【作者】 邓娟；

【导师】 胡耀波；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2010， 硕士

【摘要】 与其它金属结构材料相比,镁的密度低,阻尼性能高,电磁屏蔽性能优异,导热导电性能好,镁及镁合金的应用受到了一定关注。镁及镁合金是一种集结构与功能于一体的理想金属材料,但密排六方晶体结构的镁滑移系少,塑性成形困难;且纯镁强度低,抗拉强度仅为100MPa,弹性模量只有45GPa。塑性成形困难和强度低是影响镁及镁合金工业化应用的两个瓶颈。近年来,稀土镁合金特别是Mg-Gd系合金因其具有优异的室温和高温力学性能而得到重视。本课题通过半连续浇铸法在纯Mg基础上添加xGd(x=2,4,6%)和0.6%Zr(质量分数,下同),采用X射线荧光分析、光学显微分析、扫描电子显微镜结合能谱分析、X射线衍射分析以及透射电子显微分析等方法研究了Gd含量对不同热处理及挤压态的Mg-xGd-0.6Zr合金显微组织的影响,利用显微硬度及拉伸性能表征了材料各个状态下的力学性能,分析得到低Gd含量的Mg-Gd系合金的强化机理及塑性机理。研究结果表明:铸态Mg-xGd-0.6Zr(x=2,4,6%)合金呈明显的晶粒结构组织,没有出现Gd的枝晶偏析。随Gd含量增加,固溶态Mg-xGd-0.6Zr合金晶粒略微减小,退火态合金中第二相依次增加。退火态合金中第二相Mg5.05Gd主要有两种形貌,一种尺寸约5μm×5μm×1μm规则矩形相,一种不规则相。该Mg5.05Gd相在退火态Mg-4Gd-0.6Zr和Mg-6Gd-0.6Zr合金中质量百分比分别为2.787wt.%和5.340wt.%。随着Gd含量的增加,铸态和退火态Mg-xGd-0.6Zr合金单位质量百分比含量的强度增加率增加明显。固溶态合金中Gd的原子百分比与硬度呈直线关系,且与固溶态Mg-Gd二元合金相比,Zr的加入使单位Gd含量在镁合金中的固溶强化作用减弱。随固溶温度的增加,Mg-6Gd-0.6Zr合金的晶粒尺寸增大,420°C下合金显微硬度值最大,460°C下合金中原子基本完全固溶。Mg-6Gd-0.6Zr合金最佳固溶工艺为300°C×6h+460°C×10h。不同状态合金经挤压后,Mg-2Gd-0.6Zr和Mg-4Gd-0.6Zr合金强度相差不大,Mg-6Gd-0.6Zr合金强度最高;三种合金的延伸率都超过30%。铸态挤压Mg-2Gd-0.6Zr、Mg-4Gd-0.6Zr和Mg-6Gd-0.6Zr合金的抗拉强度与屈服强度依次为206、207、237MPa与150、145、168MPa,延伸率依次为36.8%、43.4%、33.4%。Mg-6Gd-0.6Zr合金时效10h后,塑性变化较小,强度略有提高,抗拉强度和屈服强度分别为243、175MPa,延伸率为31.7%。所有合金的断裂方式都为韧性断裂。该Mg-xGd-0.6Zr系合金强度的强化方式包括细晶强化、固溶强化、第二相强化以及形变强化四种强化方式。屈服强度与各个强化方式的贡献值可线性表示。Mg-xGd-0.6Zr合金的塑性机理主要由降低c/a比值和细化晶粒共同贡献。该合金系出现具有明显高低屈服点和屈服平台的屈服现象,主要原因是位错的增殖以及位错与固溶原子的相互作用。更多还原

【Abstract】 Compared with other metals, magnesium and magnesium alloys have attracted increasing interest for potential application due to their low density, and excellent damping capacity, electromagnetic shielding, and thermal conductivity. Magnesium and magnesium alloys are one of ideal metals exhibited simultaneously with structure and function, but magnesium with close-packed hexagonal structure has less slip systems, which lead to be weak workable, and pure magnesium has low strength, ultimate tensile strength(UTS) only 100MPa,elastic modulus only 45GPa. So low strength and weak workability are two bottlenecks that effect the application of magnesium and magnesium alloys in industrialization. Rare-earth magnesium alloys have been the focus of intense investigation for the past few decades due to those outstanding mechanical properties at room and elevated temperature, especially Mg-Gd series alloys, meeting the need of new metal materials with low density, excellent cast performance, mechanical properties and resistance corrupt in the automotive, architectural and aerospace industry.The elements xGd(x=2,4,6%) and 0.6%Zr were added to pure magnesium in this paper. The effect of Gd contents on the microstructure and properties of Mg-xGd-0.6Zr alloys in different heat treatment and extruded processes were studied by optical microscopic, scanning electron microscopic equipped with energy dispersive spectroscopy, transmission electron microscopy and micro-hardness and tensile testers.The results show that the as-cast Mg-xGd-0.6Zr (x=2, 4, 6%) alloys display grain structure and no Gd element segregation appears in the microstructure. With the increasing of Gd content, the grain size of as-quenched Mg-xGd-0.6Zr (x=2,4,6%) decrease and the second phases Mg5.05Gd contents increase. The morphology characters of the second phases Mg5.05Gd in as-annealed alloys included two kinds, one of them is regular shape, about 5μm×5μm×1μm, one of them is irregular shape. The weight percent of the second phases Mg5.05Gd in as-annealed GK4 and GK6 alloys are 2.787wt.%,5.340wt.%, respectively.For as-cast and as-annealed Mg-xGd-0.6Zr alloys, the seconde phases could make the hardness of unit Gd content obviously increase. There is a line increasing relationship between Gd atom percent and micro-hardness of as-quenched alloys and compared with the Mg-Gd seris alloys, the addition of Zr weakens the solid solution strengthen effects. The grain size of alloys after different temperature treated increase with solution temperature increase. The second phase Mg2Gd, Mg3Gd brought in due to non-equilibrium solidification during the casting process could be transformed into equilibrium phase Mg5.05Gd which could be dissolved intoα-Mg solid solution phase at 460°C solution temperature. The optimization of solid solution treatment process is 300°C×6h + 460°C×10h. Micro-hardness value of alloy treated under 420°C is highest due to interaction of solid solution strengthening and second strengthening.After alloys in different states extruded, the strength of Mg-2Gd-0.6Zr and Mg-4Gd-0.6Zr alloys are almost the same and the strength of Mg-6Gd-0.6Zr is highest. However, the elongation of all extruded alloys is over 30%. The ultimate and yield tensile strength and the elongation of Mg-xGd-0.6Zr(x=2, 4, 6%)-Ext alloys are 206、207、237MPa, 150、145、168MPa and36.8%、43.4%、33.4% respectively. After aged 10h, the ultimate and yield tensile strength of extruded Mg-6Gd-0.6Zr alloys increases slightly, to be 243、175MPa respectively, at the same time, the elongation of alloys rarely decreases, to be 31.7%. The fracture mechanism of all studied alloys is ductile fracture feature.Refinement strengthen, solid solution strengthen, precipitation strengthen and resident compression stress are contributive to the strength of Mg-xGd-0.6Zr (x=2,4,6%) alloys and yield tensile strength relates with the line relationship of four kinds of strengthen ways. The decreased c/a value and the grain refinement are helpful to high plasticity for Mg-xGd-0.6Zr alloys. The dislocation proliferation and reaction between solute atoms and dislocation can be used to explain the yield phenomenon with obvious low and upper yield points and yield flat of Mg-xGd-0.6Zr alloys.更多还原