Nitrogen Concentration Gradient Research Articles

Nitrate sources and transformations in a river-reservoir system: Response to extreme flooding and various land use

Nitrate (NO3−) pollution poses a global aquatic threat, promoting eutrophication and endangering human health. Large floods can swiftly drive significant nitrogen load into rivers and downstream water bodies, nevertheless, precisely quantifying NO3− sources in watersheds with complex land use patterns remains challenging. In June 2020, heavy and persistent rainfall triggered severe floods across southeastern China, causing Lake Qiandao, the largest reservoir in this region, to reach its highest recorded level. This study investigated NO3− sources and transformations within the Xin'an River-Lake Qiandao system, particularly focusing on the impact of the catastrophic summer flooding event. Through extensive sampling, we identified a distinct nitrogen concentration gradient: from the pristine Xin'an River source, characterized by 95 % forest cover and consistently low nitrogen levels for the initial 125 km, to a gradual rise as the river traversed human-impacted regions, culminating in peak concentrations within residential and agricultural areas. Isotopic analysis identified a general upward trend in nitrate stable isotope (δ15N–NO3–) associated with the expansion of agricultural and urban lands. Mean values of δ15N–NO3– gradually increased from headwaters and forest-dominant catchments (+2.88 ‰) to agricultural regions (+6.64 ‰) and residential areas (+7.10 ‰). A Bayesian modeling identified that during the flood season, soil erosion and chemical fertilizers were the primary contributors, collectively accounting for 74 % of NO3− sources in the Xin'an River. However, accumulated sewage in tributaries and the overflow of sewage from treatment plants, exacerbated by the flood, significantly impacted Lake Qiandao, particularly in densely populated urban areas, contributing 53 % of the total NO3− input. Additionally, NO3− levels and isotopic values in Lake Qiandao were influenced by a mixture of sources and nitrogen cycling processes, including nitrification and algae assimilation following the flood event. In contrast, during baseflow conditions, the contribution of domestic sewage and livestock wastewaters increased to 41 % in the upper river, while Lake Qiandao remained affected by non-point source pollution, with soil erosion and chemical fertilizers contributing 58 % to the total nitrogen pollution. This study sheds light on the complex dynamics of NO3− dynamics within river–reservoir systems, particularly in the context of extreme flooding, emphasizing the critical need for comprehensive water quality management strategies along the entire watershed.

Microstructures and mechanical properties of powder-filled laser welding joints composed of high-nitrogen steel

In this work, a permanent connection of a high-nitrogen steel plate is achieved by using layer-by-layer powder-filled laser welding of high-nitrogen steel powder, and the stability levels of the welding process, microstructures and mechanical properties are analyzed. Compared with other processes, powder-filled laser welding is smooth without bursts and spatters, and the nitrogen content of the welding seam is obviously improved. The welding seam consists mainly of austenite, with few ferrite and manganese oxides. There is a nitrogen concentration gradient between the inside and the upper part of the melt pool, which leads to nitrogen escape. Laser remelting exacerbates this process, resulting in more severe nitrogen loss in the bottom layer than in the upper layer of the welding seam. The differences in the nitrogen solubilities in austenite and ferrite and the high-nitrogen content in the melt pool are the main reasons for the formation of micropores. Tensile experiments and impact experiments are conducted on the welding joint. The average tensile strength and elongation of the welding joint are 1032.36 MPa and 19.70%, respectively, contributing to 94% and 53.4% of the total values for the base metal. The grain refinement and relatively high nitrogen content are the reasons for the better tensile strength relative to other processes. The average impact energy of the welding joint is 24.68 J, which is approximately 63.74% of the base metal. Pores are the main reason for the reduced impact energy of the welding joint.

