[SY-I3] Interaction of moving grain boundaries with solutes: bridging time scales between atomistics and continuum
Invited
Solutes can strongly interact with grain boundaries (GBs) and impact their thermodynamic and kinetic properties. Some of the most interesting effects of solute segregation include the segregation-induced GB phase transformations and the solute drag/pinning effect. An even more interesting is the effect of GB motion on GB phase transformations. Studying these effects by molecular dynamics is highly problematic due to the limited time scale of the method. The talk will present two alternative methods for studying phase transformations in moving GBs. One method is based on a semi-analytical discrete GB model in a binary solid solution. The regular solution model predicts first-order phase transformations in GBs, which can be shown by a GB phase coexistence line and GB spinodals of the bulk phase diagram. The model overcomes the time-scale limitation and treats both GB thermodynamics and GB dynamics within a unified framework. It gives direct access to the solute drag force and the GB free energy, which are difficult to compute by atomistic simulations. GB migration can be modeled on different timescales and over a wide range of velocities in both the transient and steady-state regimes. The simulations reveal interesting effects, such as kinetic stabilization of metastable or even unstable GB phases, dynamic GB phase transformations, and the dynamic GB hysteresis. Other interesting effects include the break-way events in which the GB leaves the segregation atmosphere behind and forms a new segregation. This new segregation atmosphere can represent the same of a different GB phase. It is shown how the discrete model reduces to phase-field model in the limit of wide segregation region. The phase-field simulations within this model yield similar results as the discrete model but have certain computational advantages. The results of this study can be broadly interpreted in terms of extremum principles of non-equilibrium thermodynamics, which predict dynamic stabilization of thermodynamically unstable but kinetically favored phases. Extension of the models to 2D and 3D systems and more accurate thermodynamic treatments will be discussed.