Dislocation Etch Pits Research Articles

Mechanisms of structure transformations in thallous nitrate crystals

Geometrical and kinetic factors in polymorphic transformations of single crystals of thallous nitrate have been investigated by optical and X-ray methods. The transformations ortho­rhombic ⇌ rhombohedral can proceed by two distinct mechanisms with different kinetics. The slower kinetics are associated with a mechanism which maintains a definite orientation relation and causes formation of plates of the new phase: it is closely similar to martensitic transformation in alloys. Dislocation etch pits can be produced at the interface. Nucleation rates for the other, ‘reconstructive’, mechanism were predominantly time-dependent in superheating, and temperature-dependent in supercooling. Nucleation appears to occur at defects. A transformation from orthorhombic to cubic can occur in good crystals at 126·5°C and proceeds by a third, continuous mechanism, despite the similarity of the stable rhombohedral structure to the cubic. It appears that the choice of a transformation mechanism can depend on amount of imperfection and can be quite sensitive to small changes in the difference between the structures.

Plastic Deformation of CsI Single Crystals

AbstractThe plastic deformation of CsI single crystals by compression is studied. Different regions of the stress‐strain curve are compared with the distribution of birefrigence bands along slip lines and with the arrangement of dislocation etch pits. The various stages of uniform deformation and of kink‐band formation are considered in detail. It is found that both diffusion and slipping processes occur, even during linear work hardening, and this leads to polygonisation. After the kink‐band formation stage not only polygonisation, but also recrystallisation is observed, which indicates an increase in the diffusion process.

Growth of Seeded Copper Crystals of Perfection Comparable to Unseeded Crystals

A graphite mold of a new design was made for growing better seeded crystals from the melt. The main features were (1) a narrow seed constriction, (2) a split mold, (3) high density graphite, (4) equal cross sections of seed and crystal, and (5) high ratio of sample to mass crucible mass. The resulting crystals were compared by two complementary techniques with those grown in a conventional mold. An improvement of at least a factor of ten was obtained. The best crystals thus far have 1−2×104 etch-pit sites/cm2 intersecting the {100} surfaces, and are free from subboundaries. This brings the level of perfection of as-grown seeded crystals up to that of unseeded ones thereby making them useful in dislocation etch-pit studies.

Dislocation Etch Pits in GaAs

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Dislocation energy and etch pits orientation on diamond octahedral faces

Dislocation energy and etch pits orientation on diamond octahedral faces

Orientation of Stacking Faults and Dislocation Etch Pits in β ‐ SiC

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Dislocation etch pit studies in annealed and deformed polycrystalline niobium

An etch pit technique has been used to study the dislocation structures in annealed polycrystalline niobium and after plastic deformation in tension at room temperature. The etch-pit density due to grown-in dislocations was found to decrease markedly with increasing annealing temperature. On tensile stressing, plastic flow was observed considerably below the upper yield point, and evidence of enhanced slip activity at grain boundaries suggested that most dislocation sources were located in these regions. The spacing of etch pits along individual slip planes were generally irregular but classical type dislocation pile-ups against grain boundaries were sometimes detected in the microstrain region. The grown-in dislocations as revealed by etching did not appear to participate in yielding, probably because they were too strongly locked by interstitial impurities. Many aspects of the etch pit patterns seen in niobium in this study were similar to previous observations made on Fe-Si alloys 3,12.

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.

A STUDY OF ETCH-FIGURES OF DISLOCATION LINES IN MOLYBDENUM CRYSTALS

实验结果表明,应用甲醇、硫酸、盐酸的混合液为电解浸蚀剂,可以在钼晶体的{100},{111}及{110}面上显示位错蚀斑,而在{111}面及{110}面上同时可以显示出排列成平行线或六方网络的蚀线,这些蚀线被证明为位错线的蚀象。根据观测结果,总结出解释位错线蚀象的经验规律:观测到的蚀象相当于一定深度内位错线在观察面的投影;蚀象的宽度决定于位错线段到原始表面的距离,距离愈远,宽度愈细,类似于一种夸张的光学透视效应,因而根据蚀象就可以直接推出位错线在空间的位置,采用多次浸蚀的方法,对于一些位错空间排列组态的实

Dislocation distributions in deformed copper single crystals

Abstract Dislocation etch pit distributions and slip lines were studied on copper single crystals oriented for single glide and deformed in tension at 4·2°K. Slip lines on the cross slip plane were observed from the earliest stages of deformation and their character changed during the transition to stage II. The distribution of forest dislocations is much more isotropic and less homogeneous than that of dislocations crossing other planes. The ratio of the forest to other dislocation densities in stage I is about 1:15 but tends to 1 in stage II. The flow stress correlates better with the forest than with main glide dislocation density; twist experiments support this conclusion.

