Abstract
The reaction between an Al–Ti melt and a carbon containing medium, i.e. pure graphite or molten aluminium containing aluminium carbide, was studied. Experiments were carried out at 1373 K, using three different experimental setups. Depending on the carbon source, titanium carbide precipitated as a layer or as free particles. Classical nucleation theory for homogeneous and heterogeneous nucleation is used to assess the surface tension. The influence of stoichiometry on surface tension and nucleation is discussed. The formation of a continuous layer, or peritecticlike precipitation of titanium carbide, is theoretically analysed. The growth of the layer is rate controlled by the mass transfer of carbon at the interface, plausibly as a result of the ordering tendencies in titanium carbide. The degeneration of precipitation into that of freely formed particles is found to be nucleation controlled. A model for degenerate precipitation is proposed, including predictions of the reaction front compositions, growth rate, and particle fractions in a dilute ternary system. The model is in good agreement with experiments, after compensating for settling of the precipitates, except in predicting the precipitated fractions.
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