Atomic-level characterization of η-twins with non-homogeneous nucleation on the E(Al18Mg3Cr2) phase in 7075 alloy: TEM observations and DFT calculations

Abstract

In this work, an uncommon type of η-twin with non-homogeneous nucleation on the surface of the E-phase is characterized, and two defects and growth processes of the twin are analyzed. According to the transmission electron microscopy image analysis, heterogeneous nucleation of the η phase at the dominant site of the E phase can lead to the formation of such η-twin only when a rather strict site-direction relationship is satisfied as well as when it is at the interface adjacent to the E phase. The formation of twin defects is related to the E phase but ultimately determined by the nature of the η phase, and the crystallographic relationship between the defect structure and the E phase is self-consistent. Combined with the analytical results from STEM, first-principles calculations show that the solute segregation at the defects reduces the total energy of the system, which favors the transition of the η phase of the heterogeneous nucleation and the formation of twins, and that solute Al is more favorable than Mg. In addition, the elemental distribution and effects in the precipitation-free zone were analyzed by EDS mapping.