摘要
镁及镁合金是一种具有低密度和高比强度等优良特性的轻金属,但其有限的变形能力和低腐蚀抗性使其应用受到了限制,镁合金材料现有的使用状况远没有充分发挥其潜在优势。镁及镁合金是一种密排六方结构金属,相对于面心立方和体心立方结构的金属,引起镁中塑性变形的滑移系较少,因此塑性差已成为变形镁合金加工与应用上的一个瓶颈。镁及镁合金在拉伸和压缩展现出不同的塑性变形机制,通常认为镁的最主要的塑性变形机制是孪晶和滑移。然而,镁及镁合金的内在塑性变形机理非常复杂,到目前为止还不是很清楚,这也是最近几年对镁及镁合金的一个研究重点。
由于镁单晶与镁合金具有类似或相同的微结构和变形特征,在微观下研究镁单晶的塑性变形机制是研究镁及镁合金的一个重要途径。本文应用分子动力学在原子尺度对镁单晶进行拉伸、压缩和剪切来研究镁及镁合金塑性变形机制,以及通过对镁单晶中不同缺陷与孪晶的相互影响研究其塑性变形机制内在本质及关系。主要内容和结论包括：
1.采用分子动力学方法模拟镁单晶在c轴拉伸和压缩时不同温度下微观变形机制及微结构演化行为。模拟结果表明：拉伸时最主要的变形机制是{1012}<1011>型孪晶和型锥面滑移；压缩时最主要的变形机制不是通常认为的压缩孪晶,而是型锥面滑移。此结果揭示了镁单晶拉压不对称的微观本质。
2.采用分子动力学方法模拟镁单晶受到剪切作用在不同温度下微观变形机制及微结构演化行为。模拟结果表明：剪切时最主要的微观变形机制是层错和相变,相变区的原子密排面与原晶粒的密排面保持平行,没有发生转角。
3.采用分子动力学方法模拟不同温度下点缺陷、线缺陷和面缺陷在{1012}孪晶长大中所起的作用。模拟结果表明：点缺陷和各类线缺陷对于孪晶运动的影响不太明显。我们所研究的面缺陷在超低温(5K)时阻碍孪晶长大,孪晶面不能越过面缺陷所在的位置；低温和室温(约150K-350K)时,当孪晶界靠近面缺陷时,面缺陷对孪晶长大阻碍作用明显,而当孪晶界滑过面缺陷头部后,面缺陷对孪晶长大阻碍作用减小；高温时(350K-580K),面缺陷对孪晶长大影响作用不大,在位错周围的原子变得较为混乱,形成许多空位及微裂纹。
Magnesium and its alloys have attracted attention in recent years as lightweight for the transportation industry due to their low density and relatively high specific strength. However, the application of magnesium alloys is restricted due to the limited forming capacity and the poor corrosion resistance. From the mechanical point of view, magnesium and its wrought alloys show a pronounced direction-dependence of plastic yielding and work hardening, as well as different yielding behavior in tension and compression, the so-called strength differential effect. This feature is due to the crystal structure of magnesium, which is a hexagonally close-packed (hcp) structure. Compared to face-centered cubic (fcc) or body-centered cubic (bcc) metals, the number of slip systems allowing plastic deformation in magnesium is limited. It is generally agreed that mechanical behavior of magnesium is sensitive to twinning and slip. Nevertheless, deformation mechanisms acting on magnesium and its alloys are sophisticated and still a subject to be discussed. Therefore, the deformation mechanism is the key rule of magnesium and its alloys in rencent years.
Because magnesium single crystal and magnesium alloys have the same or similar structures and deformation characteristics, it is an effective way to investigate the deformation behavior of magnesium and its alloys by researching the behaviors of magnesium single crystal. In this thesis, molecular dynamics simulatin at a certain strain rate is applied to investigate deformation mechanism in magnesium single crystal. The tension and the compression are applied along the c-axis direction and the shear stress is applied perpendicular to the c-direction at different temperatures. Furthermore, the influence of other defects in magnesium single crystal on the twinning behavior is discussed. The main contents and results including:
1. Molecular dynamics simulation is applied to investigate the microstructure evolution of magnesium single crystals under c-axis extension and compression at different temperatures. For extension, the {1012} twin and the pyramidal slip are found to be the main deformation mechanisms under the c-axis tension in magnesium single crystal. For compression, the pyramid slip is found to be the main deformation mechanism, instead of the twin.
2. Molecular dynamics simulation is applied to investigate the microstructure evolution of magnesium single crystals under shear at different temperatures. These simulation results indicate that stacking fault and phase transformation are the main deformation mechanisms under shear. The new phase shares the same close-packed plane with the original hcp lattice.
3. The influence of the point defect、the linear defect and the plane defect on the movement of {1012} twin in magnesium single crystal at different temperatures are investigated by molecular dynamics simulation. The simulation results indicate that the vacancy and the linear defects have no distinctly influence on the movement of twin. For the plane defect, at ultralow temperatures (5K), it will impede the {1012} twin movement; at low and room temperatures (150K-350K), the plane defect impedes twin movement when the twinboundary moves near it. However, when the twin boundary pass through the head of this line defect, the effect of this line defect hindering the twin boundaries movement can be ignored; at elevated temperatures (350K-580K), the impeding effect of this line defect on the twin movement can be ignored.
引文
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