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THE EFFECTS of DOPANTS and COMPLEXIONS on GRAIN BOUNDARY DIFFUSION and FRACTURE TOUGHNESS in Α-Al2o3

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THE EFFECTS of DOPANTS and COMPLEXIONS on GRAIN BOUNDARY DIFFUSION and FRACTURE TOUGHNESS in Α-Al2o3 THE EFFECTS OF DOPANTS AND COMPLEXIONS ON GRAIN BOUNDARY DIFFUSION AND FRACTURE TOUGHNESS IN α-Al2O3 BY LIN FENG DISSERTATION Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Materials Science and Engineering in the Graduate College of the University of Illinois at Urbana-Champaign, 2017 Urbana, Illinois Doctoral Committee: Associate Professor Shen Dillon, Chair Professor John Lambros Assistant Professor Jessica Krogstad Assistant Professor Daniel Shoemaker ABSTRACT Grain boundaries often play a dominant role in determining material properties and processing, which originates from their distinct local structures, chemistry, and properties. Understanding and controlling grain boundary structure-property relationships has been an ongoing challenge that is critical for engineering materials, and motivates this dissertation study. Here, α-Al2O3 is chosen as a model system due its importance as a structural, optical, and high temperature refractory ceramic whose grain boundary properties remain poorly understood despite several decades of intensive investigation. Significant controversy still surrounds two important properties of alumina that depend on its grain boundaries; diffusional transport and mechanical fracture. Previously enigmatic grain boundary behavior in alumina, such as abnormal grain growth, were found to derive from chemically or thermally induced grain boundary phase transitions or complexion transitions. This work investigates the hypothesis that such complexion transitions could also impact grain boundary diffusivity and grain boundary mechanical strength. Scanning transmission electron microscopy based energy- dispersive spectroscopy and secondary ion mass spectrometry are utilized to characterize chemical diffusion profiles and quantify lattice and grain boundary diffusivity in Mg2+ and Si4+ doped alumina. It has found that Cr3+ cation chemical tracer diffusion in both the alumina lattice and grain boundaries is insensitive to dopants and complexion type. We hypothesize that extrinsic point defects mostly form bound clusters and are immobile. This fact coupled with compensation by impurities makes the lattice diffusivity insensitive to dopant type. The lack of ii dopant effect on grain boundary diffusivity is difficult to rationalize, but we hypothesize that a similar mechanism as described for the lattice may be active at the boundary, although charge compensation is not necessary here. Lattice and grain boundary fracture toughness of alumina is studied by a combination in-situ transmission electron microscopy based micro-cantilever fracture and finite element simulation. These experiments allow the boundary properties to be isolated from the microstructural geometry effects that influence the measured fracture properties of polycrystals. The results suggest that samples with disordered complexions doped with either Si2+ or Y3+ at high temperature exhibit boundaries weaker than the undoped material. Whereas grain boundaries with ordered complexions doped by Y3+ are stronger than the undoped boundaries. This embrittlment phenomenon is used to address anomalous grain boundary strength versus grain size behavior that has been widely observed in the literatures. iii ACKNOWLEDGEMENT Time flies by quickly, and the first day I came to the U of I, still feels like it was yesterday. A large number of people have made unforgettable impressions and contributions during my six-years as a grad student in Urbana. One of the important people giving me the most help is my advisor, Professor Shen J. Dillon. Professor Dillon is a great advisor to do research with and is a good friend. Without his help, I would not have survived my PhD study. I was a “trouble maker” in the Dillon lab, but Professor Dillon was extremely patient and understanding during my stay with him. I gained much interest and confidence in research, and obtained much knowledge and understanding about interfaces and ceramic materials under his guidance. I would also like to thank my doctoral committee members, Professor John Lambros, Jessica Krogstad and Daniel Shoemaker for their time and effort in reviewing my dissertation, and their suggestions to my research. Specifically, Professor Lambros who provided me with significant insights and valuable suggestions for the fracture toughness part of this work. My family has always been my biggest inspiration and a constant source of strength and motivation. A loving and warm family atmosphere has helped me a positive outlook on life. My dad and mom provided me with the best education they could, always ensuring that I had the best of opportunities to study in many great schools. Although they did not hope