Phase relations and structural features in the system Ni–Zn–B

Abstract

Phase relations for the system Ni–Zn–B have been established in the isothermal section at 800°C based on X-ray powder diffraction and electron microprobe analyses of about 60 samples, which have been prepared by hot pressing of pre-annealed powder compacts of arc-melted master alloys NixBy and Zn-filings. Six ternary compounds (labeled τ1 to τ6) were found to exist, which in some cases exhibit considerable mutual solid solubilities mostly as an exchange of Ni–Zn at constant B-content, but in the case of the so-called τ-phase τ1-(Ni1−xZnx)21[Zn1−y−z□y(B4)z]2B6 (0.07≤x≤0.11, 0.07≤y≤0.53, 0≤z≤0.3) also with Zn/B substitution. Single crystal X-ray data analysis for the composition x=0.07, y=0.125, z=0.30, and a=1.05800(3) nm confirmed the Cr23C6 type with space group Fm3¯m, but revealed partial replacement of Zn-atoms by B4-tetrahedra (RF=0.014). Whereas Ni/Zn exchange (at constant B-content) ranges at about 4–5at% for τ4-Ni3ZnB2 and τ5-Ni48Zn32B20, it is below 3at% for τ2-Ni12ZnB8−x (x=0.43), τ3-Ni21Zn2B20 and τ6-Ni47Zn23B30. The crystal structures of τ2-Ni12ZnB8−x (x=0.43; Ni12AlB8 type), τ3-Ni21Zn2B20 (own type) and τ4-Ni3ZnB2 (own type) have been determined in our foregoing paper [8]. Due to the non-availability of suitable single crystals, the crystal structures of τ5-Ni48Zn32B20 (body-centered orthorhombic; a=1.6(2)nm, b=0.63(7)nm, and c=0.27(0)nm, determined from TEM) and of τ6-Ni47Zn23B30 have not been elucidated yet. Phase equilibria at 800°C are dominated by a large three-phase field (βB)+Ni2Zn11+τ3, as practically all ternary compounds form at compositions with less than about 50at% Ni. Furthermore, precise data on atom site distribution and positional parameters have been provided from X-ray single crystal refinements for Ni-borides, for which crystal structures hitherto have only been derived from X-ray diffraction photographs: o-Ni4B3−x (x=0.19; Pnma, a=1.9665(4)nm, b=0.29852(1)nm, c=0.65750(2)nm; RF=0.015) and m-Ni4B3 (C2/c, a=0.64356(3)nm, b=0.48867(3)nm, c=0.78267(3)nm, β=103.288(3)°, RF=0.021).