Characterization of the reactivity in Al-10wt.%TiC metal matrix composites by image analysis

Abstract

A metal matrix composite was made by the addition of 10 wt.% of angular TiC0.96 particles, D50 = 18 μm, to molten Al (99.9 wt.%). In order to evaluate the stability of TiC particles in Al at high temperatures, composite samples were placed in a pre-heated furnace, held for 48 hours between 600 °C and 1000°C and air quenched. The as-cast and heat-treated samples were polished and imaged in a scanning electron microscope in backscattered mode. Quantitative metallography was performed using image analysis. The segmentation method was applied to samples that showed low reactivity, i.e. those exhibiting only TiC, Al and Al4C3. In highly reacted samples, a noise deconvolution method enabled reliable measurements of the TiC and Al3Ti phases, which overlap in the gray level histogram. After a normalizing procedure, area fraction measurements showed that TiC reacts slowly in solid state and the reaction increases in liquid state with temperature to a maximum at 725°C. At higher temperatures, the reaction mechanism is different and it results mainly in the formation of Al4C3 suggesting that TiC is partially stable.