Microstructure evolution and mass transfer behavior during multi-pass laser surface nitriding process on titanium alloy

Laser surface nitriding technique is successfully applied to improve the surface modification efficiency of titanium alloys. Moreover, adjustment of nitride layer thickness can be achieved by applying multi-pass laser surface nitriding. Mass transfer behavior of laser surface nitriding could be totally different from the steady-state gas nitriding processes because it involves rapid heating and cooling procedures for many times, which is systematically analyzed in this research by conducting different number of laser surface nitriding passes and in-situ laser quenching after laser surface nitriding experiment on commercial pure titanium TA2 alloy. Results indicate that the thickness of nitridation layer increases with the increase of nitriding passes. After three passes laser surface nitriding, the depth of nitridation region can reach to about 56 μm. During multi-pass laser surface nitriding process, the pre-formed TiN layer will be partially decomposed under the thermal impact of subsequent laser processing. The decomposed nitrogen will further diffuse deeply into the substrate along the nitrogen concentration gradient and combine with the nitrogen from the environment to continually offer the nitrogen source during the entire laser nitriding process with high processing efficiency.

Effect of Sintering Temperature and Solution Treatment on Phase Changes and Mechanical Properties of High-Nitrogen Stainless Steel Prepared by MIM

High-nitrogen stainless steel (HNSS) has been widely concerned and studied owing to its excellent mechanical, corrosion resistance, and biocompatibility properties. A series of HNSS was prepared by metal injection molding (MIM) using gas atomized Fe-Cr-Mn-Mo-0.3 N duplex stainless steel powders. Both sintering and solution treatments were carried out in an N2 atmosphere. The effects of nitrogen distribution and phase transformation on the mechanical properties of MIM HNSS during sintering and solution were studied. The results show that as the sintering temperature increased, the sample density increased, but the total nitrogen content decreased. Nitrogen and Cr2N concentration gradients along the cross-section of as-sintered samples were formed after cooling. The high nitrogen content promotes the decomposition of γ: γsaturated translated to γ and Cr2N. Meanwhile, the low Mn content in austenite also decomposes γ: γ translated to α and Cr2N. After solution treatment, a single γ phase was obtained for samples sintered at 1200 to 1320 °C. For solution treatment samples sintered at 1320 and 1350 °C, their tensile strength was 988.76 and 1036.12 MPa; yield strength was 615.61 and 636.14 MPa, and elongation was 42.58 and 40.08%, respectively. These values vastly exceeded the published MIM HNSS values.

Urine osmolality predicts worsening renal function and poor prognosis in acute decompensated heart failure

