Structural properties of austenitic stainless steel in the initial state and after rf plasma nitriding

Mössbauer spectroscopy, TEM, electrical resistivity and magnetic measurements are used for investigation of structural changes of X5CrNi189 austenitic stainless steel sheet induced by rf plasma nitriding carried out at 400°C for 24 h. The initial structure formed by small grains of austenite and α′-martensite changes after nitriding into expanded austenite close to the surface and austenite with minor contents of ferrite/CrN in the bulk.     

The nitrogen-absorption isotherm for Fe–21.5 at. % Cr alloy: dependence of excess nitrogen uptake on precipitation morphology

Nitriding of Fe–21.5 at. % Cr alloy leads to a “discontinuously coarsened”, chromium-nitride/ferrite lamellar precipitation morphology in the nitrided zone. The nitrogen-absorption isotherm for this alloy with this precipitation morphology was determined at 560°C. To assure a constant precipitation morphology the Fe–21.5 at. % Cr specimen was first homogeneously pre-nitrided (at 580°C in an ammonia/hydrogen gas mixture of nitriding potential 0.103 atm^?1/2) and then de-nitrided (at 470°C in hydrogen gas atmosphere). The amount of nitrogen remaining in the de-nitrided specimen indicated that the composition of the nitride precipitates is CrN and not (Fe, Cr)N. The measured nitrogen-absorption isotherm revealed the presence of excess nitrogen in the nitrided specimen, which is a surprise in view of the coarse, lamellar precipitation morphology. The occurrence of this excess nitrogen could be ascribed to an unexpected, minor fraction of the total chromium content in the alloy present as coherent, tiny nitride platelets within the ferrite lamellae of the “discontinuously coarsened” lamellar precipitation morphology, as evidenced by transmission electron microscopy. A possible kinetic background for this unusual phenomenon was discussed.     

Continuous and discontinuous precipitation in Fe-1 at.%Cr-1 at.%Mo alloy upon nitriding; crystal structure and composition of ternary nitrides

The internal nitriding response of a ternary Fe–1 at.%Cr–1 at.%Mo alloy, which serves as a model alloy for many CrMo-based steels, was investigated. The nitrides developing upon nitriding were characterised by X-ray diffraction, scanning electron microscopy, electron probe microanalysis, transmission electron microscopy and atom probe tomography. The developed nitrides were shown to be (metastable) ternary mixed nitrides, which exhibit complex morphological, compositional and structural transformations as a function of nitriding time. Analogous to nitrided binary Fe–Cr and Fe–Mo alloys, in ternary Fe–Cr–Mo alloys initially continuous precipitation of fine, coherent, cubic, NaCl-type nitride platelets, here with the composition (Cr_½,Mo_½)N_¾, occurs, with the broad faces of the platelets parallel to the {1?0?0}_α-Fe lattice planes. These nitrides undergo a discontinuous precipitation reaction upon prolonged nitriding leading to the development of lamellae of a novel, hexagonal CrMoN₂ nitride along {1?1?0}_α-Fe lattice planes, and of spherical cubic, NaCl-type (Cr,Mo)N _x nitride particles within the ferrite lamellae. The observed structural and compositional changes of the ternary nitrides have been attributed to the thermodynamic and kinetic constraints for the internal precipitation of (misfitting) nitrides in the ferrite matrix.     

Isothermal decomposition behavior of the high nitrogen concentration γ-Fe[N] prepared from pure iron

The isothermal decomposition characteristic of the homogeneous high nitrogen austenitic samples prepared by a new multi-stage nitriding process was investigated by SEM and TEM in this paper. Lamellar-structure precipitations arranged on the decomposed austenite grain boundaries (GBs) and the flaky γ′ particles and network-structure precipitations appeared inside of the γ matrix. The extra high Vickers hardness more than 800 HV was found in the 5-h aged samples, which was different from those of the bainitic/martensitic structures in Fe-C alloys. The SAED analysis indicates the γ′ has the coherent relation with the parent γ-Fe[N] phase and the interstitial nitrogen atoms are inclined to aggregate on {1 1 0}γ′/γ planes, which also contributes to the hardness of the matrix.     

