Atomic-Scale Unique Interface Observation of η-Precipitates in Al-Zn-Mg Alloy

Al alloys have been widely used in the automobile and aerospace industry due to their lightweight property. Among them, the Al-Zn-Mg alloy is attracting attention due to its high strength. The main strengthening mechanism of Al-Zn-Mg alloy is known as precipitation hardening. The precipitation sequence of this ternary system has been reported as follows: Super-saturated solid solution → GP zones → η’ phase → η phase. The η phase possesses the space group of P6₃/mmc and is known to have 15 different orientation relationships with Al. Among the 15 differently oriented η precipitates, η_1, η₂, and η₄ have the highest proportion and the rest except for these are known to exist less than 1%. In this study, η phase is studied thoroughly at atomic-scale using probe aberration-corrected FEI Themis Z microscope and Ab-initio calculation. Besides, 3-dimensional morphology of η phase was observed using 3-D tomography with the transmission electron microscope. Although the orientation relationship of η precipitatesand Al matrix was reported, the atomic-scale observation of η₄has not been done yet. In this study, η₄precipitate, which occupies about 10% of the total precipitates, was observed in atomic scale and it was confirmed that it has a pseudo-periodic step-like interfacial structure with Mg and Zn segregation. We compared the interface energy of the simulated flat interface and experimentally observed the unique interface using ab-initio calculation. Consequently, the experimentally observed unique interface was energetically favored. We will further discuss the origin of this unique interface formation.