Tantalum Samples Research Articles

Local Ratcheting Phenomena in the Cyclic Behavior of Polycrystalline Tantalum

A direct numerical simulation of the cyclic response of a 250-grain polycrystalline aggregate over more than 1000 cycles is presented, being one of the few available simulations including a significant number of cycles. It provides unique results on the evolution of the accumulated plastic strain and ratcheting phenomena inside the grains. Even though the average stress–strain response stabilizes after 500 cycles, unlimited ratcheting is observed at some locations close to grain boundaries and triple junctions. A clear surface effect of the ratcheting behavior is evidenced based on an appropriate combination of Dirichlet, Neumann, and periodic boundary conditions. The magnitude of the ratcheting indicator is found to be significantly higher at the free surface than in the middle section of the aggregate. Both single- and polycrystalline samples of pure tantalum are tested at room temperature for identification of the parameters in the crystal plasticity model. Special attention is dedicated to modeling the static strain aging effects observed in this material.

Lower-bound dislocation density mapping in microcoined tantalum using high-resolution electron backscatter diffraction

High-resolution electron backscatter diffraction (HR-EBSD) is used to map the geometrically necessary dislocation (GND) density in the cross-sections of annealed, rolled, and microcoined tantalum foils that are typical targets used in modern laser-induced compression materials science studies. The microcoined samples are characteristic of microforming, where the deformation length scale is on the order of the grain size (~50 μm). In particular, inhomogeneities in dislocation density maps across 0.1–1 mm regions of interest are compared with expectations from slip line field approximations. The average HR-EBSD GND dislocation density measurements in various annealed and cold worked tantalum samples are compared with corresponding dislocation density approximations from microhardness measurements. GND densities in the range 1013 − 1015 m−2 are typical.

Tantalum/ Nitrogen and n-type WO3 semiconductor/FTO structures as a cathode for the future of nano devices

In the last decades an important number of research papers published on nano chip electrode and cathode electrochromic materials. Tantalum (Ta) with so high melting point can be as a good candidate for the future of nano chip devices. However, its surface has not enough trap centers and/or occupation states, so nitrogen ions exposed on Ta surafce, may solve this problem. For this purpose, in the present work, samples of tantalum with purities of 99.99% (0.58 mm thickness) were implanted by nitrogen ions. The ions’ implantation process was performed at 30 keV and also at different doses which were in the range between 1017- 1018 ions/cm2. The electrical, nano structural characteristics, sample surface topography characteristic were investigated on Tantalum nitrides (Ta/N) structures by looking at current–voltage (I–V) curves.In addition to Ta/N, WO3powders as a famous EC metal oxide, a silver metal deposited on fluorine doped tin oxide (FTO)-coated glass andmultilayer structure with using the physical vapor deposition (PVD) apparatus are formed. Some techniques such as uv- visible, Atomic Force Microscopy (AFM) and X Ray Diffraction (XRD) have been used. The obtained results show the formation of hexagonal tantalum nitride (TaN0.43), and more trap centers of sample surface (in comparison to current cathode material of EC device). The electrical resistivity of the tantalum after nitrogen implantation is also found to increase with ion doses.Therefore, Ta/N with more trap centers (rough surface) can be suggested as a good element of the future of EC and nano devices.

Spall fracture in additive manufactured tantalum

We present a series of experiments on the response of additive manufactured (AM) tantalum to dynamic loading, specifically the spall strength. Rectangular plates of AM tantalum were produced, with subsequent characterization revealing a highly anisotropic microstructure. Samples were taken from these plates to investigate the effect of anisotropy on the spall strength: the resistance to high strain-rate tensile damage. A conventional, wrought tantalum sample, possessing an equiaxed microstructure, was also tested to serve as a control. Shock loading was performed via light gas-gun flyer-plate impact experiments, with laser velocimetry on the rear of the samples to record the shock wave profiles and soft-recovery techniques to allow post-mortem analysis. In general, the AM samples were found to have a higher Hugoniot elastic limit, the dynamic yield strength under shock loading, while having a reduced spall strength, when compared to the wrought tantalum samples.

Nanostructure development in refractory metals: ECAP processing of Niobium and Tantalum using indirect-extrusion technique

A modified ECAP die by implementing the principles of indirect extrusion was fabricated to process refractory metals of Niobium and Tantalum. The main advantage of this modified technique over conventional method of ECAP is the reduction of frictional force during extrusion. Finite Element Method was utilized to demonstrate the difference between the extruding loads during the process. Several samples of Niobium and Tantalum were subjected to ECAP in different passes. Analysis of experimental results demonstrated a dramatic improvement in mechanical properties. Similarly, microstructure and XRD analyses of the samples showed noticeable enhancement in grain refinement of metals towards nanoscale.

