Resistometric and calorimetric analysis of phase transformations in AuCu alloys

The order-disorder transitions in AuCu alloys have been studied by differential scanning calorimetry and resistometric methods. We have collected data with different sets of samples of equiatomic composition, the structure of which was determined by X-ray diffraction. Grain growth occurs mostly below 300 degreesC either in rolled and nanocrystalline samples. The occurrence of ordering does not depend on the grain size, but ordering is favoured by high defect contents in the alloy. The shape of heat flow traces provide evidence that ordering and disordering transitions are diffusive at low temperature (below 385 degreesC) being exothermic and endothermic, respectively, in wide temperature ranges. The transition from AuCuI to AuCuII changes the mechanism above 385 degreesC. The transition from AuCuII to face-centered cubic (fcc) solid solution is of first order. The reverse transition from the fee structure to AuCuI occurs by nucleation and growth on continuous cooling.