Nitrogen In Solid Solution Research Articles

Hot workability of steels for forgings

Sufficient hot ductility is one of the prerequisites for the successful forming and heat treatment of steels. The influence of chemical composition (including trace elements), of soaking and deformation temperatures, strain rate and duration of deformation was to be studied in hot tensile testing. To distinguish the different embrittling mechanisms from each other, a great number of steels with systematically varied composition were examined. Test conditions were chosen so as to give maximum agreement with actual hot working operations.In the temperature range from 1 200–600°C, which was covered by this study, hot embrittlement was only found on steels containing at least one of the elements N, Nb, Pb or Bi. Embrittlement due to MnS precipitations did not occur, as the soaking temperature was limited to max. 1 315°C and the Mn/S ratio was at least 30. Nitrogen is the main cause of hot embrittlement in commercial steels. The fact that also unalloyed, aluminium‐free steels embrittle with sufficiently low strain rate shows that nitrogen in solid solution may cause embrittlement even in the absence of nitride formers. The nitride formers accelerate the embrittling process, provided the nitrides are dissolved at soaking temperature. Aluminium, however, has a retarding effect in the presence of vanadium. Embrittlement is attributed to nitrogen atoms entering into multiple voids and micropores, where they recombine to form molecules which impede the slip of dislocations, thus leading to embrittlement. A sufficient length of the deformation operation and recrystallisation being impeded by precipitations are the prerequisites for this type of embrittlement. Titanium, by binding nitrogen at an early stage, prevents precipitation. Also in the case of embrittlement by lead and bismuth, the most conclusive explanation is that atoms of these elements accumulate in voids. Embrittlement by niobium, however, is attributed to deformation‐induced precipitation, as it only occurs on cooling from soaking to test temperature and not on direct heating to test temperature.

Effects of nitriding at low nitrogen partial pressures on yield and fatigue in some stainless steels

AbstractNitriding stainless steel at nitrogen partial pressures from 0·25 to 0·95 atm and temperatures from 980 to 1160 C resulted in selective internal nitridation of the strong nitride-forming elements, Ti and Cr. Titanium nitride ahrays formed in Tibearing steels, but chromium nitride only for nitriding conditions helmr the Cr2N Solvus. A nitrided case could, therefore, consist of up to three zones, one of nitrogen in solid solution, one of precipitated TiN, and one of Cr2N or mixed rhromium-titanium nitride, depending on nitriding conditions and composition. The case was the shallmrer the greater the intervention of internal nitridation in the diffusion process. Swiace hardness, strength, and fatigue endurance were usefully increased by nitriding. The strengthening was wholly attributed to nitrogen in solid solution, the precipitates being too coarse to contribute. Thus, tensile andfatigue strengthening were directly related to case depth, which, in turn, was optimized by nitriding Ti-free steels with...

The oxidation and corrosion resistance of nitrided iron alloys

The chemistry and mechanical properties of nitrided iron alloys are well understood but their resistance to oxidation and corrosion has received less attention. Thermogravimetric and metallographic results are presented of the oxidation in air of nitrided iron, FeMo and FeCr laboratory alloys, and mild steel. Oxidation resistance is improved after nitriding by formation of a fine grain oxide which has a higher fracture strain than the coarser oxide on the corresponding annealed alloys. No void formation occurs at the oxide/metal interface. The results are discussed in terms of nucleation of oxide on dispersed incoherent nitride particles in the metal surface. The improved cohesion of the oxide on nitrided alloys is however not reflected in the results of thermogravimetric studies as the reduced interfacial voidage on the dispersoid-containing alloy allows unrestricted cation transfer from metal to oxide and the conditions are insufficiently aggressive to cause spalling on un-nitrided alloys. Experiments on nitrided mild steel demonstrate the resistance to fracture of oxides formed on nitrided alloys. The criteria for resistance to aqueous corrosion are different from those for resistance to high temperature oxidation but nitriding is shown to produce significant improvements in resistance to general and pitting attack. Potentiokinetic measurements are reported for nitrided laboratory alloys and a commercial austenitic stainless steel containing nitrogen in solid solution. This behaviour is not fully understood but is not simply related to oxide nucleation and cohesion as is the case in high-temperature oxidation.

