Single Crystals Of Different Orientations Research Articles

Influence of UO2 crystal orientation on laser ablation performance

Crystallographic orientation dependence deteriorates the performance of surface analysis methods such as secondary ion mass spectrometry (SIMS) and focused ion beam (FIB). This study explores the corresponding potential challenges of laser ablation (LA) as a powerful sampling tool for inductively coupled plasma-mass spectrometry (ICP-MS). To this end, three UO2 single crystals of different orientation as well as polycrystalline UO2 were produced and characterized. Subsequently, a ns-laser ablation system was employed to study laser-matter interaction in detail. Firing the laser continuously at 1Hz with various single shot fluence (2, 4, 6, 8, 12Jcm-2) for diverse periods created LA craters impacted by cumulative fluence between 50 and 650Jcm-2. Repeated LA experiments on the (100) plane of a UO2 single crystal at the beginning and end of the entire study revealed highly reproducible (<3%) LA rates, only limited by the fluctuation of the laser energy output of the ns-LA system. After thorough cleaning of the ablated samples, surface roughness and average depth of LA craters were determined using confocal laser scanning profilometry. Both LA rate and average depth of craters decreased exponentially with increasing single shot fluence independently of the crystal orientation. Surface roughness increased linearly with increasing cumulative fluence having largest intensification for lowest single shot fluence. Scanning electron microscope (SEM) images not only revealed the conical silhouette of LA craters, but also identified a convex meniscus at its bottom. This particular shape of the crater bottom with a deeper ring surrounding the central region is a result of melted and re-solidified UO2 generated during the LA process and the main limiting factor for the achievable depth resolution. The rapid re-solidification of the liquid phase after each single laser shot created tiles of different shape and orientation, depending on UO2 crystal orientation. Three different types of ejected particles radially distributed around the LA craters were identified by SEM, providing profound insights into laser-UO2 interaction.

Creep of single-crystals of nickel-base γ-alloy at temperatures between 1150 °C and 1288 °C

A γ-analogue of the superalloy CMSX-4 that does not contain the strengthening γ′-phase and only consists of the γ-solid solution of nickel has been designed, solidified as single-crystals of different orientations, and tested under creep conditions in the temperature range between 1150 and 1288 °C. The tests have revealed a very high creep anisotropy of this alloy, as was previously found for CMSX-4 at supersolvus temperature of 1288 °C. This creep anisotropy could be explained by the dominance of 011111 octahedral slip. Furthermore, the analysis of the creep data has yielded a high value of the creep activation energy, Qc≈442 kJ/mol, which correlates with the high activation energy of Re diffusion in Ni. This supports the hypothesis that dislocation motion in the γ-matrix of Re-containing superalloys is controlled by the diffusion of the Re atoms segregating at the dislocation core. The Norton stress exponent n is close to 5, which is a typical value for pure metals and their alloys. The absence of γ′-reprecipitation after high-temperature creep tests facilitates microstructural investigations. It has been shown by EBSD that creep deformation results in an increasing misorientation of the existing low angle boundaries. In addition, according to TEM, new low angle boundaries appear due to reactions of the a/2011 mobile dislocations and knitting of new networks.

Two-dimensional electron gas at the interface of Ba0.8Sr0.2TiO3 ferroelectric and LaMnO3 antiferomagnet

The temperature dependence of the electrical resistance has been studied for heterostructures formed by antiferromagnetic LaMnO3 single crystals of different orientations with epitaxial films of ferroelectric Ba0.8Sr0.2TiO3 deposited onto them. The measured electrical resistance is compared to that exhibited by LaMnO3 single crystals without the films. It is found that, in the samples with the film, for which the axis of polarization in the ferroelectric is directed along the perpendicular to the surface of the single crystal, the electrical resistance decreases significantly with temperature, exhibiting metallic behavior below 160 K. The numerical simulations of the structural and electronic characteristics of the BaTiO3/LaMnO3 ferroelectric−antiferromagnet heterostructure has been performed. The transition to the state with two-dimensional electron gas at the interface is demonstrated.

