Single Crystals Of Various Orientations Research Articles

A Generalization of the von Mises Criterion for a Single Crystal with a Hexagonal Crystal Lattice

A natural generalization of the von Mises flow criterion is obtained taking into account anisotropy on single-crystal materials with a hexagonal crystal lattice. Boundary-value problems are posed for plane deformation by compression with back pressure, for uniaxial tension, compression, and for pure shear. A special case of solving boundary value problems for hexagonal magnesium with a purity of 99.9% at room temperature is considered. The values of coefficients included in the fluidity function and their errors are determined. The calculated and experimental data σ0.2% for magnesium single crystals of various orientations are compared.

High temperature deformation and extended plasticity in Mg single crystals

The high temperature deformation behavior of Mg single crystals was precisely investigated using orientation imaging microscopy. For this purpose, Mg single crystals of various orientations were tensile tested in vacuum at temperatures between 473 and 673 K. A strain rate of 4.2 × 10−4 s−1 was employed. The elongations to fracture depended strongly on crystal orientation, the lowest fracture strains being associated with multiple slip. Single crystals in which single slip was activated exhibited extended ductilities corresponding to more than 1.5 in true strain. The strong orientation dependence of the ductility can also be correlated with the ease of occurrence of dynamic recrystallization (DRX), which took place in the multiple-slip specimens. The role of twinning in the initiation of DRX is also discussed.

Analysis of thermally activated formation and breakdown of Kear-Wilsdorf barriers in Ni3Ge single crystals of various orientations

The orientation dependence of the yield stress in Ni3Ge single crystals has been examined both theoretically and experimentally. The positive temperature dependence of the yield stress in the low temperature region is attributed to formation of Kear-Wilsdorf barriers. The forces driving the formation and breakdown of barriers are calculated within the framework of the Hirsch scheme. A distinctive feature of the model proposed is that the barrier is considered on the screw component of the a/2[ $${a \mathord{\left/ {\vphantom {a 2}} \right. \kern-\nulldelimiterspace} 2}\left[ {\bar 101} \right]\left( {111} \right)$$ 01](111) superdislocation in the primary octahedral plane. The major role in barrier formation belongs to anisotropy of elastic moduli, energy of antiphase boundaries in the octahedral plane, shear stresses in the cubic and octahedral planes, and friction-induced stress in the cubic plane. A comparison of predicted values of the driving force of barrier formation and breakdown with experimental values reveals their good agreement. An analysis of the orientation dependence of the driving force of barrier formation in the temperature range T = 77–293 K shows that the dependence Δ $$ \tau $$ (T) has an extremum for crystals deformed along the [ $${a \mathord{\left/ {\vphantom {a 2}} \right. \kern-\nulldelimiterspace} 2}\left[ {\bar 101} \right]\left( {111} \right)$$ 39] crystallographic direction, which is confirmed experimentally.

Stress Distribution in a Slip Trace of Deformed Ni3Ge Single Crystals

The shear‐stress distribution produced by distortion of Ni3Ge single crystals under compression is studied. The evolution of the dislocation structure during deformation of Ni3Ge single crystals of various orientations ([2¯34], [1¯11]$, [1¯39], and [001]) at T = 77, 293, 523, 673, and 873 K is analyzed. It was found that, up to failure strains, the dislocation structure is characterized by a uniform dislocation distribution. Regardless of the strain‐axis orientation, the linear relation τ = f(ρ0.5) is valid for all the test temperatures except for T = 77 K. The deviation from the linear relation at T = 77 K is due to the suppressed thermally activated slip of dislocations in nonuniform‐strain fragments at the specimen edges. In these fragments, the shear stresses are substantially reduced, and hence, the stresses produced by the dislocation cluster retard the development of slip in this trace.

The effects of sulfuric acid on the mechanical properties of ice single crystals

AbstractIce single crystals of various orientations containing various concentrations of H2SO4 up to 11.5 ppm were cut from large pucks of laboratory-grown ice. Constant-strain-rate compression tests were performed on the doped ice crystals both at −20°C at an axial strain rate of 1 × 10−5 s−1 and at −10°C at 1 × 106 s−1. The stress–strain curves showed a linearly rising stress with increasing strain, followed by a sharply declining stress after reaching a peak. With further strain, the sharp decline in stress slowed. The tests clearly showed, for the first time, that this naturally occurring impurity dramatically decreases both the peak stress and the subsequent flow stress of ice single crystals. The decrease in the peak strength was related to the square root of the concentration of H2SO4 up to 11.5 ppm, suggesting that the solubility limit of H2SO4 in ice is at least 11.5 ppm. The sulfuric acid also appeared to increase the ductility of the ice. Preliminary examination of a doped ice single crystal by synchrotron X-ray topography suggested that sulfuric acid dramatically increases the grown-in dislocation density.

