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

技术摘要：这份职业建议书的主要目标是确定纳米晶面心立方金属的表面疲劳裂纹萌生机制，作为三个材料参数(晶粒度、广义层错能曲线、氧化行为)和三个加载因素(最大塑性应变、频率、环境)的函数；纳米晶金属(晶粒度为100 nm)显示出非凡的机械性能，并可将超高强度和相当大的延展性结合在一起。然而，到目前为止，人们对它们的疲劳特性几乎没有定量的、基于机理的了解，例如与粗晶(晶粒度1微米)相比，观察到的疲劳极限有所改善。因此，这项建议试图监测纳米晶面心立方纳米棒的循环塑性，并测量作为三个加载因子的函数的纳米晶面心立方纳米棒在最先进的MEMS设备上的起始疲劳寿命。此外，这项建议寻求利用透射电子显微镜(TEM)观察来确定疲劳裂纹萌生机制，并尽可能根据上述实验参数建立操作机制(如发生频率)的相关统计数据。特别是，将进行定量的原位透射电子显微镜疲劳试验，以观察循环加载过程中疲劳损伤的累积情况。建议的研究为获得疲劳过程中的长度-尺度效应的机械洞察提供了原创性的贡献。特别是，这一研究计划有望产生一个将纳米晶面心立方金属的循环塑性特征(包括重要的不可逆机制)与表面疲劳裂纹萌生联系起来的机制模型。该模型可为预测此类材料的疲劳行为提供科学依据。非技术综述：纳米晶金属是一类很有前途的超强材料。由于晶粒度减小而导致强度增加的原因是相当清楚的，并且已经确定了几种塑性变形机制在这种晶粒度区域中运行。然而，目前还没有一个令人满意的模型将循环载荷下的塑性变形机制与由此产生的纳米晶金属的疲劳退化特性联系起来。这项建议旨在利用最先进的实验技术研究纳米晶体金属的疲劳控制机制，并利用这一理解向高中学生和教师促进科学、技术、工程和数学领域的研究和教学。特别是，PI将创建一个名为FAMED(机械工程侦探故障分析)的暑期充实计划，面向高中生，将让高中教师、研究生和本科生参与制定和实施该计划。在为期一周的课程中，学生们将以简短的讲座和动手演示的形式学习与材料失效相关的基础科学知识。他们还将有机会在理想化的诉讼案件中担任失败分析专家，这些案件的结果取决于对失败对象的正确分析。

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.