Decoration Of Dislocations Research Articles

Deformation behavior, twinning propensity model, and twinnability map for laser powder bed fused austenitic steel homogenized by hot isostatic pressing

Hot isostatic pressing (HIP) has emerged as a highly effective and extensively adopted technique for eliminating intrinsic flaws, mitigating residual stress, and addressing microstructural anisotropy in laser powder bed fused (L-PBF) metallic materials. However, the deformation behavior, strengthening mechanism, and associated mechanistic models of HIPed l-PBF austenitic steel remain largely unclear. Through multiple microstructural characterizations, this study determined that the primary deformation mechanism of HIPed l-PBF austenitic steel involved the emission of twinning partials from either the annealing twin boundaries or high-angle grain boundaries (HAGBs) with the decoration of extra dislocations and prismatic inclusions. A twinning propensity model was formulated based on the identified deformation mechanisms, revealing that the critical angle (θτ-crit) for twinning partial emissions ranged from 25.64° to 28.46° By incorporating the mathematical relationship describing the force equilibrium state between trailing and twinning partials, a twinnability map considering both the crystallographic orientation and stress state (magnitude and direction) was established to guide the twinning control for desired mechanical properties. Finally, the accuracy of the twinning model and twinnability map was effectively confirmed using the microstructural results, whereas the limitations of the twinning model and twinnability map were reasonably examined through micro-defect and texture analysis. The results of this study provided a significant theoretical framework for understanding the twinning behavior of high-performance and homogeneous l-PBF metallic materials.

PL Signatures from Decoration of Dislocations in SiC Substrates and Epitaxial Wafers

Photoluminescence (PL) signatures of 4H-SiC bare and epitaxial wafers from a surface inspection tool have been studied. Large variations in PL black or white dot densities were confirmed for comparable crystal quality and growth process conditions. Comparison with KOH etching results confirms that both PL black and white dots are tied to discrete threading dislocations. PL spectra results suggest dislocation decoration by donor-acceptor pairs.

In situ studies on the evolution of damage microstructures in tungsten under heavy ion irradiation and post annealing

The evolution of heavy-ion irradiation induced microstructures, followed by annealing experiments, was analyzed in situ and characterized systematically in tungsten via transmission electron microscopy. The formation and evolution of loop rafts, dislocation decoration, and tangled dislocation networks were analyzed during irradiation. Consisting of 1/2<111> loops, raft structures had a preferred orientation, which was dependent on loop geometry. Decoration of a near-edge line dislocation with loops led the line first to curve and then annihilate. Drastic loss of loops, loop-loop coalescence, glide of jogged line dislocations as well as untangling of dislocation networks were observed during post-annealing. The gliding of glissile loops varied with its size, while the motion of line dislocations depended on their nature. The glide distance for the small loops with a size range from 1 to 3 nm was 6 times larger than the large loops with a size range from 5 to 8 nm. A near-edge dislocation had the largest mobility, a near-screw one had the lowest mobility, and a mixed one had the mobility in between. The direct observation and comprehensive analysis of the complicated interaction among dislocations will offer insights into the bridge between the microstructural evolution and macroscopic response to irradiation damages.

Decoration and Density Increase of Dislocations in PVT-Grown SiC Boules with Post-Growth Thermal Processing

Post-growth thermal processing at higher temperature generates more BPDs (basal plane dislocations). It is observed that dislocation visibility in surface inspection tool images varies significantly even at comparable dislocation densities. Combination of dislocation decoration and light absorbance from SiC matrix by point defects or dopants has been proposed as a working hypothesis to explain dislocation visibility variations.

Depth distribution of irradiation-induced dislocation loops in an Fe-9Cr model alloy irradiated with Fe ions: The effect of ion energy

