Abstract: By expanding the Gibbs free energy of a ternary system into a Taylor series with respect to the composition,a thermodynamic criterion of spinodal decomposition is conducted as G_(xx)(δx)2+2G_(xx)δxδy+G_(yy)(δy)~2<0,from which,various conditions of spinodal decomposition are discussed.When G_(xx)<0 G_(xy)~2-G_(xx)G_(yy)<0,it is called unstable decomposition unrestricted by the direction of composition fluctuation;when G_(xx)<0,G_(xy)~2-G_(xx)G_(yy)>0,or G_(xx)>0, G_(xy)~2-G_(xx)G_(yy)>0,as well as G_(xx)=0,are all considered as unstable decomposition restricted by the direction of composition fluctuation δx and δy.The phenomenon of turning round in composition during spinodal decomposition is attempted to be explained from above point of view.The computer calculated ranges of unstable decomposition according to the criterion of spinodal decomposition on Cu-Ni-Fe ternary system is consistent with the experimental re- sults obtained from literatures.

Key words: spinodal decomposition, thermodyamic criterion, Cu-Ni-Fe