Beiträge aus dem MPI zum Nitrieren
Das Max - Planck Institut für Metallforschung, speziell die Abteilung Phasenumwandlungen unter Leitung von Prof. Dr. Ir. Eric J. Mittemeijer, beschäftigt sich u.a. mit dem Thema Nitrieren und Nitrocarburieren. Die Forschungsschwerpunkte hierzu werden am MPI für Metallforschung wie folgt gesetzt:
" This project focuses on unusual, hitherto not understood, fundamental aspects of the phase transformations involved, with an emphasis on the roles of misfit stress/distortion and the interplay of thermodynamics and kinetics.
-Nitride precipitation and coarsening; coherent and incoherent diffraction effects
-Microstructure and residual stress
-Unexpected formation of ε iron nitride by gas nitriding of nanocrystalline α iron films
-Unusual and extreme elastic anisotropy of γ'-Fe4N and of cementite, Fe3C"
Dr. Andreas Leineweber, Mitarbeiter bei Prof. Dr. Ir. Eric J. Mittemeijer, hat nitrierpraxis.de freundlicherweise eine Übersicht seiner Arbeiten zum Thema Nitrieren und Nitrocarburieren zur Verfügung gestellt.
Kontakt:
Dr. Andreas Leineweber
Max Planck Institute for Metals Research
Heisenbergstrasse 3
D-70569 Stuttgart
Germany
phone:49 (0)711 689-3365; room 3N12
email:
Diese E-Mail-Adresse ist gegen Spambots geschützt! JavaScript muss aktiviert werden, damit sie angezeigt werden kann.
Veröffentlichungen:
A. Leineweber, H. Jacobsa, F. Hüning, H. Lueken, H. Schilder, W. Kockelmann
ε-Fe N: magnetic structure, magnetization and temperature dependent disorder of nitrogen
Journal of Alloys and Compounds 288 (1999) 79–87
Abstract
ε-Fe N has been investigated by time-of-flight neutron diffraction (temperature range 4.2–618 K) and SQUID magnetometry ( 2–700 K, B <=5 T). A ferromagnetic spin structure is observed with magnetic moments oriented perpendicular to the c-axis of the exagonal nuclear structure. The magnetic saturation moment of iron is 2.2 µB at 4.2 K from neutron diffraction and 2.0 µB from magnetic measurements and decreases in a Brillouin-like manner on heating to Tc = 557° K. Above 450 K an increasing but reversible C isorder of the nitrogen partial structure is observed.
A. Leineweber, H. Jacobs, F. Hüning, H. Lueken, W. Kockelmann
Nitrogen ordering and ferromagnetic properties of ε-Fe N 3 N1+x ( 0.10<=x<=0.39) and ε-Fe (N0.80 C0.20 )1.38
Journal of Alloys and Compounds 316 (2001) 21–38
Abstract
The ε-phase iron nitrides ε-Fe N with x=0.10, 0.22, 0.30, 0.33, 0.39 and a carbonitride ε-Fe (N0.80 C0.20 )1.38 were investigated by neutron diffraction. In these phases the non-metals occupy octahedral interstices in an hcp arrangement of Fe. Type and degree of ccupational order of N and N+C, respectively were determined as a function of composition and temperature. The ordering of N is losely related to that of the ε-Fe3 N ‘ideal’ structure (space group P6, 22 ). Neutron diffraction data on ε-Fe3 N 1.22 revealed lowered symmetry (P312 ). An earlier proposed ε-Fe N type ordering (P31m) was not observed for the iron nitride samples but it was found for ε -Fe3 (N 0.80C0.20 )1.38 . Magnetisation data show that the nitride and carbonitride phases are soft ferromagnetic materials and confirmed agnetic structure data as derived from neutron diffraction.
