【作者】 韩剑；

【导师】 戴起勋；

【作者基本信息】 江苏大学 ， 材料学， 2009， 硕士

【摘要】 7xxx系铝合金具有高的比强度和刚度、易加工、较好的抗应力腐蚀能力等特性,在飞机关键部件上得以应用。本文的研究内容作为863高技术研究项目研究内容的一部分,主要以7075-T651铝合金为基础来探究热处理后7xxx系高强铝合金的疲劳断裂性能,附带进行拉伸断裂性能的测试,并使用扫描电子显微镜观察其断口形貌,分析疲劳断裂机理;同时为了改善7xxx系铝合金的断裂性能,又以7055铝合金为研究对象,通过添加稀土Y,采用坩埚炉熔炼,借助金相分析、X—射线衍射分析、扫描电子显微镜分析以及力学性能测试等分析手段,来研究稀土Y对于7055合金微观组织和力学性能的影响机理。为日后本课题后序7xxx系铝合金的断裂研究打好基础。7075-T651合金断裂性能的研究表明:合金非轧向拉伸断裂有着准解理和韧窝断裂的混合特征。疲劳试验的S-N曲线没有水平部分,只是随着应力的降低,循环周次不断增大,估算的疲劳极限为223MPa。疲劳裂纹都萌生于合金表面或内部夹杂缺陷处,裂纹扩展中产生大量疲劳辉纹,同时观察到轮胎花样,疲劳台阶,二次裂纹等其它一些疲劳特征。疲劳瞬断区都具有静拉伸断口的微观特征,表现为脆性和延性的混合断裂。合金当中的析出相粒子对合金的疲劳性能都产生一定影响。具体而言,粗大的析出相粒子可能会导致裂纹萌生,而细小弥散分布的微小析出相和对合金的疲劳性能的有着积极的影响。7055合金微合金化结果表明:将稀土元素Y加入7055合金当中可以形成YAl₃与Al₆Cu₆Y等化合物相。Y加入到7055合金后虽然牺牲较少的拉伸强度和硬度,却大幅提高了合金的韧性。同时Y的添加显著地细化合金在铸态下的显微组织,抑制了粗大共晶组织形成,使二次枝晶间距减小,原来骨骼状的第二相变得细化并在一定Y含量下出现球化。因此使部分难以消除的共晶组织在后续热处理过程中得以完全消除,减少了裂纹的扩展源,对7055合金的综合性能提升具有一定现实意义。更多还原

【Abstract】 7xxx series aluminum alloys used in key components of airplane are characterized by high specific strength and stiffness,excellent machining and SCC property.This paper reports on the tensile and fatigue fracture performances of 7075 alloy in the T651 temper were studied.The rapture mechanisms of 7075 alloy were concluded on the observation of fractography using scanning microscope.Aiming at promoting fatigue performance of 7xxx series alloy,the effects of addition of yttrium on as-cast microstructure and mechanical property of 7055 aluminum alloy were investigated using optical microscope,scanning electron microscope,X-ray diffraction,etc.The tensile fracture face of 7075-T651 alloy in its short-transverse direction displayed a mix of quasi-cleavage crack and dimple crack.There is no horizontal part on the S-N curve of 7075-T651 and the cycle numbers just increased with the reduction of stress amplitude.The fatigue crack initiations derived from the surface or inner contaminant,and lots of fatigue fringe were generated in the age of crack propagation, meanwhile other fatigue characters such as tyre patterns,fatigue sidestep and secondary crack also can be found on the fracture face.The fatigue failure region showing mixed failure mode of brittle and ductile rapture was similar with the character of tensile fracture.The precipitated phase particles of 7075 alloy have some effects on its fatigue performance.Specifically,the crack initiation resulted from the massive precipitated phase particles,while dispersion of small precipitated phase particles have positive effects on fatigue performance.The micro-alloying research showed that the Al₆Cu₆Y and YAl₃ resulting from addition of yttrium to the 7055 alloy is distributed at the grain boundary.And the 7055 alloy obtains wide-range rate of growth in its toughness with little loss in its tensile strength and hardness with the addition of yttrium,while the as-cast microstructure of 7055 aluminum alloy was fined remarkably and the formation of massive eutectic was restrained.The addition of yttrium narrowed the spacing of second arborescent crystals and skeleton-shape second phase of the 7055 alloy were sphericized and fined,so the elimination of the hard-to-remove eutectic texture reduce the crack initiation field in the subsequent heat treatment.It makes practical sense in promoting the mechanical performance of 7055 alloy.更多还原