本研究藉由改變不同的熱處理條件，探討熱處理對壓鑄與模鑄AZ91D鎂合金兩種材料顯微結構及其耐蝕性質之影響。熱處理條件主要在氮保護氣氛下，將壓鑄與模鑄AZ91D鎂合金試片進行440℃、持溫20小時之固溶化處理後，再以不同冷卻條件對合金試片進行冷卻，冷卻條件主要分為爐冷、空冷以及水淬等三種。
　　實驗結果顯示，壓鑄AZ91D鎂合金在440℃/20hr固溶處理後，在不同冷卻條件進行冷卻後，其顯微結構明顯有所不同。在空冷的條件之下，壓鑄合金試片表面有一β相析出聚集層，其顯微組織呈現條層狀的析出結構，合金試片內部則僅有少量β相之零星分佈，而經爐冷條件冷卻之合金試片，其析出之條層狀β相則佈滿於合金試片。經水冷冷卻之壓鑄合金試片，試片本身為單一α相固溶體，但是試片本身化學成分組成分佈仍不均勻，由GDS及EDS分析可知，經440℃/20hr/水冷處理之壓鑄合金試片近表層的鋁含量較內部為高，這也是在空冷冷卻時，壓鑄合金試片表面會有一β相析出聚集層產生的原因。由於模鑄AZ91D鎂合金本身鋁含量較壓鑄材為低，故在相同空冷條件冷卻下，合金試片之微結構為單一α相之固溶體，與其水冷試片結果相同；而爐冷試片則與壓鑄材相同，佈滿條層狀之β相析出。在TEM顯微組織觀察方面，發現壓鑄AZ91D鎂合金原材中，鋁元素在α與β相兩相之間分佈，其相界處有一缺鋁區的存在，而不連續析出之兩相組織則並無此現象。硬度值量量測結果顯示，具有條層狀β相析出之試片其硬度值明顯高於單一α相固溶體；經比較模鑄合金試片原材之初晶α相與經空冷或水冷冷卻所形成固溶α相之硬度值可發現，鋁元素的固溶可提高材料本身的硬度值。電化學交流阻抗的分析結果顯示，經不同熱處理或原材之鎂合金試片，其耐蝕性明顯皆不佳，而經微弧氧化(MAO)處理之鎂合金試片，其本身的電化學交流阻抗值則明顯比未經表面處理的試片高出許多，顯示經陽極化處理之鎂合金試片具有良好的耐蝕性。

　　The effect of cooling rate on the microstructure and electrochemical behavior of die cast and ingot cast AZ91D magnesium alloys during solid solution heat treatment was investigated. The heat treatment was performed at 440℃ for 20h, and then followed by furnace cooling, air cooling and water quenching, respectively. The structure was identified by XRD, the microstructure morphology was examined by OM, SEM and TEM, and the corrosion resistance was evaluated by EIS.
　　The experimental results showed that for the die-cast specimen cooled in furnace, a quantity of β phase (Mg17Al12) precipitated in a shape of lamella all over the matrix (α phase) was observed. If it was cooled in air, the β phase was precipitated at a high density near the surface. No β phase could be observed if it was quenched in water. The precipitation of β phase might help to increase the hardness of the matrix according to the micro-hardness test. For the eutectic structure of die-cast specimen, TEM analysis further indicated that a significant Al depletion zone existed between α and β interface. However, for the ingot-cast alloy, β phase could be observed distributed over the whole matrix as it was cooled in furnace, but it would form single α phase solid solution by both air cooling and water quenching after annealing at 440℃ for 20hrs. The comparison of precipitation behavior of die cast and ingot cast alloys due to different Al content and distribution in the matrix would be further discussed.
　　The EIS result indicates that all the magnesium alloys investigated in this study, regardless of the heat treatment, has a poor corrosion resistance, as compare with the commercial magnesium alloy coated with an oxide layer which was formed by micro-arc oxidation (MAO) technique.

1. 蔡幸甫，輕金屬在新世代產品的應用與商機，2002.08.
2. 蔡幸甫，電子資訊產品機構件金屬化趨勢，2003.03.
3. 蔡幸甫，鎂合金產業技術及市場發展趨勢專題調查，工研院產業經濟與資訊服務中心科技專案成果，2001.10.
4. 許光良、鄭勝元，鎂合金回收材檢測技術(Ⅳ)，鑄造科技170期，p.16，2003.
5. 王文樑，鎂合金於國外汽車產業，金屬工業34卷3期，2000.05.
6. 台灣鎂合金協會，鎂合金產業通訊25期，2004.05.
7. 曾坤三，”鎂合金壓鑄機性能評估與選用”，金屬工業33卷3期，p.59-65，1999.05.
8. 黃士宗，”鎂合金熔煉過程中氣體保護問題探討”，鑄造科技170期，2003.11.
9. M. Avedesian and Hugh Baker, Magnesium and magnesium alloys, ASM Specialty Handbook, 1999.
10. 王文寬，”鎂板的製造方法”，工業材料，p.125，2002.12.
11. 邱垂泓、楊智超，”鎂合金成型技術之發展趨勢”，工業材料，p.84-88，2001.06.
12. 劉展光，”薄壁鎂合金筆記型電腦上蓋之壓鑄模澆流道設計分析”，國立交通大學機械工程研究所碩士論文，2002.
13. 洪一中、廖嘉榕、楊智富，”合金元素對鎂合金微觀組織及機械性質之影響”，鑄工28卷4期，p.18，2002.
14. 馬寧元，”鎂壓鑄合金之設計”，鑄造科技155期，p.24，2002.
