Existence Of Dislocations Research Articles

Evolution of Stacking Fault and Dislocation during Dynamic Recrystallization of Inconel 625 Alloy

The purpose herein is to improve the understanding of microstructure evolution of inconel 625 alloy during the dynamic recrystallization (DRX) process. Herein, the evolution process of DRX is determined by the grain orientation spread (GOS) method, and 1100 °C is determined as the complete recrystallization temperature. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) techniques are used to investigate the density and morphology of dislocations and stacking faults (SFs), as well as the effects of dislocations and SFs during recrystallization. With increasing recrystallization fraction, the density of dislocations and SFs decreases gradually, the dislocation start is extended, and SFs appear on other crystal planes. The existence of dislocation and SF increases the nucleation energy and nucleation sites of recrystallization, respectively, which is beneficial to the nucleation of DRX.

3D printing of a titanium-tantalum Gyroid scaffold with superb elastic admissible strain, bioactivity and in-situ bone regeneration capability

The simultaneous achievement of admirable mechanical compatibility and osteoinduction in metallic implants can avoid stress shielding and facilitate osseointegration and osteogenesis. Herein, we reported a titanium-tantalum (Ti-Ta) Gyroid scaffold in-situ fabricated with selective laser melting (SLM), a powder-bed-fusion three-dimensional (3D) printing process, enabling superb elastic admissible strain (EAS), bioactivity and in-situ bone regeneration capability. The printed scaffold with 90% porosity exhibited a good combination of low elastic modulus (1.8 GPa) and high compressive yield strength (55.5 MPa), resulting in a superb EAS (3.03%) that is suitable for the reconstruction of cancellous bone. The mechanisms of the high EAS were ascribed to the formation of β(Ti, Ta) solid solution, ultrafine β grains accompanying with nanocrystalline α' grains, and the existence of dislocations and stacking faults. Bone-like apatite was spontaneously induced on the surface of the printed Ti-Ta alloy due to the generation of self-passivating Ta2O5 film, indicating a good biomineralization ability. Compared to pure Ti, the printed Ti-Ta alloy exhibited enhanced expression of vinculin, earlier cell extension, increased nuclei density, better cell proliferation, and the up-regulated expression of osteogenesis genes. Animal studies further validated that the printed Ti-Ta scaffold was capable to reinforce bone integration and accelerate bone regeneration. These findings provided a promising strategy for treating bone defects through 3D printing of metallic scaffolds.

Chemo-mechanical study of dislocation mediated ion diffusion in lithium-ion battery materials

A mechanically coupled diffusion model combined with finite element formulation is developed to study the influence of dislocations on ion diffusion in lithium-ion batteries. The dislocation is modeled by the regularized eigenstrain based on a non-singular continuum dislocation theory. The model is validated with the analytical solution of the stress field of edge dislocations and the solution for the stress-dependent equilibrium concentration around the dislocation. Simulation results on LiMn2O4 demonstrate strong ion enrichment and depletion on the tensile and compressive sides of an edge dislocation, respectively. A stronger influence of the edge dislocation on diffusion is found at a lower state-of-charge, which verifies the experimental observation reported in the literature. The diffusion-induced stress compensates partially the stress field of the edge dislocation and is ascertained to have a state-of-charge dependency. The existence of dislocation does not introduce obvious mobility anisotropy in the bulk material but it results in local mobility heterogeneity around the dislocation. A three-dimensional simulation of the diffusion along the edge dislocation line reveals that the pipe diffusion can be initiated or accelerated on the tensile side of the edge dislocation.

Dynamic observation of dislocation evolution and interaction with twin boundaries in silicon crystal growth using in – situ synchrotron X-ray diffraction imaging

The grown-in dislocation dynamics and interaction mechanisms with growth twins are investigated in-situ during the directional solidification of silicon crystal. The melting, solidification and cooling down process is performed in a dedicated installation at the European synchrotron radiation facility and is followed by X-ray Bragg diffraction imaging techniques (X-ray topography) at the mesoscale in real-time. Existing dislocations in the seed are observed to propagate in the up-grown crystal via replicas. They expand vertically with the moving solid-liquid interface being always aligned perpendicular to the growth front. During the solidification process when they meet a growth twin lamella (Σ3{111}), they neither pile-up nor transmit through the boundary. They are blocked by the twin, but they continue to move laterally behind the growth front due to the thermomechanical stresses in the system. The existence of dislocations at the solid-liquid interface, their evolution and interaction with twin boundaries is discussed, as growth proceeds, based on a detailed crystallographic analysis of the system.

