Dependence Of Dislocation Density Research Articles

Dislocation density in Ni 3(Al,Hf)

The temperature dependence of dislocation density and its relationship with the anomalous growth of the flow stress are presented. A pronounced density increase of about one order of magnitude in a temperature range below the peak of anomalous flow stress growth has been clearly established experimentally. The correlation with different components of the applied stress is examined and it is shown that the dislocation density is related to the stress required for dislocation multiplication.

A model describing neutron irradiation-induced segregation to grain boundaries in dilute alloys

A model describing neutron irradiation-induced grain boundary segregation at a given temperature is established for dilute alloys based on a complex diffusion mechanism and combined with Mc-Lean’s equilibrium segregation model. In the model, irradiation-enhanced solute diffusion is taken into consideration. The diffusion equations are more rigorously solved than in earlier models, so that an accurate definition of the grain boundary solute concentration is given as a function of time. The effect of the temperature dependence of dislocation density is accommodated and the estimation method for complex diffusion is reappraised. Theoretical predictions are made for segregation of phosphorus in neutron-irradiatedα-Fe. There exists a transition temperature below which combined irradiation-induced nonequilibrium and irradiation-enhanced equilibrium segregation is dominant and above which thermal equilibrium segregation is dominant. The peaks in the temperature dependence of segregation shift to lower temperatures with decreasing neutron dose rate and/or increasing neutron dose. The combined radiation-induced nonequilibrium and radiation-enhanced equilibrium peak segregation temperature is about 150 °C forP grain boundary segregation in neutron-irradiatedα-Fe at dose rate=10−6 dpa/s and dose=1 dpa. The thermal equilibrium segregation peak is around 550 °C for the same conditions. Comparison of some experimental and predicted results shows that the predictions are generally consistent with the observations.

Irradiation-induced silicon segregation in ferritic steels

Irradiation-induced segregation (IRS) of Si in ferritic steels has been modelled using a non-equilibrium segregation based method relying on describing the motion of point defect-solute complexes to grain boundaries. The model is extended from the earlier versions to account more accurately for composition changes occurring on the grain boundary plane and to account more properly for the temperature dependence of dislocation density. The results of the model indicate strong peaking of IRS at certain critical temperatures. The peak positions move as the irradiation conditions are altered and as certain microstructural features like grain size and dislocation density are changed. The model is applied in this paper to provide explanation for observed DBTT shifts after neutron irradiation of ferritic steels.

The Dependence of Dislocation Density and Cell Size on Crystallographic Orientation in Aluminum

The Dependence of Dislocation Density and Cell Size on Crystallographic Orientation in Aluminum

Microstructure characterization in electrodeposited Cu-coatings obtained at different bath temperatures

X-ray profile and texture analysis, optical microscopy and microhardness measurement were used in order to study the dependence of the microstructure formation of electrodeposited copper coatings on the bath temperature. The coatings were obtained from an acid electrolyte without any additions within a temperature range from -10 o C up to 110 o C on polycrystalline copper and amorphous carbon substrates. The obtained temperature dependence of dislocation density is similar to that of the microhardness. Above room temperature the lattice perfection and the main grain size rapidly increase. At all temperatures deposition on carbon leads to a -fibre texture, but on copper substrates a texture change of the coatings from a -fibre to a more complicated, substrate-related epitaxial texture was observed

Theory of plastic deformation: - properties of low energy dislocation structures

Work-hardening phenomena are based on the very fundamental principles (i) that at the position of every dislocation axis the respective resolved shear stress cannot exceed the friction stress, including the self-stress of bowing dislocations, and (ii) that always that structure forms which among those accessible by the dislocations minimizes stored energy per unit length of dislocation line. Such dislocation structures have been named LEDSs. The corresponding work-hardening theory, the mesh length theory, is applicable to all materials deforming via gliding dislocations and to all types of deformation. Results previously achieved with the mesh length theory are summarized, and a number of new developments are discussed. Depending on the dislocation structures formed, the work-hardening behavior differs. Easily intersecting glide causes dislocation cell structures with almost dislocation-free cell interiors delineated by dislocation rotation boundaries. Pronounced planar glide causes Taylor lattices characterized by local planar order parallel to the one or perhaps two most highly stressed glide plane(s), no systematic lattice rotations, and overall uniform dislocation density. The most widely observed basic features of work hardening are explained in general terms. Specific applications are indicated for layer-type crystals, h.c.p. single crystals, single-crystal and polycrystalline pure f.c.c. metals and α-brass-type alloys, precipitation-hardened materials and steels. Included are the different stages of work hardening, dynamical effects in low temperature plasticity, the general characteristics of grain boundary strengthening and the Hall-Petch relationship. In addition, proposed explanations for (i) glide system interactions in polyslip resulting in microbands and affecting texture formation, and (ii) creep without stress dependence of dislocation density, are discussed.

