Helical Dislocations Research Articles

Highlights in the history of X-ray topography 1

Abstract X-ray topography provides a non-destructive method of mapping point-by-point variations in orientation and reflecting power within crystals. The discovery, made by several workers independently, that in nearly perfect crystals it was possible to detect individual dislocations by X-ray diffraction contrast started an epoch of rapid exploitation of X-ray topography as a new, general method for assessing crystal perfection. Another discovery, that of X-ray Pendellösung, led to important theoretical developments in X-ray diffraction theory and to a new and precise method for measuring structure factors on an absolute scale. Other highlights picked out for mention are studies of Frank-Read dislocation sources, the discovery of long dislocation helices and lines of coaxial dislocation loops in aluminium, of internal magnetic domain structures in Fe-3 wt.% Si, and of stacking faults in silicon and natural diamonds.

Relationship between nanostructure and dielectric response of lead scandium tantalate — (I) structure and domain textures

Three ceramic preparations of lead scandium tantalate were chosen for the present study because they exhibit three different relaxor-type dielectric response behaviours. Two preparations, made in lead-rich atmospheres, so as to conserve virtually 100% occupancy of the perovskite (ABO 3) A sites, were annealed or quenched so as to yield extreme values for the long-range order parameters ( s) for (Sc, Ta) ordering on the perovskite B site; thus s = 0.93 and 0.10 for pst-o and pst-d, respectively. The third preparation, made in the classical way in the expectation that there would be significant levels of Pb atom vacancies, allowed earlier literature results for the dielectric response to be reproduced (sample pst-def; s = 0.20). Results of high-resolution and dark-field transmission electron microscopic studies and electrical property measurements are compared in order to elucidate, as directly as possible, the structure-property relationships. In particular the chemical and polar domain structures are revealed and the unusually high density of chemical domain wall (CDW) and polar domain wall (PDW) structures characterized. New types of extended defects are found in the Pb-deficient preparation, including edge, screw and helical dislocations and small (ca. 2 nm diameter) void-like defects. There is also HRTEM evidence for atomic disorder, probably due to Pb site vacancies. The different degrees of ordering of the B-site cations Sc and Ta were also characterized by HRTEM, as evidenced by chemical domain textures. Direct evidence for Pb-atom displacements was obtained for pst-o and pst-d, allowing polar nanodomain textures to be examined at atomic resolution and the local direction and sense of the polarization vector to be characterized. The results of Monte Carlo and next-nearest-neighbour Ising model simulations for the evolution of the chemical domain textures are compared with the experimental dark-field images. This allows several distinct types of chemical defect clusters to be characterized, the existence of both charged and neutral defects are implied by the simulations. A discussion of the role played by such defects in producing random local fields and finally determining the frequency dependence of the dielectric response completes the paper.

Effect of beryllium on age hardening, defect structure, and S′ formation in Al–2·5Cu–1·2Mg alloy

AbstractThe effect of micro additions of Be on the aging behaviour, defect structure, and S′ phase formation in Al–2·5Cu–1·2Mg alloy is investigated. It is shown that the addition of 0·15%Be significantly increases the peak hardness and overall precipitation rate for the alloy. An analysis of the kinetic data shows that S′ precipitation obeys the Avrami equation, and that the Be addition does not affect the growth dependent parameter or activation energy, but significantly increases the nuclei density dependent parameter, which is consistent with the higher density of S precipitates in the microstructures of the alloy containing Be. Analysis using transmission electron microscopy shows that Be reduces the dislocation loop density during the early stages of aging, which is attributed to the high Be–vacancy binding energy, and promotes formation of dislocation helices. The enhanced nucleation of S′, notwithstanding the lower density of dislocation loops, which stimulate S′ nucleation, suggests the operation...

Prevention of substrate melt-back by Se addition during liquid phase epitaxial growth of GaAs on GaAs-coated Si

Effects of Se addition to the Ga melt were investigated during the liquid phase epitaxial (LPE) growth of GaAs on GaAs-coated Si substrates. It was observed that the addition of about 0.005 wt% Se to the Ga growth melt effectively suppressed dissolution of the GaAsSi substrates into the melt during the initial stage of the growth, which is the most serious problem in LPE growth of heteroepitaxial semiconductor layers. Results of preliminary defect analysis revealed formation of helical dislocations within the LPE layer, which indicates excess point defect concentration in the layer probably due to the heavy Se doping. No clear evidence of precipitation was observed.