• Cross-sectional STEM was applied to characterize Fe-9Cr irradiated with Fe ions of 1 and 5 MeV. • The arrangement and size of dislocation loops depends on the distance from the irradiated surface. • Irradiated microstructures correlate with the profiles of displacement damage and injected interstitials. • For 5 MeV ions, decoration of dislocations with loops was found at depths smaller than 0.8 µm. • The arrangement of loops in this region resembles the case of neutron irradiation; no such region forms for 1 MeV ions. It is generally accepted that the microstructure of ion-irradiated Fe-based alloys does not only depend on the local level of displacement damage and the initial microstructure. Other factors such as the vicinity of a surface and the injected ions also play a role and may give rise to peculiar depth dependencies of the irradiated microstructure. Some investigators reported a band-like appearance indicating depth ranges of relatively uniform microstructure clearly distinguished from other ranges. Clarification is important for at least two purposes: first, to identify a depth range suitable for gaining meaningful information about the behaviour of materials exposed to neutron irradiation and, second, to correctly interpret results obtained by methods, such as nanoindentation, that integrate over extended depth ranges. A variation of the ion energy is expected to gain additional insight. In this work, two samples of Fe-9%Cr were irradiated at 300 °C with Fe 2+ ions, one sample using 1 MeV ions and another sample using 5 MeV ions. Calculations using the binary collision code SRIM indicate displacement damage peaks at depths of 0.3 and 1.3 µm for ion energies of 1 and 5 MeV, respectively. The depth distribution of irradiation-induced dislocation loops was studied by cross-sectional scanning transmission electron microscopy (STEM). Loops visible in the STEM images were found to be arranged within two bands with the positions of these bands depending on the profiles of displacement damage and injected interstitials. The first and second band exhibit noticeably different number densities and mean sizes of the loops. For the 5 MeV irradiation, an extended range between the sample surface and the first band was observed, where decoration of preexisting line dislocations with loops is dominant. This microstructure resembles cases reported for neutron irradiation. For the 1 MeV irradiation, such a range does not exist. Estimates characterizing the loop size and number density in the distinct depth ranges are provided.

In-situ WB-STEM observation of dislocation loop behavior in reactor pressure vessel steel during post-irradiation annealing

To better understand the thermal stability of dislocation loops formed by long-term neutron irradiation in reactor pressure vessel (RPV) steels, in-situ scanning transmission electron microscopy (STEM) observation was performed for a surveillance test specimen of a European pressurized water reactor (PWR). The surveillance test specimen was neutron irradiated to a fluence of 8.2 × 1023 neutrons•m−2. A membrane sample from the surveillance test specimen was annealed at 673 K and 723 K for 30 min using a heating holder, and the same area was in-situ observed under STEM. After annealing, dislocation loops with Burgers vectors of ½ 〈111〉 and 〈100〉 were quantitatively examined. When annealing temperature increased from 673 K to 723 K, the number of dislocation loops decreases, whereas the size of them becomes larger. Correspondingly, the proportion of 〈100〉 dislocation loops changes from 27% to 45%. The ratio of 〈100〉 to ½ 〈111〉 loops increases with annealing temperature rising. The evolution process of dislocation loops during annealing at 723 K was in-situ observed and analyzed to shed light on the transformation mechanism of dislocation loops going from ½ 〈111〉 to 〈100〉. It is the first time to directly observe that two ½ 〈111〉 dislocation loops collide with each other and coalesce to form a 〈100〉 dislocation loop. Moreover, small ½ 〈111〉 dislocation loops could be absorbed by large 〈100〉 dislocation loops, whereas the Burgers vector of 〈100〉 loops remained unchanged. Dislocation decoration occurs during annealing due to the interaction between dislocations and loops. The dislocations decorated by loops are pretty stable during the continuous annealing process, which is well explained by molecular dynamics simulation.

TEM Observation of Loops Decorating Dislocations and Resulting Source Hardening of Neutron-Irradiated Fe-Cr Alloys

Several open issues remain concerning the quantitative understanding of irradiation hardening in high-Cr steels. One of these issues is addressed here by correlating yield points that are observed in stress-strain curves with dislocation decoration observed by TEM for neutron-irradiated Fe-Cr alloys. It is found that both higher neutron exposure and higher Cr content promote irradiation-induced loops to arrange preferentially along dislocation lines. Consequently, the activation of dislocation sources requires unlocking from the decorating loops, thus resulting in a yield drop. This process is considered within the source hardening model as opposed to the dispersed barrier hardening model, the latter aimed to describe dislocation slip through a random array of obstacles. Microstructure-informed estimates of the unlocking stress are compared with measured values of the upper yield stress. As functions of neutron exposure, a cross-over from the dominance of dispersed-barrier hardening accompanied by smooth elastic-plastic transitions to the dominance of source hardening accompanied by yield drops is observed for Fe-9% Cr and Fe-12% Cr.

