Stress-induced Reorientation Research Articles

Stress-induced reorientation of oxygen-hydrogen pairs in a Ta75Nb25 alloy

Anelastic relaxation techniques have been applied to investigate at kHz frequencies the mobility of H and D trapped by O impurities in a Ta75Nb25 alloy. An internal friction peak has been detected at liquid nitrogen temperatures, the height of which, for a given O content nO (nO =O/(Ta + Nb)), linearly increases with increasing nH(D) (nH(D) =H(D)/(Ta + Nb)), indicating that more than one H(D) atom can be trapped by a single O impurity. The peak is associated with a wide relaxation time spectrum, which is due to the intrinsic compositional disorder of the alloy and to interactions among the OH(D) complexes. These interactions give rise to a temperature dependence of Curie-Weiss type for the relaxation strength, as well as to a decrease of the peak temperature with an increase of the H(D) content.

Stress-induced domain reorientation in urea inclusion compounds

FOR over 50 years urea inclusion compounds (UICs)-in which molecules are entrapped within the cavities of a hydrogen-bonded urea network—have intrigued chemists, crystallographers and spectroscopists1. Most previous work on these and other inclusion compounds has focused on intrachannel interactions and dynamics. We have now found that UICs can serve as useful models to probe long-range interactions and cooperative phenomena in crystals. Here we report the structure of 2,10-undecanedione/urea (1:9) (1/ urea), in which an extended hydrogen-bonded array connecting hosts and guests serves to distort the urea channel away from the hexagonal symmetry normally observed for UIC crystals. The distortion is large enough, and exhibits enough cooperativity, to generate macroscopic domains, but is small enough to allow facile domain reorientation under small compressive stresses. By incorporating a specific impurity (2-undecanone) into 1/urea, we can modify the domain size and optical properties, and make the domain reorientation spontaneously reversible when the uniaxial stress is released. All of these phenomena can be understood in terms of cooperative interactions between different channels of the hydrogen-bonded network formed by urea and its guest.

Relationship of Endurance to Microstructure of IBAD MoS2Coatings

The endurance of ion-beam-assist-deposited (IBAD) MoS2 is correlated with coating structure and orientation. Structure and orientation are determined by X-ray diffraction, while endurance is measured with a ball-on-disk tribometer operating in dry air. The IBAD MoS2 coatings contain both crystalline and non-crystalline components. Only two orientations of the crystalline component are observed: the (001) planes parallel (basal orientation) and perpendicular (edge orientation) to the surface. Endurance of the coating is not related to coating thickness or substrate chemistry, but is related to the relative amount of the two MoS2 crystal orientations. Coating endurance decreases with increasing edge (100) intensity. Furthermore, MoS2 coatings with poor crystallinity exhibit good endurance. These results are discussed in terms of a possible oxidative wear mechanism and stress-induced basal re-orientation of the non-crystalline IBAD MoS2.

Hardness and fracture toughness of tetragonal zirconia single crystals

Yttriaand ceria-stabilized tetragonal zirconia attract great interest due to their exceptional mechanical properties. Hardness tests of such materials exhibit a stress-induced martensitic transformation of metastable tetragonal ZrO 2 to the stable monoclinic form [1, 2]. The compressive stress arising around the indentation improved the mechanical properties of tetragonal zirconia as compared to the completely stabilized (cubic) zirconia (CSZ). The dilatation associated with phase transformation also causes surface uplift around the indentation, which was revealed by phase contrast optical microscopy [3]. Another possible mechanism for increasing the toughness of tetragonal zirconia is a stress-induced reorientation of ferroelast domains [4, 5]. Major investigations of such materials were performed using ceramics and polycrystals. The mechanical properties of tetragonal zirconia single crystals have not been adequately studied. We studied the hardness and fracture toughness of 3.3 mol % yttria-0.3 mol % ceria and 3.3 mol% yttria-0.3 mol% terbia-stabilized tetragonal zirconia crystals (Y-Ce-TZC and Y-Tb-TZC, respectively). The tests were performed on the (0 01) plane of crystals grown by the scull-melting technique. Single crystals doped with yttria and ceria were red, crystals grown with yttria and terbia additions were yellow. Raman spectroscopy revealed only tetragonal phase in these crystals. Fracture toughness was determined by indentation and three-point single edge notch beam (SENB) bending. Three-point bending tests were performed on 3.5 mm × 5.0 mm × 25 mm rectangular specimens, the sides of which were coincident with the (001) plane. The hardness tests were performed on a Matsuzawa MXT70 microhardness tester at 0 .1 -10N loads and on a TP-2 Vickers hardness tester at 50-300 N loads. The indentation diagonals were parallel to the (100) and (110) directions (henceforth ( 10 0) and (1 10) indentations). Indentation size and radial crack length were measured by a NU-2E microscope (Carl Zeiss Iena, Germany) using the phase contrast technique. Indentation fracture toughness was determined according to the expressions given by Anstis et al. for half-penny radial cracks [6]:

