Question

A sample of metal with many tiny grains (and hency many grain boundaries) is known to be much stronger and harder than a similar piece of metal with few or one single crystal grain.

Propose an explanation as to why it is much more difficult to deform a metal with many small grains compared to a metal with larger, fewer grains.

Answer 1

Surface defects are the irregular arrangement of atoms at the grain boundaries. In a grain the atoms are arranged in particular order, however, the directions of the arrangement are different in neighbouring grains. The boundaries are irregularly shaped with the effect that at some places atoms are too far apart while at other places they are too close than the normal distance, thus giving rise to tensile and compressive forces. These defects also cause restriction to movement of dislocations. In a structure with small grains the dislocations can move only a small distance before encountering an obstacle, i.e. grain boundary. Therefore, a metal is stronger when it has small grains structure than when it has a large grain structure.