Etch Pit Studies Research Articles

Slip sources in the surface layers of polycrystalline Fe-3 pct Si in the early stages of deformation

Results of a systematic etch-pit study of the distribution of slip sources in successive layers of silicon-iron in its early stages of deformation are presented. These indicate that slip initiates at sources associated with grain boundaries that emerge at the surface and that grain boundary sources lying at progressively greater depths are activated as the applied stress is increased. A simple model is proposed which adequately explains the observed characteristics of slip traces which are nucleated at applied stresses insufficient to propagate them across the grain. Contradictory observations on the nature of slip sources made by other workers on polycrystalline copper are also discussed.

Observation of grain boundaries

SUMMARYThe observation of grain boundaries using the optical microscope, the electron microscope and the field‐ion microscope has considerably aided our understanding of grain boundary structure. The study of etch pits in the optical microscope has proved that low angle boundaries are composed of dislocations.A large number of grain boundary observations have been made by transmission electron microscopy. Here, various classes of boundary structural features have been characterized in grain boundaries; they can be classified into three categories: (1) features which are essentially independent of the true structure of the boundary (thickness fringes, moiré fringes); (2) accidental features which result from a perturbation of the boundary by phenomena originating in the adjacent grains (dislocations, ledges); (3) features of the true structure of the grain boundary (features of the equilibrium structure‐features of the perturbed structure). It must be appreciated that certain features of the structure are not resolved because of their poor contrast in the electron microscope image.The field‐ion microscope has been used for a relatively short time in studies of grain boundaries. Nevertheless it has yielded very interesting results. Unfortunately, at the present time, it seems that the positions of the atoms may not be determined sufficiently accurately in this instrument to allow a precise determination of the structure. Only in a limited number of cases has it been possible to account for the observed features on the basis of theoretical considerations.

Cryogenic magnetic properties of secondary recrystallized thin sheet dysprosium

Secondary recrystallization was observed in 0.25mm thick dysprosium sheet having final cold reductions in the range of 40-60% prior to annealing at 1150°C in a dry helium atmosphere. X-ray and etch-pit studies revealed a predominant (0001)[ <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10\bar{1}0</tex> ] secondary recrystallized texture. Magnetization curves at 4.2 K and 77 K were in good agreement with (0001)[ <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10\bar{1}0</tex> ] single crystal magnetization curves. Coercivity measured from a peak induction of 3.1 T was 1.9 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> A/m at 77K, but increased to 16 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> A/m at 4.2K measured from a peak induction of 3.3 T. Square loop behavior was observed at 4.2 K, whereas low ratios of remanent magnetization to tip magnetization were observed at 77 K. The magnetization behavior is explained by domain models consistent with the single crystal magnetization curves, the temperature dependency of the (0001) magnetocrystalline anisotropy, and Egami's concept of an intrinsic wall coercivity in dysprosium at temperatures near 0 K.

Etch Pit Studies of GaP Liquid Phase Epitaxial Layers

In investigations of the influence of defects on the luminescence properties of compound semiconductors, the relative importance of purely structural defects, e.g., dislocations, vs. imperfections of a chemical or stoichiometric nature, such as impurities, vacancies, etc., is invariably discussed. Recent photoluminescence studies on and have shown that dislocations are an important factor only if they are decorated with impurities. For the decorated dislocations only occur when there is also present another type of etch feature called a saucer or S‐pit. In the present report further comparisons of dislocations and S‐pits are made in material typical of that used in electroluminescent devices made by liquid phase epitaxy (LPE). The etch pit studies on LPE material have been examined in terms of using S‐pits as an observable in evaluating the device potential of different LPE layers.

