Nb Addition Research Articles

Relationship between shape memory effect and density of annealing twin boundary in FeMnSiCrNi alloys microalloyed by B or Nb

Previous work showed that a strong negative dependence of shape memory effect (SME) existed on annealing twin boundaries (ATBs) in Fe-Mn-Si-based shape memory alloys (SMAs). Microalloying is expected to improve the SME by inhibiting recrystallization and reducing the density of ATBs. In this study, the effects of 0.01wt.% B or 1wt.% Nb addition on the recrystallization, and the resultant evolutions of grain boundary character distributions and SME were studied in the cold-rolled Fe-Mn-Si-based alloys. The addition of B and Nb obviously improved the SME after annealing at temperatures above 973K due to their impedance of recrystallization and the resultant lower density of ATBs. Independent of the B and Nb microalloying and recrystallization, there still existed a strong negative dependence of SME on the density of ATBs. These findings provide insight into improving the SME of Fe-Mn-Si-based SMAs through microalloying to impede the occurrence of recrystallization and reduce the density of ATBs.

The effects of Nb addition on the microstructure and properties of Cu-3.2Ti alloy

The effects of Nb addition on the microstructure and properties of Cu-3.2Ti alloy

Effect of Nb contents on the passivation behavior of high-strength anti-seismic rebar in concrete environments

In this study, the surface analysis, cross-section analysis and electrochemical analysis were used to explore the formation mechanism of Nb contents on the passive film of high-strength anti-seismic rebar in simulated concrete pore solution. The passivation experiments confirmed that the addition of Nb promoted the stability and compactness of surface passive film of Nb-containing rebar, and the passivation efficiency of Nb-containing rebar was stronger than that of CS rebar. Firstly, with the decreases of pH, the increases of Nb promoted that the outer layer of the passive film were mainly composed of Fe oxides and Fe oxyhydroxides, the inner layer were mainly composed of Fe oxides and Nb oxides. Secondly, the increases of Nb were beneficial to the formation of Nb oxides, which enhanced the passivation rate of the passive film and inhibited the degradation of Fe oxides, thus enhancing the thickness of surface passive film of Nb-containing rebar.

Influence of Cutting Surface Condition and Composition on High-Temperature Oxidation Behavior of Ti2 Al C MAX Phase Ceramics

