Ta Single Crystals Research Articles

Internal-friction study of the interstitial–substitutional effect on the deformation behaviour of Nb–O, Nb–Ta–O and Nb–Mo–O single crystals

To investigate the interstitial-substitutional interaction in dislocations, the effect of 0 on the temperature, frequency and amplitude dependence of the internal friction Q −1 in Nb–O, Nb-20 mol% Ta–O and Nb-20 mol% Mo–O single crystals has been studied (f = 1.55–8.2 Hz) in the temperature range from 298 to 1473 K. In our previous study, Nb–Mo and Nb–Ta single crystals were found to be strengthened by solute O. It was also suggested that the interstitial–substitutional interaction in dislocations contributes to the increase in their critical resolved shear stress (CRSS). In this study, Snoek-type relaxation peaks due to O are observed between 500 and 700 K in all the single crystals. The Snoek peak of Nb-20mol% Mo–O consisting of several peaks is analysed. The activation energy of the Snoek peak in Nb-20mol% Mo–O is higher than that of Nb–O. These results are attributable to the existence of the interstitial–substitutional complexes. The amplitude dependence of Q −1 at intermediate and high temperatures decreases as the O content increases. Moreover, the breakaway stress of Nb-20 mol% Mo–O still has a high value at 1200K and does not decrease much even at 1473 K. This suggests that the formation of Mo–O complexes reduces the dislocation mobility at high temperatures. From the results, the effect of the interstitial-substitutional interaction on the CRSS was discussed.

Solid-Solution Strengthening by Oxygen in Nb-Ta and Nb-Mo Single Crystals

We have investigated the high-temperature solid-solution strengthening by oxygen in Nb, Nb–Ta and Nb–Mo single crystals by compression tests with constant strain rate. Nb, Nb–20Ta, Nb–40Ta, Nb–20Mo, and Nb–40Mo single crystals with various O content were grown by use of the electron-beam floating zone melting method. It is found that Nb and Nb–Ta single crystals are strengthened by oxygen at temperatures below 1100 K, while Nb–20Mo single crystals are strengthened even at 1473 K. The critical resolved shear stress (c.r.s.s.) of Nb–Ta single crystals at 298 K increases linearly with increasing O content, irrespective of Ta content. Furthermore, the increment of c.r.s.s. of Nb–Ta single crystals is larger than that of Nb. The c.r.s.s. of Nb-Mo single crystals is higher than that of Nb for all temperatures tested, and the c.r.s.s. of Nb–20Mo single crystals also increases with increasing O content. These results indicate that the interstitial–substitutional (i–s) interaction between O and Ta or Mo affects the mechanical properties in the Nb-based alloys at high temperature.

Mechanical Loss Associated with Stress Anomaly in Ni3Al and Ni3(Al, Ta) Single Crystals

AbstractThe mechanical loss and shear modulus behaviors of Ni3Al and Ni3(Al, Ta) single crystals have been investigated in the temperature range 100 K – 1300 K. The mechanical loss spectra exhibit two temperature regimes, which are separated by a relaxation peak at nearly 950 K for a frequency of 1 Hz. This relaxation peak has been interpreted by the stress re-orientation of Al-Al elastic dipoles in the (111) octahedral plane [1, 2]. In the low temperature regime, corresponding to the anomaly domain of the flow stress, the mechanical loss of pre-deformed specimens exhibit a strong positive dependence on both the oscillation amplitude and the amount of pre-strain.Pre-deformations, which were performed either at room temperature or at 100 K, yield a broad maximum in the mechanical loss that extends from nearly 100 K up to 550 K. This maximum is observable for only strain amplitudes larger than 10−4 and entirely vanishes after heating the specimens above 550 K. The increase in mechanical loss has been attributed to the bowing of the superkinks under the action of the applied stress. The gradual and irreversible decrease in damping above 300 K is interpreted in terms of pinning of the screw dislocation segments by a thermally activated process leading to the formation of Kear-Wilsdorf locks.

High quality superconducting tunnel junctions on Nb and Ta single crystals for radiation detection

High quality Nb/Al-based tunnel junctions, fabricated with a superconducting interface onto thick single crystal x-ray absorbers of Nb and Ta are discussed. Current-voltage characteristics, recorded at 0.5 K, show a subgap current which is still dominated by thermally excited quasiparticles. The quality parameter Rsubgap/Rnormal reaches a value of several million, which is unequalled for nonepitaxially sputtered tunnel junctions. The fabrication process and some development steps, such as preparation of ultrasmooth crystal surfaces are described. Observations of x-ray photons absorbed in Nb and Ta single crystals detected by the superconducting tunnel junctions are also presented.

