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Abstract: The microstructural evolution behavior of Mg–5Si–1Al alloys modified with Sr–Sb during isothermal heat treatment was investigated in the present study. Although the morphology of eutectic Mg2Si phase varied with isothermal holding temperature increasing from 620 to 670°C, no spheridization occurred for primary Mg2Si polyhedrons in Sr–Sb-modified alloy even when the temperature reaches 660 or 670°C. Such an abnormal phenomenon of primary Mg2Si, during partial remelting isothermal treatment, might be ascribed to both the spheridization restriction effect caused by incorporation of Sb in Mg2Si and stability of octahedral primary Mg2Si crystals faced by {111} planes. [Copyright &y& Elsevier]