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The microstructure difference and enthalpy changes have been investigated by scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) during the as-cast ageing process. After the as-cast ageing process, the eutectic silicon and α-Al is in the ideal state for the alloy with 0.08 wt% mischmetal (MM) addition. The energy of the phase transformations is 5.8 J/g during the isothermal process, when Al-11%Si-0.27%Mg alloy is aged at 150°C. When the additive contents of MM are 0.03, 0.06, 0.08 and 0.1 wt%, respectively, the energies of phase transformations are 4.98, 1.39, 1.07 and 1.25 J/g correspondingly. It is shown that the energies of phase transformations decrease gradually as the MM content increases from 0 wt% to 0.08 wt%; the energies then increase upon further addition of the MM concentration from 0.08 wt% to 0.1 wt% in the as-cast ageing process. Furthermore, the beginning time of phase transformations is delayed with the MM addition.