An explanation for the formation of polyhedral abnormal grains in single-phase systems

  Morphologically distinctive grain boundaries, present in a model Ni system with cube-shaped abnormal grains, were characterized via transmission electron microscopy. It is revealed that the boundaries terminated by {100} planes are energetically and kinetically more stable than other boundaries terminated by other planes. These findings indicate that abnormal grains assume a polyhedral shape when specific planes are highly singular and their migration is governed by step formation and growth, irrespective of the orientation of matrix grains.