Abstract Background/Purpose Worsening renal function (WRF) can sometimes occur in the patients with acute decompensated heart failure (ADHF) and increase the risk of morbidity and mortality (1). In a previous study, it was reported that fractional excretion of sodium (FENa) reflects net sodium reabsorption from nephron segments and predicts WRF during treating ADHF (2). On the other hand, recently the new drugs which approach urine concentration mechanism and affect urine osmolality (U-OSM), such as tolvaptan and sodium-glucose cotransporter-2 inhibitor, have begun to be widely used as treatment of heart failure. Thus, we focused on U-OSM, which reflects not only sodium handling but also water excretion controlled by the collecting duct, and evaluated the association between WRF and U-OSM. Moreover, previous studies have demonstrated that FENa, fractional excretion of urea nitrogen and transtubular potassium concentration gradient are markers for long-term prognosis in patients with ADHF (3–5). Therefore, we also studied whether U-OSM can predict prognosis in ADHF. Methods A total of 157 patients admitted to our hospital because of a primary diagnosis of ADHF from February 2020 through July 2021 were retrospectively reviewed. U-OSM in the spot urinary samples were examined within 72 hours after admission. U-OSM was calculated based on the following validated formula (6): U-OSM = 1.07 × {2 × [urine sodium (mEq/L)] + [urine urea nitrogen (mg/dL)]/2.8 + [urine creatinine (mg/dl)] × 2/3} + 16.2. The primary outcome was the occurrence of WRF during hospitalization. WRF was defined as increased serum creatinine ≥0.3 mg/dL from baseline (7). The secondary outcome was the occurrence of ADHF readmission and all-cause death within 180 days after discharge. Results Primary Outcome. WRF developed in 46% of all patients. In the patients that developed WRF during hospitalization, U-OSM was significantly lower than in the patients without WRF (366±106 mOsm/L versus 430±128 mOsm/L; P<0.001). Receiver operating characteristic curve analysis revealed the optimal cutoff values of U-OSM was 403 mOsm/L (AUC 0.64; 95% CI: 0.56–0.72; P<0.001) to predict the WRF (Figure 1). On multivariable logistic regression analysis, U-OSM (OR, 1.99, 95% CI: 1.27–3.12; p=0.003) and serum creatinine (OR, 1.00, 95% CI: 0.99–1.00; P=0.009) were independent predictors of WRF. Secondary Outcome. There were 34 patients (22%) readmitted and 9 patients (6%) died within 180 days after discharge. ROC curve analysis revealed the optimal cutoff values of U-OSM as 349 mOsm/L (C-statistic 0.74; 95% CI: 0.65–0.83; P<0.001) to predict ADHF readmission and all-cause death within 180 days (Figure 2A). On Kaplan-Meier analysis, the secondary outcome was significantly higher in patients with U-OSM<349 mOsm/L (u-OSM≥349, 57%, U-OSM<349, 43%; HR, 0.99; 95% CI: 0.99–1.00, P<0.001) (Figure 2B). Conclusion U-OSM on admission may be a predictor of WRF and a prognostic marker in ADHF patients. Funding Acknowledgement Type of funding sources: None.

Sustainability of the rice–crayfish farming model in waterlogged land: A case study in Qianjiang County, Hubei Province, China

The rice–crayfish farming model has been rapidly developed and become an economically viable method to supply food in China in recent years. However, its environmental and economic sustainability has not been thoroughly investigated. This study uses a survey in 2016 and a field experiment in 2017 in Qianjiang, Hubei Province, China to assess the relative economics of concurrent rice–wheat (RW), rice–crayfish (RC), and crayfish monoculture (CM) models in waterlogged land areas. The field survey indicated that the RC model had a higher benefit–cost ratio (3.5:1) than the RW (2.0:1) and CM (3.1:1) models and the RC model protected farmers’ enthusiasm for grain production facing unfavourable weather conditions. The field experiment aimed to explore nitrogen management strategies in RC fields. In the experiment, four levels of nitrogen concentration gradient - 0 kg N ha−1 (0 N), 75 kg N ha−1 (75 N), 150 kg N ha−1 (150 N) and 225 kg N ha−1 (225 N), were set in a 2-year-old rice–crayfish (RC2) field, an 8-year-old rice–crayfish (RC8) field, and a RW field as a control. The field experiment results suggested that the peak rice yield in RW, RC2, and RC8 occurred when 225 N, 150 N and 75 N were used, respectively. In RC2 and RC8, however, residual feed-nitrogen that was not used by crayfish was utilized by rice plants. Thus, an optimal amount of nitrogen in RC fields was proposed to improve the nitrogen use efficiency and reduce environmental pollution by nitrogen fertilizer. Farmers use less nitrogen but have higher net income in RC than in RW and CM. It is necessary to sustainably develop integrated farming technologies (i.e., proper field configurations for rice fields) to effectively sustain rice production. The results also showed that the RC farming model was a viable diversification option for rice farmers in waterlogged land.

Effects of different nitrogen concentrations and light intensities on lipid accumulation and growth of Chlamydomonas reinhardtii

This study sets different nitrogen concentration gradients to explore the impact of nitrogen concentration on biomass and lipid accumulation of Chlamydomonas reinhardtii under different light intensities. The results showed that the conditions for the accumulation of lipid in the algae were 30μE-m-2-s-1 light intensity and 100 mg.mL-1 nitrogen concentration with the lipid content of C. reinhardtii was 39.79% and chlorophyll content was 17.08 mg.mL-1. This study provides an experimental basis for the regulation of lipid production of C. reinhardtii by using nitrogen-deficient culture and provides a foundation for further utilization of genetic engineering methods to regulate the lipid metabolism of C. reinhardtii.

