Dislocation Grain Research Articles

Kinematic hardening in a dispersion strengthened aluminum alloy: experiment and modeling

The hardening behavior of a dispersion strengthened aluminum alloy, 8009, is characterized experimentally and shown to be primarily kinematic in nature. Although the alloy contains 0.25 volume fraction of dispersoid, the microstructure is analogous to that of a material cycled to saturation in fully reversed loading fatigue in that it consists of essentially dislocation and dispersoid free grain interiors with a grain diameter of less than 0.5 μm. The high dispersoid volume fraction is associated with the grain boundaries. An attempt is made to model the hardening behavior of the alloy using the micro-mechanic model known as the generalized method of cells (GMC). Good correspondence between the model response and the actual material behavior is obtained; however, matching of flow stresses between model outputs and actual alloy behavior requires assignment of unrealistically high material properties to some model subcells. Explanations for the limited modeling success are tendered and suggestions for improvement of the physical realism of the GMC model are proposed.

Approaches For Reduction Of The Defect Density In Group III Nitride Based Heterostructures

ABSTRACTLateral epitaxial overgrowth (LEO), pendeo-epitaxy (PE), and solid-phase epitaxial recrystallization (SPER) are discussed as three approaches for reducing the defect density in group III nitride based heterostructures. Studies of the LEO GaN and PE GaN revealed, that a major factor for the defect reduction in the laterally overgrown regions is the change of the dominant growth direction - from vertical in the window regions to lateral in the regions over the mask or over the trenches, and the related threading dislocations lines redistribution. The mechanisms of defect reduction in LEO GaN and PE GaN are similar, although they arise through different process routes, and are related to the free-standing (PE) or quasi-free-standing (LEO) growth of GaN, and the associated stress redistribution. The stress distributions in the LEO and PE GaN heterostructures are calculated and compared with finite element modeling. Another approach for reduction of the defects is the SPER process and the related thermal activation for dislocation reactions and grain boundary mobility and migration. This approach is shown in the example of annealed AlN films.

Microstructure and Superplasticity of Al–Cu Alloy Melt Spun Ribbons

Microstructural change and superplasticity of Al-Cu alloy melt spun ribbons were examined. Al-10mass%Cu ribbons had small α grains containing fine dispersed θ (Al 2 Cu) precipitates and the precipitates were effective in suppressing the grain growth during deformation at high temperatures. Al-33mass%Cu ribbons formed fine lamellar structure with small colonies as melt spun and fine equiaxed dual structure composed of α and θ phases was obtained during heating before tensile test. Tensile tests were performed at initial strain rates in the range of 1.7 × 10 -4 to 1.7 × 10 -1 s -1 and at various temperatures between 300 and 793 K. Superplasticity in both alloys occurred depending on the microstructure, temperature and strain rate. The peak in elongation-strain rate curves of Al-10mass%Cu ribbons appeared around a strain rate of 4 × 10 -4 s -1 at 723 K, while the peak for Al-33mass%Cu ribbons was shifted to higher strain rate around 3 × 10 -2 s -1 at 773 K. No significant change in equiaxed grains was observed during superplastic deformation of Al-33mass%Cu ribbons and the superplastic flow was mainly provided by the α/θ interfacial sliding accompanied by an accommodation process by diffusion of a large amount of vacancies and solute atoms. In Al-10mass%Cu ribbons, plastic flow was provided not only by grain boundary sliding but also by the motion and multiplication of dislocations in grains.

Effective medium theory for ionic conductivity in polycrystalline solid electrolytes

It is usually found that for the relatively poor ion conducting solids, the ionic conductivity for the pure polycrystalline sample is higher than that of the single crystal. The difference has been suggested to be due to the presence of dislocations grain boundaries etc. in the polycrystal. In this paper we propose a theoretical model for the polycrystal including these defects. A quantitative estimate of the grain boundary contribution to conductivity is made using an effective medium theory and it is found to exhibit an Arrhenius behaviour. The results for calcium fluoride, thallium chloride and cuprous chloride show that the grain boundary conductivity is ≈ 10 5 times that of the single crystal. The ratio of activation energy for grain boundary conduction to that of the single crystal is found to be 0.5–0.6 which is consistent with results obtained from other sources.

