Strength and fracture mechanisms in beta-alumina

The fracture stress, fracture initiation energy and work of fracture have been measured for a fine grained beta-alumina ceramic. The fracture stress has been shown to be naturally distributed and can be described by the Weibull distribution function. Values of the fracture initiation energy and work of fracture were found to be similar, 25 J m^–2.The microstructure of beta-alumina has been examined using transmission electron microscopy. In thin foils of the fine grained material cleavage cracks were invariably found to occur in the basal layers. The phenomena has been explained empirically in terms of atomic bonding of the mirror plane. Contributing terms to the fracture energy t have been discussed and it is suggested that lamination and deformation at the crack tip by generation and movement of dislocations along basal planes is a significant factor.     

Subboundaries in hcp4He crystals studied by SR X-ray topography

The internal structure of hcp⁴He crystals was studied by SR (synchrotron radiation) X-ray topography. Subboundaries in the crystals appeared as black or white bands in the X-ray topographs. In one of the hcp⁴He crystals, the subboundaries turned out to be flat planes perpendicular to the basal plane. They were small-angle tilt boundaries which consisted of basal edge dislocations. The dislocation spacing in one of the subboundaries was determined to be 800 nm and the total density of the boundary dislocations to be 2.6 × 10⁵ cm^–2. The subboundaries in another hcp⁴He crystal were curved and/or branching, indicating that the crystal was strained.     

Dislocation morphology in graded heterojunctions: GaAs1−xPx

The details of the formation, propagation, interaction, and densities of misfit dislocations are combined into a simple model quantitatively predicting dislocation densities for both abrupt and graded heterojunctions. Three key concepts are introduced: (1) misfit dislocations are segmented; (2) accordingly, they must give rise to a density of inclined dislocations, n _I, that propagate through the growing layer; and (3) these inclined dislocations can bend in and out of any subsequently formed misfit plane to relieve the strain, and when bent in, serve as strain-relieving misfit dislocations. Thus, the value of n _I is expected to remain constant with thickness. Also, n _I is predicted to vary directly with the compositional gradient at the heterojunction. It is pointed out that there are two general classes of misfit dislocations, pure-edge and mixed and that their intersections, which cause the misfit dislocations to appear to bend within their plane, can be simply classified into three general types.Transmission electron microscopy was used for a comprehensive study of dislocations in a series of GaAs_1–xP_x heterojunctions prepared by a vapour phase growth technique. The main features of the above model were corroborated. The value of n _I was found to be constant with growth distance as postulated, and in quantitative agreement with prediction, n _I decreased from 4 × 10⁷ cm^–2 to 10⁶ cm^–2 as the compositional gradient decreased from 5% phosphorus/m to 0.2% phosphorus/m. Note that these values can far exceed the dislocation density of the substrates. Of particular significance, the inclined dislocations n _I were found to propagate through a constant-composition region grown on top of a compositionally graded region, so that formation of the heterojunction must affect subsequently grown layers. Finally, it is shown that the misfit dislocations are, indeed, a combination of pure-edge and mixed, and all three postulated general interactions between these dislocations are shown to occur.     

Influence of current pulses on the mobility and multiplication of dislocations in Zn

For the purpose of clarification of the mechanisms of the electron-plastic effect the influence of current pulses with a length of 2·10^–4 sec and a density of more than 20 MA/m² on the mobility of pyramidal dislocations in Zn single crystals in the area of thermally activated movement at 77 and 293 K and also on their multiplication was studied by the method of selective etching. It was shown that the increase in the rate of movement of dislocations under the action of current pulses is accompanied by multiplication of the dislocations with current densities of more, than 10² MA/m². The rules established are discussed taking into consideration the pondermotive forces, thermal effects, electron-dislocation interaction, and the work of the Frank-Reid effect under the action of the electron wind.S. Ordzhonikidze Siberian Metallurgical Institute, Novokuznetsk. Translated from Problemy Prochnosti, No. 10, pp. 48–53, October, 1989.     

Plastic deformation of copper thin foils

According to one suggested model, bending of a single crystal introduces edge dislocations of the same sign. In the present study, this model is examined by computer simulation using molecular dynamics. When a notch is present on the tension surface, Heidenreich-Shockley partial dislocations are created near the tip of the notch. In the compression surface, partial dislocations are created due to wrinkling of the crystal plane. The results of simulation shows that dislocations are more easily created in a compressive bending region than in a tension bending region or simple tension region. For shear deformation, partial dislocations are created on the highest resolved shear stress slip plane {1 1 1} and slip in the direction of highest resolved shear stress.     

