Al-12Si-3.5Cu-1.0Mg-2.0Ni alloys with modification and T6 treatment were conducted at 300 ℃ for 25～400 h.The evolution of microstructure and properties of alloy under high temperature and long time holding temperature was analyzed.The results show that the eutectic Si morphology is changed from granular to rod-like at 300℃ for 25～100 h

and grain size is increased with number decreases

and the coarse bulk Si phase appears.After 200～400 h

the size of the coarse bulk Si phase is decreased

and the eutectic Si morphology is changed from rod-like to granular

meanwhile

the size is decreased with the quantity increases.The precipitation

aggregation and growth of alloying compounds form acicular Al-Ni-Cu or Al-Cu-Ni phases.The mechanical properties of the alloy are decreased

where the tensile strength is decreased from 332.34 MPa to 185.65 MPa

and the elongation is decreased firstly and then increased.