Dislocation Growth Research Articles

Dislocation Loop Generation Differences between Thin Films and Bulk in EFDA Pure Iron under Self-Ion Irradiation at 20 MeV

In the present investigation, high-energy self-ion irradiation experiments (20 MeV Fe+4) were performed on two types of pure Fe samples to evaluate the formation of dislocation loops as a function of material volume. The choice of model material, namely EFDA pure Fe, was made to emulate experiments simulated with computational models that study defect evolution. The experimental conditions were an ion fluence of 4.25 and 8.5 × 1015 ions/cm2 and an irradiation temperature of 350 and 450 °C, respectively. First, the ions pass through the samples, which are thin films of less than 100 nm. With this procedure, the formation of the accumulated damage zone, which is the peak where the ions stop, and the injection of interstitials are prevented. As a result, the effect of two free surfaces on defect formation can be studied. In the second type of experiments, the same irradiations were performed on bulk samples to compare the creation of defects in the first 100 nm depth with the microstructure found in the whole thickness of the thin films. Apparent differences were found between the thin foil irradiation and the first 100 nm in bulk specimens in terms of dislocation loops, even with a similar primary knock-on atom (PKA) spectrum. In thin films, the most loops identified in all four experimental conditions were b ±a0<100>{200} type with sizes of hundreds of nm depending on the experimental conditions, similarly to bulk samples where practically no defects were detected. These important results would help validate computational simulations about the evolution of defects in alpha iron thin films irradiated with energetic ions at large doses, which would predict the dislocation nucleation and growth.

Effect of Graphene Sheets Aggregation on The Dislocation-Blocking Mechanism of Nanolaminated Aluminum/Graphene Composite: Molecular Dynamics Simulation Study

Aluminum/graphene nanolaminated structures have a very proper reinforcing and toughening effect on aluminum composites. Graphene layers effectively prevent the growth and movement of dislocations in the aluminum matrix. Therefore, more and shorter dislocations lines occur in the aluminum matrix between the graphene layers. In this paper, tensile tests have been performed on nanolaminated aluminum/graphene composite using molecular dynamic simulation to study the dislocation-blocking mechanism and its reinforcing and toughening effect. The nucleation, expansion, and displacement of the dislocation in the aluminum matrix were investigated under tension. The results showed that the reinforcement mechanism includes increasing displacement density and shear stress transfer. Besides, the reinforcing and toughening effects were investigated as a function of the distance between the graphene sheets (the spacing of sheets between 4-14 A). The results showed that the distance between the graphene sheets has an effective role in creating the dislocation-blocking mechanism in the aluminum matrix. Decreased graphene sheets increase the mechanical properties of the aluminum matrix due to the dislocation-blocking mechanism, which can be limited by the onset of graphene sheet aggregation. As the result, stable steps in 10-12 A distance between graphene sheets were obtained by dislocations with a yield strength of about 14 GPa and yield strain of 0/065.

Growth defects of organic crystals: A review

During the growth of organic crystals, crystal defects can be formed. Microscopic defects, which include point, line, and planar defects, appearing as vacancies, growth dislocations, striations, and others, play an important role in the mechanical properties, optical properties, and electrical conductivity of organic crystals. In addition, macroscopic defects, such as fluid inclusions and cavities, may cause toxicity during crystal dehydration and degradation while favorable cavity defects can satisfy certain functions. Herein, this paper summarizes the growth defects of organic crystals from four perspectives; formation mechanism, characterization methods, control methods, and applications. We conclude three kinds of formation mechanism which are related to diffusion of solute molecules, impurities participating in the crystal growth, and structure of solute molecules respectively. Next, the characterization methods are exhibited from the perspectives of microscopy, spectroscopy, and others for different research areas. Then, we offer some control methods of defects which include adjustment of crystallization operation parameters, assistant of ultrasound and additives, irradiation, and ion implantation. As for applications, organic crystal defects have application potential in the fields of pharmacy and materials, for example, nonlinear optical materials, energetic materials, and semiconductor materials. Finally, we put forward the future outlooks on the growth defects of organic crystals.