Ti2AlC, one of the MAX phase ceramics, has metal-like properties (electrical conductivity, high fracture toughness and good machinability) and ceramic-like properties (low density, high-temperature oxidation resistance and high bending strength). Ti2AlC ceramics can be cut with cemented carbide tools and the mechanical strength does not decrease significantly after cutting. Ti2AlC ceramics have potential as high-strength machinable ceramics for use in high temperature components. However, the oxidation resistance of Ti2AlC ceramics decreases after cutting because the cutting damages inhibit the formation of protective Al2O3 scale. There are few discussions on the effect of cutting surface conditions on oxidation resistance of Ti2AlC ceramics. The continued growth of the protective Al2O3 scale requires a sufficient Al supply during high-temperature oxidation. The oxidation resistance of Ti2AlC ceramics is reduced when a non-protective TiO2 scale forms on the surface. As one of the common methods to prevent the growth of the TiO2 scale is Nb addition in Ti-based alloys and compounds. The oxidation resistance of as-machined Ti2AlC ceramics may be improved by increasing the amount of Al and Nb additions. This study reports the influences of cutting surface conditions and composition on oxidation behavior of Ti2AlC ceramics. Commercial Ti, Al, C and Nb powders were mixed in Ti:Al:C:Nb molar ratios of 2:1.2:0.9:0 and 1.9:1.2:0.9:0.1 with the corresponding samples receiving the designations Al1 and Al1.2Nb. The powder mixture was annealed for 12 h in a vacuum at 1300°C. The synthesized powder was consolidated over 15 min using a pulsed electric current sintering technique at 1300°C, in a vacuum, and under a uniaxial pressure of 30 MPa. The phases present in the sintered samples were also identified by XRD. Milling tests were performed on a 4 x 4 mm sample surface using a 2 mm diameter end-mill mounted on a plane milling machine. Dry cutting was performed with a spindle speed, feed rate, and cut depth of 4000 rpm, 46 mm/min, and 100 μm. Threading tests were cut into an JIS-M6 bolt shape with a thickness of 3 mm with a lathe and diamond saw. The surface roughness of each sample was measured using a surface texture measuring instrument. The samples were heated in an electric furnace at 1200°C for 4 h in laboratory air. The surface and cross-sections of the oxidized samples were observed via scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX).The main peaks of all sintered samples in these XRD patterns were identified as originating from Ti2AlC and minor peaks originating from TiAl3 (Al-rich phase). The average arithmetic mean roughness (R a) values of the end-milled and threaded surfaces were 0.8 and 4.5 μm. The SEM and EDX results show that a continuous Al2O3 scale formed on the end-milled surface of Al1.2 and Al1.2Nb after the oxidation test. The results of SEM and EDX investigations indicate that a non-protective TiO2 scale had formed on the threaded surface of Al1 and a continuous Al2O3 scale with a thickness of less than 4 μm was formed on the threaded surface of Al1.2Nb after the oxidation test. The Al-rich phase provided the Al necessary for the formation of a continuous Al2O3 scale on the end-milled surface. The oxidation resistance of Ti2AlC ceramics was greatly reduced in the case of screw-like shapes with high R a values and many corners on the cutting surface. The addition of Nb improves the oxidation resistance of Ti2AlC ceramics under such cutting surface conditions. The cause of non-protective oxidation was the destruction of the Al2O3 scale because of the growth of TiO2 scale. The TiO2 scale tends to form at corners where continuous Al2O3 scale formation is difficult. One of the factors that improves the oxidation resistance of Ti2AlC ceramics with screw shapes is the suppression of TiO2 growth by Nb addition. The higher Al concentration and the addition of Nb remarkably increase the oxidation resistance of the Ti2AlC ceramics after cutting.

Anode Properties of MoSi2 Containing Nb and Si in MgCl2-NaCl-CaCl2 and Influence of Electrolytic Potential on It

IntroductionMg metal is produced by electrolysis in a molten chloride bath. Carbon-based material is used as anode. However, carbon-based anode reacts electrochemically with oxide ions dissolved in the bath, which causes its undesirable consumption. In our laboratory the anodic behavior of MoSi2 in molten chloride containing O2- has been investigated in expectation of the protection by formed oxide film on it. A SiO2 film was formed on the MoSi2 anode in molten MgCl2-NaCl-CaCl2 containing MgO, and the dissolution of MoSi2 was suppressed by the formed oxide film. It was also shown that the addition of a small amount of Nb or Si improved the anodic properties of MoSi2, but sufficient electrolytic current was hardly applied so far. In this study, the effect of a simultaneous addition of Nb and Si in MoSi2 was studied for the improvement in its anodic property.Experimental methodMoSi2 with a small amount of Nb and Si was prepared by arc melting, and then shaped cylindrically to use as working electrode. All the electrodes in this study contained 0.6 at% Nb based on our previous study. A Cu plate and a Ag/AgCl couple were used as counter and reference electrode, respectively. The potential of the Ag/AgCl electrode was calibrated with the Mg metal deposition potential measured by cyclic voltammetry. Electrolysis was conducted at 700K in an Ar atmosphere. The anodic behavior of MoSi2 was investigated by cyclic voltammetry, and potentio-static electrolysis was performed at 2.7V (vs. Mg/Mg2+, the same hereinafter). The weight change in MoSi2 anode was measured, and the surface and cross section of the MoSi2 electrode were observed and analyzed by SEM-EDX and XRD. Assuming the three anodic reactions on the MoSi2, such as oxide film formation, Mo elution and the other reactions, their current contributions were estimated from the weight change and average film thickness.Result and DiscussionThe shape of the cyclic voltammogram did not change significantly by the addition of Nb and Si in the MoSi2. A sharp anodic current peak was observed in the cyclic voltammogram around 2.0V regardless of Nb and Si addition, and an anodic current bump was observed above 2.5V. It was considered that the peak around 2.0V corresponded to the oxide film formation, and the fluctuation in the current of this peak changed with the amount of Si addition. The current bump around 2.5V is thought due to a gas generation, and its current increased with the Si addition. Figure 1 shows the influence of Nb and/or Si addition in MoSi2 on the current during the potentio-static electrolysis at 2.7V. Gas bubbles were observed immediately after the start of electrolysis regardless of the Nb and/or Si addition, and the blue fog was observed with an initial current attenuation. The current after the initial attenuation showed gradual change, and was strongly affected by the Si addition. The current became maximum at the electrode with 1.0 at% Si, and the smaller and lager Si addition seemed to cause the decrease in current. An oxide film was formed on the MoSi2 surface more uniformly at the electrode with Si than at the electrode without Si. The current contributions depended on the Si content, and it was shown that the most of current spent the reaction other than the film formation and Mo elution at the electrode with 1.0 at% Si. Since the reaction other than the film formation and Mo elution was thought a gas generation reaction, the MoSi2 seemed to behave as inert anode. From the results in this study, it is concluded that the 1.0 at% addition of Si with 0.6 at% Nb to MoSi2 is appropriate for the improvement of its anode properties.AcknowledgmentThe results of this research were obtained as a result of the commissioned work (JPNP 14004) of the New Energy and Industrial Technology Development Organization (NEDO). Figure 1

