Segregation Of Nitrogen Research Articles

On a mechanism of Snoek-like relaxation caused by C, N and H in fcc iron-based alloys

The effect of the grain boundary segregation of carbon, nitrogen and hydrogen in face-centred cubic (fcc) iron-based solid solutions on the concentration dependence of the Snoek-like relaxation strength is analysed based on the results of measurements and available experimental data. It is shown that carbon and hydrogen cause a square-like concentration dependence of the relaxation strength, whereas linear concentration behaviour occurs in nitrogen-containing solid solutions. The role of grain boundaries in the apparent square concentration dependence of the relaxation strength is demonstrated for the case of hydrogen using a comparison of results obtained for polycrystals and single crystals. Taking into account the strong affinity of carbon and hydrogen atoms with the grain boundaries and a weak segregation effect for nitrogen in the fcc iron-based solid solutions, it is concluded that the apparent square concentration dependence of the Snoek-like relaxation strength in the carbon- and hydrogen-doped solid solutions can be due to absorption of interstitial atoms at the grain boundaries, which takes some of the interstitials out of the solid solution. The conclusion is made that the Snoek-like relaxation is mainly caused by single carbon, nitrogen or hydrogen interstitial atoms creating complexes with substitutional solutes. Some additional experiments with single crystals are desirable in order to test the possible contribution of C–C pairs to the Snoek-like relaxation in austenitic steels.

Oxygen Precipitation in Nitrogen Doped CZ Silicon

Nitrogen doping of CZ silicon results in an early formation of large precipitate nuclei during crystal cooling, which are stable at 900°C. These are prone to develop stacking faults and high densities of defects inside defect denuded zones of CZ silicon wafers. Simultaneous doping of FZ silicon with nitrogen and oxygen results in two main stages of precipitate nucleation during crystal cooling, an enhanced nucleation around 800°C, which is nitrogen induced, and a second enhancement around 600°C, which depends on the concentration of residual oxygen on interstitial sites. A combined technique of ramping with 1K/min from 500-1000°C with a final anneal at 1000°C for 2h and lateral BMD measurement by SIRM provides a possibility to delineate v/G on nitrogen-doped silicon wafers. Surface segregation of nitrogen and oxygen during out-diffusion can explain the enhanced BMD formation in about 105m depth and the suppressed BMD formation in about 405m depth below the surface. The precipitate growth is enhanced in regions where nitrogen is filled up again after a preceding out-diffusion.

Segregation and enhanced diffusion of nitrogen in silicon induced by low energy ion bombardment

A sample of nitrogen-doped, single crystal Czochralski silicon was subjected to several different surface preparations. Secondary ion mass spectrometry depth profiling has shown that prolonged glancing-angle bombardment by 3–5kV Ar+ ions significantly increases the nitrogen concentration in the near surface by up to an order of magnitude over the bulk value. Concentrations are observed to be elevated over the bulk value to a depth up to 25μm. Nitrogen-implanted samples and samples with a 1nm surface nitride did not exhibit nitrogen segregation under the same conditions, but a sample with 100nm of surface nitride did exhibit ion bombardment induced drive-in. In nitride-free samples, the source of the nitrogen is indicated to be a nitrogen-rich layer in the first micron of material. The diffusion behavior of nitrogen in silicon is discussed and the Crowdion mechanism for diffusion is suggested as the enabling mechanism for the enhanced low temperature diffusion.

Modelling of grain boundary segregation kinetics that are responsible on irreversible and reversible temper embrittlement of engineering steels

There are presented the computer models of multicomponent grain boundary segregation (GBS) that are formed at quenching or tempering of engineering steels. The modelling of these processes requires different algorithms. The dynamic model of segregation at quenching is based on solution of independent diffusion and adsorption-desorption equations for various impurities in steel tied by feedback. The model of segregation under tempering considers the influence of alloying and tempering parameters on concentration and thermodynamic activity of carbon in the α-solid solution. It is based on regression analysis of experimental data on grain boundaries chemical composition obtained by Auger spectroscopy. There is established that grain boundary enrichment by harmful impurities (S and P) is possible after carbon and nitrogen segregation dissolution. The models allow to analyze the competition of elements during formation of GBS and to establish main factors responsible on development of different kinds of embrittlement in engineering steels with various alloying and content of harmful impurities.