Role of weak magnetic strength in the operation of aerobic granular reactor for wastewater treatment containing ammonia nitrogen concentration gradient

Weak magnetic field (WMF) and aerobic granular sludge (AGS) technology were both robust technologies in wastewater treatments. In this study, the AGS characteristics and nutrient removal performances were all estimated at the load of 20 to 40 mg/L ammonia nitrogen (NH4+-N) and 0 to 40mT magnetic field. Results showed that 10mT was beneficial for keeping stable structure of granules when increasing NH4+-N load, accompanied with increasing protein (PN) secretion in EPS. Besides, all the total nitrogen (TN) removal rate under 10mT reached above 90%, while they were all less than 80% under other WMF strength when loading with 40 mg/L NH4+-N. Moreover, the simultaneous nitrification and denitrification (SND) efficiency could be enhanced by WMF of 10mT. Illumina MiSeq sequencing showed that NH4+-N load changed the bacterial richness and diversity when the magnetic strength was 10mT. And Candidatus_Competibacter was identified as the main functional genes for effective operation in this system.

Structural Characterization of Fine γ′-Fe4N Nitrides Formed by Active Screen Plasma Nitriding

The paper presents the structural characterization of γ′-Fe4N nitrides produced by active screen plasma nitriding (ASPN) processes. Experiments were performed on the Fe-Armco model material at 693, 773, and 853 K for 6 h. Investigation of the properties of the substrate was realized using scanning electron microscopy (SEM, SEM–EBSD/Kikuchi lines), energy-filtered transmission electron microscopy (TEM-EFTEM), X-ray diffraction (GID, grazing incidence diffraction, micro-XRD), and secondary ion mass spectroscopy (SIMS). Results have confirmed that the γ′-Fe4N nitrides’ structure and morphology depend considerably on the nitriding process’s plasma conditions and cooling rate. In addition to that, γ′-Fe4N nitrides’ formation can be correlated with the surface layer saturation mechanism and recombination effect. It has been shown that the γ′-Fe4N structure depends considerably on several phenomena that occur in the diffusive layer (e.g., top layer decomposition, nitrogen, and carbon atoms’ migration). Our research proves that the nitrogen concentration gradient is a driving force of nitrogen migration atoms during the recombination of γ′-Fe4N nitrides. Finally, realized processes have allowed us to optimize active screen plasma nitriding to produce a surface layer of fine nitrides.

Sediment Denitrification Rate and Its Response to Exogenous Carbon and Nitrogen in the Ponds and Bottomland of the Chaohu Lakeshore Zone

From October 2018 to August 2019, three typical ponds and bottomland were selected in the Chaohu lakeshore zone, where surface sediments and overlying water samples were collected simultaneously. A set of incubation experiments, along with exogenous carbon and nitrogen concentration gradients, were conducted to analyze sediment denitrification rates and its response to carbon and nitrogen limitation. Moreover, the main influencing factors for the sediment denitrification process were identified using correlation analysis method. The results showed that ① the denitrification rates of three plant plexus sediments were 2.15-10.87, 2.08-10.65, and 2.06-10.88 mg·(kg·h)-1, with averages of 6.47, 6.97, and 6.76 mg·(kg·h)-1, respectively. Overall, there was no significant difference between them. ② In general, exogenous nitrogen addition could significantly increase denitrification rates of the three plant sediments, indicating that nitrate was the limiting factor for the sediment denitrification process. ③ Exogenous carbon addition resulted in a significant decrease in the denitrification rates of three plant plexus sediments, indicating that organic carbon inhibited the denitrification process. ④ Exogenous carbon and nitrogen added simultaneously displayed a dramatic effect on the increase of sediment denitrification rates. Except for the bottomland Pucao in October and pond Pucao in June, all other cases showed higher denitrification rates for high carbon and nitrogen concentration.

