【作者】 杨淼；

【导师】 刘耀辉；

【作者基本信息】 吉林大学 ， 材料加工工程， 2014， 博士

【摘要】 镁是最轻的结构金属，有很高的比强度和比刚度，正在成为研究热点。镁合金在汽车上的使用，有利于降低车辆的自重、提高车辆的燃油经济性、节能环保。镁合金耐蚀性差与实际应用强度较低的缺陷，正严重制约其发展，并且形成了使用中的安全隐患。为了提高镁合金使用的安全性，预测其使用寿命，本文以铸造AM50镁合金为研究对象，通过热处理、稀土（Gd、Er）添加、热加工等方法，系统地研究AM50镁合金在不同处理的条件下，腐蚀、力学性能的变化规律，在前人研究的基础上总结出预测不同处理后AM50镁合金剩余强度变化的函数公式。经过T4、T6处理、稀土Gd和Er的单一改性均能够有效提高AM50镁合金的耐蚀性和力学性能。热处理后，由于固溶和时效强化的作用提高合金的拉伸强度，α-Mg中Al元素含量提高以及β-Mg17Al12数量减少改善了合金的耐蚀性能。稀土元素的加入通过固溶强化、细晶强化和弥散强化机制，提高合金的力学性能；稀土元素通过细化晶粒，净化合金成分，减少AM50镁合金的腐蚀微电偶对数量，改善合金的耐蚀性。稀土改性能有效降低AM50镁合金剩余强度的衰减，是提高AM50镁合金腐蚀动态力学性能的有效手段。本文中稀土Gd和Er的最佳添加量分别为1%和0.5%。热挤压处理后合金的组织显著细化，拉伸强度显著提高，均超过了300MPa，韧性增加，变形能力提高；随着稀土Gd添加量的增加稀土化合物Al2Gd3相数量增多，β-Mg17Al12数量减少；热挤压处理后α-Mg和β-Mg17Al12相的数量增长，合金中腐蚀微电偶对的数量增多，合金的耐蚀性较铸造态变差；断口分析表明热挤压处理的AM50、AM50Gd1和AM50Gd2镁合金断裂模式为包含韧性和脆性双重特征的混合断裂模式，热挤压处理后添加稀土Gd量为1.0%的AM50镁合金表现出优异的腐蚀剩余强度性能。在前人工作的基础上，本文通过不同处理方法的大量数据，建立了AM50镁合金腐蚀剩余强度的数学模型，用以预测AM50镁合金在腐蚀环境下的使用寿命。剩余强度表达式如下：对不同NaCl溶液浓度下AM50镁合金在恒应力下的腐蚀蠕变性能进行了探索性的研究。腐蚀蠕变曲线在腐蚀和应力的影响下主要分为三个阶段：第一阶段为弹性变形阶段,第二阶段为恒速率稳态蠕变阶段,第三阶段为受到严重腐蚀后的加速蠕变阶；AM50镁合金的腐蚀电流和腐蚀电压随着应变量的增加先减小后增大；AM50镁合金的应力腐蚀蠕变敏感性在NaCl溶液中5%浓度范围内,随C1-离子浓度的提高而提升；断口分析表明,AM50镁合金在NaCl溶液中的蠕变断裂机制为脆性沿晶断裂,试样中除了主断裂裂纹,还出现了其他微裂纹。更多还原

【Abstract】 Magnesium is the lightest structural metal with the high specific strength and specificstiffnes, which receives widespread attention. Magnesium alloy as a auto part, whichreducing vehicle weight will help to improve the fuel economy, energy efficient andenvironmentally friendly. However, poor corrosion resistance and low strength inpractical application are seriously restrict its development and lead to sec urity risks inuse.In order to improve the security of magnesium alloy in use, meanwhile, predict itslife, as-cast AM50magnesium alloy was adopted as the target alloy for the study, by heattreatment, rare earth elements(Gd, Er) modified and hot extrusion processing. Systematicstudy with different treatments of AM50magnesium alloy were carried out for corrosion,mechanical properties. Further more, on the basis of previous work,the formula, whichpredict the residual strength of AM50magnesium alloy was summed.The T4, T6heat treatment and rare earth element (Gd and Er)could effectivelyimprove the corrosion resistance and properties of AM50magnesium alloy. The tensileproperty improved by solid solution and aging strengthening. β-Mg17Al12solid solutionin α-Mg, improve the corrosion resistance of α-Mg by increasing the content of Al inα-Mg. Meanwhile, the reducing of the β-Mg17Al12phase reduced the number of galvaniccell. The addition of rare earth element Gd/Er significantly improves the mechanicalproperties of AM50magnesium alloy by solid solution, fine grain and dispersionstrengthening. The rare earth element can refine the microstructure of the matrix, cleanthe composition of the alloy, reducing the amount of β-Mg17Al12, thus effectivelyreducing the micro-galvanic effect in corrosion process and promotes the corrosionresistance of AM50magnesium alloy. In general, rare earth modification could suppressthe decay of corrosion residual strength of AM50magnesium alloy, which is an effectivemean for the protection of the dynamic mechanical properties. The best addition is1%Gd and0.5%Er. The tensile strength of hot extrusion alloys all increased to above300MPa. Further more, their toughness and deformation capacity both improved. And the micro structure of the alloy obviously refined compare to as-cast alloys. Meanwhile, the addition of rare earth element increase the Al2Gds and reduce the amount of β-Mg17Al12phase. The hot extrusion lead increasing of α-Mg and β-Mg17Al12, which deteriorated the corrosion resistance of the alloy by increasing the micro-galvanic couples. The fracture morphology analysis shows that the fracture mode of three hot extrusion treated alloy is mixed fracture, which include the characteristics of ductile fracture and brittle fracture. Therefore, the hot extrusion treated AM50Gd1magnesium alloy shows the best corrosion residual strength property.On the basis of previous work, through a lot of data from different processing, a mathematical model of corrosion residual strength for AM50magnesium alloy is established predict the service life in corrosive environments. As follows:The corrosion creep properties of AM50magnesium alloy under constant stress with different concentrations of NaCl aquous solution was carried out exploratory. The results show that the corrosion creep is consist of three stage:deceleration creep, constant speed creep and accelerated creep. The Tafel curve analysis showed that corrosion potential and current was affected by deformation. The corrosion creep susceptibility of AM50magnesium alloy under5%concentration range in NaCl solution increases with the concentration of Cl-anion. From the fracture morphorlogy analysis, the fracture mode of corrosion creep in NaCl is brittle fracture, there were other cracks besides the main crack.更多还原