No abstract

The degree of normal segregation, occurring during directional (normal) freezing in a transverse magnetic field of 34 000 oersteds, has been determined for aluminium–copper alloys solidified for rates of [Formula: see text] to 4 inches per hour. For ingots in the composition range 0.5% < %Cu < 4.5%, solidified at rates exceeding [Formula: see text] inch per hour, the magnetic field increased normal segregation, corresponding to a maximum decrease in the effective distribution coefficient ke of approximately 13%. It is shown that the decrease in ke can occur only through a rise in the liquidus of the alloy system, which is in agreement with the theory developed to account for the inverse segregation results obtained for aluminium–copper ingots chill-cast in a magnetic field (previously reported). An increase in ke due to the field was found for 0.5% Cu ingots and also for the 4.5% Cu and 7% Cu ingots. The concentration dependence of ke is related to the concentration gradients and diffusion zone lengths, and is shown to be compatible with the thermodynamic constraints on the system undergoing the irreversible process of solidification in a magnetic field.

No abstract

T he overaging kinetics of Al-Zn-Mg-Cu alloy 7050 at 430-455 K (157-182°C) was investigated by electrical resistivity and tensile property measurements, which were supplemented by T EM and diVerential scanning calorimetry (DSC). Overaging in this alloy system was shown to follow classical coarsening behaviour as characterised by a time−1/3 particle size dependence. Changes in solute supersaturation, as measured by electrical resistivity, could be directly related through the Gibbs-T hompsonFreundlich equation to particle size variations, which were inferred from the DSC results. Strength changes, when interpreted in terms of the dislocation looping mechanism, also fit the classical coarsening theory. T he activation energy for the coarsening reaction was estimated to be about 160 kJ mol−1, which is 30-40 kJ mol−1 higher than that for solute (Mg,Zn) diVusion. T his diVerence is ascribed to the solution enthalpy of the coarsening precipitate. Precipitate dissolution under non-isothermal conditions (DSC) had an activation energy (~135 kJ mol−1) closer to that for diVusion.MST /4278T he author is in

No abstract