The Impact of Ti/Al Contacts on AlGaN/GaN HEMT Vertical Leakage and Breakdown

Enhanced leakage paths below Ti-/Al-based contacts to GaN-on-Si HEMTs have been identified and studied. Through a novel use of the quasi-static capacitance–voltage technique, the depth of these preferential leakage paths was determined to be $\sim 1.6~\mu \text{m}$ , extending down into the superlattice strain relief layer. Along these paths, the material resistivity was reduced by more than a factor of 100 compared to the uncontacted epitaxy. It is suggested that the cause of the additional leakage is decoration of dislocations. This result is important for understanding buffer transport, a critical parameter for breakdown, and buffer charge storage.

Detailed transmission electron microscopy study on the mechanism of dislocation loop rafting in tungsten

Dislocation loop rafting and dislocation decoration have been previously observed in neutron and heavy ion irradiated materials. Understanding the fundamental aspects of these phenomena assist in evaluating irradiation damage of nuclear materials. Multiple different mechanisms have been suggested to explain loop rafting. Here, we performed a detailed transmission electron microscopy study on dislocation loop rafts in heavy ion irradiated tungsten. Different imaging conditions showed that the rafts are of <111> Burgers vector type and specifically the same <111> Burgers vector variant (<11¯1>) in the particular grain analyzed. Some rafts were associated with dislocation lines while some form as a result of alignment of dislocation loops. They were shown to form at both room temperature (RT) and high temperature with stronger rafts forming at RT. These observations confirm the mechanism previously suggested by Wen et al. which explains raft formation due to loop glide, cluster-cluster and grown-in dislocation-cluster interaction with subsequent Burgers vector rotation. Similar irradiation studies on nanocrystalline tungsten showed that these materials are more resistant to raft formation at RT irradiations.

Precipitates originating from tungsten crucible parts in AlN bulk crystals grown by the PVT method

In order to evaluate the possible involvement of crucible materials in the growth of AlN bulk crystals grown by physical vapor transport, we applied growth conditions with a high vertical thermal gradient and hence high supersaturation of aluminum vapor. Under these conditions, precipitates formed causing diffuse grayish substructures at the initial growth interface and in the crystal body, decorating dislocations. Electron microscopy studies revealed that the precipitates are elongated, single‐phase particles with sizes of 50–500 nm of commensurate structure, oriented along the &lt;110&gt; direction. Chemical analysis of the precipitates showed tungsten as well as carbon and oxygen. The lattice parameters of the precipitates are in close agreement to hexagonal tungsten hemicarbide (W2C). The possible transport from the tungsten parts and its conversion into tungsten hemicarbide precipitates is discussed. We thus conclude that the W2C precipitates may contribute to the decoration of dislocations, even in growth with moderate thermal gradients.

The Impact of Dislocation Structure on Impurity Decoration of Dislocation Clusters in Multicrystalline Silicon

Light microscopy, electron backscatter diffraction and transmission electron microscopy is employed to investigate dislocation structure and impurity precipitation in commonly occurring dislocation clusters as observed on defect-etched directionally solidified multicrystalline silicon wafers. The investigation shows that poligonised structures consist of parallel mostly similar, straight, well-ordered dislocations, with minimal contact-interaction and no evidence of precipitate decoration. On the other hand, disordered structures consist of various dislocation types, with interactions being common. Decoration of dislocations by second phase particles is observed in some cases. Enhanced recombination activity of dislocations may therefore be a result of dislocation interaction forming tangles, microscopic kinks and jogs, which can serve as heterogeneous nucleation sites that enhance metallic decoration.

Effect of Mn addition on one-dimensional migration of dislocation loops in body-centered cubic Fe