High temperature deformation behavior of permanent casting AZ91 alloy with and without Sb addition

The elevated temperature deformation behavior of permanent cast magnesium alloy AZ91 with and without Sb addition has been investigated using slow strain rate (5.0 × 10^–4s^–1) elevated temperature tensile and constant load creep testing at 150°C and 50 MPa. The alloy with 0.4 wt% Sb showed a higher elevated temperature tensile strength and creep resistance due to the formation of thermal stable Mg₃Sb₂ precipitates and a smaller microstructure as well as the suppressing of the discontinuous precipitation. Plastic deformation of AZ91 based alloys is determined by motion of dislocation in basal plane and non-basal slip systems. The dislocation motion in a slip system is influenced by temperature, precipitates and other lattice defects. Dislocations jog, grain boundaries and/or precipitates are considered as obstacles for moving dislocations. The deformation twinning were founded in the creep process by TEM. Cross slip of dislocations was taken into account as the main softening mechanism for permanent cast AZ91 alloy during elevated temperature deformation process.     

Spectrum of the electron-phonon interactions of zinc single crystals induced by a basal-dislocation network

A dislocation spectrum of the electron-phonon interactions (EPI) of zinc single crystals is obtained for the first time. It is shown that dislocations are a source of hypersonic waves. Possible mechanisms of energy dissipation in vibrations of the network of basal dislocations in the point-contact zone are considered. It is noted that a change in the effective transport function of EPI results from the inertial properties of dislocations.Physicotechnical Institute of Low Temperatures, Khar'kov. Translated from Inzhenerno-Fizisheskii Zhurnal, Vol. 67, Nos. 1–2, pp. 81–85, July–August, 1994.     

Formation of dislocations in metals during the diffusion of low-solubility surface-active substances

The formation of dislocations during diffusion in solids was analyzed. Calculations relating to the diffusion of surface-active substances (SAS) in metals, at temperatures for which the Rebinder effect is manifested, were carried out. It was shown that this effect is most intensely manifested when the depth of diffusion is on the order of tens of interatomic spacings. It was established that, when SAS diffuse to such a depth, the dislocation density in metals reaches 10¹²–10¹³ cm/cm³ which leads to embrittlement and crack nucleation even at low stress levels. A diffusional model of grain boundaries, taking into account the formation of dislocations during diffusion, was analyzed. It was noted that thermal stresses contribute to the formation of dislocations when high-melting metals are tinned with low-melting metals.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 5, No. 6, pp. 643–649, November–December, 1969.Our gratitude is due to I. A. Goncharenko for his assistance in the experimental work.     

Effect of temperature on the lower yield strength and static strain ageing in low-carbon steels

Three different low-carbon steels, namely ASTM 516 Gr70 PVQ, CSA G40.21 350 WT and CSA G40.21 350 AT plates, were tensile tested at temperatures from ambient to 623 K over a range of strain rates (1.48×10^–5–1.48×10^–3 s^–1). The lower yield strength generally decreases with increasing temperature, but there is a strength plateau, or a small peak, at temperatures between 423 and 573 K. The results of computer modelling show that concentration of nitrogen in the atmospheres at dislocations decreases with increasing temperature, approaches a minimum value at a temperature around 423 K, then increases with increasing temperature for temperatures above 423 K. This effect of temperature on the concentration of nitrogen in atmospheres at dislocations results in the strength plateau or small peak.     

Structural and Transport Properties of Sn-Doped Bi2Te3 – x Se x Single Crystals

The transport properties of Bi_{2 – y} Sn _y Te_{3 – x} Se _x solid solutions are studied. The results demonstrate that doping with Sn has a strong effect on the temperature dependences of the thermoelectric power and electrical conductivity of the crystals. This suggests that the valence band of the crystals contains Sn-related resonance states. The point defects and dislocation system in Bi₂Te₃ and Bi_{2 – y} Sn _y Te_{3 – x} Se _x solid solutions are studied by transmission electron microscopy. It is shown that the predominant defects in the crystals studied, grown by the Czochralski technique, are dislocations lying in the (0001) plane. The estimated dislocation density is 10⁸ to 10⁹ cm^–2, and the primary slip plane is (0001). Electron-microscopic examination indicates the presence of stacking faults and very small dislocation loops in both Bi₂Te₃ and Bi_{2 – y} Sn _y Te_{3 – x} Se _x single crystals. Since all of the crystals are highly degenerate semiconductors, it is reasonable to assume that structural defects have an insignificant effect on their electrical properties.     

