In-Situ Analysis of Coarsening during Directional Solidification Experiments in High-Solute Aluminum Alloys
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E final solidified microstructure of alloys depends on the development of solid-phase morphology and segregation. During continuous solidification, growth and coarsening (i.e., dissolution of crystals or dendrite branches and growth of others) develop simultaneously. The final properties of an alloy are determined, in part, by the morphological evolution of the solid phase, which most often solidifies dendritically. The evolution of such ramified structure also influences microsegregation, which again affects the final properties of the solidified alloy.[1–3] Coarsening of high-order branches is driven by compositional and geometrical gradients. These gradients cause material transport, i.e., solvent transport in the liquid from highly curved regions to regions with low curvature and vice versa for the solute element in the alloy. Different coarsening mechanisms have been described: (1) radial dissolution of weak dendrites arms and thickening of large ones;[4] (2) dissolution of the root of weak arms (i.e., fragmentation);[4–6] (3) dissolution of weak dendrites from the tip toward the root;[7] and (4) coalescence between neighboring dendrite arms.[8] D. RUVALCABA, PhD Researcher, and D.G. ESKIN, Senior Scientist, are with the Materials Innovation Institute, 2628CD Delft, The Netherlands. Contact e-mail: [email protected] R.H. MATHIESEN, Assistant Professor, and L. ARNBERG, Professor, are with the Norwegian University of Science and Technology, N-7491 Trondheim, Norway. L. KATGERMAN, Professor, is with the Delft University of Technology, 2628CD Delft, The Netherlands. This article is based on a presentation given at the International Symposium on Liquid Metal Processing and Casting (LMPC 2007), which occurred in September 2007 in Nancy, France. Article published online August 14, 2008. 312—VOLUME 40B, JUNE 2009
For dendritic solidification, coarsening can be described in terms of secondary dendrite arm spacing (SDAS). During coarsening, the spacing between secondary dendrite arms and high-order branches would tend to become larger in relation to the coarsening mechanisms mentioned previously. ‘‘SDAS characterization’’ has been well accepted in the metallurgical and material science community, and it is a term usually related to the mechanical properties of the alloy. A finer microstructure, i.e., low SDASs, usually improves the mechanical properties (e.g., yield strength). Both the cooling rate and composition show profound influence over the SDAS. For example, a finer microstructure is achieved by increasing the cooling rate.[1,2] A well-established relationship between SDAS and local solidification time ts is given by SDAS ~ Atns , where A and n are constants.[9] It was found that n is between 0.28 and 0.5 for most metallic alloys.[4,9–13] This correlation enables analysis of the dependence of SDAS with the cooling rate, which is inversely proportional to ts. Coarsening in secondary dendrite arms has been well documented in the literature, where n is considered to be constant.[1–4,7–13] However, much of the analysis in coarsening
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