钢铁材料中形变诱导相变超细化机理研究

通过计算机编程建立奥氏体相中12[1 1 0]刃位错、奥氏体相中非形变区和形变区奥氏体/铁 素体相界模型.用实空间的连分数方法计算了非形变区和形变区奥氏体/铁素体相界界面能， 计算了碳、氮及微合金元素在完整晶体及位错区引起的环境敏感镶嵌能，进而讨论形变过程 中铁素体形核的难易程度，碳、氮及合金元素在位错区的偏聚及析出与铁素体细化的关系. 计算结果表明：α-Fe易于在高密度位错区（形变带、亚晶界、晶界）形核，在奥氏体形变 过程中，就会大大提高α-Fe形核率，细化铁素体晶粒；碳、氮和微合金元素易于单独或共 同 关键词： 奥氏体/铁素体相界 刃位错 形变 晶粒细化     

Constituted oxides/nitrides on nitriding 304, 430 and 17-4 PH stainless steel in salt baths over the temperature range 723 to 923 K

The progressively developed oxides and nitrides that form on nitriding 304, 430 and 17-4 PH stainless steel are analysed by X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) in this study. The experimental results show that the Cr contents and matrix structures (ferrite, austenite and martensite) play an important role in forming FeCr₂O₄, Cr₂O₃ and Fe₂O₃ oxides as well as nitrides. After a short immersion time, oxides of Cr₂O₃ and FeCr₂O₄ form in nitride films on 304 stainless steel samples. Fe₂O₃ oxide will subsequently form following an increasing immersion time. For the 430 stainless steel, Cr₂O₃ predominately forms after a short dipping time which hinders the growth of the nitride layer. As a result, this sample had the thinnest nitride film of the three for a given immersion time. After the formation of oxides, both CrN and Cr₂N were detected near the surface of the nitride films of three samples while Cr₂N phases formed in the deeper zone. The greatest amount of Fe₂O₃ oxide among the three samples was obtained on the nitriding 17-4 PH stainless steel which also had a high intensity count of N 1s.     

Structure and magnetic properties of CrN thin films on La0.67Sr0.33MnO3

High crystalline quality CrN thin films have been grown on La_0.67Sr_0.33MnO₃ (LSMO) templates by molecular beam epitaxy. The structure and magnetic properties of CrN/LSMO heterojunctions are investigated combining with the experiments and the first-principles simulation. The Nėel temperature of the CrN/LSMO samples is found to be 281 K and the saturation magnetization of CrN/LSMO increases compared to that of LSMO templates. The magnetic property of CrN/LSMO heterostructures mainly comes from Cr atoms of (001) CrN and Mn atoms of (001) LSMO. The (001) LSMO induces and couples the spin of the CrN sublattice at CrN/LSMO interface.     

Mass-transport driven by surface instabilities in metals under reactive plasma/ion beam treatment at moderate temperature

This paper presents a generalized approach to the mechanisms of oxidation, hydrogenation and nitriding of metals under ion irradiation with reactive particles at elevated temperatures. Experimental results on the plasma oxidation of bilayered Y/Zr films, the plasma hydrogenation of Mg films and the ion beam (1.2 keV N ₂ ⁺ ) nitriding of stainless steel are presented and discussed. We make special emphasis on the analysis of surface effects and their role in the initiation of mixing of bilayered films, the ingress of reactive species in the bulk and the restructuring of the surface layers. It is suggested that primary processes driving reactive atoms from the surface into the bulk are surface instabilities induced by thermal and ballistic surface atom relocations under reactive adsorption and ion irradiation, respectively. The diffusion of adatoms and vacancies, at temperature when they become mobile, provide the means to relax the surface energy. It is recognized that the stabilizing effect of surface adatom diffusion is significant at temperatures below 300–350°C. As the temperature increases, the role of surface adatom diffusion decreases and processes in the bulk become dominant. The atoms of subsurface monolayers occupy energetically favorable sites on the surface, and result in reduced surface energy.     

Improvement of erosion and erosion-corrosion resistance of AISI420 stainless steel by low temperature plasma nitriding

Plasma nitriding experiments were carried out with DC-pulsed plasma in 25% N₂ + 75% H₂ atmosphere at low temperature (350 °C) and normal temperature (550 °C) for 15 h. The composition, microstructure, microhardness profiles, residual stress profiles and electrochemical impedance spectrum analyses of the nitrided samples were examined. The influence of plasma nitriding on the erosion and erosion-corrosion resistance of AISI 420 martensitic stainless steel was investigated using a jet solid particle erosion tester and a slurry erosion-corrosion tester.Results showed that the 350 °C nitriding layer was dominated by ?-Fe₃N and α_N phase, a supersaturated nitrogen solid solution. However, nitrogen would react with Cr in the steel to form CrN precipitates directly during 550 °C nitriding, which would lead to the depletion of Cr in the solid solution phase of the nitrided layer. Both 350 and 550 °C plasma nitriding could improve the erosion resistance of AISI420 stainless steel under dry erosion, but the former showed better results. In both neutral and acid environment, while the erosion-corrosion resistance of AISI 420 was improved by means of 350 °C nitriding, it was decreased through 550 °C nitriding.     

