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Technical Review & Discussion


On the Mechanism of Modification of the Aluminum-Silicon Eutectic by Strontium: The Role of Nucleation M.M. Makhlouf, Worcester Polytechnic Institute, Worcester, MA , USA


Reviewer: There was a long discussion published on this subject in previous issues of the IJMC. Does this contribu- tion add to that earlier discussion?


(Editors note: the original paper, The Modification of Al-Si Casting Alloys: Important Practical and Theoretical As- pects, G.K. Sigworth, Alcoa Primary Metals, Rockdale, Texas was published in IJMC Spring 2008; Vol. 2 Issue 2 and the subsequent technical discussion was published in IJMC Winter 2009; Vol. 3 issue 1)


Author: The current paper is different from the earlier dis- cussion that appeared in IJMC. The current paper focuses on the role that strontium plays in the nucleation of the eu- tectic phases irrespective of what the catalyst for nucleation is. The discussion that the reviewer refers to focused on the nature of the nucleant and was initiated by Dr. Camp- bell’s suggestion that oxide films are potent nucleants for the eutectic phases. The current paper draws on the Clas- sical Nucleation Theory and highlights the role that stron- tium plays in altering the environment in which the eutectic phases form.


Reviewer: The author refers to an article (ref. 1) in the old edition of the ASM Metals Handbook on Casting. This chap- ter has been changed in the new edition of the handbook and is now very similar to the previous IJMC articles.


Author: The article in the old edition of the ASM Metals Handbook represents persisting beliefs, and I reference it only as a preamble to establishing the fact that a complete theory for the mechanism of modification of the eutectic phases in hypoeutectic Al-Si alloys must address not only the Growth aspects of the process, but also its Nucleation aspects.


Reviewer: The author should discuss briefly some of the evidence for the role of P (or AlP) as nucleant, and how Sr might change that nucleation.


Author: The nature of the nucleant is not the concern of this paper and that is why I do not discuss the role of P in as much as I do not discuss the role of oxide bi-films, or pre- eutectic iron-rich particles. I do not dispute that the known modifiers of eutectic silicon (e.g., Na, Sr, Ca, etc.) react with P (when P is present in the melt) and as a consequence they have the capability to neutralize AlP particles and thus af- fect the nucleation of the eutectic phases. However, I inten- tionally do not go in this detail as it may distract the reader from the purpose of the paper. Instead I say in the body of the article: “it [meaning Sr] hinders nucleation of eutectic silicon by decreasing the number of potential silicon em- bryos in the liquid.” And I provide ample evidence for this assertion from the published literature.


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International Journal of Metalcasting/Winter 10


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