【作者】 郑飞燕；

【导师】 赵红亮；

【作者基本信息】 郑州大学 ， 材料加工工程， 2007， 硕士

【摘要】 镁合金具有密度低、比强度和比刚度高、良好的铸造性能、电磁屏蔽能力以及易于再生利用等一系列独特的优点，被誉为“21世纪最具发展潜力和前途的材料”。其结构件在汽车、飞机、计算机、通讯等领域获得了日益广泛的应用。AZ91镁合金兼有良好的铸造工艺性能、室温力学性能和成本低等优点，是目前应用最广泛的压铸镁合金。但是，由于在凝固过程中，粗大的β相以离异共晶的形式沿晶界析出，合金的塑性较差，并且由于该合金的结晶温度间隔较宽，凝固收缩大，热裂倾向严重，致使合金铸造工艺复杂，严重阻碍了AZ91的应用和发展。因此，在AZ91镁合金的基础上开发出具有较高抗拉强度、高韧性并且能满足成形性能要求的新型镁合金显得尤为重要。实践证明，细小的等轴晶可以改善镁合金的显微组织和力学性能。本课题通过向熔体中加入Sr、B对AZ91D合金进行细化，提高其强韧性。利用光学显微镜（OM），X射线衍射仪（XRD）、差示扫描量热仪（DSC）、带能谱分析（EDS）的扫描电子显微镜（SEM）等分析手段，通过对合金拉伸性能、疲劳性能、流动性、自腐蚀电位的测试与分析，较系统地研究了Sr、B对AZ91D镁合金组织和性能的影响及其作用机理，从而为研制新型高性能镁合金提供理论依据。研究结果表明，AZ91D合金中晶粒大小约为250μm，添加0.8％Sr后，晶粒大小约为120μm，细化了60％。加入0.03B后，AZ91D-0.5Sr合金的晶粒尺寸由原来的125μm降低到添加后的80μm，细化了36％。铸态AZ91D、AZ91D-0.2Sr、AZ91D-0.5Sr合金均由α-Mg和β-Mg₁₇Al₁₂相组成。当Sr的添加量达到0.8％，合金中出现新相Al₄Sr。在AZ91D-0.5Sr合金的基础上添加B后，合金中同样出现新相Al₄Sr。由于Sr与Al的结合，导致Al₄Sr周围Al含量相对减少，晶界上共晶β-Mg₁₇Al₁₂相数量减少，从而由连续与半连续网状结构逐渐变为断续、弥散分布的带状结构。AZ91D合金的抗拉强度和伸长率分别为129MPa和1.1％。添加0.8％Sr以后，抗拉强度达160MPa，较AZ91D合金提高了23.6％；伸长率为1.3％，提高了12％。在AZ91D-0.5Sr合金的基础上添加0.09％B，合金的抗拉强度和伸长率分别为151MPa和1.6％，抗拉强度较AZ91D-0.5Sr合金提高了10.2％。铸态下AZ91D合金的断裂方式是典型的脆性断裂。合金经Sr、B变质处理后，断口呈现一定的韧窝和撕裂棱，合金的韧性变好。Sr的加入，对合金的固相线影响不大，但能明显降低合金的液相线。液相线从AZ91D合金的596℃下降到添加0.5％Sr后的566℃，结晶温度间隔减小了32℃。添加量分别为0.2％、0.5％时，Sr对AZ91D合金凝固过程的影响主要表现为降低了合金的液相线并且在合金凝固前期，富集在固／液界面前沿，从而抑制晶粒的快速长大。当合金中Sr的加入量达到0.8％时，富集在固／液生长界面的Sr与Al结合生成高熔点的Al₄Sr相，Al₄Sr相分布在α-Mg相前沿，形成溶质过冷层，从而降低了α-Mg晶粒的生长速率，细化了组织。Sr、B元素的加入能改善合金的流动性，提高其铸造性能。AZ91D合金流动性浇注试样的长度为330mm，加入0.5％Sr后，流动性浇注试样的长度为380mm，提高了21％。AZ91D-0.5Sr-0.09B合金的流动性试样长度为580mm，比AZ91D合金提高了76％。说明加入Sr、B都提高了合金的流动性，能够改善其充型能力。加入Sr和B后，合金的自腐蚀电位得到一定程度的提高。AZ91D-0.5Sr-0.09B合金的自腐蚀电位约为-1.55V，比AZ91D合金的自腐蚀电位提高了约0.04V，腐蚀稳定性得到改善。高周疲劳试验中，合金的疲劳源起源于合金熔炼过程中产生的铸造缺陷。添加0.5％Sr后试样断口上可以看到规则的海滩波纹状辉纹。AZ91D合金的疲劳断裂区显示准解理特征。添加Sr后在合金试样断口的疲劳断裂区，韧窝特征明显，撕裂棱也比AZ91D合金明显增多。这些韧窝和撕裂棱的存在能够减小疲劳裂纹扩展的速率，增加疲劳寿命，从而改善疲劳性能。Sr对AZ91D合金的细化机理为：由于Sr在镁合金中是表面活性元素，在凝固过程，Sr富集于固／液界面前沿，降低了合金中的结晶潜热，从而增大了实际过冷度，提高形核率，阻碍了晶粒的生长。在AZ91D合金中联合添加Sr和B的复合细化机理为：加入微量B后，B与熔体中的SrO和MgO发生反应。使原来被包裹住的Mg和Sr得以释放，从而有更多的Sr在熔体中发挥细化作用，并且与合金中的Al结合形成Al₄Sr。Al₄Sr的形成有利于合金组织的细化。此外，B与MgO反应，减少了熔体中氧化物的含量，净化了熔体，有利于合金综合性能的改善。更多还原

