The kinetics of L10 superstructure formation in the Cu–56Au alloy (at. %): resistometric study

Due to the improved strength properties compared to the equiatomic Cu–50 at. % Au alloy, non-stoichiometric Cu–56 at. % Au alloy can be used both in dentistry and as a corrosion-resistant conductor of weak electrical signals in tool engineering. The work studies the kinetics of the disorder→order phase transformation in the Cu–56Au alloy, during which the disordered fcc lattice (A1-phase) is rearranged into an atomically ordered one with the L1₀ superstructure. The initial disordered state of the alloy was obtained in two ways: applying plastic deformation by 90 % or quenching at a temperature of above 600 °C (i. e., from the region of the A1-phase existence). To form the L1₀ superstructure, annealing was carried out at temperatures of 200, 225, and 250 °C. The annealing duration ranged from 1 h to 2 months. Resistometry was chosen as the main technique to study the kinetics of the disorder→order transformation. The temperature dependences of the electrical resistivity of the alloy in various structural states are obtained. The authors constructed the graphs of the electrical resistance dependence on the annealing time logarithm, based on which, the rate of the new phase formation was estimated. To evaluate the structural state of the alloy at various transformation stages, the authors used X-ray diffraction analysis (XRD). The crystal structure rearrangement during the transformation is shown by the example of splitting the initial cubic A1-phase peak (200) into two tetragonal ordered L1₀ phase peaks – (200) and (002). Based on the resistometry and X-ray diffraction analysis data, the authors carried out a quantitative assessment of the rate of the disorder→order phase transformation in the alloy under the study. It is established that the values of the converted volume fraction (resistometry) and the long-range order degree (X-ray diffraction analysis) are close. The study shows that in the temperature range of 200–250 °C, the rate of atomic ordering according to the L1₀ type in the nonstoichiometric alloy Cu–56 at. % Au is maximum at 250 °C. It is identified that the disorder→order transformation in the initially quenched specimens of the investigated alloy proceeds approximately an order of magnitude faster than in preliminarily deformed specimens.