Grain Boundaries as Solitary Waves
In computer molecular dynamics “experiments” on high angle symmetric tilt grain boundaries we have discovered that these boundaries exhibit many features attributed to solitons. For example, the grain boundaries have a geometrically stable motion which is nonlinear in that it requires a finite temperature to activate and it persists over times long compared to other relaxation times. The analogy with dislocation motion naturally comes to mind since low angle grain boundaries may be regarded as rows of dislocations and there is extensive literature  on the soliton aspects of dislocations. If we had started out to look for soliton motion we might well have started out with low angle grain boundaries; however, we were originally interested in using dynamic simulation methods to obtain the thermodynamic properties of the metallurgically more important high angle grain boundaries . We attempted to measure the excess boundary entropy by observing the change relative to the perfect crystal in the spectrum of the velocity autocorrelation function for atoms near the boundary. However, we found changes far from the boundary as well, and in tracking this down we discovered that the two crystals were undergoing a relative sliding motion which was coupled with migration of the boundary . This is the motion we call soliton-like and we shall describe this motion in some detail in the following.
KeywordsEntropy Migration Soliton Autocorrelation
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