Ta Single Crystal Research Articles

High temperature oxidation behaviour of polycrystalline and single crystal Ta stabilized γ'-strengthened Co-Ni-Al-Ta-Cr alloys

Isothermal oxidation was carried out on the polycrystalline and single crystal γ'-strengthened Co-Ni-Al-Ta-Cr alloys at 800–1000 ℃. The oxidation kinetic of both alloys follows logarithmic, cubic and parabolic law at 800 ℃, 900 ℃ and 1000 ℃, respectively. The polycrystalline alloy exhibits superior oxidation resistance at 800–1000 ℃, which can be explained by the effects of grain boundary. Grain boundary diffusion enhances the formation of the protective spinel oxides layer at 800–900 ℃. The beneficial effect of the grain boundary on the spallation resistance of oxides results that the polycrystalline alloy maintains better oxidation properties at 1000 ℃.

Quasi-static compression of shock loaded, single crystal tantalum micropillars

This paper presents the mechanical behaviour of single crystal Ta (6.5 GPa–22 GPa) 1-D shocked along the three principal crystallographic directions [100], [110] and [111]. Room temperature mechanical testing was carried out on micropillars to make possible precise load and displacement measurements. The findings exhibit a marked variation in the flow curve and strain hardening between the three orientations, and relative changes are noticed with shock loading. The yield strength (YS) increases linearly as a function of shock load. The YS was highest along [111] (282 MPa) compared with the [100] and [110] orientations (194 and 206 MPa). The strain hardening rates without pre-shock loading are found to be in the order [111] > [100] > [110] and this is not altered by pre-shock loading. Deformation was accomplished by slip on {112} planes. Further, assessments are considered to identify effective plastic strain accumulation during shock loading. The accumulation of effective plastic strain was found to be similar for all directions for shock loading up to 6.5 GPa, however, above 15 GPa the effective strain was lower for [110] compared to [100] and [111] directions. An attempt is made here to assess the flow curve hardening behaviour and variation in yield strength based on a different combination of pair orientation to throw some light into materials response at high-pressure shock loading.

Predicting extreme anisotropy and shape variations in impact testing of tantalum single crystals

Recent Taylor cylinder impact tests carried out for Ta single crystals showed strong variations in dimensional changes for different crystallographic directions aligned with the cylindrical axis. In order to capture the effect of crystallography on the deformation characteristics and final shapes of the impacted cylinders, a single crystal material subroutine is adapted and embedded in the solid mechanics/dynamics Finite Element solver Abaqus to simulate the aforementioned single crystal Ta Taylor impact experiments. Details of the coupled model implementation, and insights on the role played by single crystal anisotropic flow on the deformation behavior across a broad range of strain rates and temperatures for different single crystal orientations are presented and discussed. We demonstrate the predictive capability of the adopted crystal plasticity model to capture the significant role played by crystal orientation-induced anisotropy, as well as strain hardening and adiabatic heating, on the dynamic deformation response of crystalline materials. This re-emphasizes the need of microstructure-aware models to improve the accuracy of simulations for high-consequence engineering design.

Atomic insights in crystallization of liquid Cu on single crystal Ta and amorphous Ta

The crystallization processes of liquid Cu on single crystal Ta and amorphous Ta under different undercoolings were investigated with molecular dynamics simulation. As the undercooling is not greater than 350 K, only heteronucleation process occurred in both cases. However, at a larger undercooling (400 K), the nucleation transformed from a heteronucleation mode to a mix-mode of heteronucleation and homonucleation in both cases. Interestingly, we found that the heteronucleation of Cu embryos, in the amorphous-Ta/liquid-Cu samples, was always triggered by the recrystallization of amorphous Ta; it means that crystal Ta possesses a higher nucleation potency than amorphous Ta. Further analysis indicated that the higher nucleation potency of single crystal Ta originates from its ability to induce liquid Cu atoms ordering. Liquid Cu took a longer time, on average, to nucleate on amorphous Ta than on crystal Ta at different temperatures. In addition, microstructure analysis indicated that the crystallized region contained both stacking faults and twin crystals.

