The Y-Cu-Mg system in the 0-66.7 at % Cu concentration range: the isothermal section at 400°C

Synthesis and characterization of about fifty alloys were performed in order to construct the isothermal section of the Y–Cu–Mg ternary system at 400 C in the 0–66.7 at.% Cu concentration range. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS) and X-ray powder diffraction (XRPD) techniques were used to examine microstructures, identify phases and define their compositions and crystal structures. Phase equilibria in the investigated compositional region are characterized by the absence of extended ternary solid solutions and by the presence of at least ten ternary phases. Crystal structures of the previously reported Y2Cu2Mg, Y5Cu5Mg8, Y5Cu5Mg13, Y5Cu5Mg16 and YCuMg4 phases were confirmed. A ternary phase with homogeneity range around the YCu4Mg stoichiometry was found, crystallizing in the cF24–MgCu4Sn structure type; at 400 C this phase coexists with a ternary solid solution based on the binary Laves phase Cu2Mg, which dissolves about 5 at.% Y. The equiatomic YCuMg phase was also found to exist: from the analysis of X-ray powder patterns it is suggested to crystallize in the hP9–ZrNiAl structure type (a= 0.74449(4) nm, c= 0.39953(2) nm). Two other stoichiometric ternary phases were detected, of approximate compositions Y25Cu18Mg57 and Y13Cu9Mg78, whose crystal structures are still unknown. In the Mg-rich region, a ternary phase forms characterized by a large homogeneity region.