【作者】 李智峰；

【导师】 彭立明；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2011， 硕士

【摘要】 镁合金较高的比强度、比刚度和良好的力学性能促使人们在镁合金新材料领域开展了大量的研究工作,试图开发出新型高强度镁合金材料。最近的研究表明Mg-3.0Nd-0.2Zn-Zr(wt.%)(NZ30K)合金是一种强度较高低稀土含量同时成型性能良好的镁稀土合金,该合金室温与高温力学性能、耐腐蚀性能显著优于常规商用AZ91D合金,有望在航空航天、汽车等领域得到应用。然而,作为材料关键应用特性之一,NZ30K镁合金的疲劳性能并没有进行过系统深入的研究,本文将展开这方面的研究工作。本文主要研究晶粒度大小及其分布、常规铸造成型工艺下不同显微组织对NZ30K镁合金室温拉压高周疲劳性能的影响。研究通过改变Zr含量制备不同晶粒度大小及其分布不同的重力铸造NZ30K镁合金,通过重力金属型、重力砂型、低压金属型和低压砂型四种不同铸造工艺制备具有不同显微组织的NZ30K镁合金,通过光学显微镜(OM)研究合金显微组织,通过拉伸实验测试合金室温力学性能,采用德国SincoTec MAG50KN高频疲劳机检测合金的室温高周疲劳性能,采用电子显微镜(SEM)对疲劳实验断口进行观察与表征。通过调整Zr元素的含量,可以制备平均晶粒大小分别为40μm、60μm、100μm和180μm的NZ30K-T6镁合金。随着平均晶粒尺寸的减小,合金晶粒尺寸分布范围减小并逐渐趋向于正态分布;合金室温力学性能中,屈服强度随平均晶粒尺寸的减小而升高,两者呈H-P关系,抗拉强度随平均晶粒尺寸的减小先升高后降低,在60μm时达到峰值,延伸率随平均晶粒尺寸的减小先下降而后又升高,100μm时延伸率最低,60μm时延伸率最高。NZ30K-T6镁合金达到107时的室温拉压疲劳极限强度随合金晶粒尺寸的减小大体上呈现增加趋势,平均晶粒尺寸从180μm到100μm,疲劳极限强度变化很小,增量为1MPa,而从100μm到60μm,合金疲劳强度提高比较明显,增量为12MPa,而从60μm到40μm,材料疲劳强度略有下降,下降量小于1.5MPa。NZ30K镁合金在重力金属型铸造(GPC)、重力砂型铸造(GSC)、低压金属型铸造(LPC)以及低压砂型铸造(LSC)四种不同的铸造方式下的显微组织与力学性能的差异,可以理解为合金常规凝固条件下不同冷却速率的影响。凝固冷却速率快(金属型铸造)时,合金晶粒尺寸成正态分布,且平均晶粒尺寸较小,凝固冷却速率降低到砂型铸造以后,合金晶粒尺寸分布范围更加宽泛,明显偏离正态分布,且晶粒不同区域分布不均匀,合金平均晶粒尺寸较大。对于短时拉伸性能而言,细小均匀的显微组织,有利于提高合金的力学性能,包括屈服强度、抗拉强度和延伸率。相对于提高幅度不大的屈服强度而言,抗拉强度和延伸率提高的幅度比较明显。对于室温疲劳性能而言,细小均匀的显微组织并没有显著提高合金的性能:GPC-NZ30K-T6的疲劳强度为76.39±7.04MPa,GSC-NZ30K-T6合金的疲劳强度为75.27±0.75MPa,GPC-NZ30K-T6合金的均值疲劳强度仅比GSC-NZ30K-T6提高了1.12MPa,但合金疲劳性能的波动范围却大大增加;从合金疲劳性能波动范围上讲,晶粒分布不均的组织更有利于稳定NZ30K合金的疲劳性能。NZ30K镁合金的疲劳断面观察结果表明,疲劳裂纹源总位于疲劳试验外表面附近,整个断口主要由三个特征区域组成,分别对应疲劳裂萌生区、疲劳裂纹扩展区和瞬时断裂区。裂纹源区的断口形貌总体相对平整,条纹清楚规则,断口疲劳条纹大体呈平行分布,纹理间距较小;扩展区的形貌相对于源区断面起伏有所增加,断口疲劳条纹分布杂乱;瞬时断裂区的断面高低起伏进一步增加,同时断面条纹更加不规则,断面主要由解理面组成,与室温拉伸断口类似。更多还原