Oxygen precipitation in nitrogen doped Czochralski silicon wafers. II. Effects of nitrogen and oxygen coupling

Nitrogen segregation and coprecipitation with oxygen in N-doped Czochralski (N-CZ) silicon wafers are investigated as a function of depth based on extended defect structure and chemical composition. High resolution nitrogen and oxygen secondary ion mass spectroscopy imaging revealed strong coupling of oxygen with nitrogen in annealed as well as in “as-grown” N-CZ Si wafers. In both cases, the near-surface regions appeared highly supersaturated in N and O forming a continuum of defects initiated by N-O complexes. The N and O stoichiometry depth profiles were found to depend on the material thermal history. The spatial variation of the stoichiometry ratio was also determined for precipitates using a combination of scanning transmission electron microscope (STEM) in Z-contrast mode with electron energy loss spectroscopy. The precipitate atomic and microstructures, analyzed by high resolution TEM and STEM, clearly demonstrate that second phase precipitate is precursor to a third phase that is an outer oxynitride shell. Nitrogen and oxygen cosegregation from the matrix to the precipitate interface occurs in a similar fashion as in the subsurface region. We propose a mechanism for oxygen precipitation in N-CZ Si based on N segregation to the interface while oxygen is trapped inside the oxynitride shell.

Radiation-induced nitrogen segregation during electron energy loss spectroscopy of silicon oxide–nitride-oxide stacks

Electron energy loss spectroscopy (EELS) in both fixed-probe transmission electron microscopy (TEM) and scanning TEM (STEM) was used to measure elemental profiles of Si, O, and N in the SiOx/SixNy/SiOx/poly-Si stacks deposited on silicon. The results revealed radiation-induced nitrogen segregation to both the Si/SiOx and SiOx/poly-Si interfaces; the extent of nitrogen segregation increased visibly with increasing the radiation dose. The nitrogen diffusion through the oxide layers apparently was accompanied by the diffusion of oxygen into the nitride. Low-noise artifact-free nitrogen profiles across the Si/SiOx interfaces were obtained in STEM by scanning the beam parallel to the layers. These measurements indicated lack of detectable nitrogen segregation to the interfaces.

The critical amount of nitrogen on the formation of nitrogen gas pores during solidification of 25Cr-7Ni duplex stainless steels

The effects of casting thickness, nitrogen contents, cooling rate, and Mn contents on the formation of nitrogen gas pores during solidification of 25Cr-7Ni-1.5Mo-3W duplex stainless steels (DSS) were quantitatively investigated. In the case of a sand mold, the formation of nitrogen gas pore was not affected by the thickness of castings, which ranged from 13 to 52 mm, and the critical initial nitrogen content for the formation of gas pore was 0.30 wt pct. In the case of the molds made of a stainless steel (STS) and water-cooled Cu, the critical initial nitrogen content did not change much compared to the sand mold. The amount of nitrogen gas pores increased with initial nitrogen contents of castings. The segregation of nitrogen and alloying elements was calculated with Thermo-Calc. The calculated data and the experimental results were compared to estimate the critical nitrogen partial pressure in the residual melt for the nucleation of gas pores. The effect of Mn content on the formation of gas pores was also investigated. The increase of Mn content from 1 wt pct to 2.6 wt pct changed the critical initial nitrogen content 0.30 wt pct to 0.40 wt pct.

高窒素ステンレス鋼溶接継手特性に関する研究 １ 高窒素ステンレス鋼溶接金属のブローホール抑制並びに凝固モードと耐食性の評価

High nitrogen-bearing stainless steel (HNS) is attractive material due to its superior corrosion resistance, mechanical properties as well as resource saving type material. HNS of about 1 mass% in nitrogen content manufactured by a pressurized ESR was tried to joint by conventional arc welding. Because of the nitrogen supersaturation at molten state, blowholes were easily formed in the weld metal. A new welding method was proposed by using high Cr-Mo Steels as welding consumables which had relatively large nitrogen solubility at molten state in order to suppress the blowhole generation in the weld metal. The evaluation of pitting corrosion resistance of weld metal was also investigated with taking into account the solidification mode. The welding method by using high Cr-Mo steels as the welding consumables and by controlling the dilution rate to hold the nitrogen content in molten metal less than equilibrated solubility of molten metal with a nitrogen gas at 1×105Pa was found to be effective to suppress the blowholes generation in the weld metal. The critical pitting corrosion temperature (CPT) of the weld metal portion in 6%FeCl3 solution dropped compared with that of the base metal because of microsegregation of alloy elements. Controlling the composition of the weld metal so as to solidify with FA mode, the degree of CPT drop could be minimized compared with that of other solidification modes because of nitrogen segregation enriched in austenite in which Cr, Mo were relatively depleted. The alloy elements compositions at the portion where Cr and Mo contents were minimized in the weld metal could be estimated by Thermo-calc and compared with the results of EPMA analysis. Tensile strength of the weld metal portions were 700∼900MPa and those were about 20% lower value than that of the parent metal because of the reduction of nitrogen content.

