Effect of Erbium Addition on the Microstructure and Hardness of an Al–Mg–Si Alloy

In hypoeutectic Al–Mg–Si alloys the eutectic Mg2Si solidifies as coarse, faceted flakes that act as crack initiation sites and limit mechanical properties. Rare-earth additions are known to modify faceted eutectics in aluminium alloys, but the response of eutectic Mg2Si to erbium (Er) is not well established. This work tests the hypothesis that Er, which has a large atomic radius and negligible solid solubility in aluminium, segregates to the solid/liquid interface during eutectic growth and restricts the growth of Mg2Si. A cast Al–Mg–Si alloy containing 0, 0.1, 0.3 and 0.5 wt.% Er was examined by optical microscopy and Vickers hardness testing. Er addition changed the eutectic Mg2Si from long, continuous, strongly elongated flakes into shorter, blockier features: quantitative image analysis gives an area-weighted aspect ratio falling from 6.2 in the base alloy to 4.6 and 3.5 at 0.1 and 0.3 wt.% Er. The same analysis resolved no monotonic reduction in the characteristic size of the eutectic, so the change is one of shape rather than of scale, and a progressive refinement of the eutectic is not demonstrated by these images. Mean hardness rose from 60.8 ± 4.5 to 92.4 ± 9.1 HV; the effect of Er content was significant overall (ANOVA, p = 0.006), although with three indentations per alloy only the 0.5 wt.% alloy differed significantly from the base alloy in a pairwise comparison. The loss of elongated faceted growth is consistent with a growth-restriction mechanism but is not established by these observations. No over-modification was found up to 0.5 wt.% Er.