When the sliding of the grain boundary containing particles is accommodated by grain-boundary diffusion, we evaluate the sliding rate and the stress distribution on the boundary, by taking the particle rotation and the intrinsic boundary viscosity into account. The particle rotation reduces the energy required for the grain-boundary sliding and enhances the sliding rate, whereas the boundary viscosity increases the energy for viscous sliding and reduces the sliding rate. Although the boundary viscosity causes shear stresses and induces a torque to the particle, the effect of the torque is lower than the effect of the reducing sliding rate. As a result, the increasing boundary viscosity reduces the rotation rate of particles. The present analysis also predicts the reverse rotation of particles with respect to the grain-boundary sliding, which is different from the favorable rotation by frictional sliding. The particles elongated in parallel to the sliding direction can rotate in the reverse direction at low boundary viscosities.
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