Effects of nitrogen on low-cycle fatigue properties of type 304L austenitic stainless steels tested with and without tensile strain hold

A quantitative analysis of the effects of nitrogen on high temperature low-cycle fatigue without and with tensile strain hold at 600 °C has been conducted for type 304L stainless steels. For better understanding of the role of nitrogen on grain boundary precipitation, the grain boundary segregation of nitrogen was analyzed by Auger electron spectroscopy. The nitrogen addition is found to give relatively better resistance to creep-fatigue than continuous low-cycle fatigue. This in turn improves the fatigue life. This is due to the retardation of the precipitation of carbides at the grain boundary and reduction in the density of grain boundary cavitation sites which are the main factor of grain boundary damage under creep-fatigue test.

Effect of nitrogen segregation on TED and loss of phosphorus in CZ-Si

Transient enhanced diffusion (TED) and dose loss (pile-up) are investigated for phosphorus-implanted samples covered with both oxide and nitride films. P ions were implanted into p-type (1 0 0) CZ-Si (dose 5×10 13 cm −3 , 100 keV) through a chemical vapor deposition (CVD) Si 3N 4 film. For a half of samples, Si 3N 4 was etched off and SiO 2 films were grown by CVD. Both samples were annealed for 20–360 min at 700 °C. Diffusivity of P and the dose loss were estimated based on the secondary-ion mass spectrometry (SIMS) P profiles. Both Si/Si 3N 4 and Si/SiO 2 interfaces were investigated with transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). There is no significant difference in P diffusivity between the SiO 2 and Si 3N 4 films for the present annealing condition of 700 °C for 20–360 min. Regarding dose loss, a distinct different behavior was observed. In case of the SiO 2 cover film, amount of dose decreases with the annealing time. On the other hand, amount of dose decrease with annealing time up to 180 min, but is recovered for more than 180 min in case of the Si 3N 4 cover film. From TEM and EELS analyses, it is found that nitrogen segregates at the Si/Si 3N 4 interface, resulting in recovery of dose loss.

Ultrathin oxynitride formation by low energy ion implantation

Oxynitride films have been formed by rapid thermal processing of N-implanted wafers. The formation mechanism, the chemical composition, and the physical thickness of the oxynitride films were studied by x-ray photoelectron spectroscopy (XPS). Segregation of nitrogen to the surface was performed on nitrogen ion-implant wafers under nitrogen gas. The outcome of the segregation is the formation of an ultrathin oxynitride layer ranging in thickness from 4 to 8.5 Å . Oxidation of nitrogen ion-implanted wafers, where the nitrogen is segregated beforehand and nitrogen ion-implanted wafers with no segregation prior to oxidation, is the focus of this study. XPS results showed that simultaneous segregation and oxidation forms an oxynitride film consisting of two layers where the interface is rich in nitrogen and the surface in oxygen. In the case of nitrogen segregation prior to oxidation, the nitrogen atoms in the oxynitride film, formed at oxidation temperatures less than 1000 °C, are uniformly distributed throughout the film in the form of SiOxNy. At high temperatures (∼1100 °C), the composition of the oxynitride formed by simultaneous segregation and oxidation becomes similar to that where N-segregation is performed prior to oxidation. The presence of nitrogen atoms retards significantly the diffusion of oxygen to the substrate surface thus producing uniform ultrathin films ranging in thickness from 13 to 40 Å.

Determination of the chemical width of grain boundaries of boron- and carbon-doped hot-pressed β-SiC by HAADF imaging and ELNES line-profile

A residue of sintering additives is often found at the grain boundaries in sintered ceramics. This can be used to determine the macroscopic properties of the sintered polycrystalline materials. A combination of both high-angle annular dark-field (HAADF) imaging and energy-loss near edge-structure (ELNES) line-profile methods was carried out to measure the chemical width of grain boundaries using the sintering additives. Application of both HRTEM and HAADF imaging methods for boron- and carbon-doped hot-pressed SiC leads to the identification of the structural differences either within the matrix or at the grain boundaries very clearly. Additionally, EELS analysis was also carried out to identify the chemistries and bondings at the grain boundaries. The segregation of both boron and nitrogen is clearly shown, as well as the chemical width measured at the grain boundaries by the ELNES line-profile method. Incorporation of nitrogen within the grain interior was also detected by ELNES analysis.