Experiments for in silico evaluation of Optimality of Photosynthetic Nitrogen Distribution and Partitioning in the Canopy: an Example Using Greenhouse Cucumber Plants.

Acclimation of leaf traits to fluctuating environments is a key mechanism to maximize fitness. One of the most important strategies in acclimation to changing light is to maintain efficient utilization of nitrogen in the photosynthetic apparatus by continuous modifications of between-leaf distribution along the canopy depth and within-leaf partitioning between photosynthetic functions according to local light availability. Between-leaf nitrogen distribution has been intensively studied over the last three decades, where proportional coordination between nitrogen concentration and light gradient was considered optimal in terms of maximizing canopy photosynthesis, without taking other canopy structural and physiological factors into account. We proposed a mechanistic model of protein turnover dynamics in different photosynthetic functions, which can be parameterized using leaves grown under different levels of constant light. By integrating this dynamic model into a multi-layer canopy model, constructed using data collected from a greenhouse experiment, it allowed us to test in silico the degree of optimality in photosynthetic nitrogen use for maximizing canopy carbon assimilation under given light environments.

δ15N systematics in two minerotrophic peatlands in the eastern U.S.: Insights into nitrogen cycling under moderate pollution

Availability of reactive nitrogen (Nr; mainly ammonium, NH4+, and nitrate, NO3−) via precipitation and upwelling groundwater affects carbon (C) accumulation in peat deposits and biological greenhouse gas emissions. Isotope composition of Nr was studied at Wolf Swamp (WS) and The Glades (GL), two peatlands located in western Maryland (U.S.), to provide a more robust understanding of N cycling at the ecosystem level. In recent years, both sites experienced a moderate atmospheric pollution (3–9 kg of deposited Nr ha−1 yr−1), and possible additional Nr inputs via groundwater contaminated by fertilizers/livestock. Sampling of living Sphagnum, vertical peat profiles, atmospheric deposition, and surface bog water was carried out on October 22–24, 2017. The δ15Nr signatures of vertical precipitation were negative (mass-weighted means of −7.2 and −8.1‰ at WS and GL, respectively) despite reports that NOx emitted by coal-burning power plants, a precursor of deposited nitrate, had been extremely positive (up to +26‰). Mean δ15N values at both sites increased in the order: vertical precipitation < living Sphagnum < atmospheric N2 < surface bog water < maturating peat. Intermediate δ15N values of living Sphagnum (−1.5‰ at both sites) suggested simultaneous incorporation of isotopically light atmospheric Nr and isotopically heavy Nr from bog water in the newly formed biomass. The δ15N values of bulk peat increased downcore by 4.5 and 4.9‰ at WS and GL, respectively. Gradual mineralization of organic N, mainly by nitrification and denitrification, is isotopically selective, leaving behind higher δ15N values of bulk peat at greater peat depths. Nitrogen concentration gradients and δ15N trends in vertical peat profiles at these locations therefore cannot be used as an archive of past changes in pollution levels and N isotope signatures of incoming Nr.

Tribological behaviour of plasma nitrided cast iron D6510 and cast steel S0050A under the inclined-impact sliding condition with extremely high contact pressure

