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

技术总结：本CAREER提案的首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（晶粒尺寸、广义堆垛层错能曲线、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100 nm）具有非凡的机械性能，可以将联合收割机超高强度与相当大的延展性结合起来。然而，到目前为止，很少有定量的，基于机械的理解，他们的疲劳性能，如观察到的改善疲劳极限相比，其粗粒对应物（晶粒尺寸1微米）。因此，该建议旨在监测循环塑性和测量初始疲劳寿命的纳米晶面心立方纳米梁测试与一个国家的最先进的MEMS设备，铝，铜，镍，和Au，作为三个加载因子的函数。此外，该提案旨在使用透射电子显微镜（TEM）观察来识别疲劳裂纹萌生机制，并尽可能建立作为上述实验参数的函数的操作机制（如发生频率）的相关统计数据。特别是，将进行定量原位TEM疲劳试验，以观察循环加载期间的疲劳损伤累积。拟议的研究提供了原始的贡献，以获得机械洞察疲劳过程中的长度尺度效应。特别是，这项研究计划预计将产生一个机械模型连接的特点，循环塑性（包括，重要的是，不可逆机制）的表面疲劳裂纹萌生纳米晶面心立方金属。该模型可为预测该类材料的疲劳行为提供科学依据。非技术总结：纳米晶体金属是一类有前途的超强材料。由于晶粒尺寸减小而导致强度增加的原因是相当好理解的，并且已经确定了几种塑性变形机制在该晶粒尺寸范围内起作用。然而，目前还没有令人满意的模型连接在循环载荷下的塑性变形机制和由此产生的疲劳退化性能的纳米晶金属。该提案旨在使用最先进的实验技术研究纳米晶金属的疲劳机制，并利用这种理解促进科学，技术，工程和数学领域的研究和教学，以高中学生和教师。特别是，PI将针对高中生开设一个名为FAMED（Failure Analysis for Mechanical Engineering Detectives）的暑期课程，由高中教师、研究生和本科生共同开发和实施。在为期一周的课程中，学生们将通过简短的讲座和实践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化的诉讼案件中担任故障分析专家，其结果取决于对故障对象的正确分析。

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.