Thermodynamic Calculation of Stacking Fault Energy and its Effect on Fcc→Hcp Phase Transformation in Nitrogen Alloyed Stainless Steels

The observed fcc→hcp phase transformation in some iron-based alloys, such as Fe-Mn-based shape memory alloys, is due to the formation of stacking faults, expansion of the faults by the movement of Shockley partial dislocations followed by ordered overlapping of the faults. Therefore, alloy systems with low stacking fault energy (SFE) are naturally more prone to the fcc→hcp martensitic phase transformation. Nitrogen (N) is known as an austenite stabilizing element; however, some existing experimental data in the literature have shown that it decreases the stacking fault energy of austenitic steels. Therefore, one would expect that N alloying should promote the fcc(γ)→hcp(e) martensitic transformation and thus have a destabilizing effect on austenite. To resolve the above discrepancies we have considered the effect of N on the stacking fault energy of fcc iron-based alloys. Our calculations have shown that by increasing N in solid solution, the stacking fault energy of the system initially increases to a maximum and then decreases. Using a phenomenological description for fcc(γ)→hcp(e) heterogeneous martensitic nucleation kinetics, we have demonstrated that increasing nitrogen concentration decreases the driving force for the fcc(γ)→hcp(e) martensitic transformation and increases the friction work due to both nitrogen solid solution strengthening and segregation of nitrogen to stacking faults for fcc iron-based alloys.

Mössbauer Surface Analysis of Nitrogen Ion Implanted Austenitic and Ferritic Steels

This work aims to examine the effects of ion energy and current density during nitrogen implantation of steel. The microstructure and the nitrogen distribution at the surface of austenitic and ferritic steels were compared and discussed. The surface of the austenitic X2CrNiMo17-13-2 and ferritic X1CrNiMoNb28-4-2 steel was nitrided by N - ions implantation at doses of 0.3 - 10 10 17 N + cm -2 , with an energy of 60 or 100 keV The beam current density is 1.5 - 3.5 μA cm -2 . X Ray Diffraction (XRD), Conversion Electron Mossbauer Spectroscopy (CEMS) were used to investigate the nitride phases resulting from the nitrogen implantation. The formation of Cr 2 N, α'-martensite, CR-N enriched austenite, Fe 4 N γ'-like structure, h.c.p. e and metastable f.c.c. phases is also detected depending on steel grade and nitriding conditions. Evolution of the phase formation is associated with chromium and nitrogen migration and segregation during the nitriding process.

The effect of boron on stress-relief cracking of alloy steels

High-purity vacuum-melted laboratory heats of the MnMoNiCr forging steel designated by the ASTM as A508-class 2 were studied to determine the effects of additions of boron and of aluminum on the susceptibility to stress-relief cracking (SRC). The highly deleterious effect of boron was reproduced, and the aluminum addition was found to be at least as deleterious as the boron. High-resolution examination of the fracture surfaces showed that post-fracture faceting occurred on the intergranular fracture surface of the pure steel, but not on the boron-doped steel, indicating a difference in surface composition. From previous AES studies on other steels, it was deduced that the effect of both boron and aluminum is to scavenge nitrogen and to prevent surface segregation of nitrogen with chromium at the test temperature of 550°C. This would allow more sulfur segregation to the surface and thus provide a higher surface concentration of this element, which is known to be the cause of the diffusion-controlled intergranular decohesion that occurs during SRC at high stresses, a process known as dynamic embrittlement. It is concluded that the cosegregation of nitrogen and chromium retards SRC by suppressing sulfur segregation, but this protective effect can be eliminated by scavenging of nitrogen by either boron or aluminum. It is suggested that the susceptibility of a heat of steel to SRC could be assessed by examining the surface-segregation behavior by means of AES.

Influence of nitrogen on the mechanism of austenitic stainless steel hardening

On the example of a C18N12M2 austenitic stainless steel, the influence of nitrogen (whose content varied from 0 to 0.45 wt.%) on the grain boundary hardening coefficient k h entering into the Hall-Patch equation is analyzed. High values of k h in steels with and without nitrogen are found. The data of the Auger analysis show that the hardening coefficient in the steel without nitrogen is determined by the grain-boundary segregation of carbon and oxygen. The grain-boundary hardening in the steel with nitrogen is not connected with the predominant segregation of nitrogen at grain boundaries. It is completely governed by intragranular processes—interaction of nitrogen atoms with dislocations.