Plasma nitriding as a surface modification was applied on two substrate materials: cast iron D6510 and cast steel S0050A. After measurement of the friction coefficients of the treated samples using a pin-on-disc tribotester, an inclined impact-sliding wear tester was utilized to investigate their tribological behaviour under tilting contact with extremely high contact pressure. While numerous surface fatigue cracks, severe chipping, and peeling of the compound layer were observed for the treated cast steel sample, the treated cast iron sample had far fewer surface fatigue cracks without chipping or peeling of the compound at the same test condition. The governing mechanisms of the treated cast iron sample’s superior resistance to surface fatigue failure were revealed by studying the cross-sectional hardness and nitrogen concentration profile. Energy-dispersive X-ray spectroscopy (EDS) analysis indicated that the treated cast iron sample had a smaller nitrogen concentration gradient, which led to a smaller hardness gradient as measured. The results suggest that a smaller hardness gradient between the compound layer and the diffusion zone and a thicker hardened case was able to improve the wear resistance and surface fatigue cracking resistance against high contact loads. Moreover, the smaller friction coefficient of the treated cast iron sample could also be beneficial for improving the wear resistance.

Nurse plants transfer more nitrogen to distantly related species.

Plant facilitative interactions enhance co-occurrence between distant relatives, partly due to limited overlap in resource requirements. We propose a different mechanism for the coexistence of distant relatives based on positive interactions of nutrient sharing. Nutrients move between plants following source-sink gradients driven by plant traits that allow these gradients to establish. Specifically, nitrogen (N) concentration gradients can arise from variation in leaf N content across plants species. As many ecologically relevant traits, we hypothesize that leaf N content is phylogenetically conserved and can result in N gradients promoting N transfer among distant relatives. In a Mexican desert community governed by facilitation, we labelled nurse plants (Mimosa luisana) with 15 N and measured its transfer to 14 other species in the community, spanning the range of phylogenetic distances to the nurse plant. Nurses established steeper N source-sink gradients with distant relatives, increasing 15 N transfer toward these species. Nutrient sharing may provide long-term benefits to facilitated plants and may be an overlooked mechanism maintaining coexistence and increasing the phylogenetic diversity of plant communities.

Zymoseptoria tritici development induces local senescence in wheat leaves, without affecting their monocarpic senescence under two contrasted nitrogen nutrition

Zymoseptoria tritici causes large losses in wheat yield usually related to the losses of green area. However, the issue of whether green area losses result from the local necrosis or from acceleration in the monocarpic apical senescence remains open.The present study examined whether leaf inoculation with Zymoseptoria tritici modifies apical senescence of flag leaf in wheat cultivar Soissons grown under two contrasted nitrogen nutritions.The dynamics of local and apical senescences and the pycnidia number were measured repeatedly throughout the grain filling. Local and apical senescence were adjusted to functions, the parameters of which were analyzed according to nitrogen and inoculation treatments. The intra-leaf gradient of residual nitrogen concentration was measured at plant maturity.The relative rate of apical senescence increased twofold when fertilization was withheld. Both treatments, inoculation and nitrogen, generated a large range of disease that developed to different extents, resulting from different rates and timing. Local necrosis and pycnidia number strongly varied with nitrogen nutrition but necrosis varied in the same extent as apical senescence, thus green area relative loss was unaffected by nitrogen nutrition. By opposition, apical senescence was never affected at any time by the inoculation. Residual nitrogen increased in the inoculated portions of the infected leaves, in correlation with local necrosis, whereas the residual nitrogen in the non-inoculated part of infected leaves stayed at the level of control leaves.Apical senescence was managed at the leaf tissue level, according to nitrogen availability but regardless of disease, a useful output for modelling.

Local residual stress measurements on nitride layers

In this work, local stresses in different nitrided maraging steel samples of high practical interest for industrial applications were studied through the so-called micro-slit milling method using a focused ion beam. The nitrogen concentration profiles were acquired by glow discharge optical emission spectroscopy. The residual stress state was measured on the surface and also in cross-section, i.e. examining effects of the nitrogen concentration gradient. It is shown that an enhanced lateral resolution can be achieved when a novel multiple fitting approach is employed. The results presented show an overall agreement with stress profiles obtained by X-ray diffraction. Finite Element Modeling is used to explain the apparent discrepancies. A clear correlation between the residual stress and nitriding profiles has been found and the applicability of this method is shown in particular when stress gradients are present.