Mechanisms of reactive sputtering of indium II: Growth of indium oxynitride in mixed N 2-O 2 discharges

The reactive sputtering of indium in mixed N 2-O 2 discharges was investigated using in situ optical absorption and emission spectroscopic analyses of plasma processes. All measured discharge parameters including the target sputtering rate, the target current and the sputtered indium emission intensity and neutral atom density exhibited abrupt changes with oxygen concentration at a critical value C ∗. For a total pressure of 50 mTorr C ∗ was approximately 2.5 mol.%. The abrupt change was caused by the formation of an indium oxide layer on the target surface over a very narrow range of O 2 partial pressures. The nitrogen coverage on the target was small at all gas compositions because of rapid preferential sputtering. X-ray photoelectron spectroscopy, Auger electron spectroscopy and X-ray diffraction were used to characterize the deposited film chemistry and structure as functions of the growth conditions. All films were n-type semiconductors and could be classified into the following three groups: (1) films grown at 0< C 0 2 <2.5 mol.% were InN-based alloys with the wurtzite structure containing oxygen in solid solution; (2) films grown at 2.5 mol.% < C O 2 <6 mol.% were two-phase mixtures; (3) films grown at C O 2 >6 mol.% were cubic In 2O 3-based alloys with nitrogen in solid solution. Optical band gap measurements showed that E g ranged from 1.7 to 2.4 eV for films of type (1) and from 2.4 to 3.6 eV for films of type (3).

On the determination of small concentrations of interstitial carbon and nitrogen in bcc iron

The disaccommodation of the initial susceptibility has been studied on samples of bcc iron containing both carbon and nitrogen in solid solution. Two simple methods for separate determination of concentrations of relaxators in dual solid solutions have been developed and applied to the determination of low carbon and nitrogen contents in bcc iron. Both these methods consist in the decomposition of the magnetic (or mechanical) relaxation measurements and because they are universal they can be used in the study of many other point defects in different materials.

Effect of nitrogen on the strength of thorium

The effect of nitrogen in solid solution on the plastic deformation of polycrystalline thorium was studied by measuring the flow stress from 4.2 K to 773 K at several strain rates. Nitrogen was found to behave similarly to carbon in thorium and increased the thermally activated component of the flow stress. The activation energy was 1.3 eV for overcoming nitrogen atoms in solution. Aging the higher nitrogen alloys produced an age hardening contribution to the flow stress which was a thermally activated component at about 600 K but became an athermal component at lower temperatures.

Influence of interstitial ordering of nitrogen in tantalum on electrical resistivity

In tantalum samples containing more than 1.2 at.% of nitrogen in solid solution, electrical resistivity is reduced by up to 58 per cent during annealing. It is shown that this reduction is caused by partial ordering of the nitrogen atoms in the tantalum lattice. Experimental results on the dependence of resistivity on annealing time, annealing temperature and nitrogen concentration are discussed.

Mössbauer spectroscopy of disordered and ordered iron–nitrogen solid solutions

Mossbauer spectroscopy has been used to study the distribution of nitrogen atoms in FeN solid solutions with high nitrogen content. It demonstrates the ordering of nitrogen in the b.c.t. solution at low temperatures, since a 400 kOe hyperfine environment appears. A new hyperfine interpretation of the f.c.c. solution shows a more effective contribution to the iron 3d band from nitrogen than from carbon. Hyperfine field data exhibit a localized negative and positive contribution at first and second nearest Fe neighbours respectively in the disordered b.c.t. solutions. Comparison with the case of carbon and isomer shifts favour 4s spin density oscillations consistent with the ordered Fe16N2 hyperfine data. Quadrupole shifts lead to a [001] or [110] spin polarization. La spectometrie Mossbauer permet de suivre la distribution des atomes d'azote dans les solutions solides FeN a forte teneur. Grǎce a l'apparition d'un environnement dont le champ hyperfin vaut 400 kOe, elle revele la mise en ordre de l'azote dans la solution quadratique centree a basse temperature. Une nouvelle interpretation de la solution cubique a faces centrees montre que la contribution a la bande 3d du fer est plus effective dans le cas de l'azote que dans celui du carbone. Dans la solution quadratique centree desordonnee, le champ hyperfin presente une contribution localisee qui est negative pour les atomes de fer premiers voisins et positive pour les seconds. Si l'on compare ces resultats a ceux du carbone et si l'on considere les valeurs prises par le deplacement isomerique, on doit envisager des oscillations de la densite de spin 4s confirmant les donnees hyperfines de la phase ordonnee Fe16N2. Les deplacements quadrupolaires conduisent a une polarisation de spin selon la direction [001] ou [110].

