【作者】 周磊；

【导师】 郭雅芳；

【作者基本信息】 北京交通大学 ， 固体力学， 2016， 博士

【摘要】 纳米晶(nanocrystalline, NC)金属及其合金由于其优秀的力学性质在工业界得到广泛的应用。这主要是因为纳米晶中晶粒尺寸被细化到纳米尺度,一般情况下在100纳米以下。由于微结构中的面缺陷,例如晶界、共格孪晶界、非共格孪晶界的存在,金属强度被极大提高。因为这些缺陷作为位错形核源,会在材料变形阶段提供大量位错以兼容应变。同时晶界及孪晶界对材料内部裂纹的扩展有着至关重要的影响。裂纹扩展过程中裂尖产生的各类塑性变形(相变,位错发射等),会与晶界共同作用,影响裂纹扩展。对于纳米尺度的材料而言,传统实验手段很难准确捕捉到材料内部微结构的演化过程,而利用分子动力学方法通过计算机模拟手段可以将原子尺度的缺陷演化过程清晰得以呈现,从而进行变形机制的深入研究与分析。因此,本文利用分子动力学方法,对含有晶界及孪晶界的不同种类的金属材料进行模拟,讨论了材料的宏观力学性质,并通过对其微观结构的演化进一步解释了材料的变形机制。主要研究工作取得如下进展：1.模拟并讨论了体心立方铁含有{112}(110)晶界的双晶模型在拉伸载荷作用下的变形过程以及沿晶裂纹扩展。在模拟中发现,双晶模型在拉伸载荷作用下,塑性变形机制主要为晶界面上位错逐层形核导致的相变和非晶界面位错滑移导致的含有孪晶界的折线状结构的生成。相变主要在塑性阶段初期出现,而随着应变增大,含有孪晶界的折线状结构逐渐开始主导塑性变形。双晶模型中沿晶裂纹的扩展会受到裂纹位置和温度的影响,相比于前两者应变率对裂纹扩展影响不大。2.模拟并讨论了{1012}(1011>纳米孪晶镁在单轴压缩及单轴拉伸载荷作用下的变形行为以及沿晶裂纹扩展,在考虑孪晶界间距、加载方向以及裂纹位置等因素的前提下揭示了{1012}<1011>纳米孪晶镁的塑性变形机制和不同孪晶界间距时韧脆转变的内在原因。在模拟中发现,在单轴压缩载荷作用下,{1012}(1011>纳米孪晶镁的塑性变形机制随应变增大由基面部分位错活动转变为孪晶迁移；而在单轴拉伸载荷作用下,材料的塑性变形以基面位错活动、基面层错及面心立方相变为主。当改变加载方向后,会导致孪晶界迁移机制与基面位错发射机制的竞争。{1012}<1011)纳米孪晶镁中沿晶裂纹位置的改变会对裂纹扩展机制产生影响。不同的孪晶界间距也使材料出现韧脆转变,较小的孪晶界间距会抑制沿晶裂纹扩展从而提高材料的断裂韧性。3.模拟并讨论了{1011}(1012)纳米孪晶镁在单轴压缩及单轴拉伸载荷作用下的变形过程。在考虑了是否存在预置缺陷以及孪晶界间距等因素的前提下,解释了{1011}(1012)纳米孪晶镁在不同载荷作用下的塑性变形机制,并在此基础上讨论了{1011}(1012)纳米孪晶镁的拉压不对称。在模拟过程中发现,由于{1011}(1012)孪晶界极其稳定的特征,材料会有很高的屈服强度,但塑性能力很差。当引入预置线缺陷以后,材料塑性能力有所提高。其中在单轴压缩载荷作用下,模型的塑性变形机制主要为基面位错滑移同时模型中也存在一定数量的锥面滑移。在单轴拉伸载荷作用下,基面滑移和锥面滑移为主要塑性变形机制。由于压缩模型与拉伸模型塑性机制不同的原因导致了{1011}(1012)纳米孪晶镁的拉压不对称。此外,在拉伸载荷作用下,左侧边界裂纹的裂尖附近仅有少量基面位错活动,同时由于{1011}(1012)孪晶面无法像{1012}(1011>孪晶面一样作为裂纹扩展面,材料展现了较强的断裂韧性。4.{1012)(1011>纳米孪晶与{1011}(1012)纳米孪晶由于其所包含孪晶界的不同,力学性质与塑性变形机制有显著差异。对于{1012}(1011>纳米孪晶而言,由于其内部孪晶界较易于形核基面位错,并提供大量位错活动,因此对其塑性能力有一定提高作用。同时由于{1012}(1011>孪晶界可作为裂纹扩展面,对沿晶裂纹扩展无法起到有效抑制作用。而{1011}(1012)孪晶界由于其稳定性,很难在孪晶面形核大量位错,这种性质一方面有助于提高材料的屈服强度另一方面也会导致材料较低的塑性变形能力。通过预置线缺陷,可有效改善{1011}(1012)纳米孪晶材料的塑性变形能力。此外由于{1011}(1012>孪晶界很难作为沿晶裂纹扩展面,因此提高了材料的断裂韧性。更多还原

