Etch Pit Technique Research Articles

X-Ray-Induced Dislocation Generation in Sodium Chloride

The strain distribution, induced by subjecting one-half of a sodium chloride single crystal to x rays while shielding the other half, has been investigated with a dislocation etch pit technique. Due to the expansion of the irradiated half, a strain gradient arises antisymmetrically about the plane of the interface that is sufficient in magnitude to cause plastic deformation in the protected half, but not in the irradiated half of the crystal. The plastic deformation takes the form of a number of high-density dislocation bands which tend to align in a [100] direction. The bands increase in number but not appreciably in size as irradiation proceeds. The absence of plastic deformation in the irradiated half can be explained by the fact that irradiation raises the yield strength to such a level that the strain is not sufficient to induce plastic flow. In the protected region, however, dislocations are generated and glide until they impinge, causing discrete bands to form roughly parallel to (100) planes. From an analysis of the data, an estimate of the magnitude of the strain produced at the interface and the relationship between the strain and the number of F centers generated can be obtained. The results, although in some ways unexpected, are consistent with previous experiments performed on x-irradiated alkali halides.

Calcium Tungstate. II. Observation of Dislocations

An etch-pitting technique is described for revealing the points of intersection of dislocations with the (100) and (001) surfaces of calcium tungstate single crystals; each etch pit represents one dislocation. The dislocation configuration that is responsible for the residual strain in as-grown ``a'' axis crystals and the mechanism of its growth from the melt are discussed. Preliminary results concerning the distribution of impurities in doped calcium tungstate crystals and the effect of doping on dislocation density and distribution are described. The solution behavior of calcium tungstate in various solvents and a chemical polishing procedure for calcium tungstate are presented.

Fracture on (110) Planes in LiF

A study was made of (110) fracture in LiF using birefringence and etch-pit techniques. Fracture always occurred parallel to birefringence bands, and it is suggested that the stress of these bands aids (110) crack propagation. Surfaces formed by (110) fracture appear to be flat. If (100) steps are present, as suggested by Kuznetsov, they are less than 100 Å in height. By comparing etch-pit density data with birefringence data, it is estimated that only about one in twenty etch pits was due to single, unpaired dislocations. All the rest were probably caused by dipoles.

X-Ray Studies of Epitaxial Copper Electrocrystallization

With the use of x-ray diffraction and etch-pit techniques, the imperfections in electrodeposited copper single crystals were compared to corresponding substrate imperfections. Areas of local polycrystallinity in the deposit were found to be associated with areas of misorientation in the substrate. The deposit perfection is influenced by the interaction of both dislocations and impurities in the substrate. With special purification of the electrodeposition solution, a deposit could be produced with a degree of perfection higher than that of the substrate.

Dislocation Mobility in Germanium

The stress and temperature dependence of the glide velocity of individual dislocations in pure germanium has been measured by means of an etch-pit technique. The motion may be described by a thermal activation energy which, for 60\ifmmode^\circ\else\textdegree\fi{} dislocations, varies from 1.49 eV at 8.0 kg/${\mathrm{mm}}^{2}$ to 2.25 eV at 0.8 kg/${\mathrm{mm}}^{2}$. For screws, it is 1.47 eV independent of stress. At constant temperature, the velocity varies with stress in a complex manner which is not adequately accounted for by previous theories alone.

Etch Pit Studies of Dislocations in Copper Crystals Deformed by Bending: I. Annealed Crystals; II. Irradiated Crystals

Annealed and irradiated copper crystals of low initial dislocation density were deformed by bending, and dislocation multiplication and motion were studied using an etch pit technique for observing the dislocations. For the annealed crystals there was dislocation motion and multiplication prior to macroscopic yielding, and the percentage of grown-in dislocations moved by the stress increased with the stress. The dislocations were hindered in the motion by subboundaries and by interaction with other dislocations. Relaxation effects were observed and were much greater for stresses less than the yield stress than for higher stresses. For the neutron-irradiated crystals there was no motion or multiplication of dislocations prior to yielding. The slip traces were long, with dislocations closely spaced in the trace. Although not conclusive, the evidence apparently was best interpreted in terms of source hardening. A cube-root dependance of the yield stress on neutron dose was observed. No hardening resulted from irradiation with 1018/cm2 cobalt γ rays.

Dislocations in Rutile as Revealed by the Etch‐Pit Technique

Potassium hydroxide and sodium hydroxide were found to be satisfactory for polishing various surfaces of rutile. Dislocation etch pits could be produced by alkali fusions, 85% orthophosphoric acid, and concentrated sulfuric acid. The {l 10}‐, near {111}‐, (001)‐, and {100}‐type surfaces were etched and the etch pits were analyzed in terms of their relation to the deformation systems and crystal symmetry. The correlation between etch pits and dislocations was substantiated by means of matched cleavage surfaces, existence of substructures, generation of dislocations, dislocation densities, and polygonization phenomena. Dislocations were introduced by impacting and macroscopic plastic deformation. Generation of dislocations was observed from 1050°C to as low as room temperature. The slip planes confirmed by means of dislocation traces and slip lines were {110}‐and {101}‐type planes. The slip direction corresponding to the {110)‐type plane was [OO11, the close‐packed direction. The Burgers vector, structure, and motion of dislocations in the edge orientation on the {110) [001] system were determined by analyzing the crystal structure. The Burgers vector was c [001], the lattice translation vector, and the dislocations did not dissociate into partial dislocations. Another possible slip system was the (100) [010] system.