Assessment of Quartz Grain Growth and the Application of the Wattmeter to Predict Quartz Recrystallized Grain Sizes

AbstractRelationships between the recrystallized grain size and stress are investigated for experimentally deformed water‐added quartz aggregates. For stresses ≥100 MPa there is a variation in the measured recrystallized grain size for a given stress. This variation correlates with a change in the c‐axis fabric in general shear experiments, where samples with larger recrystallized grain sizes for a given stress have dominantly prism <a> c‐axis fabrics and samples with smaller recrystallized grain sizes for a given stress have dominantly basal <a> c‐axis fabrics. The dislocation creep flow law also changes at conditions where these two c‐axis fabrics form (Tokle et al., 2019, https://doi.org/10.1016/j.epsl.2018.10.017). Using the wattmeter model (Austin & Evans, 2007, https://doi.org/10.1130/G23244A.1), different piezometric relationships are quantified for samples that develop prism <a> and basal <a> c‐axis fabrics, respectively. The wattmeter model is sensitive to grain growth kinetics; a new grain growth law for quartz is formulated based on reanalysis of microstructures in samples from previous work. The activation enthalpies and water fugacity exponents for our grain growth law and dislocation creep flow laws are the same within error, suggesting the recrystallized grain size versus stress relationships are nearly independent of temperature and water fugacity, consistent with laboratory observations. The wattmeters successfully predict the recrystallized grain size versus stress relationships of all quartzite samples from experiments with added water. These results support the use and extrapolation of the wattmeter model for both experimental and geologic conditions to investigate the stress state and grain size evolution of quartz rich rocks.

Plasma assisted molecular beam epitaxy growth mechanism of AlGaN epilayers and strain relaxation on AlN templates

Several series of high Al composition AlGaN epilayers were grown on AlN/sapphire templates by plasma assisted molecular beam epitaxy. Three growth regions, Ga droplets, intermediate Ga-rich, and N-rich, have been identified based on in situ and various ex situ characterizations, and AlGaN growth diagrams have been summarized. The Al composition is basically determined by Al supply when it is less than the effective N radical beam supply. X-ray diffraction reciprocal space maps have revealed residual strain status of AlGaN on AlN, demonstrating lattice relaxation as the layer thickness increases. The experimental critical thicknesses are observed increasing as Al composition increases, but they deviate from the theoretical values, which is attributed to the combined influence of nonequilibrium growth conditions, kinetics, phase separation and dislocation generation. Energy bandgaps were determined by optical absorption edges, suggesting a bowing parameter of 1.1 eV.

In-situ TEM investigation of unfaulting behavior of Frank loops in FCC Pd during H2+ & He+dual-beam irradiation

A faulted Frank dislocation loop (FDL) can transform into an unfaulted perfect dislocation loop (PDL) under plastic deformation, quenching or particle irradiation. Here, we report our direct observation of the unfaulting processes of FDLs in palladium (Pd) by in-situ ion irradiation in transmission electron microscopy. Results indicate that the FDL unfaulting is a combination of three processes involving the interaction between adjacent FDLs, the interaction between FDL and PDL, and the nucleation and growth of Shockley partial dislocations within FDL. Meanwhile, bubbles tend to nucleate within dislocation loops, and the PDLs capture more He/H2 over FDLs, which confirms for the first time that the irradiation-induced loops play an important role in the aging of Pd and the unfaulting strengthens this process.

