CONSTITUTIONAL VACANCIES IN NIAL

An explanation is offered, based on a simplified form of many-atom bonding theory, of why NiAl forms constitutional vacancies on its Ni sublattice when excess Al is present. The resultant increase in the number of Al atoms thereby able to bond with Ni atoms is only partially offset by the weakening of individual Al-Ni bonds, so that there is a net gain. Also, the Ni-Ni bonds, being weak in NiAl, claim only a minor penalty when an Ni site is vacated. Other defects, i.e. antisite Ni atoms and triple defects, as well as ordering of vacancies, are briefly discussed.