Microstructure, phase morphology, eutectic coupled zone and hardness of Al-Co alloys

This study examines the roles played by crystal growth rates (V) and cooling rates (T) on the microstructural evolutions of hypoeutectic (0.7 wt% Co), eutectic (1.0 wt% Co) and hypereutectic (1.5 wt% Co) Al-Co alloys. For this, these alloys have been directionally solidified by using a transient heat flow setup. It is shown that the microstructures of the 0.7 and 1.0 Co alloys are formed by alpha-Al matrices. In these alloys, a morphological reverse transition from alpha-Al matrix dendrites to cells is observed while a alpha-Al/ Al9Co2 eutectic mixture is also contained. In the Al1.5 wt% Co alloy, the prevalence of eutectic features occurs. Experimental growth laws relating cellular, dendritic and eutectic spacings to V and T are determined. Plots relating V and T to the alloys Co contents are proposed. The latter allow the range of coupled growth during transient solidification of Al-Co alloys to be examined. The hypereutectic alloy (1.5 wt% Co) demonstrates increase of about 41% in hardness as compared to the other examined alloys.