N–N interaction and nitrogen activity in the iron base austenite

The Monte Carlo computer simulation of the f.c.c. Fe–N alloy is performed using the data of abundances of different iron sites in the austenite lattice as obtained by means of Mössbauer spectroscopy. The validity of the interpretations of available Mössbauer spectra was tested based on the data of calculation of N–N interaction energies in the two first coordination spheres on the interstitial sublattice, which could satisfy the experimental data of nitrogen distribution in austenite derived from Mössbauer spectra. It is shown that no values of N–N interaction energies exist that could be consistent with the data of conversion electron Mössbauer spectroscopy (CEMS), where a thin surface layer only contributes to the Mössbauer spectra. A strong N–N repulsion (>0.14 eV) in the first coordination sphere and a soft N–N repulsion (<0.07 eV) in the second one was found to be consistent with the studies performed by means of transmission Mössbauer spectroscopy (TMS), according to which single nitrogen atoms and 180° N–N pairs exist in the nitrogen austenite. A possibility of the long-range order-like Fe₄N is shown at the energy values determined for two coordination spheres, provided a small N–N repulsion in the third coordination sphere occurs. It is also observed that the available data of nitrogen activity in Fe–N austenite are not consistent with the values of N–N interaction energies determined from the analysis of Mössbauer spectra, i.e. the available activity data do not correspond to the nitrogen distribution in the iron austenite as it follows from Mössbauer data. The concentration dependence of nitrogen activity and an effect of the N₂ gas pressure on the nitrogen solubility in Fe–N austenite are calculated using the values of N–N interaction energies in two coordination spheres.     

A comparative study of corrosion inhibition of steel and stainless steel in hydrochloric acid by N,N,N′,N′-tetramethyl-p-phenylenediamine

Protection of Metals and Physical Chemistry of Surfaces - The corrosion of mild steel (MS) and AISI type 321 stainless steel (AISI 321) in 1 M HCl solution and the inhibitive mechanism of...     

Comparative investigation of oxidation resistance and thermal stability of nanostructured Ti–B–N and Ti–Si–B–N coatings

Nanostructured Ti–B–N and Ti–Si–B–N coatings were deposited on silicon substrate by ion implantation assisted magnetron sputtering technique. To evaluate the oxidation resistance and thermal stability the coatings were annealed on air and in vacuum at 700–900°C. As-deposited and thermal-treated coatings were investigated by transmission electron microscope, selected area electron and x-ray diffraction, atomic force microscopy, Raman and glow discharge optical emission spectroscopy. Nanoindentaion tests were also performed. Obtained results show that Si alloying significantly improves the thermal stability of Ti–B–N coatings and increases their oxidation resistance up to 900°C. It was shown that formation of protective amorphous SiO₂ top-layer on the coating surface plays important role in the increasing of the oxidation resistance.     

Electrochemical and optical characteristics of the hole conducting material N,N′-diphenylbenzidine during oxidation in DMF

The electrochemical oxidation of the hole conducting material N,N′-diphenylbenzidine (DPB) in N,N-dimethylformamide (DMF) was investigated by cyclic voltammetry and spectroelectrochemistry. The results were compared to electro-oxidation in acetonitrile. Time-resolved UV–Vis spectroscopy, with spectra measured on a time scale of 20 ms by a diode array spectrometer, was used in conjunction with cyclic voltammetry to monitor the progress of electrochemical oxidation and reduction in situ and to obtain kinetic and structural information on reaction products. The oxidation of DPB in DMF is quasi-reversible at E_sw<+1.2 V (vs. Ag/AgCl), while at higher switching potentials, the oxidation is followed by chemical reactions leading to the polymerization of DPB. The radical cation DPB^+., which can be transformed into the neutral radical DPB^· through the loss of a proton, was detected in the oxidation by in situ ESR spectroscopy. A reaction mechanism was proposed for the oxidation processes. Structural changes induced in DPB at high potentials are suspected to be accountable for device failure of DPB based light-emitting diodes (LEDs) in applications.     

