Max-Planck-Institut für Intelligente Systeme (ehemals Max-Planck-Institut für Metallforschung) Stuttgart Precipitation of nitrides in iron-based binary and ternary alloys; influence of defects and transformation-misfit stresses Maryam Akhlaghi Dissertation an der Universität Stuttgart Bericht Nr. 252 November 2015 Contents 2 Precipitation of nitrides in iron-based binary and ternary alloys; influence of defects and transformation-misfit stresses Von der Fakultät Chemie der Universität Stuttgart zur Erlangung der Würde eines Doktors der Naturwissenschaften (Dr. rer. nat.) genehmigte Abhandlung Vorgelegt von Maryam Akhlaghi aus Teheran, Iran Hauptberichter: Prof. Dr. Ir. E. J. Mittemeijer Mitberichter: Prof. Dr. J. Bill Prüfungsvorsitzender: Prof. Dr. T. Schleid Tag der Einreichung: 16.09.2015 Tag der mündlichen Prüfung: 25.11.2015 MAX-PLANCK-INSTITUT FÜR INTELLIGENTE SYSTEME (ehemals MAX-PLANCK-INSTITUT FÜR METALLFORSCHUNG) INSTITUT FÜR MATERIALWISSENSCHAFT DER UNIVERSITÄT STUTTGART 2015 Contents 4 Contents I Table of contents Chapter 1. Introduction ............................................................................................................. 1 1.1. General introduction ......................................................................................................... 1 1.2. Microstructure of nitrided region ...................................................................................... 1 1.3. Nitriding of binary and ternary Fe-based alloys ................................................................ 2 1.4. Residual stress development in nitrided specimens .......................................................... 3 1.4.1. Development of hydrostatic strain in nitrided case ................................................... 3 1.4.2. Development of residual macrostresses in nitrided case and unnitrided core ........... 3 1.5. Residual stress development in nitrided specimens .......................................................... 5 1.5.1. The definition of nitriding potential (rN) ................................................................... 5 1.5.2. Stressed-solid/gas-phase thermodynamic equilibrium .............................................. 6 1.6. Outline of the thesis .......................................................................................................... 8 References ............................................................................................................................. 10 Chapter 2. Low temperature nitriding of ferritic Fe-Cr-Al alloys ....................................... 15 2.1. Introduction ..................................................................................................................... 16 2.2. Experimental ................................................................................................................... 16 2.2.1. Specimen preparation and nitriding ........................................................................ 16 2.2.2. Microstructural characterization ............................................................................. 17 2.3. Results ............................................................................................................................ 18 2.3.1. Morphology of nitrided region ................................................................................ 18 2.3.2. Simultaneous vs. successive precipitation of nitrides ............................................. 19 2.4. Discussion ....................................................................................................................... 22 2.5. Conclusions .................................................................................................................... 24 References ............................................................................................................................. 25 Chapter 3. Lattice-parameter change induced by accommodation of precipitate/matrix misfit; misfitting nitrides in ferrite .......................................................................................... 27 3.1. Introduction ..................................................................................................................... 28 II Contents 3.2. Elastic strain field in a misfitting system: lattice-parameter changes .............................. 29 3.2.1. Change of the lattice parameter of the matrix ......................................................... 29 3.2.2. Change of the lattice parameter of the misfitting phase .......................................... 30 3.2.3. Change of the lattice parameter of the assembly (matrix plus misfitting phase) ..... 30 3.2.4. Coherent vs. incoherent diffraction of matrix and misfitting precipitates; consequences for interpretation of lattice-parameter changes ............................... 31 3.3. Experimental ................................................................................................................... 32 3.3.1. Specimen preparation.............................................................................................. 32 3.3.2. Nitriding and de-nitriding experiments ................................................................... 33 3.3.3. Specimen characterization ...................................................................................... 34 3.3.3.1. Weight measurement ...................................................................................... 34 3.3.3.2. Electron probe microanalysis (EPMA)........................................................... 34 3.3.3.3. X-ray and synchrotron diffraction .................................................................. 34 3.4. Results and Evaluation .................................................................................................... 36 3.5. Discussion ...................................................................................................................... 38 3.6. Conclusions ..................................................................................................................... 43 Acknowledgements ................................................................................................................ 44 References ............................................................................................................................. 44 Chapter 4. Diffraction-line shifts and broadenings in continuously and discontinuously coarsening precipitate-matrix systems; coarsening of initially coherent nitride precipitates in a ferrite matrix ..................................................................................................................... 49 4.1. Introduction ..................................................................................................................... 50 4.2. Experimental Procedures ................................................................................................ 51 4.2.1. Specimen preparation.............................................................................................. 51 4.2.2. Nitriding experiments ............................................................................................. 52 4.2.3. Specimen characterization ...................................................................................... 52 4.2.3.1. Weight measurement ...................................................................................... 52 4.2.3.2. Light microscopy (LM) .................................................................................. 53 4.2.3.3. X-ray diffraction (XRD) analysis ................................................................... 53 4.2.3.4. Transmission electron microscopy (TEM) analysis ....................................... 53 4.3. Results ............................................................................................................................ 53 Contents III 4.3.1. Fe-2.0 at.%Cr alloy ................................................................................................. 53 4.3.2. Fe-4.5 at.%Cr alloy ................................................................................................. 56 4.4. Discussion ....................................................................................................................... 61 4.4.1. Initial state .............................................................................................................. 61 4.4.2. Continuous Coarsening ........................................................................................... 63 4.4.3. Discontinuous Coarsening ...................................................................................... 64 4.5. Conclusions ..................................................................................................................... 65 Acknowledgements ................................................................................................................ 67 References ............................................................................................................................. 67 Chapter 5. Misfit Induced Changes of Lattice Parameters in Two-Phase Systems: Coherent/Incoherent Precipitates in a Matrix ....................................................................... 71 5.1. Introduction ..................................................................................................................... 72 5.1.1. micro-and macrostrains; coherent and incoherent diffraction ................................. 72