The thermal stability of TiC particles in an Al7wt.%Si alloy

The thermal stability of TiC particles in an Al7wt.%Si matrix has been investigated by heat-treating samples between 500 and 1000 °C for 6 h. The compositions, morphologies and quantities of the reaction phases present were evaluated using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis and image analysis. Differential scanning calorimetry was used to identify the temperature intervals over which the reactions occurred. No reaction was observed when holding in the solid state, but large quantities of reaction products were observed when holding in the semi-liquid and liquid states between 600 and 800 °C, with the largest volume of reaction products being observed at 800 °C. The reaction products were identified as large Ti–Al–Si block-shaped intermetallics, with an average composition TiAl2.14Si0.38, and Al4C3 layers, which both formed at the TiC particle–matrix interface. At 850 °C and above, the rate of reaction of TiC reduced dramatically, intermetallic blocks were no longer formed and small quantities of Al4C3 were observed. DSC experiments showed that, above approximately 885 °C, the Ti–Al–Si intermetallic and Al4C3 reaction products react to form fine needles of TiC and Ti3SiC2 which replace the Al4C3 phase at the TiC particle–matrix interface.