T. Liapina, A. Leineweber, E.J. Mittemeijer, W. Kockelmann
The lattice parameters of ε-iron nitrides: lattice strains ue to a varying degree of nitrogen ordering
Acta Materialia 52 (2004) 173–180
Abstract
The effect of the cooling procedure on the lattice parameters of hexagonal e-iron nitride powders, after equilibration at elevated temperature, was studied: ε-Fe3N1+x, with x = 0:00, 0.10, 0.22 and 0.33, was annealed at 573 K and, subsequently, either quenched or slowly cooled. X-ray diffraction measurements showed for x = 0:00 and 0.10 significantly higher axial ratios c=a after quenching than after slow cooling. For x = 0:22 and 0.33 this effect was virtually absent. By means of neutron diffraction analysis applied to slowly cooled and quenched Fe3N1.00 it was shown that quenching, as compared to slow cooling, is associated with a higher (retained) disorder of the superstructure of nitrogen and a slightly smaller magnetic moment of iron. Both for slowly cooled and quenched samples new relations between the lattice parameters and the nitrogen content are given.
Andreas Leineweber and Eric J. Mittemeijer
Diffraction line broadening due to lattice-parameter variations caused by spatially varying scalar variable: its orientation dependence caused by
locally varying nitrogen content in ε-FeN0.433
J. Appl. Cryst. (2004). 37, 123–135
Abstract
The diffraction line broadening due to lattice-parameter variations caused by a position-dependent (spatial) scalar variable (e.g. the composition) was analysed theoretically. It was shown that the anisotropy of the resulting microstrain-like line broadening depends on the symmetry of the crystal system of the phase concerned. This model provides a physical basis for a special case of a previously reported phenomenological description of anisotropic microstrain broadening [Stephens (1999). J. Appl. Cryst. 32, 281±289], which is widely used in Rietveld refinement. The model presented was used to analyse the hkl dependence of the X-ray diffraction line broadening of a sample of hexagonal ε-iron nitride, FeNy0 with the average N content y0 = 0.433. The observed anisotropy of the line broadening was shown to be compatible with the known composition dependencies of the lattice parameters a(y) and c(y). From the extent of the line broadening, the standard deviation of y0 could be determined very well, as 0.008.
T. LIAPINA, A. LEINEWEBER, and E.J. MITTEMEIJER
Phase Transformations in Iron-Nitride Compound Layers upon Low-Temperature Annealing: Diffusion Kinetics of Nitrogen in ε- and γ'-Iron Nitrides
METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 37A, FEBRUARY 2006—319/330
The ε/γ'-iron nitride compound double layers with thicknesses of about 10 µm ere grown on pure alpha-Fe, by gas nitriding at 823 K, followed by quenching. The specimens were subsequently annealed at significantly lower temperatures, in the range of 613 to 693 K, for different periods of time. These heat treatments led to a redistribution of N, within the compound layer as ell as between the compound layer and the adjacent ferrite, inducing thickness changes in the -and γ'-sublayers. The microstructure and sublayer-thickness changes were analyzed by light microscopy and X-ray diffraction (XRD). The experimentally observed time and temperature dependences f the layer-thickness changes were compared with the results obtained from numerical simulations, y adopting a model based on volume diffusion in the ε-and γ'-phases and on local equilibrium at the phase interfaces. In this manner, the intrinsic diffusion coefficient of N in the γ'-phase and the integral diffusion coefficient of N in the γ'-phase were determined for the applied range of annealing temperatures.
A. Leineweber, J. Aufrecht, E.J. Mittemeijer
Phase transformations of iron nitrides at low temperatures (<700K) - application of mechanical mixtures of powders of nitrides and iron
Int. J. Mat. Res. 97 (2006) 753 - 75
Z.Q. Liu, A. Leineweber and E.J. Mittemeijer, K. Mitsuishi and K. Furuya
Electron-diffraction study on ε-iron nitride powders with various nitrogen contents: Variation of long-range nitrogen ordering
J. Mater. Res., Vol. 21, No. 10, Oct 2006 2572 - 2581
Abstract
Homogeneous ε-iron nitride powder particles of the compositions Fe3N1.00, Fe3N1.22, Fe3N1.30, and Fe3N1.39 were investigated using selected-area electron diffraction. The superstructure reflections, due to N ordering on the octahedral interstitial sites of a hexagonal close-packed (hcp) arrangement of Fe, indicate a hexagonal supercell with α` = 31/2ahcp , c`= chcp for all cases; P6322 space group symmetry for Fe3N1.00, Fe3N1.30, and Fe3N1.39; and lower symmetry (P312) for Fe3N1.22 and some of the Fe3N1.30 particles. Recognition of the occurrence of double diffraction is essential for correct interpretation of the diffraction patterns. Additionally, diffuse scattering by some of the Fe3N1.39 grains was observed.