Stress-induced reorientation of oxygen-hydrogen complexes in a Ta 75Nb 25 alloy

The anelastic behaviour of a Ta 75Nb 25 alloy containing various amounts of H and O has been investigated between 67 and 700 K at kilohertz frequencies. A relaxation process has been observed at temperatures between 74 ( f = 1.27 kHz) and 83 K ( f = 6.92 kHz), which is about two times wider than a single Debye peak. Its relaxation parameters are: W R = 0.098 eV; Γ 0 = 3 × 10 −11 s. The process appears to be related to stress-induced reorientation of O-H complexes.

Temperature dependence of the internal friction and modulus change in Y 2O 3- and M gO-doped ZrO 2 polycrystals

Internal friction and frequency measurements were carried out on Y 2O 3- and MgO-doped zirconia polycrystals at low frequencies (1.5-3.5 Hz) in the temperature interval 130–623 K. The results obtained with the first system confirm published data. In the ZrO 2:MgO system two peaks have been observed at 223 and 450 K. The first anneals out at temperatures between 473 and 573 K. The second peak was only detected for high MgO concentrations and increased thereafter. The low temperature peak is associated with oil contamination during sample cutting. The second peak can be ascribed to stress-induced reorientation of oxygen vacancy-magnesium cation pairs in the cubic phase with an activation energy of about 1.17 eV.

Stress-induced magnetization reorientation in amorphous ribbons

Amorphous magnetic alloys are magnetically soft, hence it should be easy to align the moments. The magnetization orientation can be found by Mössbauer spectroscopy, by assuming a 3: x: 1:1: x:3 area ratio for sextets. However, x rarely reaches the value 4 expected for perfect in-plane alignment. We show that it is difficult to determine the value of x for iron alloy ribbons due to their very broad spectral lines. Also thickness effects must be taken into account. Hence, the observed line areas may reflect problems with our ability to fit these spectra, rather than a general tendency for the moments to resist perfect alignment.

Internal friction peak associated with the enhanced re-orientation of split interstitials of magnesium atoms in close vicinity of the dislocation kinks in aluminium

A new internal friction peak has been observed around 125°C (| = 1 Hz) in AlMg specimens twisted and subsequently annealed at an elevated temperature. This peak appears only when internal friction measurements are taken at descending temperatures. It does not appear in the case of tensile deformation and the peak is suppressed by subsequent tensile deformation. The activation energy is found to be 0.7 ± 0.1 eV and τ 0 is 10 −8 s. The peak is very sensitive to application of a small bias-stress. It is assumed that the mechanism of this peak is the stress-induced re-orientation of an <100 > split interstitials or dumbbell of MgMg atoms enhanced by the stress field of moving dislocation kinks in aluminium.

Point defect relaxation in silicon single crystals

Abstract Isothermal internal friction measurements reveal a relaxation peak in the temperature range 360–470 K for vibration frequencies between 10−3 and 10 Hz. The temperature range and the fact that the relaxation parameters are not sensitive to the structural state of the specimens indicate a point defect relaxation mechanism. Since the controlled defects in the bulk cannot explain the relatively high relaxation strength observed, we propose a mechanism based on stress-induced self-interstitial reorientation, this type of defect being the only one to be present in a sufficiently high concentration in our samples.

Stress-induced hydrogen reorientation in a bcc Nb–50-at. %-V alloy

A stress-induced reorientation relaxation due to interstitial hydrogen in a Nb--50-at. %-V alloy has been investigated by internal friction for H concentrations between 2.5 and 28 at. %. The peak is interpreted as a Snoek-type relaxation with a distribution of activation energies across the peak from 0.05 to 0.25 eV. This energy spectrum is due to differing environments of the interstitial hydrogen owing to a variation of the chemical short-range order in the alloy. The internal-friction maximum and the values for the modulus defect are roughly linear in concentration, indicating that H clustering is not involved. The reduction of the modulus with decreasing temperature at temperatures above the peak, but not below, is associated with the reorientation phenomenon. There is no evidence for hydride formation in this alloy for hydrogen concentrations to C/sub H/ = 0.28, down to temperatures of approx.6 K.

