【作者】 刘宇；

【导师】 刘天模；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2008， 硕士

【摘要】 镁合金是一种很有吸引力的轻金属结构材料,其在交通工具、电子通信、航空航天等领域有着广阔的运用前景。镁是密排六方结构,只有有限的滑移系,室温下塑性成型能力差,同时也限制了镁合金的比强度的大幅度提高。目前细化晶粒成为优先考虑用来提高镁合金强度和塑性的一种有效手段。作为一种全新的深度塑性变形方式,在已有的研究中,对变通道角挤压工艺(CCAE)的微观组织演变、晶粒细化机制和CCAE变形后细晶粒镁合金的力学性能、热稳定性能的研究都未开展,而对CCAE变形工艺的理解也不够清晰。本文旨在研究CCAE变形过程中AZ31系镁合金的微观组织演变和变形后的力学性能以及CCAE变形工艺。重点讨论了CCAE变形过程中的晶粒细化机制和模型,变形后的室温压缩性能及断裂机制,变形后微观组织的热稳定性能,以及CCAE变形过程中的挤压力和应变。以期对AZ31系镁合金的CCAE变形机理和CCAE变形工艺本身进行初步研究和探讨。为此,选取了应用比较广泛的铸态AZ31镁合金作为研究对象。采用金相显微分析(OM)和X射线衍射分析等手段,对不同挤压温度下AZ31镁合金在CCAE变形过程中的显微组织和织构的演变规律进行了分析;采用室温力学性能测试,探讨了CCAE成型后AZ31镁合金的室温力学性能和室温下的断裂方式及断裂机理;采用透射电子显微技术(TEM),探讨了CCAE变形过程中AZ31镁合金的晶粒细化机制和模型;对经CCAE变形后的AZ31镁合金进行了退火处理,探讨了经CCAE变形后AZ31镁合金显微组织的热稳定性能;采用上限法对CCAE变形工艺进行了数值分析,对CCAE变形过程中挤压力和应变的计算进行了初步探讨。取得如下结果:AZ31镁合金经CCAE变形后,镁合金晶粒明显细化。变形后合金室温延伸率随晶粒细化而提高,屈服强度和硬度都随晶粒细化而提高,与Hall-Petch关系的趋势符合。在250~450℃温度范围内进行CCAE变形,AZ31镁合金的晶粒随变形温度的降低而减小。AZ31镁合金经CCAE热变形后,合金的室温强韧性得到综合改善。CCAE变形过程中AZ31镁合金的晶粒细化机制可以归结为在两不同内径的通道的交接处的剪切区的剪切作用引起的晶粒破碎和整个CCAE变形过程中发生的连续动态回复和再结晶(CDRR)。对于连续动态回复和再结晶,变形初期在粗晶粒内产生许多位错,位错会发生交互作用,重新排列形成位错界面以及亚晶界,而形成的位错界面以及亚晶界会进一步演化为小角度晶界和大角度晶界,镁合金得以细化。对挤压温度为250℃的挤压成型样进行晶体结构稳定性的实验,退火温度范围为200~500℃,保温时间为30min,随着退火温度的升高,晶粒不断长大,在200~275℃时,晶粒随温度上升呈平缓线性长大,从275~400℃晶粒长大趋于平缓,400℃以后,晶粒迅速长大。在这三个温度区间,其晶粒长大激活能分别为:73.6,16.7,105.1kJ/mol。在高温范围内的激活能介于纯镁的晶格自扩散能(QL=135kJ/mol)和晶界扩散能(Qgb=92kJ/mol)之间。在低温范围内,其晶粒长大激活能小于纯镁的晶界扩散能,并且与之比较接近。但是在中温范围内,晶粒长大激活能远小于QL和Qgb,其大约相当于0.18Qgb。在经过CCAE热变形后细化的晶粒在随后的退火过程中,晶粒长大的驱动力来源可能是多方面的,但最主要的而且在所有晶粒长大过程中都存在的驱动力来源是晶界的界面能。对CCAE变形过程中的挤压力和应变进行了上限法分析,发现理论计算和实验结果基本吻合。更多还原

【Abstract】 Magnesium alloys have high potential as light metal structural materials for transport, electronic communications, aeronautics, astronautics and other applications due to their high specific properties, low density and high damping capacity. However, magnesium alloys have poor formability and limit ductility at room temperature ascribed to their hexagonal closed-packed (HCP) crystal structure with limited slip systems. Grain refinement is an important practice to improve the synthetic mechanical properties of magnesium alloy. As a completely new severe plastic deformation method, in recent reserch, there is still neither a detailed discription of microstructure evolution and grain refinement mechanism during CCAE process nor an investigation on the mechanical properties and thermal stability of AZ31 magnesium alloy after deformation. Moreover, the understanding for the CCAE process is still not clear.The purpose of this paper is to investigate the microstructure evolution, mechanical properties of AZ31 magnesium alloy by CCAE. And placed an emphasis on the understanding of grain refinement mechanism and model during CCAE, the compression deformation and fracture mechanism at room temperature, the thermal stability of deformed microstructure, the extrusion force and strain of CCAE, in order to provide a preliminary investigation and discussion for the CCAE deformation mechnism and process.Therefore, in this study as-casted AZ31 magnesium alloy were elected as CCAE deformation material for investigation. The AZ31 magnesium alloy microstructure and texture evolution were analyzed by Optical microscopy (OM), the mechanical properties and fracture way and mechanism were discussed by mechanical test at room temperature, the grain refinement mechanism and model were explored by transmission electron microscopy (TEM), the thermal stability of CCAE deformed AZ31 microstructure was investigated by annealing process, and the extrusion force and strain during CCAE process were calculated by upper-bound analysis. The main results can be summarized as follows:For AZ31 magnesium alloy, the grains were refined effectively after CCAE. The ductility, strength and microhardness were improved with the grain refinement, which is consistent with Hall-Petch relationship. The effect of grain refinement was improved with lowering the CCAE temperature. Both the ductility and synthetic mechanical properties of AZ31 magnesium alloy can be improved by CCAE.Grain refinement mechnism of AZ31 alloy during CCAE can be described as grain fragmentation in the shear zone and continuous dynamic recovery and recrystallization (CDRR). For the CDRR, at the initial stage of CCAE deformation, dislocation density increases and then dislocations are arranged into dislocation boundaries and sub-grain boundaries. With further deformation, these sub-boundaries evolve to low angle grain boundaries (LAGBs) and high angle grain boundaries (HAGBs), therefore the grains can be refined.To investigate the thermal stability of CCAE deformed AZ31 magnesium alloy, the 250℃deformed specimen was annealed at 200~500℃for 30min. The grain continually grew with elevatory temperature. The grain growth appeared gentle linear trend at 200~275℃, and the growth of grain appeared gentle trend at 275~400℃, after 400℃, the grain grew rapidly. In the three temperature intervals, the active energy of grain growth were 73.6,16.7,105.1kJ/mol respectively. In the high temperature range, the activation energy is between that for lattice self-diffusion (QL=135kJ/mol) in pure Mg and that for grain boundary diffusion (Qgb=92kJ/mol). In the low temperature range, it is less than Qgb but close to it. In the intermediate temperature range, on the hand, the Q value is considerably lower than QL and Qgb, corresponding to ~0.18Qgb. The driving forces sources of grain growth may be many ways, but the main source is interfacial energy of grain boundary.The extrusion force and strain during CCAE process were calculated by upper-bound analysis, and the extrusion force calculated value is well agree with the experiment result.更多还原