Dependence of Lattice Constant and Oxygen Deficiency of CrNbO4 on Oxygen Partial Pressure

Because of the excellent oxidation resistance and superior mechanical properties at high-temperature environment, Ni-based alloy, 718 (Ni-19Fe-19Cr-5Nb-0.6Al-1Ti-3Mo) is one of the most used materials for gas turbines and related aerospace applications. Regarding to the oxidation performance of Alloy718, even though the chromium content 19 mass% is not sufficient to form a protective Cr2O3 scale on Ni-Fe-Cr alloys, the Alloy718 has excellent oxidation resistance by formation of an exclusive Cr2O3 scale. Based on our previous studies, the Nb addition to a Ni–19Fe–19Cr alloy could develop a continuous CrNbO4 layer at the metal-oxide interface, which acts as a diffusion barrier for inward oxygen and outward Fe and Ni diffusion and is favorable for rapid establishment of an external Cr2O3 scale. However, the interpretation in theprevious works is limited to insufficient information of defect chemistry in CrNbO4 oxides. To our knowledge, and the motivation for this study is that the defect chemistry of rutile CrNbO4 oxides has barely been studied to describe the composition versus oxygen partial pressure phase diagrams in this system. Therefore, the dependence of lattice constants and oxygen deficiency of CrNbO4 on oxygen partial pressure were investigated in the current study.The CrNbO4 compounds used in this study were prepared by a solid-state reaction method. The initial materials of Cr2O3 and Nb2O5 had at least a purity of 99.9%. Oxygen partial pressure was varied during the heat treatment at 1000oC, the following buffers: Ni/NiO (P O2 ≒10-11 atm), Co/CoO (P O2 ≒10-12 atm), Fe/FeO (P O2 ≒10-15 atm), H2/H2O (P O2 ≒10-17 atm), Mn/MnO (P O2 ≒10-24 atm) were used for yielding different levels of oxygen deficiency in the CrNbO4. At relatively higher oxygen partial pressure, a small quantity of oxygen vacancies (≒0.01) created under Ni/NiO (P O2 ≒10-11 atm) at 1000oC. Based on the results, maximum 0.05 per formula unit of vacancies could be created. However, a partial formation of Cr2O3 was found when reduced CrNbO4 samples were tried to be obtained below the partial pressure of Fe/FeO (P O2 ≒10-15 atm) at 1000oC (Fig. 2). This could be attributed to the valence states from Nb5+ to Nb4+, forming a solid-solution of NbO2-CrNbO4 under relatively lower partial pressure at 1000oC. Based on experimental data, an increase NbO2 dissolution in NbO2-CrNbO4 solid-solution, that is, (Nb,Cr)O2, leading to the Cr2O3 precipitation under Mn/MnO (P O2 ≒10-24 atm) at 1000oC. The present results show that CrNbO4 may initially formed and changed into (Nb,Cr)O2 solid-solution with increasing oxidation time. (Nb,Cr)O2 could coexist with Cr2O3 under low oxygen partial pressure at 1000oC.