Steady state deformation and dislocation structure of pure and Mg-doped LiF single crystals. II. Etch pit studies of dislocation structure

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Berg-Barrett X-ray topographic and etch pit studies of dislocations in tellurium single crystals

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Etch pit studies of dislocation arrangements resulting from the deformation of single crystals of copper 7.5 at.% aluminium alloys

Abstract Single crystals of copper 7.5 at.% aluminium alloys, oriented for single slip, have been deformed in tension at room temperature. Narrow bands of deformation are introduced. The densities and distributions of dislocations within the bands have been determined by the etching method. Primary dislocations of opposite sign are generated at opposite surfaces and migrate across the crystals in near-edge orientations on adjacent clusters of glide planes. This provides the first clear example of a moving forest of edge dislocations of the type envisaged by Taylor in 1934. A fraction of these dislocations are retained within the crystals in the form of extended sequences of pile-ups. Forest dislocations, generated by the activation of internal sources, are revealed by etching sections parallel to the primary planes. They occur within thin disc-shaped volumes with irregular contours. All the secondary systems may be activated within these volumes and the resulting distributions introduce effective obstacles to the further propagation of primary dislocations. The yield stress is the stress required for the activation of surface sources. The activation of internal sources requires the superposition of internal stresses, due to dislocation configurations, on the resolved component of the applied stress.

Glide Band Formation in Silicon

Birfringence and etch pit studies of the initial stages of plastic flow in octagonal-shaped silicon crystals oriented for single slip and compressed at low strain rates in the temperature range 750–800 °C show characteristics quite similar to that found for avalanche behavior and ``Luders-band'' type flow in alkali-halide crystals. The narrow glide bands taper to a single plane at opposite ends and the dislocation morphology, as revealed by their phase retardation birefringence characteristics, is in the form of large concentric loops on neighboring planes with staggered sources in the central region of the band. Calculations show that the stress field from dislocation arrays enhances the formation of neighboring arrays by double cross slip and multiplication, and show how different kinds of defects can form. The enhanced shear stress at the cross-slipped segment is significantly greater if the initial source operates on the surface. The effects of partialized dislocations and various dynamic effects which affect the begining and cessation of glide band formation and broadening are shown to be consistent with avalanche behavior.

On the formation of fatigue striations

Thin foils of an aluminium alloy containing the fracture surface were studied in the electron microscope. The fatigue striations were visible due to thickness contrast and bands of dislocations. Supplementary etch pit studies and stereographic measurements were carried out. In many cases the crack plane was close to (100) or (110). The mechanism of fatigue crack propagation is discussed in the light of the new evidence.

Initial dislocation distributions in tungsten fibre-copper composites

Etch-pit studies of composites consisting of small volume fractions of tungsten wires into which a copper matrix had been introduced by vacuum infiltration showed the dislocation density in the single crystal matrix to increase with volume fraction of fibre and to rise to high values toward the fibre-matrix interface. The dislocation distributions, which resulted from strains arising due to differential shrinkage, were interpreted in terms of a contribution, independent of fibre fraction, due to transverse strains which fall off with distance from the fibre and a second one due to axial strain which increases with fibre fraction but is invariant with distance from the fibre. Part of the axial adjustment is accomplished by interface shear.

Steady-State Flow in Marble at 500° to 800°C

The stress-strain behavior of Yule marble, both parallel and normal to the foliation, has been determined in a series of constant strain-rate extension tests at 5000 bars confining pressure, at temperatures of 500° to 800° C and at strain rates ranging from 10−3 to 10−7/sec. These data correlate well with earlier results at 25° to 500° C and l0−1 to 10−8/sec. Steady-state flow is dominant at 400° C and rates <10−7/sec, at 500° C < 10−4/sec, and at all strain rates at higher temperatures. Transient flow (work hardening) is prevalent at lower temperatures and higher rates. Transmission and reflection microscopy as well as etch-pit studies reveal that translation gliding on r, f and twinning on e are dominant in the transient region, becoming less prominent in the steady-state region at high temperatures. Polygonization by dislocation climb becomes characteristic in the steady-state flow region, along with intergranular and intra-granular recrystallization. Grain boundary sliding is present, but of minor importance. The effect of temperature and strain rate on the mechanical behavior, as well as on the correlation of subgrain formation and recrystallization with steady-state flow, is consistent with a diffusion-controlled process. Comparison of the data with several models for diffusion-controlled flow show poor correlation with the Eyring equation, e = A exp ( −E/RT) sinh (Bσ), but excellent correlation with the Weertman equation, e = C exp ( −E/RT) <σN. The activation energy E and the stress exponent N are nearly identical for both orientations of this highly anisotropic marble, and thus this material becomes nearly isotropic (mechanically) for steady-state flow. On the basis of the empirical-flow equation, extrapolations to representative natural strain rates of 3 × 10−14/sec predict stresses of 103 bars at 250° C, ranging to about 30 bars at 700° C. Work-hardening can be expected at these rates at temperatures <250° C; in this region, this marble is expected to be mechanically anisotropic. Calculated equivalent viscosities at natural strain rates range from 3 × 1022 poises at 25° to 4 × 1020 poises at 700° C.

