Isothermal Section of Ga–Ru–Cu Ternary Phase Diagram at 1073 K: Formation of New Ternary Phase, Ga₄Ru₃Cu, and Its Structural Relation with the GaRu β-Phase

Abstract

We have investigated the isothermal phase diagram of the Ga–Ru–Cu ternary system at 1073 K by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy to determine whether the stable icosahedral quasicrystal (i-phase) forms, as it does in the similar Al–Ru–Cu system, and new intermetallic phases exist. This is the first systematic investigation of the Ga–Ru–Cu system. No stable i-phase is identified in the Ga–Ru–Cu alloy system. However, we found a new ternary (τ) phase in the Ga–Ru–Cu system, which is characterized as a C-centered orthorhombic lattice with a_τ = 11.80 Å, b_τ = 6.04 Å, and c_τ = 3.07 Å. The (ideal) chemical composition of the τ-phase is Ga₅₀Ru_37.5Cu_12.5 (Ga₄Ru₃Cu). This new phase is stable up to 1073 K. Above 1173 K, the τ-phase transforms to the cubic GaRu (β) phase without any secondary phases. Thus, there is an order–disorder transition between the low-temperature τ-phase and the high-temperature cubic β-phase in this temperature range.