Beryllium Single Crystals Research Articles

Beryllium fracture observed by in situ high voltage electron microscopy

Beryllium single crystals were strained to fracture in a high voltage electron microscope and were studied using in situ and analytical techniques. The crystals were oriented so that the stress axis was parallel to the basal planes, thereby activating the prism planes of the first order during plastic deformation. Resulting dislocation cells were characterized and slip traces on external and fracture surfaces were analyzed. Microcracks were seen to initiate at dislocation cell walls and mostly continued to propagate along them. A pseudocleavage mechanism for microcracks has been discussed.

The crystallization behaviour of beryllium

The crystallization behaviour of beryllium was investigated by the examination of single crystals grown by zone melting and other directional solidification techniques. The results indicate that the solidification of beryllium from the melt does not yield a preferred growth orientation of the crystals. It is possible to grow beryllium single crystals in any orientation by the use of a seed crystal. In spite of an allotropic transformation in beryllium, the growth of subgrain free single crystal is possible with the crucibleless zone melting technique combined with the necking technique.

Compton Profile of Beryllium

An experimental and theoretical analysis of the Compton profile of beryllium is presented. The measurements on single-crystal beryllium are performed with use of synchrotron radiation (10-keV x rays). The theoretical calculation employs local density-functional pseudopotentials. The agreement between the one-electron calculation and the measured spectra is distinctly superior to previous attempts. A small directionally independent difference remains. This difference is substantially reduced by incorporating correlation effects within the homogeneous approximation.

Beryllium Wunschmaterial f�r Neutronenmonochromatoren

Compared to other commonly used monochromator materials, beryllium single crystals give a considerable higher intensity of the monochromatic beam for neutrons with a wavelength between 0.07 to 0.3 nm. For this reason it is necessary to produce single crystals without subgrains and with a low defect concentration. To date, attempts to grow such crystals have failed. In this treatise the neutron scattering properties and the crystallisation behavior of beryllium is discussed. In addition, possible future developments in the field are reviewed.

Selection De Blocs Monocristallins Dans Un Cristal De Beryllium Par Topographie Neutronique

We have characterized the crystalline perfection of beryllium single crystals produced by zone melting and identified relatively large grains by means of neutron topography. The crystalline perfection inside these grains is adequate for their utilisation as neutron monochromators.

A minimum in the resistance of beryllium

The temperature dependence of the resistance of single crystal beryllium specimens with widely varying impurity concentrations is studied in a longitudinal magnetic field over the temperature range 4.2–20 °K. It is demonstrated that the resistance changes nonmonotonically with temperature, and that application of a magnetic field of H∼10–20 kOe eliminates this anomalous resistance behavior. On the basis of the experimental data the preliminary conclusion is drawn that the observed resistance behavior is produced by magnetic impurities (the Kondo effect).

Ultrasonic attenuation in beryllium oxide

The room-temperature attenuation of longitudinal acoustic waves propagating in the c direction in single crystal beryllium oxide (BeO) has been measured from 0.3 to 1.2 GHz. The attenuation follows a frequency squared dependence with a measured attenuation of 1.1 dB/cm at 1.0 GHz. The magnitude and frequency dependence of the loss indicate an intrinsic interaction with thermally excited phonons characteristic of the ωτ ≪ 1 region. The loss level compares favorably with a value of 1.4 dB/cm predicted from thermal conductivity and crystal structure considerations [D. W. Oliver and G. A. Slack, J. Appl. Phys. 37, 1542–1548 (1966)]. Using the WE lattice loss theory [T. O. Woodruff and H. Ehrenreich, Phys. Rev. 123, 1553–1559 (1961)] a Grunheisen γ of 0.54 is calculated. This compares to longitudinal mode c-axis Grunheisen constants of 0.78 for zinc oxide and 1.51 for cadmium sulfide, binary compounds of the same crystal class. Empirical relationships were developed between crystal constants and ultrasonic attenuation for the II–VI compound hexagonal crystal class.

