| Grain boundary (GB) segregation in binary systems was traditionally classified into two types: the Langmuir-McLean (LM) and Fowler-Guggenheim (FG) types which differ in the continuity of temperature- or concentration-dependence of impurity amount at GBs. In the LM type, the segregation energy changes with concentration so radically that a sudden jump in enrichment with temperature or concentration can be seen. The concentration-dependent segregation energy is derived from nonzero interchange energy in GBs. In this aspect, a system with high tendency of de-mixing is believed to possess the LM type of GB segregation. However, the FG type had also been observed in some systems with high tendency of de-mixing and sometimes even both types can be found in a single binary system. In this work, a new energetic term, the interfacial coherent energy, which has not been considered previously in the literature, was introduced into the theory of GB segregation and a more general isotherm was obtained. It was suggested from this new isotherm that a binary system with solute segregation less than one monolayer at general GBs obey the LM type, no matter how high the interchange energy is. That means, the introduction of the interfacial coherent energy eliminates the contribution of interchange energy in grains with that in GBs. In this paper, it was shown that this theoretical prediction agrees well with published data in the GB segregation in some Cu alloys. It must be mentioned that this work is not aimed at the overthrow of the application of the FG type which is beneficial in its simplicity. |
full paper (pdf, 70 Kb)