AZ80鎂合金析出硬化與超塑性研究

Study of Precipitation Hardening and Superplasticity of AZ80 Magnesium Alloy

賴威任(Wei-Jen Lai)

指導教授 ： 王文雄

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2008年)

https://doi.org/10.6342/NTU.2008.01485

摘要

本研究主要探討AZ80鎂合金之析出硬化反應以及經不同熱軋製程處理後之超塑性特性。析出硬化實驗主要針對125-300℃之間進行研究，透過硬度測試(Microhardness test)及拉伸試驗(Tensile test)探討析出物對機械性質之影響。另外利用XRD、OM、SEM及TEM對析出物做一系列仔細的分析。超塑性實驗主要針對三種不同熱軋方向的試片進行超塑性實驗，溫度選擇在200-400℃，拉伸速率則選擇3×10-3、1×10-3及3×10-4(s-1)共三個應變速率。實驗結果則利用OM及SEM作進一步分析，了解溫度及應變速率對顯微組織演變之影響。
析出硬化實驗結果顯示，150-300℃各時效溫度材料均有明顯之硬化。其中以175℃/256 h時效處理之試片有最佳之硬度，約增加38%。顯示AZ80經時效處理硬度增加有限，不同於一般可析出硬化之鋁合金。而這主要的原因則歸咎於其析出物(Mg17Al12)之特性。經觀察，此析出物依其析出類型可分為連續及不連續析出物，按其形貌則可再細部加以區分為層狀、橢球狀、晶界間析出物、費德曼組織(Widmanstätten structure)以及不規則板狀結構。由與這些析出物均無法有效的阻擋差排移動，因此硬化效果不佳。而不同形貌之析出物在不同時效溫度其大小及密度也會有所不同，此即造成不同硬度之主因。
超塑性實驗結果顯示，材料在300℃/3×10-4s-1時有最佳之伸長率，約350%。在此溫度下伸長率激增的原因可分為兩部分：一是由於300℃接近材料再結晶溫度(Recrystallization temperature)，材料產生動態再結晶(Dynamic recrystallization)提供足夠之晶粒，使其依靠晶界滑移(Grain boundary sliding)產生大量變形；另一原因是由於300℃材料很快便析出Mg17Al12，而此溫度下大部份的析出物都在晶界處生成，因此可以有效的阻礙晶粒的成長，維持細晶組織。在這兩者同時作用之下材料得以獲得高的伸長率。

關鍵字

鎂合金,AZ80,析出硬化,超塑性,SEM,TEM

並列摘要

This study mainly focuses on precipitation hardening behavior and superplasticity of AZ80 magnesium alloy. The precipitation hardening experiment was conducted between 125-300℃. The influence of the precipitate on mechanical properties was measured by microhardness test and tensile test. The precipitate was investigated in detail by XRD, OM, SEM and TEM. The superplastic experiment focuses on three different rolling directions. The temperature ranges from 200-400℃ and the strain rates were 3×10-3、1×10-3 and 3×10-4(s-1). The results were analyzed by OM and SEM to further understand the influence of temperature and strain rate on microstructural evolution.
The result of the precipitation hardening experiment indicates that each aging temperature between 150-300℃ shows hardening effect. The highest hardness is occurred at 175℃/256 h and the hardness increment is about 38%. The result shows that the hardness increment is low compared with precipitation-hardenable aluminum alloys. This is influenced by the nature of Mg17Al12 precipitates. The precipitates can be divided into continuous and discontinuous precipitates by their forming mechanism. They can be further divided into lamellar, elliptical, intergranular, Widmanstätten structure and irregular slab by their morphologies. Because the precipitate can not effectively resist dislocation moves, the hardening effect is poor. At different aging temperature the morphology, the size, and the distribution density of Mg17Al12 precipitates are also different. These reasons then lead to different hardness.
The result of superplastic experiment shows that the material has best elongation (about 350%) at 300℃/3×10-4s-1. The rapid elongation increase at this temperature has two reasons: one is because 300℃ is close to the recrystallization temperature of the material, dynamic recrystallization then occurs and provides enough grains for grain boundary sliding which produces large deformation; the other reason is because Mg17Al12 precipitates rapidly at 300℃ and most of them are formed on grain boundaries, they can effectively impede grain growth to remain the fine grain structure. These two effects appear at the same time and contribute to high elongation.

並列關鍵字

magnesium alloy, AZ80, precipitation hardening, superplasticity, SEM, TEM

參考文獻

2. H. Watanabe, H. Tsutsui, T. Mulcai, M. Kohzu, S. Tanabe and K, Higashi, “Deformation mechanism in a coarse-grained Mg-Al-Zn alloy at elevated temperature”, International Journal of Plasticity, 17 (2001) 387-397.

3. S. Celotto and T. J. Bastow, “Study of precipitation in aged binary Mg-Al and tenary Mg-Al-Zn alloys using Al NMR spectroscopy”, Acta Materialia, 49 (2001) 41-51.

4. S.R. Agnew, P. Mehrotra, T. M.Lillo, G.M. Stoica and P.K. Liaw, “Crystallographic texture evolution of three wrought magnesium alloys during equal channel angular extrusion”, Materials Science and Engineering A, 408 (2005) 72-78.

5. Y. N. Wang, C.I. Chang, C.J. Lee, H.K. Lin and J. C. Huang, “Texture and weak grain size dependence in friction stir processed Mg-Al-Zn alloy”. Scripta Materialia, 55 (2006) 637-640.

6. M.M. Myshlyaev, H.J. McQueen, A. Mwembla and E. Konopleva, “Twinning, dynamic recovery and recrystallization hot Mg-Al-Zn alloy”, Materials Science and Engineering A, 337 (2002) 121-133.