Nitrogen In Solid Solution Research Articles

Improving strength-ductility synergy of titanium matrix composites containing nitrogen via the introduction of intragranular nano-TiB

We prepared a titanium matrix composite (TMC) with added boron nitride nanosheets (BNNSs) for strength-ductility trade-off issues. The composite, characterized by in-situ nano-TiB intragranular distribution and trace nitrogen solid solution, was prepared using rapid hot press sintering (FHP) and short-duration hot rolling. During pre-rolling heat preservation, the diffusion of boron and nitrogen resulted in the intragranular distribution of nano-TiB and nitrogen solid solution. The nano-TiB demonstrated excellent load transfer, fracture suppression, and dislocation storage capabilities at both room and high temperatures. Coupled with the nitrogen solid solution, the composite exhibited a significant enhancement in strain hardening effect compared to the titanium matrix. The composite outperformed the titanium matrix in strength and ductility at both room and high temperatures, demonstrating a notable strength-ductility synergy. This work provides a reference for designing TMCs with excellent performance at both room and high temperatures.

Investigation on Microstucutres and properties of ion nitriding layer of Al0.5CoCrFeNiTi0.25 HEAs cladding coating

To investigate the effect of ion nitriding on the microstructures and properties of high entropy alloy coatings, the Al0.5CoCrFeNiTi0.25 high entropy alloy coating was fabricated via laser cladding and subsequently subjected to ion nitriding treatment. A comparative analysis were conducted to investigate the phase compositions, microstructures and properties of cladding coatings and ion nitriding layer. Furthermore the formation mechanism of the nitriding layer was studied. XRD results revealed that Al0.5CoCrFeNiTi0.25 HEA coatings consisted of duplex FCC and BCC phases, whereas the nitriding layer was composed of high-nitrogen FCC solid solutions and nitrides such as AlN, CrN, Fe4N and TiN. Following ion nitriding, the surface morphology of nitriding layer presented “cauliflower-like” nitride particles, which became denser with the extended nitriding time. Notably, the microhardness of nitriding layer was approximately four times that of HEA coating, and the wear resistance and the corrosion resistance of cladding coating were improved significantly after ion nitriding. During ion nitriding process, N atoms reacted with the alloying elements to form nitrides, which preferentially deposited at grain boundaries and BCC phases and grew horizontally. As the deposition layer spread across the entire surface, the nitriding mechanism shifted from reaction -controlled to diffusion-controlled, with N atoms diffusing inward to form a diffusion layer with high nitrogen solid solutions. With the prolong of nitriding time from 1h to 12h, the thickness of the diffusion layer increased from 11.77 μm to 96.07 μm.

Surface melt nitriding and strengthening mechanism of plasma torched M50 bearing steel

This paper presents a new plasma torch nitriding technology for the first time to realize the surface nitriding of bearing steel for strengthening. By adjusting the plasma torch current parameters in the range of 120–160 A, M50 bearing steel was carried out at a nitriding speed of 2 s/cm2. After nitriding, the nitrogen content was increased from 0.00732 wt% in the original M50 steel up to 0.416 wt%, and the thickness of the nitrided layer was up to 2.51 mm. FeN0.076, Fe2N and Fe3N phases were formed in the steel, which was beneficial to increase the hardness and strength of bearing steel. The Vickers hardness of the original M50 bearing steel was 241 HV0.2. However, the highest surface hardness of specimen after nitriding was increased up to 778 HV0.2, which was attributed to the effect of sub-surface hardness strengthening within 2 mm from the depth of nitride surface. The average wear coefficient and the volumetric wear rate of the specimen after nitriding were decreased by about 32 % and 70 %, respectively. TEM observation showed that after nitriding, the structure of nitrided layer was martensite. The precipitates in the steel changed from large-sized, irregular and aggregated carbides (M23C6, M4C3 and M2C) to small-sized, spherical and uniformly distributed carbon‑nitrogen composite precipitates (M23(C, N)6, M7(C, N)3) in the steel sub-surface, but no obvious precipitates were formed on the surface of nitrided specimens. At depths of 1 mm and 2 mm, the size of the precipitates was reduced by about 68 % and 41 %, respectively, and the area of the precipitates was reduced from 18.4 % to 2.3 % and 3.7 %, respectively, compared to the original M50 bearing steel. It was concluded that the nitrided layer were strengthened synergistically with nitrogen solid solution, nitrogen-containing precipitates and martensite.

