Further studies on the nickel–aluminium system. I. β-NiAl and δ-Ni₂Al₃ phase fields

New lattice parameter and density results have been obtained for alloys in the β-NiAl and 6-Ni₂Al₃ phase fields of the nickel–aluminium system. The lattice parameter of the β-NiAl phase (CsCl-type) falls linearly from 2.8870 Å at 50 at.% Ni to 2.8618 Å at 66. at.% Ni, with 2.00 atoms per unit cell. On the other hand, the lattice parameter on the Al-rich side of NiAl falls linearly from 2.8870 Å to 2.8652 Å, while the number of atoms per unit cell falls from 2.00 to 1.817 by the creation of vacancies in normally nickel sites. The trigonal 6-Ni₂Al₃ phase-structure, which is essentially an extension of cubic β-NiAl, but with every third plane of nickel atoms perpendicular to the trigonal axis missing, shows a minimum in the a and c spacings at stoichiometric Ni₂Al₃. Density measurements indicate that the vacancies formed by the missing planes are progressively filled as nickel is added to Ni₂Al₃, but that a substitutional solid solution is formed on the aluminium-rich side of stoichiometric Ni₂Al₃ with aluminium replacing nickel atom by atom. In the NiAl phase, the number of valence electrons increases from 2.28 per unit cell at 61.9 at.% Ni to 3.00 at stoichiometric NiAl, and remains constant at 3.00 as the vacancies form until Ni₂Al₃ is reached, at which stage the number of vacancies will have reached a maximum when there are only 1.67 atoms per pseudo-cubic cell. The number of electrons per pseudo-cubic unit cell then begins to rise and reaches the phase boundary value of 3.12 at 37.6 at.% Ni.