SGER: Stress State Dependence of Peierls Barriers and Dislocation Kinetics at the Nanoscale

SGER：纳米尺度 Peierls 势垒和位错动力学的应力状态依赖性

• 批准号: 0439418
• 负责人: Mahadevan Khantha
• 金额: $ 6万
• 依托单位: University of Pennsylvania
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2004
• 资助国家: 美国
• 起止时间: 2004-08-01 至 2005-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0439418&HistoricalAwards=false
• 关键词: SGER; Stress; State; Dependence; Peierls

The plastic behavior of crystalline materials is governed by dislocation glide and the yield and flow stress is determined by the ability of dislocations to circumvent obstacles. Localized obstacles that can be overcome with the aid of thermal activations control temperature and strain rate dependencies. Among those, the Peierls barrier is particularly significant. It is an intrinsic property of a specific material, related to the atomic structure of the dislocation cores. It cannot be altered by treatments, such as purification, annealing etc., unless the nature of the material is altered. While the dislocation core structure affects deformation properties at all scales, it is most important at nanocale when the size of the samples and/or components becomes comparable with the extent of the critical thermally activated event of overcoming the Peierls barrier. Furthermore, at this scale other obstacles may be separated so as to diminish in importance, while the Peierls barrier relates to the inherent lattice resistance. When the dislocation core effects are important, unexpected deformation modes together with strong and often unusual dependence of the yield and flow stress on temperature and crystal orientation are common. A prominent feature is that the plastic flow is influenced by the entire stress tensor rather than only by the Schmid stress, i. e. the shear stress in the slip direction in the slip plane. An outstanding challenge is to identify and quantify how the atomic level core properties project onto the nanoscale via thermally activated motion of dislocations and then percolate through the mesoscale dynamics of dislocations up to the macroscopic flow properties. The link and interplay between the two lowest levels of this hierarchy is the focus of this Small Grant for Exploratory Research.The state-of-the-art atomistic calculations, carried out by the investigators, expose the exact tensorial stress-state dependence of the Peierls stress, which is the stress needed to overcome the Peierls barrier at 0K. However, they do not reveal the Peierls barrier alone and its stress-state dependence. Hence, the principal challenge of the proposed research is to utilize the atomic level understanding of the stress-state dependence of the Peierls stress in the theoretical analysis of the thermally activated dislocation motion. The researchers will employ for this purpose the multidimensional Kramers approach in which the thermal activation over the energy barrier is treated using the stochastic Langevin equation with associated 'friction coefficients' for the reaction and nonreaction modes. In the present context, the 'friction' represents the cumulative effect of core transformations induced by the applied stress tensor, which slows down and/or accelerates the kinetics of escape over the barrier. The exploratory aspect of this project is primarily related to how the friction coefficients can be connected to the stress-state dependence of the Peierls stress determined by atomistic simulations of the dislocation cores and related glide at 0K.The outcome of this research will be constitutive relations for the dislocation mobility that encapsulate the effects of temperature and stress induced atomic level core transformations. These relations may then be employed in modeling that ranges from nanoscale dislocation dynamics to continuum analyses of plastic yielding in single and polycrystals. The results of such analysis will accurately reflect the effect of atomic level core properties of dislocations on plastic flow at the nanoscale and via further coarse graining, also on micro and macro scale.This research that links dislocation kinetics at the nanoscale with the atomic level properties of dislocations has wide ramifications. It may provide entirely new insights into how the atomic structure of dislocations affects the dislocation kinetics at finite temperatures. This is of paramount importance, as both functional and structural materials are becoming more complex chemically and crystallographically. It is expect that the proposed research will have major impact not only as a scientific advancement but also in industrial developments when newly formulated constitutive relations become parts of modeling tools, which can be used by materials researchers, as well as device designers.

晶体材料的塑性行为由位错滑移控制，屈服应力和流动应力由位错绕过障碍物的能力决定。局部的障碍，可以克服的帮助下，热激活控制温度和应变速率的依赖性。 其中，Peierls障碍尤为重要。它是特定材料的固有性质，与位错核的原子结构有关。它不能被处理改变，例如纯化、退火等，除非材料的性质改变。虽然位错核心结构影响在所有尺度的变形性能，它是最重要的纳米尺度时，样品和/或组件的大小变得可比的临界热激活事件的程度，克服Peierls障碍。此外，在该尺度下，其他障碍物可以被分离以降低重要性，而Peierls势垒涉及固有的晶格电阻。当位错核心效应很重要时，通常会出现意想不到的变形模式以及屈服应力和流动应力对温度和晶体取向的强烈且通常不寻常的依赖性。一个突出的特点是塑性流动受整个应力张量的影响，而不仅仅是施密德应力，即。e.滑移面内沿滑移方向的剪应力。 一个突出的挑战是如何识别和量化的原子水平的核心属性项目到纳米尺度上通过热激活运动的位错，然后渗透到宏观流动性能的位错的介观动力学。这个层次结构的最低两个层次之间的联系和相互作用是这个探索性研究小额资助的重点。研究人员进行的最先进的原子计算揭示了Peierls应力的确切张量应力状态依赖性，这是克服Peierls势垒所需的应力。然而，他们并没有单独揭示Peierls势垒及其应力状态依赖性。 因此，所提出的研究的主要挑战是利用原子水平的理解的应力状态依赖的Peierls应力的热激活位错运动的理论分析。 为此，研究人员将采用多维Kramers方法，其中使用随机Langevin方程与反应和非反应模式的相关“摩擦系数”来处理能量障碍上的热活化。在本上下文中，“摩擦”表示由所施加的应力张量引起的核心转变的累积效应，其减慢和/或加速越过屏障的逃逸动力学。该项目的探索性方面主要涉及如何将摩擦系数与Peierls应力的应力状态依赖性联系起来，该应力状态依赖性通过位错核心的原子模拟和相关的滑移来确定。本研究的结果将是包含温度和应力诱导的原子水平核心转变的影响的位错迁移率的本构关系。这些关系，然后可以在建模，范围从纳米级位错动力学连续分析在单晶和多晶体的塑性屈服。这种分析的结果将准确地反映位错的原子水平核心性质对纳米级塑性流动的影响，并通过进一步的粗晶化，在微观和宏观尺度上也是如此。这项将纳米级位错动力学与原子水平性质联系起来的研究具有广泛的影响。它可能会提供全新的见解如何位错的原子结构影响位错动力学在有限的温度。这是至关重要的，因为功能和结构材料在化学和晶体学上都变得越来越复杂。 预计所提出的研究将产生重大影响，不仅作为一个科学进步，而且在工业发展时，新制定的本构关系成为建模工具的一部分，可以使用材料研究人员，以及设备设计师。