Processing, characterization and properties of copper-based composites strengthened by low amount of alumina particles

Copper-based composites strengthened by nano- and micro-sized alumina particles were fabricated by internal oxidation and mechanical alloying followed by hot-pressing. The effect of the simultaneous presence of nano- and micro-sized alumina particles on the microstructure and properties of the copper matrix was the object of this study. The inert gas-atomised prealloyed copper powder containing 0.5 wt.% Al and the mixture of inert gas-atomized prealloyed copper powder with 0.6 wt.% micro-sized alumina powder served as starting materials. Microstructure of composites was studied by X-ray diffraction analysis, scanning (SEM) and transmission electron microscopy (TEM). Microhardness, density and electrical conductivity were also applied for determination of properties. Microstructural characterization showed that nano-sized alumina particles significantly lower the grain size and inhibit the grain growth. Considerable increase of microhardness was also detected. At the maximum values (after 10 h of milling) the microhardness of both composites, Cu–0.5 wt.% Al and hybrid Cu–0.5 wt.% Al + 0.6 wt.% Al2O3, was approximately 3 times higher than microhardness of non-milled compacts processed from prealloyed copper and electrolytic copper powders. Micro-sized alumina particles play a twofold role in the strengthening of the copper matrix of the hybrid composite: together with the nano-sized particles they strengthen the matrix at shorter milling time, but with prolonged milling time the dislocation substructure formed around coarse particles serves as a trigger for the start of recrystallization processes provoking a decrease of microhardness. Both composites exhibit a much higher thermal stability at 800 °C than copper alloy processed by the method of vacuum melting and casting. The contribution of individual mechanisms such as the grain size, thermal expansion mismatch and Orowan hardening in strengthening of composites was evaluated and correlated with experimental results.