Niobium Alloys Research Articles

The influence of weld fluid flow on the formation mechanism of intermetallics and HfO2 precipitates in dissimilar electron beam welding of C-103 niobium alloy to nickel

In this research, the effect of weld fluid flow in molten pool on the formation and behavior of intermetallic compounds NbNi3 and Nb7Ni6 and HfO2 precipitates in dissimilar electron beam welding of niobium base alloy C-103 to nickel was investigated. It was observed that due to the large difference in the melting temperature of the base metals, nickel contributed 82% by volume and C-103 contributed 18% by volume in weld zone. In the weld zone and attached to the C-103 base metal, a thin and continuous intermetallic zone was formed, including the initial phase of NbNi3 and the eutectic structure of NbNi3+Nb7Ni6. In the areas after the continuous intermetallic layer in welding, the initial phase of NbNi3 and the eutectic structure of NbNi3+γ-Ni are formed. During welding, HfO2 particles in the C-103 base metal enter the molten pool due to the weld fluid flow and remain as white precipitates in the welding zone and close to C-103. In addition to these precipitates, nanometer precipitates of HfO2 are also observed in the weld, which are formed in-situ in the weld due to the reaction of hafnium with dissolved oxygen in the melt. In the central zone of the weld, continuous band-shaped intermetallic layers can be observed which have been removed from the wall of C-103 due to the fluid flow of the molten pool and moved towards the central zone of the weld. These layers act as nucleus in the center of the molten pool and solidification occurs from their surface. At first, solidification begins with the planar growth of the initial phase of NbNi3, and then cellular and columnar dendritic growth occurs. At the end of solidification, the eutectic structure of NbNi3+γ-Ni forms between cells and dendritic branches. Due to the presence of intermetallic compounds NbNi3 and Nb7Ni6 as well as HfO2 nanometer precipitates in the weld, the hardness of the weld zone was higher than that of the base metals.

The role of additively manufactured niobium alloy C103 substrate's surface finish and corner geometry on the silicide diffusion coating's performance

Nb-based C103 is an increasingly relevant material for aerospace applications due to recent advances in additive manufacturing (AM). AM enabled complex component geometries, along with the associated surface roughness, are a challenge to the survival of environmental barrier coatings (EBCs), like R512E. EBCs are necessary to provide protection for the oxidation susceptible C103 substrates in elevated temperature, oxidizing environments. This work leveraged cyclic thermal exposures to understand the role of AM surface finish on the residual stress, the oxidation protection mechanisms, and the survivability of the R512E coated C103, with insight into the corner geometry-dependent performance supported by finite element modeling (FEM). The experimental characterization indicated that the rough surface finish from AM resulted in increased voids within the build layers in the diffusion formed coating, which in conjunction with tensile residual stress along the substrate interface, negated the coating protection during thermal cycling. Conversely, the coating applied to prepared surfaces indicated survivability throughout the exposures. Furthermore, the coating's experimental corner-dependent performance was consistent with the FEM results, highlighted by increased damage around sharp corners with improved coating performance at a fillet corner. These experimental and model results inform the surface and corner preparation requirements to leverage the benefits of AM in R512E coated C103 for elevated temperature applications.

Thermal stabilizing and toughening of a dual-phase Nb alloy by tuning stabilizing element C in Nb-BCC

Niobium alloys have found extensive application in industries, such as aerospace, nuclear reactor, and emerging electronic technologies, owing to their high melting point, low density, and remarkable formability. Nevertheless, they still fall short in terms of comprehensive strength, toughness, and thermal stability when subjected to complex impacts and/or torsional forces during service. Here, a dual-phase (BCC/FCC) Nb alloy with attractive mechanical properties and thermal stability was designed by tuning stable element C in the Nb-BCC matrix assisted by hot deformation and aging processes. Our findings reveal that the formation of discontinuous carbides at the grain boundary promotes the phase transformation of the matrix from BCC to FCC (K-S orientation relationship), resulting in the formation of FCC thin layers and nano particles. This unique configuration hinders the slipping of dislocations during deformation and impedes the degeneration of microstructures during the thermal cycling process from 200 °C to 900 °C. Moreover, the discontinuous carbides at GBs provide channels to transfer dislocations between various phases and/or grains, which results in attractive mechanical properties and thermal stability. The ultimate tensile strength, yield strength, elongation, and elasticity modulus of the designed Nb alloy reach impressive values of 790.5 MPa, 436.5 MPa, 39.1 %, and 63.5 MPa, respectively. These observations provide guidelines for designing dual-phase Nb alloys with remarkable strength, toughness, and thermal stability for aerospace applications by tuning the stabilizing element C in the Nb-BCC matrix.