The effect of dislocations on the thermodynamic properties of Ta single crystal under high pressure by molecular dynamics simulation**Project supported by the National Natural Science Foundation of China (Grant No. 51231002) and the Basic Scientific Research Projects in Central Colleges and Universities (Grant No. 2018ZD10).

The thermodynamic properties of Ta metal under high pressure are studied by molecular dynamics simulation. For dislocation-free Ta crystal, all the thermodynamic properties considered are in good agreement with the results from experiments or higher level calculations. If dislocations are included in the Ta crystal, it is found that as the dislocation density increases, the hydrostatic pressure at the phase transition point of bcc→hcp and hcp→fcc decreases, while the Hugoniot temperature increases. Meanwhile, the impact pressure at the elastic–plastic transition point is found to depend on the crystallographic orientation of the pressure. As the dislocation density increases, the pressure of the elastic–plastic transition point decreases rapidly at the initial stage, then gradually decreases with the increase of the dislocation density.

Top-seeded solution growth and morphology change of RbTiOPO4:Ta single crystal

The RbTiOPO4:Ta single crystal with dimensions of 4 mm × 31 mm × 18 mm was successfully grown by Top Seeded Solution Growth Technique. It is concluded that the doping Ta element can strongly influence the growth and morphology of the RbTiOPO4 crystal. The evident morphology change of RbTiOPO4:Ta crystal with respect to RbTiOPO4 crystal has been observed and the (1 0 0) crystal face was more developed than any other crystal faces. The possible reasons of the morphology change were analyzed through experimental and theoretical methods. Several methods were tried to increase crystallographic a direction dimension of RbTiOPO4:Ta crystals. Finally, the RbTiOPO4:Ta single crystal with crystallographic a direction dimension up to 6 mm was obtained by using thicker seed crystal. This way makes it possible to get isometric RbTiOPO4:Ta crystals, which is beneficial for nonlinear optical applications due to larger area in x-y plane.

Charge fluctuations and nodeless superconductivity in quasi-one-dimensionalTa4Pd3Te16revealed byTe125-NMR andTa181-NQR

We report $^{125}$Te nuclear magnetic resonance and $^{181}$Ta nuclear quadrupole resonance studies on single-crystal Ta$_{4}$Pd$_{3}$Te$_{16}$, which has a quasi-one-dimensional structure and superconducts below $T_{\rm c}=4.3$ K. $^{181}$Ta with spin $I=7/2$ is sensitive to quadrupole interactions, while $^{125}$Te with spin $I=1/2$ can only relax by magnetic interactions. By comparing the spin-lattice relaxation rate ( $1/T_{1}$) of $^{181}$Ta and $^{125}$Te, we found that electric-field-gradient (EFG) fluctuations develop below $80$ K. The EFG fluctuations are enhanced with decreasing temperature due to the fluctuations of a charge density wave that sets in at $T_{\rm CDW}=20$ K, below which the spectra are broadened and $1/T_{1}T$ drops sharply. In the superconducting state, $1/T_{1}$ shows a Hebel-Slichter coherence peak just below $T_{\rm c}$ for $^{125}$Te, indicating that Ta$_{4}$Pd$_{3}$Te$_{16}$ is a full-gap superconductor without nodes in the gap function. The coherence peak is absent in the $1/T_{1}$ of $^{181}$Ta due to the strong EFG fluctuations.

Plastic deformation of a porous bcc metal containing nanometer sized voids

Nanoporous materials, can present an outstanding range of mechanical properties. Both molecular dynamics and dislocation analysis were used to evaluate and quantify the evolution of plasticity in a porous Ta single crystal containing randomly placed voids with 3.3nm radii and average initial porosity of 4.1%, when subjected to uniaxial compressive strain. Nanovoids act as effective sources for dislocation emission. Dislocation shear loops nucleate at the surface of the voids and expand by the advance of the edge component. The evolution of dislocation configuration and densities were predicted by the molecular dynamics calculations and successfully compared to an analysis based on Ashby’s concept of geometrically-necessary dislocations. Resolved shear stress calculations were performed for all bcc slip systems and used to identify the operating Burgers vectors in the dislocation loops. The temperature excursion during plastic deformation was used to estimate the mobile dislocation density which is found to be less than 10% of the total dislocation density.