Effects of Nb/Ti additions and heat treatment on the microstructure evolution and hardness of as-cast and directionally solidified NiAl–Cr(Mo) alloy

The effects of the addition of Nb and Ti and heat treatment on the microstructure evolution and hardness of the as-cast and directionally solidified NiAl–Cr(Mo) alloy were studied. Nb mainly solubilizes in the Cr(Mo) phase while Ti basically solubilizes in the NiAl phase. Precipitated phases of Cr2Nb and Ni2AlTi are formed and mainly distributed at the cell boundary. With the increasing Nb content, the primary Cr(Mo) phase increases significantly and does not show dendritic morphology. The eutectic coupled growth is suppressed and the lamellar microstructure becomes irregular. Fully eutectic microstructure with no primary Cr(Mo) phase is obtained in the directionally solidified NiAl–Cr(Mo)–Nb and NiAl–32Cr–6Mo–1Ti–1Nb alloys while dendrites + eutectic is observed in the directionally solidified NiAl–32Cr–6Mo–3Ti–3Nb alloy. After heat treatment, Nb and Ti are redistributed. The eutectic microstructure becomes relatively uniform and the precipitated phases become fine and dispersed. The addition of Nb and Ti can increase the hardness of the alloy due to the solid solution strengthening and precipitation strengthening. As the Nb content increases, the hardness further increases. The hardness of the directionally solidified alloys is higher than the as-cast alloys attributing to the regular microstructure. After heat treatment, the hardness of the all alloys decreases due to the varying microstructure morphology and phase distribution, especially the coarsening of NiAl precipitates and the existence of dislocations in Cr(Mo) phase.

Experimental study on micro-crack initiation in photovoltaic polycrystalline silicon wafer

Brittle fracture of polycrystalline silicon wafer is critical to long-term reliability of electronic devices and solar cells for its wide use as components or substrates in semiconductor industry. In order to observe its initiation under loads, classical tensile test was modified by introducing the poly-methyl methacrylate (PMMA) layer to determine critical stress of the micro-crack initiation in the wafer. The method is verified by the finite element computation and then the fracture criterion of the maximum principal stress criterion for the wafer material is deduced from a series tests on the material strength. It was found that the existence of dislocations and impurities in the wafer significantly influence the threshold stress levels which is modeled by the Weibull distribution. Especially, micro-cracks are more likely to initiate in the dislocation position but not in the impurity position. The proposed method can be further extended for studying the tensile fracture and related issues on such brittle materials as ceramics, quartz, and ice.

Effect of annealing on the microstructures and deformation behaviour of Ti–50.7at.%Ni shape memory alloy

In this paper Ti–50.7at.%Ni as a commercial material was examined to analyse the effect of annealing temperature on the microstructure and deformation behaviour. It was found that the microstructure and deformation behaviour of as-received material was affected by the existence of dislocations that was introduced during manufacturing process. The presence of martensite variant and the B2 austenite broadening in XRD spectra of as-received material verified this conclusion. However, annealing treatment has gradually eliminated this effect. The plateau region on stress–strain curve which was absent in the as-received material started to appear and get longer with increasing annealing temperature. Maximum tensile strength was observed in the sample annealed at 400 ℃ for 1 h. Annealing treatment above 500 ℃ transformed the B19′ martensite to B2 austenite, which leads to a decrease in the tensile strength of the material. Nearly perfect superelasticity was found in the as-received material and sample annealed at 300 ℃.

Analysis of mechanical properties of nanocrystalline Al+α-Al2O3 composites using molecular dynamics simulation

In this article, mechanical properties of nanocrystalline Al+α-Al2O3 composites are investigated using molecular dynamics simulations. The configurations of matrix and volume fraction of α-Al2O3 may affect the mechanical properties of the particle reinforced metal-matrix composites and are taken into account. The potentials for the Al+α-Al2O3 system developed by Xin Lai et al. are adopted to depict the interactions between Al and α-Al2O3. Monocrystal Al and Bicrystal Al based α-Al2O3 particle reinforced nanocomposites are modelled respectively. Results show that: (1) volume fraction of the particles has no explicit effects on the elastic modulus and ultimate strength in both monocrystal Al and bicrystal Al based matrix nanocomposites, (2) disappearance of valley in the stress-strain curve of bicrystal Al results from existence of dislocation in matrix of various orientations.