Mobility of Edge Dislocations on {112} Slip Planes in 3.25% Silicon Iron

The possibility that {112} planes in 3.25% silicon iron are active slip planes is investigated. This is found to be the case for certain axial orientations when the deformation temperature is above approximately 184°K. Below this temperature, the {110} plane is the observed operative slip plane. Utilizing a dislocation site etch-pitting technique and a method of introducing dislocations into the material devised by Stein and Low, measurement is made of the velocity of edge-type dislocations on {112} planes in 3.25% silicon iron single crystals. The velocity of these dislocations is found to be very sensitive to the applied stress in the range of velocities examined which is from 10−7 to 10−2 cm/sec. Three temperatures of testing are investigated: 198°, 233°, and 298°K. A method for the determination of the operative slip plane in 3.25% silicon iron as a function of the deformation temperature and the orientation of the deformed samples is proposed. This results in the observation that the stress to cause any given dislocation velocity increases more rapidly with lowering deformation temperature when slip occurs on {112} planes rather than on {110} planes.

Role of Surfaces in Plastic Flow of NaCl Single Crystals. II

The effects of surface sources, ozonized surface films, and surface dissolution, on the nature of plastic flow in NaCl single crystals were studied using etch pit and birefringence techniques. Slip steps increase in depth and become rounded as the slip band broadens. This is also often accompanied by a bending of the steps as much as 3° off 〈100〉. These effects are explained in terms of both internal and surface cross-slip mechanisms. Ozonization of NaCl for short periods restricts the action of surface sources and for longer times provides barriers to the egress of dislocations. The ``Suzuki effect'' (formation of a work-hardened surface layer) is found to depend upon the existence of a multiplicity of surface sources such as is produced by polishing on wet silk. Cross-slip of screw dislocations at the surface is found to be greater and flow more homogeneous when the surface is dissolved during deformation. This is proposed as a likely mechanism to explain the ``Joffe effect'' when atmospheric variables are eliminated.

Dislocations and Plastic Flow in NaCl Single Crystals. I

Dislocation multiplication and motion in NaCl was studied using etch pit and birefringence techniques. The observations show that dislocations in glide bands of NaCl are in the form of large concentric loops with sources staggered in the center of the bands. They propagate and multiply by cross-slip and are braked through intersection jogs. The slip distance parallel to b is estimated to be 2–3 mm and that normal to b 0.5–1 mm, at a stress of 30 g/mm2 in crystals having 5×104 dislocations/cm2. ``Deformation bands'' form normal to the operating system during the latter part of stage I by ``glide polygonization'' of edge dislocations on closely spaced slip planes. These limit dislocation motion to shorter distances in stage II. Stage III appears when significant flow occurs on a secondary system oblique to the primary operating one.

On the Yield Stress of Copper Crystals

99.999% copper crystals were deformed in tension using an Instron tensile tester, and the dislocation density and arrangement in the crystals were determined before, during, and after the deformations using an etch pit technique. For crystals of low initial dislocation density, it was found that a large amount of dislocation multiplication occurred prior to yielding. Experimental relationships of dislocation density versus applied stress and versus shear strain were determined. It was found that the yield stress was not related to the initial dislocation density or arrangement. The yield stress was postulated to be determined by the stress necessary to break the gliding dislocations through impurity atom barriers in the crystal.

The density and distribution of dislocations in deformed copper crystals

The density and distribution of dislocations in copper crystals deformed in tension have been studied using etch-pitting techniques. The dislocation density is found to be roughly proportional to the square of the flow stress and in quantitative agreement with other experimental and theoretical results. The dislocations are distributed in a rough cell structure after sufficient strain. Interaction between primary and secondary dislocations leads to the formation of dense dislocation networks, which grow and coalesce to form the boundaries of the cell structure. Glide polygonization occurs in large regions of lattice curvature where dislocations of one sign accumulate. The kink bands that separate regions of reverse curvature appear to originate at subboundaries that are effective dislocation barriers.

Dislocation Etch Pits on the Basal Plane of Zinc Crystals

An etch pit technique has been developed that reveals the sites of dislocations which intersect the basal plane surface of zinc crystals. Pits can be formed by either Cl2, HCl, Br2, HBr, or HI dissolved in an appropriate organic solvent. Etchants made with Cl2 or HCl produce hexagonal pyramidal pits whose edges are parallel to 〈101̄0〉 directions; but Br2, BHr, or HI etchants produce hexagonal pits whose edges are parallel to 〈112̄0〉 directions. The technique is useful for observing glide which occurs on nonbasal planes. The dislocation etch pits are seen to form a ``polygonization'' pattern in a crystal that was annealed after deformation. A discussion is presented on the mechanism of etch pit formation and the factors which might determine the shape of the pits.