A channeling investigation of the structural modifications in uranium dioxide submitted to swift-ion irradiation and aqueous corrosion

This paper presents a study of the influence of swift-heavy-ion irradiation on the behavior of UO 2 towards aqueous corrosion. Single crystals of various orientations are irradiated with 340-MeV Xe ions and subsequently leached in deionized water at 180°C. The channeling technique is used to quantify the structural modifications induced by both irradiation and leaching. The U and O sublattices are investigated with standard Rutherford Backscattering (RBS) and the resonant scattering on 16O at 3.045 MeV. Channeling angular scans are reproduced with Monte Carlo simulations. Irradiation induces a polygonization of the fluorite structure. Leaching leads to an incorporation of additional O atoms in the surface region of the UO 2 crystal which form a U 4O 9-like structure.

Orientation dependence of plastic deformation in NiAl single crystals

Compression stress-strain tests were performed on NiAl single crystals of various orientations at room temperature and complemented by observations of slip traces and dislocation structures. In ‘soft’ oriented samples (far from the [100] direction), 〈100〉 slip vectors are activated at a critical resolved shear stress of ≈90 MPa. The 〈100〉 slip is very homogeneous as witnessed by a faint and uniform slip line pattern. The yield stresses of soft orientations obey the Schmid law. For ‘hard’ orientations (within 3 ° from the [100] direction), the yield stress for 〈100〉 slip becomes too large and 〈111〉 slip is activated instead. The 〈111〉 slip is highly inhomogeneous and gives rise to distinct glide bands. The strain concentrated in these bands remains constant during deformation indicating that the plastic strain is carried by mobile dislocations at the periphery of the glide bands. We suppose that the difference in the slip homogeneity is due to highly different mobilities of 〈100〉 and 〈111〉 dislocations.

Anisotropy of transformation characteristics of Cu-base shape memory alloys

Shape memory alloy single crystals undergoing stress induced thermoelastic martensitic transformations exhibit strong anisotropy of deformation characteristics when loaded along various crystallographic axes. In the present work, CuAlNi and CuAlZnMn single crystals of various orientations were investigated by thermomechanical tension/compression tests. In particular, the orientation dependence of transformation strains and stress-temperature critical conditions for martensitic transformation were studied. Results have been summarized in stress-temperature phase diagrams which provide sufficient basis for micromechanical modeling of Cu-based polycrystals. Experimentally measured transformation strains were found to be in reasonable agreement with predictions based on the phenomenological theory of martensite crystallography.

Dislocation Slip Conditions and Structures in Copper Single Crystals Exhibiting Instabilities in Creep

Microstructural aspects of compressive creep at 723 to 773 K and its instabilities in copper single crystals of various orientations are investigated. Metallography, transmission electron microscopy, electron- and Laue X-ray diffraction were used to study dislocation slip. A computer simulation of a change of crystal orientation due to straining was applied to analyse dislocation slip conditions.

Cross-sectional TEM and corrosion studies of Al and N implanted copper

ABSTRACTPoly- and single crystalline copper was implanted with aluminium and nitrogen at doses ranging from 1016 to 5 × 1017 at/cm2 and energies of 170 keV. The corrosion resistance of the implanted surfaces was tested by exposure to an H25-containing atmosphere. The amount of copper sulphide formed was measured by chrono potentiometric reduction. The amount of corrosion products was markedly reduced (up to a factor 50) by high dose implantations of aluminium. The microstructure of the implanted copper was examined by Transmission Electron Microscopy of cross-sectioned specimens. A deep damage layer far exceeding the ion range was observed. XTEM-pictures of aluminium implanted copper single crystals of various orientations suggest a channeling mechanism for this deep damage layer. In situ annealing of the specimens in the TEM showed that most of the implantation damage is removed at 600°C except for an array of dislocations at the end of the damage range.