The role of addition of Mo and Nb on microstructure, phase and mechanical properties in tungsten heavy alloys

This study attempts to analyse the effect of Nb and Mo addition on sinterability, microstructural features, phase evolution, and mechanical properties in a W-Ni-Fe composition-based alloy. The base alloy (90W-7Ni-3Fe) and the alloy compositions with Mo and Nb addition of (1.25, 2.5, & 3.75 wt%) independently were sintered in a reducing atmosphere at 1500 °C for 60 min. Nb was selected for its lower thermal conductivity and specific heat, whereas Mo was selected for its grain refinement capabilities. Microstructural analysis by SEM revealed tungsten grain refinement from 28.60 µm for the base alloy to 18.21 µm upon 3.75 wt% Mo addition, while Nb addition, on the contrary, leads to a slight tungsten grain growth to 32.75 µm. The grain size increase was directly proportional to contiguity variation and inversely proportional to dihedral angle variation. Microstructure and phase analysis revealed Nb addition resulted in the formation of NbO2 along with the emergence of Nb rich – Fe lean phases (relative to matrix) with channel and stripped morphology. A maximum hardness value of 385.78 ± 25.4 HV1 was found for the alloy with 3.75 wt% Nb addition, while the least hardness value of 296.65 ± 9.15 HV1 was found for the base alloy. An increment in compressive strength was seen with the addition of Mo and Nb to the base alloy, further Nb addition resulted in a higher compressive strength as compared to Mo addition in the alloy. The addition of Nb, a novel aspect of this study, could serve as a foundation for its inclusion as a viable alloying element in tungsten heavy alloy.

Impact of Nb content on the morphology and properties of Ti (C0.5N0.5)-FeCrMo-based green cermets

A series of TiCN-FeCrMo-xNb (x= 2,4,6,8 wt.%) was prepared through pressureless vacuum sintering at a sintering temperature of 1500°C. This study aims to fabricate TiCN-high chromium Fe-based cermets and investigate the effect of incorporating strong carbide-forming elements such as Nb on the microstructure, phase, and mechanical properties. The microstructure and composition of the sintered cermets were investigated by Scanning electron microscope (SEM) in combination with an energy dispersive spectrometer (EDS) respectively, and detailed investigation of phase constitution was carried out using thermodynamic calculations and X-ray diffraction (XRD) measurements. Microstructure tends to become finer with increasing Nb additions (up to 4 wt.%); however, when Nb additions increase to 6-8 wt.%, ceramic particles become much bigger with a bimodal distribution of Ti(C,N) particle size. Microstructure study reveals that the addition of Nb inhibits the grain growth by diffusion of Nb in the Ti(C,N) during solid-state sintering and leads to refining the microstructure. Based on these findings, optimal Nb addition (up to 4 wt.%) in the Ti(C,N)-FeCrMo -based cermets plays the role of an inhibitor in the dissolution-reprecipitation process, inhibiting the precipitation of brittle (Ti, M) C, N rim phases. The mechanical properties of the Ti(C,N)-FeCrMo -based cermet increased with 2-4 wt.% Nb addition and decreased sharply with further increasing the Nb content. The maximum hardness and fracture toughness have been achieved for the cermet with 4 wt.% Nb with 1322 ± 61 HV30 and 8.56 ± 0.17 MPa.m1/2 respectively.