Electric spark alloying of steel, titanium and niobium alloys using rotating electrode method

The article presents the results of a study of electrospark coatings on metals and alloys using the rotating electrode method. The influence of various parameters on the formation of an electrospark coating on steel 20, titanium VT1-0, niobium and niobium alloy 5VMTs, on titanium VT1-0, niobium and niobium alloy 5VMTs with subsequent siliconization has been studied. The dependences of the growth of the thickness of the electrospark coating with increasing doping time, current in the circuit, and electrode rotation speed were obtained. It has been established that with increasing thickness of the electrospark coating, the thickness of the silicide layer increases. Electric spark alloying leads to an increase in the heat resistance of steel 20 with an aluminum coating by 1.5—3.0 times, of titanium by 2—9 times, and of titanium with a nichrome coating by 2 times. The heat resistance of titanium with a copper electrospark coating after siliconization increases up to 2—10 times, depending on the thickness of the electrospark layer and the siliconization temperature.

Evaluation of High-Vacuum Annealing and Hot Isostatic Pressing on the Microstructure and Properties of an Additively Manufactured Niobium Alloy

Evaluation of High-Vacuum Annealing and Hot Isostatic Pressing on the Microstructure and Properties of an Additively Manufactured Niobium Alloy

Evaluation of Simulated Heat Affected Zone of Molybdenum- and Niobium Alloyed Ultra-High-Strength Steels

Gleeble 3800-thermomechanical simulator was used to simulate the heat affected zone of quenched and tempered 0.16 wt.% C steels with variation of molybdenum-and niobium contents. The purpose of the study was to evaluate the effect of alloying content on the properties of the coarse-grained zone of HAZ region (CGHAZ) and partially re-austenitised inter-critical zone (ICHAZ) with two different t8/5 times (5 s and 15 s). Results showed that Mo and Nb decreased the amount of softening in the HAZ-region, especially with longer t8/5 -time (15 s). 0Mo steel had mixed microstructure of bainite and martensite in the CGHAZ region with t8/5 time of 15 s, which led to higher degree of softening compared to other steels. Shorter t8/5 time of 5 s produced martensitic microstructure in CGHAZ region in all cases leading to higher hardness values. Impact energy values at-40 °C were at least 34 J/cm2 in all cases. Highest hardness values in the ICHAZ-region were achieved in the case of 0.5Mo steel. Also, at-40 °C impact energy values of ICHAZ were at least 34 J/cm2 in all cases, however Mo-free steel achieved clearly higher impact energies in ICHAZ region, which is result from softer microstructure with relatively low hardness compared to other steels. Overall, it can be concluded that longer t8/5 time can be used, which corresponds to higher heat input in welding, if Mo and/or Nb alloying is used.

Bimetallic structure of TZM and NbZr1 fabricated by wire-based directed energy deposition

The outcome of this study highlights the feasibility of additively manufactured bimetallic structures from refractory alloys for high-temperature applications. Using a wire-based directed energy deposition (DED) process, a bimetallic structure composed of molybdenum alloy (TZM) and niobium alloy (NbZr1) was successfully fabricated. The TZM-NbZr1 interface displayed no significant defects, such as cracks or delamination, and showed no presence of intermetallic compounds. A few pores were observed on the deposited NbZr1 side near the interface. During uniaxial tensile testing, the specimen’s failure occurred on the NbZr1 side in close proximity to the interface, resulting in an ultimate tensile strength of 249 ± 126 MPa and an elongation of less than 6 %.

