Abstract
Spinodal decomposition of hypermonotectic Al–6 wt.%Bi, Al–8 wt.%Bi and Al–6 wt.%Bi–8 wt.%Zn alloys has been investigated using synchrotron radiography. In the case of the 6 and 8 wt.%Bi binary alloys undercoolings of 70 and 110 K, respectively, were required to initiate the L→L1+L2 reaction, which appeared to occur very close to the monotectic reaction temperature. The nucleated L2 droplets were set in collective size-dependent motion by forces coupled to external fields (gravity and imposed temperature gradient) as well as forces arising due to internal fluctuations of the system. With experimental conditions similar to those realized during strip casting of the same materials, it was found that the size-dependant droplet velocity field combined with Stokes drag at the L1–L2 interfaces as well as attractive and repulsive diffusion-coupling between adjacent L2 droplets, yield complex meso- to microscale hydrodynamics. The hydrodynamics are the dominating mechanisms for L2 droplet coagulation, and are accordingly decisive for the final size distribution and geometrical dispersion of the soft Bi-rich component in the cast material. A different decomposition mode was observed in the Al–6 wt.%Bi–8 wt.%Zn ternary alloy, with the L2 droplets undergoing an immiscible–miscible–immiscible transition. In contrast to what was found for the binaries, L2 domains formed at relatively small undercoolings, and very little droplet motion was observed, as all L2 domains nucleated and remained on the crucible walls until they encroached on the monotectic front. At small distances from the monotectic front a Zn-rich solute boundary layer preceding the α-Al, caused the L2 domains to dissolve as Bi–Zn–Al regains complete miscibility upon reaching a critical Zn-concentration. In the shallow mush region behind the monotectic reaction, a high Zn solid solubility and a relatively fast diffusion of Zn in α-Al combine to cause a rapid diminishing Zn concentration in the mush liquid, restoring Bi-immiscibility and consequentially a secondary nucleation of L2 droplets in the mush.
Highlights
Spinodal decomposition of hypermonotectic Al–6 wt.%Bi, Al–8 wt.%Bi and Al–6 wt.%Bi–8 wt.%Zn alloys has been investigated using synchrotron radiography
The proximity to full video-microscopic resolutions opens the way for studies of the dynamics of liquid–liquid phase separation and the current work reports on the first use of in situ synchrotron x-ray video microscopy to observe Al–Bi and Al–Bi–Zn alloys during directional solidification, under conditions that should be reasonably analogous to the vertical strip casting process [4]
The region of the phase diagram between Tbin(c0) and Tmon is known as the liquid two-phase zone (LTPZ)
Summary
The phase diagram for the Al–Bi system can be seen in figure 1. The microstructure development model by Wu et al [9] only investigates L2 transport on a macroscopic scale and does not include any criteria for coalescence of the L2 phase, which later will be shown to be extremely important with respect to the final size distribution of the L2 phase. This model does not include any of the diffusion controlled mechanisms for droplet motion. The important hydrodynamic forces that determine droplet motion are not well described on typical casting length and timescales and to date have not been successfully modelled
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