Grain sizes of alloys in the semi-solid state are very important during isothermal heat treatment.In this paper

magnesium ZK60 semi-solid billets were prepared by near-liquids casting and equal channel angular extrusion

respectively.Isothermal grain coarsening of as-cast and extruded billets has been investigated by Optical micrograph and image dissector camera system.The formation mechanics of intragranular liquid droplets has also been studied.The results show that grain coarsening rates of both as-cast and extruded billets are decreased with increase in holding time at the same temperature

when the width of the semi-solid interval ranges from 0.68 to 0.85.The coarsening rate constant K of as-cast billet decreases from 570 μm3/s（solid fraction equals to 0.65） to 319 μm3/s（solid fraction equals to 0.8）

and then it increases to 859 μm3/s（solid fraction equals to 0.85） during isothermal heat treatment.However

the coarsening rate constant K of extruded billets decreases from 146 μm3/s（solid fraction is 0.65） to 98.8 μm3/s（solid fraction equals to 0.85） during isothermal heat treatment。The formation of intragranular liquid droplets of as-cast billets originates from eutectic of intragranular

which is produced by non-equilibrium freezing

and entrapped liquid

which is created by coalescence of solid particles.