Plastic flow in polycrystal states in a binary mixture

Using molecular dynamics simulation, we examine the dynamics of sheared polycrystal states in a binary mixture composed of small and large particles in two dimensions. We vary the composition c of the large particles and the shear rate (gamma) over dot. to realize changeovers among crystal, polycrystal, and glass. We find large stress fluctuations arising from sliding motions of the particles at the grain boundaries, which occur cooperatively to release the elastic energy stored. These stress fluctuations decrease as the system crosses over from polycrystal to glass. The dynamic processes are visualized with the aid of a sixfold angle alpha(j)(t) representing the local crystal orientation and a disorder variable D-j(t) representing a deviation from the hexagonal order for particle j.