Elastic-Plastic Transition in Copper Crystals as Determined by an Etch-Pit Technique

Copper (99.999%) crystals with a dislocation density of 50/mm2 have been prepared. These crystals were stressed by applying a pure bending moment, and they were etched with a dislocation etch either before and after or while the stress was applied. The motion of dislocations was determined by observing the size and nature of the dislocation etch pits. The resolved stress necessary to move grown-in dislocations was about 4 g/mm2. Examples of dislocation motion under stress, then return motion when the stress was removed, and of multiple motion under stress were observed. Multiplication of dislocations occurred at a resolved stress of about 18 g/mm2. The observed phenomena are discussed in terms of simple dislocation theory.

Non-basal slip and twin accommodation in zinc crystals

An etch pit technique was used to observe dislocation patterns on the basal plane of zinc monocrystals. It was confirmed that at room temperature non-basal slip occurs on the {112̄2} pyramidal planes. Observations were made on {101̄2} mechanical twins that go partially through the crystal. Such twins often terminated in a group of slip bands at 90° and 30° to the twin trace; the orientation of these slip bands is consistent with pyramidal slip. At other twins a row of dislocations emanates from the tip; these dislocations lie on the {1012} twin habit planes. When a crystal is annealed, the twins tend to become parallel-sided, and they recede leaving a trail of dislocations in the matrix crystal.

The relationship between plastic flow and the fracture mechanism in magnesium oxide single crystals

Abstract Magnesium oxide crystals plastically deformed under a three-point load develop internal slits coplanar with the (110) slip plane and parallel to the [001] bending axis in the tension region. Using etch pit techniques it has been shown that the slits lie along the edge of a (110) slip band and are confined between two adjacent orthogonal (110) slip bands. It is proposed that under certain circumstances edge dislocations moving in one of the (110) slip planes pile into the barrier provided by a wider (110) slip band and coalesce to form a slit nucleus along the lines of intersection. This nucleus lies parallel to the (110) plane and grows by cleavage over this plane until it meets another (110) slip band where it becomes halted. Thus cracks can be nucleated where slip bands intersect but, more important, they can also be stabilized by slip bands. If microcracks form in the early stages of plastic flow their growth is unrestricted and the material is brittle. If the crystal is first plastically defo...

The dislocation structure of slip bands in iron

The dislocation structure of slip lines in silicon iron has been examined by an etch-pit technique. It has been observed that the arrays when viewed normal to the edge component are straight but not coplanar over long distances while the arrays where the screw component emerges from the surface show much curvature. In bend specimens the increase in strain from the neutral plane toward the surface is accomplished mainly by the lateral growth of the first bands formed rather than by the formation of new slip bands. Observations made in the slip plane itself demonstrate that the motion of the screw components is considerably more limited than that of the edges. Evidence is presented for the existence of interaction between the edges and screw components of adjacent loops in agreement with the theoretical predictions. The observations can be most adequately explained by the double cross-slip model of the formation of a slip band. Evidence is presented for the transfer of portions of a loop from one plane to another as this model requires.

On the formation of dislocation substructure during growth of a crystal from its melt

Dislocation networks in as-grown lithium fluoride crystals were studied by etch pit and decoration techniques in an attempt to obtain experimental evidence concerning the origin of grown-in dislocations. Several mechanisms for formation of new dislocation loops at or near a growing crystal face were considered. The kinds of distribution observed, and the insensitivity of the grown-in networks to the growth rate and temperature gradient in the solid, were considered to be consistent with the hypothesis that stresses are responsible for most of the grown-in dislocations.

Arrangements of Dislocations in Plastically Bent Silicon Crystals

Dislocations introduced into single crystals of silicon by plastic bending at an elevated temperature have been studied quantitatively by the etch-pit technique. The average etch-pit density after deformation is approximately two to three times higher than the calculated dislocation density. Prolonged annealing of the deformed samples at a temperature close to the melting point results in polygonization or the alinement of etch pits perpendicular to the active slip planes. Satisfactory agreement was obtained between the densities of etch pits in the annealed and polygonized specimens and the predicted dislocation densities. Piled-up horizontal arrays of etch pits have been observed in lightly bent samples. The value of yield stress obtained from the geometrical distribution of these arrays is an order of magnitude below the reported experimental values. Dislocation lines in single crystals deformed at 1000°C have been revealed by the technique of copper decoration. Examination of the decorated samples by infrared transmission microscopy revealed that the dislocation lines lie in the [211] direction.

On the Polygonization of Rock Salt Crystals

Ionic crystals can be plastically bent under water to a great extent. This phenomenon has been known as Joffe effect, the mechanism being not fully elucidated. In the present work the X-ray diffraction method and etch pit techniques have been used in order to investigate the arrangement of crystallites in the bent and annealed rock salt. Both results show, with good correspondence, that the crystal consists of many polygonized crystallites which have the form like letter L , with small angle boundaries between the neighbours. The mean thickness of these crystallites was about 0.02∼0.03 mm, when the radius of curvature of bending was about 20 mm.