Field emission study from tantalum surfaces micro-structured with femtosecond pulsed laser irradiation

This paper presents our results on surface micro-structuring of tantalum samples using femtosecond (fs) pulsed laser irradiation and its effect on field emission performance. Micro-structuring of Ta targets has been carried by varying laser fluence in the range 0.35–0.55 J/cm2 while keeping target scan speed constant at 25 µm/s. Laser-treated surfaces have shown the formation of high density micro-protrusions and an oxide phase mainly consisting of crystalline Ta2O5. Our results showed substantial improvement in field emission performance of laser micro-structured surfaces in comparison to pristine Ta surface. Areal number density of micro-protrusions decreases from 8.8 × 105 to 3.5 × 105 protrusions/cm2 as incident laser fluence varied from 0.35 to 0.55 J/cm2. Ta sample micro-structured with 0.55 J/cm2 laser fluence has shown low turn on field (~ 4.0 ± 0.6 Vµm− 1) and high-field enhancement factor (4500 ± 500).

Inferring Strength of Tantalum from Hydrodynamic Instability Recovery Experiments

Hydrodynamic instability experiments allow access to material properties at extreme conditions, where strain rates exceed 105 s−1 and pressures reach 100 GPa. Current hydrodynamic instability experimental methods require in-flight radiography to image the instability growth at high pressure and high strain rate, limiting the facilities where these experiments can be performed. An alternate approach, recovering the sample after loading, allows measurement of the instability growth with profilometry. Tantalum samples were manufactured with different 2D and 3D initial perturbation patterns and dynamically compressed by a blast wave generated by laser ablation. The samples were recovered from peak pressures between 30 and 120 GPa and strain rates on the order of 107 s−1, providing a record of the growth of the perturbations due to hydrodynamic instability. These records are useful validation points for hydrocode simulations using models of material strength at high strain rate. Recovered tantalum samples were analyzed, providing an estimate of the strength of the material at high pressure and strain rate.

The Effects of Nitrogen on Structure, Morphology and Electrical Resistance of Tantalum by Ion Implantation Method

In this paper, samples of tantalum with purities of 99.99% (0.58 mm thickness) were implanted by Nitrogen ions. The ions’ implantation process was performed at 30 keV and also at different portions which were in the range between 1 × 1017 and 1 × 1018 ions/cm2. The electrical characteristics were investigated on tantalum nitrides and Ta structures by current–voltage. The samples’ surface morphology was also studied through the atomic force microscopy. Through the application of the X-ray diffraction technique, the microstructure of the modified surfaces was obtained after ion implantation. Results of the experiments show the formation of hexagonal tantalum nitride (TaN0.43), as well as the fact that ion dose increases, more interstitial spaces are occupied by nitrogen atoms in the target crystal. The electrical resistivity of the tantalum after nitrogen implantation is found to increase with ion doses. Experimental data demonstrated that different nitrogen dose in ion beam powerfully affects microstructure, phase formation, surface morphology and resistivity of the tantalum. The changes in nitrogen ions were found to be responsible for variation in the resistivity values.

Variations in Hardness with Position In One Dimensionally Recovered Shock Loaded Metals

The microstructural and mechanical response of materials to shock loading is of the utmost importance in the development of constitutive models for high strain-rate applications. However, unlike a purely mechanical response, to ensure that the microstructure has been generated under conditions of pure one dimensional strain, the target assembly requires both a complex array of momentum traps to prevent lateral releases entering the specimen location from the edges and spall plates to prevent tensile interactions (spall) affecting the microstructure. In this paper, we examine these effects by performing microhardness profiles of shock loaded copper and tantalum samples. In general, variations in hardness both parallel and perpendicular to the shock direction were small indicating successful momentum trapping. Variations in hardness at different locations relative to the impact face are discussed in terms of the initial degree of cold work and the ability to generate and move dislocations in the samples.

