Abstract
Surface abrasion, sometimes followed by polishing, can create an altered surface layer (ASL) with thickness of a few hundreds of nanometers, which has different microstructure than the underlying bulk. Such an ASL typically contains ultrafine subgrains, and second phase particles that result from unusual phase transformations. ASLs have been shown to exhibit a different corrosion resistance than the underlying matrix, and can be either more susceptible or more resistant to corrosion. In this work, the ASL microstructure of AA7055 was studied at the early stages of natural aging right after surface abrasion using transmission electron microscopy (TEM) and atom probe tomography (APT). For the as-abraded ASL, the pre-existing η'/η precipitates and grain boundaries were deformed and aligned along the abrasion direction. However, for the abraded and 7-day natural aged bulk sample, ultrafine subgrains formed in the localized region of the ASL, and MgxZny and Al2Cu particles precipitated at the subgrain boundaries. To accelerate the precipitate kinetics of the ASL, the bulk abraded sample was heated at 50 oC for 1 h; more subgrains and Al2Cu particles formed in the whole ASL. Potentiodynamic polarization testing was performed on the abraded and aged bulk samples during immersion in NaCl solution. With increasing post-aging temperature and time, the breakdown potential of the ASL increased.
Acknowledgements: focus ion beam and TEM were conducted at Center for Electron Microscopy and Analysis at The Ohio State University. APT was conducted at Center for Nanophase Materials Sciences at Oak Ridge National Laboratory, which is a U.S. DOE Office of Science User Facility.