Modeling of the dynamic inelasticity of tantalum single crystal under ramp wave loading

In a previous study, the behavior of single crystal tantalum under ramp wave loading along the [100] and [110] orientations was characterized. The principal objective of the present study is to gain some insights on the observed single crystal behavior particularly on its precursor response and strong orientation dependence, and the implication of the macroscopic behavior on the possible underlying deformation mechanisms. The approach used to achieve this objective is through the material model development and numerical simulation. A continuum model developed in a previous work for polycrystalline tantalum was first modified to describe the experimental data and extract the material information associated with the data. A rigorous finite deformation single crystal model based on dislocation slip was then developed to gain physical insights into the possible deformation mechanisms. The slip systems considered were the {110}〈111〉 and {112}〈111〉 systems. Dislocation density and its evolution by nucleation or multiplication were incorporated as a key mechanism for describing the precursor behavior in both models. The orientation dependence was modeled through the assumption of anisotropic dislocation nucleation. In the continuum model, different nucleation rates were assumed for the [100] and [110] orientation. In the single crystal model, this anisotropy is assumed to be associated with the twinning/antitwinning asymmetry of the BCC crystals. The precursor for the [100] orientation is attributed mainly to the slips along the antitwinning direction and that for the [110] is to the slips along the twinning direction. The anisotropic dislocation nucleation leads to the orientation dependence of the rate sensitivity of single crystal Ta and the subsequent deformation behavior. Both models were demonstrated to be able to generate reasonably consistent results and to capture the observed material features. Through the developed models, a reasonable understanding was achieved for the evolution of stress, strain, strain rates, strength, temperature, and stress strain relations for single crystal tantalum under ramp wave loading and the possible correlation between the macroscopic behavior and microscopic deformation mechanisms.

Dynamic yielding of single crystal Ta at strain rates of ∼5 × 105/s

A magnetic loading technique was used to produce planar ramp loading of [100] and [110] orientations of single crystal tantalum to peak stresses of either ∼18 or ∼86 GPa for applied plastic strain rates of about 2 × 106/s. It was found that the dynamic elastic limit varied only slightly for factor-of-2 changes in the resulting elastic strain rates near 5 × 105/s. For wave propagation in the [100] direction, the dynamic elastic limit varied from 4.18–3.92 GPa for corresponding sample thicknesses of 0.625–1.030 mm and exhibited a slight rate dependence for the strain rate region studied. For [110] compression, the elastic limit was essentially independent of propagation distance, but exhibited a significant sample-to-sample variation; the elastic limit for this orientation varied from 2.49–3.18 GPa over sample thicknesses of 0.702–1.023 mm, with an average and standard deviation for the data of 2.93 ± 0.27 GPa. There was no apparent rate dependence in this case for the strain rates examined.

An atomistically-informed dislocation dynamics model for the plastic anisotropy and tension–compression asymmetry of BCC metals

Atomistic simulations have shown that a screw dislocation in body-centered cubic (BCC) metals has a complex non-planar atomic core structure. The configuration of this core controls their motion and is affected not only by the usual resolved shear stress on the dislocation, but also by non-driving stress components. Consequences of the latter are referred to as non-Schmid effects. These atomic and micro-scale effects are the reason slip characteristics in deforming single and polycrystalline BCC metals are extremely sensitive to the direction and sense of the applied load. In this paper, we develop a three-dimensional discrete dislocation dynamics (DD) simulation model to understand the relationship between individual dislocation glide behavior and macro-scale plastic slip behavior in single crystal BCC Ta. For the first time, it is shown that non-Schmid effects on screw dislocations of both {110} and {112} slip systems must be implemented into the DD models in order to predict the strong plastic anisotropy and tension–compression asymmetry experimentally observed in the stress–strain curves of single crystal Ta. Incorporation of fundamental atomistic information is critical for developing a physics-based, predictive meso-scale DD simulation tool that can connect length/time scales and investigate the underlying mechanisms governing the deformation of BCC metals.