Vanadium nitrides: reactions of vanadium oxides with lithium in the presence of nitrogen

The reaction of vanadium oxides with liquid lithium containing dissolved nitrogen provides a method for the preparation of nitrides of vanadium. The oxides V2O5, VO2, V2O3, and VO are reduced by liquid lithium to the metal. Depending on the quantity of nitrogen available, further reaction then occurs to give the α phase solid solution of nitrogen in vanadium, the nitrides V3N and VN, or the ternary nitride Li7VN4.

Effect of multiple-mechanical-thermal-treatment on tensile properties of niobium

Effects of multi-mechanical-thermal-treatments (mmtt) on the room temperature tensile properties of niobium containing oxygen and nitrogen in solid solution were investigated. The mmtt was based on the strain-ageing process involving pre-straining just beyond the Lüders strain and ageing at 200° and 350°C for one hour. At 200°C the free dislocations introduced by the pre-straining were locked by oxygen atoms and at 350°C by nitrogen atoms. With increasing number of mmtt cycles lower and upper yield stresses and ultimate tensile stress increased, while total elongation decreased. At a retained total elongation of 7.5% (five mmtt cycles) the lower yield stress increased by 35% on ageing at 200°C, and 59% on ageing at 350°C. Nitrogen, as in the case of solution hardening, was more effective than was oxygen in strengthening niobium during mmtt. The hardening effect was attributed to a gradual build-up of a uniform dislocation structure with increasing number of mmtt cycles.

Relationship between Structure and Sputtering Parameters in NbN Films

The structure of thin films of niobium nitrides was examined by reflection electron diffraction techniques. The nitride films were obtained by reactive rf sputtering of the niobium at different power levels and partial pressures of nitrogen. The films were deposited on oriented sapphire substrates maintained either at room temperature of at 400 °C. Crystal structures of three distinct phases were observed and identified as (1) α-bcc niobium with nitrogen in solid solution, (2) β-hexagonal Nb2N, and (3) δ-fcc NbN. In this study, the γ–Nb4N3 composition could not be obtained as a single phase but was always found to occur as a mixture with the δ phase. The films prepared at both high partial pressures of nitrogen and rf powers had strong textures that were dependent also on the film thickness. The electron-diffraction data showed that niobium nitride films with successive structure transformations similar to that in the bulk state can be realized and that the transformations can be correlated to the sputtering parameters used.

Enthalpy of formation of the nitrogen solid solution in ?-zirconium and of zirconium nitride in their homogeneity regions

1. A study was made of the enthalpy of formation of the nitrogen solid solution in zirconium and of zirconium nitride within their homogeneity regions. It was established that the enthalpy of formation changes linearly within the homogeneity region of each phase. 2. The fact that nitrogen content within the homogeneity region of each phase affects the rate of change in enthalpy of formation is ascribed to differences in the character of chemical binding. This manifests itself also in differences in the chemical properties of the nitrogen solid solution in zirconium and zirconium nitride in their homogeneity regions.