INTERNAL FRICTION DUE TO DISLOCATIONS IN MOLYBDENUM AT KILOCYCLE FREQUENCY

用Marx方法(频率为86kc/s及102kc/s)测量了区熔提纯钼单晶及工业纯钼多晶的内耗振幅依赖关系。钼单晶在2000℃氢气炉中退火后可测得Granato-Lücke型的内耗-振幅曲线。微量冷加工后内耗增大,并使内耗振幅曲线上出现转折平台(或称极大值),与铝在低温低振幅下测得的结果相似。根据位错蚀坑的观察结果,初步认为此转折平台的出现是与冷加工在晶体中产生的未被钉扎的新位错有关,并求得转折平台处内耗的增值与新位错密度成正比的关系。此外还做了温度和时效(200—300℃)对内耗振幅曲线的影响,随着温度的降低和时效时间的增长,内耗明显下降。以上的结果我们用推广的GL理论进行了讨论。测量了冷加工对工业纯钼多晶的内耗振幅曲线的影响,基本符合GL理论。

BaTiO3 Single Crystals Subjected to Mechanical Impulses

Etching with fusion of NaOH delineates dislocations in the BaTiO3 crystals and by this use, the behavior of mechanically-treated crystal is investigated. When the crystal is subjected to a pointed impulse, a characteristic a-domain pattern appears near the point of contact. Annealing the mechanically-shocked crystal above 1000°C removes the pattern, while two parallel columns or parallel crosses of dislocation etch pits appear. It is supposed that the elastic strain may be released by taking place of a plastic flow upon heating. The part surrounded by parallel columns exhibits an anomalous birefringence, almost unchanged across the Curie point.

Grown-In Dislocations in Calcium Tungstate Crystals Pulled from the Melt

Etch pits on (001) surfaces of calcium tungstate crystals, formed on treatment with hot dilute solutions of hydrochloric acid, were identified to be dislocation etch pits. The technique is able to follow the motion of dislocations during annealing by the formation of new sharp-bottomed pits adjacent to flat-bottomed pits at the initial dislocation positions. Inclusions, both solid and gaseous, are an important source of dislocations in crystals of calcium tungstate grown from the melt.

Sliding and Wear in Ionic Crystals

Wear tests by sliding single crystals of calcium fluoride, sodium chloride, magnesium oxide, and lithium fluoride on their natural cleavage faces beneath a weighted indenter are described. The slip systems responsible for the resultant deformation were determined and details thereof studied microscopically with the aid of etchants to reveal dislocation etch pits. In calcium fluoride, the structure of the material in the wear track is interpreted as a damage structure called ``friction damage.'' Etching revealed clusters of point defects left behind by moving screw dislocations. Subsequent annealing in inert atmospheres produced microscopically visible cavities a short distance below the wear track. Their possible origin and role in surface deterioration and wear are discussed. In calcium fluoride, surface cracks emanating from the wear track lie along ordinary cleavage faces; in rock-salt-type crystals, however, they are interpreted as dislocation cracks on the dodecahedral planes.

On kink kinetics in crystal dissolution

A new acidic etchant, consisting of hydrofluoric acid and ferric fluoride, is described for producing dislocation etch pits on lithium fluoride. The geometry of pits produced by solutions containing concentrated hydrofluoric acid, ferric fluoride and water has been investigated by interference microscopy. These observations, with others of independent investigators, substantiate basic postulates concerning the effect of the etchant properties, undersaturation and inhibitive power, on the nucleation and subsequent motion of kinks in crystal ledges. At the present time, the interdependence of the environmental parameters allows only a qualitative description of their effect on crystal dissolution.

Etch Patterns and Dislocation Etch Pits on Germanium with KI-I2 Redox System

Dissolution of Ge proceeds through the oxidation of Ge by iodine. The etching rate is several hundred µg/cm2hr with the activation energy being approximately 0.22eV/molec. and it is in the order: {100}>{110}>{111}. The etchant develops three-, two- and four-fold symmetrical patterns, bounded by {111} planes of the slowest etching rate, on the surfaces {111}, {110} and {100}. Several varieties of etch pits show the anomalies of dislocation.

Domain Wall Caught in Dislocations in Ferroelectric Glycine Sulfate Crystals

Simultaneous observation of both dislocation etch pits and ferroelectric domain walls was made on b-face of ferroelectric glycine sulfate crystals by etching technique with water. Negative end of the domain is pebbled while positive end is smooth. The dislocation etch pits are well discernible from the “pebbles” on the etched face. It was revealed that under applications of a d.c. electric field of about 30 v/cm a domain wall is caught in dislocations during the domain wall motion resulting in zig-zag shape. Evidences were obtained that also in as-grown crystals domain walls were caught in dislocations. It is assumed that dislocations in which a domain wall is caught are edge dislocations emerging out of b-face.

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.

A new method for the study of dislocation etch pits

A new method of etching is described. A comparative study of etch pits produced by the new method of etching as well as the usual method is made. It is found that the pits grow more in depth when etched by the dynamic method than when etched by the static method. It is observed that the structures of the pits produced by these methods also differ slightly. The depths of the pits have been measured and it is established that in the dynamic method of etching the development of pits takes place more in depth than in extension. It is shown that the surface defects extend only up to a few molecular layers and therefore, when etched, the pits nucleated at these defects develop only in extension with increase in the time of etching and ultimately interefere with one another, thus producing an effect as if they are washed away. The pits nucleated at the dislocations penetrate deeper into the body of the crystal. Perfect correlation has been established in regard to the individual isolated pits produced on the matched faces even after polishing both of them. And thus the dislocation etch pits are clearly distinguished from the etch pits nucleated at other surface defects in a characteristic manner.

Plasticity and Dislocation Etch Pits in CaF2

Plasticity and Dislocation Etch Pits in CaF₂