15. I. J. Polmear, “Light Alloys-Metallurgy of the Light Metals”, p127.
16. C. H. Caceres, C. J. Davidson, J. R. Griffiths, C. L. Newton, “Effects of solidification rate and aging on the microstructure and mechanical properties of AZ91 alloy”, Materials Science and Engineering A325, pp.344-355, 2002.
17. Arne K. Dahle, Young C. Lee, Mark D. Nave, Paul L. Schaffer, David H. StJohn, “Development of the as-cast microstructure in magnesium Aluminium alloys”, Journal of Light Metals, Vol. 1, pp.61-72, 2001.
18. E. Ovrelid, P. Bakke, T. A. Engh, Light Metals, pp.141-154, 1997.
19. D. Turnbull, “Theory of cellular precipitation”, Acta Metallurgica, Vol. 3, p55, 1955.
20. D. B. Williams, E. P. Butler, “Grain boundary discontinuous precipitation reactions”, International Metals Reviews, No.3, 1981.
21. H. I. Aaronson, C. S. Pande, “A synthesis of mechanisms for initiation of the cellular (or discontinuous precipitation ) reaction”, Acta Mater., Vol. 47, No. 1, pp. 175-181, 1999.
22. D. A. Porter, K. E. Easterling, “Phase transformations in metals and alloys”, second edition, p322.
23. S. P. Gupta, “Kinetics of discontinuous coarsening of cellular precipitate in a Ni–8.5 at% Sn alloy”, Acta Metall., Vol.35, No.3, pp.747-757, 1987.
24. S. P. Gupta, R. Nakkalil, “Kinetics of discontinuous coarsening of cellular precipitate in a Ni–8 at% In alloy”, Acta Metall. Mater., Vol.38, No.10, pp.1871-1882, 1990.
25. G. P. Geber, “An APFIM / TEM investigation of the discontinuous precipitation in a Ni In alloy”, Applied Surface Science 87/88, pp.234-242, 1995.
26. J. C. Colmenero, K. Akune, “Discontinuous precipitation in a Zn 1.6 wt% Al alloy”, Materials Characterization, Vol. 37, pp.123-130, 1996.
27. S. Abdou, G. Solorzano, M. El-Boragy, W. Gust, B. Predel, “In Situ study of Discontinuous precipitation in Al 15at.% Zn”, Scripta Materialia, Vol. 34, No. 9, pp.1431-1436, 1996.
28. D. Duly, J. P. Simon, Y. Breche, “On the competition between continuous and discontinuous precipitation in binary Mg Al Alloys”, Acta Metall. Mater., Vol. 43, No.1, pp.101-106, 1995.
29. M. Frebel, K. Behler, “The growth kinetics of the discontinuous precipitation reaction in Mg Al alloys”, Metallurgical Transactions A, Vol. 8A, p621, 1977.
30. D. Bradai, P. Zieba, E. Bischoff, W. Gust, “Correlation between grain boundary misorientation and the discontinuous precipitation reaction in Mg 10 wt% Al alloy”, Materials Chemistry and Physics, Vol. 78, pp.222-226, 2002.
31. L. M. Klinger, Y. J. M. Brechet, G. R. Purdy, “On velocity and spacing selection in discontinuous precipitation Ⅰ. Simplified analytical approach”, Acta Mater., Vol. 45, No.12, pp.5005-5013, 1997.
32. L. Klinger, Y. Brechet, D. Duly, “On the influence of non-steady state velocity on interlamellae concentration profiles in discontinuous precipitation”, Scripta Materialia, Vol. 37, No. 8, pp.1237-1242, 1997.
33. Rajan Ambat, Naing Naing Aung, W. Zhou, “Evaluation of microstructural effects on corrosion behaviour of AZ91D magnesium alloy”, Corrosion Science, Vol.42, p.1433, 2000.
34. Guangling Song, Amanda L. Bowles, David H. StJohn, “Corrosion resistance ofaged die cast magnesium alloy AZ91D”, Materials Science and Engineering A366, p.74, 2004.
35. Guangling Song, Andrej Atrens, Xianliang Wu, Bo Zhang, ”Corrosion behaviour of AZ21, AZ501 and AZ91 in sodium chloride”, Corrosion Science, Vol.40, p.1769, 1998.
36. 鍾時俊、Oleg Demine、翁榮洲，微電弧氧化表面處理原理與應用，工業材料194期，pp. 176-180，2003.
37. Y. Han, S. H. Hong, K. W. Xu, “Porous nanocrystalline films by plasma electrolytic oxidation”, Surface and Coatings Technology, Vol.154, pp.314-318, 2002.
38. A. L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S. J. Dowey, “Plasma electrolysis for surface engineering”, Surface and Coatings Technology, Vol.122, pp.73-93, 1999.
39. Yong Han, Seong-Hyeon Hong, Kewei Xu, “Synthesis of nanocrystalline titania films by micro-arc oxidation”, Materials Letters, Vol.56, pp.744-747, 2002.
40. 汪建民，”材料分析”，材料科學叢書2，1998.
41. 洪敏雄，”工程材料實驗(Ⅰ)–金屬材料實驗，材料科學叢書3，1999.
42. Bernard A. Boukamp, “A package for impedance/admittance data analysis”, Solid State Ionics 18 & 19, pp.136-140, 1986.
43. H. Okamoto, J. Phase Equilibria, Vol.19, p598, 1998.
44. G. Song, A. Atrens, D. StJohn, J. Nairn and Y. Li, “The Electrochemical Corrosion of Pure Magnesium in 1N NaCl”, Corrosion Science, Vol.39, p.855, 1997.