Impact of Threading Dislocations in GaN Power Switching Devices

Thick epitaxial (3-15 microns) GaN power switching devices are known to contain a high density of crystal defects, and especially threading dislocations in the epitaxial layer. The impact of these defects on device performance and reliability is a topic of ongoing research. This paper presents state-of-the-art methods for analyzing threading dislocation density, growth, and quantification of their impact on leakage current, blocking voltage, and peak E field. The effects of substrate type, epitaxial thickness, and processing techniques on dislocation density have been investigated to provide a roadmap for design and performance optimization of switching two terminal devices. Experimental and simulated data are used in conjunction to model dislocation impact on electrical performance characteristics and to identify techniques to minimize defect density.The experimental results have been achieved for 3 and 10 micron thick epitaxy as well as initial results for 15 micron epitaxy. The substrate was bulk GaN, and the epitaxy was grown by HPVPE by Kyma Corporation. The DMOS test structure investigated showed state of the art current voltage characteristics for a 10 micron n- epitaxy. Significant deviations were observed between the measured doping profile and the simulated profile, using a critical field of 3 MV/cm. For the DMOS device with epi doping of 8 × 1015 \U0001d450\U0001d45a−3 and epi thickness of 8 \U0001d707\U0001d45a. The device breaks down at approximately 2020 \U0001d449. At 2000 \U0001d449, the peak field is roughly 3.11 \U0001d440\U0001d449/\U0001d450\U0001d45a. For a device with epi doping of 8 × 1015 \U0001d450\U0001d45a−3 and epi thickness of 15 \U0001d707\U0001d45a. The device breaks down at approximately 3020 \U0001d449. At 3000 \U0001d449, the peak field is roughly 3.14 \U0001d440\U0001d449/\U0001d450\U0001d45a. The observed trend is significant since it shows that by increasing the epi thickness, also results in an increase in the peak field. For thin GaN epi of 3 microns, a peak breakdown field of 2.77 MV/cm was measured. These three date points will allow one to determine a relationship between peak critical field and epi thickness, which is a major requirement for designing vertical GaN high voltage switches.We report images from transmission x-ray topographs and TEM investigations as well as the correlation with critical breakdown field measurements. The transmission x-ray topograph shows that the GaN is characterized by a nearly uniform distribution of strain centers which are likely bundles of threading screw and edge dislocations. Strain free centers are then regions of relatively low dislocation densities. The threading dislocations are part of dislocation loops which are present in the GaN substrate and migrate through the interface. The cross-sectional TEM image shows partial dislocation loops through the thickness of the GaN sample.Figure: X-Ray Topography of GaN/GaN showing a nearly uniform distribution of strain centers. The Grazing incidence topograph is similar to the transmission image, the grazing image also shows the nearly uniform distribution of strain centers. The X ray image shows that individual threading dislocations are not resolved showing a relatively low dislocation density in those regions. Figure 1

Reduced dislocation growth leads to long lifetime InAs quantum dot lasers on silicon at high temperatures

We describe the effectiveness of filter layers, which displace misfit dislocation (MD) formation away from the active region, in improving high temperature reliability of epitaxially integrated InAs quantum dot lasers on on-axis silicon substrates. We find that inserting these “trapping layer (TL)” filters at either 80 nm or 180 nm from the active region substantially reduces device degradation at 60 °C. After 3000 h of continuous operation, the best trapping-layer-free device shows a 55% increase in threshold current while the best trapping layer (TL) devices each show less than a 9% increase. We explain these findings by correlating changes in individual device performance to changes in misfit dislocation (MD) structure. All MDs in devices without TLs show evidence of recombination enhanced dislocation climb (REDC); in contrast, adding trapping layers at 180 nm or 80 nm reduces the fraction of electrically active MDs to 9% and 1%, respectively. Reliability data after 3000 hours suggest that incorporating trapping layers a shorter distance from the active region (80 nm) is more effective than incorporating these layers further away. We conclude by identifying the mutually and self-reinforcing failure processes associated with REDC that TLs significantly remediate: increasing dislocation line length, increasing point defect densities, and increasing junction temperature. Overall, understanding and controlling crystal defects continues to be the most impactful avenue toward integrating light sources on photonic integrated circuits and closing the gap with native-substrate lasers.

Impact of Threading Dislocations in GaN Power Switching Devices

Thick epitaxial (3-15 microns) GaN power switching devices are known to contain a high density of crystal defects, and especially threading dislocations in the epitaxial layer. The impact of these defects on device performance and reliability is a topic of ongoing research. This paper presents state-of-the-art methods for analyzing threading dislocation density, growth, and quantification of their impact on leakage current, blocking voltage, and peak E field. The effects of substrate type, epitaxial thickness, and processing techniques on dislocation density have been investigated to provide a roadmap for design and performance optimization of switching two terminal devices. Experimental and simulated data are used in conjunction to model dislocation impact on electrical performance characteristics and to identify techniques to minimize defect density.

