Controlled Nucleation Method （CNM） is applied to AZ91 magnesium alloy to produce thixotropic structure suitable for semisolid casting.Pouring temperature and cooling rate are the two important solidification conditions that can be used for microstructure control in CNM process.Both have a significant effect on the as-cast microstructure and therefore on semisolid microstructure.Low pouring temperature produces fine non-dendritic as-cast structure.After partial remelting and isothermal holding

it develops into a thixotropic structure.High pouring temperature leads to a coarse dendritic as-cast structure

which turns into an irregular solid network in semisolid state.High pouring temperature can combine with high cooling rate to achieve a thixotropic structure.