Elucidation of the one-dimensional (1-D) motion of dislocation loops is important for describing the microstructural development of materials under irradiation. In this study, the effect of Mn on radiation-induced microstructure evolution in body-centered cubic Fe was experimentally investigated by focusing on the migration of dislocation loops. Pure Fe and Fe–1.4Mn alloy were irradiated with Fe3+ ions to introduce dislocation loops. In pure Fe, inhomogeneous distribution of loops in the vicinity of the residual dislocation was observed. However, in Fe–1.4Mn, isolated dislocation loops were homogeneously distributed in a high number density. In situ transmission electron microscopy during annealing revealed that 1-D motion of dislocation loops occurred in pure Fe at 623K, while 1-D motion of dislocation loops occurred minimally in Fe–1.4Mn annealed at temperatures below 773K. These results indicate that 1-D motion of dislocation loops play a key role in producing the differences in the microstructures between pure Fe and Fe–1.4Mn. In pure Fe, dislocation loops were mobile and trapped in the strain field of a dislocation, leading to the formation of loop decoration of dislocations. However, in Fe–1.4Mn, dislocation loops were less mobile and dislocation loops were homogeneously formed in high density in the matrix. The migration of dislocation loops by Mn solute is strongly suggested as one of the key mechanisms of microstructure development in irradiated Fe–Mn alloy.

Electrochemical Evaluation of Radiation-Induced Segregation in Austenitic Stainless Steels with Oversize Solute Addition

Effect of different levels of oversize element, cerium, on radiation-induced segregation (RIS) in type 316 stainless steel was investigated. The effect of prior cold-work on RIS was also investigated. Samples with 0.00, 0.01, and 0.04 wt.% cerium were irradiated to 0.70 dpa using 4.8 MeV protons at 300 °C. Characterization of proton-irradiated specimens was carried out using electrochemical potentiokinetic reactivation (EPR) test followed by atomic force microscopic examination. The specimen with prior cold-work (without cerium addition) showed the lowest EPR values indicating the lowest chromium depletion in this material. The specimen with 0.04 wt.% cerium showed the lower EPR value as compared to the specimen with 0.01 wt.% Ce. The irradiated specimen with prior cold-work showed linear features after the EPR tests and such features were attributed to decoration of dislocations, generated due to prior cold-work, by point defects produced during irradiation. The resistance to RIS offered by cold-work (linear features) has been more effective as compared to that by the addition of oversize solute addition.

Grain boundary energies and cohesive strength as a function of geometry

Cohesive laws are stress-strain curves used in finite element calculations to describe the debonding of interfaces such as grain boundaries. It would be convenient to describe grain boundary cohesive laws as a function of the parameters needed to describe the grain boundary geometry; two parameters in two dimensions and five parameters in three dimensions. However, we find that the cohesive law is not a smooth function of these parameters. In fact, it is discontinuous at geometries for which the two grains have repeat distances that are rational with respect to one another. Using atomistic simulations, we extract grain boundary energies and cohesive laws of grain boundary fracture in two dimensions with a Lennard-Jones potential for all possible geometries which can be simulated within periodic boundary conditions with a maximum box size. We introduce a model where grain boundaries are represented as high symmetry boundaries decorated by extra dislocations. Using it, we develop a functional form for the symmetric grain boundary energies, which have cusps at all high symmetry angles. We also find the asymptotic form of the fracture toughness near the discontinuities at high symmetry grain boundaries using our dislocation decoration model.

Influence of dislocation decoration with mobile donors on exciton luminescence in ZnO crystals

AbstractIn ZnO single crystals, the influence of grown‐in and introduced by mechanical damage dislocations on room temperature exciton luminescence was investigated. It was found that the higher dislocation density the weaker the luminescence and the lower the intensity of its short‐wavelength side with respect to that of long‐wavelength one. After introduction of dislocations, gradual quenching of exciton luminescence and the suppression of its short‐wavelength side took place. Observed effects were accounted for by the formation of optical absorption “tail” due to decoration of dislocations with mobile shallow donors, which resulted in the increase of reabsorption of emitted light in the crystal. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Orthodox etching of HVPE-grown GaN

Orthodox etching of HVPE-grown GaN in molten eutectic of KOH+NaOH (E etch) and in hot sulfuric and phosphoric acids (HH etch) is discussed in detail. Three size grades of pits are formed by the preferential E etching at the outcrops of threading dislocations on the Ga-polar surface of GaN. Using transmission electron microscopy (TEM) as the calibration tool, it is shown that the largest pits are formed on screw, intermediate on mixed and the smallest on edge dislocations. This sequence of size does not follow the sequence of the Burgers values (and thus the magnitude of the elastic energy) of corresponding dislocations. This discrepancy is explained taking into account the effect of decoration of dislocations, the degree of which is expected to be different depending on the lattice deformation around the dislocations, i.e. on the edge component of the Burgers vector. It is argued that the large scatter of optimal etching temperatures required for revealing all three types of dislocations in HVPE-grown samples from different sources also depends upon the energetic status of dislocations. The role of kinetics for reliability of etching in both etches is discussed and the way of optimization of the etching parameters is shown.