Andreas Leineweber, Tatiana Liapina, Thomas Gressmann,Marc Nikolussi and Eric J. Mittemeijer
Phase transformations and interstitial atom diffusion in iron-nitride, iron-carbonitride and iron-carbide layers
Advances in Science and Technology Vol. 46 (2006) pp. 32-41
Abstract.
α-Iron foils were exposed to various gas atmospheres containing all or a number of the components NH3, CO, H2 and N2 for different periods of time at 550°C. In this way surficial compound layers were generated which contain different iron nitrides (ε, γ’), iron carbonitride (ε) and/or iron carbide (cementite, Fe3C). These compound layers were used to study phase transformations associated with N- and/or Cdiffusion processes in the corresponding phases. These studies involved (a) the layer-growth kinetics of cementite and (b) various solid-state phase transformations occurring in compound layers upon annealing in vacuum.
T. Liapina, A. Leineweber and E.J. Mittemeijer
Phase Transformations in ε-/γ’-Iron Nitride Compound Layers in the Temperature Range of 613 K – 693 K
Defect and Diffusion Forum Vols. 237-240 (2005) pp. 1147-1152
Abstract.
ε/γ'-iron nitride (ε-Fe3N1+x, γ'-Fe4N) compound layers with thicknesses of about 10 μm were grown on pure α-Fe by gas nitriding at 823 K followed by quenching and were annealed at different temperatures in the range of 613 K – 693 K for different periods of time. These heat treatments led to a redistribution of nitrogen within the compound layer as well as between the compound layer and the adjacent ferrite, inducing thickness changes of the ε- and γ'-layers. The changes were analysed by light microscopy, electron probe microanalysis and X-ray diffraction. Models to describe and interpret the phase transformations in the ε/γ'-iron nitride compound layers as a function of time and temperature are discussed.
Z. Q. LIU, A. LEINEWEBER, K. MITSUISHI, K. FURUYA
Transmission electron microscopy study on the superstructure and the precipitation of γ -Fe4 in initially homogeneous ε-iron nitride powders
J MATER SCI 41 (2006) 2673–2677
Annealed and quenched ε-Fe2−3N powders with an initially homogeneous composition of Fe3N1.0 were studied by transmission electron microscopy (TEM). TEM specimens were successfully prepared from the powder using a sandwiching technique. The superstructure in ε-powders was identified as ε`-Fe3N type (space group P6322), and no other type of superstructure was observed. γ'-Fe4N nitride precipitated from ε powders as individual grains during the annealing process, which is different from the typical fine lamellar structure observed in bulk iron-nitride samples. The observed orientation relationships between γ' and ε grains are also different from that reported in the lamellar structure of bulk iron-nitride samples. This suggests that in the powder investigated by us there is no one specific orientation of the precipitated γ' grains with respect to the parent ε grains.