In situ UV–vis and Raman spectroscopic studies of the electrochemical behavior of N,N′-diphenyl-1,4-phenylenediamine

The electrochemical behavior of the N,N′-diphenyl-1,4-phenylenediamine (B3) in acid solution has been investigated by in situ UV–vis and Raman spectroscopies. The electrogenerated species semiquinone form (B3⁺) and monoprotonated structure of B3 (B3⁺) have been characterized as the main products of B3 oxidation. The N,N′-diphenyl-1,4-benzoquinonediimine (B2Q1) species has also been characterized in B3 neutral solution and the B3⁺ proton loss was attributed to proton absence in solution. Finally, a mechanism has been proposed concerning the electron and proton transfer that occurs during the oxidation process.     

Synthesis and characterization of uniform nanoparticles of γ-Mo2N for supercapacitors

Uniform nanoparticles of molybdenum nitride were synthesized by temperature-programmed reaction(TPR) using MoO3 and ammonia as reactants. This material was characterized by X-ray diffractometry(XRD), transmission electron microscopy(TEM), scanning electron microcopy(SEM) and cyclic voltammetry(CV). Results show that the material consists of a pure phase of γ-Mo2N nanoparticles with average diameter of about 16 nm. The material presents a specific capacitance of 172 F/g in 1 mol/L H2SO4 electrolyte at a scan rate of 1 mV/s and the potential window is broadened to 1.1 V (-0.6 to 0.5 V). At the 6 000th cycle, the material remains 94.9% and 94.7% of the initial capacitance in 1 mol/L H2SO4 and KCl solution, respectively. A possible mechanism comprising surface control and diffusion control is proposed to explain the effect of scan rates on specific capacitance.     

Preparation and microwave dielectric properties of Csf/Si3N4 composites

The CJSi3N4 composites were prepared by hot-press sintering method using α-Si3N4 power, short carbon fibers and La2O3-Y2O3 sintering additives. The mechanical and microwave dielectric properties of CjSi3N4 composites were studied and discussed. The results show that the addition of the short carbon fibers can not destroy the relative density of the sintered samples, but it deteriorates the flexural strength of the sintered samples, so the flexural strength of the silicon nitride matrix is the highest among the samples. The real part （ε3 and the imaginary part （ε＇3 of the permittivity of CsfCSi3N4 composites greatly increase with increasing voltmae fraction of the short carbon fibers, achieve the maximum 73.1 and 101.5, respectively. A strong frequency dependence of the imaginary part （ε″） of the permittivity is observed.     

Optimization of friction and wear behaviour of Al–Si3N4 nano composite and Al–Gr–Si3N4 hybrid composite under dry sliding conditions

The tribological behaviour of gravity die stir cast LM6 alloy with graphite (Gr) and silicon nitride nanoparticles was investigated. Al–Gr–Si₃N₄ hybrid composite, Al–Si₃N₄ nanocomposite and Al–Gr nanocomposites were separately fabricated to investigate their frictional and wear characteristics under dry sliding conditions. EDS was used to ensure the uniform presence of nano Si₃N₄ and graphite in the cast. L9 orthogonal array method was chosen to conduct the experiments to study the effect of different applied loads (20, 30 and 40 N) and sliding distances (1, 2 and 3 km). The results showed that the respective wear rate and coefficient of friction (COF) decreased by 25% and 15% for hybrid composite when compared with those of Al–Si₃N₄ nanocomposite whereas the wear rate and COF of Al–Gr was found to be very minimal. The micro Vickers hardness of the hybrid composite was 14% more than that of the simple nanocomposite and there was not much notable variation for Al–Gr and Al–Si₃N₄ nanocomposite materials. Scanning electron microscope was used to analyze the worn surface and subsurface, from which it was noted that the predominant wear mechanisms observed were abrasive for nanocomposite and both abrasive and adhesive mechanism for hybrid composite. Analysis of variance (ANOVA) and F-test were used to check the validity model and to determine the significant parameters affecting the wear rates.     