Habit, dislocation densities, and microhardness of NaClO3‐NaBrO3 mixed crystals

AbstractResults of a systematic study of growth habit, dislocation densitics, and microhardness of the NaClO3—NaBrO3 mixed crystals are presented. A drastic change in habit of NaBrO3 for small additions of NaClO3 was observed. The composition dependence of dislocation density for this system is found to be highly non‐linear. A non‐linear composition dependence of microhardness, with positive deviations from linearity was also observed. This increase in hardness is correlated with the enhanced dislocation density in the mixed crystal.

Effect of magnesium content on the stress exponent and effective stress in the steady state creep of Al-Mg alloys

The stress exponent of steady state creep,n, and the internal (σ i) and effective stresses (σ e) have been determined using the strain transient dip test for a series of polycrystalline Al-Mg alloys creep tested at 300° C and compared with previously published data. The internal or dislocation back stress,σ i, varied with applied stress,σ, but was insensitive to magnesium content of the alloy, being represented by the empirical equationσ i=1.084σ 1.802. Such an applied stress dependence ofσ i can be explained by using an equation forσ i of the formσ i α(dislocation density)1/2 and published values for the stress dependence of dislocation density. Values of the friction stress,σ f, derived using the equationσ e/σ=(1−c) (1−σ f/σ), indicate thatσ f is not dependent on the magnesium content. A constant value ofσ f can best be rationalized by postulating that the creep dislocation structure is relatively insensitive to the magnesium content of the alloy.

Crystal growth progress in response to the needs for optical communications

The requirements of optical communications and the characteristics of SiO2 fibers are used to set the wavelengths for LED's, lasers and detectors. These wavelengths are shown to dictate single crystalsubstrates and epitaxial materials for optical communication devices. Recent interest focuses on the 1.3 and 1.5 μm region with InP and GaSb substrates and (GaIn)(AsP) epi-layers. Liquid encapsulated Czochralski growth of substrates is reviewed. Important recent advances include methods for twin free growth, new iso-electronic dopants, dislocation free material and analysis of dislocation density dependence on thermal strain. Liquid phase epitaxy (LPE), vapor phase epitaxy (VPE), metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) for epi-layers are reviewed and compared. Important advances include quantitative distribution coefficient studies for major constituents in LPE, diffusion theory of heterostructure formation by LPE, the realization that classic thermodynamics could be used to explain success and failure in deposition on curved surfaces, extension of the slider deposition apparatus from the LPE concept to vapor growth; and the beginnings of distribution coefficient work in the vapor phase. Areas for further work include the acquisition of better equilibrium data (phase diagrams, distribution coefficients, etc.) for all systems, and quantitative studies of the effect of growth rate on the distribution of major constituents and impurities. Opportunities abound for turning art into science with real profit for technology.

Growth of dislocation-free silicon films by molecular beam epitaxy (MBE)

The dependence of dislocation density on various growth conditions for Si MBE films is investigated. Dislocation density is determined by chemical etching and SEM observation. High temperature heat treatment generates dislocations in substrates, and film dislocation density increases with an increase in the substrate dislocation density. The dislocation generation in the substrate is prevented by using a substrate heater designed not to induce thermal stress. The film dislocation density decreases exponentially with an increase in the preheating time of the substrate. The density is proportional to exp (E/kBT) where T is growth temperature. The value of the activation energy is 3.5 eV. Dislocation-free films can be grown reproducibly at 860 °C on (100)-oriented substrates preheated to 1210 °C for 10–20 min.

On the stress sensitivity parameter in high temperature creep of Al-3 wt% Mg

Stress dependence of creep rates, internal and effective stress, as well as the apparent instantaneous strain associated with sudden stress changes during the steady-state creep of Al-3 wt% Mg were studied. The transition in stress exponentn is attributed to the change in the stress dependence of dislocation density and does not necessarily correspond to the change in the creep mechanism. The effective stress exponentn* is considered as the identifying parameter for the process controlliag the creep mechanism. The absence of the instantaneous plastic strain upon stress changes during creep shows that creep is mainly controlled by viscous glide motion of dislocations.

Effects of the Crystal Diameter on the Dislocation Density in KCl Crystals Grown by the Czochralski Method

AbstractThe dependence of dislocation density on the crystal diameter of the Czochralski grown KCl crystals (both pure and Sr doped) was investigated by the method of the etch pit and X‐ray topograph. (1) The dislocation density drastically decreased with the decrease in diameter (D) for small D (D < 2 mm, Fig. 2). (2) The dislocation density in the Sr doped crystal was lower than that of pure component in the diameter examined. (3) Many helical dislocations and dislocation loops were observed in the thin crystals (D ∼ 0.5 mm) of the pure KCl and Sr doped one, respectively, by X‐ray projection topography. These results were discussed in connection with the dislocation generation mechanisms due to thermal stresses or supersaturated point defects.