Anisotropy of Fracture Toughness of Piezoelectric Ceramics

The anisotropy of fracture toughness of PZT‐ and barium titanate‐based piezoelectric ceramics was studied using doubletorsion and indentation methods. The influence of elastic anisotropy and the piezoelectric effect in calculating the Kc value are discussed. The fracture toughness in the polarization plane is greater than that in the perpendicular plane. A model is suggested to explain the observed anisotropy; it is based on assumption of a stress‐induced domain reorientation zone near the crack tip.

Trapping and isotope effects of deuterium in Nb-5 at.% Ti

The anelastic relaxation spectrum of the Nb-5 at.% Ti system has been investigated for various deuterium concentrations, cD, between 0.07 and 1.76 at.% in the temperature range 1.2-300K. Three relaxation processes located around 170, 100 and 50K have been observed at a vibration frequency of 20 kHz, and a marked frequency-dependent rise of dissipation is measured below 5K. The maxima increase in height and shift in temperature with increasing deuterium content, but the shifts of the high- and low-temperature peaks are of opposite sign. The peak at the lower temperature grows with the square of cD and becomes the predominant process at higher concentrations. The relaxation processes are attributed to the stress-induced reorientation of titanium-deuterium complexes. These observations are interpreted under the assumption that interaction with Ti substitutionals shifts the energies of the trapping sites occupied by deuterium atoms randomly.

Uniaxial stress measurements on the 1039.8 meV zero-phonon line in irradiated silicon.

A vibronic band in Si with its zero-phonon line at 1039.8 meV is a product of neutron irradiation and annealing at 500 to 800 K. We show that uniaxial stresses perturb the zero-phonon line as expected for a tetragonal centre. Since the TA(X) phonons are particularly important in forming the phonon sideband, we suggest that the tetragonal symmetry is a result of a naturally axial centre, and is not a Jahn-Teller distortion of a T d centre. No stress-induced reorientation of the centre occurs at small stress perturbations, but when the zero-phonon line splitting is comparable with the TA(X) energies the high energy stress-split component decreases rapidly in intensity. The zero-phonon line is negligibly affected by magnetic fields of up to 5 Tesla.

Interstitial-solute complexes in an irradiated Al-Fe alloy

The internal friction spectra of preferentially orientated polycrystalline and single-crystal samples of a dilute Al-Fe alloy have been measured after electron irradiation at 4.7K and after annealing at various temperatures. Several low-temperature internal friction peaks were found that are attributed to the stress-induced reorientation of self-interstitial-Fe complexes. At least three different defect configurations with large binding and migration energies co-exist in the sample after annealing at 80K. All four major peaks anneal simultaneously, providing strong evidence of vacancy migration in stage III. Defects with tetragonal or (100)-orthorhombic symmetry are not consistent with the observed orientation dependence of the major peaks; trigonal defects ((111) symmetry axis) possess the highest symmetry in agreement with the three largest peaks. Previous interpretations of channelling and Mossbauer measurements based on the formation of isolated, (100)-tetragonal, mixed-dumbbell interstitials in Al-Fe alloys are not consistent with the present internal friction results.

Deformation behaviour and shape memory effect of near equi-atomic NiTi alloy

The mechanical shape memory effect associated with the martensitic-type transformation which occurs in polycrystalline Ti-50.3 at. % Ni alloy has been investigated using the techniques of transmission and optical microscopy. Deformation of initially partially transformed material within the recoverable strain range was found to occur by: (1) stress-induced transformation of the most favourably oriented existing martensite variants at the expense of adjacent unfavourably oriented variants and retained high temperature phase (2) stress-induced re-orientation of favourably oriented martensite by utilizing the most favourably oriented twin system, and (3) stress-induced twin-boundary migration within the martensite. The reverse transformation during heating restores the original grain structure of the high-temperature phase in a highly coherent manner. It was concluded that deformation modes limited to those involved in the transformation process and the reversibility of the transformation give rise to the memory effect.

