INFLUENCE OF RAPID SOLIDIFICATION ON THE MICROSTRUCTURE OF ALUMINIUM RICH HYPEREUTECTIC Al-Co ALLOYS

A hypereutectic alloy in the Al rich region of the Al-Co system having a cobalt content of 9.71at.%Co was selected in the composition range where the Al9Co2 cobalt aluminide crystallizes directly from the melt without involving a peritectic reaction. Two different cooling rates during solidification, namely similar to 5x10(6) degrees C/s obtained by melt spinning and similar to 5 degrees C/s obtained by die casting have been applied. X-ray diffraction has ascertained the presence of the equilibrium phases, fccAl and monoclinic Al9Co2, - irrespective of the cooling rate applied during solidification. No metastable phases have been produced by rapid solidification. SEM electron microscopy has indicated a dendritic morphology for the primary Al9Co2 crystals in both casting conditions. This morphology may be associated with a presumed low value of the entropy at melting for the Al9Co2 compound. The eutectic mixture (Al+Al9Co2) located in the interdendritic space seemed to have a globular morphology. The grain size refinement of the primary Al9Co2 dendrites induced by rapid solidification was estimated to be similar to 200 times by considering the average thickness of the dendrite stems that was similar to 100 mu m at slow cooling rate and similar to 0.5 mu m at rapid cooling rate. The dendritic morphology and grain size refinement induced by rapid solidification recommends the alloy as a possible candidate for making Co skeletal catalysts through a de-alloying process by alkali leaching.