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1、摘 要镁合金因其质量轻、比强度高，在汽车工业中的应用越来越广泛，因此如何安全使用镁合金已受到人们的普遍关注。疲劳是各种工程构件服役期间的主要失效形式之一，对于镁合金结构件亦不例外。因此，研究镁合金的疲劳变形和断裂行为不仅具有理论意义，而且也具有一定的实用价值。本文主要针对不同处理状态的挤压变形AZ61合金的低周疲劳行为进行了系统的研究，以期为此种镁合金的抗疲劳设计和合理使用提供可靠的理论依据。低周疲劳实验结果表明，挤压态和时效态AZ61镁合金在0.3%的外加总应变幅下表现为循环稳定其后发生循环应变硬化，在其他外加总应变幅下则呈现循环应变硬化，而固溶态以及固溶+时效态AZ61镁合金在所有外加总应
2、变幅下均表现为循环应变硬化；时效处理可以有效地提高挤压变形AZ61镁合金在较低外加总应变幅区间的疲劳寿命，而固溶+时效处理则降低挤压变形AZ61镁合金的疲劳寿命；对于不同处理状态的挤压变形AZ61镁合金，其弹性应变幅、塑性应变幅与疲劳断裂时的载荷反向周次之间呈线性关系，并可分别用Basquin和Coffin-Manson公式来描述，同时其循环应力幅与塑性应变幅之间亦呈线性关系；不同处理状态的挤压变形AZ61镁合金在较高的外加总应变幅下进行疲劳变形时，其循环滞后回线上压缩变形部分的宽度大于拉伸变形部分的宽度，且最大拉伸应力明显高于最大压缩应力，即表现出明显的拉-压不对称循环变形行为。疲劳断口形貌
3、分析结果表明，在总应变控制的疲劳加载条件下，不同加工处理状态的热挤压AZ61镁合金的疲劳裂纹均是以穿晶方式萌生于试样表面，然后以穿晶方式扩展并呈现解理断裂特征。关键词：镁合金，热处理，循环应力响应，疲劳寿命，疲劳断裂 / 文档可自由编辑打印Low-Cycle Fatigue Behavior of Extruded AZ61 Magnesium AlloyAbstractDue to their light weight and high specific strength, magnesium alloys have been widely used in automobile indust
4、ry. Thus, how to utilize safely magnesium alloys has been paid much more attention. Fatigue is a main failure form of various structural components during operation. For the magnesium alloy components, the same case is also true. Therefore, the investigation concerning fatigue and fracture behavior
5、of magnesium alloys is of both academic and practical significance. In this investigation, the strain-controlled fatigue deformation and fracture behaviors of extruded AZ61 alloys with different treatment states has been studied in order to provide a reliable theoretical foundation for both fatigue
6、resistant design and reasonable usage of this magnesium alloy.The results of low-cycle fatigue tests reveal that the as-extruded and aged AZ61 alloys exhibit the stable cyclic stress response followed by cyclic strain softening at the total strain amplitude of 0.3%, while show the cyclic strain hard
7、ening at other imposed total strain amplitudes. For the extruded AZ61 alloys subjected to solid solution and solution plus aging treatment, the cyclic strain hardening can be observed at all total strain amplitudes used in this investigation. It is noted that the aging treatment can enhance the fati
8、gue lives of the alloy at lower total strain amplitudes, while the solution plus aging treatment leads to a reduction in the fatigue lives of the extruded AZ61 alloy. For the extruded AZ61 alloys with different treatment states, the relations between elastic strain amplitude, plastic strain amplitud
