Grain refinement during solidification of aluminium alloys
Understanding the crystallization process during solidification is an essential step to tailor the mechanical properties of solidified materials. The physical processes that govern crystallization are grain nucleation and the subsequent grain growth. In the industrial production process of aluminium alloys the addition of TiB2 particles along with solute titanium is widely used to enhance the nucleation rate and control the grain growth during solidification. This procedure is generally referred to as grain refinement. Although the mechanisms responsible for grain refinement are extensively studied in last decades, a comprehensive understanding is still lacking due to experimental difficulties in monitoring the grain nucleation and growth in situ. The question of particular interest is how added TiB2 particles along with solute titanium results in grain refinement, while no grain refinement is observed if one of the two is missing. In this project we have used synchrotron X-ray diffraction [1,2], neutron diffraction [3,4,5], and small angle neutron scattering  to investigate the mechanism responsible for grain refinement in Al-Ti-B alloys.
In our synchrotron X-ray diffraction study [1,2] we have investigated the nucleation and the growth of individual aluminium grains during solidification. An aluminium alloy with solute titanium (0.1 wt.%) and added TiB2 particles (0.1 wt.%) is chosen as a model system. Time resolved X-ray diffraction measurements during solidification of this alloy are performed at the ESRF. The evolution of 2D diffraction images during solidification is shown in Figure 1. Before solidification the short-range order in the liquid gives rise to diffuse scattering, indicated by two broad rings L1 and L2 on the 2D image. During solidification we observe the appearance of bright spots, which correspond to Bragg peaks from individual grains of the solid phase that grow within the liquid phase
In Figure 2a the time evolution of nucleation process and the solid phase fraction is shown for solidification during continuous cooling at rate of 1 K/min. The results demonstrate that the nucleation process is limited to the initial stage of the solidification and is complete at a solid phase fraction of about 20%. In fact the growth of nucleated grains leads to a significant release of latent heat that limits the undercooling available to activate later nucleation events as the transformation proceeds. Figure 2b shows the evolution of the grain radius of an individual aluminium grain as a function of temperature during solidification. The observed growth behaviour of the aluminium grains is controlled by the diffusion of solute titanium and the release of latent heat. As titanium has a strong affinity for the solid phase, its concentration in the melt decreases as the solidification proceeds. A careful analysis of the measured diffraction patterns shows a limited number of weak diffraction spots, originating from a TiAl3 that appears before the start of the solidification process. In Figure 2b the evolution of an individual TiAl3 grain is shown. The TiAl3 phase is found to form about 10 K above the experimental solidification temperature of aluminium. At the nucleation temperature of the aluminium the intensity of the TiAl3 grain starts to decrease, and finally vanishes near the end of the transformation. Due to its better nucleation efficiency, the formation of the TiAl3 phase in the presence of both solute titanium and TiB2 particles was long proposed, but due to the meta-stable nature of the TiAl3 phase there was no experimental evidence available.
These experiments provide the first in-situ information about the nucleation and growth kinetics of individual grains within the melt during solidification and confirm the mechanism responsible for grain refinement during solidification for the investigated aluminium alloys.
-  N. Iqbal, N.H. van Dijk, S.E. Offerman, M.P. Moret, L. Katgerman, and G.J. Kearley, Real-time observation of grain nucleation and growth during solidification of aluminium alloys, Acta Materialia 53 (2005) 2875-2880. [PDF]
-  N. Iqbal, N.H. van Dijk, S.E. Offerman, N. Geerlofs, M.P. Moret, L. Katgerman, and G.J. Kearley, In-situ investigation of the crystallisation kinetics and the mechanism of grain refinement in aluminium alloys, Materials Science and Engineering A 416 (2006) 18-32. [PDF]
-  N. Iqbal, N.H. van Dijk, V.W.J. Verhoeven, W. Montfrooij, T. Hansen, L. Katgerman, and G.J. Kearley, Experimental study of ordering kinetics in aluminum alloys during solidification, Acta Materialia 51 (2003) 4497-4504. [PDF]
-  N. Iqbal, N.H. van Dijk, V.W.J. Verhoeven, T. Hansen, L. Katgerman, and G.J. Kearley, Periodic structural fluctuations during the solidification of aluminum alloys studied by neutron scattering, Materials Science and Engineering A 367 (2004) 82-88. [PDF]
-  N. Iqbal, N.H. van Dijk, T. Hansen, L. Katgerman, and G.J. Kearley, The role of solute titanium and TiB2 particles in the liquid-solid phase transformation of aluminum alloys, Materials Science and Engineering A 386 (2004) 20-26. [PDF]
-  N. Iqbal, N.H. van Dijk, C. Dewhurst, L. Katgerman, and G.J. Kearley, SANS investigation on the solidification of aluminum alloys, Physica B 350 (2004) e1011-e1014. [PDF]
- Naveed Iqbal (2005), Solidification, real-time investigation of grain nucleation and growth during liquid to solid phase transformation of aluminium alloys [PDF]
- N. Iqbal, N.H. Van Dijk, S.E. Offerman, M.P. Moret, L. Katgerman, and G.J. Kearley, View on grain nucleation and growth kinetics during solidification, Highlights 2005 of the European Synchrotron Radiation Facility, (2006), 37-38. [WWW]
- Presentation at the Fall Meeting 2004 of the Materials Research Society in Boston [PowerPoint]