(Invited) The Impact of Oxide Precipitates on Minority Carrier Lifetime in Czochralski Silicon

Oxide precipitates form in silicon for microelectronic and photovoltaic applications, and act as strong recombination centres. We have measured the injection-dependence of minority carrier lifetime in ~50 samples from p-type and n-type Czochralski silicon wafers with a wide range of precipitate densities. We find that all the data can be parameterized in terms of two independent Shockley-Read-Hall centres. The first is at EV + 0.22eV, and has a capture coefficient for electrons 157 greater than that for holes. The second is at EC - 0.08eV and has a capture coefficient for holes ~1,200 greater than that for electrons. The density of the centres is approximately dependent on the precipitate concentration. The existence of dislocations and stacking faults around the precipitates increases the density of both centres.

Atom probe tomographic study of a friction-stir-processed Al–Mg–Sc alloy

The microstructure of a twin-roll-cast Al–4.5Mg–0.28Sc at.% alloy after friction-stir processing, performed at two tool rotational rates, was investigated by atom probe tomography. Outside the stir zone, the peak-aged alloy contains a high number density (∼8.0×1023m−3) of ∼1.5nm radius Al3Sc (L12) precipitates with a minor Mg content, providing an increase of ∼600MPa in the Vickers microhardness. In the stir zone of the sample processed at 400rpm rotational rate, the microhardness increase is mainly due to grain refinement, rather than precipitate strengthening, because the Al3Sc precipitates, with spherical lobed cuboids and platelet-like morphology, grow and coarsen to a 10–20nm radius. The Sc supersaturation across the stir-processed zone has a concentration gradient, which is higher on the retreating side and lower on the advancing side of the friction-stir tool. Hence, after aging at 290°C for 22h, the microhardness increase within the stir zone also displays a gradient due to precipitate strengthening with varying precipitate volume fractions. In the stir zone for the sample processed at 325rpm rotational rate, the microhardness increase is also predominantly due to grain refinement, as coarse Al3Sc precipitates form heterogeneously at grain boundaries with a platelet-like morphology. The hardness remains unchanged after a 290°C aging treatment. This is because the Al3Sc precipitates are highly heterogeneously distributed due to a combination of a small Sc supersaturation (0.05at.%) in the matrix, the existence of dislocations, and a large area per unit volume of grain boundaries (∼4–6×106m−1).

Microstructure characterization of ZrB2–SiC composite fabricated by spark plasma sintering with TaSi2 additive

Abstract Dense ZrB 2 –SiC composite was synthesized by spark plasma sintering with 10 vol.% TaSi 2 additive. When sintered at 1600 °C, core–shell structure was found existing in the sample. The core was ZrB 2 and the shell was (Zr,Ta)B 2 solid solution. This result was ascribed to the decomposition of TaSi 2 and the solid solution of Ta atoms into ZrB 2 grains. The solid solution process probably decreased the boride grain boundary active energy, contributing to the formation of coherent structure of grain boundaries. Additionally, the existence of dislocations in the boride grains indicated that the applied pressure also imposed an important effect on the densification of composite. When sintered at 1800 °C, owing to the atom diffusion, Ta atoms homogeneously distributed in the boride grains, leading to the disappearance of core–shell structure. The boundaries between (Zr,Ta)B 2 grains, as well as between boride grains and SiC particles, were still clear without amorphous phase existing.

Highly Stable Spatio-Temporal Mechanical Characterization of Nanocontact between Sharp Tips Using Electrostatic Microactuator inside Transmission Electron Microscope

A microelectromechanical systems-in-transmission electron microscope (MEMS-in-TEM) setup was established to characterize mechanical properties of a nanostructure captured or generated between tips, while observing its shape and deformation. This setup achieved a stable actuation for several tens of minutes with sub-nm accuracy, and a precise TEM observation of 0.2 nm in spatial resolution. The displacements of a tip-moving actuator with and without the nanostructure were measured from TEM images; the difference between them indicates a force applied to the nanostructure. The force was obtained by multiplying the displacement difference with a spring constant of supporting beams of the tip. Here, we performed an approach-formation-retraction-fracture experiment of a gold nanocontact between tips under TEM observation over 10 min at the actuation speed of 0.1 nm/s. The force during the retraction-fracture process was measured. The maximum force was 66 nN due to the work hardening by the existence of dislocations. This setup will be a powerful tool to examine the role of atomic scale structure for the mechanical characteristics and the extremely-low-speed kinetics.