Challenge of fabricating difficult-to-nitride materials: Formation of martensitic phase in (Fe0.8Co0.2)8N foil based on industrially suitable gas-nitriding process

Nitrogen-martensitic phase (Fe0.8Co0.2)8N was successfully synthesized by using an industrially suitable gas-nitriding process of a bulk foil. So far, the nitrogen-martensitic phase of the bulk material has not been synthesized at such a high Co content. We found that this is because the nitride was easily denitrided by elevating temperature. In this work, by exploiting a NH3 gas-nitriding process combined with quenching at a rapid cooling rate (>800 °C/s) in NH3 atmosphere, we found that nitrogen stayed at the surface layer of the foil. By using cross-sectional laser microscopy, the nitride region was observed as a 7-μm-thick layered shape at the surface of the 100-μm-thick foil. An x-ray diffraction technique revealed that the nitride layer was a martensitic phase that was characterized as a body-centered tetragonal structure with c/a = 1.04. These findings can be applied to nitriding and surface treatments for alloy systems in which a nitrogen solid solution is hardly formed. Our developed method is promising because the martensitic phase is expected to be formed in whole bulk by further optimizing parameters clarified in this work.

Effect of nitrogen content on grain boundary engineering and corrosion resistance of 316LN stainless steel

Nitrogen solid solution can effectively enhance the strength of steels, but its effect on grain boundary engineering and the corrosion mechanism of steels is still unclear and systematic investigation is needed. In this study, 316LN steel specimens with five nitrogen concentrations were melted and subjected to compression and annealing. The grain boundary character distribution was investigated by electron backscatter diffraction. The corrosion behavior was investigated by the electrochemical polarization curves, electrical impedance spectroscopy, and X-ray photoelectron spectroscopy. The results showed that nitrogen promotes the formation of low-Σ boundaries when the pre-strain is not above 10%. The polarization resistance increases with increasing nitrogen content. The quantitative relationship between the nitrogen content and the pitting potential was obtained. Nitrogen favors the formation of oxides, hydroxides, and NH3/NH4+ on the passivated surface. The effect of planar slip on grain boundary engineering and the influence of NH4+ on the corrosion resistance are discussed.

Microstructure, tensile properties and fracture behavior of high-Nb TiAl composites reinforced by nitrogen solid solution and in-situ boride

TiAl-based composites have broad application prospects in the aerospace and automotive fields due to their excellent high-temperature properties. In this study, the B/N synergistic effect on microstructural evolution, mechanical properties and fracture behavior was investigated by addition of nano-BN particles to Ti-46.8Al-8.0Nb alloy (at%). It was found that the B reacted with the Ti and Nb in the high-Nb TiAl matrix to form needle-shaped borides of TiB, TiB2 and NbB, which in turn reduced the lamellar colony size by inhibiting the movement of grain boundaries during the sintering process, while the N atoms diffused into the octahedral interspaces of the γ and α2 lattice. The lamellar colony size can be greatly reduced from 351 ± 144 μm to 44 ± 10 μm. The as-sintered microstructure can be gradually transferred from fully lamellar (FL) to dual phase (DP) with increment of nano-BN content. Composite containing 0.75 at% nano-BN exhibit the highest ultimate tensile strength at room temperature, reaching 660 ± 12 MPa. At high temperatures, the composite containing 1.50 at% nano-BN exhibits the highest ultimate tensile strength, reaching 517 ± 9 MPa. The mechanism of improved mechanical properties is attributed to grain refinement strengthening, thermal mismatch strengthening, Orowan strengthening, solid-solution strengthening.

