Microstructural stability and microhardness of ultrafine grained and nanostructured Cu-5vol.%Al2O3 composite lumps/powders produced by high energy mechanical milling

Ultrafine grained lumps and a nanostructured powder of Cu-5vol%Al₂O₃ composite were produced using two high energy mechanical milling routes respectively. The milled composite materials were heat treated at 150, 300 and 500 degC for 1 hour, respectively, to determine the microstructural stability and micohardness changes of the materials as a function of the heat treatment condition. For the Cu-5vol.%Al₂O₃ composite lumps produced using route 1 (12 hours milling), after heat treatment at 150 degC, the Cu grain sizes decreased from the range of 100-250 nm to the range of 50-180 nm due to recrystallisation, but its average microhardness also decreased from 224 HV to 212 HV due to reduction of dislocation density. For the 24 hours milled Cu-5vol%Al₂O₃ powder produced using Route 2, the Cu grain sizes increase slightly from the range of 40-180 nm to the range of 50-200 nm, and as the result of this grain coarsening and decrease of dislocation density, the average microhardness decreased from 270 HV to 257 HV respectively. Further increasing the annealing temperature to 300 degC caused the grain sizes of the 12 hours milled lumps to increase to the range of 50-350 nm, and those of the 24 hours milled powder to 60-300 nm, both resulting in a decrease in the average microhardness to 207 HV for the lumps and 248 HV for the powder. Increasing the annealing temperature from 300 to 500 degC caused a much more significant increase of the Cu grain sizes of both the lumps and the powder, and a significant decrease in the microhardness of the 24 hours milled powder particles to 216 HV. However, the microhardness of the lumps decreases very little to 196 HV, suggesting the significant reinforcement effect of the Al₂O₃ nanoparticles.