Structure determination of Mg5Si6:particles in Al by dynamic electron diffraction studies

Science

Volumn 277, Issue 5330, 1997, Pages 1221-1225

  Zandbergen, H W ^a   Andersen, S J ^a,b   Jansen, J ^a,c  

^a DELFT UNIVERSITY OF TECHNOLOGY   (Netherlands)

^b Senter Industriell Tekn F   (Norway)

^c UNIVERSITY OF AMSTERDAM   (Netherlands)

Author keywords

[No Author keywords available]

Indexed keywords

ALLOY; ALUMINUM; ELEMENT; MAGNESIUM SILICATE; NANOPARTICLE;

ARTICLE; CHEMICAL COMPOSITION; CORROSION; CRYSTAL STRUCTURE; ELECTRON DIFFRACTION; ELECTRON MICROSCOPY; MICROSCOPE IMAGE; MOLECULAR INTERACTION; PRECIPITATION; PRIORITY JOURNAL; STRUCTURE ANALYSIS;

EID: 0030874912     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.277.5330.1221     Document Type: Article

References (28)

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4. G. A. Edwards, G. L. Dunlop, M. J. Couper, in Proceedings of the International Conference on Aluminum Alloys, Georgia Institute of Technology, Atlanta, GA, 11 to 16 September 1994 (Georgia Institute of Technology, Atlanta, 1994), p. 620.
5. S. J. Andersen, Met. Mater. Sci. 26A, 1931 (1995).
6. D. Van Dyck and M. Op De Beeck, in Proceedings of the 12th International Congress on Electron Microscopy, International Federation of Electron Microscope Societies, Seattle, WA, 12 to 18 August 1990 (San Francisco Press, Seattle, WA, 1990), pp. 26-27.
7. W. Coene, G. Janssen, M. Op De Beeck, D. Van Dyck, Phys. Rev. Lett. 69, 3743 (1992).
8. 8) determined by x-ray single-crystal diffraction or neutron powder diffraction. The MSLS refinement resulted in very similar (a difference in atom positions less than 0.01 nm and on average less than 0.003 nm) atom positions. Putting one or more atoms at the wrong position gives much higher R values.
9. The material investigated was Al with 0.2 weight % Fe, 0.5 weight % Mg, 0.53 weight % Si, and 0.01 weight % Mn, extruded as described in (5) and aged at 185°C for 5 hours.
10. The information limit is not circular but elliptical (0.135 nm by 0.150 nm), because of the anisotropic response of the microscope to external vibrations.
11. Other reasons to cool the specimen are the reduction of the electron beam induced contamination and amorphization. Unfortunately, commercial sample cooling holders do not allow real HREM imaging; thus it is difficult to do electron diffraction and HREM on the same precipitates.
12. obs) is the standard deviation of the reflection.
13. EDX element analysis was done with a Link EDX element analysis system, and the mineral forterite was used to calibrate the Cliff-Lorimer factor for these two elements.
14. K. Matsuda, S. Tada, S. Ikeno, T. Sato, A. Kamio, Scr. Mater. 32, 1175 (1995).
15. S. Andersen et al., in preparation.
16. W. Coene, A. Thust, M. Op De Beeck, D. Van Dyck, Ultramicroscopy 64, 107 (1996).
17. -1) frequency, respectively.
18. For the TFEWR, series of 20 HREM images were recorded with focus increments of 5.2 nm. The starting focus was about -70 nm. The images were recorded with a 1024 by 1024 pixel slow scan charge-coupled device camera.
19. Further analysis of this exit wave, such as the atomic structure at the interface between β″ and the Al matrix and the structure and the origin of the defect in the β″ precipitate, is given elsewhere (15). 20. Because of the overlap with the Al reflections, the 10% discarded reflections are rather random for the structure of the precipitate, which implies that the accuracy of the structure determination is not substantially hampered by this omission.
20. The space group could have been determined by convergent beam electron diffraction, but given the small size of the precipitates, this method was not used.
21. To approximate a kinematic refinement with the MSLS program, a very small thickness (for example, 1 nm) or a very low occupancy should be used. The latter approach was used because in this way the excitation error of the diffraction spots is property taken into account. This excitation error occurs because, in contrast with x-ray single-crystal diffraction in which each diffracted beam is measured at its maximum (reflections are in exact Bragg condition), most diffracted beams in electron diffraction are not in exact Bragg condition. In the calculation of the kinematic R values, a 1 % occupancy of all atom sites and the thicknesses obtained for the dynamic refinement were used.
22. It would be better to do a complete refinement with the addition of an Al matrix in front and after the precipitate and to refine the thicknesses of these two layers as well. This, however, is not yet possible with the MSLS package.
23. For the simulated exit wave, the following parameters were used: defocus spread, 10 nm, which is used to impose an information limit of about 0.14 nm; specimen thickness, 10 nm; convergence angle, 0.1 mrad; and crystal tilt, 1.5° about the [201] direction. The specimen tilt and thickness were roughly estimated from the loss of symmetry in the surrounding Al matrix. The agreement between the calculated and the experimental exit wave can be improved by adjustment of the twofold astigmatism and beam tilt, but this would be merely a cosmetic improvement without giving any proof of the reliability of the model.
24. Al.
25. D. L. Dorset, Structural Electron Crystallography (Plenum, New York, 1995).
26. W. Coene and A. J. E. M. Jansen, Scanning Microsc. Suppl. 6, 379 (1992); H. W. Zandbergen, D. Tang, D. Van Dyck, Ultramicroscopy 64, 185 (1996).
27. W. Coene and A. J. E. M. Jansen, Scanning Microsc. Suppl. 6, 379 (1992); H. W. Zandbergen, D. Tang, D. Van Dyck, Ultramicroscopy 64, 185 (1996).
28. The Al-Mg-Si specimen was supplied by HYDRO Aluminum AS (Sunndalsøra, Norway). Stichting Technische Wetenschappen is acknowledged for financial support. We thank D. van Dyck and M. op de Beeck for discussions. S.J.A. is indebted to Norsk Hydro Al for financial support.