A DIFFERENTIAL SCANNING CALORIMETRY STUDY OF SOLID-STATE REACTIONS IN AA6061-SIC, AA6061-AL2O3 AND A357-SIC COMPOSITES FABRICATED BY MEANS OF COMPOCASTING

Phase formation and dissolution in AA6061-SiC, AA6061-Al2O3 and A357-SiC composites have been investigated by means of differential scanning calorimetry (DSC). The composites were fabricated by means of molten metal mixing, extruded and heat treated. The DSC studies were carried out on material in the as-quenched and naturally and artificially aged conditions. In the AA6061 materials the effects of the reinforcement are similar to those observed by other authors. Some remarkable features of the results for the composites are: a smaller amount of Guinier-Preston (GP) zones, a faster precipitation kinetics of the intermediate phases (beta'' and beta'); and the absence of the peak related to the formation of the beta phase. These results are all a consequence of the largely different thermal expansion coefficients of the ceramic particulates and the host alloy. in contrast, the differences between A357 monolithic and composite materials are much less noticeable. This should be a consequence of the large amount of free Si in A357 which being in the form of particles, makes the monolithic material behave much like a composite.