Evolution of β-phase precipitates in an aluminum-magnesium alloy at the nanoscale

Aluminum alloys in the 5xxx series are susceptible to sensitization due to the formation of beta (Al3Mg2) at grain boundaries at moderate to low temperatures. Little is known about the mechanism of beta phase formation, which is thought to be preceded by the metastable phases beta '' and beta'. Using high-resolution transmission electron microscopy (HRTEM), energy dispersive x-ray spectroscopy (EDS), and precession electron diffraction (PED), we determine a parameter space for beta phase precipitates at various sensitization temperatures and investigate their growth habits and local matrix strain states along grain boundaries. Our findings reveal that metastable beta-related phases are present at low aging temperatures, while the equilibrium beta phase is present at temperatures well above the previously described solvus of similar alloys. Furthermore, the phases were found to prefer particular grain boundary planes and contribute to the local grain boundary strain state differently. Overall, these findings present a unified view of beta phase evolution and its contribution to lattice strain environments in aluminum magnesium alloys, which serves as a foundation for use in a range of temperatures and environments. (C) 2019 Published by Elsevier Ltd on behalf of Acta Materialia Inc.