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

技术概述：本CAREER方案的总体目标是确定纳米晶面心立方金属的表面疲劳裂纹起裂机制，并将其作为三个材料参数（晶粒尺寸、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（晶粒尺寸为100纳米）具有非凡的机械性能，可以结合超高的强度和可观的延展性。然而，到目前为止，对其疲劳特性的定量的、基于力学的理解还很少，比如与粗晶材料（晶粒尺寸为1微米）相比，观察到的疲劳极限有所提高。因此，本研究旨在监测纳米晶面心立方纳米梁的循环塑性并测量其引发疲劳寿命，该纳米梁采用最先进的MEMS设备对Al、Cu、Ni和Au进行测试，作为三种载荷因素的函数。此外，本建议寻求利用透射电子显微镜（TEM）观察来确定疲劳裂纹的起裂机制，并尽可能建立相关的运行机制（如发生频率）作为上述实验参数的函数。特别是，将进行定量的原位TEM疲劳试验，以观察循环加载过程中的疲劳损伤积累。提出的研究为获得疲劳过程中长度尺度效应的机理见解提供了原创性贡献。特别是，该研究计划有望产生一个机制模型，将循环塑性的特征（包括，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。该模型可为预测该类材料的疲劳性能提供科学依据。非技术概述：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经被很好地理解，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，目前还没有一个令人满意的模型将纳米晶金属在循环载荷下的塑性变形机制与由此产生的疲劳退化性能联系起来。本提案旨在利用最先进的实验技术研究纳米晶金属的控制疲劳机制，并利用这一理解促进科学、技术、工程和数学领域的研究和教学，面向高中学生和教师。特别是，PI将以高中学生为对象，创建一个名为“famous（机械工程侦探故障分析）”的暑期充实项目，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实践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.