Microstructural characterization of Ni3Al processed by reactive atomization and deposition

Reactive atomization and deposition (RAD) is a new processing technique that has been developed to synthesize dispersion-strengthened materials. In this process, atomization,in situ reaction, and consolidation are combined into a single step by spray atomization and deposition with a reactive gas. The matrix material selected for this study is an Ni3Al + Y + B alloy in combination with N2-O2 atomization gas. The as-deposited microstructures reveal a spheroidal grain morphology, a banded structure, and a γ + γ′ mosaiclike structure. The formation of the γ + γ′ mosaiclike structure is attributed to an annealing effect during deposition. Matrix-lattice mismatches of 0.5 to 1 pct at the γ/γ′ interface and {100} growth orientations of γ′ phase are deduced from microscopic observations. The formation of the banded structure is attributed to the high cooling rate that is inherent to RAD processing. Anticipated dispersoids, such as Y2O3, Al2O3, and Y3Al5O12 are identified using transmission electron microscopy (TEM). Dislocation pileups and grain boundary pinning are observed in the vicinity of oxide dispersoids. The origin and movement of dislocations in the as-deposited materials may be attributed to the residual stresses that originate from thermal gradients and the large amount of deformation experienced by the solid and semisolid droplets during impact. The preliminary results and analyses reported here suggest that the high thermal stability of the RAD processed Ni3Al using N2-15 pct O2 may be attributed not only to the hindering effect of oxide dispersoids on grain boundary mi-gration, but also to the high cooling rate experienced by the droplets during atomization and the short annealing effect experienced by the material during deposition.

Axisymmetric Textures in Alumina

The occurrence of axisymmetric textures prevails over a wide range of processing approaches for alumina-based materials. Mechanisms ranging from dislocation slip, recrystallization, anisometric grain rotation and grain growth all can contribute to the development of textures with similar characteristics. This paper documents neutron texture measurements of axisymmetric textures in hot-pressed, hot-forged and tape-cast alumina. Strong basal textures were introduced in the tape-cast samples during long term annealing in finer, non-tabular aluminas if glass and dopant additions are introduced. The implications of these results versus prior reports of textures in alumina and alumina matrix composites are discussed.

Thermal nucleation of grains and cavities in deformed metals

During cold-working of metals a certain part (2 - 10%) of the exerted work is stored in the body. The stored energy is borne predominantly by dislocations in grains. The dislocation structure can significantly influence the diffusive processes and grain boundary migration at elevated temperatures. The aim of this paper is to quantify the interaction of dislocations with grain boundaries and free surfaces. The analysis makes it possible to discuss the influence of the dislocation structure on the kinetics of nucleation of creep cavities. At the end of the paper the author reopens the problem of the nucleation of new grains by means of thermal fluctuations and finds arguments supporting the idea.

Image force on a lattice dislocation due to a grain boundary in b.c.c. metals

Abstract The elastic interaction energy ΔE between a dislocation and a grain boundary is calculated for different geometrical configurations and for different anisotropic b.c.c. metals. For a given grain boundary (R, θ) and for a given Burgers vector of the dislocation, the interaction energy may be attractive or repulsive according to the dislocation line and it always occurs in a set of directions for which ΔE = 0. By using the integral formalism, the general condition for which ΔE = 0 is derived and verified for some particular cases. A good correlation is found between the order of magnitude of the interaction energy ΔE and the order of magnitude of the anisotropy factor H. Finally, the distances up to which the image forces F 1 = ΔE/d (d is the distance between the dislocation and the grain boundary) may overcome the Peierls forces in different metals are calculated for two temperatures, 100 and 300 K, and practical consequences of the ‘dislocation—grain boundary’ interaction are pointed out.

Creep Behavior of MoSi2-SiC Composites

AbstractUsing a cylindrical indenter, the indentation creep behavior of hot pressed and HIPed MoSi2-SiC composites containing 0-40% SiC by volume, was characterized at 1000-1200°C, 258-362 MPa. The addition of SiC affects the creep behavior of MoSi2 in a complex manner by pinning grain boundaries during pressing, thus leading to smaller MoSi2 grains; by obstructing or altering both dislocation motion and grain boundary sliding; and by increasing the overall yield stress of the material. Comparisons are made between indentation and compressive creep studies. It is shown that under certain conditions, compressive creep and indentation creep measurements yield comparable results after correcting for effective stresses and strain rates beneath the indenter.

