CAREER: Fundamental Investigation of Surface Fatigue Crack Initiation Mechanisms in Nanocrystalline FCC Metals

职业：纳米晶 FCC 金属表面疲劳裂纹萌生机制的基础研究

• 批准号: 1255046
• 负责人: Olivier Pierron
• 金额: $ 55万
• 依托单位: Georgia Tech Research Corporation
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-05-15 至 2019-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1255046&HistoricalAwards=false
• 关键词: CAREER; Fundamental; Investigation; Surface; Fatigue

TECHNICAL SUMMARY:The overarching objective of this CAREER proposal is to identify the surface fatigue crack initiation mechanisms in nanocrystalline face-centered-cubic metals as a function of three material parameters (grain size, generalized stacking fault energy curve, oxidation behavior) and three loading factors (maximum plastic strain, frequency, environment); Nanocrystalline metals (grain size 100 nm) exhibit extraordinary mechanical properties and can combine ultra-high strength with considerable ductility. However, there is so far little quantitative, mechanistic-based understanding of their fatigue properties, such as the observed improved fatigue limit compared to their coarse grained counterparts (grain size 1 micrometers). Accordingly, this proposal seeks to monitor cyclic plasticity and measure initiation fatigue life on nanocrystalline face-centered-cubic nanobeams tested with a state-of-the-art MEMS device, for Al, Cu, Ni, and Au, as a function of three loading factors. In addition, this proposal seeks to identify the fatigue crack initiation mechanisms using transmission electron microscopy (TEM) observations, and whenever possible, establish relevant statistics of the operating mechanisms (such as frequency of occurrence) as a function of the aforementioned experimental parameters. Particularly, quantitative in-situ TEM fatigue testing will be performed to observe fatigue damage accumulation during cyclic loading. The proposed research offers original contributions to obtain mechanistic insight into the length-scale effects in fatigue processes. Particularly, this research program is expected to yield a mechanistic model linking the characteristics of cyclic plasticity (including, importantly, irreversibility mechanisms) to surface fatigue crack initiation in nanocrystalline face-centered-cubic metals. Such a model can provide a scientific basis for predicting the fatigue behavior of this class of materials. NON-TECHNICAL SUMMARY:Nanocrystalline metals are a promising class of ultra-strong materials. The reasons for the increase in strength due to decreasing grain size are fairly well understood, and several plastic deformation mechanisms have been identified to operate in this grain size regime. However, there is currently no satisfying model linking the plastic deformation mechanisms under cyclic loading and the resulting fatigue degradation properties of nanocrystalline metals. This proposal seeks to investigate the governing fatigue mechanisms of nanocrystalline metals using a state-of-the-art experimental technique, and to use this understanding to promote research and teaching in the fields of Science, Technology, Engineering, and Mathematics to high school students and teachers. Particularly, the PI will create a summer enrichment program, entitled FAMED (Failure Analysis for Mechanical Engineering Detectives), targeted for high school students, that will involve high school teachers, graduate and undergraduate students to develop and implement it. During the one-week-long program, the students will learn about the fundamental science related to the failure of materials in the form of short lectures and hands-on demos. They will also have a chance to act as failure analysis experts in idealized litigation cases whose outcome depends on the correct analysis of a failed object.

技术摘要：本职业提案的首要目标是确定纳米晶面心立方金属中的表面疲劳裂纹萌生机制，作为三个材料参数（晶粒尺寸、广义堆垛层错能量曲线、氧化行为）和三个载荷的函数因素（最大塑性应变、频率、环境）；纳米晶金属（晶粒尺寸 100 nm）表现出非凡的机械性能，可以将超高强度与相当大的延展性结合起来。然而，迄今为止，对其疲劳特性的定量、基于机理的了解还很少，例如与粗晶对应物（晶粒尺寸为 1 微米）相比，观察到的疲劳极限有所改善。因此，本提案旨在监测 Al、Cu、Ni 和 Au 的循环塑性并测量使用最先进的 MEMS 设备测试的纳米晶面心立方纳米梁的初始疲劳寿命，作为三个函数的函数负载因素。此外，该提案旨在利用透射电子显微镜（TEM）观察来识别疲劳裂纹萌生机制，并尽可能根据上述实验参数建立操作机制（例如发生频率）的相关统计数据。特别是，将进行定量原位 TEM 疲劳测试，以观察循环加载过程中的疲劳损伤累积情况。拟议的研究为获得疲劳过程中长度尺度效应的机械洞察提供了原创性贡献。特别是，该研究计划预计将产生一个机械模型，将循环塑性特征（重要的是不可逆机制）与纳米晶面心立方金属的表面疲劳裂纹萌生联系起来。这样的模型可以为预测此类材料的疲劳行为提供科学依据。非技术摘要：纳米晶金属是一类很有前途的超强材料。由于晶粒尺寸减小而导致强度增加的原因已被很好地理解，并且已经确定了在这种晶粒尺寸范围内运行的几种塑性变形机制。然而，目前还没有令人满意的模型将循环载荷下的塑性变形机制与纳米晶金属的疲劳退化性能联系起来。该提案旨在利用最先进的实验技术研究纳米晶金属的疲劳机制，并利用这种理解来促进高中生在科学、技术、工程和数学领域的研究和教学和老师。特别是，PI 将针对高中生创建一个名为 FAMED（机械工程侦探的故障分析）的夏季强化项目，该项目将由高中教师、研究生和本科生参与开发和实施。在为期一周的课程中，学生将以简短的讲座和动手演示的形式了解与材料失效相关的基础科学。他们还将有机会在理想的诉讼案件中担任失效分析专家，其结果取决于对失效对象的正确分析。