Spinodal ordering in the equiatomic AuCu alloy

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Abstract
To identify spinodal ordering in AuCu alloys, electron microscopy and electron and X-ray diffraction examinations were employed on the equiatomic composition of AuCu alloy using direct ageing. The spinodal ordering temperature T′s and the coherent phase boundary T s were determined to be 630 and 648 K respectively. Three distinguishable phase transition mechanisms were found. (1) The ordering proceeds at grain boundaries by a heterogeneous nucleation mechanism after showing an incubation period at a temperature between the transition temperatures T c and T′c. (2) The ordering is induced in the grain interior by a homogeneous nucleation mechanism between T′c and T c. (3) The spinodal ordering is brought about below T s. The microstructure induced by the spinodal ordering is characterized by the tweed structure. The direct-ageing method is effective in distinguishing the ordering processes with or without an incubation period, and in eliminating confusion in microstructures by the unavoidable ordering which occurs during quenching.

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