Stress and Strain Rate Effects on Incipient Spall in Tantalum

Spall fracture is a high-rate tensile damage phenomenon associated with impulsive and shock-load events. Typically, the material undergoes a sequence of compression followed by release into high rate (on the order of 104 s-1 and up) tension, causing voids to nucleate and grow, which can then coalesce into a crack and the material fails. We present a series of experiments on high purity, well characterized tantalum samples subjected to shock-loading via gas-gun plate impact. Through careful selection of the flyer-plate velocity and material we have independent control over the peak compressive stress and the tensile strain rate in the sample. At all times, the spall damage remains incipient, i.e. in the early stages of void formation and the material does not fully fracture. Velocimetry was used on the rear of the sample to record the wave-profiles and determine spall strength. Soft recovery and sectioning of the samples allowed the internal damage to be observed, quantifying the damage amount, distribution, and relationship to microstructural features with both optical and electron based microscopy.

On the evaluation of dislocation densities in pure tantalum from EBSD orientation data

We analyze measurements of dislocation densities carried out independently by several teams using three different methods on orientation maps obtained by Electron Back Scattered Diffraction on commercially pure tantalum samples in three different microstructural states. The characteristic aspects of these three methods: the Kernel average method, the Dillamore method and the determination of the lattice curvature-induced Nye’s tensor component fields are reviewed and their results are compared. One of the main features of the uncovered dislocation density distributions is their strong heterogeneity over the analyzed samples. Fluctuations in the dislocation densities, amounting to several times their base level and scaling as power-laws of their spatial frequency are observed along grain boundaries, and to a lesser degree along sub-grain boundaries. As a result of such scale invariance, defining an average dislocation density over a representative volume element is hardly possible, which leads to questioning the pertinence of such a notion. Field methods allowing to map the dislocation density distributions over the samples therefore appear to be mandatory.

Effect of laser processing parameters on mechanical properties of porous tantalum fabricated by laser multi-layer micro-cladding

PurposeAdditive manufacturing (AM), a method used in the nuclear, space and racing industries, allows the creation of customized titanium alloy scaffolds with highly defined external shape and internal structure using rapid prototyping as supporting external structures within which bone tissue can grow. AM allows porous tantalum parts with mechanical properties close to that of bone tissue to be obtained.Design/methodology/approachIn this paper, porous tantalum structures with different scan distance were fabricated by AM using laser multi-layer micro-cladding.FindingsPorous tantalum samples were tested for resistance to compressive force and used scanning electron microscope to reveal the morphology of before and after compressive tests. Their structure and mechanical properties of these porous Ta structures with porosity in the range of 35.48 to 50 per cent were investigated. The porous tantalum structures have comparable compressive strength 56 ∼ 480 MPa, and elastic modulus 2.8 ∼ 9.0GPa, which is very close to those of human spongy bone and compact bone.Research limitations/implicationsThis paper does not demonstrate the implant results.Practical implicationsIt can be used as implant material for the repair bone.Social implicationsIt can be used for fabrication of other porous materials.Originality/valueThis paper system researched the scan distance on how to influence the mechanical properties of fabricated porous tantalum structures.

Epitaxial growth of tantalum carbides by low carbon flow carburizing

The carburizing of tantalum samples under different carbon flow rates has highlighted the influence of the carbon flow rate on the structure of tantalum carbides. XRD analyses enabled the identification of surface structures that were revealed by optical micrographs. Four carbon flow rates were tested. The lowest carbon flow rate produced a Ta2C layer that grew epitaxially on the tantalum substrate according to the relationship: {101¯}Ta// {1¯101}Ta2C; 〈101¯〉Ta// 〈17¯17010〉Ta2C. An increase in the carbon flow rate resulted in the appearance of TaC nuclei through the carbon enrichment of the Ta2C layer. These nuclei appeared in the form of islets and highly oriented needles. EBSD (Electron Back Scattered Diffraction) analyses and pole figures revealed that the carbide layers grew epitaxially towards their substrates. The Ta2C layer grew according to the relationship {101¯}Ta// {1¯101}Ta2C while the TaC structures nucleated according to the following relationships: {111}TaC//{0001}Ta2C; 〈111〉TaC//〈0001〉Ta2C; {100}TaC// {202¯3}Ta2C; 〈100〉TaC// 〈909¯8〉Ta2C and {110}TaC// {101¯3}Ta2C; 〈110〉TaC// 〈909¯16〉Ta2C. The highest carbon flow produced stoichiometric TaC grains at the surface, which appeared to be equiaxed. The diversity of the TaC crystallographic orientations stems from the underlying Ta2C layer, the TaC grains having nucleated epitaxially from the Ta2C layer. This misorientation of the Ta2C grains with respect to Ta is governed by the exceeding of a carbon flow limit.

Achieving surface chemical and morphologic alterations on tantalum by plasma electrolytic oxidation.

