【作者】 张真；

【导师】 汪明朴；

【作者基本信息】 中南大学 ， 材料物理与化学， 2011， 博士

【摘要】 镁是地球上储量最丰富的元素之一,而镁合金是其最主要的利用方式,它具有较高的强度、比刚度、减震性、电磁屏蔽性、易切削性和易回收性等一系列优点,被誉为“21世纪绿色工程结构材料”。但由于镁及镁合金具有HCP晶体结构,加工成型性不佳,这严重阻碍了在高速交通运输领域、航空航天领域和电子信息领域有广泛应用前景的变形镁合金的开发。针对镁合金难以加工成形这一难题,本文以工业上广泛应用的AZ31B商业镁合金为原料,对镁合金塑性变形的相关基础问题进行了研究。全文先后进行了等温恒速压缩热模拟和轧制实验,对不同变形条件的组织结构进行了观察,并分别构建了AZ31B镁合金低温、高温下的变形方程,同时对该合金轧制过程的组织织构演变、孪生、动态回复及动态再结晶行为进行了详细的研究,得出以下主要结果：1 AZ31B合金的等温压缩试验结果表明,样品取向对流变曲线的影响在低温条件下较为明显,这种影响来源于镁合金板材所具有的基面织构,是由压缩过程中不同类型的孪生行为所造成的,同时,取向造成的差异在临界应变εc后可以忽略,并且随着温度升高时,样品取向对压缩行为的影响逐渐减小。2 AZ31B合金不同变形条件的组织观察结果显示,合金低温变形时以基面滑移和机械孪生为主,部分孪晶交截处产生了动态再结晶；而高温下的变形组织则表现出连续动态再结晶和不连续动态再结晶的特征,其流变曲线可用加工硬化、过渡、软化和稳态流变四个阶段加以描述。同时,流变曲线的峰值应力随变形温度降低和应变速率增加而增大。3 AZ31B合金等温压缩塑性变形的流变行为可分别用双曲正弦函数修正的Arrhenius关系描述,所求得的本构方程分别为：低温ε=5.6[sinh(0.0296676)]4.5exp(-94300/RT)高温ε=5.7×107[sinh(0.0296676)]2.7exp(-127500/Rt)4 AZ31B合金铸轧板单道次热轧过程中,变形量较小的阶段,孪生起主导作用,分别产生了透镜状的{1012}孪晶和细长的{1011}窄孪晶,其中{1012}孪晶的产生使得基面织构突然增大,但此类孪晶内部不易发生塑性变形,因而不会发生再结晶；而细长的{1011}窄孪晶形核后,其内部的位错滑移能够被大量激活激活,因而容易成为动态再结晶的优先形核点。热轧中等应变阶段,回复和不连续动态再结晶起着重要的作用,晶粒内部形成了各种形貌的亚晶组织和锯齿状特征的晶界,基面滑移和基面位错在此过程中起着十分重要的作用。随着变形的继续增大,非基面滑移能够更为广泛的发生,晶粒内部形成了位错胞状缠结组织,为连续动态再结晶提供了结构基础。5剪切带是镁合金板材轧制过程中必经阶段,这是由外应力状态所决定的；但是不同的组织、织构状态会对剪切带的形成方式起到重要影响。织构和晶粒尺寸对剪切带形成机制的影响可以归结为：基面织构越强,孪生在剪切带中所起的作用就越大；反之,动态再结晶所起的作用就越大。晶粒尺寸越大,孪生在剪切带形成中期的作用越显著；反之,动态再结晶在剪切带形成中起的作用越明显。6本文利用离散极图对10%和20%热轧样品中孪晶类型及主要孪生要素进行分析,并对所有等价孪生系的Schmid因子进行了计算,结果表明,AZ31B合金热轧过程中起主导作用的{1012}、{1011)孪生行为均取决于宏观外应力,并遵从Schmid定律；同时少量的{1012}孪晶会起着协调局部应变、使晶粒变形均匀的作用。7本文采用了一种新的Euler空间分析方法对10%和20%热轧过程中的孪生行为进行了分析,此方法将孪生过程考虑为沿晶体学坐标轴的连续旋转过程,通过比对孪晶系Euler指数计算值和测量值来确定孪晶类型,并根据晶粒的Euler坐标计算各孪生系的Schmid因子大小。此方法与极图分析方法得出的结论基本吻合,其优势在于可以严格的确定孪生系的各个要素,并精确的进行取向相关的计算,如孪生Schmid因子大小等。更多还原

【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.更多还原