Quantification of 3-D effects of microstructure on fatigue crack initiation and early growth in planar slip alloys

This project addresses the quantitative relationship between short fatigue crack resistance and the twist component of crack plane deflection across either the particle-matrix interface or grain boundary, in order to quantify short fatigue crack growth in 3 dimensions in a planar slip alloy, such as AA2026, 2524 and 2099 Al alloys. A focused ion beam will be employed first to fabricate a micro-notch which has a controlled twist angle with the primary slip plane in a coarse grain of the alloy, and second to make serial cross-sections of fractured particles found in the sample surface after fatigue. The resistance to the micro-crack growth from the notch will be extracted from the measurement of crack growth rate as a function of the twist angle, which is critical for the development of a quantitative microstructure-based model for short fatigue crack growth. The 3 dimensional effects of the surface particles on fatigue crack initiation and early growth will be quantitatively studied from reconstruction of the 3 dimensional geometries of the particles using the serial cross-sections. The twist angle and driving force along the particle interface will be calculated from EBSD measurement and the reconstructed 3 dimensional geometries of these particles. Incorporation of both the resistance and driving force in PI’s crystallographic model, the growth behavior of the micro-crack from a particle in the surface will be quantified in 3 dimensions in the alloys. Consequently, a microstructure-based model will be established to calculate the S-N curve for an alloy, given the grain structure and texture in the alloy. Therefore, using this model, the desirable microstructure and texture will be identified for the optimum high cycle fatigue properties in the alloys. The technical merits of this project include: 1) Quantification of the relationship between crack resistance and crack plane twist angle across a grain boundary, 2) incorporation of the 3 dimensional effects of microstructure in a crystallographic model for quantification of short fatigue crack growth, 3) the model will allow identification of the desirable microstructure and texture for the optimum resistance to fatigue damage in an alloy, and 4) revelation of the mechanism for the difference in micro-crack growth behaviour between different particles on surface. Broader Impacts: The improvement of the fatigue properties of Al alloys, as a result of this project, can have significant impact on the aerospace and automotive industries, and as such the proposed research could lead to a wider impact on society; 2) the alloy industry as a whole will also benefit from the methodology, to be developed in this project, since it will advance alloy design by identification of the desirable microstructure and texture that lead to optimum fatigue resistance in high performance alloys; and 3) development of 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 the findings from the project, for use in his MSE courses. Undergraduate students will also participate in this project to make 3-D animation models for crack growth across grain boundaries, 3-D microstructure and X-ray diffraction, etc. These models will be made available on a webpage, together with the latest results from the project, for access by the public. The PI will organize symposium series on fatigue damage in metallic materials at the TMS annual meetings, to disseminate effectively the findings from this project among the researchers from the research community and engineers from alloy industry.