In order to clarify the combined effect of solution and dispersion hardenings at high temperatures, the internal stress, the effective stress and dislocation density in Al-3 at% Mg-1 at% Mn alloy have been measured and compared with those in Al-3 at% Mg alloy. The measurement has been carried out by applying the stress-relaxation technique to the steady-state tensile deformation at temperatures from 573 to 723 K and at strain rates from 3×10⁻⁵ to 3×10⁻³ s⁻¹.
It is found that the additive rule of hardenings by solute Mg atoms and dispersed Al₆Mn particles holds in the low Z region, but the effect of dispersion hardening decreases with the increase of Z, where Z is the Zener-Hollomon parameter. This behavior is explained by the relative value of the inter-dislocation spacing to the inter-dispersoid spacing. In the low Z region where the dislocation spacing is larger than the dispersoid spacing, nearly all dislocations undergo the resistance of dispersoids, and the additive rule holds. In the high Z region, however, the dislocation spacing becomes smaller than the dispersoid spacing, and the part of dislocations not directly contacting with the dispersoids increases as Z increases, resulting in the decrease of the effect of dispersion hardening.