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The effect of quench transfer time on microstructure and localized corrosion behavior of 7050-T6 Al alloy



The microstructure and localized corrosion behavior of the 7050-T6 Al alloys treated with different quench transfer time were investigated. Optical microscope observations show that the volume fraction of the recrystallized grains increases slightly with prolonging quench transfer time. Scanning electron microscope observations reveal that the stable η (MgZn2) phase nucleates and precipitates on grain boundaries in the process of transferring to quench. Further observations, using transmission electron microscope, found that the size, nearest neighbor distance, and copper content of the grain-boundary precipitates increase with quench delay. As a result, the open-circuit potentials and charge transfer resistance (Rt) of the alloys reduce with increasing transfer time, while the susceptibility to intergranular corrosion (IGC) and strength loss after exfoliation corrosion tests increase sharply. In addition, the IGC network appearance changes from large network to fine network structure, due to the different temperature range where very rapid η phase precipitation takes place between on grain boundaries and on sub-grain boundaries.