An ultrafine-grained Al6063/Al2O3 (0.8 vol.%, 25 nm) nanocomposite was prepared via powder
metallurgy route through reactive mechanical alloying and hot powder extrusion. Scanning
electron microcopy, transmission electron microscopy, and back scattered electron diffraction
analysis showed that the grain structure of the nanocomposite is trimodal and composed of
nano-size grains (<0.1 μm), ultrafine grains (0.1–1 μm), and micron-size grains (>1 μm) with
random orientations. Evaluation of the mechanical properties of the nanocomposite based on
the strengthening-mechanismmodels revealed that the yield strength of the ultrafine-grained
nanocomposite ismainly controlled by the high-angle grain boundaries rather than nanometric
alumina particles. Hot deformation behavior of the material at different temperatures and
strain rateswas studied by compression test and compared to coarse-grained Al6063 alloy. The
activation energy of the hot deformation process for the nanocomposite was determined to be
291 kJ mol−1, which is about 64% higher than that of the coarse-grained alloy. Detailed
microstructural analysis revealed that dynamic recrystallization is responsible for the observed
deformation softening in the ultrafine-grained nanocomposite.