Work jardening and the pressure dependence of dislocation density and arrangements in shock loaded nickel and copper

Work jardening and the pressure dependence of dislocation density and arrangements in shock loaded nickel and copper

Creep Behavior of Al-2.2 at%Mg Alloy at 573 K

Stress dependence of creep rates and the level of internal stress, as well as transient behavior upon sudden stress changes during the steady-state creep were studied. The stress exponent for steady-state creep rates, n, depended on the stress level. In the stress region lower than 7 MPa, n was about 5 and the inernal stress was nearly equal to the applied stress. The steady-state creep in this region is controlled by a recovery process, though dislocations glide viscously. In the stress region between 7 and 30 MPa, n was about 3. In the stress region higher than 30 MPa, n became again larger than 3. In these two regions, where the internal stress was obviously less than the applied stress, the role of viscous gliding of dislocations becomes important and cannot be neglected in the discussion of the total process of creep. Transition from the middle to the high stress regions was found to arise from the change in the stress dependence of dislocation density. These results indicate that the change in n does not necessarily correspond to the change in the creep mechanism.

Relation of dislocation density in different slip systems to work hardening parameters for magnesium crystals

Using transmission electron microscopy, the dependence of dislocation density, ϱ, on stress, τ, and strain, ϵ, has been studied in Mg crystals deformed by basal and pyramidal slips. It has been shown that for the slip systems investigated the dependence τ(ϱ) may be written in the form τ ∼ Σ i α i G i b i√ ϱ i, where α i is the coefficient whose value takes account of the type of dislocation interaction appearing during deformation. The strain dependences for the rate of changing the density of different dislocations have been found. Based on the measurement data on the density of thin slip bands in the (0001) 〈11 20〉 system, the density of basal mobile dislocations providing the onset of an easy glide stage has been estimated.

Size effect of dislocation density in Sb crystals grown by the bridgman method in a hard crucible

AbstractA dislocation density dependence on crystal transverse dimensions has been investigated for crystals of Sb grown in split graphite moulds at rates of 1.5; 8.5 and 15 cm/h. The dependence is shown to be the same for all growth rates which agrees with an earlier suggested theoretical equation.

1591. Dependence of dislocation density on the growth rate of copper single crystals (USSR): V V Sanadze et al, Izv VUZ Fiz, No 7, 1970, 127–130 ( in Russian)

1591. Dependence of dislocation density on the growth rate of copper single crystals (USSR): V V Sanadze et al, Izv VUZ Fiz, No 7, 1970, 127–130 ( in Russian)

Statistical model for the description of the dependence of dislocation density on growth rate in the presence of additives in KCl crystals

Statistical model for the description of the dependence of dislocation density on growth rate in the presence of additives in KCl crystals

The influence of grain size on the nature of portevin-lechatelier yielding

Abstract When the grain size dependence of dislocation density is applied to Cottrell's theory of the Portevin–Lechatelier effect, the result indicates that the critical strain ∊0, necessary to initiate serrated flow, should depend on the grain size μ, at constant strain rate, according to the relationship ∊0 = (constant)μ p , where p=n/(m + β). The parameters n and (m + β) are evaluated by dislocation density measurements and strain rate experiments respectively to give ∊0 ∝ μ½. Direct experimental determination of ∊0 as a function of grain size is in agreement with that predicted theoretically, indicating that the effect arises from the influence of grain size on the average velocity of mobile dislocations. The period and amplitude of the serrations decrease with increasing grain size. This effect is consistent with the idea that each serration corresponds to a yield point followed by a local Luders strain. The period and amplitude of the serrations decrease with increasing grain size. This effect is co...

Dislocation vectors related to the ductility of beryllium

Specimens subjected to prescribed amounts of deformation by rolling have been examined by thin foil electron microscopy and the observed dislocation structures related to the variation of ductility with temperature, particular attention being focused on the brittle-to-tough transition in beryllium. A typical feature of the specimen examined was the heterogeneous high-density dislocation arrays which consisted of networks and of tangles with associated prismatic loops. The loops are elongated in the 〈101̄0〉 direction with a Burgers vector 1 3 〈112̄0〉, the presence of more than one oriented set being indicative of their formation during simultaneous slip in more than one basal direction. Apart from the expected dependence of dislocation density upon strain, no obvious differences were found between the dislocation structures in beryllium rolled at 250° C, 450° C and 650° C. A significant feature of the present work was, however, the identification of non-basal, 1 3 〈112̄3〉, slip vectors in these materials together with the observation that no such vectors could be located in metal deformed at room temperature. Some dislocations were observed to move in the electron microscope and where these were amenable to identification they were found to have a 1 3 〈112̄0〉 Burgers vector. The apparent absence of moving 1 3 〈112̄3〉 dislocations implies that non-basal dislocations may experience a higher Peierls-Nabarro force than basal dislocations. No evidence was found to suggest that impurities are the prime reason for the low-temperature brittleness of beryllium and it is proposed, therefore, that the ductility-temperature behaviour may be more satisfactorily explained by consideration of the Peierls-Nabarro forces acting upon the different types of dislocation.