Strain rate sensitivity of Al-based composites reinforced with MnO2 additions

Fine-grained Al-based composites reinforced with MnO2 particles were manufactured by means of powder metallurgy (PM) and mechanical alloying (MA) methods. It was found that the applied powder consolidation methods, including KOBO extrusion, did not induce any chemical reaction between thermodynamically unstable components. However, it was shown that addition of magnesium to the Al-matrix initiated a reaction in the vicinity of MnO2 particles that resulted in the nucleation and growth of nano-sized aluminum-magnesium oxides. This led to a local refining of structural components. The most intense refining of structural components was observed for the MA Al-MnO2 composite. Strain rate sensitivity (SRS) of as-extruded materials was tested in compression in the range 293-773 K. SRS was determined by making a rapid change in the basic true strain rate from ε̇=1.2·10-3 to ε̇=1.2·10-2. It is found that SRS did not practically depend on strain. The highest value of SRS was observed for the PM Al-MnO2-Mg composite. SRS of PM materials evidently increases with deformation temperature; however, it becomes smaller above a temperature of ∼600 K. For the MA Al-MnO2 composite, tested at high temperatures, primary mechanical alloying resulted in relatively low increase of SRS with temperature that also becomes smaller above ∼600 K. Suppression of the increase in SRS at high temperatures can be attributed to the specific features of grain boundaries created by the adhesive bonding between powder particles. This could hinder grain boundary sliding mechanisms and micro-cracks development at the compound interfaces.