T. Gressmann, M. Wohlschlögel, S. Shang, U. Welzel, A. Leineweber,
E.J. Mittemeijer, Z.-K. Liu
Elastic anisotropy of γ'-Fe4N and elastic grain interaction in γ'-Fe4N1-y layers on α-Fe: First-principles calculations and diffraction stress measurements
Acta Materialia 55 (2007) 5833–5843
Abstract
The three independent single-crystal elastic-stiffness constants Cij of cubic γ'-Fe4N (face-centred cubic (fcc)-type iron substructure) have been calculated by first-principles methods using the density functional theory: C11 = 307.2 GPa, C12 = 134.1 GPa and C44 = 46.0 GPa. The Zener elastic-anisotropy ratio, A = 2C44/(C11 - C12) = 0.53, is strikingly less than 1, implying <100æ> as stiffest directions, whereas all fcc metals show A > 1. This elastic anisotropy is ascribed to the ordered distribution of N on the octahedral interstitial sites. X-ray diffraction lattice-strain measurements for a set of different hkl reflections recorded from γ'-Fe4N1-y layers on top of α-Fe confirmed the ‘‘abnormal’’ elastic anisotropy of γ'-Fe4N1-y. Stress evaluation, yielding a compressive stress of about -670 MPa parallel to the surface, was performed on the basis of effective X-ray elastic constants determined from the calculated single-crystal elastic constants Cij and allowing a grain interaction intermediate between the Reuss and the Voigt models.
Andreas Leineweber
Mobility of nitrogen in ε-Fe3N below 150 °C: The activation energy for reordering
Acta Materialia 55 (2007) 6651–6658
Abstract
Previously quenched (from 573 K) ε-iron nitride powder of a composition close to Fe3N, which shows partial disorder in the interstitial N superstructure, can be reordered by annealing between 373 and 408 K. The kinetics of the reordering can experimentally betraced by the axial ratio (chcp/ahcp), as measured by X-ray powder diffraction, and can be described by a first-order rate law for the time-dependence of (chcp/ahcp) with an Arrhenius-type temperature dependence of the rate constant. The determined activation energy of 144 ± 5 kJ mol-1 can be associated with specific jumps of N atoms from disorder to order sites. The order-of-magnitude of the preexponential factor of (7 ± 10) · 1013 s-1 can be related with the vibration frequency of the N atom in an octahedral site.
Marc Nikolussi, Andreas Leineweber, Ewald Bischoff, Eric Jan Mittemeijer
Examination of phase transformations in the system Fe - N - C by means of nitrocarburizing reactions and secondary annealing experiments; the α + ε two-phase equilibrium
Int. J. Mat. Res. 98 (2007) 11 1086 - 1092
T. Gressmann, A. Leineweber, E.J. Mittemeijer, M. Knapp
X-ray diffraction analysis of an ε/γ' ironnitride compound double layer
Z. Kristallogr. Suppl. 26 (2007) 223-228
Abstract.
An ε-Fe3N/γ' -Fe4N double compound layer grown on α-Fe was investigated by Xray diffraction analysis. The anisotropic diffraction-line broadening observed by highresolution synchrotron diffraction measurements and the diffraction-line shifts as measured for hexagonal ε layer can be quantitatively explained by the presence of both a lattice parameter gradient associated with a concentration gradient and a stress gradient within the layer. Investigation of the γ' layer revealed an elastic anisotropy unusual for fcc type metals.
T. Gressmann; A. Leineweber; E. J. Mittemeijer
X-ray diffraction line-profile analysis of hexagonal ε-iron nitride compound layers: composition– and stress–depth profiles
Philosophical Magazine
Vol. 88, No. 2, 11 January 2008, 145–169
Abstract
Two hexagonal ε-Fe3N1+x layers grown on α-Fe substrates by nitriding in NH3/ H2 gas atmospheres were investigated by high-resolution X-ray powder diffraction using synchrotron radiation employing systematic tilting of the diffraction vector with respect to the specimen surface. Considering all recorded reflections simultaneously, the complicated diffraction profiles obtained were analyzed using a model incorporating hkl-dependent (anisotropic) and tilt angle ( v )-dependent diffraction-line broadening and diffraction-line shifting. The diffraction-line broadening is mainly ascribed to the nitrogen concentration– depth profile within the layers causing depth-dependent strain-free lattice parameters, whereas the line shifts are predominantly caused by the stress– depth profile originating from the concentration-dependence of the coefficients of thermal expansion of the " phase, with stress parallel to the surface, which is of tensile nature at the surface and of compressive nature at the ε-/γ’ interface. This stress gradient additionally leads to a v dependence of the line broadening. Fitting of the microstructure and diffraction model led to determination of microstructure parameters, which can be related to the different sets of treatment conditions applied for the ε-iron nitride layer growth.