Bulk crystal growth, spectroscopic, hirshfeld surface analysis, physicochemical and quantum chemical investigations on 2-ethylimidazolium d-tartrate single crystal

The ‘2-ethylimidazolium d-tartrate (2EIMDT)’ C9H14N2O6 is synthesized and confirmed by experimental NMR, FTIR methods and powder XRD analysis. The HRXRD study reveals that the grown 2EIMDT is an ideally perfect single crystal without having any internal structural grain boundaries. It has good optical transmittance in the wavelength region of 250–1100 nm. The thermal stability of the crystal is ascertained up to 150 °C and the second harmonic generation activity of the crystal was studied by Kurtz-Perry powder technique. Etching analysis reveals the growth pattern and dislocation density of the crystal. A detailed analysis of the intermolecular interactions via Hirshfeld surface analysis and fingerprint plots revealed that the 2EIMDT structure is stabilized mainly by formation of O…H/H…O and H…H hydrogen bonds. The quantum-chemical computations were made to find the molecular properties. The HOMO-LUMO, molecular electrostatic potential, natural bond orbital, nonlinear optical behaviour of the 2EIMDT were studied with the same basis set in the gas phase, theoretically.

Effect of the degree of supercooling on growth mechanism of Cu6Sn5 in pure Sn/Cu solder joint

The growth mechanism of Cu6Sn5 intermetallic compound (IMC) at Sn/Cu interface reflowed under 250 °C, 275 °C, and 300 °C for a duration of 60 s and undergoing highly pressurized air action (HP), water cooling (WC), air cooling (AC) or furnace cooling (FC) in the aftermath has been outlined. In addition, synchrotron radiation imaging technique has been utilized to in situ observe interfacial Cu6Sn5 grains growth. It has been revealed that the morphologies of Cu6Sn5 grains under HP and AC are both scallop-like, whereas the samples processed with HP and FC bear plane structure and prismatic morphologies. These revelations indicate the secondary growth of Cu6Sn5 grains during the cooling stage as the significant determinant of the final IMC morphology. Thus, continuous growth on the rough surface (M1), spiral dislocation growth (M2), and 2D nucleation and growth (M3), have been proposed to explain the observed morphologies of interfacial Cu6Sn5 grains. M1 has been recognized as the principal mechanism responsible for IMC morphology undergoing water cooling, whereas combination of M1 and M2 decide the morphological feature of interface resulting from air cooling. Finally, all of the three mechanisms (M1, M2 and M3) account for the IMC morphology corresponding to furnace cooling. As all of these three growth mechanisms are affected by the degree of supercooling (ΔT), it is inferred that the ΔT can influence the secondary nucleation of IMC at the cooling stage. With the fact that the homogeneous nucleation occurs at larger ΔT, and the relative favorability for heterogeneous nucleation at smaller ΔT; the growth mechanisms can be mapped with the degree of undercooling. Consequently, the IMC growth mechanisms proposed and their association with ΔT, can be used to obtain an appropriate thickness and morphology of IMC to improve the reliability of solder joints.

In-situ carbide-reinforced CoCrFeMnNi high-entropy alloy matrix nanocomposites manufactured by selective laser melting: Carbon content effects on microstructure, mechanical properties, and deformation mechanism

The fabrication of high-entropy alloy (HEA) matrix nanocomposites by additive manufacturing (AM) is challenging due to that the control of defect-low sample having even distribution of reinforcement via AM is extremely hard. In this study, we investigated the effect of carbon content on the microstructure evolution, tensile properties, and deformation mechanisms of Cx(Co20Cr20Fe20Mn20Ni20)100–x (x = 0.5, 1.0, and 1.5 at.%) HEA matrix nanocomposites additively manufactured by selective laser melting (hereafter referred to as SLM-built C-HEAs). SLM-built C-HEAs showed epitaxial growth grains, dislocation networks, and nano-sized carbides. In addition, with an increase in carbon content, the number density of nano-sized carbides, and the average grain sizes and columnar widths increased. In addition, the strength, work hardening rate, and elongation of SLM-built C-HEAs were enhanced as the carbon content increased. Dislocation networks in the as-built samples hindered the dislocation motion in the early to later stages of deformation, thus leading to high back stresses in SLM-built C-HEAs. Deformation twins were also formed in the three samples, because the critical stress for twinning was similar to the flow stresses at an early stage of deformation of SLM-built C-HEAs. Further, the yield strengths of SLM-built C-HEAs were predicted using six strengthening mechanisms that considered the microstructural factors. Based on the above findings, we discussed the correlations between the microstructure, mechanical properties, and deformation mechanisms of SLM-built C-HEAs with different carbon contents.

