Structural Investigation of Noncrystalline Nickel-Phosphorus Alloys

The structure of noncrystalline electrodeposited Ni–P alloys, 73.8–81.4 at.% Ni, has been investigated by x-ray scattering and by physical density measurements. The x-ray interference functions, I(k), are qualitatively inconsistent with those calculated for fcc-, hcp-, and Ni3P-type crystallites. Calculated radial distribution functions RDF(r) indicate that the alloys have a better defined short-range order than that observed in liquid noble metals above their melting points. The observed I(k) are very similar to the I(k) calculated by Dixmier, Doi, and Guinier in Physics of Noncrystalline Solids, J. A. Prins, Ed. (North-Holland Publ. Co., Amsterdam, 1965), p. 67 from their model. However, the parameters needed to fit the experimental results are inconsistent with the atomic sizes expected for nickel and phosphorus. The noncrystalline alloys are between 0.6% and 1.4% less dense than the corresponding mixtures of fcc Ni and Ni3P, both of which are essentially close packed. A grain boundary density deficit model has been developed which relates the fractional density difference between the microcrystalline and macrocrystalline forms of a close-packed metal to the minimum average microcrystal grain size. The fractional density differences of between 0.6% and 1.4% observed for the noncrystalline Ni–P alloys would correspond to minimum average microcrystal grain sizes D of between 133 and 57 Å. Stored elastic energy calculations indicate that rms strains 〈ε2〉1/2 greater than 0.03 and due to simple compressive and dilatory stresses are inconsistent with the reported energy of transformation of noncrystalline Ni–P to crystalline nickel and Ni3P. Crystallite size broadening with D≥57 Å and strain broadening with 〈ε2〉1/2≤0.03 are insufficient to produce fcc, hcp, or Ni3P model I(k) consistent with the observed Ni–P I(k). The high densities of the noncrystalline Ni–P alloys suggest that the alloys have a continuous structure rather than one in which internal boundaries separate small well-ordered regions. These limitations on structural models for noncrystalline Ni–P alloys may apply to the other noncrystalline metallic alloys with similar diffraction patterns.