オーステナイト系ステンレス鋼中の水素による内部摩擦

The temperature dependence of internal friction was measured from 100 to 350 K at a frequency of about 700 Hz for hydrogen-charged 310S, 316 and 304 stainless steels. A characteristic relaxation peak due to dissolved hydrogen was found at about 280 K, on which there is little information in the literature. The characteristics of this 280 K peak are as follows: (1) The internal friction peak grew up with hydrogen charging and went to decay with room-temperature aging, the time responses of which were consistent with a diffusion-controlled process. The peak hight increased not only with the absorbed volume of hydrogen but also with the nickel content of alloys. (2) The peak temperature, in contrast to the peak hight, did not depend much on the absorbed volume of hydrogen and the compositon of alloys. From the relation between peak temperature and test frequency, the activation energy was estimated to be 11.8 kcal/mol and the frequency factor to be 1.3×1012 s−1. Both of these values agreed quite well with the corresponding values for hydrogen diffusion in austenitic stainless steels. (3) Apparently, the 280 K peak could be regarded as a Snoek-type relaxation occurring in the fcc lattice, which might be explained in terms of the stress-induced reorientation of hydrogen atom pairs or clusters under the influence of nickel in Fe-Cr-Ni alloys.

Effect of hydrogen on the low-temperature deformation properties of neutron-irradiated vanadium

The influence of hydrogen on the low temperature deformation characteristics of neutron-irradiated vanadium single crystals has been investigated in compression in the temperature range from 77 to 550 K. The addition of hydrogen to neutron-irradiated vanadium causes a local perturbation in the temperature dependence of yield stress that is similar to unirradiated vanadium. A mechanism based on the stress induced reorientation of small hydrides by the stress fields of moving dislocations has been put forward to account for this observation. In contrast, the presence of hydrogen prior to irradiation results only in a general rise in the yield stress. The disappearance of the local perturbation is attributed to the trapping of hydrogen atoms to form stable complexes during irradiation, thereby eliminating the phenomenon of stress-induced reorientation of hydrides. The fact that the hydrogen does not reduce the thermal component of the yield stress implies that the cancellation of neutron-produced defects by hydrogen is unlikely. The effect of prestraining prior to neutron irradiation resulted in a reduction of the increase in the yield stress due to the neutron irradiation. This reduction is probably due to the fact that the additional dislocations provide an increased sink density.

The Snoek peaks in niobium-25 at. % tantalum alloy

The effect of the addition of 25 at. % tantalum as substitutional solute on the internal-friction spectrum of niobium containing oxygen and nitrogen as interstitial solutes has been studied at a frequency of 1 Hz. Tantalum has been chosen because it hasa) the same atomic size,b) higher shear modulus,c) about the same chemical affinity towards nitrogen andd) higher chemical affinity towards oxygen than niobium. The relaxation spectrum of the alloy contains two main peaks, which correspond to the oxygen and nitrogen Snoek peaks in unalloyed niobium, and consist of four and two constituent peaks respectively. The additional peaks arise from relaxation processes caused by stress-induced reorientation of oxygen and nitrogen atoms in the neighbourhood of tantalum atoms. Since there is no difference in the atomic diameters of niobium and tantalum atoms, the associated activation energies for the relaxation processes causing the additional peaks are affected by the differences in their shear moduli and their chemical affinities towards oxygen and nitrogen.

Substitutional-interstitial interactions in niobium-titanium alloys: An internal friction study

The influence of additions of interstitial oxygen and nitrogen on the internal friction spetrum of the niobium-1 at-% titanium alloy was studied. The nature of the various observed relaxation processes introduced by the presence of substitutional titanium is discussed. A thermodynamic analysis was carried out for two pronounced interaction peaks attributed to the stress-induced reorientation of single oxygen or nitrogen atoms around single titanium atoms and the respective binding energies were estimated. It was also found that, over the range of oxygen concentration studied, interstitial oxygen is completely removed from random migration by substitutional titanium atoms acting as trapping centres, whilst nitrogen population is always partitioned between mobile and trapped atoms even at very low relative nitrogen concentrationsC N/C Ti. A possible reason for the different behavior of oxygen and nitrogen is suggested.

Low temperature deformation characteristics of group Va metal-hydrogen single crystals

An investigation was undertaken to determine the influence of hydrogen on the low temperature deformation characteristics of Group Va metal single crystals in a temperature range 20–473 K. High purity vanadium, niobium and tantalum containing hydrogen exhibit low-temperature perturbations in the temperature dependence of their yield stresses. On the basis of an analysis of the activation parameters, and the fact that the local perturbation in the temperature dependence of yield stress occurs at critical temperatures and below the solvus, a mechanism which could explain the results is the stressinduced reorientation of the hydride platelets due to the stress fields of the moving dislocations. Additional observations were made to demonstrate that neither precipitation hardening nor a trapping phenomenon can account for the local perturbation in the temperature dependence of yield stress.