Early Stage of Ordering in Stoichiometric AuCu Alloy

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Abstract
Very early-stage AuCu I ordering in a stoichiometric AuCu alloy was examined from a point of view of spinodal ordering. Coexistent state of ordered and disordered phase was studied by X-ray diffractometry (XRD), transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) on specimens which were aged at the temperature between 653 and 623 K directly quenched from a solution treatment temperature. Spinodal ordering temperature, Ts, was estimated to be about 630 K. Change in the XRD profiles was discontinuous above Ts, while continuous below that temperature. TEM observations indicated that the ordering process at higher temperatures than Ts was a nucleation and growth mechanism followed by twinning. Instead, the homogeneous ordering produced at lower temperatures, was spinodal ordering associated with tweed contrast due to the arrangement of fine ordered domains parallel to the ⟨101⟩ directions. HREM observations for the coexistence of the ordered and disordered phases indicated that the interface became diffuse at the temperature close to Ts in spite of nucleation and growth ordering. Furthermore, a distinct habit plane did not exist in the early stage of the spinodal ordering process. Changes in the axial ratio as a function of aging time followed an identical curve regardless of the ordering mechanism. These results could be qualitatively explained by a simple model of Landau’s expansion of the free energy density.

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