What are the Grain boundaries

Grain boundaries disrupt the motion of dislocations by a material, so reducing crystallite size is a common way to improve strength. Since grain boundaries are defects in the crystal structure they tend to reduces the electrical and  thermal conductivity of the material. The high interfacial energy and relatively weak bonding in most grain boundaries often makes them preferred sites for the onset of corrosion and for the precipitation of latest phases from the solid. They are also significant to many of the mechanisms of creep.