Open-atmosphere laser nitriding of austenitic steels to form wear-resistant surfaces

This study presents an interesting result based on the laser beam irradiation of austenitic steels in an open atmosphere, which enables the formation of a surface layer comprising a nitrogen solid solution rather than an oxide. First, a focused nanosecond pulsed Nd:YAG laser beam with a laser energy of 400 mJ·pulse−1 was used to irradiate an AISI 316 stainless steel. This formed a surface layer of ~3 μm thickness with ~12 at.% nitrogen solid solution, thereby enhancing the hardness of the AISI 316 stainless steel surface to ~380 HV, nearly twice that of the untreated surface. Consequently, wear resistance was improved by preventing plastic deformation and abrasive wear. Thereafter, similar laser beam irradiation of Inver42® and Kovar® steels resulted in the formation of nitrogen solid solution layers with a thickness of ~1.5 μm, which is lower than that of AISI 316 steel. Furthermore, their nitrogen contents were reduced to ~6 at.%. Consequently, the improvement in the surface hardness of the Inver42® and Kovar® steels was less than that of the AISI 316 stainless steel. These differences were attributed to their alloy compositions; for instance, Cr, which has a high affinity for nitrogen, might trap the available nitrogen, thereby contributing to an increase in the nitrogen content.

High-strength and high-toughness austenitic stainless steels based on type 316LN at 4.2 K

High-strength and high-toughness prototype steel forgings, N2, N3, and N4, were developed based on type 316LN as a cryogenic structural material. By extrapolating the proportional relationship between carbon plus nitrogen content (C + N) and 0.2 % proof stress (σ0.2) in the range of approximately 0.10–0.25 mass% C + N content, the C + N content was successfully designed to achieve the target σ0.2. To suppress the degradation of the plane-strain fracture toughness (KIC(J)) while increasing the nitrogen solid-solution content, the alloying level was designed by considering the stability of the austenite phase and stacking fault energy. The steel forgings showed a good balance of strength and toughness at 4.2 K, i.e., N2 (σ0.2 = 1142 MPa, KIC(J) = 228 MPa√m), N3 (σ0.2 = 1226 MPa, KIC(J) = 219 MPa√m) and N4 (σ0.2 = 1296 MPa, KIC(J) = 215 MPa√m). The σ0.2 and KIC(J) of ITER-grade 316LN (316LNL, 316LNM, and 316LNH) thick plates with low interstitial (C + N) content of 0.13–0.17 mass% were also evaluated at 4.2 K. Then, the effects of nitrogen content on strength and fracture toughness were successfully clarified in the full range of interstitials concentrations. An alloy design with higher Ni, Mn, and Mo contents in the chemical composition range of 316LN would be desirable for improving the fracture toughness by increasing σ0.2 due to the higher nitrogen content.

Outstanding strength-ductility balance in a Ti–N alloy by tuning ball milling time

The trade-off between strength and ductility is a prevalent issue in strengthening commercially pure titanium (CP–Ti). This study presents a method to prepare ultra-high strength Ti–N alloys with adequate ductility using ball-milling (BM) and spark plasma sintering (SPS) techniques, based on nitrogen solid solution and grain refinement strengthening. The effect of ball milling time of mixed powders on mechanical properties of Ti–N alloys was studied systematically. Quantitative analysis reveals that solid solution strengthening and grain refinement strengthening worked synergistically to enhance yield strength of alloys. The Ti–N alloys with the most favorable mechanical properties demonstrated a yield strength of 1606 MPa and a compressive plastic strain of 14.44 %. The development of novel Ti–N alloys provides a viable strategy for the advancement of high-performance titanium alloys using ubiquitous elements.

Nitrogen isotope marker experiments in austenitic stainless steel for identification of trapping/detrapping processes at different temperatures

Expanded austenite – austenitic stainless steel containing a colossal amount of nitrogen in solid solution – is still some kind of conundrum with the nitrogen atom transport from the surface into the bulk being enhanced for higher nitrogen content. At the same time, atomic ordering due to an enhanced affinity between chromium and nitrogen is observed.Here, nitrogen transport experiments are performed at different temperatures using 14N and 15N as isotopic tracers. The depth distribution of each of these isotopes as well as the total nitrogen content is investigated with time-of-flight secondary ion mass spectrometry (ToF-SIMS) and glow discharge optical emission spectroscopy (GDOES), respectively. In parallel, phase information is obtained using in situ X-ray diffraction (XRD) during nitriding (for time-resolved data) and during sputter depth profiling (for depth-resolved data).During nitriding, no delayed transport of nitrogen, i.e. trapping, is observed for the expanded austenite. However, as soon as CrN precipitates are formed, nitrogen trapping is a dominant factor with an almost perfect correlation between precipitation and trapping. Cluster ion analysis in SIMS confirms this effect with the abundance of Cr2+ ions significantly increased compared to FeCr+ ions in the presence of precipitation. As nitrogen transport and CrN formation occur in parallel at higher temperatures, further experiments to separate the CrN formation at higher temperatures from the nitrogen transport at lower temperatures have been performed.

