A NEW TYPE OF DEFECT LATTICE IN THE Al-Ni SYSTEM

Abstract

A thorough investigation of the structural changes in the δ phase of Al-Ni alloys has been taken by measuring the lattice spacings, densities, and intensities of diffraction lines. The determination of the lattice spacings shows that, within the narrow homogeneity range less than four atomic per cent, a and c decrease at first with the increase of the Ni content, while after the ideal composition Ni 2 Al 3 , they increase with the increase of the Ni content; in other words, both a and c reach minimum simultaneously at the ideal composition. All density and intensity measurements prove unequivocally that when the Ni content is less than that in the ideal composition, the number of atoms contained in each unit cell always remains five, the Al atoms substituting at random some of the Ni atoms at the centres of the pseudo-cubes; and when the Ni content exceeds that in the ideal composition, the number of atoms per unit cell is more than five, the superfluous Ni atoms filling up randomly part of the interstices forming centres of the pseudo-cubes in the ideal structure.The principal factor governing these changes is the mean number of valence electrons in the fundamental structural unit. In the δ phase, the mean number of valence electrons per fundamental structural unit, the pseudo-cube, could not be greater than three.Complementary to the corresponding β phase in the same system, the δ phase forms a new type of defect lattice in alloy phases.