A micro-macro description for pseudoelasticity of NiTi SMAs subjected to nonproportional deformations

A micro-macro constitutive model is developed and applied to the prediction of the pseudoelasticity (PE) of NiTi shape memory alloys (SMAs) subjected to nonproportional loadings. It is based on the work by Gall et al. and takes into account different elastic properties of the austenite and martensite. In the constitutive model for a single crystal, the activation of forward and inverse martensitic transformations and the volume fraction of each of the 24 martensite variants (MVs), and the interaction between the MVs are considered. The constitutive behavior of polycrystalline NiTi SMAs is obtained by assembling that of single crystals of different orientations with finite element method (FEM). The corresponding numerical algorithm and FEM model are developed, in which an additional step is adopted to get rid of the MVs that may irrationally be activated during transformation calculation, which directly reduces the dimension of the Jacobian and enables the convergence and stability of the numerical implement of the model, especially in the cases of complex multiaxial nonproportional deformation. A thin-walled cylindrical model is suggested for finite element analysis, in which the texture microstructure in the materials formed during the cold-drawn processes can be considered. It is shown that the MVs activated under tension are different from those under compression, and when subjected to shear deformation the MVs activated contain those appearing during tension, and more MVs activated may account for the larger slope in the shear stress-strain curve. The PE of NiTi SMAs subjected to uniaxial strain and nonproportional strain by some paths in ε−γ/3 plane is simulated, and the satisfactory agreement between the simulated and experimental results demonstrates the validity of the approach developed.

Strain rate and orientation dependencies of the strength of single crystalline copper under compression

Molecular dynamics (MD) simulations are used to model the compression under uniaxial strain of copper single crystals of different orientations at various temperatures and strain rates. Uniaxial strain is used because of the close resemblance of the resulting stress state with the one behind a shock front, while allowing a control of parameters such as strain rate and temperature to better understand the behavior under complex dynamic shock conditions. Our simulations show that for most orientations, the yield strength of the sample is increased with increasing strain rate. This yield strength is also dependent on the orientation of the sample, but less dependent on temperature. We find three regimes for the atomistic behavior around the yield: homogeneous dislocation nucleation, appearance of disordered atoms followed by dislocation nucleation, and amorphization. Finally, we show that a criterion solely based on a critical resolved shear and normal stress is insufficient at these strain rates to determine slip on a system.

Studies of topological features of the HFCVD surface of a nanocrystalline diamond film using a scanning tunneling microscope with a diamond tip

Structural features of HFCVD nanocrystalline diamond films have been considered and a physicomathematical model of the atomic structure of its surface has been constructed. The film has been studied in air using a scanning tunneling microscope with a tip of a boron-doped semiconducting diamond single crystal. Fractal analysis of images of the film surface from scanning tunneling microscope has shown the presence of diamond single crystals of different orientations having a mean size of 31 nm with a fractal dimensionality of 2.36 and 2.73.

Vertex Dynamics Simulations of Orientation Dependent Recovery in Deformed Aluminium Single Crystals

A 2-dimensional vertex dynamics simulation is applied to the annealing behaviour of deformed Aluminium single crystals having different orientations. It is observed in experiments that deformed single crystals of different orientations - typically the common rolling textures like Goss (110)[001], Brass (110)[1-12], Cube (001)[100] – exhibit remarkably different rates of recovery. It is suggested that this difference arises from the deformation microstructures, with sub-grain boundaries of various misorientation values. The sub-grain boundary mobilities and energies, being strong functions of the boundary misorientation, thus affect the recovery rates. This effect is illustrated using vertex dynamics simulation on the same orientations and schematic deformation substructures as above. Good agreement is obtained for the orientation dependency of recovery.

