Defect Structures and Long-Range-Order Parameters in Off-Stoichiometric Ni3Al

The deviation of the composition from stoichiometry in the Ll2-type intermetallic compound Ni3Al has been systematically investigated in relation to the change in lattice parameter, density, and long-range-order parameter, S. Special attention is paid for such crystallographic features as the kind of defect structure and the atom sites in off-stoichiometric composition. It is concluded that substitution occurs and maximum order retains at both side of stoichiometry. Therefore it is obvious that at the Ni-rich side all Al atoms occupy cube corner sites and the Ni atoms occupy all face centre sites and vacant cube corner sites of the unit cell, while at the Al-rich side all Ni atoms occupy face centre sites and the Al atoms occupy all cube corner sites and vacant face centre sites of the unit cell.