Influence of γ'N and ɛ'N phases on the properties of AISI 304L after low-temperature plasma nitrocarburizing

The surface of austenitic stainless steel AISI 304L has been hardened by adding nitrogen and carbon through the plasma nitrocarburizing process. Plasma nitrocarburizing at low temperatures eliminates the formation of nitrides and carbides of chromium, thereby increasing the surface hardness without compromising its corrosion resistance. This is attributed to the presence of a hard case consisting of a solid-solution of nitrogen and carbon occupying interstitial sites in an “expanded” austenite phase. The expanded austenite is further characterized as nitrogen-expanded austenite (γN) and carbon-expanded austenite (γC). In the present work, austenitic stainless steel AISI 304L samples were plasma nitrocarburized by varying the N2 content (%N2) in N2-H2-C2H2 feed gas mixture and treatment times (3, 5, and 7 h) at 673 K and 500 Pa. The samples were characterized by means of XRD, microindentation, XPS, EIS, and potentiodynamic polarization testing, for studying the microstructural, mechanical, and electrochemical properties. The XRD analysis revealed the presence of ɛ'N and γ'N in addition to γN and γC when treated with higher %N2 gas for 5 and 7 h. From XPS, the binding energies of undefined FexN were obtained which were found to lie very close to the defined FexN for higher %N2 gas. Surface hardness significantly improved after the plasma nitrocarburizing processes irrespective of the %N2 and treatment times compared to the untreated AISI 304L. The presence of CrN precipitation on the surface of samples treated with the 75% N2 for 5 and 7 h were observed from XPS, which caused higher corrosion rate. Among the treated samples, the process with 50% N2 and 5 h exhibited the best corrosion resistance with high surface hardness.

Strong and ductile titanium via additive manufacturing under a reactive atmosphere

Pure metals tend to have a superior malleability compared to alloys. However, the applicability of pure metals is limited by their low strengths and hardness properties. In this study, strong and ductile pure Ti was synthesized via an approach involving powder modification and additive manufacturing (AM) under a reactive atmosphere. Specifically, pure Ti was processed by laser powder bed fusion in an environment containing a mixture of argon and nitrogen gases. In-situ high-energy synchrotron X-ray diffraction analysis reveals that in-situ nitrogen strengthening of the pure Ti occurs during the reactive AM of Ti. Furthermore, the presence of a nitrogen solid solution leads to the formation of high-strength pure Ti (yield strength of ∼979 MPa and ultimate tensile strength of ∼1058 MPa, respectively). This material exhibits excellent uniform elongation (11%) due to strong work hardening caused by the interaction of interstitial elements and submicrostructure. The proposed reactive AM approach paves the way for in-situ strengthening of pure metals and alloys.

Tribological Behavior of Titanium-Sintered Composites with Ring-Shaped TiN Dispersoids

The wide applicability of titanium (Ti) has prompted the analysis to improve its mechanical strength through the addition of different alloying elements. Among these, Ti materials with pre-mixed pure Ti and titanium nitride (TiN) powders as the starting materials have exhibited improved mechanical properties and tribological performance. In this study, the tribological properties of Ti matrix composites with ring-shaped TiN dispersoids were evaluated. The materials were fabricated from pre-mixed pure Ti powder and core–shell structured Ti–(N) powder, which were prepared by heat treatment at 1273 K under N2 gas. The tribological behavior of the Ti–TiN composites was studied by varying the applied load using a ball-on-disk wear test under oil lubrication conditions. The initial familiarity period of the Ti–TiN composites decreased. Subsequently, compared to the pure Ti specimen employed as a reference material, the friction coefficient was significantly lower and more stable. This is attributed to the ring-shaped, hard TiN dispersoids, which prevented the adhesion phenomenon and improved the oil film formability owing to the increase in microhardness and abrasive wear resistance of the nitrogen solid solution in the core region.

