Real Time Synchrotron Microradiography of Dendrite Coarsening in Sn-13 Wt Pct Bi Alloy
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INTRODUCTION
DURING alloy solidification, the primary solid phase takes a branched morphology under most circumstances because of constitutional undercooling.[1] The average distance between side arms, i.e., dendrite arm spacing (DAS), imposes a significant effect on the mechanical properties of castings. It was found that the ultimate tensile strength (UTS), the yield strength, and the elongation of castings increased when the DAS decreased, and the final DAS also had an effect on heat treating processes such as homogenization in eliminating microsegregation of castings.[2] One of the important features of dendritic solidification is dendrite coarsening during dendrite growth; i.e., DAS increases with solidification time. This was first revealed by Kattamis et al.[3] in their studies of solidification of Al-Cu alloys. The DAS was observed to increase during solidification, indicating that some dendrite coarsening mechanism(s) came into play during solidification. In Kattamis’ study, dendrite coarsening in Al-Cu alloys during isothermal holding and continuous cooling was investigated by interface quenching. Samples were water quenched when they were either cooled or held in the two-phase region. Metallographic examination on the quenched samples revealed that coarsening of side branches did occur during solidification as well as during isothermal holding. The DAS increased with time of solidification or holding. Based on their observations, two coarsening models were proposed, dendrite thinning (radial remelting) and dendrite fragmentation (remelting at the root of the dendrite arms). B. LI, Post-doctorate, is with the Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, MD 21218, USA, Contact e-mail: [email protected] H.D. BRODY, Professor, is with the Department of Metallurgy and Materials Engineering, University of Connecticut, Storrs, CT 06269-3136, USA. A. KAZIMIROV, Beamline Staff Member, is with the Cornell High Energy Synchrotron Source, Ithaca, NY 14853, USA. Manuscript submitted January 13, 2006. Article published online March 27, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
In the observations of solidification in transparent NH4Cl aqueous solution, Kahlweit[4] suggested that coarsening by dendrite thinning did not occur readily. Fragmentation was indeed observed in some experiments. Other than remelting by thinning, smaller dendrites tended to remelt from dendrite tips toward dendrite roots, i.e., axial remelting, while the tip radii remained practically unchanged. Based on Kahlweit’s model, Kattamis et al.[5–8] established a numerical model to calculate the evolution of the dendritic interfacial area per unit volume (the ratio of interfacial area to volume) SV. The evolution of the SV ratio with respect to solidification time in Al-Cu alloys was measured and compared with the numerical model, and the comparison showed that the agreement was satisfactory for short coarsening times but less satisfactory for longer holding times. Kirkwood et al.[9] also studied the DAS in Al-Cu allo
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