Synergetic modification effects on primary Mg2Si in Al-20Mg2Si alloy induced by the co-addition of beryllium and antimony

•Primary Mg2Si is modified from dendrites to truncated octahedrons with 0.5 wt% Be-Sb.•Be-Sb modifies Mg2Si by controlling the nucleation and growth process simutaneouly.•The formation of Mg3Sb2 enhances heterogenous nucleation sites of primary Mg2Si.•Be refines the primary Mg2Si by adsorption-poisoning mechanism.•0.5 wt% Be-Sb modified alloy exhibits an improvement in mechanical properties.