Microstructure evolution and mechanical properties of laser additive manufactured Ti6Al4V alloy under nitrogen-argon reactive atmosphere

It is known that the reactive nitrogen atmosphere can effectively modulate the microstructure of laser powder bed fusion (L-PBF) of Ti6Al4V alloys. In the present work, high nitrogen contents of 15 and 25 vol% were applied in argon atmospheres to investigate the influence on microstructure and mechanical performances. The enhanced nitrogen solid solution was achieved with nitrogen content ranging 374 ppm and 456 ppm (parts per million) in the Ti6Al4V alloys, respectively. Besides, the EBSD (electron back-scattered diffraction) characterization shows the significant refinement of acicular martensite α′ and the EBSD phase reconstruction shows the interrupting epitaxial growth of the β phase with the increasing nitrogen content in the atmosphere. The KAM (kernel average misorientation) diagrams with an increasingly higher value indicate an increased micro-strain and dislocation density at higher nitrogen content in the atmosphere. Meanwhile, the high magnification TEM further confirms the martensite α’ refinement and also the formation of β lamella at 25 vol% nitrogen content in L-PBF atmosphere. Such microstructure evolution can be mainly attributed to the more severe thermal cycling with a higher thermal gradient and cooling rate under the atmosphere with higher nitrogen content. By applying the 25 vol% nitrogen in the atmosphere, the L-PBF Ti6Al4V alloys exhibit a significant increase in maximum compressive strength of ∼1885 MPa and maximum compressive strain of ∼10%, respectively. The underlying strengthening mechanism can be attributed to the refinement of martensite structure, solid solution, and higher dislocation density. The present work shows that the atmosphere can effectively modulate the microstructure and improve the mechanical property of L-PBF alloys.

Nanosized Cr-N clustering in expanded austenite layer of low temperature plasma-nitrided Fe-35Ni-10Cr alloy

Expanded austenite (γN) formed by low temperature nitriding of austenitic stainless steel is a N-enriched fcc containing Cr-N short range ordering, but no direct observation of Cr-N clustering has been reported. In the present study, the nanostructure of an Fe-35Ni-10Cr (at%) alloy plasma-nitrided at 673 K was investigated using transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP) technique in this study. Nanosized Cr-N clusters were directly observed in the γN accompanied with strong streaks in selected area diffraction, obvious modulated structure in TEM and Cr-N rich regions observed by 3DAP. Thermodynamic calculations suggested that Cr-N clusters were formed by spinodal decomposition due to a strong Cr-N attractive interaction. The hardness of the γN layer was much higher than that of Fe-N austenite steel, which indicates that the hardness is not only due to nitrogen solid-solution hardening but also to a synergetic effect of the coexistence of Cr and N.

STRUCTURE OF COMMERCIAL TITANIUM SUBJECTED TO LOW-TEMPERATURE ION NITRIDING

The structural-phase state of commercial titanium of grades ВТ1-00 (VT1-00) and ВТ1-0 (VT1-0) has been studied in the initial state and after various types of low-temperature ion nitriding. In the initial state, the VT1-00 and VT1-0 alloys have a single-phase α-Ti structure with a hexagonal close-packed crystal lattice. The hardness of titanium is 140–150 HV 10. It has been shown that ion-beam nitriding of VT1-00 alloy at low temperatures of 350 and 450 °С leads to the formation of thin (up to 5 μm) nitrogen-hardened layers with a hardness of 160–180 HV 0.05. As a result of ion implantation at temperatures of 500 and 550 °C, a nitrogen-modified layer with a microhardness of 190–220 HV 0.05 is formed in the surface layers of the VT1-00 titanium alloy, containing a solid solution of nitrogen in the α-Ti matrix phase. Nitrogen implantation of the VT1-00 alloy at a temperature of 620 °C leads to the formation of titanium nitrides TiN0.26, ε-Ti2N, η-Ti3N2-x in the surface layer of titanium alloy. The microhardness of VT1-00 titanium treated with nitrogen ions at 620 °C increases to 360 HV 0.05. In the case of ion-plasma nitriding (IPN) of VT1-0 titanium at 550 °C for 5 h, a nitrogen-modified layer up to 20 μm deep is recorded in its surface layer, containing isomorphic phases: α-Ti and titanium nitride TiN0.26. The microhardness of the VT1-0 titanium alloy subjected to IPN is 190 HV 0.01.