The influence of the solid electrolyte on the impedance of hydrogen oxidation at patterned Ni electrodes

In order to investigate the role played by the solid electrolyte in the H 2 oxidation reaction, patterned Ni electrodes on YSZ (yttria stabilized zirconia) single crystals with different crystallographic orientations, on polycrystalline YSZ and on polycrystalline LSGM ((La,Sr)(Ga,Mg)O 3 − x) were studied in H 2 + H 2O gas as a function of partial pressure of H 2O in the temperature range of 673–823 K. Symmetrical cells using YSZ single crystals exhibit a single almost perfect electrode semicircle in the Nyquist plot. The corresponding polarisation resistance (R p) depends on the surface orientation of the YSZ single crystals with a maximum of R p for < 110> and a minimum for < 100>. The capacitance obtained from the impedance arc is almost temperature independent but different for the three surfaces. In the case of polycrystalline YSZ a depressed impedance arc results with the capacitance being between the extreme values measured for single crystals of different orientation. A modified situation is found for patterned Ni electrode on LSGM: the capacitance of the single arc depends on temperature and is much larger than in the case of YSZ cells. The polarisation resistance on polycrystalline LSGM is much smaller than on polycrystalline YSZ while activation energies of the polarization resistance are still similar. Capacitances and resistances are discussed in terms of possible mechanisms.

Effect of Grain Boundaries and Grain Orientation on Structure and Properties

The evolution of deformation microstructures in metals follows a universal pattern of grain subdivision. However, the structure in the grain boundary region may be different from that in the grain interior, although a characteristic region cannot be identified for polycrystals with medium to high stacking fault energy. In the grain interior, the dislocation structure is predominantly composed of almost planar boundaries (geometrically necessary boundaries) and cell boundaries (incidental dislocation boundaries) forming a cell block structure. For grains with grain sizes reaching down to about 4 μm deformed in tension and by rolling, a clear correlation has been established between the characteristics of the deformation structure and the orientation of the grain in which it evolves. A similar correlation is observed for single crystals of different orientations. Such correlations form the basis for a general analysis of active slip systems and for modeling of the flow stress and flow stress anisotropy of polycrystalline samples.

Effect of orientation on the cyclic deformation behavior of silver single crystals: Comparison with the behavior of copper and nickel single crystals

This paper reports and discusses the effects of orientation on the cyclic deformation behavior of typical face-centered cubic (fcc) copper, nickel and silver single crystals, including the cyclic stress–strain (CSS) curves and dislocation configurations. Firstly, the CSS curves of silver single crystals of different orientations showed a clear plateau region over the strain range γ pl = 8.0 × 10 - 5 – 7.0 × 10 - 3 , however, there were two saturation shear stresses of 18–21 and 25–26 MPa, corresponding to the lower and upper plateau, respectively. After that, the dislocation configurations from differently oriented silver single crystals are well summarized as follows: (1) persistent slip band (PSB) ladders or walls appear in [ 2 ¯ 3 9 ] , [0 1 1] and [ 4 ¯ 5 9 ] silver single crystals; (2) a labyrinth is the main dislocation structure in the [ 1 ¯ 8 1 8 ] silver single crystal at high strain amplitudes; (3) vein and cell structures form in the [ 2 ¯ 3 3 ] silver single crystal at low and high strain amplitudes, respectively; (4) interaction between the primary and secondary PSBs arises in the [ 1 ¯ 4 1 4 ] silver single crystal. Finally, combined with the results of copper and nickel single crystals, it can be concluded that the effects of orientation on the cyclic deformation behavior of these three kinds of single crystals follow a general principle. That is the orientation-dependent dislocation configuration can be divided into three regions in the stereographic triangle, including the central, [0 0 1] and [ 1 ¯ 1 1 ] regions.