Mechanical Behavior of Ti–Zr–Ni Quasicrystals during Nanoindentation

The parameters and mechanisms of deformation of a Ti41.5Zr41.5Ni17 quasicrystal and a W + 12 at. % Ta single crystal under nanoindentation conditions were studied and compared. It was found that, initially, the deformation of the quasicrystal is elastoplastic; however, beginning from a certain critical load, the deformation acquires a steplike character with alternating segments of slow elastoplastic deformation and rapid plastic deformation. A qualitative model is proposed for the plastic deformation of quasicrystals during nanoindentation.

Lattice rotations during compression deformation of a [011] Ta single crystal

The slip-system symmetry along 〈011〉 in body-centered cubic materials is such that the 〈111〉 Burgers vectors on potentially active {110} and {112} slip planes lie within a single {011} plane. In the absence of friction, this affords the opportunity for (macroscopically) homogeneous large-strain deformations by compression. The constitutive response of four specimens in the [011] orientation deformed to 10, 20, 30, and 40% engineering strain, exhibited an up-turn in the true stress–true strain curves at approximately 25%. The primary objective of this investigation was to determine the degree of inhomogeneity of the deformation resulting from the platen/sample interface friction, as related to material modeling. To accomplish this, a simple analytical model was adapted and implemented within a 3D finite-element code to simulate the influence of friction on the measured constitutive response and estimate the associated lattice rotations in the specimen corners. Orientation imaging microscopy (OIM) was applied to characterize the lattice rotations and other mesoscopic features of the deformed microstructures. OIM revealed organized patterns of alternating crystal orientation (deformation bands) that evolved as a function of strain.

Strain-amplitude-dependent mechanical loss at intermediate temperatures in a Ni3(Al, Ta) single crystal

Strain-amplitude-dependent mechanical loss at intermediate temperatures in a Ni3(Al, Ta) single crystal

Orientation Imaging Microscopy Investigation of the Compression Deformation of a [011] Ta Single Crystal

High-purity tantalum single crystal cylinders oriented with [011] parallel to the cylinder axis were deformed 10, 20, and 30 percent in compression. The samples took on an ellipsoidal shape during testing, elongated along the [100] direction with almost no dimensional change along [011]. Two orthogonal sectioning planes were selected for characterization using Orientation Imaging Microscopy (OIM): one in the plane containing [100] and [011] (longitudinal) and the other in the plane containing [011] and [011] (transverse). OIM revealed patterns of alternating crystal rotations that develop as a function of strain and exhibit evolving length scales. The spacing and magnitude of these alternating misorientations increase in number density and decrease in spacing with increasing strain.

The role of electron structure of substrates in interaction of adsorbed atoms on furrow faces of transition metals

Structure, work function ϑ and binding energy q of Dy and La layers adsorbed on tantalum single crystal Ta (112) plane (surface) were investigated using low-energy electron diffraction (LEED) and contact potential difference (CPD) techniques. A significant influence of temperature of the substrate on the work function dependences ϑ ( n), where n is the adsorbate surface density, have been revealed for La at 0 < θ < 1.5 and for Dy at 0.5 ⩽ θ ⩽ 1.5, where θ is the adsorbate adatom concentration (La, Dy) ratio to the atomic surface density of the substrate [Ta(112) plane]. The effects on the indirect interaction between adatoms through the substrate have been estimated to be considerably weaker than in the case of Mo and W as a substrate.

X‐ray fluorescence analysis using intensive linear polarized monochromatic x‐rays after bragg reflection

AbstractTo obtain intense linear polarized monochromatic X‐rays a single crystal where Bragg reflection occurs at an angle 2θ=90° is used. A number of suitable crystal materials is listed which fulfil the requirements of the Bragg equation for the wavelengths of standard X‐ray tubes and the angular condition. As the first result of a new construction a detection limit of 1 ng Ca was determined using a Cr anode and a Ta single crystal (002) plane as polarizer.