Grain boundary diffusion in high-Zn-content Al alloys: Evidence of grain boundary phase transition induced by Zn segregation

Grain boundary (GB) phase (or complexion) transitions can influence a broad range of interface-related properties in Al alloys. GB diffusion of Zn in coarse-grained pure Al, AlZn, and AlZnMgCuLi alloys is studied as a highly sensitive probe of the occurrence of GB complexion transitions in the high-Zn-content Al alloys. The co-existence of two kinds of GBs, including low-sigma coincidence site lattice (CSL) GBs and random high-angle GBs, is manifested through the appearance of two distinct diffusion contributions in the penetration profiles, ‘‘slow’’ and ‘‘fast’’, respectively. The relatively “fast” diffusivities are similar in all materials studied, whereas the “slow” diffusivities are found to be enhanced by an order of magnitude in the Zn-containing alloys. The potential influences from low-angle GBs and low-sigma special GBs are analyzed, and this enhancement is attributed to the coupling effect of disconnections and Zn precipitation at the low-sigma CSL GBs, dominantly the near-coherent Σ3 boundaries. The obtained results advance the understanding of GB complexion transitions accounting for the whole spectrum of internal interfaces ranging from low-angle to high-angle GBs and including low-sigma special boundaries of the high-Zn-content Al alloys.