Physics of Dislocation Patterning and Size Effects in Plasticity

位错图案物理和可塑性尺寸效应

• 批准号: 0116090
• 负责人: Robin Selinger
• 金额: $ 19.24万
• 依托单位: Catholic University of America
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2001
• 资助国家: 美国
• 起止时间: 2001-08-01 至 2006-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0116090&HistoricalAwards=false
• 关键词: Physics; Dislocation; Patterning; Size; Effects

This grant supports theoretical research on the elastic-plastic response of crystalline solids. The objectives of the project include: (1) characterizing and understanding the thermodynamics driving coalescence of dislocations into ordered structures in two dimensions, and (2) investigating the origins of size effects in plasticity.Dislocation patterning has been extensively modeled as a non-equilibrium process governed by competing kinetics, but the thermodynamics driving the coalescence of dislocations into ordered structures is not well understood. This project considers the simple case of screw dislocations under shear in two dimensions, and models their coalescence into slip bands using both simulation and theory. Simulation studies are carried out using an idealized model of a crystal under anti-plane shear with constant strain rate, a system that is a close analog of the two-dimensional XY rotor model under driving boundary conditions. It is conjectured that slip-band formation in 2D represents a non-equilibrium quench into two-phase coexistence between defect-rich and defect-poor phases. Characterization of the underlying equilibrium phase diagram, scaling behavior, and flow properties of the defect-rich phase will be used to derive constitutive laws for plasticity in two dimensions. Dislocation patterning in three-dimensional crystalline solids is far more complex, involving dislocation entanglement and a host of inherently 3D mechanisms. While no 2D model can accurately describe a real 3D material, this work will provide at least qualitative insight into the competition between energy and entropy in evolution of dislocation microstructures. The project's second goal is to gain insight into the origin of size effects in plasticity. Recent experiments show that some mechanical properties of crystalline solids vary with sample size in the range below ~ 100 microns. Size effects are observed in torsion and bending but not in simple tension, suggesting that strain gradients play an important role. Efforts to build strain gradient effects in continuum plasticity theory point to the importance of a characteristic length scale whose origin is not fully understood. To find out what mechanisms are involved, simulation studies of dislocation patterning will be carried out using both the idelaized two-dimensional model and more realistic molecular dynamics in two and three dimensions. Geometries will include (1) a ductile slit crack loaded in shear, (2) torsion/bending of a beam, and (3) pullout of a thin fiber from a soft metal matrix. In each case multiple simulations will be preformed for samples of different sizes and the results will be compared to the predictions of continuum strain gradient theories.%%%This grant supports theoretical research on the elastic-plastic response of crystalline solids. The objectives of the project include: (1) characterizing and understanding the thermodynamics driving coalescence of dislocations into ordered structures in two dimensions, and (2) investigating the origins of size effects in plasticity. Successful completion of this project will increase our knowledge of the mechanical propoerties of materials, including their failure.***

该资助支持结晶固体弹塑性响应的理论研究。 该项目的目标包括：（1）表征和理解驱动位错合并成二维有序结构的热力学，以及（2）研究塑性尺寸效应的起源。位错图案已被广泛建模为受竞争动力学控制的非平衡过程，但驱动位错合并成有序结构的热力学尚不清楚。 该项目考虑了二维剪切下螺旋位错的简单情况，并使用模拟和理论将它们的合并建模为滑移带。 仿真研究是使用恒定应变率反平面剪切下的理想晶体模型进行的，该系统与驱动边界条件下的二维 XY 转子模型非常相似。 据推测，二维滑带的形成代表了富缺陷相和贫缺陷相之间的两相共存的非平衡淬灭。 富缺陷相的基本平衡相图、缩放行为和流动特性的表征将用于推导二维塑性的本构定律。 三维晶体中的位错图案要复杂得多，涉及位错缠结和许多固有的 3D 机制。 虽然没有 2D 模型可以准确描述真实的 3D 材料，但这项工作至少将提供对位错微结构演化中能量和熵之间竞争的定性洞察。该项目的第二个目标是深入了解可塑性中尺寸效应的起源。 最近的实验表明，结晶固体的一些机械性能随样品尺寸在约 100 微米以下的范围内变化。 在扭转和弯曲中观察到尺寸效应，但在简单拉伸中观察不到尺寸效应，这表明应变梯度起着重要作用。 在连续塑性理论中建立应变梯度效应的努力表明了特征长度尺度的重要性，其起源尚不完全清楚。 为了找出其中涉及的机制，将使用理想化二维模型和更现实的二维和三维分子动力学进行位错图案的模拟研究。 几何形状将包括（1）剪切载荷下的延性狭缝裂纹，（2）梁的扭转/弯曲，以及（3）从软金属基体中拉出细纤维。 在每种情况下，将对不同尺寸的样品进行多次模拟，并将结果与​​连续应变梯度理论的预测进行比较。%%%这项资助支持结晶固体弹塑性响应的理论研究。 该项目的目标包括：（1）表征和理解驱动位错合并成二维有序结构的热力学，以及（2）研究塑性中尺寸效应的起源。 该项目的成功完成将增加我们对材料机械性能（包括其失效）的了解。***