Modulating NH3 oxidation and inhibiting sulfate deposition to improve NH3-SCR denitration performance by controlling Mn/Nb ratio over MnaNbTi2Ox (a = 0.6-0.9) catalysts

The MnaNbTi2Ox (a = 0.6–0.9) catalysts for NH3 selective catalytic reduction denitration were prepared using the co-precipitation method. Among them, Mn0.7NbTi2Ox exhibits well low-temperature catalytic performance, wide activity temperature range (180–480 ℃), and worthy resistance to SO2 even with H2O. XRD was used to investigate the structure of MnaNbTi2Ox, in which Mn and Nb oxides highly dispersed in the MnaNbTi2Ox catalysts and Nb can dope into the crystal lattice of TiO2. XRF and XPS results show Nb can affect the transfer of electrons to Mn4+, and changing the Mn/Nb ratio can regulate the Mn4+ content on the MnaNbTi2Ox catalysts. H2-TPR, NH3 and NO oxidation results verify that Nb inhibits the oxidation capacity of MnaNbTi2Ox, and altering the Mn/Nb ratio can get appropriate oxidation property, which facilitates low-temperature NH3 activation and limit non-selective oxidation for NH3. In-situ DRIFTS results show Nb-OH bonds can provide new Brønsted acid sites, and both Lewis and Brønsted acid sites are active. Furthermore, Nb addition prevents sulphate deposition on the catalyst. The effect of Mn/Nb on catalytic performance, N2O formation and inhibition, SO2 poisoning, SO2 effect on NH3-SCR, and the enhancement of SO2 tolerance are also analyzed.

Micro abrasion in Fe-Cr-C-Nb alloys samples: The role of niobium

This paper addresses to analyze the role played by Niobium (Nb) in the microstructure of Fe-Cr-C alloys being tested in micro abrasion conditions (ball abrasion test). Three alloys with differences in Cr content and one of them with 5% Nb have been prepared as weld tracks under Fe-C substrate. The resulting microstructures contain carbides embedding in a metallic matrix (MMC). Significant changes in mechanical properties are expected depending on the morphology, size and distribution of carbides. Results pointed out to an improvement in wear resistance of Fe-Cr-C-Nb alloy although it showed lower values of hardness and volume fraction of carbides in comparison to the other alloys. The coefficient of friction was also affected by Nb addition. Such results evidence the possibility of using such alloy in applications that require high performance in wear resistance as mining and agriculture equipment.

Microstructural and Oxidation Effects of Nb Additions to U3Si2

U3Si2 is a long term, accident-tolerant nuclear fuel candidate for light-water reactors because of its superior thermal conductivity and increased uranium density when compared to traditional uranium dioxide (UO2). While reducing internal thermal stresses and increasing efficiency, U3Si2 exhibits energetic oxidation during certain off-normal and accident scenarios, which include coolant or steam exposure. To mitigate this, Nb is investigated as an alloy constituent to enhance corrosion resistance and increase mechanical strength. The work presented investigates the response of Nb-alloyed U3Si2 to steam atmospheres. A thermogravimetric analysis is conducted in flowing steam to T > 1000 °C to assess oxidation resistance. The phase characterization of as-melted, thermally annealed and post-oxidation compositions with up to 12 vol% Nb by powder X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy is reported.

