Fundamental Understanding of Deformation Mechanisms in Nanocrystalline Superplasticity

纳米晶超塑性变形机制的基本理解

• 批准号: 0703994
• 负责人: Amiya Mukherjee
• 金额: $ 32.5万
• 依托单位: University of California-Davis
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-07-01 至 2013-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0703994&HistoricalAwards=false
• 关键词: Fundamental; Understanding; Deformation; Mechanisms; Nanocrystalline

TECHNICAL: This is a renewal project designed to gain a fundamental understanding of the deformation mechanisms of superplasticity in nanocrystalline materials. The phenomenon of sliding along the grain interfaces is one of the dominant rheological characteristics in superplasticity. The ultra-fine grained nanocrystalline materials contain a very large density of interfaces and, therefore, they offer a unique opportunity to study the role of grain boundary sliding in superplasticity. Using high pressure torsional straining to produce porosity and contaminant free nanocrystalline materials from ingot stock, the project outlines an experimental program to study the superplastic deformation of a model nanocrystalline material. Preliminary mechanical results on nanoscale materials suggest that the experimental goal is achievable. However, the theoretical micro-mechanisms for such observation of nanocrystalline superplasticity are far from being clear. Experimental investigations during the previous NSF grant have revealed high flow stress, extensive strain hardening, and a correlation between microstructural instability and onset of enhanced plasticity in nanomaterials. This renewal project attempts to uncover the underlying reasons for such observations. A specially designed and instrumented tensile testing device has been constructed for conducting precision mechanical tests under protective environment to prevent oxidation during testing. The nanostructure before and after deformation will be investigated using TEM/HREM. Particular emphasis will be paid to in-situ tensile TEM observation at the superplastic temperature range, which will be complimented at the University of Southern California by molecular dynamic simulation studies. The intellectual merit lies in our attempt to correlate the microstuctural information with the mechanical data obtained from such nanoscale materials in the context of elevated temperature plasticity in really diminished length scales. Special emphasis will be given to the difficulty of intra-granular dislocation generation inside the matrix in truly nanoscale structure and its implication to slip accommodation processes in current models of nanocrystalline plasticity at elevated temperatures. The role of grain boundary as a source of dislocations in the interface sliding process as the grain size decreases to nanoscale dimensions will be emphasized. NON-TECHNICAL: Determining whether the observed increase in superplastic strain rate and/or decrease in superplastic temperature with decreasing grain size is a general phenomenon or not, has important technological implications in our society. Increasing superplastic strain rate will decrease forming time and will make it an economically viable process. Lowering superplastic forming temperature will allow utilization of some of the existing forming technology for shop-floor practice in industrially significant intermetallic structural materials. Outreach activities would provide educational outreach to the Davis community through ongoing programs for high school students in the Davis/Sacramento area. The program has involved a number of lectures on the basics of Materials Science and Nanotechnology for students who came to visit campus and our Metallography in our laboratory. Another area of our outreach activity was targeted to the public at large of all ages through participation in Nanoscape project at the Exploratorium in San Francisco. This program aimed to increase public awareness of nanoscience by involvement in the creation of several large-scale, scientifically accurate, and visually compelling models of nanoscale molecules and crystals. Our task was to help the public understand how these geometries hold promise of new technologies.

技术：这是一个更新项目，旨在对纳米晶材料超塑性变形机制有一个基本的了解。沿晶粒界面滑动的现象是超塑性的主要流变特征之一。超细晶粒纳米晶材料包含非常大的界面密度，因此，它们为研究晶界滑动在超塑性中的作用提供了独特的机会。该项目概述了利用高压扭转应变从铸锭生产出多孔且无污染的纳米晶材料的实验计划，以研究模型纳米晶材料的超塑性变形。纳米级材料的初步力学结果表明实验目标是可以实现的。然而，这种观察纳米晶超塑性的理论微观机制还远未明确。之前的 NSF 资助期间的实验研究揭示了纳米材料中的高流动应力、广泛的应变硬化以及微观结构不稳定性和塑性增强之间的相关性。这个更新项目试图揭示这种观察结果的根本原因。我们构建了专门设计和仪器化的拉伸测试装置，用于在保护环境下进行精密机械测试，以防止测试过程中的氧化。将使用 TEM/HREM 研究变形前后的纳米结构。将特别强调超塑性温度范围内的原位拉伸 TEM 观察，南加州大学将通过分子动力学模拟研究对此进行补充。智力上的优点在于我们尝试将微观结构信息与在真正减小的长度尺度的高温塑性背景下从此类纳米级材料获得的机械数据相关联。将特别强调在真正的纳米级结构中基体内部产生晶内位错的困难及其对当前高温下纳米晶塑性模型中滑移适应过程的影响。当晶粒尺寸减小到纳米级尺寸时，晶界作为界面滑动过程中位错源的作用将被强调。非技术性：确定观察到的超塑性应变率增加和/或超塑性温度随晶粒尺寸减小而降低是否是普遍现象，对我们的社会具有重要的技术意义。提高超塑性应变率将减少成形时间，并使其成为经济上可行的工艺。降低超塑性成形温度将允许利用一些现有的成形技术进行工业上重要的金属间结构材料的车间实践。外展活动将通过针对戴维斯/萨克拉门托地区高中生的持续项目，向戴维斯社区提供教育外展活动。该计划包括为参观校园和我们实验室金相学的学生提供一系列有关材料科学和纳米技术基础知识的讲座。我们的外展活动的另一个领域是通过参与旧金山探索博物馆的 Nanoscape 项目，面向各个年龄段的广大公众。该计划旨在通过参与创建几个大规模、科学准确且视觉上引人注目的纳米级分子和晶体模型来提高公众对纳米科学的认识。我们的任务是帮助公众了解这些几何形状如何承载新技术的希望。