Modification of the silicon phase and mechanical properties in Al-40Zn-6Si alloy with Eu addition

Abstract The influence of Eu additions upon the silicon phase and mechanical properties of Al-40Zn-6Si alloys have been investigated via scanning electron microscopy (SEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The primary Si was discovered to be remarkably refined and modified to nodular with 0.5% Eu addition. Meanwhile, the plate-like eutectic Si was modified to a fibrous structure. The effect of Eu on nodular primary Si was clarified from five aspects: (a) the adsorption of single Eu atomic column at the twin boundary, inducing stacking faults and parallel twins, (b) the adsorption of single Eu atomic column and triangle-like Eu-rich clusters at the junction of Si twins, inducing multiple twins, (c) the adsorption of Eu linear distribution along the twin boundary, inducing poisoning of twin plane re-entrant edge (TPRE), (d) the adsorption of Eu linear distribution at special grain boundary ∑9 twin, reducing the lattice distortion and grain boundary energy, (e) the solute entrainment within Si. These results prove that the widely accepted impurity induced twinning (IIT) and poisoning of TPRE mechanisms are valid for nodular primary Si. By addition of 0.5%Eu, the ultimate tensile strength and elongation were enhanced by 31% and 358% respectively.