Ly, Ramatou (2019-11). Processing and Microstructure Effects on the Strength and the Localized Corrosion Resistance of Ultra-Fine Grained Al-Mg-Si Alloys. Doctoral Dissertation.
Thesis
Improving the strength, and the corrosion resistance of aluminum alloys simultaneously through aging treatment, and grain refinement is a challenge as these two properties are mutually exclusive. In this work, we investigated the effects of shear banding, dynamic recrystallization, and precipitation on the corrosion susceptibility, and the strength of an ultra-fine grained (UFG) AA6061 aluminum alloy. For that, we extruded the material by equal channel angular pressing (ECAP) up to seven passes following route Bc combined with a post-aging treatment at 100 ?C. We first characterized the microstructure after processing in depth before investigating corrosion susceptibility by accelerated immersion tests, and electrochemical testing. We found that after 3 ECAP passes, the AlFeSi intermetallic compounds are fragmented and closely spaced along the shear direction and within the shear planes of the material making this direction and plane highly susceptible to pitting. Besides, the shear bands are preferred regions for precipitation after post-aging treatment. Therefore the corrosion susceptibility of UFG AA6061 alloys is tailored by the shear bands network of the material, and the precipitation within the bands.The immersion and electrochemical tests reveal that filiform corrosion, and intergranular corrosion (IGC) are severe when the material is partially recrystallized and peak aged. Also, the bulk potentiodynamic response is significantly affected by the bulk composition, and the precipitation within the shear bands. In addition, passivation is easier after ECAP regardless of the post-aging treatment. However, the passive film is less stable. We also found that dynamic recrystallization decreases the susceptibility to IGC even at peak aging by reducing the volume fraction and the length of the shear bands. Thus in UFG AA6061 aluminum alloys, the energy stored in the grains seems to influence the susceptibility to IGC more than grain boundary precipitation.
