Gold-copper, AuCu

Figure 3.22. Unit cell representations of two varieties of AuCu superlattices. For the AuCu II superlattice, M refers to the length of repeat unit, and APB indicates the antiphase boundaries between adjacent periodic arrays. Republished with the permission of the International and American Associations for Dental Research, from Determination of the AuCu Superlattice Formation Region in Gold-Copper-Silver Ternary System , Uzuka, T. Kanzawa, Y. Yasuda, K. J. Dent. Res. 1981, 60, 883 permission conveyed through Copyright Clearance Center, Inc.

Binary Alloys. The material Au-20wt%Ag is used for low-voltage electrical contacts. Gold-copper alloys form the ordered phases AusCu [60748-60-9], AuCu [12006-51-8], and AuCus [12044-96-1]. Gold-nickel alloys decompose into gold-rich and nickel-rich solid... 

Even when complete miscibility is possible in the solid state, ordered structures will be favored at suitable compositions if the atoms have different sizes. For example copper atoms are smaller than gold atoms (radii 127.8 and 144.2 pm) copper and gold form mixed crystals of any composition, but ordered alloys are formed with the compositions AuCu and AuCu3 (Fig. 15.1). The degree of order is temperature dependent with increasing temperatures the order decreases continuously. Therefore, there is no phase transition with a well-defined transition temperature. This can be seen in the temperature dependence of the specific heat (Fig. 15.2). Because of the form of the curve, this kind of order-disorder transformation is also called a A type transformation it is observed in many solid-state transformations. 

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