Crystallographic shear planes in nanocrystalline SnO2 thin films by high-resolution transmission electron microscopy

Atomic structures of crystallographic shear planes (CSPs) in nanocrystalline thin films of semiconductor SnO₂ were investigated by high-resolution electron microscopy. The films were prepared by electron beam evaporation in high vacuum (10^–6 torr) and followed by annealing in synthetic air at 700 °C for 1–2 H. CSPs with the displacement vector of [1/2 0 1/2] were observed in the planes parallel to (¯101), (110) and (¯3¯21). Most of the CPSs were found to terminate or interact with each other within SnO₂ crystallites. Partial dislocations exist at terminal places of CSPs or along intersecting lines of CSPs. CSP steps were also observed. Structural models of these defects have been proposed. Based on analysis of experimental data, it has been suggested that the Sn/O ratio at CSPs which are not parallel to their displacement vector, at cores of partial dislocations and at CSP steps, is higher than that of the perfect structure, that is, these defects are able to provide extra free electrons with the films.     

Transmission electron microscopy observations of antiphase boundaries in CdTe

Specimens from CdTe crystals, prepared by two different methods were examined by transmission electron microscopy. Antiphase boundaries were observed in both the {110} and {311} planes. Comparatively regularly ordered partial dislocations with a Burgers vector b=a/6 112 were found in both cases. The distance between partial dislocations is about 6–8 nm. The probable formation mechanism of these boundaries is discussed.     

Electrical properties of high purity tin dioxide doped with antimony

The electrical conductivity of high purity tin dioxide doped with antimony was studied at temperatures of 900 to 1200° C and partial pressures of oxygen between 10^–8 and 1 atm. For specimens having a dopant concentration over 1 × 10¹⁹Sb cm^–3, the electrical conductivity decreased slightly with temperature and independent of oxygen partial pressure. The electrical conductivity of specimens having a dopant concentration under 1 × 10^–8Sb cm^–3 increased with temperature and with decreasing partial pressure of oxygen. The significance of the dopant and the thermally created defects is discussed.     

Dislocation analysis of die-cast Mg–Al–Ca alloy after creep deformation

Tensile creep tests were combined with detailed transmission electron microscopy in order to characterize the dislocation movements during creep and to explain the creep properties of the Mg–Al–Ca AX52 die-cast alloy at 473 K and stresses from 15 to 70 MPa. TEM observations indicate that dislocations are generated within the primary α-Mg grain in the die-casting process, which consist of both the basal and non-basal segments. The basal segments of dislocations are able to bow out and glide on the basal planes under the influence of a stress, and the jogs follow the basal segments with the help of climb during creep. The creep mechanism for the alloy is deduced as dislocation climb due to the formation of sub-boundaries during creep, while the easy glide of the basal segments of dislocations is controlling the creep rates immediately after the stress application of creep tests.     

Nanoscale dynamic wetting and spreading of molten Ti alloy on 6H–SiC

We have investigated nanoscale features at the reactive wetting front of the molten Ag–27.4 wt.% Cu–4.9 wt.% Ti on 6H–SiC using video movies recorded in situ on a high-temperature stage of a high-resolution transmission electron microscope and also proposed a model of a chemical reaction at each tip. One of the features of reactive wetting and spreading at 1073 K in 4 × 10⁻⁵ Pa was the discontinuous motion of the tip, and the halting time depended on the thickness of an amorphous Si–O layer on SiC, which can be explained by the time needed for the decomposition of the layer by Ti atoms to form TiC nanoparticles since Ti atoms in the molten alloy sufficiently rapidly diffuse to the tip on the SiC surface. Molten Ti and TiC nanolayers preceded the Ti₅Si₃ nanolayer at the tip. The reaction required to form the TiC nanolayer is also the rate-determining step for spreading. The contact angle of the tip increased up to 60–80° when the tip halted, whereas the tip decreased down to 10° on the nonbasal plane and 20° on the basal plane of SiC when it traveled rapidly. The high traveling angle of the molten tip on the basal polar plane of SiC indicates a high interfacial energy between Ti and SiC(0 0 0 1).     