【Abstract】 Nanocrystalline (NC) metals and their alloys have been wildly used in the applied industries for their superb mechanical behaviors. The enhancement of the strength and ductility are mainly due to the refinement of the grains size to a nanometer level, usually under 100nm. Plane defects such as grain boundaries (GBs), coherent twin boundaries (CTBs) and incoherent twin boundaries (ITBs), play a crucial role on the plastic deformation of the polycrystalline metal. They serve as defects nucleation sources which can accommodate the plastic deformation very well. Dislocation activities may also be hindered by the GBs, CTBs or ITBs which improves the dislocation storages abilities. As a result the ductility would be highly enhanced by the rich volume of dislocation. Moreover, the plane defects significantly exert effect on the intergranular and the transgranular crack propagations. Due to the dislocation emission from the plane defects or crack, crack tips may be badly blunted, and subsequently crack propagations are impeded. This feature are benefit to the materials fracture toughness. Thus it is very important to investigate the materials which contain plane defects, and reveal their deformation mechanisms. Molecular dynamics (MD) method are competent in presenting microstructure evolutions details in atomistic scale by simulating models. Thus in this work, molecular dynamics simulations are employed to study the mechanical behaviors and plastic deformation mechanisms of bicrystal and nanotwinned materials. The main results are listed as follow:1. α-Fe bicrystal models which contains the{112}(110) grain boundary are simulated under tensile loadings. Plastic deformation mechanisms and intergranular crack propagations are investigated. It is found that phase transition induced by the partial dislocation slipping along the grain boundary interface and subgrains generations are the main plastic deformation mechanisms. Phase transition can be captured mainly in the early stage of the plastic deformation, and with the strain increasing, subgrains gradually dominate the deformation. Temperature and strain-rates exert influences on the mechanical behaviors of the bicrystal models. Relative high temperature and high strain-rates are benefit to the subgrains generations which enhances the materials ductility. Intergranular crack propagation are affected by the crack position and temperatures due to the different plastic deformation mechanisms around the crack tip. It is found that subgrains generations ahead of the crack can prevent cleavage fracture especially under the high temperature.2. {1012}<1011) nanotwinned magnesium with different orientations and twin boundaries spacing are studied. Intergranular crack propagation are also concerned considering crack position and twin boundaries spacing effects. When the tensile loadings is perpendicular to the twin boundaries interfaces, basal dislocation emission, stacking faults and fcc phase transition are the main plastic deformation mechanisms. With the loadings orientations changing, twinning dislocation inducing twin boundaries migration and basal dislocation motion compete to dominate the deformation process. Different crack positions lead to various crack propagation pattern due to the dislocation nucleation abilities. Moreover a ductile to brittle transition is captured by changing the twin boundaries spacing. Essentially, it is the dislocation densities that influences the materials fracture toughness.3.{1011}{1012) nanotwinned magnesium with pre-existing line defects and different twin boundary spacing are studied to investigate the mechanical behaviors and plastic deformation mechanisms. Because of the stability of the{1011}(1012) twin boundary, it can hardly serve as defects nucleation sources. Thus the model without pre-existing defects yields under a high strain and possesses very low average flow stress. The introduction of the line defects can remarkably enhances the ductility of the nanotwinned materials under both compressive and tensile loadings. Basal and pyramidal slip are the main plastic deformation mechanisms. However due to the different volume of the slip, the tension-compression asymmetry are capture during the simulation processes.4. Because of the different twin boundaries in{1012}{1011} and{1011}{1012} nanotwinned magnesium, materials have distinct mechanical behaviors and plastic deformation mechanisms. This can be attributed to the different abilities of generating dislocations along twin boundaries interfaces and twin boundaries impeding crack propagation.更多还原