First phase formation at interfaces: effective heat of formation model for ternary systems

The effective heat of formation (EHF) model, which was used successfully for predicting the first phase formation in binary systems (Pretorius et al., Mater. Sci. Eng., R10 (1993) 1) is modified and applied to ternary systems. In ternary systems the composition of the interface layer cannot be related to the lowest temperature eutectic point as it is done in the conventional EHF model. Thus, unlike binary systems, for ternary interaction it is impossible to predict the first phase, as having maximum effective heat/free energy of formation at the concentration of the lowest temperature eutectic. Application of the EHF model to the first phase formation during interface reactions in ternary systems requires a knowledge of the diffusion path within the interface intermixed layer. However, qualitative analysis of the experimental results can be made using EHF model adopted for ternary systems, without knowledge of the diffusion path. Instead of three-dimensional EHF-diagrams, their projections on a compositional triangle can be used. A simple method to construct such projections is proposed. The previously obtained experimental results on the initial stages of interaction between non-oxide ceramics and metals are in a good correlation with the modified EHF model.