Feasibility of electrocaloric refrigeration for the 4–15 K temperature range

The feasibility of a solid state type of refrigeration, which utilizes the electrocaloric effect in certain dielectric materials, has been investigated. The study was limited to the temperature range where the refrigerator would absorb heat from a load at about 4 K and reject heat to a reservoir at about 15 K. Heat switches would be required for such a refrigerator and two types were studied. One type was a multiple-leaf contact switch, the other a magnetothermal switch of single crystal beryllium. Many electrocaloric materials were studied but none was found with a sufficiently large reversible electrocaloric effect for a practical refrigerator. The largest effects were seen in a SrTiO 3 ceramic, followed by a KTaO 3 single crystal. Temperature reductions of about 0.3 K at 10 K were observed during depolarization from fields of 20 kV cm −1. A theoretical model, based on the electret behaviour of impurity-vacancy dipoles is postulated to interpret the anomalous dielectric behaviour of the materials investigated. Another theoretical model, based on the lattice dynamics of displacive dielectrics, is postulated to explain the observed temperature changes seen in such materials. The model points out that at 4 K the entropies of displacive type materials are probably too low for practical refrigeration. An investigation of certain order-disorder dielectrics is suggested.

Lattice location study of6Li implanted into Be

The authors have shown experimentally using the ion beam channelling lattice location technique, that lithium atoms ion-implanted into single-crystal beryllium metal occupy substitutional sites. This result disagrees with expectations based on considerations of chemical potentials and local electron densities. The disagreement is thought to arise from a dependence of the final lattice site of Li upon the ion implantation process itself. It is argued that there is a high probability for the capture of Li by a Be vacancy created by the Li as it comes to rest and that, under these conditions, a substitutional site for lithium in beryllium is probably.

The peculiar effect of forest dislocations on single twin layer development in zinc and beryllium single crystals

This is an investigation of the effect of different types of forest dislocation on the rate of twin layer broadening, V n, in zinc and beryllium crystals, and on the velocity of the twinning dislocation movement, V t, in zinc crystals under the action of a constant external shear stress. Increasing the forest basal dislocation density, ϱ b, was found to result in increasing V n and reducing V t, while increasing the forest pyramidal dislocation density, ϱ p, causes V n to decrease. An analysis in terms of crystal geometry shows that the dualism of the influence of the basal dislocations stems from the fact that they behave as twinning dislocation sources whose density, increasing with ϱ b, leads to higher V n. The decrease in the effective stress, τ ∗, with increasing ϱ b is estimated. An analysis of the experimental data yielded the relation V t (τ ∗) and an estimate of the activation volume, which amounted to 6 × 10 −21 cm 3. The close coincidence of the activation volumes as obtained from V t (τ ∗) and V n (τ) suggests that the rate-controlling mechanism of the twin layer development in zinc crystals with large forest basal dislocation density is the twinning dislocation inhibition. In Be crystals, the increasing V n effect is observed during untwinning. In Be twinned crystals, electron microscopy revealed twinning dislocations with a density of about 10 5 cm −1 at the twin boundaries and a large forest basal dislocation density inside the twin ( ca. 10 8 cm −2).

Oscillatory channeled-ion scattering yield in beryllium

Strong oscillations in the Ruterford-backscattering yield of energetic /sup 4/He ions channeling in the basal plane of a beryllium single crystal have been observed. The wavelength of the oscillation is in reasonable agreement with a particle trajectory calculation using an analytical continuum potential, including the effects of thermal lattice vibrations, and is in good agreement with expectations from Monte Carlo simulations by Barrett.

