MICROSTRUCTURE EVALUATION OF IRON NITRIDE INTERSTITIAL COMPOUND, AS A CANDIDATE FOR PERMANENT MAGNETIC MATERIAL by PARIVASH MORADIFAR Submitted in partial fulfillment of the requirements For the degree of Master of Science Thesis Advisor: Prof. Dr. rer. nat. habil. Frank Ernst Department of Materials Science and Engineering CASE WESTERN RESERVE UNIVERSITY May 2016 ! Committee Approval Sheet CASE WESTERN RESERVE UNIVERSITY SCHOOL OF GRADUATE STUDIES We hereby approve the thesis of PARIVASH MORADIFAR candidate for the Master of Science* Committee Chair FRANK ERNST Committee Member DAVID MATHIESEN Committee Member PETER D. LAGERLOF Date of Defense January 2016 ! *We!also!certify!that!written!approval!has!been!obtained!for!any!proprietary!material!contained!therein.! 2! Dedication This thesis is gratefully dedicated to my beloved parents, Farideh Barati Hezaveh and Mohammad-Esmaeil Moradifar, my beloved sister, Paria Moradifar, for their unconditional love, support, and guidance, without which this thesis would have never been finished. 3! Table of Contents Table of Contents .................................................................................................................... 4! List of Figures .......................................................................................................................... 6! List of Tables ........................................................................................................................... 9! Abstract .................................................................................................................................. 12! Chapter 1 -! Introduction ................................................................................................. 15! Chapter 2 -! Background ................................................................................................. 18! 2.1! Permanent Magnet Materials ..............................................................................................18! 2.2! Iron-Nitrogen (Fe-N) System ...............................................................................................19! 2.2.1! Interstice sites in iron atom (BCC and FCC) structure ...................................................................20! 2.2.2! Nitrogen austenite (γ- (Fe-N)) ........................................................................................................21! 2.2.3! Nitrogen-martensite (α') .................................................................................................................22! 2.2.4! γ'-Fe4N ............................................................................................................................................25! 2.2.5! Ordered martensitic nitride (α''-Fe16N2) .........................................................................................26! 2.3! α''-Fe16N2, a phase with a possible giant magnetization ....................................................28! 2.4! TEM Sample Preparation of Powder Particles ..................................................................34! 2.5! Purpose of Study ...................................................................................................................36! Chapter 3 -! Materials and Experimental Methods ...................................................... 38! 3.1! Materials and Nitriding Process ..........................................................................................38! 3.2! Characterization Technique ................................................................................................41! 3.2.1! X-Ray Diffraction (XRD) ...............................................................................................................41! 3.2.2! Vibrating Sample Magnetometer (VSM) .......................................................................................43! 3.2.3! Scanning Electron Microscope (SEM) ...........................................................................................43! 3.2.4! Transmission Electron Microscope (TEM) ....................................................................................45! 4! Chapter 4 -! Results .......................................................................................................... 46! 4.1! XRD Results ..........................................................................................................................46! 4.2! VSM Results ..........................................................................................................................50! 4.3! TEM Sample Preparation Results .......................................................................................52! 4.4! TEM Results ..........................................................................................................................56! 4.4.1! TEM observation of quenched sample ...........................................................................................56! 4.4.2! TEM observations of cryomilled sample .......................................................................................67! 4.4.3! TEM observations heat-treated sample ..........................................................................................75! Chapter 5 -! Discussion .................................................................................................... 90! 5.1! XRD Results ..........................................................................................................................90! 5.2! VSM Results ..........................................................................................................................92! 5.3! TEM Sample Preparation Results .......................................................................................94! 5.4! TEM Results ..........................................................................................................................95! 5.4.1! Quenched sample ............................................................................................................................95! 5.4.2! Cryomilled sample ..........................................................................................................................97! 5.4.3! Heat-treated sample ........................................................................................................................98! Chapter 6 -! Conclusion ................................................................................................. 102! Chapter 7 -! Future Work .............................................................................................. 106! 5! List of Figures Figure 1: Applications of permanent magnets .............................................................................. 15! Figure 2: History of permanent magnetic materials [1] ................................................................ 16! Figure 3: China rare earth exports pricing .................................................................................... 17! Figure 4: Magnetization versus magnetic field strength (M vs. H) curve [8] ............................... 19! Figure 5: Iron-Nitrogen system [9] ............................................................................................... 20! Figure 6: Interstitial sites in (a) FCC and (b) BCC iron [10] ........................................................ 21! Figure 7: Structure of nitrogen-austenite ...................................................................................... 22! Figure 8: Nitrogen-martensite (α') with BCT structure (a=b≠c, α=β=γ=90°) [10] ...................... 24! Figure 9: Lattice parameters variation of iron-nitrogen martensite (α') and iron-carbon martensite with interstitial content [16] .................................................................................................. 25! Figure 10: γ'-Fe4N structure [17] .................................................................................................. 26! Figure 11: Structure of α''-Fe16N2 [21] ......................................................................................... 27! Figure 12: (a) Nitriding reactor, (b) Schematic of nitriding reactor by D.Matthiesen .................. 39! Figure 13: Schematic of producing bulk samples of α''-Fe16N2 ................................................... 40! Figure 14: Cryomilling machine ................................................................................................... 41! Figure 15: Lift-out protocol of a slice of a single particle ............................................................ 44! Figure 16: XRD diffractogram of AHC 100.29 as-received and simulated diffractogram of α- iron ......................................................................................................................................... 46! Figure 17: simulated XRD diffractograms of γ-iron, α'-Fe8N, and α''-Fe16N2 ............................. 47! Figure 18: XRD patterns of quenched sample, cryomilled sample and heat-treated sample ....... 49! Figure 19: Superimposed XRD diffractograms of cryomilled sample and heat-treated sample .. 50! Figure 20: VSM curves of four samples of (a) As received sample (b) Quenched sample (c) Cryomilled sample (d) Heat-treated sample .......................................................................... 51! 6! Figure 21: SEM image of cross-sectional embedded particle powders in the epoxy resin (Durcupan) ............................................................................................................................