Max-Planck-Institut für Intelligente Systeme (ehemals Max-Planck-Institut für Metallforschung) Stuttgart Nitriding of iron-based binary and ternary alloys: microstructural development during nitride precipitation Sai Ramudu Meka Dissertation an der Universität Stuttgart Bericht Nr. 239 November 2011 Nitriding of iron-based binary and ternary alloys: microstructural development during nitride precipitation von der Fakultät Chemie der Universität Stuttgart zur Erlangung der Würde eines Doktors der Naturwissenschaften (Dr. rer. nat.) genehmigte Abhandlung vorgelegt von Sai Ramudu Meka aus Kuntoji (Gangavathi)/ Indien Hauptberichter: Prof. Dr. Ir. E. J. Mittemeijer Mitberichter: Prof. Dr. J. Bill Prüfungsvorsitzender: Prof. Dr. E. Roduner Tag der Einreichung: 01.09.2011 Tag der mündlichen Prüfung: 17.11.2011 MAX-PLANCK-INSTITUT FÜR INTELLIGENTE SYSTEME, STUTTGART (ehemals MAX-PLANCK-INSTITUT FÜR METALLFORSCHUNG) INSTITUT FÜR MATERIALWISSENSCHAFT DER UNIVERSITÄT STUTTGART 2011 Contents 1 Introduction 7 1.1 Gaseous nitriding - thermodynamics ............................................................................................... 8 1.2 Microstructure of nitrided zone ....................................................................................................... 9 1.3 Alloying element nitrides in the ferrite matrix ............................................................................... 11 1.4 Excess nitrogen .............................................................................................................................. 13 1.5 Development of iron-nitride compound layers on pure iron .......................................................... 14 1.6 Development of iron-nitride compound layers on iron-based alloys ............................................. 15 1.7 Pore formation in the nitrided zone: metastable nature of the Fe-N phases .................................. 16 1.8 Nitriding of iron-based ternary alloys ............................................................................................ 17 1.9 Scope of this thesis......................................................................................................................... 17 1.10 Methodology .................................................................................................................................. 18 1.11 Outline of the thesis ....................................................................................................................... 18 2 The emergence and disappearance of a high density of microcracks in nitrided Fe-4.65at.%Al alloy 21 2.1 Introduction .................................................................................................................................... 22 2.2 Experimental .................................................................................................................................. 22 2.3 Results and discussion ................................................................................................................... 23 2.3.1 X-ray diffraction: AlN phase identification ............................................................................. 23 2.3.2 Microstructure of nitrided Fe-4.65at.%Al alloy ....................................................................... 24 2.4 Conclusions .................................................................................................................................... 31 3 Unusual nucleation and growth of γ′ iron nitride upon nitriding Fe-4.75at.%Al alloy 33 3.1 Introduction .................................................................................................................................... 34 3.2 Experimental .................................................................................................................................. 34 3.2.1 Specimen preparation ............................................................................................................... 34 3.2.2 Nitriding ................................................................................................................................... 35 3.3 Microstructural characterization .................................................................................................... 37 3.3.1 Light optical and scanning electron microscopy ...................................................................... 37 4 Contents 3.3.2 Transmission electron microscopy ........................................................................................... 37 3.3.3 Electron probe micro analysis .................................................................................................. 38 3.3.4 X-ray diffraction ....................................................................................................................... 39 3.4 Results and discussion ................................................................................................................... 39 3.4.1 Unusual morphology of γ′ iron nitride in the surface region .................................................... 39 3.4.2 Nano-sized γ′ precipitates in diffusion zone ............................................................................. 40 3.4.3 Nano-sized cubic AlN precipitates in diffusion zone ............................................................... 43 3.4.4 Unexpected formation of ε-iron nitride .................................................................................... 45 3.4.5 Diffraction-line broadening ...................................................................................................... 48 3.4.6 Role of AlN precipitation ......................................................................................................... 49 3.4.7 The negligible solubility of Al in γ′ .......................................................................................... 51 3.5 General discussion: mechanism of γ′ formation in Fe-Al alloy ..................................................... 55 3.6 Conclusions .................................................................................................................................... 57 4 Unusual precipitation of amorphous silicon nitride upon nitriding Fe-2at.%Si alloy 59 4.1 Introduction .................................................................................................................................... 60 4.2 Experimental details....................................................................................................................... 61 4.2.1 Specimen preparation ............................................................................................................... 61 4.2.2 Nitriding and denitriding .......................................................................................................... 61 4.3 Microstructural characterization .................................................................................................... 62 4.3.1 Light microscopy and scanning electron microscopy .............................................................. 62 4.3.2 Transmission electron microscopy ........................................................................................... 62 4.3.3 Quantitative analysis of composition and nitrogen uptake ....................................................... 63 4.3.4 X-ray diffraction ....................................................................................................................... 63 4.3.5 Microhardness measurement .................................................................................................... 64 4.4 Morphology: light optical and scanning electron microscopy ....................................................... 64 4.5 Amorphous nature of the silicon-nitride precipitates ..................................................................... 67 4.6 Ideally weak nitriding kinetics; hardness and nitrogen-concentration depth profiles .................... 71 4.7 Nitrogen solubility of ferrite matrix and composition of the amorphous nitride precipitates ........ 75 4.8 Initial stage of nitride precipitation and “self-catalysis”: non-monotonous hardness change and increasing nitrogen-uptake rate with nitriding time ................................................................ 77 4.9 Technological application of silicon-nitride precipitation in ferrite .............................................. 79 4.10 Conclusions ................................................................................................................................... 80 Contents 5 5 Nitride formation and excess nitrogen uptake upon nitriding ferritic Fe-Ti-Cr alloys 83 5.1 Introduction .................................................................................................................................... 84 5.2 Experimental .................................................................................................................................. 85 5.2.1 Specimen preparation ............................................................................................................... 85 5.2.2 Nitriding ................................................................................................................................... 86 5.2.3 X-ray diffraction ....................................................................................................................... 87 5.2.4 Microhardness measurement ...................................................................................................