Ultrasonic Study of Creep in Sodium Chloride

The attenuation and velocity change of a 10-MHz sound wave and the shear strain ε have been measured simultaneously during transient creep in single crystals of sodium chloride. Quantitative determinations of the dislocation den ity A and average dislocation segment length L were obtained from the ultrasonic measurements. A was found to be approximately proportional to ε, and during a given run, L went through a maximum several minutes after a run was started. A model which meets all the limitations imposed by the simultaneous ultrasonic and strain measurements is proposed, according to which transient creep in sodium chloride proceeds by a two-step process. First, edge dislocations move quickly away from the dislocation source, giving an elongated loop composed primarily of two screw segments. The slower screw portions of the loop then move to give the observed strain. The velocity of the screw dislocation thus determines the strain rate ε̇. This model is supported by an interpretation of directly observed shapes of dislocation loops in alkali halide crystals. As the increase in the internal stress σG, determined from the increase in A during a run, was found insufficient to account for the decrease in ε̇, the mobile screw-segment length Ls must decrease. An atomistic theory of transient (logarithmic) creep was developed from this finding. From the theory, the screw-dislocation-segment length Ls was found to be the same order-of-magnitude as that found ultrasonically, and the derived screw-dislocation velocity was found to be in good agreement with that determined independently from an etch-pit study.

Pressurization effects in chromium

Abstract Microstructural changes resulting from pressurization treatments have been studisd using transmission electron microscopy and an etch-pit technique; details of the etch-pit technique are given in an Appendix. ‘Free’ dislocations are produced at Cr2O3 particles in the chromium matrix due to the difference in compressibilities. Diffraction contrast analysis shows that the dislocation loops are prismatic and of vacancy type; their number is in reasonable agreement with strain calculations using compressibility data. The effect of pressurization on the tensile load/elongation curves for single crystals has been determined. The magnitude of the yield drop is decreased and the hardening rate for ‘single-slip’ orientations is increased, showing that the numerous pressure-induced dislocation sources lead to multiple slip from the onset of plastic flow. Etch-pit studies of the effects of various repeated pressurization and pressurization-ageing cycles correlate with changes in tensile characteristics.

Etch-Pit Studies of the Dislocation Structures Developed during Creep Deformation of Copper Single Crystals

Dislocation arrangements and substructure formation due to high-temperature creep deformation were investigated in copper single crystals by means of the etch-pit technique. Etch-pit observation was made on the surfaces parallel to the primary slip plane, (111), and to the critical slip plane, (\bar111).At an early stage of transient creep, in some regions, which will be called “region A”, subgrains elongating in the direction of the deformation band (10∼20μ×50∼200μ in size) were seen in the (111) observation. Cell structures (cell size : ∼100μ) were observed in the other regions, which will be called “region B”, adjacent to region A. Regions A and B were bordered along the plane of the deformation band, (\bar101). With the progress of transient creep deformation, the width of the long subgrains in region A increased and the cells in region B changed into well-developed subgrains, decreasing in their size. At the end of transient creep the width of the long subgrains in region A and the size of subgrains originating from the cell structures in region B were 20∼40μ and ∼50μ, respectively. The facts show that the structures in regions A and B tend to become similar with creep deformation. During steady-state creep, little change occurred in the shape and size of the substructures.Many primary dislocations were observed within the subgrains and the cells in transient creep. The primary dislocations, which were generated during instantaneous elongation, decreased in transient creep and increased slightly in steady-state creep.