Some Effects of Nitrogen Content on the Stress Corrosion Susceptibility of AIS1 Type 304 Austenitic Stainless Steel

Abstract The effects of nitrogen content on the susceptibility of AISI Type 304 stainless steel to stress corrosion cracking have been studied by exposing U-bend specimens of the alloy containing various amounts of nitrogen to boiling magnesium chloride solutions. It was found that nitrogen in solid solution in Type 304 stainless steel reduces the resistance of the alloy to stress corrosion. Increasing amounts of nitrogen cause shorter times to crack initiation and more rapid failures by increasing the rate of propagation. The susceptibility of Type 304 is further increased by thermally aging the stressed U-bend specimens at 154 C before exposing them to the corrosive medium. The observations from this investigation seem to support those past theories which suggested that the increased susceptibility of austenitic stainless steels with higher nitrogen contents was due to the formation of nitrogen-rich areas within the austenitic matrix.

Electrical Resistivity Studies of Iron–Nitrogen Solid Solutions

AbstractThe electrical resistivity of iron–nitrogen solid solutions prepared from high purity iron has been studied as a function of applied longitudinal magnetic fields up to 60 kOe at 4.2°K. It is demonstrated that the presence of interstitial nitrogen increases the resistivity of iron and alters the shape of resistivity vs. magnetic field curves, particularly at low fields. By extrapolating the data to B = 0, where B is the internal magnetic induction, it has been possible to determine that the residual resistivity contribution per 1 at% nitrogen in solid solution in iron is about 0.1 μΩcm. At 78°K the value is approximately 7.0 μΩcm. Such a large positive deviation from the Matthiessen rule is characteristic for iron alloys with another transition metal as a solute. Magnetoresistivity data of iron‐nitrogen solid solutions (but not pure iron) very approximately obey the Kohler rule.

Equilibrium vapour pressures of carbon monoxide + nitrogen solid solutions

Vapour pressures of carbon monoxide + nitrogen solid solutions have been measured in the region of the λ-transition (55–63°K) by a condensation technique. Measurements on bulk solutions were not practicable for T < (0.9 times the temperature of the triple point) because very long equilibrium times were encountered. Equilibrium pressures for three solutions of fixed vapour concentration (7.5, 20 and 39 % nitrogen) are reported and the excess Gibbs energies have been derived as a function of temperature. Deviations from ideal behaviour are small in both the α and β solid phases. Conventional vapour pressure measurements on a bulk specimen of pure carbon monoxide (∼1 cm3) are also described and the results compared with those of other workers; the agreement is good.

On the effect of partially permeable interfaces in ultrasonic testing: Mundry, E. Part II. Model tests and applicability. Materialprufung, 9, no. 4, p. 120 (1967)

We present a detailed analysis based on both experimental and modeling approaches of the unique silicon nitride precipitation sequence recently observed in ferritic Fe–Si alloys upon nitriding. At 570 °C, silicon nitride forms as an amorphous phase of size-dependent cuboidal morphology which results from the symmetry of matrix crystal structure. These amorphous precipitates are stable over remarkably long treatment durations. However, we demonstrate here that it is possible to trigger a transition to the precipitation of a crystalline modification of Si3N4 by switching to a denitriding medium at the same temperature. This change in structure is associated with a change from the cube-like morphology to a hexagonal prism shape. This unique phenomenon, driven by a classical nucleation and growth process, can be explained by the shift of stress-relief mechanism related to the change of atmosphere. The plentiful availability of nitrogen atoms during nitriding allows a strong relaxation of the precipitation-induced stress, which is energetically more favorable to the amorphous phase. Upon annealing in a low nitrogen activity medium, nitrogen in solid solution diffuses outward and no longer relieves the precipitation-induced stress. In that configuration, the crystalline modification of Si3N4 becomes more stable owing to its lower associated stress. It precipitates in a hexagonal prism shape, which is the equilibrium shape dictated by the crystal symmetries of both the matrix and the precipitates.