Study on burgers vector of dislocations in KDP (010) faces and screw dislocation growth mechanism of (101) faces.

We modified the conventional etching-optical method to measure dislocation direction in a KDP crystal. As burgers vector of dislocation in the KDP crystal must match the minimum periodic vector of the crystal lattice, we suggest that dislocations with a burgers vector of [101], [102] and [103] exist. Atomic force microscopy was employed to characterize the morphology of growth spirals on the hillock of (101) faces. Multi-spirals consisting of more than two element steps with a height of 0.5 nm which is equal to (101) face interplanar distances were observed. We propose the multi-spiral structure is determined by the burgers vector of the corresponding dislocation, and constructed a geometric model of the crystal with screw dislocation to derive the relationship. Growth spirals on the (101) face present a particular triangular morphology and we proved that the triangle structure is formed by connected steps in the 1/2[111] and [010] direction. Micropipes form when the magnitude of the dislocation's burgers vector exceeds 1 nm, as predicted by BCF theory. Interaction between dislocations was observed also.

Conversions of a stacking fault to threading dislocations in homoepitaxial diamond growth studied by transmission electron microscopy

In this paper, we present how configurations of stacking faults (SFs) in (001)-oriented single crystal diamond substrates changed and extended in the film during homoepitaxial diamond growth by chemical vapor deposition, which was investigated using transmission electron microscopy. In the early stages of the film growth, the SFs in the substrate converted to threading dislocation (TD) rows along the end of the (111) SF planes appearing on the substrate surface, where the direction of the end of the SFs on the surface was [1-10]-[-110]. These TDs then traveled nearly in the same direction tilted by 10° to [-110] from the [001] direction without divergence, while extending with slight divergence within ∼7° in the [110]-[-1-10] direction. The diffraction contrasts observed under several two-beam conditions demonstrated that the SF in the substrate was intrinsic type and had a (111) slip plane with a displacement vector R=-a/3[111], where a is the lattice constant of diamond. At the same time, all the TDs converted from the SFs were edge type and their Burgers vector was b = a/2[1-10], perpendicular to the [001] direction.

Dissymmetry of Collective Crystal Growth

The statistical regularities of collective crystal growth under isotropic/anisotropic conditions are described based on the adsorption-fluctuation dislocation growth model. A version of realization of the Curie symmetry principle for an open stationary nonequilibrium system, described by random processes, is presented. The form of manifestation of the Curie principle is shown to depend not only on the physical conditions of the crystallization process but also on the structure of transitions between morphotypes (a Markov chain).

Relationships between welding parameters, aging conditions, and weld properties in AA7075-T6 friction stir welds

The influence of processing parameters and several combinations of natural as well as artificial aging on the mechanical properties of friction stir welded AA7075-T6 is analyzed in the present investigation. Different sets of welding parameters were employed to obtain joints characterized by different peak temperatures and thermal cycles. The joint obtained with the highest welding speed has guaranteed the best mechanical properties. The latter were shown to be strongly correlated to heat input, which influences both densities of dislocations in nugget zone grains and growth and dissolution of strengthening precipitates in the heat-affected zone that establish the mechanical characteristics of friction stir welds. On the other hand, artificial aging has drastically reduced the strain at break due to strain localization in the heat affected zones while it has proved its effectiveness in stabilizing the mechanical properties when exposed to further natural aging at room temperature.

Effects of grain size and temperature on mechanical properties of nano-polycrystalline Nickel-cobalt alloy

Influences of grain size and temperature on the mechanical properties of nano-polycrystalline Ni-Co alloy were investigated with molecular dynamics simulations. It is found that the critical grain size of Hall-Petch relationship is 4.3 nm, at which the maximum flow stress of 4.83 GPa is obtained. In samples with the d > 4.3 nm, the average flow stress increases with the decrease of d, according with the Hall-Petch relationship, caused by the dislocation glide and growth of deformation twin with the breaking of individual grain boundaries. For samples with the d < 4.3 nm, the evolution of flow stress and d is consistent with the inverse Hall-Petch relationship, attributed to the softening caused by the rotation of grain and migration of grain boundary, in which small grain merging can be observed, promoting grain boundary movement, and the extrusion of atoms leads to thinning of grain boundary. In addition, under high temperature loading, the fraction of grain boundary atoms increases, while the fraction of grain interior atoms decreases gradually. Also, the grain boundary will melt with the increase of the atomic disorder degree and the density of partial dislocation decreases. Simultaneously, the resistance to grain growth of samples at high temperature is lower than that of samples at low temperature. These conclusions have positive significance for the design of this fascinating nano-polycrystalline alloy.

