【作者】 刘生发；

【导师】 王仲范；

【作者基本信息】 武汉理工大学 ， 车辆工程， 2004， 博士

【摘要】 本文以Mg-Al-Zn系最具有代表性的AZ91合金（Mg-9Al基）的组织细化作为研究方向，以溶质元素（Sr、Ca、Ce、Nd），原位TiC_p和电磁搅拌对AZ91的组织和性能的影响为主要研究内容，探讨AZ91的化学成分、组织结构和性能变化之间的关系，以提高合金的力学性能和改善腐蚀性能为目的。 研究了Sr对AZ91合金组织和性能的影响。结果表明，适量的Sr能细化合金的a晶粒。Sr增加合金的密度，有利于减少合金的显微缩松。Sr提高铸态与时效合金的室温强度性能，对延伸率的影响不大。探讨了Sr细化组织的机理。 研究了Ca对AZ91合金组织和力学性能的影响。少量的Ca可明显细化a晶粒并提高铸态与时效合金的室温强度性能，但对延伸率不利。提出了Ca细化组织的机制是：凝固过程中固／液界面前沿富集的Ca强烈抑制a晶粒的生长。 系统地研究了Ce、Nd对AZ91合金组织与性能的影响。结果表明，Ce、Nd均能显著细化合金的显微组织。同时，Ce、Nd与合金中的Al结合形成针状的Al3Ce和粒状的Al₃Nd化合物，两种化合物主要分布在晶界处。稀土元素的加入使β-Mg₁₇Al₁₂相由连续网状变为非连续分布。Ce、Nd明显提高铸态和时效合金的室温力学性能，对延伸率提高有限。提出了Ce、Nd细化组织的机制是：结晶过程中固／液界面前沿富集的Ce、Nd引起成分过冷增大，导致晶核数量增加。 首次系统地研究了适量的Ce、Ca、Sr对AZ91合金复合细化的工艺与机理。采用液淬方法和金相观察探讨了复合细化机制。 使用基于SHS原理的铸造接触反应法成功制备了Al-TiC中间合金。对反应进行了热力学计算，研究了反应合成的工艺参数。 首次系统地研究了以Al-TiC中间合金为AZ91合金的晶粒细化剂对显微组织和力学性能的影响。结果表明，原位TiC颗粒明显细化合金的a晶粒并提高合金的强度性能。探讨了原位TiC颗粒细化合金的机制。 首次研究了电磁搅拌对AZ91和AZ91+0.8％Ce合金显微组织的影响。结果表明，合适的输入电压使合金的a枝晶组织发生球团化和细化，β相的数量明显增多，使元素Zn在β相中的偏聚倾向降低。探讨了电磁搅拌和稀土Ce复合细化组织的机制。 采用静态失重法和极化曲线研究了Ce、Nd和Sr对AZ91合金在NaCl水溶液中的腐蚀性能。结果表明，Ce、Nd明显降低合金的腐蚀速率，而Sr对合金的腐蚀性能不利。探讨了显微组织与腐蚀性能之间的关系。更多还原

【Abstract】 Structural refinement during solidification of AZ91 alloy in Mg-Al-Zn system has been studied with a focus on the effects of solute elements, such as Sr, Ca, Ce, Nd, in situ TiC particulate and electromagnetic stirring on microstructures and mechanical properties. The relations of chemical composition, microstructures and properties are discussed to improve its mechanical properties and corrosion resistance.The effects of Sr on microstructures and properties of AZ91 alloy are investigated. The results show that Sr addition leads to the α grain refinement, the increase in density, that is, decrease of microporosity. In addition, Sr addition results in improving the ambient strength properties and little influencing elongation for as-cast and T6 condition AZ91 alloy. The role of Sr in grain refinement of AZ91 alloy is also discussed.The microstructures and mechanical properties of AZ91 alloy added minor Ca are investigated. The results indicate that the α grain is fined, the room temperature strength properties are improved, but elongation becomes poor due to Ca addition. The mechanism of grain refinement of Ca on AZ91 alloy is attributed to Ca at the solid/liquid interface front during solidification strongly restricts the α grain growth.The influences of Ce and Nd on the as-cast microstructures and mechanical properties of AZ91 alloy are systematically studied. The results reveal that microstructures are markedly fined by Ce and Nd addition, Ce and Nd are combined with Al and respectively form Al3Ce needles and Al3Nd particles near the grain boundaries. Meanwhile, the morphology of β -Mg17Al12 phase are changed from continuous network to discontinuous one. Ce and Nd addition significantly improve ambient strength properties of as cast and T6 condition AZ91 alloy, and slightly increase its elongation. The mechanism of grain refinement of Ce and Nd on AZ91 alloy is mainly attributed to the increasing of constitutional undercooling in a diffusion layer ahead of the advancing solid/liquid during solidification, which results in the increase of the number of nucleation because nucleants in the melt are more likely to survive and be activated in the constitutionally undercooled zone.The mechanism of grain refinement and technique of Ce addition combined Ca or Sr are systematically investigated. Water-quenching method and metallographic observation are used to study the mechanism of grain refinement.A master alloy of Al-TiC is successfully prepared by cast contact reaction based on SHS principle. Thermodynamic calculation of chemical reaction is made to study process parameters of reactive synthesis.The effects of the level of Al-TiC on microstructures and mechanical properties of AZ91 alloy are systematically investigated. The results show that in stiu TiC particulate can fine the α grain, which results in improving strength properties. In addition, the role of TiC particulate in grain refinement of alloy is discussed.The effects of electromagnetic stirring on microstructures and mechanical properties of AZ91 and AZ91+0.8%Ce alloy are studied. The results show that electromagnetic stirring with appropriate input voltage has an important role in the refinement and spheroidization of the α phase, and reduces the amount of β -Mg17Al12 and the segregation tendency of Zn element in the β-Mg17Al12 phase. The mechanism of structural refinement of electromagnetic stirring and Ce element on alloy is presented.The effects of corrosion resistance of Ce Nd and Sr on AZ91 alloy immersed in NaCl solution are investigated using static weight loss test and polarization curve measurement. The results show that Ce and Nd can significantly reduce corrosion rate of AZ91 alloy, but Sr is disadvantageous to corrosion resistance. The relation between the microstructures and corrosion resistance is discussed.更多还原