热变形对连续铸轧AZ31B镁合金微观组织及力学性能的影响

【作者】 李树梅；

【导师】 汪明朴；

【作者基本信息】 中南大学 ， 材料物理与化学， 2010， 硕士

【摘要】 本论文对均匀化后的连续铸轧AZ31B镁合金板坯进行了热压缩试验及热轧试验,并分别建立了AZ31B合金高温和低温的流变应力本构方程；并利用维氏硬度检测、光学显微镜、透射电子显微镜、X射线衍射分析、EBSD等技术对其性能,显微组织结构及孪生,动态再结晶行为进行了研究,结果表明：1.AZ31B镁合金高温压缩变形真应力-真应变曲线具有动态再结晶特征,可用加工硬化、过渡、软化和稳态流变四个阶段加以描述。峰值应力随变形温度降低和应变速率增加而增大。变形温度为100℃且应变速率大于0.1s-1时试样发生宏观开裂；2.AZ31B合金高温和低温压缩变形流变行为可分别用双曲正弦函数修正的Arrhenius关系和Z参数描述,所求得本构方程分别为：低温ε=5.6288[sinh(0.029667σ)]4.5430 exp(-94352／RT)高温ε=5.7195×107[sinh(0.029667σ)]2.658 exp(-127524／RT)3.通过金相组织发现,AZ31B合金低温变形时以基面滑移和机械孪生为主,高温变形以位错滑移,交滑移及攀移为主；低温下动态再结晶机制为孪生诱导再结晶机制,而高温下则为连续动态再结晶机制；4.TEM观察发现低温变形时孪晶交截区内有大量的位错,且DRX核心在此形成,表明AZ31B镁合金低温孪生动态再结晶存在孪晶-孪晶交截形核的动态再结晶模式。5.热轧实验表明,AZ31B合金板坯具有良好的热轧性能：在375℃条件下热轧开坯,一道次压下量可高达60%。随着热轧变形量的增加,AZ31B合金板坯退火后的晶粒减小,当变形量为60%时,300℃退火1小时后晶粒尺寸仅为4.3μm。6.当晶粒尺寸较大时,小变形阶段,AZ31B合金的塑性变形机制主要为孪生机制。随着变形量的增加,孪晶数量逐渐减少,孪生机制逐渐让位与位错滑移机制；7.将变形60%的AZ31B合金于375℃退火30min,获得晶粒尺寸为8μm。再对其进行小变形,此时,主导变形机制为位错滑移。而将变形60%的AZ31B合金于460℃退火2小时使之与铸轧均匀化后晶粒尺寸相当,再将其热轧10%,组织再次出现大量孪晶,说明粗大晶粒容易发生孪生而小晶粒更易滑移变形。8.受{1012}孪生影响,小变形量时基面织构迅速减弱,而后,随着位错滑移逐渐增加,基面织构增强；变形量增加到50%时,基面织构出现向轧向延伸的趋势,说明此时已经发生了非基面位错的滑移。更多还原

【Abstract】 AZ31B Mg alloy plate blanks which were fabricated by twin-roll casting technology has been homogenized, and then hot compression tests and hot-rolling tests were carried out to analyze the hot deformation behavior of AZ31B Mg alloys. The flow stress constitutive equations for high temperature and low temperature were established respectively, and the performance and microstructure of the alloy as well as its dynamic recrystallization mechanisms have been analyzed by Vickers hardness testing, optical microscope, transmission electron microscope, X-ray diffraction, and Electronic Back Scattering Diffraction. Finally, the conclusions were obtained are as follows:1. The true stress-strain curves of AZ31B magnesuim alloy which have a characteristc of dynamic recrystallization are composed of four different stages, i.e., work hardening, transition, softening and steady stages. The peak stress increases with the decrease of deformation temperature and the increase of strain rate. The specimens cracked when the temperatures lower than 100℃and strain rates higher than 0.1s-1.2. The flow stress constitutive equations for high temperature and low temperature can be described by the Arrhenius equation which is ameliorated by hyperbolic sine fuction and Z parameter, and the equations are obtained as follows: Low temperatureε=5.6288[sinh(0.029667σ)]4.5430 exp(-94352/RT) High temperatureε=5.7195×107[sinh(0.029667σ)]2.658 exp(-127524/RT)3. Through OM observing, it was found that the main mechanisms for low temperature of AZ31B alloy are basal plane slip and twinning, while dislocation slip, cross slip and climb for high temperature; the dynamic recrystallization mechanism is twining dynamic recrystallization at low temperature. However, continuous dynamic recrystallization mechanisum is operating when the temperature is higher.4. Twins intersected with each other and tangled dislocations located on twins-intersected zone, further more, a few DRX nuclei and tiny DRX grains were observed in the twinning band and twins-intersections, these imply that the main places where DRX nuclei formed are in twin boundaries and twins-intersection zones.5. The hot-rolled experiments represented that the AZ31B alloy sheet has good hot-rolled property that it can reduce by 60% one pass when be rolled at 375℃. With the increase of reduction, the scale of as-annealing grains decrease gradually, and the grains were only 4.3μm, when the reduction reached up to 60%.6. When the grain size is large, the main deformation mechanism of the AZ31B alloy is twinning during the low reduction stage. In pace with the increase of the reduction, the dislocation gliding become the dominant mechanism instead of twinning.7. The grain size is 8μm when the 60%-reduction AZ31B alloy sheet has annealed at 375℃for 30min. The small grain size hindered the twinning, so the dominant mechanism during 10% reduction stage becomes the dislocation gliding. However, changing the annealing conductions to enable the grain size to grow up to 20μm then the dominant deformation mechanisms go back to twinning again, these explain that twinning is easier than dislocation gliding in coarse grains while the opposite in tiny grains.8. Influenced by{1012} twins, the texture of (0001) plane get down suddenly during small deformation, and then get up slowly with the increase of deformation.更多还原