Synthesis of carbon-encapsulated nickel nanoparticles

Carbon-encapsulated metal nanoparticles (CEMNs) were obtained by the catalytic decomposition of hydrocarbons (CH₄, C₂H₆, C₂H₄) on nanocrystalline nickel. Nanocrystalline nickel was obtained by precipitation from nickel nitrate solution, followed by calcination and reduction under hydrogen. A small amount of structural promoters (aluminium and calcium oxide) was added to avoid recrystallisation of fine nickel particles at elevated temperatures. Reduction and carburisation of the samples was carried out in a differential reactor with mass control. The rate of decomposition of methane, ethane and ethylene on nanocrystalline nickel was measured and the apparent activation energy of the process was determined. The obtained samples were characterised using the XRD, SEM and HRTEM methods.     

Mechanical properties of the alloy 36NKhTYu in relation to the mechanisms underlying the precipitation of the γ′-and η-phases

The kinetic characteristics of processes taking place during the aging of the alloy 36NKhTYu at 700–850?C were studied; these included an increase in the diameter of theγ′-phase particles formed in regions of continuous decomposition, an increase in the thickness of theγ′-lamellas formed in regions of discontinuous decomposition, and an increase in the volumetric proportion of material in which continuous precipitation of theγ′-phase was occurring. The formation of theη-phase was also studied (the degree of discontinuous precipitation of theη-phase depended on the quench temperature); interstitial stacking faults appeared, their formation and growth being associated with the precipitation of titanium carbide. With increasing period of aging the mechanism underlying the interaction of the dislocations with theγ′-particles changed; initially the particles were intersected by superdislocations; later they were encircled by single dislocations. The discontinuousγ′-phase precipitation which occurred on aging the material after quenching from 1280?C (in the case of fine-grained samples) was ascribed to the fact that insufficient carbon was available to provide carbide particles for pinning the grain boundaries.     

The effect of deformation and quenching temperature on the mechanism of decomposition in steel EI702

It is shown that the mechanism of decomposition in steel EI702 is determined by the quenching temperature and the degree of prior deformation. On aging after quenching from low temperatures or after heavy deformation, the principal mechanism of decomposition, in which the '-phase is formed, is discontinuous precipitation. However, on aging after quenching from high temperatures, the principal mechanism is continuous decomposition. It is found that discontinuous decomposition in steel E1702 is a very desirable process, since by accelerating aging it greatly increases the strength characteristics. The nature of the effect of the quenching temperature on the mechanism of decomposition in steel E1702 is discussed.     

Emission spectroscopy detection of N2 active species in plasma nitriding process

The present paper is a review of recent works related to the optical emission spectroscopy applied to the determination of ground state densities of N atoms in Ar-N₂ flowing post-discharges. The effect of small quantities of H₂ in the Ar-N₂ discharge on N atom densities within the post-discharge has been analysed. The authors demonstrate that optical emission of Na impurity may be applied to the determination of the vibrational temperature of N₂(X,v) states at the temperatures usual for nitriding of metals in Ar-N₂ post-discharge. From the diagnostic of Ar-N₂ post-discharges, it is clearly specified that nitriding of iron base alloys in a flowing post-discharge reactor is originating in N atoms, especially when a few H₂ molecules are admixed into the Ar-N₂ discharge. Finally, correlation between the N-atom density and the thickness of the iron nitrided layers when H₂ is introduced into the Ar-N₂ discharge are given. Dedicated to Prof. Jan Janča on the occasion of his 60^th birthday.     

Structural and phase transitions in nitrided layers of iron alloys during severe cold deformation

A nanostructuring procedure similar to that proposed previously for iron alloys with carbides, nitrides (γ′-Fe₄N, TiN), and oxides, was implemented for X22 fcc alloy and X18H8 austenitic stainless steel. The procedure is based on the deformation-induced dissolution of disperse CrN nitride particles in the alloy matrices and the formation of supersaturated solid solutions of nitrogen, followed by the precipitation of secondary nanonitrides inhibiting the grain growth in the matrix during heating.     