I take a hard mode of life, e.g. go to graduate school, they always provided me with their never-ending support and helped me overcome any predicament that I had come across during my grad life. I am so grateful that I grew up together with my sweet sister, Chang. I often found myself with iv lack of time to talk with her after I came to the US, but I have a feeling she was always telepathic and provided me with comfort during times of distress. I had a happy childhood with my grandparents. Unfortunately, they passed away during my study in the US. If they were still alive, I believe they would be very happy to know that their beloved granddaughter will graduate and become a doctor. I know they will bless me always in heaven. I am so grateful that I have many great friends at U of I. Audrey is the most important friend. She is so considerate and sweet, and accompanied me through some of the hardest phases of my grad life. Shiya, Wenxuan and I are from our home city, Shenyang, and we made the “female engineering student team”. The afternoon teatime during weekend will always be a memory to cherish. Alan is a friend to complain about each other, but he always helped and provided me with the strength when I was struggling in the life (but I still think he is very childish in many aspects). We had fun-filled crazy encounters. Jian and Kedi are also great friends who helped me in my research, shared happiness in life and always gave me sweet wishes during Chinese holidays. Li Gao helped me a lot in finding jobs and gave me very useful suggestion about job interview. Hueihuei taught me to say “no” to people’s unreasonable demands, which is hard for me. Selena is a good example to look up to for being a wonderful female graduate student. I would also like to thank my labmates, who make our group a great team, including the previous labmates, Kyong Wook Noh, Salman Arshad, Ke Sun, Yin Liu, Shimin Mao, Bo huang, Daniel Anderson and Xuying Liu, and the current labmates, Jenny Tang, Gowtham Jawaharram, John Vance and Yonghui Ma. They are also wonderful friends. v I would like to thank all of the staff members in Frederick Seitz Materials research laboratory (FS-MRL). They were very helpful and patient in training me to use the advanced instruments. Doctor Tim Spila gave me much help and suggestions on SIMS, so that I had chance to become one of few users of SIMS at UIUC. Although he said he gave me some hard times, I know he is very kind and I appreciate that he helped me to run SIMS even during the weekend. Steve Burdin is a super nice and skillful staff member, who helped to repair many instruments that I needed to use. Doug Jeffers helped me solve many experiment problems, which were outside of his duty. Jim Mabon is another a very nice staff who helped work with the different TEMs. Fubo Rao and Tao Shang not only taught me about sample fabrication, but also gave much help in life. Rick Haasch is a nice friend to talk just about anything, not just XPS and Auger. There are many other great people at MRL but I cannot list all of their names here. Without their professional training and help, it would be impossible to conduct these experiments. vi TABLE OF CONTENTS CHAPTER 1 INTRODUCTION ......................................................................................................1 1.1 OVERVIEW ....................................................................................................................1 1.2 STATEMENT OF THE PURPOSE...........................................................................................15 1.3 REFERENCES ................................................................................................................17 CHAPTER 2 THEORIES OF GRAIN BOUNDARY DIFFUSION AND FRACTURE TOUGHNESS .............23 2.1 GRAIN BOUNDARY DIFFUSION ..........................................................................................23 2.1.1 Basic models for grain boundary diffusion ..........................................................23 2.1.2 Kinetic classification of diffusion in polycrystals ..................................................25 2.1.3 Secondary Ion Mass Spectrometry Analysis of Grain Boundary Diffusion ..............30 2.2 FRACTURE TOUGHNESS...................................................................................................32 2.2.1 Models of fracture ............................................................................................33 2.2.2 Linear Elastic Fracture Mechanics ......................................................................33 2.2.3 Stress Intensity Factor .......................................................................................34 2.2.4 J integral and relation to stress intensity factor...................................................35 2.3 REFERENCES ................................................................................................................37
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