Low-temperature solubility of copper in beryllium, in beryllium-aluminum, and in beryllium-silicon using ion beams

Ion implantation and ion backscattering analysis have been used to measure the solubility of copper in beryllium over the temperature range 593 to 1023 K, and to determine the effect on the copper solubility of aluminum and silicon impurities. The binary data extend 280 K lower in temperature than previous results, while the ternary measurements are unique. The information is pertinent to the use of copper for solution strengthening of beryllium. Diffusion couples were formed by ion implantation of copper into single-crystal beryllium at room temperature, followed where appropriate by implantation of aluminum or silicon. The samples were then annealed isothermally, and the time-evolution of the composition-vs-depth profile, determined by ion backscattering analysis, yielded the solubility of copper. Measurements at exceptionally low temperatures were facilitated by the short diffusion distances, approximately equal to 0.1 mu m, and the use of neon irradiation to accelerate diffusion. The resulting binary data for the solubility C/sub 0/ of copper in beryllium merge smoothly into previous results at higher temperatures. The combined data, covering the temperature range 593 to 1373 K, are well described by C/sub 0/ = (12.6 at. pct) . exp (-842 K/T). In the ternary regime, the effects of aluminum andmore » silicon on the solubility of copper were found to be small.« less

Electrical and thermal magnetoconductivities of single-crystal beryllium at low temperatures and its use as a heat switch

The effects of transverse magnetic fields up to 955 kA/m (12 kOe) on the electrical and thermal conductivities of single-crystal beryllium have been measured between 2 and 300 K. Most of the measurements were made on a sample with a resistance ratio of1340. This sample was pure enough so that the intrinsic electronic thermal resistivity could be measured for the first time. It was found to have the usual T2 behavior. The current and heat flow were along the hexagonalc axis of the crystal, while the thermal and electrical conductivities were studied as a function of the angle of the magnetic field in the basal plane. Below about 50 K the thermal conductivity could be reduced by several orders of magnitude by applying the magnetic field. The lattice conductivity, extrapolated from the measurements in the magnetic field, is given by k =γT2, where γ=1.6×10−4 W/cm K3. This value is in reasonable agreement with that obtained from measurements of beryllium alloys. The use of single-crystal beryllium as a heat switch for temperatures below about 30 K is discussed.

Influence of copper additions on the basal cleavage in beryllium single crystals at room temperature

In order to study the influence of Cu-alloying on the susceptibility of Be to cleavage along the basal planes, a series of single phase alloys containing up to 5 at. pct Cu in Be were prepared and tested at room temperature by the DCB technique. Copper additions were found to influence strongly the plastic events induced by the shear stresses acting at the crack tip. The crack propagation energy increases with Cu content up to about 1.2 at. pct Cu and decreases with further additions. The maximum value attained at 1.2 at. pct Cu is about seven times that of pure Be. This remarkable initial increase in the crack propagation energy can be accounted for on the basis of the higher energy needed to activate basal and prism slip due to solid solution hardening. In alloys containing more than about 1.2 at. pct Cu, twinning is found to be the most prominent deformation mechanism. The propensity to twin is observed to increase with increasing Cu content, and it is assumed that this reflects a corresponding decrease in the twinning energy. Since twin boundaries are highly susceptible to cleavage cracking, a decrease in the crack propagation energy results which has the effect of increasing the tendency of Be to fracture in a brittle manner in the alloys with higher Cu contents.

Phase equilibria and diffusion in the Be-Al-Fe System Using High-Energy Ion Beams

Phase boundaries enclosing the α-Be region of the Be-Al-Fe system have been investigated by a new approach which utilizes high-energy ion beams. Ion implantation of aluminum and/or iron into single-crystal beryllium was used to create a desired second phase in the first ≃0.1 μm of the sample. Under continued isothermal annealing the second phase dissolved into the underlying bulk α-Be, and this process was monitored nondestructively by ion backscattering. Analysis then gave the solid solubility and the diffusivity of the species under consideration. The solubility of iron in beryllium was measured between 773 (170 appm) and 1123 (1600 appm); above 973 the present data merge smoothly into previous results from the literature. A new upper bound was established for the solubility of aluminum below the Al-Be eutectic: ≲ 70 appm at 873 K. In the ternary regime, the solubility of iron in the presence of excess aluminum was measured for the first time; below 1123 it is ≲ 100 appm.

