鎂合金在高溫下的應用一直以來都是被探討的課題，但往往提升高溫機械性質的方法諸如添加稀土元素等方式價格過於昂貴且來源受到侷限。本篇論文嘗試以添加矽進入鎂合金中得到高溫穩定相Mg2Si，利用相具有高熔點(1086oC)、低密度、高硬度、成本低廉等特點來改善鎂合金高溫性質。本實驗先以Thermo-calc計算各種相的析出溫度並以此作為融煉參數的考量，透過特殊的雙氣體霧化製程製備原位析出的高矽鎂合金複合材料粉末，利用此製程能得到尺寸細小(3μm)且分佈均勻近似球形的Mg2Si，而且能夠減少傳統鑄造中偏析狀況嚴重的脆性相Mg17Al12。將此粉末透過簡單熱壓後，利用背向式擠型製程將粉末鍵結後，進一步利用擠型過程當中發生的動態再結晶現象細化基底相鎂晶粒尺寸，得到鎂晶粒尺寸約為2~3μm。最終可以得到鎂基底相晶粒尺寸細小，且析出相Mg2Si尺寸細小且分佈均勻的複合材料，並對此材料做加工性分析、室溫及高溫機械性質的探討，並和傳統鑄造合金做比較。本實驗一併探討不同的擠型製程參數對微觀結構及室溫與高溫機械性質的影響，並計算n值推論其主導的變形機制，並利用此機制對微觀結構的演進做一簡單的探討。透過加工性分析可以知道以粉末熱壓進行背向式擠型其加工性區間略大傳統鑄造於而所需應力皆小於傳統鑄造，可以提升工業上的應用性。室溫機械性質方面雖因Mg2Si使材料脆化而使強度提升不如預期，但高溫機械性質方面明顯有提升，透過OM及SEM觀察後，均勻分布且尺寸細小的Mg2Si明顯可透過zener pinning 及 orowan bowing等方式來提升高溫強度，最高可以提升40%以上同時並具有未添加矽材料一半的伸長率。

This object of this thesis is replacing the rare earth by silicon which can produce a thermostable phase, Mg2Si, which has advantages such as high melting point (1086oC),low density, high hardness and low cost to improve the mechanical properties of magnesium at elevated temperature.
The melting parameters is calculated by the simulation of Thermo-calc and then synthesize composite powders of magnesium of high silicon content through special process, Dual jet atomizing, which can produce in-situ composite powder of well distributed Mg2Si with small size(3μm) and near spherical shape. This process can further inhibit the formation of the weak and segregated phases, Mg17Al12.
The microstructure and bonding of these composite powders is refined by backward extrusion through continuous dynamic recrystallization mechanism and the refine grain size is 2~3μm. The analysis of workability and mechanical properties of ambient and elevated temperature are done on these composites which has fine grain size and well distributed Mg2Si and then compare with conventional casting methods.
This composite has good industrial application because its workability is larger than conventional casting and with lower flow stress. The mechanical property at ambient temperature is weakening by large amount of Mg2Si due to embrittlement but highly promoting by offering pinning effect and orowan mechanism at elevated temperature. The percentage of strengthening can attain 40% above with loosing 50% elongation comparing with composite without silicon.

1. B.L. Mordlike, T.E., <Magnesium properties application potentials>. Material science & Engneering A, 2001. A 302: p. 37-45.
2. A. LUO , M.O.P., <review cast magnesium alloy for elevated temperatre applications.pdf>. MATERIAl SCEINCE, 1994. 29: p. 5259-5271.
3. CaO, H.a.M.W., Effect of Microstructure on Mechanical Properties of As-Cast Mg-Al Alloys Metallurgical and Materials Translations a-Physical Metallurgy and Material Science, 2004. 35 A: p. 309-19.
4. Yakubtsov, I.A., et al., Effects of heat treatment on microstructure and tensile deformation of Mg AZ80 alloy at room temperature. Materials Science and Engineering: A, 2008. 496(1-2): p. 247-255.
5. Luo, A.A., Recent magnesium alloy development for elevated temperature applications. International Materials Reviews, 2004. 49(1): p. 13-30.
6. HIGASHI, M.M.K.K.K., <Tensile strength ductility and fracture of magnesium-silicon alloys.pdf>. MATERIAl SCEINCE, 1996. 31: p. 1529-1535.
7. Aikin, B.J.M. and T.H. Courtney, Modeling of particle size evolution during mechanical milling. Metallurgical Transactions A, 1993. 24(11): p. 2465-2471.
8. Avedesian, M.M., H. Baker, Magnesium and magnesium alloys. ASM International. Handbook Committee: ASM specialty.
9. Chen, Y.M., et al., Modeling of atomization rate during gas atomization. Acta Materialia, 1998. 46(3): p. 1011-1023.
10. Chawla, N.C.a.K.K., Metal Matrix composite. 2006.
11. Lloyd, D.J., Aspects of fracture in particulate reinforced metal matrix composites. Acta Metallurgica et Materialia, 1991. 39(1): p. 59-71.