BackgroundSearch for materials that may either replace titanium dental implants or constitute an alternative as a new dental implant material has been widely studied. As well, the search for optimum biocompatible metal surfaces remains crucial. So, the aim of this work is to develop an oxidized surface layer on tantalum using plasma electrolytic oxidation (PEO) similar to those existing on oral implants been marketed today.MethodsCleaned tantalum samples were divided into group 1 (control) and groups 2, 3, and 4 (treated by PEO for 1, 3, and 5 min, respectively). An electrolytic solution diluted in 1-L deionized water was used for the anodizing process. Then, samples were washed with anhydrous ethyl alcohol and dried in the open air. For complete anodic treatment disposal, the samples were immersed in acetone altogether, taken to the ultrasonic tank for 10 min, washed again in distilled water, and finally air-dried. For the scanning electron microscopy (SEM) analysis, all samples were previously coated with gold; the salt deposition analysis was conducted with an energy-dispersive X-ray spectroscopy (EDS) system integrated with the SEM unit.ResultsSEM images confirmed the changes on tantalum strips surface according to different exposure times while EDS analysis confirmed increased salt deposition as exposure time to the anodizing process also increased.ConclusionsPEO was able to produce both surface alteration and salt deposition on tantalum strips similar to those existing on oral implants been marketed today.

Effect of strain rate and dislocation density on the twinning behavior in tantalum

The conditions which affect twinning in tantalum have been investigated across a range of strain rates and initial dislocation densities. Tantalum samples were subjected to a range of strain rates, from 10−4/s to 103/s under uniaxial stress conditions, and under laser-induced shock-loading conditions. In this study, twinning was observed at 77K at strain rates from 1/s to 103/s, and during laser-induced shock experiments. The effect of the initial dislocation density, which was imparted by deforming the material to different amounts of pre-strain, was also studied, and it was shown that twinning is suppressed after a given amount of pre-strain, even as the global stress continues to increase. These results indicate that the conditions for twinning cannot be represented solely by a critical global stress value, but are also dependent on the evolution of the dislocation density. In addition, the analysis shows that if twinning is initiated, the nucleated twins may continue to grow as a function of strain, even as the dislocation density continues to increase.

Response and representation of ductile damage under varying shock loading conditions in tantalum

The response of polycrystalline metals, which possess adequate mechanisms for plastic deformation under extreme loading conditions, is often accompanied by the formation of pores within the structure of the material. This large deformation process is broadly identified as progressive with nucleation, growth, coalescence, and failure the physical path taken over very short periods of time. These are well known to be complex processes strongly influenced by microstructure, loading path, and the loading profile, which remains a significant challenge to represent and predict numerically. In the current study, the influence of loading path on the damage evolution in high-purity tantalum is presented. Tantalum samples were shock loaded to three different peak shock stresses using both symmetric impact, and two different composite flyer plate configurations such that upon unloading the three samples displayed nearly identical “pull-back” signals as measured via rear-surface velocimetry. While the “pull-back” signals observed were found to be similar in magnitude, the sample loaded to the highest peak stress nucleated a connected field of ductile fracture which resulted in complete separation, while the two lower peak stresses resulted in incipient damage. The damage evolution in the “soft” recovered tantalum samples was quantified using optical metallography, electron-back-scatter diffraction, and tomography. These experiments are examined numerically through the use of a model for shock-induced porosity evolution during damage. The model is shown to describe the response of the tantalum reasonably well under strongly loaded conditions but less well in the nucleation dominated regime. Numerical results are also presented as a function of computational mesh density and discussed in the context of improved representation of the influence of material structure upon macro-scale models of ductile damage.

Tantalum and vanadium substitution in hexagonal K0.3WO3 bronze: synthesis and characterization

Abstract Polycrystalline samples of tantalum and vanadium single and double substituted hexagonal potassium tungsten bronzes (K-HTB’s) with nominal compositions of K0.3(W6+ 0.7W5+ 0.3–yTa5+ y)O3 (0≤y≤0.3), K0.3(W6+ 0.7W5+ 0.3–y V5+ y)O3 (0≤y≤0.18) and K0.3(W6+ 0.7W5+ 0.3–yTa5+ y/2V5+ y/2)O3 (0≤y≤0.3) were synthesized by solid state reactions in quartz tubes at 10–7 MPa and 1073 K. The applied synthesis condition allowed K0.3WO3 to crystallize in space group P6322, confirmed by X-ray powder diffration and Raman spectroscopic analyses. In this K-HTB composition, W5+ could fully be replaced by Ta5+, whereas V5+ could only be substituted up to y=0.16. The degree of W5+ substitution was explained in terms of second-order Jahn–Teller (SOJT) distortion of the d0 cations W6+, Ta5+ and V5+. The applied distortion index also demonstrates why a complete substitution of W5+ in K0.3(W6+W5+)O3 was allowed by a concomitant sharing of Ta5+ and V5+, which are statistically distributed on the W5+/W6+ sites. As W5+(d1) is not SOJT susceptible, it is also shown that the concentration of W5+ in tungsten bronzes plays an important role in the local WO6 octahedral symmetry as well as in its coordination.

