A cast Mg–3Gd–0.6Zr alloy with microscale growth twins was designed successfully. After the alloy was annealed at 350°C for 1 h, the volume fraction of the growth twins was increased by approximately 120%. Gd solute atoms segregated on both sides of twin boundaries and facilitated their formation in the as–cast alloy. The formation mechanism of growth twins in the as–cast alloy was discussed in five aspects: the supersaturation of Gd solute atoms, the segregation ability of Gd, the composition of Gd, the nucleate sites of growth twins, and the total energy of the system. The growth twins elicited remarkable toughening effect but a moderate strengthening effect on the alloy.
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growth twins was designed successfully. After the alloy was annealed at 350°C for 1 h, the volume fraction of the growth twins was increased by approximately 120%. Gd solute atoms segregated on both sides of twin boundaries and facilitated their formation in the as–cast alloy. The formation mechanism of 
growth twins in the as–cast alloy was discussed in five aspects: the supersaturation of Gd solute atoms, the segregation ability of Gd, the composition of Gd, the nucleate sites of growth twins, and the total energy of the system. The growth twins elicited remarkable toughening effect but a moderate strengthening effect on the alloy.