12. Ibrahim, I.A., F.A. Mohamed, and E.J. Lavernia, Particulate reinforced metal matrix composites — a review. Journal of Materials Science, 1991. 26(5): p. 1137-1156.
13. Mabuchi, M. and K. Higashi, Strengthening mechanisms of MgSi alloys. Acta Materialia, 1996. 44(11): p. 4611-4618.
14. Mabuchi, M., K. Kubota, and K. Higashi, Effect of hot extrusion on mechanical properties of a MgSiAl alloy. Materials Letters, 1994. 19(5–6): p. 247-250.
15. T.G. Nieh, J.W.a.O.D.S., Superplasticity in metals and ceramics. 1997.
16. Kim, W.J., et al., Superplasticity in thin magnesium alloy sheets and deformation mechanism maps for magnesium alloys at elevated temperatures. Acta Materialia, 2001. 49(16): p. 3337-3345.
17. Lloyd, D.J., Particle reinforced aluminium and magnesium matrix composites. International Materials Reviews, 1994. 39(1): p. 1-23.
18. Hirai, K., et al., Effects of Ca and Sr addition on mechanical properties of a cast AZ91 magnesium alloy at room and elevated temperature. Materials Science and Engineering: A, 2005. 403(1–2): p. 276-280.
19. Mohri, T., et al., Microstructure and mechanical properties of a Mg-4Y-3RE alloy processed by thermo-mechanical treatment. Materials Science and Engineering: A, 1998. 257(2): p. 287-294.
20. Lü, Y., et al., Effects of rare earths on the microstructure, properties and fracture behavior of Mg–Al alloys. Materials Science and Engineering: A, 2000. 278(1–2): p. 66-76.
21. Srinivasan, A., U.T.S. Pillai, and B.C. Pai, Microstructure and mechanical properties of Si and Sb added AZ91 magnesium alloy. Metallurgical and Materials Transactions A, 2005. 36(8): p. 2235-2243.
22. REED-HILL, R.E., PHYSICAL METALLURGY PRINCIPLES. Third ed. 1992, International Thomson Publishing Company
23. Weertman, J., Theory of Steady‐State Creep Based on Dislocation Climb. Journal of Applied Physics, 1955. 26(10): p. 1213-1217.
24. Lou, Y., et al., Deformation behavior of Mg–8Al magnesium alloy compressed at medium and high temperatures. Materials Characterization, 2011. 62(3): p. 346-353.
25. Barnett, M.R., Influence of deformation conditions and texture on the high temperature flow stress of magnesium AZ31. Journal of Light Metals, 2001. 1(3): p. 167-177.
26. Barnett, M.R., et al., Influence of grain size on the compressive deformation of wrought Mg–3Al–1Zn. Acta Materialia, 2004. 52(17): p. 5093-5103.
27. Tan, J.C. and M.J. Tan, Dynamic continuous recrystallization characteristics in two stage deformation of Mg–3Al–1Zn alloy sheet. Materials Science and Engineering: A, 2003. 339(1–2): p. 124-132.
28. Sakai, T., et al., Dynamic and post-dynamic recrystallization under hot, cold and severe plastic deformation conditions. Progress in Materials Science, 2014. 60(0): p. 130-207.
29. Yang, X., H. Miura, and T. Sakai, Dynamic evolution of new grains in magnesium alloy AZ 31 during hot deformation. Materials Transactions, 2003. 44(1): p. 197-203.
30. Schey, J.A., INTRODUCTION TO MANUFACTURING PROCESS. Mechanical Engineering Series. 2000, Singapore: McGraw-Hill Higher Education.
31. Castle, A.F. and T. Sheppard, Hot-working theory applied to extrusion of some aluminium alloys. Metals Technology, 1976. 3(1): p. 454-464.
32. Chen, C. and C.Y. Tsao. Spray forming of AZ91 magnesium alloys with and without Si addition. in Materials Science Forum. 2005. Trans Tech Publ.
33. Dubé, D., et al., Characterization and performance of laser melted AZ91D and AM60B. Materials Science and Engineering: A, 2001. 299(1–2): p. 38-45.
34. Guan, Y.C. and W. Zhou, Calorimetric analysis of AZ91D magnesium alloy. Materials Letters, 2008. 62(30): p. 4494-4496.
35. Lavernia, E. and T.S. Srivatsan, The rapid solidification processing of materials: science, principles, technology, advances, and applications. Journal of Materials Science, 2010. 45(2): p. 287-325.
36. Watanabe, H., et al., Superplastic deformation mechanism in powder metallurgy magnesium alloys and composites. Acta Materialia, 2001. 49(11): p. 2027-2037.