Plastic Deformation

· The yield stress of polycrystalline materials is found to increase as the average grain size decreases.
· Dislocations generated within a grain pile up at the grain boundary and increase the shear stress on slip systems in the adjacent grains due to their long range elastic forces. The number of dislocations that can be piled up on a slip plane in a given grain is grain size dependent.

From: Guy & Hren, "Elements of Physical Metallurgy,"
Addison Wesley (1974)

· The local stress at the grain boundary depends upon the number of dislocations in the pile up. This local stress adds to the external stress and can activate Frank-Reed sources in the adjacent grains causing plastic deformation in those grains.