SOLID-STATE SOLUBILITY OF ALUMINUM IN THE DELTA-NI2SI NICKEL SILICIDE

Powder mixtures of Al, Ni and Si with compositions located on a line j oining the nickel silicide delta-Ni2Si to the previously reported tern ary compound Ni5.4Si2Al1 were cold-pressed, flash melted by electron b ombardment, annealed for 300 h at 1000-degrees-C under 1 atm of purifi ed argon and characterized by optical metallography, electron micropro be analysis and X-ray diffraction. Analysis of 529 unique X-ray reflex ions collected on a crystal of pure delta-Ni2Si and Rietveld refinemen t of its powder diffraction spectrum entirely confirmed the structural data previously reported for this stoichiometric compound (space grou p Pbnm, Z = 4) and allowed a precise refinement of its cell parameters : a = 7.0664(4) angstrom; b = 5.0088(3) angstrom; c = 3.7321(2) angstr om. Moreover, it has been shown that the structure of delta-Ni2Si can accommodate Al atoms up to a content of 20.9 +/- 0.5at.%, giving rise to a ternary solid solution with a wide homogeneity range. Full profil e refinement ent of X-ray powder spectra has indicated that the format ion 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 delta-Ni2Si framework, the cell parameters a, c and t he cell volume increase while parameter b decreases. On the basis of t hese results, the ternary compound Ni5.4Si2Al1 (or Ni1.8Si0.67Al0.33) appears as a particular composition in the homogeneity range of a soli d solution that can be designated by the general formula delta-Ni2-xSi 1-yAly.