Effect of irradiation on the deformation of copper and copper-aluminium alloys

The effect of low-temperature irradiation (<70°C) on the deformation of pure copper, Cu-0.09 wt% Al alloy, and Cu-0.19 wt% Al alloy was investigated. Tensile specimens were prepared from single crystals of various orientations, and were exposed to a neutron fluence of 6.85 ×1019 n/cm2(E>1 MeV) at a temperature less than 70°C. All irradiated and unirradiated specimens were pulled. A large increase in critical shear stress due to irradiation was observed; the increase was smaller in Cu-0.19 wt% Al alloy than in pure copper and Cu-0.09 wt% Al alloy. Ultimate shear stress and shear strain were less influenced by irradiation. Yield points were observed in all irradiated specimens. The yield drop was large in irradiated pure copper, and decreased with the increase of aluminium content in copper-aluminium alloys. All unirradiated specimens showed a high work-hardening coefficient (n) in the beginning of the deformation, followed by a lower value. By irradiation, the first value drastically decreased, while the second remained nearly constant. Shear stress and shear strain were influenced by crystal orientation.

Orientation relationships of zinc oxide on sapphire in heteroepitaxial chemical vapor deposition

The anisotropy in ZnO heteroepitaxial growth by CVD is studied on spherical and lens-shaped sapphire substrates. The layers of ZnO form maps containing many regions of single-crystals of various orientations. By combining equivalent types of these regions, six different modes in the orientation relationships between ZnO and sapphire are distinguished and classified. The effects of growth temperature and reaction gas velocity on the kinds and the extent of these regions are also investigated. The results, drawn by stereographic projection, may provide a useful guide for selecting and preparing substrate planes to obtain specific ZnO planes of high quality, i.e. smooth surface, high perfection, high electron mobility etc.

Effect of growth rate on the dislocations structure of aluminium single crystals of various orientations

X-ray diffraction topography and the etch figure method were used to investigate the dislocation structure of single crystals of pure aluminium (99.999%) grown from the melt by the Bridgman method with 〈100〉, 〈111〉, 〈210〉 and 〈100〉 (near the pole) orientations of the solid-liquid interface with a growth rate range from 4 to 15 mm/min. It has been shown that the dependence of the dislocation density on the growth rate is determined by the crystallographic orientation of the solid-liquid interface. Thus, for crystals with 〈111〉 orientation the dependence of the dislocation density on the growth rate is weak, for crystals with 〈100〉 orientation it is very strong, while for crystals with 〈210〉 orientation this dependence is extremely large. The results obtained are explained on the basis of existing concepts about the anisotropy of mobility of the solid-liquid interface in the directions of preferred crystal growth.

The influence of strain rate on the low cycle fatigue properties of single crystals and polycrystals of two ferritic alloys

The influence of strain rate on the cyclic plastic deformation behaviour of two high purity body-centered cubic iron-based alloys (Fe-3% Si and Fe-26% Cr-1% Mo) was studied paying particular attention to the relation between the behaviour of polycrystalline specimens and that of single crystals of various orientations. At high strain rates (about 10 −1 s −1) most single crystals exhibit stress asymetries and pronounced shape changes as a consequence of the observed slip asymmetry. At low strain rates (about 10 −6 s −1) such single-crystal effects are markedly reduced. For polycrystalline material the strain rate dependence of the single-crystal fatigue behaviour is correlated with the crack initiation mode and the relative values of the internal and effective stresses σ i and σ ∗ . At intermediate strain rates (about 4 × 10 −3 s −1 for the FeSi alloy, and at 4 × 10 −5 s −1 for the Fez.sbnd;Si alloy) σ i ≈ σ ∗ and both transgranular and intergranular crack initiation are observed. At lower strain rates σ ∗ < σ i and crack initiation takes place transgranularly along slip bands. At higher strain rates σ ∗ > σ i and crack initiation is intergranular owing to the large crystal shape changes.

The anisotropy of Al-4 Wt Pet Cu single crystals tested in plane-strain compression

Plane-strain compression tests were made on Al-4 wt pet Cu single crystals of various orientations in solution-treated and overaged conditions. Shear stress-shear strain curves were constructed using the appropriate Taylor factors, corrected for lattice rotation and observed lateral spreading. These results are compared with data from an earlier study on crystals of the same alloy aged to produce ϑ′. The anisotropy of the shear stress-shear strain curves was found to be the greatest in the normally aged material (ϑ′) and the least in the overaged material(ϑ′).

