A simple image intensifying system for Berg-Barrett topography

A simple imaging system to facilitate crystal orientation with respect to the x−ray beam for Berg−Barrett topography has been constructed. This system consists of an image intensifier tube with an x−ray phosphor deposited on its fiber−optic input plate and means to manipulate and indicate the crystal orientation. Applications of the system to studies of both crystalline and biological samples are given

Stress State Required for Pyramidal Dislocation Movement in Zinc

A tension or compression stress in such a direction that basal slip is minimized can produce second-order pyramidal slip bands in zinc single crystals. The stress required to initiate pyramidal dislocation motion is not sensitive to temperature. However, dislocation velocity at a given stress is sensitive to temperature and the very small dislocation velocity at low temperatures has lead to an erroneous estimate of a ``starting stress'' for pyramidal dislocations. Dislocation velocity at a constant temperature was found to be a function of the magnitude, but not the sense of the resolved shear stress

Dynamical x-ray diffraction from nonuniform crystalline films: Application to x-ray rocking curve analysis

A dynamical model for the general case of Bragg x-ray diffraction from arbitrarily thick nonuniform crystalline films is presented. The model incorporates depth-dependent strain and a spherically symmetric Gaussian distribution of randomly displaced atoms and can be applied to the rocking curve analysis of ion-damaged single crystals and strained layer superlattices. The analysis of x-ray rocking curves using this model provides detailed strain and damage depth distributions for ion-implanted or MeV-ion-bombarded crystals and layer thickness, and lattice strain distributions for epitaxial layers and superlattices. The computation time using the dynamical model is comparable to that using a kinematical model. We also present detailed strain and damage depth distributions in MeV-ion-bombarded GaAs(100) crystals. The perpendicular strain at the sample surface, measured as a function of ion-beam dose (D), nuclear stopping power (Sn), and electronic stopping power (Se) is shown to vary according to (1–kSe)DSn and saturate at high doses

Twinning and Slip in Zinc by Indentation

Observations of twinning and slip deformation caused by indentation of zinc reveal that extensive slip on the basal and second-order pyramidal systems takes place at loads up to 5 kg. Prismatic punching through 1-cm crystals is observed at indentation loads in excess of about 2.5 kg. It is concluded that the stress at the tip of the twins cannot be obtained by use of an elastic stress analysis

Strain modification in coherent Ge and SixGe1–x epitaxial films by ion-assisted molecular beam epitaxy

We have observed large changes in Ge and SixGe1–x layer strain during concurrent molecular beam epitaxial growth and low-energy bombardment. Layers are uniformly strained, coherent with the substrate, and contain no dislocations, suggesting that misfit strain is accommodated by free volume changes associated with injection of ion bombardment induced point defects. The dependence of layer strain on ion energy, ion-atom flux ratio, and temperature is consistent with the presence of a uniform dispersion of point defects at high concentration. Implications for distinguishing ion-surface interactions from ion-bulk interactions are discussed