Phenomena of Intergranular Liquid Film Formation in Technology

The phenomena of the first order phase transition (two-dimensional melting) of grain boundary at temperatures 0.6 - 0.9 TS0 (of the solid state melting point), discovered by the author (1971), is a fundamental property of solid crystalline materials. This finding leads to a principal revision of the scientific concepts of the solid state of substance. The phenomenological description and justification of the finding are developed. The generalized equation of Clausius-Clapeyron type for two-dimensional phase transition was obtained by applying the mathematical tools of the film thermodynamics. The equation has been used for calculating the grain boundary phase transition(GBPhT) temperature TSf of any metal, which TSf value lies within the range of (0.55 - 0.86) TS0. The investigation outcomes are applied to develop the methodology for more effective hard coating formation by synthesis of nanosize nitrides and carbonitrides in surface layers of steels and nickel alloys using a thermo-chemical processing (TChP). Production of an overall nitrogen concentration gradient from 4% to 0.5% at within surface layers leads to formation of modified coatings with a stepped change in properties. The mechanical behavior of new tools at the industrial tests indicated a higher heat resistance (nickel alloys), high resistance to surface wears and fragile breaks-down (chromium tool steels). A short overview of the results of some graded alloys characterization is presented.

A combinatorial study of the mechanical and magnetic properties of a gradually nitrided austenitic stainless steel single crystal

The mechanical and magnetic properties of a nitrided austenitic stainless steel are studied using a combinatorial approach. Plasma nitriding of a [100]-oriented 316L single crystal is carried out using a loose shadow mask to produce an in-plane lateral gradient of nitrogen concentration that extends up to 100 μm. The local mechanical and magnetic properties across the gradually nitrided area are resolved by nanoindentation and the polar magneto-optic Kerr effect, respectively. The hardness, reduced Young's modulus and remanence qualitatively depict the nitrogen profile, suggesting that the nitrogen concentration is a central effect for these observed dependencies. Conversely, the coercivity exhibits a non-monotonic behaviour due to the interplay between magnetic anisotropy and the strength of the induced ferromagnetism. Fingerprints of the expected transition from a nitrogen supersaturated solid solution to a multiphase nature of expanded austenite are evidenced along the gradually nitrided area.

Swelling effect on stress induced and concentration dependent diffusion of nitrogen in plasma nitrided austenitic stainless steel

The kinetics of plasma nitriding of austenitic stainless steel (ASS) below temperatures of nitrides formation and mechanisms of nitrogen penetration are analyzed by proposed kinetic modeling. By fitting of experimental nitrogen depth profiles it is shown that the main processes are nitrogen concentration gradient and internal stresses induced diffusion with concentration dependant diffusion coefficient and also swelling. Internal stress induced diffusion is responsible for plateau formation in nitrogen depth profile which is characteristic for nitriding process. In order to fit experimental curves of different samples (with different nitriding duration time) with fixed diffusion coefficient it is necessary to include concentration dependant diffusion and swelling process. It is shown that diffusion coefficient depends on concentration according to Einstein–Smoluchowski relation. The swelling thickness is the square root function of nitriding time.

Nitriding aluminum alloys by N-multicharged ions implantation: Correlation between surface strengthening and microstructure modifications

This paper deals with the surface strengthening of aluminum alloys by means of a new process allowing multi-charged nitrogen ion implantation. X-ray photoelectron spectroscopy, grazing-incidence X-ray diffraction and atomic force microscopy were used to study microstructural changes involved by implantation. This microstructural study revealed the formation of AlN and AlONγ due to the low nitrogen concentration gradient obtained with multi-charged implantation. Nanoindentation and wear tests were performed to evaluate the mechanical properties of implanted surfaces. A significant improvement of wear resistance was observed as a consequence of the nitride protective layer formation. The observed surface hardening is attributed to both AlONγ and AlN formations and to the precipitation-induced stress.