Elastic moduli and internal friction of cold worked copper single crystals

The change of the elastic moduli and the internal friction of copper single crystals of various orientations during deformation at 78 K was measured. First the elastic moduli change due to the fact that the crystal orientation changes upon plastic extension. An analogous effect is the change of the moduli due to the change of orientation of part of the crystal by mechanical twinning. Finally the dislocations produced by the deformation give rise to a decrease of the moduli and an increase of the internal friction. The contribution of these mechanisms to the observed effects is discussed. The dislocation contribution is analyzed in terms of the Koehler-Granato-Lücke model yielding the density and line lengths of the contributing dislocations. The result is compared with literature data from transmission electron microscopy, leading to the conclusion that the dislocation part of the modulus effects and internal friction is due to the small fraction of total dislocation density lying between the dense dislocation tangles and having line lengths of the order of 1 μm.

The Standard Potential of the Two‐Phase Cu Amalgam and Other Cu Electrodes and Calculation of CuSO4Activities in Aqueous Solutions

AbstractThe potential difference between two‐phase copper amalgam electrodes and pure, solid copper electrodes (single crystals of various orientations, strained and unstrained polycrystalline sheet) was measured in the range 15 to 26°C. At 25°C the two‐phase amalgam was 4.78 mV more positive than the solid electrodes which all gave identical results. From the potential difference and its temperature coefficient, μ0andS0of the intermetallic compound CuHg, were calculated. A discussion on previous determinations of copper standard potentials, the activity of CuSO4in solution, and the standard potential of the Hg, Hg2SO4|SO42‐electrode is included. The following purely empirical relationship is proposed between mean activity coefficient γ and concentrationm0of CuSO4solutions in the range 3 · 10−2&lt;m0&lt; 3 · 10−1mol kg−1: γ = 0.0502m0−1/2.

Deformation of high purity tantalum single crystals at 4.2 K

High purity Ta single crystals of various orientations were deformed in tension and compression at 4.2 K. The expected (011) slip was the dominant mode of deformation in tension, except for orientations near [011], where twinning and some anomalous ( 1 01) slip were observed. In tension, stress-strain curves were always serrated owing to the occurrence of adiabatic slip. In compression, the dominant mode of deformation was twinning; slip was only observed to a small extent for orientations near [001]. The occurrence of adiabatic slip in tension obscured any possible effects of normal stress on the critical resolved shear stress for slip. However, a pronounced increase of twinning stress with increasing normal stress was observed in compression. This can be explained in terms of the effect of normal stress on the nucleation of twins by the transformation of the screw dislocation core into a three-layer twin configuration. The addition of up to 4% Re to Ta eliminated twinning at 77K, and promoted the occurrence of slip at 4.2 K for orientations displaying only twinning in unalloyed specimens. This effect is most probably related to effects of solute atoms on the extended screw dislocation core.

Fatigue hardening in niobium single crystals

Nb single crystals of various orientations were cyclically deformed in tension-compression under strain control. At low strain amplitudes all crystals oriented for single slip and some oriented for multiple slip showed a two stage hardening. When present, the first stage was characterized with almost no cyclic work hardening. The rate of hardening in the second stage increased with strain amplitude and the amount of secondary slip; however in all cases it was considerably less than θ II in unidirectional deformation. In crystals oriented for single slip kink bands developed on their side faces during rapid hardening stage which resulted in considerable amount of asterism in Laue spots. A cyclic stress-strain curve independent of prior history was found to exist which was also independent of crystal orientation. Furthermore, this curve differed only slightly from that of polycrystalline Nb obtained from data in literature.

Influence of normal stress on yielding behaviour in Fe -3 ·2% Si alloy single crystals

Specimens of Fe-3·2 wt% Si alloy single crystals of various orientations, both with 18 ppm C and decarburized, have been deformed in compression (\(\dot \varepsilon \) ∼ 10−4 s−1) at different temperatures between 125 K and 293 K. It has been found that the magnitude of CRSS, the choice of the slip planes and the shape of the stress-strain curves depend on the angle between the compression axis and the Burgers vector (angleξ). The stress normal to the maximum resolved shear stress plane is strongly altered on changing the angleξ. The discussion of the obtained experimental results seems to indicate that the normal stress influences the structure of screw dislocation core and subsequently the dislocation mobility.