Microstructure and its influence on refining performance of AlTiC master alloys

Abstract

The microstructures of four different AlTiC master alloys produced through a new method involving the reaction of pure Ti and carbon in an Al melt have been examined using XRD, SEM, EPMA, and TEM, and their refining efficiencies tested. Individual TiC particles were found to be single crystals with polyhedral or spherical morphologies. In addition to the aluminium matrix, Al - 5Ti - 0.3C (wt-%) refiners contain TiC and TiAl₃ phases, whereas Al - 8.4Ti - 1.8C contains only TiC. Three types of agglomerating behaviour of TiC particles, namely discrete particles, homogeneously distributed clusters, and compact blocks along grain boundaries, were found in master alloys produced at different temperatures, and these further influence the refining performances. Ti was found to play an important role in the refinement of Al by AlTiC refiners; observation of grain centres showed that TiC particles are located at the grain centre and the excess Ti segregates to them and forms a roselike structure around them. It is clear that the refinement of Al by AlTiC results from the combined action of TiC and Ti, and it is possible that AlTiC and AlTiB share some common rules in refining Al. Possible refinement mechanisms have been discussed based on microstructure observations and previous studies.