Vacancy and solute co-segregated η1 interface in over-aged Al-Zn-Mg alloys

Graphical We present a combined first-principles and scanning transmission electron microscopy (STEM) study of the η₁/Al interfaces in an over-aged Al-Zn-Mg alloy. As one of the major η-MgZn₂ precipitate types, η₁ tends to form highly coherent interfaces with Al matrix, along with various solute segregation patterns on its interfacial layers. The interface phase diagram suggests either an Mg-rich or a Zn-rich interface, depending on the chemical potential range of Zn. Further segregation calculations, however, strongly suggest the STEM Z-contrast imaged interface to be a vacancy and solute co-stabilized Mg-rich structure that is favored at relatively low Zn chemical potentials. Based on our experimental and calculation results, the profound thermodynamics origin of the η₁/Al interface and its intricate segregation behavior is clarified and described using a generalized structure model. The η₁/Al interface structure and segregation at higher Zn chemical potentials are also predicted for future experimental validation.