Microstructure and mechanical properties of a laser treated Al alloy

An AlCuMg alloy, Al 2024-T3, was exposed to laser treatments at various scan velocities. In this paper the microstructural features and mechanical properties are reported. As far as the mechanical property is concerned a striking observation is a minimum in the hardness value at a laser scan velocity of 12cm/s. Usually an increasing hardness with increasing laser scan velocities is reported in the literature. This remarkable property could be explained based on the microstructural features observed by transmission electron microscopy. It turned out that depending on the laser scan velocity nucleation and growth of precipitates could either be enhanced or suppressed. In addition at low scan velocities helical dislocations are observed which may contribute to an increase of the hardness. After subsequent shot peening, in all cases the formation of precipitates was observed, independent of the laser scan velocities originally applied. This phenomenon of precipitation, induced by shot peening afterwards is most striking at a high concentration of alloying elements in solid solution.

High-resolution electron microscopy of dislocations in CoSi 2 with the C1 structure

Weak-beam and high-resolution electron microscopy of dislocations in CoSi 2 with the Cl structure, introduced at room temperature by pulverization, has shown that dislocations with the 〈100〉 Burgers vector and those with 1/2〈110〉 are dissociated into partial dislocations by the following reactions: 〈100〉 1/2〈100〉 + 1/2〈100〉 on {001} 1/2〈110〉 1/4〈111〉 + 1/4〈111〉 on {110} 1/2〈110〉 1/6〈211〉 + 1/6〈121〉 on {111} Stacking faults and micro-twins have also been observed by high resolution electron microscopy. Evidence that vacancies are mobile at room temperature has been obtained by observations of helical dislocations.

Microstructure and mechanical properties of a 2091 AlLi alloy—I. Microstructure investigated by SAXS and TEM

The microstructure of the 2091 alloy (AlLiCuMgZr) alloy has been studied and compared to that of two simpler AlLi and AlCuMg alloys. The SAXS, in situ SAXS and TEM techniques have been used for heat treatments at room temperature and at 150°C. The time evolution of the size and of the volume fraction of the δ′ precipitates and of the Cu-rich GPB zones have been determined. The δ′ precipitates are shown to follow a classical Lifschitz-Slyosov-Wagner law. Their interfacial energy has been estimated. S and S′ precipitates have also been characterized, respectively on grain boundaries and on helical dislocations, by TEM.

Large 〈110〉-segmented helical dislocations in natural diamond

Abstract Helical dislocations composed of very straight 〈110〉 line segments (helical shape due to climb from original screw orientation) have been observed by transmission electron microscopy in a region of diamond where growth had occurred on non-faceted, near-{100}-orientation surfaces. Of the six 〈110〉 segments forming each helix turn, the two that are pure edge segments normal to the helix axial Burgers vector are about ten times longer than the other segments, thereby producing strong flattening of the helix in the cube plane containing the helix axis. One of 30 such helices counted in a thinned specimen area of 3 × 104 μm2 was studied in detail. Its major and minor diameters were 5 and 0·6 μm respectively. The helix axis was marked by a line of vesicles, diameters from about 20 to about 130 nm, producing matrix strain contrast attributed to fluid under pressure within the vesicles. Diffraction contrast analysis showed the helix to be of interstitial character, and the vacancy volume emitted to form ...

Radiation-induced helical dislocations in Co3Ti

Abstract Fine scale segmentation of screw superdislocations in Co3Ti is shown to arise from the transformation of these into helical dislocations upon electron irradiation. Details of this transformation are analysed under weak-beam conditions.

