STRUCTURE-MAGNETISM CORRELATIONS AND CHEMICAL ORDER-DISORDER TRANSFORMATIONS IN FERROUS L10-STRUCTURED COMPOUNDS A Dissertation Presented By Nina Cathryn Bordeaux to The Department of Chemical Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the field of Chemical Engineering Northeastern University Boston, Massachusetts April 15, 2015 ACKNOWLEDGMENTS There are so many people I am grateful to for getting me to this point. First and foremost, I would like to thank my advisor, Professor Laura H. Lewis for taking me on and teaching me so much. You’ve challenged me to “ask the right questions” and I am a better scientist for it. Thank you for believing in me and for all of the ways you’ve gone beyond the expected for me, from taking me to the Taj Mahal to attending my wedding! Many thanks to my committee members, Professor Sunho Choi, Professor Teiichi Ando, and Professor Katayun Barmak, for taking the time to read my Dissertation and to serve on my committee. Thanks to Dr. Ando for teaching such a wonderful kinetics course that provided me with the foundational knowledge for much of this Dissertation. Special thanks to Dr. Barmak for being a collaborator in this research and for all of the time you’ve invested in discussing results and analysis methods. Thank you for your patience and your guidance. I want to thank Professor Joseph Goldstein for serving on my proposal committee. You provided invaluable guidance in this project and I am so grateful for all that you taught me about meteorites and scientific research. Thank you to Dr. Peter Stephens and Dr. Jean Jordan-Sweet at the NSLS for all of their help with synchrotron experiments. To all of my labmates, thank you for your comradery and commiseration. Thank you to Felix, Radhika, Melissa, Pegah, Luke, Ana, Ian, and Mahboobeh. You’ve been so great to work with! Thank you Luke for helping to get me to the finish line. Special acknowledgement goes to Radhika, who has been a great friend to me and a source of support since the beginning. Many thanks to Ana for long talks on FeNi and L10, afternoon coffee and chocolate breaks, and always being willing to lend an ear. I need to thank my husband, Eric Mueller, for loving and supporting me, especially through the Dissertation writing process. I love you. Most importantly, thanks to my mom, Margaret Ho, for being my role model. You’ve exemplified strength, independence, and intelligence. Thank you for encouraging me to pursue a career in engineering. Everything I’ve accomplished is because of you. Lastly, thank you to Northeastern University, especially for the Dissertation Completion Fellowship that has enabled me to finish this work. ii ABSTRACT Understanding chemical order-disorder transformations and their effects on structure and magnetic properties of ferrous L10 intermetallic systems has been a longstanding problem in materials science and solid state chemistry. Herein, the kinetics of ordering and disordering of the L10 structure in the FePd and FeNi systems are explored in correlation with their structure and magnetic properties. In many systems, the chemically ordered L10 structure forms through an ordering transformation from a chemically disordered parent phase, which is often a face-centered cubic (fcc, A1) solid solution of the two elements (see unit cells of both structures in Figure 1). L10 FeNi (a.k.a. tetrataenite) is only found naturally in meteorites that form over 4.5 billion years; to date, it has not been produced in bulk by laboratory methods. The challenge for manufacturing L10 FeNi is that it has a critical temperature of 320 °C, below which the fcc chemically disordered phase transforms into the L10 chemically ordered phase [1]. Below this critical temperature, the atomic mobilities of Fe and Ni are extremely low (1 atomic jump per 2,600 years [2]) therefore the atomic movement necessary for chemical ordering is limited. On the other hand, the L10 structure forms readily in FePd making it a useful model system in which to study the order-disorder transformation [3] (it is noted, however, that the high cost of palladium makes FePd an impractical material for bulk industrial applications). In this dissertation, laboratory-synthesized L10 FePd was used as a test-bed system while a variety of meteorites were used as natural sources of L10- structured FeNi. The effect of the cooling rate on the A1→L10 chemical ordering transformation in FePd is investigated to elucidate the mechanism of chemical ordering. The effect of the heating rate on the L10→A1 chemical disordering transformation in iii FeNi is investigated to gain understanding of the phase’s stability and order-disorder transformation mechanism. Analysis of the magnetic properties of L10 FeNi and L10 FePd indicates that the materials have theoretical magnetic energy products comparable to those of today’s best permanent magnets; thus, results from this work may aid future synthesis efforts for advanced permanent magnet applications. L1 structure A1 structure 0 Figure 1: Unit cells of binary alloy AB with (a) the chemically disordered fcc (A1) structure and (b) the chemically ordered L10 structure. iv TABLE OF CONTENTS 1 Introduction ...................................................................................................... 