Solidification microstructure and mechanical properties of an Al-11wt%Si alloy modified with Nb and Sb

Abstract

It is important to understand the effects of adding Nb and Sb in order to improve the applied properties of Al–Si alloys. Thus, a nonsteady-state directional solidification technique was used in the present study to evaluate the effects of Nb and Sb microaddition and the solidification cooling rate on the dendritic microstructure of an Al-11 wt% Si alloy and their relationships with the tensile and hardness properties. Sb was shown to reduce the primary dendritic spacing (λ1) at the same solidification velocity or cooling rate, whereas Nb did not have these effects. Modification of the eutectic Si was shown to occur at high cooling rates, where the acicular Si transformed into fibrous, branched, and globular forms, instead of being attributed to the addition of alloying elements. The hardness profile for the Nb-containing alloy was the highest among all alloys examined in this study. The tensile properties were correlated with the changes in λ1 along the length of the directionally solidified castings. Al–Si alloy with added Nb obtained the best performance in terms of the ultimate tensile strength and elongation to fracture due to the better distribution of Si combined with aluminum-rich phase strengthening in solid solution with Nb.