Defect structures in nickel and SUS304SS formed by the collapse of cavitation bubbles

A mercury target in high-power spallation neutron sources is subjected to pressure waves induced by a proton beam. The subsequent formation and collapse of cavitation bubbles lead to cavitation damage on the target vessel, especially the beam window. The cavitation damage in Ni and austenitic stainless steel SUS304SS were studied by using an electro-Magnetic IMpact Testing Machine (MIMTM) developed to simulate the damage. The existence of dislocations, stacking fault tetrahedra and vacancies was detected by positron annihilation lifetime measurements in Ni, and non-cellular dislocation structures were observed by transmission electron microscopy in Ni and SUS304SS. In addition, a high density of twins was observed in SUS304SS. These results were compared with those of high-speed compression tests using a high-speed projectile, proving that the cavitation damage caused by MIMTM corresponded to high-speed deformation.

The growth of Sn whiskers with dislocation inclusion upon electromigration through a Cu/Sn3.5Ag/Au solder joint

Electromigration through a Cu/Sn3.5Ag/Au solder joint results in rotation of the intermetallic compounds (IMCs). The Sn whiskers induced within the unreacted solder are believed to be due to the mechanical stress induced by this IMC rotation. Microstructural investigation and the electron diffraction pattern , revealed by transmission electron microscopy , indicate that the crystalline Sn whiskers grew with inclusion of prominent dislocations. The existence of dislocations within the Sn whiskers suggests that whisker growth experiences turbulence.

21004 MIMTMを用いた水銀キャビテーションによる金属の壊食損傷構造(原子力と材料・構造・組織(1),OS.4 原子力と材料・構造・組織)

A mercury target in high-power spallation neutron sources is subjected to the pressure wave caused by proton beam. Subsequent formation and collapse of cavitation bubbles lead to cavitation damage on the target vessel, especially the beam window. The cavitation damage in Ni and austenitic stainless steel SUS304SS were studied by using MIMTM (electro-Magnetic IMpact Testing Machine) developed to simulate the damage. The existence of dislocations, stacking fault tetrahedra and vacancies was detected by positron annihilation lifetime measurement in Ni and non-cellar dislocation structures were observed by transmission electron microscopy in Ni and SUS316SS. High density of twins was also observed in SUS304SS. These results were compared with those of high speed compression test by using high speed projectile and proved that the cavitation damage by MIMTM corresponded to high speed deformation.

Effect of threading dislocations on carrier mobility in AlGaN/GaN quantum wells

The various scattering mechanisms induced by dislocations have been reviewed andadapted to the case of threading dislocations in AlGaN/GaN quantum wells. Thesescattering mechanisms can be classified into two categories, the first one issuingstraightforwardly from the dislocation strain field, the other one being due tothe Coulomb potential created by electrons trapped on the energy states thatdislocations may create in the GaN band gap. For the first category of mechanisms(strain field effects), we indicate that edge dislocations can only be connected withthe so-called deformation potential, the piezoelectric coupling being ruled outbecause of the particular geometry of the threading dislocation strain fields. Weshow that the dislocation deformation potential can only be responsible for avery weak, even negligible, effect on the carrier mobility. Then, after a survey ofthe various results found in the literature concerning the possible existence ofdislocation energy states we conclude that dislocations are responsible for theexistence of shallow acceptor states below the conduction band and propose amodel for describing the potential associated with such states when filled byelectrons. More particularly, we show that the linear dislocation charge densityresulting from the trapped carriers at dislocation states can not be uniform, as it issystematically assumed in the literature, and we propose a description of this linearcharge density as a function of the dislocation energy state position and of thevarious features characterizing the quantum well. Using the scattering potentialinduced by such a spatially-dependent dislocation charge density together withthe usual scattering mechanisms allows us to give an estimation of their effecton the free carriers’ mobility. We particularly show that at low carrier density(∼1012 cm−2) the mobility is mainly determined by the combination of dislocation scatteringmechanisms and intrinsic scattering mechanisms. Finally we suggest that our model couldbe employed for determining the position of the dislocation energy level in the gap.

