A mechanically milled, powder-metallurgy aluminum alloy used in the present work has a composition of Al-1.5Mg-4.0Cu-0.8O-1.1C (in mass%) and is called IN9021 produced by Incomap Co. This alloy is similar to a material used in previous works [1–4] in composition. A plate (4 mm thick) was machined from an extruded rod and rolled at 423 K by 75% in thickness reduction. Tensile specimens were machined to have the gauge length of 6 mm, 4 mm in width and 1 mm in thickness with the gauge length direction in parallel to the rolling direction. They were annealed at 773 K for 1 h and then water quenched. The final structure was fully recovered and consisted of fine grains of about 0.5 mm in diameter [11]. The grain coarsening did not occur due to the presence of fine, uniformly dispersed Al2O3 and/or Al4C3 particles [12].