摘要
本研究以商用AZ91鎂合金作為基礎合金，利用添加少量Sn與高擠型比作用，作為發展輕量高強度的鍛造鎂合金Mg-9Al-Zn-xSn (x = 0-3)。Sn添加會形成Mg2Sn析出物抑制鑄造中，共晶相-Mg17Al12形成，這些未能析出的鋁原子，將固溶在α-Mg樹枝相內造成晶格常數縮小，使得XRD曲線呈現繞射峰偏移現象，同時也造成樹枝相溶化溫度降低。利用Mg2Sn析出物，在均質化與預時效兩種熱處理與擠型的多重晶粒細化與抑制晶界移動作用，可以獲得5μm細晶粒的微細結構。對照傳統鑄造AZ91進行同樣均質化與時效處理，添加3wt.%-Sn所產生析出強化與微結構細化作用，其抗拉強度可以到達379 MPa，伸長率仍有7.7% ，但是經預時效後擠型，由於析出物的粗化現象造成延性些微下降。因此，室溫拉伸顯示Mg-9Al-Zn透過預時效後擠型可以獲得較好機械性能，但是添加Sn則是以均質化後擠型在強度與延性擁有較好平衡，其中Mg-9Al-Zn-3Sn的機械性性能最好。此外，細小Mg2Sn析出物在高溫下抑制α-Mg晶粒成長，穩定細晶結構，使合金具超塑性能。但是從超塑性破斷面觀察，細小Mg2Sn析出物阻礙晶界滑移，成為孔隙形成位置，造成在高應變速率具有較低延伸率。從實驗結果計算活化能，Mg-9Al-Zn-3Sn可達到126 kJ/mol.，高於Mg-9Al-Zn的114 kJ/mol.，顯示高含量Sn所形成Mg2Sn同時阻礙晶界滑移，因此在添加1wt.%與2wt.%Sn有較高超塑性性能。

In this study, low-weight, high-strength and ductile wrought Mg-9Al-Zn-xSn (x =1-3) Mg alloys were developed, using a conventional cast AZ91 base alloy specially with the addition of small amounts of Sn and the application of high-ratio extrusion. In the as-cast state, by Sn alloyed, the additional precipitation of Mg2Sn suppressed the homogeneous nucleation of detrimental eutectic -Mg17Al12 but drove the dissolution of more Al into primary -Mg, as verified from the shift of XRD peaks and a lowered liquidus temperature.
In the high-ratio extruded Mg-9-Al-Zn-xSn alloys particularly with 2-3 wt.% Sn and with either pre-homogenization or pre-homogenization/pre-aging, the size of coarse grains was very effectively reduced to only 5 m in consequence of multiplied recrystallization and retarded boundary migration by a large amount of small Mg2Sn precipitates. Compared with a cast or homogenized/aged cast AZ91 alloy, the intense precipitation hardening and the effective microstructure refinement accordingly yielded markedly improved mechanical properties including a high strength of about 379 MPa and elongation of about 7.7%; a slight decrease in ductility was found only in the extruded Mg-9Al-Zn-3Sn alloy with pr-aging due to detrimental precipitate coarsening. These small Mg2Sn particles effectively retarded α-Mg grain growth, which would assist the alloys in retaining fine microstructures and developing high superplasticity. Small particles, in particular Mg2Sn as clearly seen on the fractured surfaces, impeded grain boundary sliding, which would detrimentally induce cavity formation and result in relatively low elongation at high strain rates.

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