Incubation time for iron-nitride layer formation upon gaseous nitriding of iron-based alloys

A model was developed to predict quantitatively the influence of alloying element (Me) dissolved in the ferrite (α) matrix on the incubation time for iron-nitride layer formation upon gaseous nitriding of iron-based alloys. The model incorporates the coupled, concurrent processes of inward diffusion of nitrogen and the depth dependency of the time dependency of the precipitation of alloying-element nitride particles in the α matrix. Experimental results were obtained by gaseous nitriding of an Fe-2.23 at.% V alloy. The incubation time for iron-nitride formation on Fe–Me alloy is generally much larger than that for iron-nitride formation on pure iron due to a pronouncedly lesser rate of increase of dissolved N content at the surface of Fe–Me alloy. The extent of segregation of N at the MeN/α-Fe interfaces has distinct influence on the incubation time.     

Ordering Effects and Hyperfine Interactions in Fe–N Austenites

Using the high accurate ab-initio FLAPW method and the cluster expansion technique, interatomic N–N interactions for Fe–N austenite were calculated. The interactions were used for calculation of temperature dependence of the short range order for Fe₁₀ N austenite. For two model structures with different nitrogen distributions, the hyperfine interactions were calculated. It was revealed, that EFG might be nonzero on nuclei even for Fe atoms that do not have nitrogen atoms at the first coordination shell. This finding has to be considered for interpretation of Mössbauer spectra of austenite Fe–N.     

Synthesis and characterization of micrometric ceramic powders for electro-rheological fluids

Micrometric lamellar ceramic powders of the displacive ferroelectric oxide Bi₄Ti₃O₁₂ were synthesized by co-precipitation of bismuth nitrate and ammonia titanyl solutions followed by a heat treatment. It was found that a complete thermal decomposition is reached at 1000 °C. Structural and thermal evolution of these ceramic powders were studied by X-ray diffraction, thermogravimetry and differential thermal analysis. The homogeneity in size and morphology of these ferroelectric particles are appropriate to prepare electro-rheological fluids. One of these fluids was prepared by dispersing the powders in silicone oil; the complex cluster structure formed by the particles, under an applied AC electric field, was observed.     

Effect of nitriding time on secondary recrystallization behaviors and magnetic properties of grain-oriented electrical steel

The effect on secondary recrystallization behaviors and magnetic properties of grain-oriented electrical steel of nitriding time from 0 to 240 s in the acquired-inhibitor method has been studied. It was found that the volume fraction of nitride precipitates increased with increasing nitriding time. However, the average diameter of the nitride precipitates decreased with increasing nitriding time. Two kinds of nitride precipitates were found to have formed after primary recrystallization annealing. A fine rod-shaped precipitate was found to be Si₃N₄ and and a coarse, lozenge-shaped precipitate was MnSiN₂. Moreover, primary grain size decreased with increasing nitriding time due to retarding of the grain growth by precipitates. After secondary recrystallization annealing, the specimen that was nitrided for 30 s obtained the largest volume fraction of abnormal growth grains and largest area percentage of Goss grains. Conversely, specimens that were nitrided more or less than 30 s demonstrated poor secondary recrystallization and obtained low area percentage of Goss grains. Furthermore, the optimum nitriding time to obtain the best magnetic properties was 30 s. In addition, the optimum nitrogen content was 150 ppm.     

Characterization of iron nitrides prepared by spark erosion,plasma nitriding,and plasma immersion ion implantation

The effect of the nitrogen uptake in α-iron upon spark erosion in gaseous and liquid ammonia, plasma nitriding, and plasma immersion ion implantation is studied. The resulting phases and hyperfine parameters, measured by the Mössbauer spectroscopy, are discussed from the point of view of initial conditions of their preparation and subsequent heat and/or mechanical treatment. Spark erosion in the ammonia gas produces fine particles with the dominating ferromagnetic α-Fe phase (50%). The 20% of specimen volume form α′-Fe and α′′-Fe₁₆N₂ phases. The last 30% occupy the γ′-Fe₄N, ferro- and paramagnetic ε phases, and γ-Fe(N). Nitriding in the liquid ammonia allows to incorporate the higher content of nitrogen into α-iron particles which results in the formation of paramagnetic ε(ζ)-Fe₂N phase. This phase also dominates the surface of α-iron specimen implanted by nitrogen using plasma immersion ion implantation at 300°C/3 h, where high uptake of nitrogen (approx. 30 at%) is reached. Plasma nitriding at 510°C results in the formation of γ′-Fe₄N phase.