The morphology and defect characteristics of vertically pulled MgAl2O4 single crystals

Optical, etch pit and X-ray topographic studies of magnesium aluminate spinel (MgAl2O4) have shown that undoped single crystals vertically pulled from stoichiometric melts can exhibit a high degree of crystalline perfection. The principal defect is strain arising from the formation of {111} facets on the solid/liquid interface. This effect is inhibited by suitable control of interface shape, but is greatly enhanced by the presence of chromium which segregates non-uniformly between the facets and the bulk of the crystal. Chromium also causes strain in the impurity-rich bands corresponding to growth striations.

On the slope of etch pits

It is pointed out, by means of a literature review, that etch pits formed at the sites of singular defects in an otherwise slowly dissolving surface are usually shallow, composed of faces misoriented from that surface by only a few degrees. An etch pit study of the (0001) zinc surface dissolved in alcoholic hydrochloric acid solutions supports the theory that the slopes of etch pits are controlled by the dissolution kinetics of the crystal. Pits widen at a rate independent of the type of defect attacked, but the slopes are dictated by the relative rates of dissolution at the defect sites.

Single-crystal growth from the vapour and etch pit studies in arsenic

Strain-free single crystals of the rhombohedral (α) semimetallic form of arsenic have been grown from the vapour. Large crystals can only be grown when the deposition temperature is above a critical value of about 700°C. Temperature fluctuations must be kept within 0·2 degc to prevent development of a severe mosaic structure. Dislocations have been examined by etching techniques. Either trigonal or hexagonal pits may be produced at the same site of emergence of dislocations on the (111) cleavage planes; the etch determines the pit shape.

High‐Temperature Creep and Dislocation Etch Pits in Polycrystalline Beryllium Oxide

Compressive creep of high‐density polycrystalline beryllium oxide was investigated in the range 1850° to 2050°C. Creep rate was dependent on the applied stress to the 2.5 power, and the apparent activation energy for creep was 145 kcal/mole. Etch pit studies showed that the dislocation density in tested specimens was two orders of magnitude greater than that in assintered material. The diffusion process controlling creep was ascribed to volume diffusion of the anion. The deformation behavior was governed by dislocation motion.

Morphological features of hexagonal ferrites

Depression growth spirals and vicinal-hill type spirals have been discovered on basal faces of single crystal hexagonal ferrite samples. Electron microscopy studies have resolved sub-unit-cell etch steps on “process etched” and on HCl etched surfaces. A variety of crystalline imperfections have been noted and etch pit studies have indicated the number and kinds of dislocations present.

Thermal‐Mechanical History and the Strength of Magnesium Oxide Single Crystals: II, Etch Pit and Electron Transmission Studies

Mechanical tests show that the room‐temperature flow strength of magnesia single crystals varies with heat treatment. Flow strength depends on the size, density, and distribution of precipitate particles which inhibit the motion of dislocations. The present paper describes optical and electron transmission microscope evidence for a precipitation reaction in magnesium oxide. The observations correlate fairly well with mechanical test data. Mechanical tests also show that mobile dislocations are locked by heating above 600°C. The room‐temperature yield strength increases with time and temperature particularly when the aging temperature exceeds 1000°C. Studies to determine the nature of this locking mechanism are described. Etch pit studies show that dislocations aged above 600°C etch at a slower rate than fresh dislocations. This corresponds precisely with the change in mechanical behavior and both effects are attributed to impurity diffusion to dislocation lines. Electron transmission studies indicate a redistribution of point defects, left by moving dislocations, above 600°C. Eventually these result in a change in dislocation configuration which is considered responsible for the strong locking observed above 1000°C.