OBSERVATIONS ON THE MICROSTRUCTURE AND PERFORMANCE OF AN AL-TI-C GRAIN-REFINING MASTER ALLOY

Al-Ti-C grain-refining master alloys are of increasing importance in a luminium casting because they are believed to introduce a smaller volu me fraction of insoluble particles into the melt than conventional Al- Ti-B master alloys are. The main microstructural features of an Al-6wt .%Ti-0.02wt.%C master alloy have been examined using X-ray diffraction and optical, scanning and transmission electron microscopy. In additi on to the aluminium matrix, two principal intermetallic phases were id entified, namely titanium aluminide and titanium carbide. The titanium aluminides were found to contain small amounts of dissolved vanadium and to possess a range of morphologies from well faceted to rough and irregular. A combination of microanalysis and electron diffraction was used to show that clusters of submicron particles, rich in Ti but wit h virtually no Al, were TiC crystals. Individual particles were found to be single crystals with an octahedral morphology. A simple thermody namic analysis of the stability of TiC in the Al-rich corner of the Al -Ti-C phase diagram is presented which confirms the stability of TiC d uring master alloy synthesis. Using standard test procedures, the Al-T i-C alloy proved to be an effective grain refiner when added to commer cial purity aluminium, and it is proposed that undissolved TiC particl es were most likely to have been the heterogeneous nuclei.