Anelastic Relaxation of Point Defects in Cubic Crystals

Point defects in solids can give rise to anelastic relaxation provided that the defects behave as elastic dipoles. Experiments with single crystals give information on the atomic configuration of the point defects. Measurements of the orientation dependence of the relaxation strength allow determination of the defect symmetry and the dipole shape factor δλ = |λ 1 -λ 2 |. This is demonstrated for two examples: (i) The Snoek relaxation of O and N in Nb and Ta single crystals: The δλ values for O and N are in the range of 0.6 to 0.7. The tetragonal defect symmetry is confirmed by experiments on Nb-O crystals with various orientations. (ii) Defects in cubic, Y 2 O 3 - or CaO-stabilized ZrO 2 : The oxygen vacancies created for charge compensation are associated with dopant cations thus forming elastic (and electric) dipoles. The dipoles exhibit trigonal symmetry.

Preferential sputtering and oxidation of NbTa single crystals studied by low-energy ion scattering

To study preferential sputtering and oxidation, three monocrystalline Nb x Ta 1 − x(110) alloys ( x = 0.25, 0.5, 0.75) as well as pure single crystals of Nb and Ta have been prepared by electron beam floating zone melting (EBFZM) and investigated by low-energy ion scattering (LEIS). After sputter cleaning, LEIS showed the enrichment of tantalum at the surface of all alloys. After exposure to oxygen, linear relationships between the oxygen and both the niobium and tantalum signals demonstrate that matrix effects do not influence the LEIS signals for these systems. The oxygen coverages of the different alloys have been determined by calibration against the Ni(100)- O c(2 × 2) surface.

Activation Volumes in the Yield Strength Anomaly Domain of Ni3(Al,Ta).

AbstractNi3(Al,Ta) single crystals have been deformed in compression in the temperature range of the flow stress anomaly (293–780K). The strain-rate sensitivity (SRS) of the flow stress has been characterised by using a technique of repeated stress relaxations that allows for the measurement of the true (or effective) activation volume (Veff). When measured at the conventional critical resolved shear stress (CRSS), Veff exhibits as a function of temperature a sharp discontinuity close to 470K. When the temperature is held constant (420K), the discontinuity of Veff occurs along the stress-strain curve at approximately 3% strain; the stress for both discontinuities is approximately the same. These results suggest a change in the rate controlling mechanism that is dependent on stress as much or more than temperature.

On the relationship between unusual mechanical properties and deformation substructures in ordered Ni 3Al

Ni 3(Al,1%Ta) single crystals have been deformed in compression in the temperature range of the flow stress anomaly (300–770 K). The strain rate sensitivity of the flow stress has been examined by the stress relaxation technique. The core structures of the superdislocations on (111) and (001) planes have been observed using weak beam transmission electron microscopy (TEM). The TEM observations and the strain rate sensitivity tests both define two temperature regimes over the investigated temperature range. The importance of using weak beam image simulations to derive actual antiphase boundary energies from the experimental observations has been emphasized.

電子線衝撃溶解法によって作製したＴａ単結晶の圧縮効果に及ぼす結晶方位の影響

電子線衝撃溶解法によって作製したＴａ単結晶の圧縮効果に及ぼす結晶方位の影響

電子線衝撃溶解法によって作製したＴａ単結晶の機械的性質におよぼす結晶方位の影響

Effects of cold roll and compression on Ta single crystals grown by electron beam melting methods were systematically studied.The rolling effect was determined using X-ray techniques and microphotography at successive stages of roll-ing, and the operating slip systems were determind from observations of slip traces on the side and front surfaces rolled. The mean grain size and the mean strain were determined by using the Hall method. The rolling texture was determined by the X-ray pole figure method at successive stages of rolling.Stress-strain curves of various Ta single crystals were obtained in order to clarify the effect of compressive deformation. The following results were obtained: According to the X-ray investigations and surface observa-tions, the mechanical structure of Ta single crystals subject to cold rolling is destroyed in the order, {110}- and , {111}- , and {100}- and .The work-hardening effect of Ta crystals, examined by using compression tests, was determined to be, small and in the order, of directions , and . The cold-rolled texture of Ta single crystals is grouped in {100}- and {111}- orientations, and it is the same as that of Fe single crystals.

