Solid states solubility of aluminum in the δ-Ni 2Si nickel silicide

Abstract

Powder mixtures of Al, Ni and Si with compositions located on a line joining the nickel silicide δ-Ni 2Si to the previously reported ternary compounds Ni 5.4Si 2Al 1 were cold-pressed, flash melted by electron bombardment, annealed after 300 h at 1000°C under 1 atm of purified argon and characterized by optical metallography, electron microprobe analysis and X-ray diffraction. Analysis of 529 unique X-ray reflections collected on a crystals of pure δ-Ni 2Si and Rietveld refinement of its powder diffraction spectrum entirely confirmed the structural data previously reported for this stoichiometric compound (space group Pbnm, Z=4) and allowed a precise refinement of its cell parameters: a=7.0664(4) Å; b = 5.0088(3) A ̊ ; c = 3.7321(2) A ̊ . Moreover, it has been shown that the structure of δ-Ni 2Si can accommodate Al atoms up to a content of 20.9 ± 0.5at.%, giving rise a ternary solid solutions with a wide homogeneity range. Full profile refinement of X-ray powder spectra has indicated that the formation of this solid solution proceeds mainly by substitution of Si atoms by Al atoms, while Ni vacancies may appear. For increasing amounts of Al atoms in the δ-Ni 2Si framework, the cell parameters a, c and the cell volume icrease while parameter b decreases. On the basis of these results, the ternary compounds Ni 5.4Si 2Al 1 (or Ni 1.8Si 0.67Al 0.33) appears as a particular composition in the homogeneity range of a solid solutions that can be designated by the general formula δ-Ni 2− x Si 1− y Al y .