Fundamental Understanding of Deformation Mechanisms in Nanocrystalline Superplasticity

技术：这是一个更新的项目，旨在获得纳米晶体材料的超塑性变形机制的基本理解。沿晶界滑动现象是超塑性流变学的主要特征之一。超细晶纳米晶材料含有非常大的界面密度，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。利用高压扭转应变技术从铸锭坯中制备孔隙度和无污染的纳米晶材料，该项目概述了一个实验方案来研究模型纳米晶材料的超塑性变形。在纳米尺度材料上的初步力学结果表明，实验目标是可以实现的。然而，这种纳米晶超塑性现象的理论微观机制尚不清楚。在之前的NSF资助中，实验研究揭示了高流动应力，广泛的应变硬化，以及微观结构不稳定性与纳米材料增强塑性之间的相关性。这个更新项目试图揭示这些观察的潜在原因。为了在保护环境下进行精密力学试验，防止试验过程中氧化，设计了专门的仪器化拉伸试验装置。采用TEM/HREM对变形前后的纳米结构进行研究。特别强调的是在超塑性温度范围内的原位拉伸透射电镜观察，这将在南加州大学的分子动力学模拟研究中得到补充。智力上的优点在于我们试图将微观结构信息与从这些纳米级材料中获得的力学数据联系起来，这些材料在温度升高的情况下具有真正缩小的长度尺度。特别强调的是，在真正的纳米尺度结构中，在基体内部产生颗粒内位错的困难及其对当前纳米晶体高温塑性模型中的滑移调节过程的影响。当晶粒尺寸减小到纳米尺度时，晶界作为位错源在界面滑动过程中的作用将被强调。非技术：确定观察到的超塑性应变率的增加和/或超塑性温度的降低随着晶粒尺寸的减小是否是一种普遍现象，在我们的社会中具有重要的技术意义。提高超塑性应变率将缩短成形时间，使之成为一种经济可行的工艺。降低超塑性成形温度将使一些现有的成形技术应用于工业上重要的金属间结构材料的车间实践。外展活动将通过正在进行的戴维斯/萨克拉门托地区高中生项目，向戴维斯社区提供教育外展。该计划包括为来参观校园的学生和我们实验室的金相学学生提供一些关于材料科学和纳米技术基础知识的讲座。我们拓展活动的另一个领域是针对所有年龄段的公众，通过参与旧金山探索博物馆的纳米景观项目。这个项目旨在通过参与创建几个大规模的、科学上准确的、视觉上引人注目的纳米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。

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.