The asymmetry of the flow stress in Ni 3(Al,Ta) single crystals

Flow stress measurements were performed on single crystalline Ni 3(Al, Ta) as a function of temperature, orientation, strain rate and sense of the applied uniaxial stress to check the predictions of the Paidar et al. model [ Acta metall. 32, 435 (1984)]. It was found that the critical resolved shear stress (CRSS) for (111)[1̄01] slip depends not only on the test temperature and orientation of the samples, as other investigators have previously observed, but also on the sense of the applied stress. The orientation dependence of the tension/compression asymmetry, including the regions where the asymmetry is a maximum (positive), a minimum, and where it disappears, is as predicted by the model. The applied stress changes the activation enthalpy of cross slip primarily through its effect on constricting the Shockley partials during cross slip and only secondarily on directly promoting (111) to (010) cross slip. A maximum attainable CRSS for (111)[1̄01] slip, the saturation stress, is also in agreement with the model. It was also found that the CRSS for (111)[1̄01] slip is strain rate independent, but the CRSS for (001)[1̄01] slip shows a strong positive strain rate dependence. The temperature at which the peak in the flow stress vs temperature curve occurs increases with increasing strain rate and decreases with increasing ratio of RSS on (001)[1̄10] divided by that on (111)[1̄01]. When the deformation occurs by (001)[1̄10] slip the stress-strain curve exhibits clearly defined, continuous yield points.

Application of the Energy-Dispersive X-ray diffraction method to determine static and dynamic displacements close to D in Ta single crystals

The energy-dispersive X-ray diffraction method has been used to measure integrated Bragg intensities from TaDx single crystals as a function of D concentration and scattering vector. The observed attenuation can be described in terms of a thermal and a static Debye–Waller factor (DWF). From the static DWF, for small scattering vectors, the displacements u1 = 0.095 (5) Å of the four Ta atoms closest to the D atom are obtained and, for large scattering vectors, the change of the mean-square thermal displacement of these atoms Δv21 = −[5.5 (3)] × 10–4 Å2.

Time-dependent measurement of pion-muon channelling

The paper introduces the concept of the “pion age” (=the time spent by a positive pion in a crystal before decaying) and shows that the dependence ofπ +/μ + lattice steering on pion age may be measured in the so-called 17 MHz mode of the Swiss Institute of Nuclear Research (SIN). By means of this new technique it is demonstrated thatπ + implanted into Ta single crystals at 80 K take up their final lattice positions (tetrahedral interstices) in less than 4 ns. Further potential applications of the technique are discussed.

Dislocation Core Structure and the Anomalous Yield Behavior of Li2Ordered Alloys At Elevated Temperatures

ABSTRACTIn many LI2ordered alloys the flow stress increases with increasing temperature and is in this “anomalous” regime strongly dependent on orientation and sense of the applied stress. These dependences can be predicted from the nature of the dissociation and core structure of the 1/2<101> screw superpartials in these alloys. Computer modelling shows that two different configurations, a glissile one on {lll} planes and a sessile one on {010} planes, exist and both are described here in detail. The anomalous increase of the flow stress may then be explained by an increasing amount of core transformations from the glissile to sessile forms as the temperature increases. The theoretical model for the immobilization of screw dislocations by this mechanism is then discussed and its validity illustrated by comparison with experimental results on Ni3(Al, Ta) single crystals.

X-ray investigation of the mean square thermal displacements in VDx, NbHx and TaH x interstitial alloys

X-Ray measurements of the integrated Bragg intensities from V, Nb and Ta single crystals as a function of hydrogen ( D) concentration and temperature have been carried out. Two different methods were applied: (i) the usual angular dispersive method using MoK α1 characteristic radiation; and (ii) the energy dispersive method using the white spectrum of the Mo tube and an intrinsic Ge detector. From the results the thermal Debye-Waller factor for the three metals and its change with hydrogen concentration are determined. The mean square of the thermal displacements ( u 2) decreases with hydrogen ( D) concentration. For NbH x ( D x ) this agrees well with ( u 2) values as determined from measured phonon density of states spectra. The results are also given in terms of a Debye temperature depending on H( D) concentration.

Depolarisation studies of positive muons in copper, vanadium, niobium and tantalum single crystals

The behaviour of positive muons ( mu +) has been studied in Cu, V, Nb and Ta single crystals, using the muon spin rotation ( mu SR) technique. The measurements were performed in the temperature regime 5-365K, in transverse external magnetic fields of 50-5500 G. In FCC Cu the mu + diffusional motion can be described in terms of quantum mechanical jump processes of a small polaron among octahedral interstitial sites. An orientation average leads to an activation energy of E=79.2+or-3.1 meV, a pre-exponential factor of D(0)=(1.66+or-0.33)*10-8 cm2 s-1 and a tunnelling matrix element of J=(18.3+or-1.8)*10-6 eV. The mu + total volume expansion was found to be Delta V=3.91 AA3. Furthermore, the electric field gradient (EFG) at the mu + nearest-neighbour nuclei has been determined as q=0.30 AA-3. In BCC V, Nb and Ta the mu + diffusion is trap-limited. As a function of the temperature the mu + probe different classes of trapping sites associated with host metal impurities. At low temperature, before decaying, the mu + always end up in a trapped state which shows that the intrinsic mu + motion between traps is fast and seems to reflect properties of a band diffusion.