Films with nanocomposite structure αFe(N) +ZrN for soft magnetic applications

Film nanocomposite structure αFe(N) + ZrN is able to ensure properties required for application of the films in modern magnetic microelectronics. To form the nanocomposite structure, the Fe79-78Zr10N11-12 films, which were prepared by long-duration (100 and 250 min) reactive magnetron sputtering characterized by the extremely low film deposition rate (7 nm/min), were subjected to annealing (at 300-600°C for 1 h.). In the case of sputtered films, the formation of the mixed structure consisting of two nanocrystalline (the supersaturated nitrogen solid solution in αFe and nonstoichiometric Zr1N1-X nitride) and amorphous phases was shown by x-ray diffraction analysis and conversion Mössbauer spectroscopy. To be the prerequisite for the nanocomposite structure, the phase composition of sputtered films should be metastable and slightly shifted towards the equilibrium. The latter is realized when a proper relationship of thermodynamic and kinetic features of the system is established in the course of deposition of the films. The evolution of the prerequisite state during annealing leads to the equilibrium state characterized by the nanocomposite structure (αFe(N) + Zr1N1-X). Static magnetic properties of the as-sputtered films and the films subsequently annealed at 500°C were measured, and the structural justification of the reached properties was presented. The analysis of the quantitatively estimated structural parameters of the annealed films, taking into account the fundamental concepts of the soft magnetic properties (saturation magnetization and coercive field), provides an opportunity to choose purposefully the conditions of the prerequisite phase state and desirable composite structure. The metastable state formed in the as-sputtered films is substantiated based on fundamentals of the materials science and metal physics. The films after annealing at 500°C show a coercive field of less than 159 A/m and a saturation induction of at least 1.4 T.

Nanograin size, high-nitrogen austenitic stainless steels

Abstract A new class of high-strength steels has been developed by combining solid solution hardening and grain boundary hardening in high-nitrogen austenitic stainless steels. Average grain diameters from 50 to 250 nm can be achieved, and yield strength levels of more than 2000 MPa in the solution-annealed condition have been measured. The effect of nitrogen solid solution hardening and grain boundary hardening is systematically explored.

Corrosion and tribocorrosion behavior of Ti6Al4V/xTiN composites for biomedical applications

The corrosion and tribocorrosion behavior of Ti6Al4V/xTiN (x=0, 5, 10 and 15, vol.%) composites fabricated by solid-state sintering and their relationship with the microstructure and microhardness were investigated. Simulated body conditions such as a temperature of 37 °C and a simulated body fluid were used. The main results demonstrated a microstructural change caused by the α-Ti stabilization due to solid-solution of nitrogen (N) into the titanium (Ti) lattice, producing a maximum hardening effect up to 109% for the Ti64 matrix by using 15 vol.% TiN. Corrosion potentials of composites changed to more noble values with the TiN particle addition, while corrosion current density of samples increased as an effect of the remaining porosity, decreasing the corrosion resistance of materials. However, changes to a less passive behavior were observed for samples with 15 vol.% TiN. The non-passive behavior of composites resulted in the reduction of the potential drops during rubbing in tribocorrosion tests. Besides, an improvement of up to 88% of the wear rate of composites was seen from the solid-solution hardening. The results allowed to understand the relationship between composition and sintering parameters with the improved tribocorrosion performance of materials.

Study on development policy for new cryogenic structural material for superconducting magnet of fusion reactor

A fusion DEMO will require large-scale cryogenic structure including TF coil cases. Because of huge electromagnetic forces, extra thick plates and/or wrought products will be supplied. Since the midsection of the huge block is weaker than the block surface region, the design yield stress must be determined taking account of this lower strength part. To search the manufacturing process to improve the midsection strength, the crystal refinement strengthening and the precipitation strengthening are considered together with the carbon and nitrogen solid solution strengthening. XM-19 was focused based on the variation of the yield stress and the fracture toughness, a 100 mm thick block and a 30 mm thick plate were trial produced, and strength and the fracture toughness at the midsection were evaluated. This study will present the experimental data and discuss the development policy for a new cryogenic structural material for a fusion reactor.