【作者】 张洁；

【导师】 吴国华； 刘文才；

【作者基本信息】 上海交通大学 ， 材料科学与工程， 2016， 硕士

【摘要】 镁锂合金是目前已知的密度最小的金属结构材料,也是目前材料科研和应用领域的研究热点。镁锂合金不仅在比强度、比刚度方面表现卓越,而且还有良好的加工性能、焊接性能、较好的电磁屏蔽性能、抗震阻尼、导热率等诸多优点,在航空航天、国防军事方面有着广泛的需求,在汽车工业、电子3C、医疗器械等领域也有这广阔的应用前景。但是,镁锂合金绝对强度偏低,热力学稳定性差、材料制备的昂贵和困难等缺点,使得镁锂合金的实际应用受到制约。对镁锂合金的强化和热力学稳定性展开研究具有重要的科研价值和实际意义。本论文选择兼具较好的强度、塑性,同时有着良好减重效果的双相Mg-8Li合金作为基础合金,通过添加Al、Zn、Nd、铈镧稀土等合金元素制备了Mg-8Li-3Al-2Zn-xNd,xRE系列合金。通过添加Zn、Gd元素制备了Mg-8Li-xZn-yGd合金,并对相应成分合金进行了挤压。本文利用金相显微镜、XRD、SEM、电子万能实验机等分析手段对合金的微观组织、拉伸性能及断裂行为进行了分析,研究了合金元素以及挤压变形对镁锂合金微观组织和力学性能的影响。并对不同稀土元素含量的铸态Mg-8Li-3Al-2Zn-xRE合金的时效行为进行了研究。实验结果表明,Mg-8Li-3Al-2Zn的基体主要由α-Mg、β-Li两相组成,合金中同时生成了AlLi和MgLi₂Al相,密度不超过1.57g/cm³,屈服强度和抗拉强度分别为160MPa和227MPa,延伸率达到18%左右,显示了较好的综合力学性能。在加入0.5%Nd元素后,合金的α-Mg相得到细化,并且由枝晶状变为条状,生成了不规则形状的Al₂Nd相,合金除屈服强度提高10MPa外,延伸率、抗拉强度没有明显的变化。热挤压工艺使得合金出现了动态再结晶现象,合金的性能提高明显,Mg-8Li-3Al-2Zn抗拉强度提高到接近250MPa,延伸率提高到24%。在Mg-8Li-3Al-2Zn-0.5Nd的基础上,研究了铈镧混合稀土RE对铸态合金组织性能的影响,发现,随着RE的增加,合金中的α-Mg相被细化,占比减少,而基体中出现了大量的Al-RE相,在合金中,Al-Ce相、Al-La相、Al-Nd相不单独存在,而是聚在一起形成块状相,或网状相。随着RE添加量的增加,合金的强度逐渐下降,延伸率逐渐上升,Mg-8Li-3Al-2Zn-0.5Nd-3RE的延伸率超过了Mg-8Li的延伸率,达到35%以上。出现这一现象的主要原因是RE元素对合金组织的细化作用,并且由于生成Al-RE相导致的固溶在基体内的Al元素大量减少,中和了有Al-RE金属间化合物的增加带来的合金塑性降低的影响。对铸态合金在不同温度下的时效行为进行了研究发现,铸态Mg-8Li-3Al-2Zn及相应添加稀土的合金试样在时效条件下硬度、相组成变化较小,热力学稳定性较佳。在对Mg-8Li-x（3Zn-Gd）合金的研究中发现,Zn、Gd元素的添加极大地细化了Mg-8Li细长枝晶状的α-Mg相,随着Zn、Gd的添加,合金的密度逐渐增加,生成的准晶相I相和W相逐渐由弥散分布变为聚集在β-Li相晶界上的网状结构。在固溶强化和弥散强化的作用下,合金强度随元素含量的增加而上升,其中铸态Mg-8Li-9Zn-3Gd合金抗拉强度最高,达到259MPa左右,但是合金延伸率降至4.6%,合金也由塑性断裂变为解理断裂。挤压变形对提高Mg-8Li和Mg-8Li-3Zn-1Gd的强度、有明显的帮助,尤其Mg-8Li-3Zn-1Gd在挤压后延伸率超过了Mg-8Li,达到44.2%,导致这一现象的原因是合金元素对Mg-8Li晶粒的细化以及挤压过程中发生动态再结晶。Mg-8Li-6Zn-2Gd和Mg-8Li-9Zn-3Gd在挤压工艺之后,合金的强度较铸态有所下降,其可能的原因是原分布在晶界的网状相破碎的后形成流线织构,对变形时位错的移动阻碍作用下降。更多还原

