THERMODYNAMICS OF INTERFACIAL SEGREGATION IN SOLUTE DRAG

The solute drag was discovered in connection with grain boundary migra tion during recrystallization, and it was suggested that the effect is caused by segregation of impurities to grain boundaries. This idea wa s later er;tended to the migration of phase interfaces during phase tr ansformations. However. in a phase transformation? the phases often ha ve different compositions. In order to maintain steady state condition s at the phase interface, a concentration profile must exist inside th e moving interface if it has a finite width. This is realized by diffu sion inside the phase interface, which consumes a part of the driving force. This concept does not necessarily involve the segregation to th e boundary explicitly but is nevertheless often referred to as a solut e drag effect. However, a complete solute drag model for phase transfo rmations should of course also include the effect of phase-boundary se gregation. Furthermore, the gradient energy may also need to be consid ered. In the present work, thermodynamics of interfacial adsorption wi ll be discussed in conjunction with a solute drag model for phase tran sformations, recently modified to take into account the interfacial se gregation of alloying elements. It mill be shown that the classical in terfacial adsorption treatment can be consistently included in the new modified solute-drag model. The gradient energy will be discussed, an d an expression to evaluate its value from the Gibbs energy function w ill be presented. (C) 1998 Elsevier Science S.A. All rights reserved.