Influence of copper additions on the basal cleavage in beryllium single crystals at room temperature

Influence of copper additions on the basal cleavage in beryllium single crystals at room temperature

Temperature dependence of the microhardness of beryllium single crystals and Be-Cu alloys

1. In Vickers hardness tests with a pyramidal indentor the microhardness of beryllium single crystals and Be−Cu alloys along the c axis reaches values of 1000–1200 kg/mm2 at low temperatures, which are comparable with the values for some crystals with covalent bonds. 2. With increasing temperatures and alloying with copper the anisotropy of the microhardness decreases, but the absolute values remain high up to 400°K. The high anisotropy of the hardness of beryllium is due to its lattice and not to the presence of impurities. 3. The results obtained and previous results on the critical shear stresses in different slip systems indicate strong anisotropy of the bonding forces in beryllium crystals and beryllium alloys with copper.

Shock−wave compression of single−crystal beryllium

Beryllium single crystals were shock loaded by planar impact along the a and c axes as well as along off−axis directions. The orientation of primary (basal), secondary (first−order prism), and tertiary (second−order pyramidal) slip systems, in addition to the relative magnitudes of their yield stresses, permits the study of dynamic yield behavior on each system separately. Slip on the primary system is activated by shock loading in directions which make angles of ϑ=23°−61° with respect to the c axis. The dynamic shear yield strength on the basal plane varies from 0.6 to 1.9 kbar depending on the shock−propagation direction: this effect is thought to be due to differences in compressive strain perpendicular to the basal plane. In all single crystals the rise times of the elastic waves are on the order of a few nanoseconds in contrast to the relatively long rise times reported for shock compression of polycrystalline beryllium. Dynamic yielding on secondary and tertiary planes results in significant stress relaxation behind the elastic wave front. This suggests that dislocations on these planes are undergoing rapid multiplication on a submicosecond time scale. The method of characteristics for rate−dependent material response is used to study dislocation multiplication and plastic wave propagation in crystals shock loaded along the a axis. The phenomenon of multiple plastic wave propagation is also observed and compared with finite−difference solutions for plane shock−wave propagation in anisotropic media. The shock−wave behavior of single−crystal beryllium is more complicated than one would expect on the assumed basis of elastic−perfectly plastic response of the three known slip systems.

Electron Microscopy of Shock Loaded Polycrystalline Beryllium

Uniaxial compression tests and hydrostatic tests at pressures up to 27 kbars have been performed to determine operating slip systems in single crystal and polycrystal1ine beryllium. A recent study has been made of wave propagation in single crystal beryllium by shock loading to selectively activate various slip systems, and this has been followed by a study of wave propagation and spallation in textured, polycrystal1ine beryllium. An alteration in the X-ray diffraction pattern has been noted after shock loading, but this alteration has not yet been correlated with any structural change occurring during shock loading of polycrystal1ine beryllium.This study is being conducted in an effort to characterize the effects of shock loading on textured, polycrystal1ine beryllium. Samples were fabricated from a billet of Kawecki-Berylco hot pressed HP-10 beryllium.

The optical absorption of evaporated beryllium films

The optical absorption spectrum of evaporated beryllium films has been measured using an UHV ellipsometric technique. The beryllium films were formed on sapphire substrates held at RT and about 20K but the observed spectra appear to be largely independent of the substrate temperature. It is believed that the data in both cases are to be associated with disordered microcrystalline or amorphous beryllium. It is therefore surprising that present data are so similar to the spectrum obtained on bulk single crystal beryllium by Weaver et al (1973). The paradox is resolved if it is assumed that the details of the electron band structure of crystalline as well as disordered beryllium are governed by short range order; theoretical arguments by Rousseau et al (1970) support this point of view.