Determining the pattern of direction and distribution of intermetallic phase in the eutectic of the weld material after electron-beam welding of titanium and niobium alloys

This paper reports a study whose object was material of the weld. The nature of changes in the microstructure of the weld material, which are caused by changes in the supplied energy, alloying elements and heat removal from the melt area, was investigated. Welding was performed with an electron beam at Uacc=60 kV, Ieb=90 mA, with an elliptical sweep of 3×4 mm. The speed of electron beam movement veb was varied from 7 to 15 mm·s-1. The temperature of the experimental welded samples T0 was varied from 300 K to 673 K. Ti-TiB alloy (a microcomposite alloy with reinforcing TiB fibers) was welded with Ti-TiB alloys, T110, and with niobium. One of the tasks of welding this alloy was to preserve and optimize the structure of this type in the weld. Grinding of boride fibers, loss of their initial orientation, and formation of a dendritic or cellular microstructure was observed in the weld. Using the methods of raster electron microscopy and micro-X-ray spectral analysis, the microstructure of the weld material was investigated and the dimensional characteristics of TiB fibers under different welding conditions were determined. The analysis of changes in the microstructure of the weld material, the average length ᶏ and the thickness ȩ of the boride fibers in the material of the joints made at different velocities of electron beam movement and initial temperatures T0 was carried out. It was established that the growth of the ratio ȩ/ᶏ from 0.04–0.07 to 0.1–0.27 is accompanied by significant changes in the microstructure and the mechanism of formation of eutectic phases. It is shown that the process that determines the formation of the microstructure of the weld material was the eutectic breakdown with the determining influence of the temperature gradient, crystallization rate, supercooling, concentration inhomogeneities, and alloying impurities.

Corrosion analysis of welded Nb-1% Zr-0.1% C alloy in lead-bismuth eutectic solution

This study aims at checking the suitability of Nb-1% Zr-0.1% C alloy as coolant channel for carrying molten lead-bismuth eutectic (LBE). Molten LBE is used to carry heat from the reactors to heat exchangers in the compact high temperature reactors. Corrosion properties of the base metal (BM), heat-affected zone (HAZ) and fusion zone (FZ) of the weld of niobium alloy were compared at the temperature of 1050 °C in LBE solution in open-air condition. The presence of lead and bismuth at grain-boundary and triple junctions of the grain boundaries is evitable in both the BM and FZ, and the material loss can be attributed to oxidation corrosion. Nb2O5 and NbO2 being brittle and permeable oxides of niobium did not inherit the characteristics of hindering further oxidation-corrosion at elevated temperature. The corrosion rates of FZ, HAZ and BM were observed to be equal to 24.16 mm/year, 20.83 mm/y and 27.80 mm/y, respectively, and this difference occurred mainly due to the formation of nitrides of zirconium (ZrN) and carbides of zirconium (ZrC) and niobium (NbC). ZrO2 could be confirmed by the micro-hardness profile, which appeared to be flat throughout the FZ and BM. The formation of these carbide and nitride phases was confirmed through XPS and XRD analyses. The differences of surface roughness values expressed in root mean square (Sq.) values, determined before and after the corrosion, were found to be equal to 0.267 and 0.230 for the base metal and weld zone, respectively. It signifies that FZ was more corrosion-resistant than BM. It is not recommended to use this alloy in open-air condition for the coolant channels, due to the high corrosion rate in the BM, FZ and HAZ, which leads to material loss beyond the acceptable limit.

Accelerated fracture healing by osteogenic Ti45Nb implants through the PI3K–Akt signaling pathway