An in situ TEM study of nucleation and growth of the Ω phase in an Al-4.0Cu-0.5Mg-0.5Ag alloy

Addition of small amounts of Ag to Al-Cu-Mg alloy with high Cu:Mg ratios stimulates precipitation of a new phase, designated Ω, which has an orthorhombic structure and forms as thin coherent hexagonal plates on {lll} α planes. This phase significantly increases the age hardening characteristics of the alloy, particularly at elevated temperatures. The mechanism of nucleation of Ω phase is not known but is believed to be fundamentally different from that of θ’ and S’ phases which also form in these alloys. The presence of Ag and Mg only in trace quantities makes the problem of finding the actual mechanism of nucleation of Ω phase difficult.An in situ hot stage TEM study was conducted on an Al-4.0Cu-0.5Mg-0.5Ag(wt.%) alloy in order to understand the nucleation and growth behavior of Ω phase. Two types of initial microstructures were used: 1) homogenized, solution-treated and cold water-quenched, 2) as-quenched and aged for 1 hr. at 190°C. The as-quenched structure shown in Fig. 1(a) consists of a high density of small dislocation loops, some helical dislocations and a few straight dislocations. When this foil was heated in the microscope, observable precipitation occured at about 205°C.

A study on dislocation lines in an In-doped LEC GaAs crystal by IR light scattering and transmission microscopy

Defects, especially dislocations in an In-doped GaAs crystal grown along [001] by liquid encapsulated Czochralski (LEC) method, were microscopically visualized by infrared light scattering tomography to study growth history and mechanism of the crystal. Here, three types of dislocations were found: (i) Grown-in dislocations, in which helical and bent dislocations were included. (ii) Dislocations introduced within liquid-solid interfaces. Some ends of the dislocations penetrated into the just-grown ingot and slipped on the (100) planes due to thermal stresses, because the ingot was still growing with a large temperature gradient from its molten phase. The opposite end terminals remained within the interfaces at their initial stages but some of them reacted between themselves to diminish their density or to make a linkage with the terminals. The terminals and some other dislocation ends acted as origins of the grown-in dislocations. (iii) A lot of dislocations were introduced into the ingot from its outer-side surface by thermal stresses during its cooling stage after the growth.

The Formation of Helical Dislocations in Silicon Substrates During Epitaxial Deposition of β- SiC

ABSTRACTA study is presented of the dislocation content of (001) Si substrates after the formation of thin epitaxial “buffer layers” of beta-SiC by the pyrolysis of propane. Helical dislocations are observed running parallel to the interface and extending for many microns. The dislocation helices emanate from the substrate, have 1/2<110> Burgers vectors which are inclined to the interface, and do not lie on the {111} glide planes. This is consistent with the extension of dislocations in the substrate by climb. The extended segments are subsequently pinned and further climb gives rise to the observed helical configuration. These climb effects are presumably due to an excess of vacancies which are produced when Si interstitials migrate to the deposit in order to form SiC.

Surface lattice in α-keratin filaments

A model for the surface lattice of the intermediate filaments (IF) in α-keratin is developed on the basis of quantitative measurements of the distribution of intensity in the X-ray diffraction pattern. A key feature of the model is the presence of a well-defined helical dislocation in an otherwise regular surface lattice. A similar feature has recently been reported to be present in microtubules. The diffraction pattern predicted by the model for α-keratin IF was calculated and compared with the observed pattern and found to explain many of the salient features. Attempts to model the distribution of scattering matter on the lattice were hampered by the paucity of data but some key features of the distribution were uncovered using trial and error methods.

Growth induced lattice defects in Ba(NO 3) 2 crystals

Combined X-ray topographic and surface microtopographic studies were made on barium nitrate crystals grown from the aqueous solution at known supersaturations on etched and unetched seeds. The following relations were confirmed: (1) Inclusions trapped on the etched seed surface act as sites for both generating new dislocations and inhibiting the propagation of seed dislocations. (2) Higher supersaturation leads to the generation of helical dislocations even at room temperature, and development of wider planar defects. (3) Dislocations change their directions when supersaturation is changed. (4) One-to-one correlation was established between the summits of growth pyramids and outcrops of dislocations with screw components. However, not all screw dislocations outcropped on the growing surface are active as growth centres. As supersaturation increases, the number of dislocations active as growth centres diminishes.

