Creep Behavior of High Temperature Alloys for Generation IV Nuclear Energy Systems A dissertation submitted to the Division of Research and Advanced Studies of the University of Cincinnati in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Department of Mechanical and Materials Engineering of College of Engineering and Applied Science University of Cincinnati by Xingshuo Wen M.S., University of Cincinnati, Ohio, USA, 2009 B.S., Shaanxi University of Science and Technology, Shaanxi, China, 2005 March, 2014 Committee: Dr. Vijay K. Vasudevan (Chair) Dr. Relva C. Buchanan Dr. Rodney D. Roseman Dr. Vesselin N. Shanov Abstract The Very High Temperature Reactor (VHTR) is one of the leading concepts of the Generation IV nuclear reactor development, which is the core component of Next Generation Nuclear Plant (NGNP). The major challenge in the research and development of NGNP is the performance and reliability of structure materials at high temperature. Alloy 617, with an exceptional combination of high temperature strength and oxidation resistance, has been selected as a primary candidate material for structural use, particularly in Intermediate Heat Exchanger (IHX) which has an outlet temperature in the range of 850 to 950°C and an inner pressure from 5 to 20MPa. In order to qualify the material to be used at the operation condition for a designed service life of 60 years, a comprehensive scientific understanding of creep behavior at high temperature and low stress regime is necessary. In addition, the creep mechanism and the impact factors such as precipitates, grain size, and grain boundary characters need to be evaluated for the purpose of alloy design and development. In this study, thermomechanically processed specimens of alloy 617 with different grain sizes were fabricated, and creep tests with a systematic test matrix covering the temperatures of 850 to 1050°C and stress levels from 5 to 100MPa were conducted. Creep data was analyzed, and the creep curves were found to be unconventional without a well-defined steady-state creep. Very good linear relationships were determined for minimum creep rate versus stress levels with the stress exponents determined around 3-5 depending on the grain size and test condition. Activation energies were also calculated for different stress levels, and the values are close to 400kJ/mol, which is higher than that for self-diffusion in nickel. Power law dislocation climb- glide mechanism was proposed as the dominant creep mechanism in the test condition regime. Dynamic recrystallization happening at high strain range enhanced dislocation climb and are i believed to be responsible for the monotonically increasing creep rates. Apart from dislocation creep, diffusional creep in existence at low stress level in fine-grained (ASTM 8) material also contributed partly to the creep rates. A reasonable prediction on the long term performance of alloy 617 was also made by extrapolation method using optimized parameters based on creep test data. Furthermore, microstructure characterization was performed utilizing Optical Microscopy (OM), Scanning Electron Microscopy (SEM), Electron Backscattered Diffraction (EBSD), Transmission Electron Microscopy (TEM) and related analytical techniques on samples from both before and after creep, with special attention given to grain size effects, grain boundary type, and dislocation substructures. Evidences for dislocation climb and dislocation glide were found through detailed dislocation analysis by TEM, proving the dislocation climb-glide mechanism. The formation of subgrain boundary, the changes in boundary characters and grain sizes was confirmed by EBSD analysis for dynamic recrystallization. The effects of initial grain size and grain boundary character distribution on the creep behavior and mechanism were also evaluated. Through the results obtained from this experimental study, new insights were provided into how changes in microstructure take place during high temperature creep of alloy 617, creep mechanism at different conditions was identified, and the creep deformation model was discussed. The results will also serve to technological and code case development and design of materials for NGNP. ii © Copyright 2014 XINGSHUO WEN All rights reserved. Acknowledgements The completion of this Ph.D. would not be possible without the encouragement and support from many individuals. First and foremost, I would like to express my sincere gratitude to my advisor Prof. Vijay K. Vasudevan, who provides his guidance, inspiration, patience, and support throughout my study in University of Cincinnati and, more importantly, in life. Without him, it would be impossible for me to accomplish this long journey. I would also like to thank the committee members Prof. Relva C. Buchanan, Prof. Rodney D. Roseman, and Prof. Vesselin N. Shanov for their precious time and valuable suggestions in completing my dissertation. I would like to thank DOE-NEUP (Program# 09-013, Sub-contract# DE-FX07-00088635) for funding this work. Part of the creep tests were conducted in Idaho National Laboratory, and Metcut Research Inc. also contributed to some of the low stress tests. The microstructural analysis was carried out in Advanced Materials Characterization Center, University of Cincinnati. I would like to acknowledge Dr. Laura Carroll and Dr. Richard Wright from Idaho National Laboratory and Dr. T.-L. (Sam) Sham from Oak Ridge National Laboratory for providing the test specimens and creep data. Their insightful discussions and valuable inputs in this research project are very helpful to my research. Special thanks also go to Randy Lloyd from INL for coaching me to setup creep tests. I would also like to thank my friends and colleagues, Dr. Hyojin Song, Dr. Amrinder Gill, Dr. Yixiang Zhao, Dr. Chaswal Vibhor, Dr. Chang Ye, Dr. Behrang Poorganji, Gokul Ramakrishnan, Sethuraman Subramanian, Abhishek Telang, and Deepthi Tammana, who shared much of their experience in research, helped me with experiments, and had numerous meaningful discussions. iv I am grateful to University of Cincinnati and Department of Mechanical and Materials Engineering for providing an excellent education and research environment, and for financial support. My thanks are due to the faculty and staff, who contributed to my study and life. Last but not least, I would like to deeply thank my father for his encouragement and patience. To my wife Yuanyuan Niu, who is always by my side with constant support and endless love, I am so lucky to have you in my life, and I could not go thus far without you. v Table of Contents 1 Introduction .................................................................................................................................. 1 1.1 Background .......................................................................................................................... 1 1.2 Material Selection for Intermediate Heat Exchanger ........................................................... 4 1.3 Motivation of Research ........................................................................................................ 5 1.4 Objective and Scope ............................................................................................................. 6 2 Literature Review ......................................................................................................................... 8 2.1 Chemistry and Microstructure of Nickel Base Superalloys ................................................. 8 2.2 Strengthening Mechanisms of Nickel Base Superalloys ................................................... 13 2.2.1 Solid Solution Strengthening ................................................................................... 13 2.2.2 Precipitation Strengthening ...................................................................................... 13 2.2.3 Grain Boundary Strengthening ................................................................................ 17 2.3 Background of Alloy 617 ................................................................................................... 18 2.3.1 Chemistry and Microstructure of Alloy 617 ............................................................ 18 2.3.2 Mechanical Properties of Alloy 617 ........................................................................ 20 2.3.2.1 Tensile Property .......................................................................................... 21 2.3.2.2 Fatigue Strength .......................................................................................... 24 2.3.2.3 Creep-fatigue Property ................................................................................ 25 2.3.2.4 Creep Behavior ............................................................................................ 27 3 Experimental Details .................................................................................................................. 39 3.1 Specimens .......................................................................................................................... 39 3.2 Creep Testing ..................................................................................................................... 39 3.2.1 Creep Testing Equipment ........................................................................................ 39 3.2.2 Creep Testing Condition .......................................................................................... 42 3.3 Microstructure