Quantification of 3-D Effects of Microstructure on Fatigue Crack Initiation and Early Growth in Planar Slip Alloys

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TECHNICAL SUMMARY:The PI of this research project proposes to develop a microstructure-based model to quantify fatigue crack initiation and early growth in a planar slip alloy, such as AA2026, 2524, and 2099 Al alloys, first, by identifying the quantitative relationship between short fatigue crack resistance and the twist component of crack plane deflection across either the particle-matrix interface or grain boundaries and, second, by calculating the growth rate of a micro-crack initiating from a particle and taking into account both the driving force and resistance in three dimensions in these alloys. A focused ion beam will be employed to fabricate a micro-notch having a controlled twist angle with the primary slip plane in a coarse grain of the alloy and to make serial cross-sections of fractured particles found in the sample surface after fatigue. The resistance to micro-crack growth from the notch will be extracted from measured crack growth rate data as a function of the twist angle. Incorporation of both the resistance and driving force in the PI's crystallographic model will enable quantification in three dimensions of the growth behavior of the micro-crack from a particle in the surface of the alloys. Using this model will allow identification of the desirable microstructure and texture for optimum high-cycle fatigue properties in the alloys. The model will also help to elucidate the mechanism for the observed differences in micro-crack growth behavior among different particles on a surface in the alloys.NON-TECHNICAL SUMMARY:This project is expected to result in improved fatigue properties in aluminum alloys, which may have significant impact on the aerospace and automotive industries. In this regard, the proposed research has potential for wider societal impacts. The alloy industry as a whole will also benefit from the methodology to be developed, as identification of the desirable microstructure and texture that lead to optimum fatigue resistance in high-performance alloys will advance alloy design. Further, this research project will help to develop well-trained young technical graduates for the aluminum, automotive, and aerospace industries. The PI will integrate the research work into his teaching activities by developing two projects, related to project findings, for use in his materials science and engineering courses. Undergraduate students will also participate by creating three-dimensional animation models of crack growth across grain boundaries, three-dimensional microstructure, and X-ray diffraction. These models will be made available on a website, together with the latest research results, for access by the public. The PI will also organize a symposium series on fatigue damage in metallic materials at The Mineral, Metals & Materials Society annual meetings to disseminate effectively research findings among members of the research community and engineers from the alloy industry.