【Abstract】 Magnesium alloy is widely used for the higher specific strength, specific stiffness and good mechanical property. A lot of studies of Mg alloy are conducted to develop new Mg alloy with higher strength. The latest study revealed that Mg-3.0Nd-0.2Zn-Zr (wt. %) alloy is a magnesium rare earth alloy with high strength, low rare earth content and good formability. The alloy has a better mechanical property at both room and high temperature and corrosion resistance property than AZ91D alloy. It is expected the alloy will be used in the field of aerospace, automobile etc. However, fatigue property of NZ30K alloy hasn’t been investigated systemly as a key factor of material, which will be focused by this article.The dimension and distribution of grain size, the influence of high cycle fatigue property of NZ30K alloy by different microstructure of normal formability process were determined. Different grain size and distribution of gravity casting NZ30K magnesium alloy can be made by adjusting the Zr content. NZ30K Mg alloys with different microstructure were made by four type of formability casting process: gravity permanent cast (GPC), gravity sand cast (GSC), low pressure permanent cast (LPC) and low pressure sand cast (LSC). Microstructure, tensile property, high cycle fatigue property at room temperature and fatigue fracture surface were investigated by optical microscope (OM), tensile test machine, SincoTec MAG50KN and scanning electric microscope (SEM).NZ30K-T6 alloy with grain sizes 40μm, 60μm, 100μm and 180μm were made by adjusting the Zr content. The range of grain size distribution is getting smaller and going to be normal distribution with the decrease of average grain size. Yield strength increases with grain size reducing, and shows H-P relation. Tensile strength increases first and then decreases with the decrease of grain size, and gets its peak at 60μm. Elongation decreases first and then increases with the decrease of grain size, and the result shows the min. value at 100μm, and max. value at 60μm. The fatigue strength of NZ30K-T6 alloy up to 107 cycles increases at room temperature with the the decrease of grain size. The variety of fatigue strength is a little with the range of grain size from 180μm to 100μm, only 1MPa enhancement. The obvious increment is 12MPa when the avearge grain size decreases from 100μm to 60μm. While the fatigue strength decreases a little from 60μm to 40μm, the reduce volume is less than 1.5MPa.The difference of microstructure and mechanical properties for NZ30K magnesium alloys cast by GPC, GSC, LPC and LSC could be understood as the influence of different cooling rate of solidification condition. If the solidification cooling speed is fast (metal mold cast), grain size distribution is normal distribution, and the average grain size is smaller. When the solidification cooling rate decreases to sand cast, the grain size distribution range is broader and deviating from normal distribution obviously, and the grain size distribution is uneven in different area and the average grain size is larger. For tensile properties, tiny and uniform microstructure is helpful for improving mechanical properties of alloy, including yield strength, tensile strength and elongation. Compare with the yield strength, the increases of tensile strength and elongation are more remarkably. For fatigue properties at room temperature, there is no large increase because of tiny and uniform microstructure. The fatigue strength of GPC-NZ30K-T6 is 76.39±7.04MPa, and that of GSC-NZ30K-T6 is 75.27±0.75MPa. The average value of fatigue strength for GPC-NZ30K-T6 is just improved 1.12MPa comparing with GSC-NZ30K-T6. However, the fluctuation range of fatigue strength property becomes wider. Considering of fluctuating range of alloy fatigue properties, the uneven grain size distribution is favorable for stabilize the fatigue property of NZ30K alloy.SEM investigation results of NZ30K alloy fatigue fracture show that fatigue cracks initiation specimen surfaces. The fracture surface consists of three typical areas, which are fracture initial area, crack propagation area and collapse area. The surface morphology of fracture initial area is generally smooth. The stripe is clear and regular, and the distribution of fracture surface stripe is parallel, with short the stripe space. The surface morphology of crack propagation area shows more fluctuation comparing with fracture initial area, and the stripe distribution is disordered. The surface morphology of collapse area shows more fluctuation than the above two areas, and the stripe distribution is more disordered. The surface morphology of collapse fracture region is similar to those of tensile fracture surface, which are all characterized by cleavage planes.更多还原