Further studies on the nickel–aluminum system. II. Vacancy filling in β and δ-phase alloys by compression at high temperatures

The combined action of pressure and temperature on aluminium-rich alloys in the β-NiAl field produces an increase in density and lattice parameter as a result of vacancy-filling. Thus at 43.7 (4) at.% Ni, a pressure of 78 kbar at 1000 °C will increase the density of the alloy from 5.16 to 5.37, with a concomitant increase in lattice parameter from 2.8593 to 2.8712 Å and an increase in atoms/unit cell from 1.77 (5) to 1.87 (7). Similar density changes and increases in atoms/unit cell occur in the trigonal δ-Ni₂Al₃ phase structure, but in alloys on the aluminium-rich side of stoichiometric Ni₂Al₃ vacancy-filling can actually lead to a reduction in both the a and c parameters. This may result from promoting electrons in the 3d shell of nickel into the conduction band, thereby reducing the nickel atom radius.