Formation and characteristic corrosion behavior of alternately lamellar arranged α and β in as-cast AZ91 Mg alloy

Formation and characteristic corrosion resistance of alternately lamellar arranged alpha and beta in as-cast AZ91 Mg alloy were investigated as an independent micro-constituent identity. As-cast AZ91 presented three microstructural entities, i.e. alpha-Mg grain, (alpha+beta) lamellae and coarse beta particle, and each had its own Al content and microstructural morphology. The lamellae occurrence was due to the precipitation of beta particle from the divorced eutectic Al-rich-alpha phase during solidification, because the Al composition can not exceed its maximum solubility. The evidences that were obtained from electrochemical tests, microcorrosion morphology and hydrogen evolution rate certified that the (alpha+beta) lamellae was beneficial to corrosion resistance, which was different from the reported deleterious influence for its original eutectic Al-rich-alpha phase. This different corrosion behavior was explained to be ascribed to the changes in fine structure and local composition that resulted in combined electrochemical effects of the changes in alpha and beta phases on the corrosion. (C) 2018 Elsevier B.V. All rights reserved.