TECHNICAL: This CAREER project addresses the resistance of grain boundaries to short fatigue crack growth and the effects of crystallographic texture and grain structure on the fatigue properties in high performance alloys. Currently, none of the existing models are able to take into account the 3-dimensional effects of microstructure in calculating short fatigue crack growth, and, therefore, their success is limited. This research is built on the PI's recent success in identifying the twist and tilt components of crack plane deflection at grain boundaries as the key factors that control the growth behavior of short cracks across the boundaries. In this research, a uniquely designed fatigue experiment on a single crystal alloy with a fine notch will be carried out in order to reveal the quantitative relation between crack plane deflection and the resistance to crack growth. The effects of texture on the fatigue properties will also be studied in details with electron backscatter diffraction in new generation high strength aluminum alloys that have different textures. The data obtained from all these experiments will be used to develop a 3-d model to quantify the growth behavior of short fatigue cracks by taking into account 3-d interaction between grain boundaries and the crack, and the effects of texture in the alloys. It is anticipated that the results derived from this project will 1) quantify the resistance of grain boundaries to short fatigue crack growth, 2) identify the optimum texture that leads to the more balanced mechanical properties, especially the fatigue properties, and 3) develop a 3D model for better simulation of short crack growth hence improving the methodology for life prediction of key engineering components. NON-TECHNICAL: Quantitative understanding of the interaction of a short fatigue crack with grain boundaries is critical to design of safer engineering structures such as airplanes and spacecrafts, and for more sustainable use of materials. In this project, the research work will be integrated into the PI's teaching activities, and the findings from this research project will also be utilized to promote materials education. A new course on crystallographic texture aimed at upper level undergraduate students and beginning graduate students will be developed to bridge the gap between extensive research and insufficient education in the field of texture. Currently, few universities offer such as a course in the U.S. In addition to graduate students, undergraduate students will also be trained in texture theory and research by participating in experimental and theoretical activities in this project. As an outreach activity in this project, the PI will also develop an on-line self-study course on texture in order to promote education on texture beyond the PI's university and help to train the technical personnel in materials industry about texture and the importance of its control during processing of metallic materials. Feedback about these courses will be actively sought and used to further improve the effectiveness of these courses.
The main results obtained from this project that are scientifically significant include: 1) establishment of the equations for quantification of the resistance from a grain boundary against short fatigue crack growth in alloys, and 2) a quantitative microstructure-based model developed to simulate short fatigue crack growth in 3 dimensions in an alloy. These results provide a solid foundation for further research work to understand thoroughly and quantitatively the growth behaviors of short fatigue cracks, especially their relationships with the local microstructure in engineering alloys. They could pave the way to the development of models to quantify more satisfactorily the total life of an alloy component in engineering structure, and to design advanced alloys that have the optimum resistance against fatigue damage. The research work conducted in this project could also advance both life prediction and alloy design methodologies, which will improve the safety of airplanes and other engineering structures. Two undergraduate students from mechanical engineering were trained in materials engineering by participating in this project to make several 3 dimension animation models for some concepts and processes in materials science education. Five Ph.D. graduate students were trained in materials technologies by participating in this project. One of these students has graduated and works in an US aluminum company. During this project, materials engineering was promoted among K-12 school students in Kentucky through demonstrating metal fracture by those graduate students who participated in this project in the annual E-day exhibitions held by the Engineering College at the University of Kentucky.
