Abstract
Microstructural investigation of extremely strained samples, such as severely plastically deformed (SPD) materials, by using conventional transmission electron microscopy techniques is very challenging due to strong image contrast resulting from the high defect density. In this study, low angle annular dark field (LAADF) imaging mode of scanning transmission electron microscope (STEM) has been applied to study the microstructure of a Mg-3Zn-0.5Y (at%) alloy processed by high pressure torsion (HPT). LAADF imaging advantages for observation of twinning, grain fragmentation, nucleation of recrystallized grains and precipitation on second phase particles in the alloy processed by HPT are highlighted. By using STEM-LAADF imaging with a range of incident angles, various microstructural features have been imaged, such as nanoscale subgrain structure and recrystallization nucleation even from the thicker region of the highly strained matrix. It is shown that nucleation of recrystallized grains starts at a strain level of revolution (earlier than detected by conventional bright field imaging). Occurrence of recrystallization of grains by nucleating heterogeneously on quasicrystalline particles is also confirmed. Minimizing all strain effects by LAADF imaging facilitated grain size measurement of nm in fully recrystallized HPT specimen after .
Highlights
In recent years severe plastic deformation (SPD) has been commonly employed to improve the mechanical properties, e.g. strength, ductility and toughness, of metallic materials
Microstructural evolution, especially grain refinement by recrystallization during severe plastic deformation of a Mg-3Zn-0.5Y alloy processed by high pressure torsion (HPT) at room temperature was studied by conventional transmission electron microscopy (TEM) as well as scanning transmission electron microscope (STEM)-low angle annular dark field (LAADF) imaging technique with an optimized annular detection angle
Specimen tiltdependent channeling effect studies demonstrate that sources of contrast from the twin are not same in High angle annular dark field (HAADF) and LAADF imaging modes
Summary
In recent years severe plastic deformation (SPD) has been commonly employed to improve the mechanical properties, e.g. strength, ductility and toughness, of metallic materials. STEM has a considerable advantage over conventional TEM because various signals emitted from the same specimen can be collected simultaneously by utilizing different detectors having different shapes and sizes and effectively used for efficient imaging with high contrast. The ADF detector is in the upper position in HAADF mode to collect high angle scattered electrons which contain Z-. The image contrast is sensitive to the displacement of the ADF detector and certain features of the specimen can be enhanced by collecting a specific portion of the scattered signals, a technique which is extremely useful for imaging inhomogeneous and deformed samples. STEM imaging within a particular angular range or with a certain camera length to observe grain fragmentation and recrystallization initiation in severely plastically deformed microstructures is demonstrated with a number of illustrations. Based on experimental observations of diffraction effects, micrographs recorded with camera lengths of 80 mm are termed HAADFSTEM images, whereas those with camera lengths greater than 80 mm are termed LAADF images in this paper
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