Characterization of flux-grown PZN-PT single crystals for high-performance piezo devices

Relaxor ferroelectric Pb(Zn(1/3)Nb(2/3))O(3-x)PbTiO(3) (PZN-PT) and Pb(Mg(1/3)Nb(2/3))O(3)-PbTiO(3)(PMN-PT) single crystals are the potential candidates for future high-performance piezoelectric devices due to their exceptionally high dielectric and piezoelectric properties. Characterization on flux-grown PZN-PT single crystals of different orientations revealed that PZN-(6-7)%PT single crystals show good homogeneity in dielectric and electromechanical properties and composition. When poled in [001] direction, these crystals exhibit high longitudinal-mode properties with dielectric constant (K(T)) approximately equal to 7000, piezoelectric coefficients (d(33)) approximately equal to 2800 pC/N, and electromechanical coupling factors (k(33)) > or = 0.92. For [011]-cut crystals, optimally poled PZN-7%PT single crystal exhibits very high transverse-mode dielectric and piezoelectric properties with K(T) > or = 5000, d(32) approximately equal to -3800 pC/N and k(32) > or = 0.90. [011]- poled PZN 6%PT has d(32) approximately equal to -3000 pC/N and comparable k(32) and K(T) values. In comparison with melt-grown PMNPT single crystals, flux-grown PZN-PT single crystals show good compositional homogeneity, superior and consistent dielectric and electromechanical properties, and higher depolarization temperatures (TDP).

The influence of crystalline anisotropy on the evolution of the crystallization front during directional solidification

The evolution of the crystallization front of transparent crystals of succinonitrile and pivalic acid in the course of directional solidification in the field of a temperature gradient (Bridgman method) is investigated experimentally. The influence of crystalline anisotropy is examined using single crystals of different orientations. Bulk (cylindrical) and planar single crystals of the materials under investigation are studied and compared for the first time. It is established that the crystallographic direction of growth plays an important role and determines the structure of the crystallization front at different stages of its evolution. New manifestations of the dynamic effects responsible for the development of the nonstationary periodic structure are revealed.

Surface vs. volume stoichiometry of MBE grown CuInS 2 films on Si

Molecular beam epitaxy CuInS 2 layers were grown on Si single crystals of different orientation. The stoichiometry was varied between 0.5 and 1.5 for the nominal Cu/In ratio and the film thickness was limited to 150 nm. Composition analysis was done by in situ photoelectron spectroscopy (PES), Rutherford backscattering (RBS) and X-ray diffraction (XRD). PES was used to determine the surface composition, RBS monitored the volume contribution and XRD probed phase formation. It was found that the surface composition differed significantly from that of the volume: instead of the expected variation along the Cu 2S–In 2S 3 binary, the actual composition varied along the Cu 2S–InS line at the surface (S-deficiency). The role of sulfur incorporation into the growing film is discussed.

Grain orientation dependence of microstructure in aluminium deformed in tension

The evolution of deformation microstructure in medium to high stacking fault energy fcc metals has been described in terms of a general framework of grain subdivision involving the formation of rotates volume elements for all deformation modes. The rotated volume elements are ordinary dislocation cells and cell blocks, where a cell block contains a group of cells. The cell blocks are bounded by dislocation boundaries accommodating the lattice misorientations between neighboring cell blocks that deform with different slip system combinations and/or by different strains or strain amplitudes. An important aspect of the deformation structure development is the pronounced dependence on the crystal orientation. This has been shown by the formation of different types of deformation microstructures, for which the dislocation boundaries formed during deformation are characterized by their different morphology, spacing, crystallographic orientation and misorientation angle, in single crystals of different orientations deformed in tension and in rolling. Studies of polycrystal behavior are much less extensive but an orientation effect has been observed on a macroscopic scale in specimen deformed in rolling and on a microscopic scale in specimens deformed in tension and in rolling. The previous studies have shown a need for a more precise characterization of the effectmore » of grain orientation on the microstructural evolution of polycrystalline metals. Such a characterization, if successful, could lead to prediction of the evolution of the deformation microstructure in order to establish quantitative correlation between microstructure, crystallographic texture and properties of polycrystals. The present study only concerns the microstructure, which has been characterized in pure polycrystalline aluminium deformed in tension at room temperature.« less

