The system ErNiAl–ZrNiAl at 600 °С and the high-temperature high-pressure modification of ErNiAl

Abstract

A series of samples along the line ErNiAl–ZrNiAl was synthesized by arc-melting, and annealed at 600 °C or at 1400 °C / 8 GPa. Phase and structural analyses were performed based on X-ray powder diffraction data collected at room temperature on a diffractometer STOE Stadi P (Cu K α 1 radiation, angular range 6° ≤ 2 θ ≤ 110°, scan step 0.015°). The structures were refined by the Rietveld method, using the program DBWS. The formation of limited solid solutions Er 1-0.79(4) Zr 0-0.21(4) NiAl and Zr 1-0.77(3) Er 0-0.23(3) NiAl, which both crystallize with ZrNiAl-type structures, was observed. It was found that high-temperature high-pressure annealing of the three-component (ErNiAl) and four-component (Er 2 ZrNi 3 Al 3 ) samples caused a transformation from the low-temperature modification of the compound ErNiAl with ZrNiAl-type structure to its high-temperature high-pressure modification with MgZn 2 -type structure. The latter showed low solubility of Zr (Er 0.91(3) Zr 0.09(3) NiAl). The structure of ht-hp ErNiAl (MgZn 2 -type) belongs to the family of Friauf-Laves phases: the coordination polyhedra of the Er atoms are 16-vertex Friauf polyhedra, and those of the Ni and Al atoms are 12-vertex icosahedra. The structure of the solid solutions Er 1- x Zr x NiAl and Zr 1- x Er x NiAl (ZrNiAl-type structure) belongs to the family of structures with trigonal prismatic coordination of the smallest atoms (Ni). The structures of the investigated compounds contain similar nets of atoms, in particular nets of triangles and pentagons. They are characterized by high packing coefficients (space filling), which reach 82 %. The highest value was observed for the compound ErNiAl in the sample annealed at 1400 °C / 8 GPa. It may be noted that high pressures generally increase the density, whereas high temperatures have the opposite effect. Key words : aluminide, X-ray powder diffraction, solid solution, crystal structure.