Cubides Gonzalez, Yenny Paola (2020-04). Role of Microstructural Factors on the Corrosion Behavior of Mg-9Al-1Zn Alloys Under Different Processing Conditions. Doctoral Dissertation.
Thesis
Magnesium and its alloys exhibit low density and high specific strength making them excellent candidates for a variety of applications in automotive, aerospace, biomedical and electronics fields. Despite these potential applications of magnesium alloys, there are still some major challenges that need to be overcome. Magnesium alloys exhibit poor formability, relatively low strength compared to conventional steel and aluminum alloys and poor corrosion resistance. Processing routes involving severe plastic deformation (SPD) and/or heat treatment methods have been extensively developed to produce magnesium alloys that exhibit superior mechanical properties than traditional cast structures. These processing methods can lead to compositional, microstructural and crystallographic changes that can have a drastic impact on the corrosion behavior of magnesium alloys. Thus, this study aims to provide more details that correlates microstructure evolution during processing with corrosion performance of Mg-9Al-1Zn (AZ91) alloys with emphasis in the role of grain size, grain structure, and morphology and distribution of cathodic secondary phases. The results revealed that AZ91 alloy exhibit good mechanical properties combined with high corrosion resistance after microstructure modification through equal channel angular pressing (ECAP). Enhanced corrosion resistance of the fine-grain AZ91 alloy is related to presence of a more protective oxide layer, formation of a more adherent and compact layer of corrosion products, and development of more uniform corrosion with shallow corrosion pits. The micro-galvanic corrosion mechanism of a peak-aged AZ91 alloy was also investigated. It was found that corrosion simultaneously initiates in the interior of ?-Mg grains and in the local ?-Mg phase within the ? + ? lamellar precipitate. In addition, it was also found that the ?-Mg17Al12 phase/?-Mg matrix interface acts as a barrier against corrosion propagation. Finally, the corrosion susceptibility of recrystallized and un-recrystallized grains in ECAPed AZ91 alloys was studied. From this study, it was revealed that corrosion preferentially initiates and propagates in the recrystallized grains owing to the higher microchemistry difference between the ?-Mg17Al12 precipitates and the adjacent ?-Mg matrix that induces a strong microgalvanic coupling between these phases. It was also concluded that the superior corrosion resistance and strength of the ECAPed alloys as increasing the strain level was attributed to the combination of smaller grain size, higher volume fraction of refined grains and higher volume fraction of uniformly distributed fine ?-Mg17Al12 precipitates.
