever silica is involved. Other sources than the deoxidation (or reoxidation) process itself, such as slag entrainment and refractory erosion which

A

take place during the fcibrication of steel, tapping end teeming processes, can also be responsible for the existence of silicate inclusions in the

metallic bath. It is thought that this type of reaction can also occur

in the solid ingot during heat treatment to the detriment of an aluminium refined grain metal structure, for example, at grain boundaries and

defect sites. Eventually, on the metal side of the interface of the silica rich (or vitreous) inclusion/metal, aluminium segregates and, in

turn, it will react with the inclusion. <■'

However, the concentrations involved in steels are so dis­ similar to those used in the present work that it is not appropriate to

,In other ways the work represents a significant appraisal of reaction mechanisms likely to be of importance in aluminium processing

and silica fibre reinforced aluminium. ^

5.2.1. Aluminium processing

Consistently molten aluminium, particularly when required of high purity, runs the risk of contamination as a result of reacting with

, silica and silicates, used as furnace linings or even thermocouple ,

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sheaths . This work has underlined the need to avoid the use of

silica or silicate materials in contact with the molten aluminium if silicon contamination of the melt is to be avoided. Interestingly the reaction will be severe at the normal holding temperature but if the temperature of .themolten aluminium was raised to 1000°C then contamination seems less likely. It is however, worth noting this fact as it would not

!

be logical in the normal course of industrial practice. Obviously it is not practicable or desirable to consider such a superheat of aluminium during its processing particularly in the light of energy requirements. 5.2.2 Silica fibre reinforced aluminium

During the last two decades there has been periodic interest in the strengthening of aluminium by fibre reinforcement with silica. Whilst there is little load transfer when the composite is elastically deformed at room temperature, because the elastic moduli of aluminium and silica are so similar, at elevated temperatures, the lower coefficient of elast­ icity of tdie silica does give rise to appreciable load transfer to the strong silica fibres. Another practical advantage of the system is that the aluminium is strengthened and the density is reduced by the silica additions.

V

Two techniques have been developed for the fabrication of aluminium-silica fibre composites. The molten aluminium or aluminium alloy may be simply infiltrated between the silica fibres and allowed to

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