Microscopic View on Grain Nucleation and Growth Kinetics During Solidification of Aluminum Alloys
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Q7.12.1
Microscopic View on Grain Nucleation and Growth Kinetics During Solidification of Aluminum Alloys N. Iqbal 1, N. H. van Dijk 1, S. E. Offerman 1,2, M. Moret 3, L. Katgerman 2, and G. J. Kearley 1 1 Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands 2 Department of Materials Science and Engineering, Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands 3 ESRF, 6 Rue Jules Horowitz B. P. 220, F - 38043, Grenoble cedex, France
ABSTRACT X-ray diffraction with hard X-rays (E = 70 keV) was used to investigate the grain nucleation and grain growth during solidification of a grain refined Al-0.3Ti-0.02B (wt.%) alloy. The investigations showed for the first time the nucleation profile during solidification and how nucleation rate increases with cooling rate. The results indicate that the nucleation process is complete for solid fraction below 30 %, irrespective of the cooling rate. This is explained in terms of the release of latent heat during solidification. The growth of individual aluminium grains during solidification is experimentally observed and compared to model predictions for the diffusion limited grain growth. The experimental results are only in agreement with the theory in the first stage of the transformation. The difference between the experiment and the theory is discussed qualitatively. INTRODUCTION Al-Ti-B master alloys have been used commercially for the grain refinement of aluminium alloys for over last 30 years. These master alloys contain micrometer size TiB2 and TiAl3 particles together with solute titanium. When added to molten aluminium alloys these particles enhance the nucleation of α-aluminum grains while the partitioning of solute titanium at the solid-liquid interface controls the grain growth during solidification. This grain refinement process results in a reduced grain size and improved mechanical properties of the final solid product [1]. Despite intensive study, a rigorous mechanism explaining the grain refinement of aluminium alloys by Al-Ti-B master alloys remains a matter of controversy [2,3]. It is observed that as few as 1 % of the particles added to the melt nucleate the aluminum grains. The remaining particles are of no inherent value to the final product, however the agglomeration of these particles may induce defects resulting in poor surface properties of the solidified material. The prevailing aim is therefore to understand the nucleation kinetics and growth of individual grains during solidification of these grain refined aluminium alloys. This can help to develop more efficient grain refiners, to control the microscopic structure during solidification, and improve the mechanical properties of the solidified material.
Q7.12.2
EXPERIMENTAL Time dependent X-ray diffraction measurements were carried out during the solidification of an Al-0.3Ti-0.02B (wt.%) alloy. The investigated sample was prepared from an Al-5Ti-0.2B (wt.%) commercial master alloy (KBM AFFILIPS). The sample was kep
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