The comparison of the different adsorption states of non-metals on the iron surface

The adsorption states of sulphur, phosphorus, carbon and nitrogen on the single crystal Fe(111) surface have been studied using the Auger electron spectroscopy. The AES spectra for high surface concentration of non-metals differ from these obtained for atomic adsorption states. The change of the AES line shape has been observed for all non-metals under the study. The observed changes have been explained on the basis of the model of the electronic structure and interpreted as a formation of the Fe–X–X bonds. For carbon and phosphorus a formation of three dimensional structure is observed. For sulphur adsorption and nitrogen segregation only a formation of X2 has been observed. The formation of X–X bonds weakens the iron-non–metal interaction. The effect of the coadsorbed electron-acceptor element (oxygen) on the state of the chemisorbed non-metal (sulphur) has been discussed, either.

Cold resistance of low-carbon Mn−Ni−Mo−V steels for pressure vessels

1. The growth in the managanese content in low-carbon Mn−Ni−Mo−V steels from 0.86 to 2.3% increases their strength and diminishes their ductility and toughness. There is a threshold manganese content above which the impact toughness (cold resistance) drops intensely. For a heattreated steel this threshold corresponds to 1.4% Mn; for a normalized steel it is 1.5% Mn. 2. The growth of the concentration of nickel from 0.46 to 3.0% increases the strength of the steel. The growth of the nickel concentration to 3.0% at 0.6–1.0% Mn does not affect the ductility parameters of the steels and at 1.3–1.4% Mn causes their decrease. 3. As the nickel content increases from 0.46 to 2.9% (at 1.3% Mn), the impact toughness at a negative temperature increases monotonically. At 1.4% Mn, a nickel addition in an amount of up to 2.5% does not affect the impact strength at a negative temperature. At 2.7–2.9% Ni, it decreases. 4. The reduction of the carbon concentration from 0.12 to 0.035% and that of aluminum to 0.016% causes a decrease in the impact toughness, especially at a negative temperature. The introduction of 0.04–0.06% Al into steels with 0.12 and 0.035% C increases the impact toughness at all the test temperatures. The decrease in the impact toughness is connected with the development of the process of nitrogen segregation in the solid solution, and its growth is caused by the formation of aluminum nitrides and the respective reduction of the nitrogen content in the solid solution. 5. The reduction of the carbon content in Mn−Ni−Mo−V steels to 0.08–0.12% and the use of quenching with tempering as the base heat treatment provides a material with enhanced strength, ductility, and cold resistance suitable for pressure vessels.

Influence of heat treatment on grain boundary microstructure in a Ni–16Cr–10Fe–0·022C model material

A nickel based Ni–16Cr–10Fe–0·022C (wt-%) model material has been investigated principally by atom probe field ion microscopy and transmission electron microscopy. The grain boundary structure was modified by applying four different heat treatments of 550, 600, and 700°C for 1 h and 850°C for 5 h. The Gibbsian interfacial excess of boron, carbon and nitrogen segregation to grain boundaries (GBs) was measured after the heat treatments. The segregating elements were unevenly distributed along the GBs, indicating the importance of the internal GB microstructure. In the material heat treated at 700°C intergranular precipitates of Cr7 C3 , Cr23 C6 , and Cr2 (C,N) were observed. The stability of these carbides is discussed. The heat treatment at 850°C resulted in the formation of Cr7 C3 only. Indications of oxygen clustering at GBs and at carbide/matrix interfaces were obtained.

Effect of nitrogen and sulphur on the segregation of molybdenum on single crystalline Fe–3.5%Mo–N,S alloys

This work describes an experimental study of the effects of nitrogen and sulphur on the surface segregation of molybdenum in an Fe–3.5%Mo alloy. It is found that the presence of small concentrations of bulk dissolved nitrogen induce a strong synergetic segregation (cosegregation) of molybdenum and nitrogen. This effect is strongest for the (100)-oriented alloy, where an epitaxially stabilized MoN surface compound is formed. The synergetic effect of segregated sulphur on the segregation of molybdenum is found to be rather weak. This finding is consistent with the recent work of A. Rolland, M.M. Montagono, J. Cabané, Surf. Sci. 352–354 (1996) 206.