Instability of TiC particles in aluminium melts inoculated with an Al-Ti-C grain refiner

Microstructural observations on super purity aluminium inoculated with an Al-Ti-C refiner confirm that the nucleant TiC particles present in the refiner are thermodynamically unstable on holding in the melt. The rate of conversion of TiC to Al4C3 is characterised using measurements of the titanium content of the aluminium; the conversion is slow enough not to impede grain refinement under normal conditions, but it is strongly temperature dependent. The mechanism of the conversion is examined and shown to be consistent with a progressive rather than a sudden fading of refiner performance. Based on estimates of the evolving size distribution of TiC particles in the melt, grain size predictions are made and shown to be in qualitative agreement with the fading of refinement seen experimentally.