1 1.1 Motivation ................................................................................................. 1 1.2 Overview of Research ............................................................................... 3 1.3 Organization of Dissertation ..................................................................... 6 2 Formalism and Critical Literature Review ....................................................... 8 2.1 General Overview of Phase Transformations ........................................... 9 2.1.1 Thermodynamics of Phase Transformations ....................................... 9 2.1.2 Kinetics of Structural Phase Transformations ................................... 17 2.2 Order-Disorder Transformations ............................................................. 26 2.2.1 Chemical Order and Disorder ........................................................... 26 2.2.2 Magnetic Phase Transitions .............................................................. 30 2.3 Critical Literature Review ....................................................................... 39 2.3.1 Overview of L10 FePd ....................................................................... 39 2.3.2 Overview of L10 FeNi (Tetrataenite)................................................. 47 3 Experimental Methods and Techniques ......................................................... 55 3.1 FePd Synthesis and Processing ............................................................... 55 3.2 Meteorite Sample Preparation ................................................................. 56 3.3 Composition and Homogeneity Determination Using Scanning Electron Microscope Energy-Dispersive X-ray Spectroscopy (SEM-EDS) ........... 57 3.4 Characterization of Microstructure and Structure Using Optical Microscopy and X-ray Diffraction (XRD) ............................................... 59 3.5 Methods of Magnetic Domain and Bulk Magnetism Characterization... 66 3.5.1 Magnetic Domain Characterization Using Magnetic Force Microscopy (MFM) ........................................................................... 66 3.5.2 Bulk Magnetic Characterization Using Vibrating Sample Magnetometry (VSM) ....................................................................... 68 3.6 Calorimetric Methods .............................................................................. 71 4 FePd Results and Discussion .......................................................................... 73 4.1 Characterization of the Structure, Magnetism, and Thermal Stability of L10 FePd.................................................................................................... 74 4.1.1 Results from the Characterization of Structure, Magnetism, and Thermal Stability of L10 FePd .......................................................... 76 v 4.1.2 Discussion from the Characterization of Structure, Magnetism, and Thermal Stability of L10 FePd .......................................................... 83 4.1.3 Significance of Work from the Characterization of Structure, Magnetism, and Thermal Stability of L10 FePd ................................ 89 4.2 The Effect of Cooling Rate on the A1→L10 Chemical Ordering Transformation in FePd ............................................................................ 90 4.2.1 Results from the Effect of Cooling Rate on the A1→L10 Chemical Ordering Transformation in FePd ..................................................... 91 4.2.2 Discussion from the Effect of Cooling Rate on the A1→L10 Chemical Ordering Transformation in FePd ................................... 101 4.2.3 Significance of Work from the Effect of Cooling Rate on the A1→L10 Chemical Ordering Transformation in FePd ................... 120 5 FeNi Results and Discussion ........................................................................ 123 5.1 Correlations of Structure and Magnetism of L10 FeNi ......................... 123 5.1.1 Results on the Correlations of Structure and Magnetism of L10 FeNi.. ......................................................................................................... 124 5.1.2 Discussion on the Correlations of Structure and Magnetism of L10 FeNi ................................................................................................