Blocking experiments with positive muons and positrons arising from the decay of pions and muons implanted in single crystals

We report on the first observation of channeling phenomena in a blocking experiment with 4.12 MeV positive muons (μ +) resulting from the decay of thermalized pions (π +) implanted into Ta single crystals. The channeling manifests itself as a peak in the μ + intensity (relative height 1.25, fwhm 0.3°) along the 〈100〉 direction of the Ta crystals. In addition, first results of a similar experiment involving the blocking of relativistic positrons from the decay of muons (μ +) implanted into an Au crystal are presented. We describe the principles of the method together with the experimental set-up and discuss conclusions to be drawn from the experimental findings as well as potential applications of the technique.

Nuclear-acoustic-resonance determination of the strain electric-field-gradient tensorSand its temperature dependence in Ta single crystals

The first quantitative nuclear-acoustic-resonance study of the temperature dependence of the tensor $\mathit{S}$ is presented for a cubic metal. The fourth-rank tensor $\mathit{S}$ relates the electric field gradient (EFG) at a nuclear site to the elastic strain field. For cubic metals, its temperature dependence allows a conclusive test of actual theoretical EFG models. A comprehensive discussion of the tensor $\mathit{S}$ is given and, by applying the screened potential approach of Nishiyama et al. [Phys. Rev. Lett. 37, 357 (1976)] to a lattice with both thermal and ultrasonic vibrations, simple expressions are derived for $\mathit{S}$ and its temperature dependence. In addition, the rotational contribution to the ultrasound-induced EFG is discussed in an appendix. With an internal calibration by the Alpher-Rubin effect the following experimental results were obtained for Ta at 300 K: $|{S}_{44}|=(6.33\ifmmode\pm\else\textpm\fi{}0.29)\ifmmode\times\else\texttimes\fi{}{10}^{22}$ V/${\mathrm{m}}^{2}$ and $\frac{({S}_{11}\ensuremath{-}{S}_{12})}{2{S}_{44}}=\ensuremath{-}0.65\ifmmode\pm\else\textpm\fi{}0.03$. For both components of $\mathit{S}$ a decrease with temperature is observed. With regard to the empirical ${T}^{1.5}$ law of the static EFG the decrease can be described by $S(T)=S(0)(1\ensuremath{-}B{T}^{1.5})$ with $B=(7\ifmmode\pm\else\textpm\fi{}3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ ${\mathrm{K}}^{\ensuremath{-}1.5}$. This result strongly supports the "phonon model" of Nishiyama et al. which attributes the temperature dependence of the nuclear electric quadrupole interaction in metals to the effect of lattice vibrations.

Implanted π + mesons as novel probes in monocrystalline solids

We report on the first observation of the lattice steering (“channeling”) of 4.12 MeV positive muons (μ +), resulting from the decay of thermalized positive pions (π +) implanted in Ta single crystals. The steering effect manifests itself as a peak in the μ + intensity (relative height 1.25, FWHM 0.3°) in a 〈100〉 crystal direction. Conclusions on the behaviour on π + in Ta as well as potential future applications of the technique are discussed.

Effect of zone-refining on orientations of recrystallized grains formed in rolled and annealed pure Mo and Ta single crystals

The effects of the number of zone passes on the orientations of recrystallized grains are studied in rolled and annealed Mo and Ta crystals with a (111)[1̄1̄2] orientation. The recrystallized texture of Mo which has been zone-refined only once is mainly composed of (511)[1̄05] and (310)[001] components, whereas that of Mo which has been zone-refined three times is composed of a sharp single (110)[001] component. In Ta which has been zone-refined once, the recrystallized texture is composed of (110)[001] and (1̄1̄5)[5̄5̄2̄] components, the latter having a twin relationship with the rolled matrix. In Ta which has been zone-refined three times, the (110)[001] component is still predominant but the (110)[001] component decreases and the (1̄1̄5)[5̄5̄2̄] component increases with increasing number of zone passes. Most of the recrystallized grains are in good coincidence with the rolled matrix.