On deformation-induced continuous recrystallization in a superplastic AlLiCuMgZr alloy

The microstructural changes of a warm rolled AlLi alloy occurring during static annealing and superplastic deformation at 515°C were studied by means of transmission electron microscopy. Deformation induces a continuous recrystallization with a rapid subgrain growth and a rapid increase in boundary misorientations. The higher strain rate results in a faster subgrain growth and a finer recrystallized grain size. The increasing rate of boundary misorientations and the strain at which the average misorientation reaches about 20° increase with increasing strain rate. The increase in boundary misorientations is proportional to the subgrain growth during the whole static annealing process. Deformation results in a more rapid increase in boundary misorientation with subgrain size than static annealing. Dislocation gliding plays an important role before the formation of high angle grain boundaries during superplastic deformation. The absorption of dislocations into subgrain boundaries results in a more rapid increase in boundary misorientation during deformation. Thus, the mechanism of the deformation-induced continuous recrystallization is suggested to be the generation of dislocations in grains and the absorption of gliding dislocations into subgrain boundaries.

The effect of plastic deformation on fracture morphology of the sigma phase in the NbTiAl system

It has been observed that in bulk specimens indentation-induced plastic deformation at room temperature causes intergranular fracture in the σ-phse of the NbTiAl system. The TEM study of the indented regions has revealed that only intergranular microcracks, which develop when two slip bands impinge on both sides of a boundary or when a slip band arrives at a grain boundary triple point, are resulted by the dislocation—grain boundary interaction. Consequently, upon subsequent loading of the indented speciments the crack propagates intergranularly in these regions. In the undeformed σ-phase, the accidental cracks that develop during the thin foil preparation have been found to follower a predominantly transgranular path, similar to the bulk specimen, with no dislocation activity at their tips. In contrast, in the plastically deformed regions the accidental cracks follow the path along slip bands. This phenomenon is believed to be a thin-foil effect.

Raman microprobe analysis of chemically revealed extended defects in GaAs

The Raman microprobe is used for characterising chemically revealed defects in as-grown GaAs. Different microstructures were scanned with 1 mu m spatial resolution. Emphasis is paid to the breaking down of the Raman-backscattering selection rules. Precipitates and twins are studied in semi-insulating GaAs. Coupled phonon-plasmon modes are analysed in Te-doped GaAs (ND-NA approximately=1017 cm-3); dislocation grain boundaries and hillocks are scanned by the Raman microprobe, revealing misorientation and free carrier effects.

Mechanisms of Inelastic Deformation and Stress Relaxation in Thin Metallizations Bonded to Hard Substrates

ABSTRACTThe yield strength of metallic thin films bonded to hard substrates can be significantly higher than is customary for bulk samples of the same metal. This is related to the constrained nature of the deformation. The constrained deformation, as well as the commonly observed crystallographic texture of thin films, places restrictive conditions on the mechanisms of deformation that produce stress relaxation. In narrow aluminum based metallizations used as interconnects in large scale integrated circuits thermal stress induced voiding provides an effective means for stress relaxation. For these interconnects, the stress state is tensile after excursions to higher temperatures; the stresses relax mainly by dislocation glide and grain boundary sliding during the cooldown, while the longer term relaxation is governed by stress-induced voiding and dislocation creep.

Mechanisms of Inelastic Deformation and Stress Relaxation in Thin Metallizations Bonded to Hard Substrates

AbstractThe yield strength of metallic thin films bonded to hard substrates can be significantly higher than is customary for bulk samples of the same metal. This is related to the constrained nature of the deformation. The constrained deformation, as well as the commonly observed crystallographic texture of thin films, places restrictive conditions on the mechanisms of deformation that produce stress relaxation. In narrow aluminum based metallizations used as interconnects in large scale integrated circuits thermal stress induced voiding provides an effective means for stress relaxation. For these interconnects, the stress state is tensile after excursions to higher temperatures; the stresses relax mainly by dislocation glide and grain boundary sliding during the cooldown, while the longer term relaxation is governed by stress-induced voiding and dislocation creep.

