Combining experimental study with theoretical analysis, this paper focus on the formation mechanism of microstructure in rapidly directionally solidified Al-Pb and Al-Bi immiscible alloys. The main works and results are summarized as follows:
Homogeneous Al-Pb alloys are prepared by using the rapid directional solidification technique. The effects of the Pb content and the solidification rate on the microstructure, especially on the size and distribution of the Pb-rich particles formed during the liquid-liquid transformation, are analyzed. The microstructure evolution during the liquid-liquid decomposition is calculated. Both the numerical results and the experimental ones indicate that the average size of the Pb-rich Particle increases with the Pb content of the alloy and decreases with the solidification velocity.
The rapid directional solidification experiments have been carried out with Al-Pb alloys in the magnetic fields of different strengths. The effects of Pb content, the solidification rate and magnetic strength on the microstructure, especially on the size and distribution of the Pb-rich particles formed during the liquid-liquid transformation, are analyzed. It indicates that the magnetic field has a great effect on the fluid flow and the temperature profile. With the increase of the magnetic strength, the efficient viscosity of the melt increases while the fluid flow decreases. This causes a steeper temperature profile and, therefore, a relatively rapid cooling rate. The microstructure evolution during liquid-liquid transformation of Al-Pb alloys solidified in a magnetic flied is calculated. The numerical results agree with the experimental results well.
The redistribution method of the solute during the solidification of the matrix liquid is investigated using the rapid directional solidification followed by rapidly quenching using the Al-3.4wt%Bi immiscible alloy. It is indicated that the solute is rejected to the melt in front of the solid/liquid interface. The nucleation of the minority phase droplet takes place when the supersaturation is high enough. The nucleated droplets grow and migrate under the effect of the temperature gradient and the specific gravity difference between the two liquids. They move towards and are gradually engulfed by the solid/liquid interface when their sizes are big enough.
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