9、e and reversals to failure are linear and can be described by Basquin and Coffin- Manson equations, respectively. In addition, a linear relationship between cyclic stress amplitude and plastic strain amplitude is also noted for the extruded AZ61 alloys with different treatment states. It has also be
10、en observed that when the extruded AZ61 alloys with different treatment states is subjected to fatigue deformation at the higher total strain amplitudes, the width of the s-e hysteresis loop in the tensile direction is greater than that in compressive direction, and the maximum tensile stress is sig
11、nificantly higher than the maximum compressive stress. The alloys exhibit the pronounced anisotropic deformation behavior in the direction of tension and compression.The observations on fracture surface of fatigued specimens reveal that for extruded AZ61 alloys with different treatment states, the f
12、atigue cracks initiate in a transgranular mode at the surface of fatigue specimens, and propagate transgranularly. In addition, the cleavage fracture feature can be found in the fatigue crack growth region.Key Words：Magnesium Alloy，Heat Treatment，Cyclic Stress Response，Fatigue Life，Fatigue Fracture目
13、 录摘 要IAbstractII第一章 绪论11.1 镁合金的分类11.2 镁合金的特点及其应用31.3 镁合金的疲劳变形行为51.4 镁合金的疲劳断裂行为81.5 影响镁合金疲劳行为的因素91.6 提高镁合金疲劳性能的途径131.7 本课题的提出及意义14第二章 实验材料、设备与方法152.1 实验材料152.2 实验设备152.3 实验内容与方法152.3.1 镁合金的热挤压152.3.2 镁合金的热处理152.3.3 显微组织观察162.3.4 疲劳试样制备162.3.5 低周疲劳实验162.2.6 断口形貌观察17第三章 实验结果分析183.1 合金的显微组织183.2 合金的循环应力
14、响应行为193.2.1 不同处理状态合金的循环应力响应曲线193.2.2 热处理对合金循环应力响应行为的影响223.3 合金的疲劳寿命行为263.3.1 不同处理状态合金的疲劳寿命行为263.3.2 热处理对合金疲劳寿命的影响293.4 合金的循环应力-应变行为303.4.1 循环应力-应变曲线303.4.2 循环滞后回线323.5 合金的疲劳断裂行为37第四章 实验结果讨论383.6.1 循环应力响应行为403.6.2 疲劳寿命行为413.6.3 循环应力-应变行为423.6.4 疲劳断裂行为43第四章 结论45参考文献46在学研究成果50致 谢51第一章 绪论1.1镁及镁合金概述镁是目前实
15、际应用中最轻的金属结构材料。纯镁的密度为1.74 g/cm3，常见镁合金的密度从1.3-1.9 g/cm3不等1姜。在目前已获得工程应用的金属结构材料中，镁合金的密度是最低的，仅为铝合金的2/3，钢的1/4。金属镁不仅质轻而且含量丰富、分布广泛，约占地壳总重量的2%，广泛分布于陆地、海洋和湖泊中。纯镁的切削性能十分良好，但机械性能低，不能做结构材料使用。此外由于镁的电化学活性很强，电动序在常用金属中占最低一位，所以抗蚀性很差，在空气中易氧化，在潮气、海水及酸、盐溶液等特殊环境中易腐蚀，与其它金属接触时还会发生接触腐蚀。因此在工业上，纯镁除了少部分用于化学工业，仪表制造及军事工业外，主要用于制造
16、镁合金，及生产镁铝合金的合金元素。目前世界上的原镁产量约有50%用于制造镁合金，33%用于制造铝合金。其余的用做生产某些合金的还原剂、脱氧剂及变质剂等。镁及镁合金是典型的密排六方结构材料，其弹性模量E为45GPa，切变模量G为17GPa，比弹性模量E/r为25GPacm3/g，均小于铝（相应值分别为71GPa、26GPa、27GPacm3/g）。与其它金属材料以及工程塑料相比，镁合金具有很高的比强度和比刚度2姜。镁合金还具有远大于其它合金的阻尼减震性能。例如镁合金KIA(含0.7%镁)在20时的减震能量不仅远高于铝合金，而且还是灰口铸铁的4.6倍AM50。此外，镁合金的低熔点、低动力学粘度、低比热容、低相变
