Surface:

If it's a free surface, the dislocation can penetrate through the surface easily, but, if it's a coating or oxide layer, it will be hard for dislocation to glid out of the surface

Grain boundary:

In discrete dislocation dynamic simulation, some authors take grain boundary as a impenetrable, such as a rigide wall, But in other papers, grain boundary was thought to be a penetrable wall.

 So,Is there somthing or simillar property for us to derive.