Influence of β-Stabilizing Nb on Phase Stability and Phase Transformation in Ti-Zr Shape Memory Alloys: From the Viewpoint of the First-Principles Calculation

In the present study, the effect of the Nb element on the lattice parameters, phase stability and martensitic transformation behaviors of Ti-Zr-based shape memory alloys was extensively investigated using the first-principles calculation. The lattice parameters of both the β parent phase and α′ martensite phase gradually decreased with Nb content increasing. For the α″ martensite phase, the lattice constant (a) gradually increased with the increase in Nb content, whereas the lattice constants (b and c) continuously decreased due to the addition of Nb. Based on the formation energy and density of state, β→α′ martensitic transformation occurred, as the Nb content was not more than 12.5 at.%. However, the Ti-Zr-Nb shape memory alloys with a Nb content higher than 12.5 at.% possessed the β→α″ martensitic transformation. However, both the largest transformation strain and sensitivity of critical stress to temperature (dσ/dT) can be optimized by controlling 12.5 at.% Nb in the Ti-Zr-Nb shape memory alloy, which was favorable to obtaining the largest elastocaloric effect.

Effect of Soil Amendments on the Aggregate Stability, Biological Activity, and Yield of Canola (Brassica napus L.) in Degraded Soils

ABSTRACT Recycling organic waste and water treatment residues in nanoform is a promising technology in various farming systems, and its effect on aggregate stability, biological activity, and canola production in degraded soils is largely unknown. Hence, we evaluated the effect of nano-biochar (nB) and nano-water treatment residues (nWTR) at levels 50, 100, and 250 mg kg−1 as compared to soil without amendments on the aggregate stability, biological activity, and canola yield in degraded soil. The results showed that the structure coefficient (SC) values are higher when nB and nWTR are added at 100 mg kg−1 soil than at levels 50 and 250 mg kg−1. In our study, the high aggregate stability in the soil treated with nB and nWTR is likely due to their higher content of nutrients, organic carbon, clay, and CEC. The seed weight of the canola plant increased with increasing nB by more than 64%, while it decreased with increasing nWTR. Microbial biomass carbon (MBC) and the activity of dehydrogenase (DHA) and catalase (CLA) increased significantly in the treated pots with the addition of nB and nWTR at various rates. The results of the pot experiment recommended that the applications of nWTR and nB could be used as a promising strategy to improve soil quality and crop yield while also increasing the recycled efficiency of WTR and rice straw.

Effect of Nb and Ti additions on microstructure, hardness and wear properties of AlCoCrFeNi high-entropy alloy

The use of Nb and Ti in the development of high-performance alloys is strategically important for technological advancement. The effect of Nb and Ti additions on the microstructure, hardness and tribological properties of the High Entropy Alloy (HEA) AlCoCrFeNi was evaluated. The addition of 0.6 mol of Nb led to the formation of Laves phase, resulting in a B2/Laves type hypoeutectic microstructure. Conversely, increasing Ti concentration led to the formation of a dendritic microstructure, with interdendritic region composed of Cr-rich BCC-A2 and Fe-Ti rich Laves phase. The average hardness values obtained were 790 HV, 597 HV and 731 HV for AlCoCrFeNiNb0.6, AlCoCrFeNiTi0.5 and AlCoCrFeNiTi1.2 HEAs, respectively. The high hardness was attributed to the formation of the Laves phase and Cr-rich phases, in addition to solid solution hardening, as confirmed by the increase in the lattice parameter calculated by Rietveld refinement method for both B2 and BCC-A2 phases. While the three compositions exhibited comparable friction coefficient, the alloy containing Nb showed superior wear behavior. This was evidenced by the lower volume of material loss and the lower wear rate, which can be attributed to the fine and homogeneous hypoeutectic microstructure observed in the ingot’s cross section. The wear mechanisms were studied, characteristics features of abrasion and adhesion wear were observed.

Enhanced oxidation resistance in nano-lamellar CoCrFeNiNbx (0.45 ≤ x ≤ 0.55) eutectic high entropy alloy during cyclic oxidation at 700–1100 °C

We report the superior oxidation resistance of nano-lamellar CoCrFeNiNbx (x = 0.45, 0.50, 0.55 atom ratio) eutectic high entropy alloy (EHEA) at 700–1100 °C. The as-cast microstructure comprises of nano-lamellar eutectic FCC+Laves phases. The oxidation kinetics followed the parabolic rate in between 700–800 °C, whereas, near-parabolic kinetics is followed at 900 °C. The addition of Nb improves the oxidation and spallation resistance of the EHEAs. A protective outer-oxide layer containing Cr2O3, FeCr2O4-type spinels, whereas, an inner-oxide layer containing Nb2O5, CrNbO4 were formed during oxidation. The mechanism of protection and the superiority of EHEAs as high temperature alloy are explored.

