Phase transformations in Ni + Al and Ni + Al + Cr mixtures during mechanochemical synthesis and subsequent heating

Abstract

Mechanochemical processing of elemental mixtures with the compositions Ni75Al25, Ni70Al25Cr5, and Ni75Al20Cr5 (5 at % Cr in the mixtures instead of the equivalent amount of Ni or Al) leads to the formation of nanocrystalline nickel-based solid solutions (crystallite size in the range ≃ 7–12 nm). Comparison of experimentally determined lattice parameters of the solid solutions with Vegard’s law values and with the lattice parameters evaluated using the Bozzolo-Ferrante rule, which takes into account the bulk moduli of constituent elements, suggests that the atoms in the solid solutions are bonded more strongly. Heating the synthesized ternary solid solutions in a calorimeter to 1000°C leads to the formation of an ordered γ′-phase (L12). Analysis of the relative intensity ratio of superlattice and fundamental reflections indicates that the Cr atoms always reside in the Al sublattice, independent of the composition of the starting mixture. When 5 at % Cr is incorporated instead of Ni, the chromium atoms force out aluminum from the Al sublattice, and the Ni deficiency in the Ni sublattice is compensated by the Al atoms. The ordered phases remain nanocrystalline (crystallite size in the range ≃ 40–70 nm).