Specimens of Al–1%Mg2Si–X%Cu alloys were isothermally heat treated at several temperatures between 573 K and 848 K for enough long periods. The resistivity in equilibrium state was estimated by extrapolation of t−1/3 plots at 573 K and 623 K, and at other higher temperatures by simply averaging measured values after long enough time. For comparison, the equilibrium resistivity was calculated for the each temperatures of heat treatment under assumptions that 0.02%Ti is always in solution and that Mg2Si, Cu and Fe would dissolve in Al up to equilibrium concentration in pseudo-binary or binary system. Though interactions between different species of solute atoms, except between Mg and Si, were completely neglected in above simple and rough assumption, the calculated resistivity coincided with experimental values within 0.5 nΩm. This is exactly same as previously observed resistivity fluctuation in present alloys quenched from 848 K, which was interpreted by the microsegregation during solidification. This coincidence suggests that amount of Cu mingling in Mg–Si or Al–Fe precipitates will be very small in the equilibrium state. If Cu bearing precipitates were observed, total amount of Cu in the precipitates will be too small to decrease the resistivity of solid solution. In another report on 473 K aging behavior of present alloys, it was suggested that solute Cu would enhance degree of supersaturation during the aging. However in the equilibrium state, the Cu addition up to 0.72% seems not to elevate so much the temperature of perfect dissolution of 1% Mg2Si.
