Electrical resistivity studies on the precipitation behaviour of AA6111

Electrical resistivity measurements have been used to study the precipitation behaviour of AA6111 as a function of temperature. For all temperatures investigated, the resistivity increases initially upon aging. At higher temperatures, the resistivity reaches a peak value and then decreases with further aging. The initial increase in resistivity occurs in two stages. The first stage is characterized by a rapid increase in resistivity and is followed by the second stage where the rate of resistivity increase is much lower. This increase in electrical resistivity was attributed to clustering reactions involving Mg and Si atoms. The activation energy for the transition from fast to slow reactions was measured to be 0.45 +/- 0.05 eV. The influence of quench rate and temperature on the precipitation behaviour was also investigated. After solution treating at 560 degrees C, samples were either: i) water quenched to room temperature followed by ageing at 120 or 180 degrees C, or ii) directly quenched into an oil bath at 120 or 180 degrees C. Quenching directly into the oil bath resulted in an increase of grain boundary precipitation. The evolution of resistivity for the two quenching routes was similar, although the resistivity for the oil quenched samples had a lower value reflecting the initial loss of solute to grain boundary precipitation. The evolution of the yield stress at a given temperature for the two quenching conditions was found to be very similar.