A comparative study of microstructure,oxidation resistance,mechanical, and tribological properties of coatings in Mo–B–(N), Cr–B–(N) and Ti–B–(N) systems

M–B–(N) (M = Mo, Cr, Ti) coatings were obtained by the magnetron sputtering of MoB, CrB₂, TiB, and TiB₂ targets in argon and in gaseous mixtures of argon with nitrogen. The structure and composition of the coatings have been investigated using scanning electron microscopy, glow-discharge optical emission spectroscopy, and X-ray diffraction. The mechanical and tribological properties of the coatings have been determined by nanoindentation, scratch-testing, and ball-on-disk tribological tests. The experiments on estimating the oxidation resistance of coatings were carried out in a temperature range of 600–1000°С. A distinctive feature of TiB₂ coatings was their high hardness (61 GPa). The Cr–B–(N) coatings had high maximum oxidation resistance (900°С (CrB₂) and 1000°С (Cr–B–N)) and possessed high resistance to the diffusion of elements from the metallic substrate up to a temperature of 1000°С. The Mo–B–N coatings were significantly inferior to the Ti–B–(N) and Cr–B–(N) coatings in their mechanical properties and oxidation resistance, as well as had а tendency to oxidize in air atmosphere after long exposure at room temperature. All of the coatings with nitrogen possessed a low coefficient of friction (in a range of 0.3–0.5) and low relative wear ((0.8–1.2) × 10^–6 mm3 N^–1 m^–1.     

Mössbauer study and thermodynamic modeling of Fe–C–N alloy

Mössbauer spectroscopy and Monte Carlo computer simulation have been combined to understand the reason for the solid-solution stability of Fe–0.93 C–0.91 N alloy (mass%). Interstitial concentrations in austenite and ferrite were determined on the basis of X-ray diffraction measurements of the lattice dilatation. The hyperfine structure of Mössbauer spectra was analyzed to identify different atomic configurations in solid solutions and determine their fractions. Thereafter Monte Carlo simulation of the interstitial distribution in ferritic and austenitic solid solutions was performed, and values of the interstitial–interstitial interaction energies were obtained for the first and second coordination spheres in austenite and the first to the fourth coordination spheres in ferrite. Simulation shows that in both austenitic and ferritic phases the interaction of interstitial atoms is characterized by a strong repulsion within the first two coordination spheres. Experimental data and simulated interstitial distributions are consistent and complementary. It is concluded that the absence of interstitial clusters prevents carbide and nitride precipitates and causes the higher thermodynamic stability of Fe–C–N solid solutions as compared with Fe–C and Fe–N ones.     

Application of the cluster variation method to ε-Fe2N1-z and ζ-Fe2N: ordering of interstitial atoms

The regular and the irregular trigonal prism approximations of the cluster variation method (CVM) are applied to investigate the ε-Fe₂N_1−z and the ζ-Fe₂N phases. The ε-Fe₂N_1-z/ζ-Fe₂N phase boundary, the types of interstitial ordering and the lattice parameters of the ε and ζ phases are calculated and compared with the available experimental data. In the composition range from 25 to 33 at.% N, long-range order occurs. The changes in the N distributions are associated with the strong repulsive interactions between neighbouring N atoms. The CVM results show that with increasing the N content towards 33 at.% N, the repulsive N–N interactions favour the distortion of the iron host lattice, thereby increasing the nearest-neighbouring distance between occupied sites, i.e. transformation to the ζ-Fe₂N phase. The calculated interstitial site occupations are in good agreement with those proposed on the basis of experimental data obtained by Mössbauer spectroscopy and neutron diffraction.     

Effect of halide anions and temperature on initiation of pitting in Cr–Mn–N and Cr–Ni steels

Abstract

The pitting corrosion of Cr₁₈Mn₁₂N and Cr₁₈Ni₉ steels in halide solutions (F^?, Cl^?, Br^? and I^?) has been investigated. The study involved cyclic potentiodynamic polarisation tests with subsequent examination of the specimens by both optical and scanning electron microscopy. Values of the critical concentrations of halide ions, [X^?]_cr, beyond which pitting occurs, as well as breakdown potentials for pitting in chloride solution, have been established. In addition, the effect of the temperature over the range of 5–80°C on the critical chloride ion concentration [Cl^?]_cr has been investigated and it has been found that temperature has a negligible effect beyond 40°C.     

