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Basic Research on the Plastic Deformation Behavior of AZ31B Mg Alloy

Author: ZhangZhen
Tutor: WangMingPu
School: Central South University
Course: Materials Physics and Chemistry
Keywords: AZ31B magnesium alloy isothermal compression hot-rolling twinning dynamic recovery dynamic recrystllization
CLC: TG146.22
Type: PhD thesis
Year: 2011
Downloads: 163
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Abstract

Magnesium is one of the most abundant elements on the earth. Mg alloy, which is the mostly application mode of mg resources, has high strength and specific-strength, good damping property, electromagnetic shielding and free cutting property etc. It has a good prospect for application in the field of modern transportation, aeronautics-astronautics and information technology. And it is regarded as’the green engineering structure materials’in the 21th century. However, Mg alloy exhibits limited ductility due to its hcp structure, which slows the development of mg alloy.Aiming to improve the poor formability of Mg alloy, the thesis concentrated on deformation mechanism of Mg alloys. AZ31B commercial alloy was chosen as experiment material in the present work. Uniaxial compression and hot-rolling experiments were conducted, and the deformed microstructure in different working condition was observed in the present work. In addition, constitutive equations for low and high temperatures deformation was constructed, and twinning, dynamic recovery and dynamic recrystallization were analyzed as well. The main results are as follows:1. Isothermal compression experiment shows that the differences between different oriented samples were more distinctive at low temperature. The differences came from the different types of twinning, which was influenced by the basal texture. Furthermore, the differences were imperceptible afterεc, and with the increase of working temperature, the differences became less.2. Microstructure observation indicated that basal slip and twinning dominated at low temperature, meanwhile DRX and CDRX play an important role at higher temperatures. The flow curve displayed four different stages, which could be described as strain-hardening, transition, softening and steady flow respectively. The peak stress usually decreases with the increase of temperature and with the decrease of strain-rate.3. The plastic flow behavior could be expressed in Arrhenius relationship, and the constitutive equation could be given asLow temperatureε=5.6[sinh(0.029667σ)]4.5 exp(-94300/RT)High temperatureε=5.7×107[sinh(0.029667σ)]2.7exp(-127500/RT)4. Twinning played the most important role in the initial strain stage of rolling process, producing thick lenticular{1012} twins and thin {1011} twins. Extensive initiation of{1012} twins increase the basal texture significantly, however dislocation slip could not easily proceed in the twinned region, thus arresting DRX in these twinned region. Dislocation slip could be easily activated in{1011} twinned region, providing preferred sites for DRX. In the moderate strain stage, some subgrain and serrated grain boundaries were produced, which indicated the process of DRV and DDRX. With the increase of rolling reduction, non-basal slip was activated more widely, forming dislocation cellular structures, which may build a structure foundation for CDRX.5. Shearing band formation was an essential stage in rolling process, which was decided by external stress state. However, elements such as grain size, texture condition would easily exert an influence on shearing bands forming mechanism. Their influences could be summarized as follows,Sharper basal texture could promote mechanic twinning, while weaker texture and small grain size may facilitate DRX, making them the most important element respectively in the shearing bands formation.6. Twinning types and their elements were analyzed based on Discrete Pole Figure (DPF) for the 10% and 20% rolled samples. It indicated that the dominating twinning system usually followed Schmid law, meanwhile some minor twins was able to activated accomdating the complicated strain concentration and making strain more homogeneous through the whole microstructure.7. A new twinning analysing method based on Euler space was adopted for for 10% and 20% rolled samples, in which twinning processes were considered consecutive rotation along crystal axes. Twinning types could be decided comparing the detected Euler index with calculated ones, and the Schmid factor could be rigorous defined. This method is successful in rigorous twinning identification and Schmid factor calculation. The results were similar with those drawn from DPF method.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metallic materials > Non - ferrous metals and their alloys > Light non-ferrous metals and their alloys > Magnesium
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