Segregation of the alloying elements at the Grain boundaries (GBs) in a polycrystalline materials leads to a nanoscale chemical and structural variations along the GB region that can significantly alter the material’s performance. Hence, the experimental investigation of the atomic structure and chemistry of the grain boundaries by resolving the sites and chemical identities of the atoms comprising the interface is crucial to fully understand the implication of the atomic segregation phenomenon. Herein, we present the atomic scale analysis of ordered-structures induced by segregation of Cu atoms in Al 7075 alloy GBs along with the detailed examination of microstructural features (texture and grain boundary character distribution) by employing advanced microscopic characterization techniques: STEM-HAADF imaging, spectroscopic (EDS and EELS) analysis, and scanning precession electron diffraction (PED).
The <111> strong texture and dominant low coincidence site lattice (CSL) GBs were revealed in the films. Results demonstrated the segregation of Cu in all types of GBs, but with varying segregation patterns. Atomic-scale structure of each type of GBs showed two types of Cu GB segregation behaviors namely, point (highly segregated atomic column surrounded by low segregated columns, misorientation < 28o) and parallel array (two highly segregated columns opposite to each other across the interface surrounded by low segregated columns, misorientation > 28o). In addition to the single atom-interstitial hollow site (point) segregation behavior (predicted in theoretical studies and assumed to analyze the segregation effects), substitutional core site segregation of two columns of Cu atoms forming an ordered-structures with a parallel array of segregating atoms along the interface has been demonstrated. This is the first time that this type of parallel array segregation behavior has been reported. Furthermore, based on intensity profiles and scanning transmission electron microscopy Z-contrast principle, non-uniformly (high and low) segregated mixed atomic columns were predicted at the grain boundaries. We believe the knowledge of this type of experimental insight of the atomic scale arrangements of adsorbate on the GBs can potentially help to develop strategies for engineering the design of alloy compositions.
5:00 PM–7:00 PM Apr 23, 2019 (US - Arizona)
PCC North, 300 Level, Exhibit Hall C-E