Unveiling the strengthening effect of Nb in Ti3SiC2/Ti25Zr25Ni25Cu25 + Nb/GH3536 brazed joint

In order to achieve the ceramic/metal brazed joint with excellent mechanical properties, many kinds of high-entropy alloy (HEA) filler have been developed to acquire the joint. However, in some HEAs with high content of active element, such as Ti25Zr25Ni25Cu25 amorphous HEA, the metallurgical reaction between active element and base materials resulted in the formation of continuous intermetallics (IMCs) layer in the brazing seam inevitably, embrittling the brazed joint. In this study, Nb element was added into Ti25Zr25Ni25Cu25 amorphous HEA filler to improve the microstructure of Ti3SiC2/GH3536 brazed joint, promoting the application of Ti3SiC2 and GH3536 in the aerospace industry. After detailed characterizations, the strengthening effect of Nb in the brazed joint was unveiled. The formation of Nb-base solid solution in the brazing seam was achieved owing to the addition of Nb, which effectively inhibited the generation of large brittle IMCs bulk. Moreover, the Nb-strengthened ZrSi2 and Ni2Ti phases were generated in the reaction zone, which was also favor in promoting the mechanical properties of brazed joints. When Ti25Zr25Ni25Cu25 amorphous HEA filler with 20 vol% Nb was applied to braze Ti3SiC2 to GH3536, a maximum value of 85.36 ± 2.39 MPa for shear strength was achieved. This study illustrates the optimization mechanism of Nb in Ti25Zr25Ni25Cu25 +Nb composite filler and promotes the further application of Ti25Zr25Ni25Cu25 in brazing system.

Passivation performance of Nb microalloyed rebar in concrete carbonation environments with different pH

In this study, XPS sputtering depth, SEM and electrochemical tests (CV, EIS, M-S, i-t, DPP) were used to study the structural composition and formation mechanism of surface passive film of Nb microalloyed rebar in SCPS with different pH. The results showed that after passivation for 10 d in SCPS with different pH, compared with CS rebar, the stability and compactness of surface passive film of 34Nb rebar gradually increased with the decreases of pH. Firstly, with the decreases of pH, the outer layer of surface passive film of 34Nb rebar was composed of Fe oxides and Fe hydroxides, and the inner layer was composed of Fe oxides and Nb oxides, thus increasing the mass ratio of Fe3+/Fe2+ and Nb2O5/(NbO2 + NbO). Secondly, with the decreases of pH, the addition of Nb promoted the formation of Fe oxides in 34Nb rebar, obtaining the excellent Rct, Ecorr and Ep. Finally, with the decreases of pH, the addition of Nb promoted the enrichment of Nb oxides in 34Nb rebar, inhibiting the degradation of Fe oxides, and the passive film exhibited the P-N type semiconductor, thus significantly decreasing the carrier density and enhancing the passivation rate of 34Nb rebar.

The Influence of the Hot-Rolling Temperature on the Microstructure and Mechanical Properties of Ti-Nb Microalloyed 21%Cr Ferritic Stainless Steel

Microalloying and heat treatment are essential processing techniques for ferritic stainless steel (FSS). Three different compositions of 21%Cr FSS with 0.28Ti, 0.21Ti + 0.05Nb, and 1.05Ti + 0.17Nb were prepared. The interaction effects of the Nb and Ti contents and hot-rolling annealing on the microstructure, mechanical properties, and precipitate phases of FSS were studied. The microstructure, crystal structure, and precipitation phase of steel at 930, 980, and 1030 °C with Ti-Nb microalloying were investigated using an optical microscope (OM), X-ray diffractometer (XRD), and scanning electron microscope (SEM). The room-temperature tensile properties, surface roughness, and hardness were tested separately. This study found that the composite addition of Ti and Nb had a dual effect of fine-grain strengthening and precipitation strengthening. The 1.05Ti + 0.17Nb steel specimen had a moderate grain size and the best uniformity after hot rolling at 980 °C. The tensile strength and elongation were 454 MPa and 34.2%, which achieved an optimal balance between strength and plasticity.

