Effects of cooling rate and isothermal solid solution process on the morphology and volume fraction of grain boundary β phase in AZ91+1 SiC alloy were investigated by copper mould spray casting and subsequent isothermal solid solution to reveal its formation mechansim.The results show that the high cooling rate of spray casting can refine the grain size significantly in comparison with furnace cooling.The formation of supersaturated solid solution can reduce the solute segregation degree

which is beneficial to inhibiting the formation of β phase.With the decrease of inner diameter

the enhanced non-equilibrium effect leads to the further decrease of β phase.The volume fraction ofβphase with diameter of 2 mm is only 4.3%

reduced by 66.1% than that of ones with 8 mm in diameter.After isothermal treatment at 320 ℃ for 2 h

the volume fraction of β phase in quenched alloy with diameter of 8 mm is decreased to 9.1%.As the increase of holding time

its value is further reduced to 5.5% for 4 hand varied sliGHTly after 8 h.The amount ofβphase is decreased greatly with solid solution temperature increase

where a complete solution structure can be obserevd after holding at 400℃ for 2 hdue to the disappearance of β phase.