【Abstract】 Magnesium alloys have many special advantages, such as low density, high specific strength, high specific rigidity, good casting capability, electromagnetic shielding performance and easy recycling etc. So they are known as ’the green project materials having development potential and best future in the 21st century. Magnesium alloys are extensively used in automobile, airplane, computer and communication equipments etc. AZ91 magnesium alloy which is widely used as the die-casting magnesium alloy, has many advantages, such as good casting property, high ambient temperature strength and low cost. However, because the coarseβ-Mg₁₇Al₁₂ phase precipitates along the grain boundary as the form of divorced eutectic when concreting. Because of the wide solidification temperature range, great solidification shrinkage, bad tendency of hot tearing, the molten technics of magnesium alloy become complex. The application and development of magnesium alloys is still confined. Therefore, it is important to develop some new magnesium alloys with high mechanical properties, high toughness, good figuration properties on the basis of AZ91 alloy.The practice proves that the alloys have fine equiaxed dendrite can improve the microstructure and mechanical properties. In this paper, Sr and B are added into the AZ91D alloy to carry through composite, refine microstructure, and then improve the intensity and toughness. Effects and mechanism of Sr and B addition on the microstructures and properties have been investigated using modern analysis and test facilities such as Optical Microscopy （OM）, X-ray Diffraction（XRD） , Differential Scanning Calorimetric（DSC）, Scanning Electron Microscopy（SEM） with Energy Dispersive Spectrometer （EDS） and whilst the test of their tensile test, fatigue test, fluidity, corrosion potential, polarization curves. It offers some experimental basement and theoretical reference for developing novel high-powered magnesium alloys.The results show that the grain size of AZ91D alloy was about 250μm, and the grain size AZ91D-0.8Sr alloy was 120μm, which was reduced by 60%.After added 0.03%B, the grain size was reduced from 125μm to 80μm, which was reduced by 36%. The main phases in the as cast AZ91D, AZ91D-0.2Sr, AZ91D-0.5Sr alloys areα-Mg andβ-Mg₁₇Al₁₂ phase. The new phase, Al₄Sr was found in AZ91D-0.5Sr alloy with B addition. Because of the combine of Sr and Al, the content of Al around the Al₄Sr was reduced. And then the content ofβ-Mg₁₇Al₁₂ was reduced, the figure changed from net-structure to spiccato and disper.The ultimate tensile strength and elongation of AZ91 alloy were 129MPa and 1.1%, respectively. After the addition of 0.8% Sr, the ultimate tensile strength reached 160MPa, which has increased 23.6%; The elongation of AZ91D-0.8Sr was 1.3%, which has increased 12%. The ultimate tensile strength and elongation of AZ91D-0.5Sr-0.09B alloy were 151MPa and 1.6%, respectively. The ultimate has increased 10.2%.Failure of AZ91D alloys was brittle through cleavage or quasi-cleavage fracture. The fracture of AZ91D with Sr and B additions obvious features the combination of quasi-cleavage and local gliding fracture. The toughness was improved.The liquidus temperature of AZ91D alloy decreased markedly with the addition of Sr, but the solidus temperature changed rearly. The solidus temperature fell from 596℃of the AZ91D alloy to 566℃after 0.5% Sr addition. The solidification range has decreased 32℃.The influence of 0.2%Sr and 0.5%Sr addition to AZ91D alloy at the process of solidification was decreased the solidus temperature and enriched on the solid/liquid interface. So it can restrain the growing up of the grain. Al and Sr form the high melting point phase Al₄Sr with 0.8% Sr addition. The Al₄Sr enriched on the solid/liquid interface and formed the super-cooling layer, restrained the growing up of the grain. So it can refine the microstructure.The results of fluidity showed that the fluidity is improved by adding Sr and B, then improved the die-casting properties. The length of fluidity sample of AZ91D was 330mm, and the length of fluidity sample of AZ91D-0.5Sr was 380mm, which has increased by 21%. The length of fluidity sample of AZ91D-0.5Sr-0.09B was 580mm, which has increased by 76%.The results of the corrosion potential of the alloys show that the corrosion potential of the alloys with Sr and B addition. The corrosion potential of alloy with 0.5% Sr and 0.09%B addition was about -1.55V, which was 0.04V higher than that of AZ91D alloy.The results of high cycle fatigue test show that the fatigue crack of the alloys initiates at gas hole and inclusions of alloy inside. The fatigue striation areas have been found in the fatigue fracture surface of specimens after 0.5Sr added. The fatigue fracture zone of the fatigue fracture surface in AZ91D alloy shows the mixed-rupture characteristics of quasi-cleavage and dimple. More dimples and lacerated ridges were found after the 0.5Sr addition. These dimple and lacerated ridges can decrease the velocity of the fatigue crack propagate, improve the fatigue properties.The mechanism of refining about Sr to AZ91D alloy is that, Sr enriched on the solid/liquid interface when concreting because it is the activity element on magnesium alloy. This will reduce the latent heat and increase the undercooling, and so increase the nucleation rate, thereby block the growth of the grain.The mechanism of complex refining about Sr and B to AZ91D alloy is that, B reacted to SrO and MgO in the molten metal. So the Mg and Sr oxid were released, and more Sr could be used to refine the grain or form the high melting point phase Al₄Sr with Al.4 Furthermore, the reaction of B to MgO in the melting, has reduced the content of oxid, consequently cleanse the molten metal. So it can improve the compositive properties.更多还原