Equilibrium segregation in a ternary solution: A model for temper embrittlement

Abstract Whereas experimental evidence has established that temper embrittlement is due to the simultaneous intergranular segregation of some impurities I (Sb, P, As Sn, ...) and of metallic alloying elements M (Ni, Cr, Mn, ...), all the theories of this phenomenon have failed so far to rationalize the role of the alloying elements, i.e. essentially to explain why they are necessary for the segregation of the impurities to occur. The thermodynamical formalism of equilibrium segregation, when taking into account the interaction of M and I atoms in an iron base solid solution can explain this behaviour. The simplest analytical treatment of such interaction based on the regular solution model is developed. The values of the interaction coefficients between the various atoms, as evaluated from the enthalpies of mixing of the intermetallic compounds, appear consistent with the value deduced from segregation measurements. The model is shown to allow a comprehensive interpretation of the segregation behaviour of various binary and ternary solutions, in particular the iron base systems. Eventually the role of bi-dimensional compound formation in the segregation process is considered.