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 Ni x B y and Zn-filings. Six ternary compounds (labeled τ 1 to τ 6 ) 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 τ 1 -(Ni 1− x Zn x ) 21 [Zn 1− y − z □ y (B 4 ) z ] 2 B 6 (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 Cr 23 C 6 type with space group F m 3 ¯ m , but revealed partial replacement of Zn-atoms by B 4 -tetrahedra ( R F =0.014). Whereas Ni/Zn exchange (at constant B-content) ranges at about 4–5 at% for τ 4 -Ni 3 ZnB 2 and τ 5 -Ni 48 Zn 32 B 20 , it is below 3 at% for τ 2 -Ni 12 ZnB 8− x ( x =0.43), τ 3 -Ni 21 Zn 2 B 20 and τ 6 -Ni 47 Zn 23 B 30 . The crystal structures of τ 2 -Ni 12 ZnB 8− x ( x =0.43; Ni 12 AlB 8 type), τ 3 -Ni 21 Zn 2 B 20 (own type) and τ 4 -Ni 3 ZnB 2 (own type) have been determined in our foregoing paper [8] . Due to the non-availability of suitable single crystals, the crystal structures of τ 5 -Ni 48 Zn 32 B 20 (body-centered orthorhombic; a =1.6(2) nm, b =0.63(7) nm, and c =0.27(0) nm, determined from TEM) and of τ 6 -Ni 47 Zn 23 B 30 have not been elucidated yet. Phase equilibria at 800 °C are dominated by a large three-phase field (βB)+Ni 2 Zn 11 + τ 3 , as practically all ternary compounds form at compositions with less than about 50 at% 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- Ni 4 B 3− x ( x =0.19; Pnma , a =1.9665(4) nm, b =0.29852(1) nm, c =0.65750(2) nm; R F =0.015) and m- Ni 4 B 3 ( C 2/ c , a =0.64356(3) nm, b =0.48867(3) nm, c =0.78267(3) nm, β =103.288(3)°, R F =0.021).