Dislocation multi-junctions and strain hardening

At the microscopic scale, the strength of a crystal derives from the motion, multiplication and interaction of distinctive line defects called dislocations. First proposed theoretically in 1934 (refs 1-3) to explain low magnitudes of crystal strength observed experimentally, the existence of dislocations was confirmed two decades later. Much of the research in dislocation physics has since focused on dislocation interactions and their role in strain hardening, a common phenomenon in which continued deformation increases a crystal's strength. The existing theory relates strain hardening to pair-wise dislocation reactions in which two intersecting dislocations form junctions that tie the dislocations together. Here we report that interactions among three dislocations result in the formation of unusual elements of dislocation network topology, termed 'multi-junctions'. We first predict the existence of multi-junctions using dislocation dynamics and atomistic simulations and then confirm their existence by transmission electron microscopy experiments in single-crystal molybdenum. In large-scale dislocation dynamics simulations, multi-junctions present very strong, nearly indestructible, obstacles to dislocation motion and furnish new sources for dislocation multiplication, thereby playing an essential role in the evolution of dislocation microstructure and strength of deforming crystals. Simulation analyses conclude that multi-junctions are responsible for the strong orientation dependence of strain hardening in body-centred cubic crystals.

Evaluation of Polycrystalline Silicon for Solar Cells by Small p-n Diode Array

ABSTRACTA small p-n diode array was fabricated on a polycrystalline Si substrate and the electrical characteristics were measured for each small diode to evaluate the distribution of energy conversion efficiency in the substrate. The crystal qualities in conjunction with the electrical characteristics were also evaluated. We found large variations in measuring the current-voltage (I-V) characteristics of the p-n diode. We also observed variations in quality even in diodes without any grain boundaries at the p-n junction. Therefore, we evaluated crystalline quality using various techniques to compare the diode characteristics. We found clear evidence in photo-luminescence (PL) mapping, where grains, including degraded diodes, were darker in the mapping, implying lower PL intensities than the others. The PL spectra obtained from the “dark grains” included D-lines indicating the existence of dislocations. We could conclude that the electrical characteristics of p-n diodes were not only affected by grain boundaries but also by crystalline defects evaluation such as dislocations. We observed a Secco-etched surface for crystalline defects evaluation using an optical microscope. The origins of etch pits were also determined by transmission electron microscope (TEM) and three different types of defects were confirmed.

On the Interaction of Dislocations with Impurities in Silicon

Dislocations and impurities in silicon have been widely investigated since many years, nevertheless many questions on this subject remain still unsolved. As an example, theory, models and experimental phenomena provide evidence of the existence of shallow bands in silicon induced by the dislocation strain field. Nevertheless, only deep bands, likely associated with contamination at dislocations, have been detected up to now by junction spectroscopy. The present contribution reviews several results, obtained by the authors, on dislocation impurity interactions and their effects on the electronic properties of defect states in silicon. Point and extended defects introduced in p-type Cz Si by oxygen precipitation and plastic deformation have been investigated with electrical methods. Different materials (oxygen precipitated and deformed Cz Si and Fz Si) were examined in order to separate the role of oxygen precipitation, plastic deformation and metallic contamination on non-radiative electronic transitions at defect centers. A deep hole trap, named T1, has been associated to dislocation-related impurity centers, while additional deep traps have been related to contamination by grown-in transition metals and to clusters involving oxygen atoms. Moreover, experimental results obtained by junction spectroscopy assessed the existence of dislocation related shallow states. These were found to be located at 70 and 60 meV from the valence and conduction band edge, respectively.

Magnetoelasticity in dysprosium–scandium superlattices

Abstract We have studied the magnetoelastic stress parameter, B γ ,2 , associated with the shear deformation within the basal plane of the (0 0 0 1) Dy (20 A)/Sc ( t =20–60 A) superlattices (SLs). The extrapolated B γ ,2 (0 K) values change from 1.45 to 0.9 GPa when t changes from 60 to 20 A, respectively. All SLs show a thermal behavior of B γ ,2 affected by volume and interfacial contributions. The scandium thickness dependence of the volume contribution shows a surprisingly logarithmic behavior that indicates the existence of dislocations in the Dy blocks.