The key to managing fracture is to achieve stable internal fixation, and currently, biologically and mechanically appropriate internal fixation devices are urgently needed. With excellent biocompatibility and corrosion resistance, titanium–niobium alloys have the potential to become a new generation of internal fixation materials for fractures. However, the role and mechanism of titanium–niobium alloys on promoting fracture healing are still undefined. Therefore, in this study, we systematically evaluated the bone-enabling properties of Ti45Nb via in vivo and in vitro experiments. In vitro, we found that Ti45Nb has an excellent ability to promote MC3T3-E1 cell adhesion and proliferation without obvious cytotoxicity. Alkaline phosphatase (ALP) activity and alizarin red staining and semiquantitative analysis showed that Ti45Nb enhanced the osteogenic differentiation of MC3T3-E1 cells compared to the Ti6Al4V control. In the polymerase chain reaction experiment, the expression of osteogenic genes in the Ti45Nb group, such as ALP, osteopontin (OPN), osteocalcin (OCN), type 1 collagen (Col-1) and runt-related transcription factor-2 (Runx2), was significantly higher than that in the control group. Meanwhile, in the western blot experiment, the expression of osteogenic-related proteins in the Ti45Nb group was significantly increased, and the expression of PI3K–Akt-related proteins was also higher, which indicated that Ti45Nb might promote fracture healing by activating the PI3K–Akt signaling pathway. In vivo, we found that Ti45Nb implants accelerated fracture healing compared to Ti6Al4V, and the biosafety of Ti45Nb was confirmed by histological evaluation. Furthermore, immunohistochemical staining confirmed that Ti45Nb may promote osteogenesis by upregulating the PI3K/Akt signaling pathway. Our study demonstrated that Ti45Nb exerts an excellent ability to promote fracture healing as well as enhance osteoblast differentiation by activating the PI3K/Akt signaling pathway, and its good biosafety has been confirmed, which indicates its clinical translation potential.Graphic abstract

Materials characterization of Ti6Al4V to NbZr1 bimetallic structure fabricated by wire arc additive manufacturing

This study investigated the microstructures and mechanical properties of a bimetallic structure (BS) of a titanium alloy (Ti6Al4V) and a niobium alloy (NbZr1) manufactured by wire arc additive manufacturing (WAAM). Three BS thin-walls were deposited with different heat input conditions: low (180 amperes (A)), medium (200 A), and high (220 A). Microstructural characterization at the interface showed that the Niobium (Nb) diffusion on the Ti6Al4V side increased as the heat input was increased. The interfacial microstructure comprised an island area and a dendritic zone with solid solutions of (β Ti and Nb) and (α + β Ti and Nb), respectively. The hardness ranged from 99 to 367 Vickers Hardness (HV), with a considerable decrease at the interface due to the migration of Nb. Maximum ultimate tensile strength of 567 MPa was achieved for the low heat input sample, and the failure occurred at the NbZr1 side. The fractography revealed intergranular and transgranular fracture morphologies, indicating a brittle failure. The results showed that the proposed BS has a good bonding strength, and neither defects (e.g., pores and cracks) nor intermetallic compounds were observed at the interface.

A strong-ductile niobium alloy enhanced by eutectic Nb2C

Niobium alloys are widely applied in aerospace technologies. Traditional niobium alloys suffer from insufficient high-temperature strength or limited room-temperature plasticity. Here, we develop novel Nb2Mo0.5W0.5Cx eutectic niobium alloys by introducing refractory alloying elements and carbide inspired by design concept of multiprincipal alloy and carbide eutectic alloys. The Nb2Mo0.5W0.5Cx alloys contain numerous eutectic structures consisting of body-centered cubic (BCC) solid solution and carbide phases, in which the carbide phase transfers from Nb2C to NbC with the increase of C addition. By optimizing the composition, a Nb2Mo0.5W0.5C0.25 niobium alloy consisting of BCC and Nb2C with strong bonded semi-coherent interface exhibits a high yield strength of 1.27 GPa, compressive strength of 2.03 GPa, and good plasticity with a fracture strain of 17.8%. Solid solution strengthening from alloying elements and second phase strengthening from carbide phases contribute to the high strengths, while the plasticity is improved mainly from fine grain strengthening and strong bonding of phase interface by reducing the interfacial energy and slowing down the crack propagation. This study offers a strategy for designing the high-performance refractory alloys to bypass the content limitation of alloying elements and enhanced ceramics that generally reduce plasticity.

Preparation of ultrafine-grained Nb-1Zr alloy by increasing the time of large plastic deformation during friction stir processing

The grain evolution mechanism of Nb-1Zr alloy during friction stir processing (FSP) was studied under three large plastic deformation conditions: high-temperature-short-time, high-temperature-long-time, and low-temperature-long-time. The niobium alloy grains can be continuously refined during processing. By increasing the time for large plastic deformation during friction stir processing, ultrafine-grained niobium alloys can be prepared with only a single pass of processing. To the best of the authors' knowledge, this phenomenon has been confirmed for the first time and can be used as an efficient processing strategy for grain refinement by FSP.

