Effect of RE elements on the structure and impact toughness of sand-cast Zn-12% Al alloy

Abstract

The effect of rare earth (RE) elements on the structure and impact toughness of sand-cast Zn-12wt.%Al (ZA-12) alloy was investigated systematically as the content of mischmetal (Ce + La > 75 wt.%) was varied. RE elements appear mostly in the form of clusters between the primary β dendrites or at the boundary of the eutectic η phases in faceted compounds, and in small amounts in the core of the primary β dendrite or in the eutectic η phases in the form of a solid solution. The secondary dendrite arm spacing (DAS u) of the primary β dendrites decreases and the eutectic β phase changes from rods to short rods or granules with increasing RE content. The RE elements, and particularly the RE compounds, absorb the Fe impurity in the alloy. The impact toughness of sand-cast ZA-12, whether as-cast or heat-treated, tends to increase as the content of the RE elements increases, and is about twice that of the RE-free alloy at around 0.12 wt.% RE. It is suggested that the improvement of the impact toughness is related to the refinement of the grain size and the absorption of the Fe impurity by the RE elements in the alloy.