Understanding the role of Nanomorphology on Resistance Evolution in the Hybrid Form-Fuse Process for Conformal Electronics

Manufacturing conformal electrically conductive circuits on rigid freeform surfaces currently require compromises between process scalability, cost, and thermal tolerance and geometric complexity of the object material. The Form-Fuse process operates by printing silver nanoparticle interconnects on planar polymer sheets followed by sequential vacuum forming and Flash Light Sintering (FLS) and can overcome the above issues. We investigate the role of nanoparticle morphology on this process by using nanowires (NW) in combination with nanoflakes (NF) and nanospheres (NS) as the printed nanoparticles. The characterization of FLS temperature, electrical resistance, morphology, and optical absorption along with electromagnetic and molecular dynamics modeling yields the following insights. The forming-induced resistivity rise is greater for higher NS and NF content due to localized embedding of NWs into the polymer. This embedding also reduces blowoff defects during FLS to create an expanded defect-free FLS window. The resistance reduction due to FLS is also greater for the mixed NW-NF and NW-NS cases, thanks to the change in localized optical absorption and interparticle fusion kinetics upon the introduction of NFs and NSs into a NW ensemble. The change in fusion kinetics is driven by enhanced dislocation growth between nanoparticles, despite surface and grain boundary diffusion trends contrary to the neck growth trends. We show that Form-Fuse can achieve similar or lesser electrical resistivity than state-of-the-art conformal 3D printing while enabling greater scalability and structural material capability and retaining the capability to handle complex shapes. We discuss the impact of our findings on the nanoparticle material cost and capabilities of Form-Fuse compared to other conformal electronics manufacturing processes.

Analysis of Cubic Boron Nitride Single Crystal Defects Growth under High Temperature and High Pressure

Cubic boron nitride (cBN) single crystals are synthesized under high temperature and high pressure in the Li-based system. The growth defects on hexagonal and triangular (111) surfaces of cBN single crystals after rapid cooling are discussed systemically for the first time using the atomic force microscope. Some impurity particles, triangle cone hole defects, lamellar-fault structures, and big steps are obvious on the surfaces of cBN single crystals. The formation mechanism of these defects is analyzed briefly at the synthetic process of cBN single crystals, and the growth mechanism of cBN single crystals transform from the two-dimensional growth to dislocation growth mechanism under high temperature and high pressure.

Multiscale simulation of grain refinement induced by dynamic recrystallization of Ti6Al4V alloy during high speed machining

During high speed machining (HSM), the strong thermal-mechanical coupling can lead to the microstructure evolution in the deformation zone of workpiece. Grain refinement may occur, which has great effects on the mechanical behavior, and even on the fatigue strength and corrosion resistance of the machined surface. The development of multiscale models to predict the microstructure evolution is gaining rising interest. This study aims to investigate the grain refinement induced by dynamic recrystallization (DRX) occurring in HSM of Ti6Al4V, through finite element (FE) and cellular automata (CA) methods. An orthogonal cutting model for HSM of Ti6Al4V is developed combining a modified Johnson-Cook constitutive model (TANH) and Johnson-Mehl-Avrami-Kolmogorov (JMAK) DRX model. The CA model is proposed considering dislocation density evolution, grain nucleation and growth. The 2D mesoscopic microstructure evolution is simulated successfully by the CA model in which the input deformation parameters come from the FE simulations of the orthogonal cutting process. Finally, the grain size and microstructure morphology calculated by both FE and CA methods are compared with those characteristics obtained experimentally by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Identical microstructure predictions from both CA and FE methods show a reasonable agreement with the TEM results, on the condition that twinning and phase transformation are not considered in the simulations. This work proves that the combination of FE and CA methods is an effective approach to achieve a more comprehensive understanding of the microstructure evolution and its effect on mechanical behavior during HSM. It shows that the rise of both DRX volume fraction and DRX grain size finally results in the slightly decreasing of average grain size of serrated chips with the increase of cutting speed, which leads to the strain softening phenomenon of flow stress.