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.