【Abstract】 Mg-Li alloys are well known for their high specific stiffness,and specific strength.They also possess such advantages as good machinability,good magnetic shielding performance and high damp resistance.Mg-Li alloys can be widely used in fields like aerospace exploration,aviation industry,automotive,3C application,and medical aid.While the wide use of Mg-Li alloys is hindered by relatively low strength and low thermodynamic stability.Research on such fields can be beneficial for both theory and application.In this thesis,Mg-8Li alloy is chosen as a basic alloy for its good comprehensive mechanical properties to develop alloys with high performance.Mg-8Li-3Al-2Zn-xNd,Ce-riched RE alloys and Mg-8Li-x（3Zn-Gd）are prepared by steel diecasting.Effects of Nd and RE on microstructure and properties are studied.Influence of extrusion process was studied too.Aging behavior of Mg-8Li-3Al-2Zn-xRE is studied as well.Analysis methods such as SEM,XRD,tensile test,OM are applied.The results show that Mg-8Li-3Al-2Zn is mainly contains two phases of nearly the same ratio,α-Mg andβ-Li..AlLi,MgLi₂Al phase are also detected.The density of is lower than pure Mg,and has a good property with the UTS higher than 220MPa and elongation of 18%.With Nd addition,shape ofα-Mg is refined from d dendritic to ribbon-shaped.Al₂Nd phase is formed while change in mechanical properties is not large.The content of Nd got in experiment may be responsible for it.After hot extrusion,DRX appeared and refined theβ-Li.Phase greatly.Both strength and elongation are enhanced.Mg-8Li-3Al-2Zn-0.5Nd-xRE was prepared to investigate RE influence on alloy.Results showed that with RE addition growing bigger,ratio of Al-RE phase increases andα-Mg is refined.As the tensile test shows,RE deceased alloy strength and improved ductility,which is not usually the case.Elongation of Mg-8Li-3Al-2Zn-0.5Nd-3RE is even bigger than that of Mg-8Li.It can be explained for the refinement ofα-Mg and cost of Al for Al-RE phase formation.Aging study conducted show that the aging softening is not obvious in as-cast Mg-8Li-3Al-2Zn-0.5Nd-xRE alloys.Zn and Gd addition in L8 resulted in formation of I-phase and W-phase,which can be strengthening for alloy.Zn,Gd addition resulted in great refinement inα-Mg.Strength increases as elongation decreases.Solid soluting strengthening and formation of I phase and W phase is responsible.When Zn and Gd are 9 and 3 wt%respectively,strength reaches the highest of 259MPa,while the elongation is as low as 4.7%.Hot extrusion is good for both strength and elongation of the alloys,too.The extruded Mg-8Li-3Zn-1Gd has a elongation higher than Mg-8Li,for which refinement ofα-Mg and DRX inβ-Li phasemay be responsible.After extrusion,strength of Mg-8Li-6Zn-2Gd and Mg-8Li-9Zn-3Gd is decreased compared to that of as-cast alloys.The possible reason for the decrease is the crushed particle secondary phase has poorer interaction with dislocation glide.更多还原