| 透射电镜原位拉伸研究纳米片层孪晶结构Cu和纳米晶Ni的形变机理 | |
| Alternative Title | In situ TEM investigation of deformation mechanism in nanoscale growth twin Cu and nanocrystalline Ni |
| 王艳波 | |
| Subtype | 博士 |
| Thesis Advisor | 隋曼龄 |
| 2008-04-22 | |
| Degree Grantor | 中国科学院金属研究所 |
| Place of Conferral | 金属研究所 |
| Degree Discipline | 材料物理与化学 |
| Keyword | 纳米孪晶片层 纳米晶 原位透射电镜 位错发射 孪晶界移动 晶粒旋转 |
| Abstract | 纳米材料制备和形变时，极易引入孪晶。孪晶界作为一种特殊的大角晶界，它的界面能要比一般晶界低得多，会产生一些特殊的变形行为。近年来，许多研究组广泛而深入地研究孪晶界面如何提高材料的强度，对于它如何提高材料的塑性和延伸率方面仍然缺乏研究和认识。特别对孪晶界面是否发射位错、孪晶界是否移动等争论颇多。另外关于纳米晶材料形变机制的研究也只是建立在分子动力学模拟的基础之上，实验证明就显得迫在眉睫了。为此，本文利用透射电镜原位拉伸技术结合高分辨电子显微学和衍衬原理研究了纳米片层孪晶Cu和纳米晶Ni的形变机理，主要结果包括以下几个方面： 纳米片层孪晶铜沿不同方向拉伸时，孪晶界面都阻碍位错运动，增加材料的强度，同时孪晶界面也可作为位错源发射位错。另外，孪晶端头和生长台阶处是位错源的有利位置，所发射的肖克莱（Shockley）不全位错未必在最大的施密特（Schmid）因子面上滑移，这取决于生长过程中储存的Shockley位错的类型。这些位错的发射以及它们的增殖极大地提高了材料的塑性和延伸率。 观察到扩展位错穿过孪晶界面时位错反应的全过程：Dγ + γA→ Dγ + γγ′ + γ′A→ DA → BD′ + Cδ → Bγ′ + γ′D′ + Cδ。这也表明晶格位错穿过普通晶界的三个准则同样适用于孪晶界面。 探讨了从共格孪晶界面演变到孪晶界发射位错的三个阶段：第一阶段是带有不可动的Frank位错的孪晶台阶的出现；第二个阶段是带有台阶结构的非共格孪晶界发射位错阶段；第三个阶段是穿过孪晶界面的位错与孪晶界面发射位错共存阶段。并提出不可动弗兰克（Frank）位错的存在和它对在孪晶界面上运动的Shockley不全位错的钉扎是孪晶界作为位错源的有利条件。并且还使用系列双束衍衬像确定了孪晶界面所发射位错的柏氏矢量。 通过原位高分辨电镜，首次观察到孪晶界面的移动，这种移动是通过形变产生的 Shockley 不全位错在孪晶界上的滑移完成的。孪晶界移动是纳米片层孪晶铜在塑性变形初期的主要形变方式。实验中还发现 Shockley 不全位错在较低 Schmid 因子面上（孪晶界面）运动，与分子动力学模拟预测的结果不同。分析表明这差异主要来自于分子动力学模拟计算的高应力状态。纳米片层孪晶样品的特殊结构，即高密度的晶界/孪晶界交点，有利于发射不全位错造成孪晶界的移动。 利用纳米束电子衍射技术和系列暗场像技术原位拉伸形变平均晶粒尺寸为20 nm的电沉积纳米晶Ni，观察到较小晶粒的旋转和晶粒合并长大的全过程，并建立了物理模型来解释形变诱发的晶粒旋转和晶粒合并长大现象。为纳米晶材料晶界协调变形机制提供了有利的实验证据。通过传统的透射电镜(TEM) 技术和X射线衍射(XRD) 实验，对比研究了块体纳米晶Ni形变前后的微观结构变化，排除了原位拉伸样品的二维薄膜效应，证明了晶粒旋转和晶粒合并长大为块体纳米材料形变的本征特性之一。 |
| Other Abstract | Twins can be easily introduced into nanocrystalline materials during their preparations and deformations. Twin boundary (TB) is an especial high angle grain boundary and will lead to particular deformation behaviors because of its lower interface energy than that of general grain boundary. Recently, the effect of TBs on advancing material’s tensile strength has been studied widely and intensively by many investigation groups. However, the study of increasing plasticity and tensile ductility is still absent, especially for that the dislocations are whether or not been emitted from TBs and that twin boundary migration is whether or not occurred during deformation. In addition, the investigation on deformation mechanisms in nanocrystalline (nc) materials is only limited to molecular dynamics simulations, the experimental evidences on deformation mechanisms is in need. Therefore, in this dissertation, the deformation mechanisms on nanoscale growth twin Cu and nc Ni have been investigated by means of in situ transmission electron microscopy (TEM) straining technique associated with high resolution TEM and diffraction contrast principle. The main results are as follows: The strength of nanoscale growth twin Cu is enhanced because the movement dislocations are blocked by TBs when it was strained along different directions. At the same time, the situations of twin ends and twin steps are also helpful to Shockley dislocation emissions, which do not necessarily move on slipping plane with the maximum Schmid factor. The results are attributed to deposited Shockley dislocation types in twin ends and twin steps. The dislocation emissions and their propagations will apparently increased material’s plasticity and tensile ductility. The dislocation reactions of extended dislocation crossing through TBs were discussed, i.e., Dγ + γA→ Dγ + γγ′ + γ′A→ DA → BD′ + Cδ → Bγ′ + γ′D′ + Cδ. At the same time, it is also indicated that three criteria of lattice dislocations crossing through GBs is fit for TBs. Three stages of dislocation emissions from TBs were discussed: The first stage is the steps formation with sessile Frank dislocation; The second stage is that incoherent TBs with steps act as dislocation sources; The third stage is crack tip dislocations join the dislocation emission processes. In addition, it is also revealed that the existence of a sessile Frank partial step and its obstruction for glissile Shockley dislocations in TBs is good for TBs to serve as dislocation sources. The dislocation burgers vector was determined by serials of dual beam diffraction contrast images. The first direct evidence of twin boundary migration via Shockley partial dislocation slipping in TBs was provided via TEM, such migration is the dominant deformation mechanism in the initial stage of plastic straining. It is also revealed that Shockley dislocations move in slip plane with lower Schmid factor (TB plane). This behavior is discussed in comparison with molecular dynamics simulations (high stress). The unique characteristics of the sample microstructure with high density TB/GB intersections are helful to twin boundary migration from Shockley emissions. Nano-beam electron diffraction and series of dark field images techniques were used to investigate the deformation mechanisms of nc Ni (average grain size 20 nm) in response to in situ tensile deformation under TEM. The experiments exhibit the complete processes of individual grain rotation and neighboring grain rotation/growth, and propose a physical model of deformation-induced grain rotation and grain growth. This provides real time and compelling evidences for GB-mediated deformation mechanism in nc materials. At the same time, these results were confirmed further by ex situ TEM observation and XRD experiments on deformed sample, eliminating two dimensions effect of in situ tensile sample. |
| Pages | 120 |
| Language | 中文 |
| Document Type | 学位论文 |
| Identifier | http://ir.imr.ac.cn/handle/321006/16849 |
| Collection | 中国科学院金属研究所 |
| Recommended Citation GB/T 7714 | 王艳波. 透射电镜原位拉伸研究纳米片层孪晶结构Cu和纳米晶Ni的形变机理[D]. 金属研究所. 中国科学院金属研究所,2008. |
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