Temperature-dependent surface modification of Ta due to high-flux, low-energy He+ ion irradiation

This work examines the response of Tantalum (Ta) as a potential candidate for plasma-facing components (PFCs) in future nuclear fusion reactors. Tantalum samples were exposed to high-flux, low-energy He+ ion irradiation at different temperatures in the range of 823–1223 K. The samples were irradiated at normal incidence with 100 eV He+ ions at constant flux of 1.2 × 1021 ions m−2 s−1 to a total fluence of 4.3 × 1024 ions m−2. An additional Ta sample was also irradiated at 1023 K using a higher ion fluence of 1.7 × 1025 ions m−2 (at the same flux of 1.2 × 1021 ions m−2 s−1), to confirm the possibility of fuzz formation at higher fluence. This higher fluence was chosen to roughly correspond to the lower fluence threshold of fuzz formation in Tungsten (W). Surface morphology was characterized with a combination of field-emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). These results demonstrate that the main mode of surface damage is pinholes with an average size of ∼70 nm2 for all temperatures. However, significantly larger pinholes are observed at elevated temperatures (1123 and 1223 K) resulting from the agglomeration of smaller pinholes. Ex situ X-ray photoelectron spectroscopy (XPS) provides information about the oxidation characteristics of irradiated surfaces, showing minimal exfoliation of the irradiated Ta surface. Additionally, optical reflectivity measurements are performed to further characterize radiation damage on Ta samples, showing gradual reductions in the optical reflectivity as a function of temperature.

Investigation on activation volume and strain-rate sensitivity in ultrafine-grained tantalum

The activation volume and strain-rate sensitivity (SRS) of a plastic deformation process and their relationship are investigated in the present work. Through theoretical modeling based on the movement of both isolated kinks and kink pairs in a dislocation line, a new form of theoretical relationship between the strain-rate sensitivity and activation volume was developed, which reduces to the conventional and frequently used form of the same relationship when SRS is approaching zero. The strain rate jump test during the necking stage of tensile testing process is validated through combined theoretical analytical approaches, for obtaining strain rate sensitivity of materials with short uniform tensile stage, such as UFG metals. To validate the theoretical approaches and modeling presented in this work, strain-rate jump tests and repeated stress relaxation experiments were conducted, to measure the SRS and activation volume of commercial pure ultrafine-grained tantalum prepared by equal channel angular pressing. The experimental relationship between the strain-rate sensitivity and the activation volume of these tantalum samples fits well with the new form of theoretical relationship developed in this investigation. Experimental data from literature on nickel confirms the new form as well. The new form of theoretical relationship between strain-rate sensitivity and activation volume can be applied for both bcc and fcc metals as long as the major plastic deformation mechanism is dislocation gliding.

Analysis of shockless dynamic compression data on solids to multi-megabar pressures: Application to tantalum

Magnetically-driven, planar shockless-compression experiments to multi-megabar pressures were performed on tantalum samples using a stripline target geometry. Free-surface velocity waveforms were measured in 15 cases; nine of these in a dual-sample configuration with two samples of different thicknesses on opposing electrodes, and six in a single-sample configuration with a bare electrode opposite the sample. Details are given on the application of inverse Lagrangian analysis (ILA) to these data, including potential sources of error. The most significant source of systematic error, particularly for single-sample experiments, was found to arise from the pulse-shape dependent free-surface reflected wave interactions with the deviatoric-stress response of tantalum. This could cause local, possibly temporary, unloading of material from a ramp compressed state, and thus multi-value response in wave speed that invalidates the free-surface to in-material velocity mapping step of ILA. By averaging all 15 data sets, a final result for the principal quasi-isentrope of tantalum in stress-strain was obtained to a peak longitudinal stress of 330 GPa with conservative uncertainty bounds of ±4.5% in stress. The result agrees well with a tabular equation of state developed at Los Alamos National Laboratory.