Ultrasonics assemble plastic components: Stafford, R.D. Metalworking production, 111, no. 29, p. 47 (1967)

We present a detailed analysis based on both experimental and modeling approaches of the unique silicon nitride precipitation sequence recently observed in ferritic Fe–Si alloys upon nitriding. At 570 °C, silicon nitride forms as an amorphous phase of size-dependent cuboidal morphology which results from the symmetry of matrix crystal structure. These amorphous precipitates are stable over remarkably long treatment durations. However, we demonstrate here that it is possible to trigger a transition to the precipitation of a crystalline modification of Si3N4 by switching to a denitriding medium at the same temperature. This change in structure is associated with a change from the cube-like morphology to a hexagonal prism shape. This unique phenomenon, driven by a classical nucleation and growth process, can be explained by the shift of stress-relief mechanism related to the change of atmosphere. The plentiful availability of nitrogen atoms during nitriding allows a strong relaxation of the precipitation-induced stress, which is energetically more favorable to the amorphous phase. Upon annealing in a low nitrogen activity medium, nitrogen in solid solution diffuses outward and no longer relieves the precipitation-induced stress. In that configuration, the crystalline modification of Si3N4 becomes more stable owing to its lower associated stress. It precipitates in a hexagonal prism shape, which is the equilibrium shape dictated by the crystal symmetries of both the matrix and the precipitates.

The solid solubility of nitrogen in Nb and Nbrich NbHf, NbMo and NbW alloys: Part I: The binary system NbN

Using a dynamic leak method, a Sieverts apparatus and a combination of X-ray diffraction and microscopical techniques, a study has been made of the interstitial solid-solubility of nitrogen in niobium as a function of pressure and temperature. It has been found that Nb can accommodate 9.48 at.% N interstitially at 2200°C and 3 × 10 −1torr N 2, but the amount which can be retained in solution at room temperature is very much smaller, being about 1.0 at.% depending on the quenching rate. The results are fully in accord with those of Gebhardt, Fromm and Jakob, and of Cost and Wert who obtain a relationship x = 6.2 × 10 −4 (p N 2 ) 1 2 exp( 46,000 RT ) for the amount x, in at.% of nitrogen in solid solution, p N 2 being the nitrogen pressure in torr, R the gas constant and T in degrees Kelvin. The lattice parameter of Nb is increased from a =3.2986 A ̊ to 3.3060 Å with 1.05 at.% N retained in the lattice.

Kinetics of N2 Desorption from the Surfaces of Iron–Nitrogen Solid Solutions

The kinetics of nitrogen desorption from iron surfaces was studied between 400 ° and 550 °C. The nitrogen desorbed isothermally into the gas phase was made available to the surface by diffusion from within the iron at the desorption temperature. The method of rate measurement involved the collection and volumetric measurement of gas samples at low pressures from specimens only a few square centimeters in area. Specimens were prepared by nitrogenizing zone-refined iron with NH3–H2 mixtures having a range of N2 fugacity between 10 and 3000 atm. The surfaces were then cleaned by ion bombardment in a bakeable high-vacuum system.The kinetics of nitrogen desorption from iron was observed to be a surface-controlled process, which showed first-order dependence upon the nitrogen concentration in the iron when conventional ion-bombardment cleaning procedures were employed. However, it was discovered that carbon monoxide is generated in glass systems during ion bombardment and can contaminate the iron surface. When ...

On the relaxation strength of the snoek peak

A calculation was made on the relaxation strength Δ of the Snoek peak when a single crystal specimen is strained by a torsional stress. For a specimen with 〈100〉 orientation Δ vanishes and for a specimen with 〈111〉 orientation Δ takes a non-zero constant value independent of the shape of cross-section of the specimen. However, for a specimen with 〈110〉 orientation Δ varies from zero to a finite value depending on the shape of cross-section of the specimen. These calculated values are compared with those obtained for a tensile stress. Two simplified cases (the uniform stress and the uniform strain approximations) were considered for a random polycrystalline sample of cylindrical shape. In these approximations the relaxation strength under a torsional stress is larger than that under a tensile stress, when the same specimen is measured at the same frequency and temperature. Confirmation of these predictions was obtained from experiments on the single and poly-crystals of α-iron containing nitrogen in solid solution. A general survey of experimental results on polycrystalline specimens shows that the closest agreement with the calculations is obtained when data on large grained samples are compared with the calculations based on the uniform stress approximation.