Determination of the AuCu superlattice formation region in gold-copper-silver ternary system.

The composition and temperature range of the AuCu-type superlattice formation in the Au-Cu-Ag ternary system was determined by X-ray powder diffraction. Four distinguishable phase transformations were detected, i.e., 1) alpha (disordered FCC phase) leads to AuCu I, 2) alpha leads to AuCu II, 3) alpha leads to alpha 1 (Cu-rich FCC phase) + alpha 2 (Ag-rich FCC phase). and 4) alpha leads to AuCu3. The stable temperature range of the AuCu I-type superlattice was restricted to a lower temperature range by adding silver to stoichiometric and off-stoichiometric Au-Cu alloys. Depending on the increase in silver content, the co-existing region of the AuCu II-type superlattice and the silver-rich alpha 2 phase extended to the lower temperature and over a wide composition range.