Theoretical analysis based on the calculation of phase diagrams was employed for Fe-Si-Co and Fe-Si-AI ordering systems to clarify the necessity for the occurrence of phase separation in Fe-base ternary ordering systems. The free energy of Fe-base ternary ordering alloys where B2 and D03 ordered structures are formed is evaluated statistically using a pairwise interaction approximation up to second nearest neighbours, taking into account not only the atomic interaction but also the magnetic interaction; based on the Bragg-Williams-Gorsky model. The calculated phase diagrams are consistent with the experimentally obtained ones. The phase diagram calculation in this work is useful to predict the equilibrium states of the ternary ordering systems. The phase separation in ordering alloys is caused by the contribution of excess free energies due to ordering. The influences of ferromagnetism on the two-phase regions are also demonstrated.
IntroductionIt has long been recognized that the phase separation of the supersaturated solid solution occurs only in alloy systems with positive interaction parameters between the nearest neighbour atoms. Recently, however, several experimental studies have revealed that the phase separation actually occurs in the ordering solid solutions such as Fe-A1 [-1-6], Fe-Si [7-10], Cu-Zn [11,12], Cu-Mn-A1 [13], and several Fe-base ternary ordering alloys [14-18], which have negative interaction parameters. The experimental evidence described above has urged us to change the conventional concept that phase separation-type and ordering-type alloys have opposite characteristics for each other.In order to understand solute atom clustering in ordering alloys, it is important precisely to evaluate the free energies of ordered and disordered solid solutions. For the binary ordering alloys, theoretical approaches [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] have been proposed taking into account the interatomic interactions between not only the nearest neighbours but also the second and higher order neighbours. In theoretical treatments of this sort, even if the first-order interaction parameter is negative, phase separations are expected to occur when the second and higher-order interaction parameters are positive.As regards ternary ordering Fe alloys, we have already found that the two-phase regions exist in the several Fe-rich ternary ordering alloys, and proposed experimental phase diagrams. In the Fe-Si-Co system [-16], in addition to A2, B2 and D03 single-phase regions, the B2 + D03 two-phase region is found in the two separate composition regions: one is the region connected to the B2 + D03 region of the Fe Si binary system; and the other exists in the isolate corn-