J. Aufrecht, A. Leineweber, J. Foct and E.J. Mittemeijer
The structure of nitrogen-supersaturated ferrite produced by ball milling
Philosophical Magazine
Vol. 88, No. 12, 21 April 2008, 1835–1855
Abstract
Highly supersaturated solid solutions of nitrogen in ferrite (bcc) were produced by ball milling of various powder mixtures of α-iron and ε-Fe3N1.08. The microstructure and the crystal structure of the product phases were examined as a function of nitrogen content using X-ray powder diffraction, high-resolution electron microscopy and Mo¨ ssbauer spectroscopy. It was found that the grain size decreases with increasing nitrogen content. Unexpected shifts of the reflections in the X-ray powder diffraction patterns of the supersaturated N-ferrites, depending on the hkl values of the reflections and nitrogen content, were observed. These shifts cannot be explained by tetragonal distortion of the bcc unit cell, but they are in accordance with the occurrence of a certain type of stacking faults on bcc {211} planes. This result, together with the observation of some isolated fcc crystals (by high-resolution electron microscopy) and a drop in microstrain for high nitrogen contents, demonstrates that unconventional deformation mechanisms are operative in these materials below a certain grain size, leading to a breakdown of the classical Hall–Petch relation for mechanical strengthening.
M. Nikolussi, A. Leineweber *, E.J. Mittemeijer
Microstructure and crystallography of massive cementite layers on ferrite substrates
Acta Materialia 56 (2008) 5837–5844
Abstract
Upon gaseous nitrocarburizing, massive cementite layers were grown on ferrite substrates. By means of electron backscatter diffraction, it was shown that the Bagaryatsky orientation relationship holds between the cementite grains of the compound layer and the ferrite grains of the substrate. The experimental data exhibit a preference for specific variants of this orientation relationship: those variants corresponding to relatively low cementite–ferrite misfit-strain energy are favoured. Furthermore, the orientation relationship of neighboring cementite grains grown on the same ferrite grain allows the establishment of low-energy grain boundaries between these cementite grains. In contrast, at ferrite-grain boundaries intersecting the substrate surface, high-energy cementitegrain boundaries occur between cementite grains on adjacent ferrite grains. The latter cementite-grain boundaries promote carbon grain-boundary diffusion through the cementite, which is experimentally supported by the observation of relatively large cementitelayer thicknesses at these locations.
A. Leineweber, E.J. Mittemeijer
Cementite-layer formation by ferritic nitrocarburising
HTM J.Heat Treatm. Mat. 63(2008)6 305 - 314
Abstrct
Massive cementite surface-layers can be prepared upon gaseous nitrocarburising of α - iron substartes , where the ammonia contained in the gas atmospere suppresses the otherwise occuring metal dusting and sooting reactions. The growth kinetics and properties of such cementite layers have been reviewed, in particular in view of possible delibrate generation of such layers in the course of vommercial heat treatment processes.
Marc Nikolussi, Andreas Leineweber, Eric Jan Mittemeijer
Growth of massive cementite layers; thermodynamic parameters and kinetics
J Mater Sci (2009) 44:770–777
Abstract
Massive, pure cementite layers were grown on ferrite substrates by nitrocarburising in a dedicated NH3/ H2/CO/N2 containing gas atmosphere at temperatures in the range of 783–843 K. From the parabolic layer-growth constants, an ‘‘apparent’’ activation energy for cementitelayer growth of 109 ± 12 kJ/mol was obtained. This ‘‘apparent’’ activation energy can be subdivided into a positive contribution due to the activation energy for (tracer) diffusion of carbon in cementite and a negative contribution due to the temperature dependence of the difference of the carbon activity in cementite at the surface and at the interface cementite/ferrite.
