AbstractHardness H, interjlake spacingλ, and tensile properties are reported for Al–12·7Si and Al–12·7Si–0·2Sb (all wt-%) eutectic alloys directionally solidified at growth velocities of up to 250μm s−1and under temperature gradients in the liquid of up to 12·9 K mm−1. The hardness is related to interflake spacing by the equation H=Ho+Kλ−0·2, where Hois the initial hardness of the alloy. This behaviour contradicts previous results, which suggest that a Hall–Petch relationship is followed. The tensile properties are shown to follow similar behaviour, confirming that hardness shows the same dependence as proof stress on interflake spacing. However, the nature of the relationship depends on the Si morphology and caution should be exercised in using hardness or interflake spacing to indicate proof stress.MST/1585