Table 1. Undercooling Caused by Addition of Strontium to an Al-7%Si Alloy19
embryos in the liquid. The significant undercooling that accompanies solidification of these alloys when they are properly modified with strontium suggests that the second effect dominates.
, and as the end of nucleation approaches and growth begins, a large fraction of the strontium atoms would have been rejected into the continuously diminishing eutec- tic liquid. These strontium atoms (now at a relatively high concentration in the eutectic liquid) poison the growth of the atomic silicon layers by becoming adsorbed onto the surface steps and kinks. The adsorbed strontium atoms induce twin- ning in the silicon crystals by altering the stacking sequence of the atomic layers as the newly added layers seek to grow around the adsorbed strontium atoms. This is the well known Twin Plane Re-entrant Edge (TPRE) mechanism which was first introduced by Hamilton and Seindensticker to explain the growth of germanium dendrites 22
As solidification progresses, strontium is continuously re- jected into the eutectic liquid ahead of the solid/liquid in- terface because of its low solubility in both silicon5,6 aluminum10,21
and and was later adopted
by Lu and Hellawell to explain the growth of eutectic silicon in chemically-modified Al-Si alloys.23
REFERENCES
1. Granger D. and Elliott R., ASM Metals Handbook, 9th ed., ASM Int., Metals Park, OH, vol. 15, p 162 (1988).
2. Bernal J.D., Nature, “A Geometrical Approach to the Structure of Liquids”, vol. 183, Issue 4655, pp 141-147 (1959).
3. Kobayashi K. and Hogan L.M., “The Crystal Growth of Silicon in Al-Si Alloys” Journal of Materials Science, vol. 20, no. 6, pp 1961-1975 (1985).
4. Ino S. and Ogawa S., “Multiply Twinned Particles at Earlier Stages of Gold Film Formation on Alkalihalide Crystals” Journal of the Physical Society of Japan, vol. 22, no. 6, pp 1365-1374 (1967).
5. Okamoto H., Journal Phase of Equilibria and Diffusion, vol. 27, no. 2, p 204 (2006).
6. Itkin V.P. and Alcock C.B., Bulletin of Alloy Phase Diagrams, vol. 10, no. 6, p 630 (1989).
7. Turnbull D. and Fisher J.C., Journal of Chemical Physics, vol. 17, p 71 (1949).
8. Campbell J., International Journal of Cast Metals, vol. 3, no.1, p 65 (2009).
International Journal of Metalcasting/Winter 10
9. Crosley P.B. and Mondolfo L.F., Modern Castings, vol. 46, p 89 (1966).
10. Shankar S., Riddle Y.W., and Makhlouf M.M., “Nucleation Mechanism of the Eutectic Phases in Aluminum–Silicon Hypoeutectic Alloys” Acta Materialia, vol. 52, Issue 15, pp 4447-4460 (2004).
11. Liu L., Ph.D. Thesis, “Paramètres Métallurgiques Contrôlant L’évolution Microstructurale Dans Les Alliages De Fonderie Al-Si-Mg and Al-Si-Cu” University of Quebec at Chicoutimi, p 248 (2003).
12. Song K., Kikutchi T.,Yoshida M., and Nakae H., Nippon Kinzoku Gakkaishi, vol. 58, no. 12, p 1454 (1994) (in Japanese).
13. Nakae H., in “Solidification and Casting”, B. Cantor and K. O’Reilly eds., Institute Of Physics Publishing, Bristol, UK, p 326 (2003).
14. Gruzleski J.E and Closset B.M., “The Treatment of Liquid Aluminum-Silicon Alloys,” American Foundry Society, Des Plaines, IL, p 31 (1990).
15. Verhoeven J.D., Fundamentals of Physical Metallurgy, John Wiley and Sons, New York, NY, p 225 (1975).
16. Bian X., Wang W., and Qin J., Materials Science Forum, vol. 331, p 349 (2000).
17. Shankar S., Prakash SV., Malik M., Jeykumar M., Walker M.J, and Hamed M., Proceedings of 113th Annual Meeting, San Francisco, CA, p 173 (2009).
TMS
18. McDonald S.D., Dahle A.K,. Taylor J.A, and St. John D.H., “Eutectic Grains in Unmodified and Strontium- Modified Hypoeutectic Aluminum-Silicon Alloys”, Metallurgical and Materials Transactions A, vol. 35, no. 6, pp 1829-1837 (2004).
19. Makhlouf M.M., Proc. of the Int. Summer School on High Integrity Die Casting, WPI, Worcester, MA (2008).
20. Closset B., Dugas H., Pekguleryuz M., and Gruzleski J.E., The Aluminum-Strontium Phase Diagram, Metallurgical and Materials Transactions A, vol. 17, no. 7, p 1250-1253 (1986).
21. Alcock C.B. and Itkin V.P., Bulletin of Alloy Phase Diagrams, vol. 10, no. 6, p 614 (1989).
22. Hamilton D.R. and Seindensticker R.G., Journal of Applied Physics, vol. 31, p 1165 (1960).
23. Lu S. and Hellawell A., “The Mechanism of Silicon Modification in Aluminum-Silicon Alloys:
24. Impurity Induced Twinning”, Metallurgical and Materials Transactions A, vol. 18, no. 10, pp 1721- 1733 (1987).
49
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85