KINETICS OF SHORT-RANGE ORDERING IN ALPHA-CU-AL ALLOYS

The ordering behaviour in quenched alpha-Cu-Al was investigated by differential scanning calorimetry (DSC) under rising temperature conditions and by electron diffraction. It was found that the ordering processes can be better explained in terms of a homogeneous short-range order (SRO) model rather than a heterogeneous disperse order (DO) model as previously interpreted for the same type of experiments. The DSC traces indicate that the ordering takes place in two stages: the stage 1 ordering at lower temperatures is associated with the migration of excess vacancies and the stage 2 ordering at higher temperatures is associated with the migration of equilibrium vacancies. At higher temperatures, a marked surge of energy absorption occurs (stage 3) which is attributed to the destruction of order. For furnace-cooled alloys, only stage 3 appears. The relative dominance of stages 1 and 2 is influenced by the quenching temperature, the quench rate, the density of vacancy sinks and the sample shape before quenching. A relationship describing the overall SRO kinetics for both stages together, in terms of either the reacted fraction or the first SRO parameter, is proposed. A method for evaluating boundary values for this parameter is developed, making use of the features displayed by the DSC thermograms. The mobility of the vacancies, which controls their life-time and the short-range ordering rate, was evaluated by computing frequency factors and activation energies. Values for activation energies controlling the short-range ordering rate are somewhat smaller than the effective values obtained for stage 1 from the DSC traces, suggesting that the presence of solute-vacancy complexes may be important mainly as the aluminium concentration increases. From estimations of solute-vacancy and divacancy binding energies, it is inferred that divacancy formation is unlikely in the alloys under study. Process frequency factor values are in very good agreement with those calculated for a short-range order state developed during two-stage ordering by a vacancy mechanism. A flow diagram of the factors controlling the fractional increase of short-range order during anisothermal experiments is proposed.