Precipitation and irradiation damage in proton-irradiated palladium-chromium alloys

Irradiation damage of Pd-Cr alloys containing 15, 20 and 25 at % Cr was studied over the temperature range 100 to 550° C, primarily in samples irradiated to a dose of 0.7 d.p.a. The solubility limit in this range of temperatures varies from 22 to 38% Cr, and precipitation of a phase having the L12 crystal structure was observed in unirradiated samples of the 25% Cr alloy aged at temperatures as low as 100° C. Octahedrally shaped voids, with faces parallel to {1 1 1}, were found only in the 25% Cr alloy irradiated from 350 to 550° C, but not in the other alloys at any temperature. The undersized chromium atoms migrate to point-defect sinks during irradiation, resulting in solute segregation and, eventually, precipitation under certain conditions. The precipitation of the L12 phase, which is the thermodynamically stable phase at higher chromium contents, was irradiation-induced at dislocation loops, voids and grain boundaries in the more concentrated undersaturated alloys. This irradiation-induced precipitation was observed in the mid-range and surface regions of the samples containing 20 and 25% Cr, but not in those containing 15% Cr. The behaviour of these alloys is compared with Pd-Fe and Ni-Si alloys, which have also been proton-irradiated in this laboratory, and the similarities and differences are noted and discussed.

On the possibility of path restricted diffusion of hydrogen in bcc iron

Summary A simple model which restricts site occupancy in the diffusive path due to the binding of hydrogen to dislocations gives good agreement with available diffusivity data. The concentration of solute in the vicinity of the dislocation cores is high enough to restrict solute migration at room temperature. This suggests that hydrogen atoms diffuse in solute atmospheres restricted to regions near dislocation cores and grain boundaries.

New studies of X-ray diffraction line profiles from aggregates of distorted crystallites I

The fourth central moment of an X-ray diffraction profile from an aggregate of distorted crystallites has been expressed by Mitra (1964a) as a function of the crystallite size, strain and strain gradients in the specimen. While the usual methods of line profile analysis yield information regarding either the apparent strain or the rms strain, the present study provides additional information regarding strain distribution in the form of strain derivatives and rms displacements of atoms over a given distancet in the direction of study. The strain parameters like 〈ee′〉, 〈ee″〉 have been obtained from fourth moment of the strain profile against range plots. The strain parameters thus obtained have subsequently been used to determine the rms displacements of the atoms. Alloys of copper and zinc at different stages of cold working and annealing have been studied by this method. The results have been discussed in the light of dislocation distribution, polygonisation and grain growth as well as distortion waves in the distorted crystals.

Dislocations and Energy of Grain Boundaries in Crystal Structures with Covalent Bonds

AbstractIn covalent bounded crystal structures the construction of grain boundaries with low lattice deformation is possible by ring dislocations. The present work deals new aspects of the formation of binding rings. There will be introduced structure units of the grain boundaries as binding tetrahedrons enabling the formation of 5–7 and 5–7–n · 8‐ring dislocations. Due to these dislocations grain boundaries with rotation angles θ between 0° and 180° can be constructed. The dislocation density in the grain boundaries does not increase monotonously with the tilt angle θ, but reaches maxima for discrete rotation angles (θ = 26.53°, 109.47°, and 148.49°). It can be shown, that dangling bonds in these grain boundaries exist only in special angular ranges.

A dynamic lattice dislocation grain boundary sliding mechanism

Abstract An analysis of grain boundary sliding is made based on the hypothesis that the rate-limiting process involves, overall, the climb-limited movement along grain boundaries of lattice dislocations, which originate in the grain interiors. Reasonable values are calculated for the contribution of grain boundary sliding to the overall strain and for the stress exponent of the boundary sliding rate.

Flux pinning in high-current-density superconductors

A major application of superconducting wire materials is the generation of magnetic fields, often in large volumes, with particular strenth, homogeneity, and field gradients. To fabricate superconductors which can carry high current densities at high temperatures and fields, flux pinning, by crystal inhomogeneities, must be understood. This paper attempts to answer two questions about flux pinning. The first addresses the nature and strenght of the elementary interaction force (f) between one flux line (FL) and one obstacle; the second, the correct summation of these elementary interactions between the obstacles in a unit volume and the FL to the (total) volume pinning force F /SUB v/ = B X J /SUB c/ . The discussion is confined to NbTi and A15 superconductors such as Nb/sub 3/Sn and V/sub 3/Ga. Important pinning sites in these superconductors are dislocation walls, precipitates, small inclusions, voids, grain boundaries, and bubbles. A series of mathematical models which have been used in the past are presented and synthesized into a more sophisticated explanation of pinning.