Realizing Ultrahigh Near-Room-Temperature Thermoelectric Figure of Merit for N-Type Mg3(Sb,Bi)2 through Grain Boundary Complexion Engineering with Niobium.

Despite decades of extensive research on thermoelectric materials, Bi2Te3 alloys have dominated room-temperature applications. However, recent advancements have highlighted the potential of alternative candidates, notably Mg3Sb2-Mg3Bi2 alloys, for low- to mid-temperature ranges. This study optimizes the low-temperature composition of this alloy system through Nb addition (Mg3.2-xNbx(Sb0.3Bi0.7)1.996Te0.004), characterizing composition, microstructure, and transport properties. A high Mg3Bi2 content improves the band structure by increasing weighted mobility while enhancing the microstructure. Crucially, it suppresses detrimental grain boundary scattering effects for room-temperature applications. While grain boundary scattering suppression is typically achieved through grain growth, our study reveals that Nb addition significantly reduces grain boundary resistance without increasing grain size. This phenomenon is attributed to a grain boundary complexion transition, where Nb addition transforms the highly resistive Mg3Bi2-rich boundary complexion into a less resistive, metal-like interfacial phase. This marks the rare demonstration of chemistry noticeably affecting grain boundary interfacial electrical resistance in Mg3Sb2-Mg3Bi2. The results culminate in a remarkable advancement in zT, reaching 1.14 at 330 K. The device ZT is found to be 1.03 at 350 K, which further increases to 1.24 at 523 K and reaches a theoretical maximum device efficiency (ηmax) of 10.5% at 623 K, underscoring its competitive performance. These findings showcase the outstanding low-temperature performance of n-type Mg3Bi2-Mg3Sb2 alloys, rivaling Bi2Te3, and emphasize the critical need for continued exploration of complexion phase engineering to advance thermoelectric materials further.

Tailoring microstructural stability and stress rupture properties of a solid-solution strengthened Fe-Ni-based superalloy via niobium addition

The influence of niobium (Nb) addition on the microstructural stability and 750 °C/135 MPa stress rupture properties of a solid-solution strengthened Fe-Ni-based superalloy designed for fourth-generation (Gen IV) nuclear reactors was systematically studied. The results showed that the stress rupture life first increased and then decreased with the increase of Nb content. The alloy with the addition of 0.5 wt% Nb exhibited the best stress rupture properties, with a stress rupture life of 164 h. Compared to Nb-free alloy, the stress rupture life has been increased by 382 %. The enhanced creep performance of this alloy was closely related to its excellent high-temperature microstructural stability, which can be explained as follows: homogeneous and optimal grain size distribution, elemental Nb provided good solid-solution strengthening effects, fine granular secondary carbides relieved stress concentration and strengthened grain boundaries (GBs), dynamic precipitation of nano-sized NbC carbides effectively pined dislocations. Dynamic recrystallization (DRX) behavior can be observed in all alloys. The mechanisms related to DRX behavior were discussed, and it was deemed that the smaller average grain size, higher blocky NbC volume fraction, and increased stacking-fault energy (SFE) may be responsible for the facilitated DRX in the alloy with higher Nb content, which greatly caused local softening and consequently, decreasing the creep resistance of the alloy. Important guidance for the further design and development of γ′-free Fe-Ni-based superalloys for Gen IV nuclear reactors with good high-temperature microstructural stability and creep performance could be summarized from the present study.