Creep of CMSX-4 single crystals of different orientations in tension and compression

Creep tests of γ′ strengthened superalloy CMSX-4 single crystals of different orientations were performed at 950 °C, in both tension and compression at constant load (initial stress 250 MPa) as well as at constant stress (250 MPa). It was found that the creep resistance depends strongly on the crystal orientation. For creep in tension it decreases in the sequence 〈111〉, 〈001〉, 〈011〉 and for creep in compression it decreases in the sequence 〈001〉, 〈111〉, 〈011〉. Constant stress tests in tension generally reveal a higher creep resistance than the constant load tests in tension. In compression the differences in the mode of testing lead only to small and unsystematic differences in creep behavior. Comparison of constant stress tests in tension and compression shows that the creep curves practically coincide at the early stages of the test. In tension there is later tertiary creep terminating in fracture. In compression no tertiary creep occurred within the test run up to a strain of about 2%. In all the tests a massive rafting, i.e. directional coarsening of γ′ particles, was observed.

The effect of high temperature rolling on structure of copper single crystals of different orientation

The work brings the data about the substructure development in single crystals of copper deformed up to about 50% of cross section reduction in rolling at high temperature. No recrystallization process was found. Instead a tendency toward inhomogeneous deformation manifesting itself in crystallographicaly oriented microbands was observed. The results are discussed in terms of the role of dynamical recovery on development of structure and its evolution in the microbands areas.

Visualization of internal domain structure of silicon iron crystals by using neutron radiography with refraction contrast

It is shown that visualization of individual domain walls and domain structure inhomogeneities in single crystals of ferromagnets is possible by using the method of neutron radiography with refraction contrast. Images of the internal domain structure of bulk (≈ 1 cm) silicon iron single crystals of different orientations are obtained.

Unstable internal friction in Bismuth and Antimony single crystals

AbstractIn the temperature range of 6 to 300 K an effect of the high amplitude ultrasound waves on amplitudedependences of decrement and modulus defects in the antimony and bismuth single crystals of different orientation is studied. The character of changes of the decrement and modulus defect amplitude dependences under the influence of the high amplitude ultrasound indicates that dislocation multiplication, obstacle break‐through by dislocation pile‐ups or deformation twinning may be observed in crystals depending on the ultrasound propagation direction and the temperature interval investigated.

Variation of low temperature internal friction at microplastic deformation of high purity molybdenum single crystals

AbstractAmplitude and temperature spectra of ultrasound absorption in weakly deformed high purity molybdenum single crystals of different orientations were measured. The results were discussed in terms of parameter changes related to quasiparticle‐ or dislocation oscillations, respectively, dislocation‐point defect interactions as well as defect generation at microplastic deformation.

Effect of neutron irradiation on the deformation of copper and internally oxidized copper-aluminium alloy

The effect of neutron irradiation on the deformation of pure copper and of internally oxidized Cu-0.09 wt% Al alloy was studied. Single crystals of different orientation were grown from pure copper and Cu-0.09 wt% Al rods, and the alloy crystals were internally oxidized. Tensile specimens were prepared from the crystals, and were irradiated to a neutron fluence of 5.5 × 10 19 n/ cm 2 ( E > 1 MeV) at a temperature below 70°C. All control and irradiated specimens were tensile-tested. The addition of second-phase particles was found to have considerable effect on the deformation of irradiated copper. The increase in resolved critical shear stress was less in internally oxidized Cu-0.09 wt% Al (Cu-Al 2O 3) than in pure copper, while the change in ultimate shear stress and shear strain was much less. Yield point phenomena appeared in all crystals, but the drop in shear stress at the yield point was less in Cu-Al 2O 3. Two values of work-hardening coefficient ( n) appeared in all unirradiated crystals, the first, a slightly larger one, was substantially decreased by irradiation, while the second was nearly unchanged. The decrease in n by irradiation was considerably larger in Cu-Al 2O 3.