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

Abstract

This study examines the roles played by crystal growth rates (V) and cooling rates (Ṫ) 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 α-Al matrices. In these alloys, a morphological reverse transition from α-Al matrix dendrites to cells is observed while a α-Al/ Al9Co2 eutectic mixture is also contained. In the Al- 1.5 wt% Co alloy, the prevalence of eutectic features occurs. Experimental growth laws relating cellular, dendritic and eutectic spacings to V and Ṫ are determined. Plots relating V and Ṫ 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.