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Materials Frontier No.165

Title:  The hardening precipitates in high strength aluminum alloys studied by atomic-resolution electron microscopy
Speaker: Prof. Jianghua Chen, Cheung Kong Scholars, School of Materials Science and Engineering, Hunan  University
Date/Time: 2015-09-21,2pm
Venue: Room 308, Material Building A
Invitor: Xiaoqin Zeng

Developments of high-strength aluminum alloys have always faced a difficult problem: owing to their small size, the early-stage strengthening precipitates are difficult to characterize in terms of composition, structure and evolution. Here we employ atomic-resolution transmission electron microscopy (TEM) imaging and first-principles energy calculations to address these problems. Recent years, we have investigated tens of typical high strength aluminum alloys, such as 2xxx (AlCuMg) and 7xxx (AlZnMgCu) alloys, with different compositions and with varying thermal processes for their property-structure-process correlations. Using advanced aberration-corrected high-resolution TEM (HRTEM) and aberration-corrected scanning TEM (STEM), much of our attention has been paid to revisit the strengthening precipitates in these important alloys and to clarify the controversies left in the past about their precipitation behaviors. Our study demonstrates the followings:
(1) Atomic-resolution imaging in HAADF-STEM can provide straightforward structure models at the atomic-scale, whereas atomic-resolution imaging in HRTEM with rapid quantitative image simulation analysis can provide the refined structures with high precision beyond the resolution limitation of the microscope. The combination of the two techniques can be more powerful in solving difficult structure problems in materials science.
(2) Most of the early-stage precipitates are highly dynamic in both composition and structure. Typically, having their characteristic genetic skeletons to guide their evolution, these dynamic precipitates initiate, mature and grow with thermal aging following characteristic evolution paths. The fine precipitation scenarios revealed are rather different from previous understandings in the textbooks and literatures published thus far.

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