A microscopic study of preferred orientation and adhesion of solid copper particles on (0001) sapphire substrates

The microscopic phenomena of solid copper particles equilibrating on (0001) sapphire surfaces have been investigated by scanning electron microscopy. Very sharp and clear silhouettes of the particles are developed. At temperatures between 900 and 1080° C, with the oxygen partial pressure lower than 10^–17 atm, the copper particles tend toward their equilibrium shape as the {111} and {110} preferred faces develop eventually into truncated octahedra or tetrakaidecahedra. Recrystallization of the particles originating at the particle-substrate interface by a heteroepitaxial mechanism is discussed. The {111} plane of the copper particles does not grow parallel to the (0001) sapphire surface as previously reported, but rather with about 20° of tilt, suggesting epitaxial growth of the Cu {112} plane on the (0001) sapphire surface. The surface energy of solid copper decreases with the increase of temperature and/or oxygen partial pressure while the work of adhesion between solid copper and (0001) sapphire can be enhanced by increasing the temperature and/or decreasing the oxygen partial pressure.     

Effect of nitrogen on the dynamic strain ageing behaviour of type 316L stainless steel

The dynamic strain ageing (DSA) behaviours of type 316L stainless steels containing different nitrogen contents (0.01–0.15 wt% N) were studied in tension under varying strain rates (1 × 10^–2–2 × 10^–4s^–1) and the test temperatures (R.T.–1023 K). The temperature range for DSA was moved to higher temperature for increasing nitrogen contents. The critical strain, _c for the onset of serration increased with nitrogen content at 773 K and then became almost constant at 873 K. Type A and B serrations were observed at 873 K with the value of the strain required to effect the transition from type A to type B serration increasing for nitrogen contents upto 0.1 wt% and then becoming saturated. The activation energy for DSA was 23.4–26.2 kcal mol^–1 (97.8–109.5 kJ mol^–1) at the onset and 65.0–76.6 kcal mol^–1 (271–320.2 kJ mol^–1) at the end of serration. The lower activation energy was related to vacancy diffusion and the higher activation energy was attributed to the diffusion of chromium to dislocations. The activation energy for DSA was slightly increased with nitrogen addition. DSA was retarded by an increase in the nitrogen content since nitrogen reduced the chromium diffusion to dislocations due to a strong interaction between the nitrogen and chromium.     

Transmission electron microscopy of dislocations in nickel oxide single crystals

The dislocation structure of NiO single crystals used in diffusion studies has been examined by transmission electron microscopy. The crystals contain dislocations (the dislocation density being 4 × 10¹² m^–2) that are probably a result of the growth process. The dislocations have a Burgers vector ofa/2 110. On annealing at temperatures above 1400° C the density is reduced to 7 × 10¹¹ m^–2, most of the dislocations forming low angle boundary arrays. The dislocation density was found to be much greater in the vicinity (within 1m) of a mechanically polished surface.     

Cathodoluminescence studies of dislocation motion in IIb–VIb compounds deformed in SEM

Continuous observation of dislocation motion on the specimen surface of II_b–VI_b semiconducting compounds (CdS and CdTe) has been made using a scanning electron microscope (SEM) with the cathodoluminescence (CL) by use of a deformation apparatus installed in the SEM chamber. With the application of stress, SEM-CL patterns revealed an increasing density of dark spots that formed dark stripes along slip traces. For both CdS and CdTe crystals, regardless of the slip system involved, the dark spots corresponding to individual dislocations appeared in the SEM field without displaying their moving state and seldom disappeared, indicating that dislocations are immobilized after they travel a certain distance from the source with a high velocity. The investigation of slip-band growth in CdS showed that screw bands on the basal plane exhibit the photoplastic effect, whereas those on the prismatic plane do not, in accordance with the macroscopic deformation tests.     

Weak beam observations of dissociation of pure screw dislocations in Ni3Al

The dissociation of superdislocations in Ni₃Al, which have a pure screw character, has been recorded on weak beam images at the g(5g) imaging condition. All screws were observed to dissociate on an {001} habit plane bounding APB faults. By computer modelling, the dissociation distance (d^CSF) of the Shockley partial dislocations that bound a complex stacking fault is shown to be less than 1.5 nm. This is the first reported case of fourfold dissociation bounding CSF + APB + CSF faults with a non-planar configuration and pure screw character. The energy values deduced from the measured spacing show quite large difference from the results obtained from dissociation with a planar system. It is questionable to use the linear anisotropic elasticity theory for such small splitting and nonplanar configuration. In addition our simulations demonstrate that a g(ng) condition is a wise choice for imaging partial separation.