The alloying process and mechanical properties of Al–4mass%Cu/Al
2O
3 (0–5 vol%) composite powders produced by mechanical alloying method (MA) were investigated by scanning electron microscopy, X-ray diffraction and Vickers microhardness tests. The particle size of MA powders increased at early stage (welding predominant stage), then decreased (fracturing predominant stage) by repeating of welding and fracturing, and finally the MA powders became the fine eqiaxed powder particles (steady state). Dispersed Al
2O
3 particles in the MA powder made the eqiaxed powder particles fine. The hardness of Al–4 mass%Cu and Al–4 mass%Cu/Al
2O
3 MA powders steeply increased due to the work hardening at the initial stage of milling. The hardness of Al–4 mass%Cu powders was saturated with a certain value, while Al–4 mass%Cu/Al
2O
3 powders gradually increased. The gradual increase in hardness was caused by the homogeneous dispersion of Al
2O
3 particles into powder matrix. The addition of Al
2O
3 powders remarkably refined the MA powder particle size, and accelerated alloying, where a lattice parameter decreased with milling time and the formation of supersaturated solid solution proceeded. It has been considered that the Al
2O
3 particles in the MA powder activated the repeating process of the welding and fracturing during MA, which increased Al/Cu interfaces and the alloying is accelerated.
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