Grain and nanoparticle coarsening of an ultrafine structured Cu–5 vol.%Al2O3 nanocomposite during isochronal annealing

• The microstructure of ultrafine structured Cu–5 vol.%Al2O3 is thermally very stable.
• The high thermal stability is due to the pinning effect of Al2O3 nanoparticles.
• Grain boundary and Al2O3 nanoparticle strengthening plays a major role.
• The softening temperature of the ultrafine structured Cu–5 vol.%Al2O3 is high.

Cu grain growth, Al2O3 nanoparticle coarsening and changes in microhardness of an ultrafine structured Cu–5 vol.%Al2O3 nanocomposite during 1 h isochronal annealing over a temperature range of 500–900 °C were investigated. The material was produced using high energy mechanical milling of a mixture of Cu powder and Al2O3 nanoparticles followed by powder compact extrusion of the milled powder at 900 °C. The grains of the ultrafine structured Cu matrix of the nanocomposite exhibited a very high thermal stability, with their average size increasing from 263 to 336 nm after annealing at 900 °C for 1 h. This is likely due to the pinning effect of the Al2O3 nanoparticles on the migration of Cu grain boundaries. With annealing at 500, 600 or 900 °C, the volume fraction and average size of the Al2O3 nanoparticles increased slightly. Annealing at 700 or 800 °C did not cause any change of the volume fraction of the Al2O3 nanoparticles, but led to a slight decrease of the average size of the Al2O3 nanoparticles from 100 to 95 nm. The microhardness of the samples did not decrease monotonically with increasing annealing temperature. Analysis of the contributions of various strengthening mechanisms reveals that hardening caused by the grain boundaries and nanoparticles plays a major role in sustaining the hardness (and strength) of the as-extruded and annealed ultrafine structured Cu–5 vol.%Al2O3 nanocomposite samples.