Machine learning guided optimal composition selection of niobium alloys for high temperature applications

Nickel- and cobalt-based superalloys are commonly used as turbine materials for high-temperature applications. However, their maximum operating temperature is limited to about 1100 °C. Therefore, to improve turbine efficiency, current research is focused on designing materials that can withstand higher temperatures. Niobium-based alloys can be considered as promising candidates because of their exceptional properties at elevated temperatures. The conventional approach to alloy design relies on phase diagrams and structure–property data of limited alloys and extrapolates this information into unexplored compositional space. In this work, we harness machine learning and provide an efficient design strategy for finding promising niobium-based alloy compositions with high yield and ultimate tensile strength. Unlike standard composition-based features, we use domain knowledge-based custom features and achieve higher prediction accuracy. We apply Bayesian optimization to screen out novel Nb-based quaternary and quinary alloy compositions and find these compositions have superior predicted strength over a range of temperatures. We develop a detailed design flow and include Python programming code, which could be helpful for accelerating alloy design in a limited alloy data regime.

The effect of Si addition in NbTiZr coatings

Nb based alloys are considered the most appropriate to replace Ni alloys for new generation aircrafts, since Ni alloys can no longer meet the extreme environmental requirements. Processing Nb based alloys as coatings might bring a wider use of alloys depending on the available understanding of their processability. This investigation takes an important step to process Niobium alloys containing high hardness low toughness silicide compounds as multilayer coatings, taking advantage of in-situ synthesis of the alloy during the deposition of powder mixtures. Procedures started by preparing elemental powder mixtures adding Si to a NbTiZr solid solution composition with good processability. In-situ synthesis of silicides occurs during the deposition of (Nb24Ti18Si + 1Zr) (at.%) powder mixtures by Plasma Transferred arc hardfacing (PTA). Si accounts for the hardness increase of the deposited coatings and their reduced mass gain. The addition of Si into the powder mixtures induced the formation of primary Nb5Si3 that competes with the primary Nb(Ti,Si,Zr)ss. Three solidification paths are identified to be competing as the PTA torch moves and the melt pool solidifies. The thermal cycles of multilayer coating processing favour the epitaxial growth of Nb5Si3 at the remelted interface between layer and induce a coarsening of the silicides in the microstructure. Phase distribution in coatings, with the more ductile and toughness phases surrounding the silicides, contributes for the confinement of cracks within the larger silicides formed after the deposition of multilayer coatings.

The microstructure analysis, mechanical properties, and fracture behavior of electron beam welded C-103 niobium-based refractory alloy

This study involves investigation of metallurgical and mechanical properties of C-103 niobium alloy welded by electron beam. ICP, XRD and EDS method were used to determine the composition and phase contents of the alloy. Electron beam welding was carried out on specimens with various currents and transverse speeds. To examine macro- and micro-structural characteristics of the welded samples, stereographic, optical, electron scanning microscopy and EDS methods were used. Also, mechanical properties including micro-hardness profile and tensile strength were measured. It was observed that C-103 alloy has HfO2 precipitates with two different morphologies of spherical coarse and fine particles. Within the weld zone, primary precipitates were dissolved and new nanometer-sized ones were formed regularly throughout this zone. Morphology of the formed precipitates was mostly spherical and rarely filament-like. Due to greater number and smaller size of precipitates in weld zone, hardness of weld zone was more than HAZ and base metal zone. Tensile strength of welded sample was 317.5 MPa which is equal to 55% of tensile strength of base metal. Fracture of all welded samples occurred within weld zone which can be referred to enlargement of grains in this zone. The features presented on fracture surface of unprocessed C-103 alloy, such as cross section reduction, formation of dimples, opaque appearance of fracture surface indicate ductile nature of fracture. However, emerging of relatively smooth and sharp surface, abundant vein patterns and cleavage planes obviously indicate brittle fracture. HfO2 precipitates, as well as dissolved oxygen in the matrix, significantly affected metallurgical and mechanical properties of welded C-103 alloy samples.