Properties and fracture processes of Si3N4/ Si3N4 joints brazed using Cu–Zn–Ti filler alloy

Abstract

Si₃N₄ ceramic was jointed to itself using a filler alloy of Cu-Zn-Ti at 1123-1323 K for 0.3-2.7 ks. Ti content in the Cu-Zn-Ti filler alloy was varied from 5 to 20 at.-%. The effect of brazing parameters, such as brazing temperature, holding time and Ti content, on the mechanical properties and facture processes of the Si₃N₄/Si₃N₄ joint were investigated. The results indicated that the increased brazing temperature, holding time and Ti content increase the thickness of the interfacial reaction zone in the Si₃N₄/filler alloy, and the size and amount of the reaction phases in the filler alloy. Their increases lead to increasing shear strength of the joint. The fracture behaviour of the Si₃N₄/Si₃N₄ joint greatly depends on the microstructure of the joint. A suitable thick reaction zone with reaction phases yields the high strength of the Si₃N₄/ Si₃N₄ joint.     

Structural Stability,Electronic Structure and Mechanical Properties of Li–N–H System

Ab initio calculations are performed to investigate the ground state properties, structural phase transition,electronic structure and mechanical properties of lithium nitride(Li3N), lithium imide(Li2NH) and lithium amide(LiNH2).The computed ground state properties like equilibrium lattice constant, cell volume, valence electron density, cohesive energy, bulk modulus and its derivatives are in good agreement with available experimental data. The structural phase transitions from a-P6/mmm to b-P63/mmc phase at a pressure of 17.5 GPa in Li3N and cubic(Fm3m) to hexagonal(P63/mmc) phase at a pressure of 102 GPa in lithium imide(Li2NH) are observed. A new high pressure hexagonal(P63/mmc)phase is predicted for Li2NH. Electronic structure reveals that Li3N and LiNH2 are semiconductors, whereas Li2 NH is an insulator. The calculated elastic constants indicate that these materials are mechanically stable at ambient condition.     

Predictions of high-pressure structural, electronic and thermodynamic properties of α-Si3N4

The plane-wave pseudo-potential method within the framework of first-principles technique is used to investigate the fundamental structural properties of Si3N4 . The calculated ground-state parameters agree quite well with the experimental data. Our calculation reveals that α-Si3N4 can retain its stability to at least 45 GPa when compressed below 300 K. No phase transition can be seen in the pressure range of 0-45 GPa and the temperature range of 0-300 K. Actually, the α→β transition occurs at 1600 K and 7.98 GPa. Many thermodynamic properties, such as bulk modulus, heat capacity, thermal expansion, Gru¨neisen parameter and Debye temperature of α-Si3N4 were determined at various temperatures and pressures. Significant differences in these properties were observed at high temperature and high pressure. The calculated results are in good agreement with the available experimental data and previous theoretical values. Therefore, our results may provide useful information for theoretical and experimental investigations of the N-based hard materials like α-Si3N4.     

Cyclic deformation and fatigue behaviour of 7Mo–0.5N superaustenitic stainless steel—slip characteristics and development of dislocation structures

The present work concerns the development of dislocation structures and surface slip markings during cyclic straining of a superaustenitic stainless steel. The composition of the tested material was Fe–25Cr–22Ni–7.6Mo–3Mn–0.46N (wt%). Two total strain amplitudes, 2.7×10⁻³ and 1.0×10⁻², were employed and specimens were investigated at specific numbers of cycles corresponding to certain stages on the cyclic hardening/softening curve. For both strain amplitudes, the developed dislocation structures are strongly planar and with increasing strain amplitude, the slip mode gradually changes from single slip to multiple slip. The short range ordering between Mo and N, as indicated by an atom probe investigation, is broken down during strain cycling leading to increased slip planarity. Early stages of cycling show dislocation multiplication. With increasing number of cycles, the dislocations are gradually grouped together in planar bands with high dislocation density, surrounded by dislocation-poor areas. The evolution of such bands is associated with decreasing effective stresses, while the internal stresses are only slightly reduced. Macroscopic slip bands, similar to PSBs, are formed upon prolonged cycling at the high amplitude. The slip markings created on the specimen surface show strong similarities with the bands of localised slip observed in the dislocation structures of the bulk.     