Vacancy Emission from and the Absorption to a Helical Dislocation in Aluminum

Vacancy Emission from and the Absorption to a Helical Dislocation in Aluminum

Dislocations in energetic materials Dislocation characterization and post-growth motion in single crystals of cyclotrimethylene trinitramine

Abstract Dislocations in solution-grown crystals of cyclotrimethylene trinitramine (RDX) have been observed and characterized by transmission X-ray topography. The dislocations were found to have the following Burgers vectors: [001], [010], [100], 〈011〉 and 〈110〉. With the exception of one pure edge dislocation type they were all of mixed character. A few of the dislocations were straight and showed characteristics typical of growth induced dislocations. Most, however, exhibited irregular line directions indicative of post-growth motion. In addition, a number of narrow dislocation helices were observed. The observed dislocation configurations are discussed in terms of climb, cross-slip and combined climb/slip mechanisms. Evidence is presented for the existence of an alternative (001) slip plane additional to the (010) plane previously defined by microhardness indentation.

Helical dislocations in Sn-doped GaP epitaxial layers and their characterization by transmission electron microscopy

Abstract A simple and rapid method is proposed for the analysis of helical dislocations by means of their inside-outside contrast behaviour on TEM micrographs and stereomicroscopy. This method can be applied in a straightforward manner to helices resulting originally from both screw dislocations and mixed dislocations. In general, this method corresponds substantially to the method given by Kamejima et al. (1979), although it is based on the conventions proposed by Foll and Wilkens in 1975. A discussion is given of preliminary results obtained by applying this method to helices in heavily Sn-doped GaP layers grown by liquid-phase epitaxy from Sn solution on (001)-oriented GaAs substrates. It was determined experimentally that not only pure screw dislocations but also mixed dislocations had been converted into helices.

Optical microscopic studies on grown and etched surfaces of flux grown LaAlO3 crystals

Surface structures on as-obtained flux-grown crystals of LaAlO3 have been investigated. Strictly oriented square, circular and rhombus shaped pointed, as well as flat-bottomed etch pits are observed. Etch pits along lineage boundaries, intersecting low-angle tilt boundaries and helical dislocations are illustrated and described. Different orientation of etch pits reveal twinning in LaAlO3 crystals. Microdisc patterns and flux inclusions are also observed. The etch patterns on the as-obtained LaAlO3 crystals are explained to be as a result of the flux cleaning operation of crystals in HNO3. Experiments on etching established HNO3 to be a dislocation etchant for LaAlO3 crystals. Dislocation etching kinetics of the HNO3-LaAlO3 surface system are investigated for the freshly identified sites as well as for sites having a previous history of etching. Data obtained on the effects of etching time, etchant concentration and temperature on the dislocation etch rates, are analysed. The results obtained are presented and discussed.

A High-Temperature X-Ray Lang Camera

A high-temperature X-ray Lang camera has been designed to examine metal single crystals with a low dislocation density during the heating or cooling processes. The furnace for the camera was set on a goniometer-hand, therefore the setting of specimen is only slightly restricted. The camera can be used to take a topograph at any temperature up to 500°C with desired heating or cooling run. The spatial gradient of temperature in the exposing area of the specimen is less than 1°C/mm, and the fluctuation of temperature during an exposure of about 30 hours is less than ±0.5°C. Using this camera, several topographs were taken during a heating process of high purity aluminum single crystal. As the result it was ascertained that a screw dislocation converts to a helical dislocation by the generation of vacancies during the heating process.

Precipitation of tellurium in (Hg, Cd)Te alloys

The precipitation of tellurium in alloys of Hg1−xCdxTe with x = 0.2 to 0.3 prepared under conditions of excess tellurium by the solid state recrystallization method have been studied using chemical defect etching and transmission eloctron microscopy. Precipitation of tellurium occurred during the quench from the recrystallization anneal. During the initial part of the quench, precipitation occurred as a result of nucleation of precipitates on pre-existing dislocations. This precipitation resulted in the formation of dislocation helices with precipitates in the interior of the helix loops. Both monoclinic and trigonal phases of tellurium were found in these precipitate defects, as well as the twinned matrix phase. Later in the quench, an additional nucleation mechanism, either homogeneous or heterogeneous on an impurity occurs resulting in the formation of complex defects in the bulk. During a subsequent post-anneal at temperatures of approximately 300°C, the tellurium precipitates dissolve, and interstitial perfect and faulted dislocation loops are formed. Both the precipitation process and the subsequent post-annealing result in considerable multiplication of dislocations.