Transmission Electron Microscopic Investigations of Grain Boundary Beta Phase Precipitation in Al 5083 Aged at 373 K (100 °C)

The fine-scale microstructure of an Al 5083 alloy sensitized at 373 K (100 A degrees C) for 3, 7, 14, 30, 45, and 90 days has been investigated using transmission electron microscopy (TEM) to study the evolution of the beta phase (Al3Mg2) at grain boundaries. In fully sensitized Al 5083, the beta phase (Al3Mg2) mostly forms heterogeneously at grain boundaries. TEM observations showed that the grain boundary precipitation of beta phase was discrete up to 14 days of aging, and grain boundaries were fully covered by the beta phase after 30 to 45 days of aging. The early stages of beta growth on the grain boundaries are consistent with the collector-plate mechanism. We find no evidence of either Mg depletion or segregation at the grain boundaries. The initial growth rates as well as the thicknesses after long aging times are greater than can be accounted for by bulk diffusion of Mg to the grain boundaries. We suggest that dislocation pipe diffusion is necessary to account for the beta growth rates.