New Complex Phase in the Copper-Gold System

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Abstract
At the composition 31.6 at. % gold, the equilibrium diagram shows a two‐phase mixture of ordered and disordered material extending from 320°C to 350°C. However, a long anneal in this interval results in an x‐ray pattern showing superstructure reflections with strong satellites. The satellite pattern is obtained both by lowering and raising the temperature, and hence is believed to represent an equilibrium structure. The satellite pattern is interpreted in terms of a Cu3Au II type structure, which is derived from the ordered Cu3Au I by introducing antiphase domain displacements (a2+a3)/2 at regular intervals along the a1 axis, exactly analogous to the development of CuAu II from CuAu I. At 31.6 at. % gold, the period is 18a1 (9 a1 between consecutive domain boundaries). Between 320°C and 335°C, there is a two‐phase mixture of Cu3Au I and Cu3Au II. From 335°C to about 345°C the structure is that of Cu3Au II. From about 345°C to 350°C there is probably a two‐phase mixture of Cu3Au II and disordered material, but this has not been established. The satellite pattern is also obtained at the composition 29.2 at. % gold, and it is likely that the Cu3Au II type structure exists over a range of compositions on the gold rich side of Cu3Au.