The ternary system Yb–Cu–Mg: Isothermal section at 400 °C in the range from 0 to 67 at.% Cu

Abstract

Phase relations in the ternary system Yb–Cu–Mg have been established for the isothermal section at 400 °C based on X-ray powder diffraction, metallography, SEM and EPMA techniques on about 30 alloys, which were prepared in sealed Ta-crucibles followed by a homogenisation treatment. Phase equilibria in Yb–Cu–Mg are characterized by the absence of Yb solubility in CuMg 2 and of Mg solubility in YbCu 2. In the compositional range 0–67 at.% Cu five ternary compounds were observed in addition to the Cu-rich ternary phase that is already reported in the literature (τ 1-YbCu 9Mg 2). The section Cu 2Mg–YbCu 2 shows an extended homogeneity range for binary Cu 2Mg up to 8 at.% Yb where Mg atoms are substituted by Yb. At increasing Yb-content τ 2-(Yb 1− x Mg x )Cu 2 (ordered variant of the Cu 2Mg-type) forms with a homogeneity region extending from x = 0.45 to x = 0.63. A ternary phase of stoichiometry Yb 2Cu 2Mg (τ 3) was detected, whose crystal structure is still unknown. Three compounds were found to form near the concentration line with 66.7 at.% Yb: τ 4-Yb 4Cu 1− x Mg 1+ x (0 ≤ x ≤ 0.25, Gd 4RhIn-type), τ 5-Yb 23Cu 7Mg 4 (almost stoichiometric, with a unique structure type) and τ 6-Yb 2Cu x Mg 1− x ( x = 0.2, structure still unknown). Atom order in the crystal structures of YbMg 2 and τ 5-Yb 23Cu 7Mg 4 (Yb 23Cu 7Mg 4-type; P6 3/ mmc; a = 1.1866(2); c = 2.34371(5) nm) was determined by direct methods from X-ray single crystal counter data.