Does the refinement/ modification of the eutectic silicon has any thing to do with the increase in the viscosity of the Solute-rich liquid, which affect the fluidity of the last liquid to solidify.
As the phase diagram suggests, Al-Bi, and Bi-Si are immissible systems. Does the Bi-rich liquid pockets effect the growth of eutectic silicon by hindering the Si-atoms on further adding-up to the neighboring Si crystals, during the solidification of the eutecctic ?
The effect of oxide films is also significant during the solidification as the bi-films will be pushed towards the liquid by the growing dendrite front (Please correct me if this is not true).
Article Discussion of “On the Refinement Mechanism of Silicon in Al-...
As I understand it, after modification with Sr or Na the bifilms are effectively deactivated (I think this occurs because the AlP itself is deactivated, which originally formed on the bifilms, nucleating the Si, which subsequently grew around the AlP and then out along the surface of the bifilms).
This means the bifilms become redundant, floating freely in the melt in a modified Al-Si alloy while the silicon now grows as a finely spaced coupled eutectic.
The residual melt concentrates in various segregated solutes as you suggest. However these will have practically zero effect on the fluidity of the melt. The bifilms are another matter. Their presence is damaging, blocking intergranular flow paths, making the liquid apparently viscous, and creating porosity.
This rather complicated scenario was originally disentangled during a discussion between my good friend, Professor Murat Tiryakioglu of N Florida University, and myself. I believe it to be the only explanation of the action of modification which fits all the facts.
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