Dislocation Velocity Measurements in Copper and Zinc

Studies of dislocation dynamics in a number of different materials have been reported, but no direct measurements in FCC or HCP metals have appeared in the literature. Studies of slip band growth in the basal system of zinc at this laboratory have indicated that individual dislocations achieve relatively high velocities at the yield stress. This implies that only very short duration loading (µsec) of low dislocation density crystals (<1000 cm^(-2)) will permit direct measurement of the dynamics of motion of individual basal dislocations in zinc. Suitable single crystal specimens of 99.999+% copper and zinc were prepared for this study. A torsion loading system was developed to apply single, short duration stress pulses to the specimens. This system generates a zero mode torsional wave front with a rise time of 2 µsec in a 1.25 cm diameter elastic rod. The loading wave front, after passing through a specimen crystal, is reflected at a free surface normal to the cylindrical axis. The reflected wave unloads the specimen, and the duration of the stress pulse at any point is just the round trip travel time of the wave front from that point to the free surface. The torsion strain on the cylindrical surface of the elastic rod is monitored using semiconductor strain gages. The stress on a cross section of the specimen crystal varies linearly from zero on the cylindrical axis to a maximum at the surface. Dislocations were observed before and after stressing. Copper cylinders with [100] axes, and 4, 3mm wide flats on {100} surfaces were tested at room temperature. Dislocation displacements up to 100µ into regions initially free of dislocations were measured on the {100} flats using the double etch technique. The dislocations were of mixed edge screw orientation. The behavior of fresh dislocations produced by scratching and of isolated aged dislocations was not significantly different. The displacement was found to be a linear function of distance from the free end, therefore, the displacement was linearly proportional to the time of stress application. A nearly linear relationship between dislocation velocity and stress was found, stresses from 2.8 Mdyne/cm^2 to 23.1 Mdyne/cm^2 produced velocities from 160 cm/sec to 710 cm/sec. Zinc crystals with a [0001] cylindrical axis and (0001) end surfaces were used to study basal dislocation mobility at room temperature. One end surface was scratched prior to stressing to produce fresh edge dislocations. The end surfaces were then bonded to elastic rods and a torsion pulse was applied. The bond on the scratched end of the crystal was removed, and displacement of dislocations from the scratches was observed using the Berg-Barrett X- ray diffraction technique. Dislocation displacement was found to be directly proportional to radial position from the cylindrical axis, which implies that the basal dislocation velocity is directly proportional to stress. Dislocation displacements up to approximately 400µ were measured. Interaction between different basal dislocations in these tests was negligible. The maximum test stress of 17.2 Mdynes/cm^2 produced a dislocation velocity of approximately 600 cm/sec. The mobility of basal dislocations in zinc was considerably reduced by aging for eight hours after scratching. This is attributed to the accumulation of jogs along the dislocations which were within 5µ of the observation surface. These results indicate that the dislocation drag in copper and the basal system in zinc is relatively small compared to that in other materials in which direct observations have been made. The drag agrees well with that predicted for phonon interaction with the moving dislocations, and with the value deduced from internal friction measurements in copper. Tests are in progress to see if the dislocation velocity increases as the phonon interaction is reduced at lower temperatures. Slip on the {1212} 〈1213〉 system of zinc occurs by the formation and growth of slip bands. A dislocation etch and the Berg-Barrett technique were used to observe the growth of the slip bands produced by compression pulse loading of specimens in the [0001] and 〈1210〉 directions. The majority of slip bands are nucleated at substructure boundaries, and the bands grow at essentially equal rates in all directions on the {1212} slip planes. Dislocation velocity at room temperature was found to be proportional to stress to the 8.7th power. The velocity at a given stress decreases with a decrease in temperature, and also decreases as the initial dislocation density increases. This indicates that motion of {1212} 〈1213〉 dislocations is assisted by thermal activation and that forrest dislocations may be the most significant obstacles to their motion. Quantitative studies of the temperature and stress dependence are underway

Deformation of Drill Pipe Held in Rotary Slips

This paper presents an analysis of measurements of drill-pipe deformation in the slip area. Drill pipe was loaded in excess of the load at the minimum yield strength with the pipe in VARCO Type SOL and Type SDXL slips. A VARCO Type MSS solid master bushing was used with the type SDL slips and the master bushing of a VARCO KMPC unit was used with the Type SDXL slips. Measurements were made on the reduction in pipe diameter in the slip area as a function of load. Values of load which caused inelastic deformation of the pipe are determined and compared to the values previously calculated from theory

Etching of High Purity Zinc

A method of etching high purity zinc to reveal various etch figures on {101¯0} planes is presented in this paper. Etch figures are formed by polishing in a dichromic acid solution after the introduction of mercury to the crystal surface. No measurable aging time is required to form etch figures at newly formed dislocation sites when mercury is on the surface prior to deformation. The mercury concentrates at the sites where etch figures form and may be removed by vacuum distillation and chemical polishing before it appreciably affects the purity of the bulk of the crystal

Stereographic X-ray reflection topography of dislocations in zinc

X-ray reflection topographs were taken of a zinc surface oriented about 1° from a basal plane. Basal dislocations are revealed in the topographs, and their apparent depth was determined using stereo pairs of topographs. The apparent depths observed in a complimentary pair of topographs using 1013 and 1013 reflections were significantly greater than those observed in an asymmetric pair of topographs in which the same 1013 reflection was used. This difference is attributed to shifts of the image with respect to the dislocation position. Quantitative estimates of the image shifts and the actual depths of the observed dislocations are obtained from the measurement of apparent depths. Dislocations are visible over the range of depths from 1.7 to 4.5 µm

Minimization of Spring Torque in Flexible Connections to Instruments

Precision electromechanical measuring instruments of various kinds often require establishing electrical connection to the suspended part of the instrument, with minimum introduction of disturbing torques. Requirements are especially demanding in the case of gyroscopes. Gyroscopes of high-precision type, as used in inertial-guidance systems and in advanced flight-control systems, generally take the form of a gimbal member which contains the rotor and which is supported relative to the case by some form of minimum-friction bearings. Usually, at least three electrical connections are required (for the polyphase rotor drive motor), which take the form of wires or ribbons extending from the case to the gimbal member. Disturbing-torque levels about the gimbal axis or axes must be kept to extremely low values; the total tolerable disturbing torque from all sources whatever is often of the order of one dyne-cm or less. Special attention must be given to the electrical lead-in connections (a) to minimize mechanical hysteresis effects, which result in erratic bias changes, and (b) to minimize spring rate. While spring coercion is a conservative type of force, it nevertheless must he kept very small, otherwise a shift of null of the pickoff which detects gimbal displacement will result in a steady spring bias