TEM STUDIES OF PRECIPITATION PROCESSES IN ALLOYS. A. J. Tolley 1,2 E. Zelaya 2 and V. Radmilovic 3 1 Centro Atómico Bariloche, Comisión Nacional de Energía Atómica, Centro Atómico Bariloche R8402AGP S.C. de Bariloche, Argentina. 2 CONICET. 3 National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, University of California, Berkeley. The transmission electron microscope is a unique instrument in which diffraction may be combined with imaging and microanalytical techniques for detailed characterization of materials. In the following, several examples of the evolution of microstructure during precipitation processes in metallic alloys are described. Figure 1 illustrates the combination of bright field imaging and composition profiling using X ray spectroscopy to study the microstructure near a grain boundary (GB) in an Al-Cu alloy with microalloying additions of Si and Ge [1]. Figure 1(a) shows the microstructure near a GB. The insets show the [001] zone axis diffraction patterns of the regions next to the GB (lower left hand corner), where θ” precipitates are found, and far from the GB (upper right hand corner), where θ´ and Si-Ge precipitates are observed. This image exemplifies the role of Si-Ge precipitates in stimulating the nucleation of the θ´ phase away from the GB. Figure 1 (b) shows the Si and Ge composition profile across the GB, where a local depletion of both elements can be observed. However, the Si-Ge precipitate free zone extends beyond the depleted zone, indicating that the lack of Si-Ge precipitates near the GB is associated with vacancy depletion rather than solute depletion. Figure 2 illustrates the combination of microdiffraction and high resolution imaging to study the structure of an ion irradiation induced precipitate in Cu-Zn-Al. The microdiffraction pattern shows additional reflections that correspond to a close packed structure with 2H or 4H stacking sequence. The high resolution image shows that both stacking sequences are found within the same particle [2]. Figure 3 shows a series of high angle annular dark field (HAADF) images obtained in scanning- transmission (STEM) mode, where Z contrast is used to study the evolution of the core-shell structure of Al 3 (Sc,Zr) precipitates in an Al-Sc-Zr alloy during annealing at 450ºC. In these precipitates, Zr segregates to the shell. The core-shell structure is found even after very long annealing time, during which significant growth of the precipitates takes place [3]. Figure 4 illustrates the combination of dark field imaging, high resolution imaging, HAADF/STEM imaging and electron energy loss elemental mapping to characterize precipitates in an Al-Li-Sc-Zr alloy. This alloy was subjected to a double ageing treatment of 18 h at 450ºC followed by 4h at 190ºC. During the high temperature anneal, Li remains in solid solution while Al 3 (Sc,Zr) precipitates are formed. During the low temperature anneal, Al 3 Li grows on the Al 3 (Sc,Zr) particles. Elemental mapping clearly shows the Sc rich core surrounded by a Li-rich shell. The high resolution image shows a clear L1 2 type contrast only within the shell, due to the different scattering from Li and Al atom columns [4]. References [1] D. Mitlin, V. Radmilovic, J.W.Morris Jr. and U. Dahmen, Metall. Mater. Trans 34A (2003) 735-742. [2] E. Zelaya, A. Tolley, A. Condó, F. C. Lovey, P.F.P. Fichtner and P. Bozzano, Scripta Mater. 53 (2005) 109–114. [3] V. Radmilovic, A. Tolley, Z. Lee, and U. Dahmen. Proc. 4 th Balcan Conf. on Metallurgy, 2006. [4] V. Radmilovic, A. Tolley, E. Marquis, M. D. Rossell, Z. Lee, U. Dahmen, Scripta Materialia 58, (2008) 529-532. Acta Microscopica, Vol. 18, Supp. C, 2009 165