Comparing bio-tribocorrosion of selective laser melted Titanium-25% Niobium and conventionally manufactured Ti-6Al-4 V in inflammatory conditions

The present study aims to evaluate the biocompatibility and bio-tribocorrosion potential of selective laser melted (SLM) Titanium-25% Niobium (Ti-25 Nb) alloy. Further the suitability of the Ti-25 Nb implant was assessed by benchmarking with conventionally manufactured commercial Ti-6Al-4 V implant. Ball-on-plate tests were performed on these alloys while submerging them in an acidic sodium lactate (ASL) medium. Simultaneously, the open circuit potential (OCV) of the two alloys were measured before, during and after sliding the indenter. Characterization of the surface morphology and composition after the test revealed that SLM fabricated Ti-25 Nb exhibits superior wear resistance and hence has greater resistance to depassivation than commercial Ti-6Al-4 V implant. Biocompatibility of the implants, in terms of cell adhesion, cell proliferation, osteogenic differentiation and Alkaline Phosphatase (ALP) activity were also studied on the surfaces of the worn alloys. Results indicate that the worn Ti-25 Nb surface is more conducive for the healthy growth of cells and facilitates more pronounced ALP activity than that of Ti-6Al-4 V, over a duration of 14 days. The superior bio-tribocorrosion performance and biocompatibility of Ti-25 Nb is attributed to SLM induced development of compressive residual stresses, smoother worn surface, higher compactness of the Nb oxides over the alloy surface and negligible toxicity of Nb.

Enhanced strength and plasticity in a Nb2MoWC0.5 alloy via eutectic dilute carbide

Refractory niobium alloys are widely used in aero-engine combustion chambers and rocket propulsion systems. Conventional Nb-based alloys often suffer from insufficient high-temperature strengths or poor room-temperature plasticity. Here we report a strategy to develop a novel Nb2MoWC0.5 niobium alloy with a good strength-plasticity synergy by introducing eutectic NbC carbide. Microstructure characterization reveals that the as-prepared alloy exhibits a hypoeutectic microstructure, in which low-energy eutectic interface remains stable under high-temperature compression up to 1473 K. The stable low-energy metal-carbide interface enables not only a superior high-temperature strength (>1 GPa at 1473 K and 0.61 GPa at 1673 K) but also possesses good room-temperature plasticity (a true plastic strain of 16%) via microcrack tip blunting. The dilute NbC carbide with alloying elements has low stacking fault energy and improved high-temperature plastic deformation. This strategy provides theoretical guidance for designing next-generation high-performance high-temperature alloys to bypass the limitation of alloying elements in conventional refractory alloys.

Surface hardening through oxygen diffusion in niobium: The defining role of stress inhomogeneity in tensile embrittlement

Surface hardening, through oxygen diffusion, enforced significant tensile embrittlement of a commercial niobium (Nb) alloy (C-103). This was explored with microscopic digital image correlation and crystal plasticity finite element (CPFE) modeling. In particular, the presence of an oxygen-rich surface layer provided a gradient in hardness and elastic stiffness. These coincided with an increase in yield and tensile strength but a significant drop in ductility. The latter was reflected in the strain localization(s) and crack(s) on the hard gauge region, which ultimately led to a brittle failure. Full-field CPFE simulations were conducted on the actual microstructures. Our model, without crack and damage initiation, predicted extreme stress inhomogeneity during the tensile deformation. In brief, ∼5 times higher stress concentration was noted in the oxygen-rich hard surface grains. This appeared to be responsible for the subsequent crack initiation and embrittlement.

Study on the Equivalence of Metallic-Cerium-Simulated Uranium-Aerosol Generation under Fire

Uranium aerosols are released from uranium-containing materials in high-temperature environments caused by nuclear accidents or other processes. Research on the generation characteristics of uranium aerosols under such conditions is an important part of nuclear-safety analysis. In this experiment, the similarity between metal cerium aerosols and uranium material aerosols was evaluated from the aspects of particle size distribution and source term. Combined with the experiment data, the effect of air flow rate and sampling time is discussed. The calculation result of the air release fraction (ARF) is 6.07 × 10−3–4.8 × 10−2, and the respirable fraction (RF) is 0.810–0.978, respectively, showing that the size distribution of particles and ARF of the cerium aerosol are different from the results of the uranium aerosols in the literature, while the RF is similar to the results obtained by using the uranium–niobium alloy in the literature.