Metal precipitation at grain boundaries in silicon: Dependence on grain boundary character and dislocation decoration

Synchrotron-based analytical microprobe techniques, electron backscatter diffraction, and defect etching are combined to determine the dependence of metal silicide precipitate formation on grain boundary character and microstructure in multicrystalline silicon (mc-Si). Metal silicide precipitate decoration is observed to increase with decreasing atomic coincidence within the grain boundary plane (increasing Σ values). A few low-Σ boundaries contain anomalously high metal precipitate concentrations, concomitant with heavy dislocation decoration. These results provide direct experimental evidence that the degree of interaction between metals and structural defects in mc-Si can vary as a function of microstructure, with implications for mc-Si device performance and processing.

Gettering of transition metal impurities during phosphorus emitter diffusion in multicrystalline silicon solar cell processing

We have investigated the gettering of transition metals in multicrystalline silicon wafers during a phosphorus emitter diffusion for solar cell processing. The results show that mainly regions of high initial recombination lifetime exhibit a significant lifetime enhancement upon phosphorus diffusion gettering. Nevertheless, transition metal profiles extracted by secondary ion mass spectrometry in a region of low initial lifetime reveal significant gradients in Cr, Fe, and Cu concentrations towards the surface after the emitter diffusion, without exhibiting a significant enhancement in the lifetime. In a region of higher initial lifetime, however, diminutive concentration gradients of the transition metal impurities are revealed, indicating a significantly lower initial concentration in these regions. From spatial maps of the dislocation density in the wafers, we find that lifetime enhancements mainly occur in regions of low dislocation density. Thus, it is believed that a generally higher concentration of transition metals combined with an impurity decoration of dislocations in regions of high dislocation density limit the initial lifetime and the lifetime after the phosphorus diffusion, in spite of the notable gettering of transition metal impurities towards the surface in these regions. Furthermore, after a hydrogen release from overlying silicon nitride layers, we observe that only regions of low dislocation density experience a significant lifetime enhancement. This is attributed to impurity decoration of the dislocations in the regions of both high dislocation density and high transition metal impurity concentration, reducing the ability of hydrogen to passivate dislocations in these regions.

Dislocation decoration and raft formation in irradiated materials

Experimental observations of dislocation decoration with self-interstitial atom (SIA) clusters and of SIA cluster rafts are analysed to establish the mechanisms controlling these phenomena in bcc metals. The elastic interaction between SIA clusters, and between clusters and dislocations is included in kinetic Monte Carlo (KMC) simulations of damage evolution in irradiated bcc metals. The results indicate that SIA clusters, which normally migrate by 1D glide, rotate due to their elastic interactions, and that this rotation is necessary to explain experimentally-observed dislocation decoration and raft formation in neutron-irradiated pure iron. The critical dose for raft formation in iron is shown to depend on the intrinsic glide/rotation characteristics of SIA clusters. The model is compared with experimental observations for the evolution of defect cluster densities (sessile SIA clusters and nano-voids), dislocation decoration characteristics and the conditions for raft formation.

Tunnel-recombination currents and electroluminescence efficiency in InGaN/GaN LEDs

The mechanism of injection loss in p-GaN/InGaN/n-GaN quantum-well LEDs is analyzed by studying the temperature and current dependences of external quantum efficiency in the temperature range 77–300 K and by measuring transient currents. The data obtained are interpreted in terms of a tunnel-recombination model of excess current, which involves electron tunneling through the potential barrier in n-GaN and the over-barrier thermal activation of holes in p-GaN. At a low forward bias, the dominant process is electron capture on the InGaN/p-GaN interface states. At a higher bias, the excess current sharply increases due to an increase in the density of holes on the InGaN/p-GaN interface and their recombination with the trapped electrons. The injection of carriers into the quantum well is limited by the tunnel-recombination current, which results in a decrease in efficiency at high current densities and low temperatures. The pinning of the Fermi level is attributed to the decoration of heterointerfaces, grain boundaries, and dislocations by impurity complexes.