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March 17, 2003


Dr. Velimir Radmilovic
National Center for Electron Microscopy, Lawrence Berkeley Laboratory
University of California, Berkeley
"TEM and Calphad Assisted Alloy Design"

Abstract.  Aluminum based alloys are materials of choice for future aerospace and automotive applications.  However, despite extensive work on these alloys, the principles underlying their physical properties are still not fully understood, and alloy design has been mostly empirical. In this work, we combine structural and chemical characterization by transmission electron microscopy with computer phase diagram calculation to develop a systematic understanding of the relationship between microstructure and properties and to aid in the design of a new Al-Cu-Si-Ge alloy. Atom probe tomography and high resolution electron microscopy revealed that in the very early stage of aging Al-Si-Ge alloys, clustering of Si and Ge atoms takes place followed by precipitation at structural defects. Electron microscopy observations demonstrated that precipitates are multiply twinned throughout the aging treatment. Any twinned section of the precipitate no longer maintains low-index interfaces with the matrix, and consequently goes from a crystallographic to a spherical interface with the matrix. This explains the equiaxed shape of the Si-Ge precipitates. CALPHAD calculation predicted and X-ray microanalysis confirmed a decreasing concentration of Ge within the equilibrium precipitate as the aging temperature was increased. In the quaternary Al-Si-Ge-Cu alloy, Si-Ge particles quickly nucleate and grow during elevated temperature aging (they are detected after as little as 30 min. at 190�C). The Si-Ge particles then act as nucleation sites for � precipitates, resulting in a peak aged microstructure consisting of a dense distribution of � attached to Si-Ge.

Vanderbilt University