Cyclic deformation and fatigue behaviour of 7Mo–0.5N superaustenitic stainless steel—stress–strain relations and fatigue life

The cyclic deformation characteristics and fatigue behaviour of a superaustenitic stainless steel with composition Fe–25Cr–22Ni–7.6Mo–3Mn–0.46N (wt%) have been investigated. Detailed studies were performed on cyclic hardening/softening behaviour, hysteresis loops, waveform, fatigue lifetime, and internal as well as effective stresses during cyclic straining in the total strain amplitude range 2.7·10⁻³–1.0·10⁻². Special attention is paid to the role of nitrogen and the interaction between nitrogen and molybdenum. Immediate cyclic softening takes place at small strain amplitudes, whereas hardening occurs during the first few cycles at large strain amplitudes followed by softening. For all strain amplitudes a virtually stationary state develops after about 10% of the lifetime with only a weak decrease of the peak stresses. In the cyclic stress–strain curve the material hardens linearly during multi step testing, whereas single step testing leads to excessive hardening at the largest strain amplitudes. During strain cycling the internal stresses develop like the total stresses, while the effective stresses decrease with increasing number of cycles for all strain amplitudes and also diminish with increased strain amplitude. This behaviour is discussed in terms of developing dislocation structures, studied in an accompanying paper. A double slope behaviour in Coffin–Manson diagrams is observed. The fatigue lifetime resembles that of AISI 316 with 0.29 wt% nitrogen at high strain amplitudes but is shorter at lower strain amplitudes. However, in stress controlled situations the superaustenitic material is superior.     

Structure and properties of selected (Cr–Al–N,TiC–C,Cr–B–N) nanostructured tribological coatings

The paper will present the state-of-art in the process, structure and properties of nanostructured multifunctional tribological coatings used in different industrial applications that require high hardness, toughness, wear resistance and thermal stability. The optimization of these coating systems by means of tailoring the structure (graded, superlattice and nanocomposite systems), composition optimization, and energetic ion bombardment from substrate bias voltage control to provide improved mechanical and tribological properties will be assessed for a range of coating systems, including nanocrystalline graded Cr_1−xAl_xN coatings, superlattice CrN/AlN coatings and nanocomposite Cr–B–N and TiC/a-C coatings. The results showed that the superlattice CrN/AlN coating exhibited a super hardness of 45 GPa when the bilayer period Λ was about 3.0 nm. Improved toughness and wear resistance have been achieved in the CrN/AlN multilayer and graded CrAlN coatings as compared to the homogeneous CrAlN coating. For the TiC/a-C coatings, increasing the substrate bias increased the hardness of TiC/a-C coatings up to 34 GPa (at −150 V) but also led to a decrease in the coating toughness and wear resistance. The TiC/a-C coating deposited at a −50 V bias voltage exhibited an optimized high hardness of 28 GPa, a low coefficient of friction of 0.19 and a wear rate of 2.37 × 10⁻⁷ mm³ N⁻¹ m⁻¹. The Cr–B–N coating system consists of nanocrystalline CrB₂ embedded in an amorphous BN phase when the N content is low. With an increase in the N content, a decrease in the CrB₂ phase and an increase in the amorphous BN phase were identified. The resulting structure changes led to both decreases in the hardness and wear resistance of Cr–B–N coatings.     

On the oxidation ofα-Fe andε-Fe2N1−z : I. Oxidation kinetics and microstructural evolution of the oxide and nitride layers

The oxidation of -Fe and -Fe₂N_1–z at 573 K and 673 K in O₂ at 1 atm was investigated by thermogravimetrical analysis, X-ray diffraction, light-optical microscopy, scanning electron microscopy and electron probe X-ray microanalysis. Upon oxidation at 573 K and 673 K, on -Fe initially -Fe₂O₃ develops, whereas on -Fe₂N_1–z initially Fe₃O₄ develops. In an early stage of oxidation the oxidation rate of -Fe₂N_1–z appears to be much larger than of -Fe. This can be attributed largely to an effective surface area available for oxygen uptake, which is much larger for -Fe₂N_1–z than for -Fe due to the porous structure of -Fe₂N_1–z as prepared by gaseous nitriding of iron. The development of a magnetite layer in-between the hematite layer and the -Fe substrate, at a later stage of oxidation, enhances layer-growth kinetics. After 100 min oxidation at 673 K the (parabolic) oxidation rates for -Fe and -Fe₂N_1–z become about equal, indicating that on both substrates the oxide growth is controlled by the same rate limiting step which is attributed to short-circuit diffusion of iron cations. Oxidizing -Fe₂N_1–z increases the nitrogen concentration in the remaining -iron nitride, because the outward flux of iron cations, necessary for oxide growth, leads to an accumulation of nitrogen atoms left behind.