Effect of ECAP and heat treatment on microstructure and mechanical properties of a SiC reinforced Al-Cu alloy

In this study, the production and optimization of an aluminum matrix composite prepared by powder metallurgy, Equal-channel angular pressing (ECAP) and a subsequent heat treatment is investigated. A gas atomized powder (composition approximately similar to the commercial AA2017 alloy) was high energy ball milled with 5 vol.-% SiC particles (0.2 to 2 mu m), hot iso-statically pressed and extruded to square bars. Optical, scanning-and transmission electron microscopy are performed to characterize the corresponding microstructural evolution. The mechanical properties are determined by quasi-static tensile testing and hardness measurements. The experimental results show that the extruded condition exhibits a relatively homogeneous particle distribution with a good interface quality. However, the subsequent solid-solution heat treatment results in a significant increase of porosity due to remnant stearic acid (milling agent for the production of the composite powder) which was only partly removed during degassing. This leads to a significant degradation of the mechanical properties. This study also indicates that ECAP-processing of aluminum matrix composites with particle sizes below 1 mu m can in principle lead to significant increases in strength by altering the relevant defect structures. The heat treatment temperature considered here (170 degrees C) results in a softening due to simultaneous recovery and over-aging. But the first results presented here may well point out directions for further studies on microstructural optimization of particle reinforced aluminum alloys by ECAP.