Regions Of High Dislocation Density Research Articles

Effect of Pre-Deformation on Precipitation in Al–Zn–Mg–Cu Alloy

AbstractIn this work, strengthening effects and evolution of precipitates in a pre-deformed Al–Zn–Mg–Cu alloy during ageing were investigated using Vickers hardness measurements, tensile tests, and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). It was found that all cold rolled conditions had higher mechanical strength than the non-deformed condition for all ageing times and that this effect increases at higher deformation ratios. It was also found that the non-deformed condition has a higher age hardening response than that of the cold rolled conditions. A homogeneous precipitate distribution was observed in the non-deformed condition, while the cold rolled conditions contained non-uniformly distributed precipitates due to the introduced dislocations. This led to larger precipitate sizes and a reduction in the precipitate number densities in the pre-deformed conditions. HAADF-STEM analysis revealed differences in the fraction of different precipitate types between the non-deformed and the cold rolled conditions. η', η2, and disordered η phase were observed in the non-deformed condition, while η', η2 and the newly identified Y phase were observed in the cold rolled conditions. The disordered η phase contained structural units of the η1 phase and was associated with reducing the lattice misfit between this phase and the Al matrix. Formation of the Y phase was related to an accelerated nucleation rate in the regions of high dislocation density. Graphical abstract

A dislocation density-based meshfree computational framework for solid phase processing

This work proposes a computational modeling framework capable of predicting the behavior of materials when subject to solid phase processing (SPP). A macroscopic dislocation density based constitutive model with internal state variables correlated to microscopic dislocation densities is incorporated into a meshfree smoothed particle hydrodynamics (SPH) framework able to handle large deformations and high temperatures typical of what a material undergoes during SPP. Unlike phenomenological constitutive models that typically neglect material microstructural contributions to macroscale response, the dislocation density based constitutive model improves significantly the predictivity of our modeling framework in a wider range of conditions and allows the multiscale material responses to be simulated simultaneously. With dislocation density based model parameters calibrated to AA1100-O aluminum test data, the generality of the proposed framework is tested on two SPP techniques, namely friction extrusion (FE) and friction stir processing (FSP). Simulation results with different process parameters are systematically validated by experimental data for both cases. The validated models are then used to elucidate the inherent physics associated with SPP by revealing the distributions of temperature, strain, strain rate, and dislocation density, stick-slip contact conditions, and heat generation rates. Model-predicted high dislocation density regions in the processed AA1100-O also agree well with the dynamically recrystallized grain refinement zones observed from experiments.

Reducing GaN crystal dislocations through lateral growth on uneven seed crystal surfaces using the Na-flux method

In recent years, we have achieved low threading dislocation density in GaN wafers by using the Na-flux multi-point seed technique. However, the resulting wafers exhibit regions of high dislocation density, exceeding 105 cm−2 at the coalescence boundary where pyramidal crystals merge. In this study, we discovered that annealing seed crystals at 900 °C generated an uneven surface with approximately 50 μm of GaN decomposition, and growing GaN on the thus-generated uneven surfaces induced lateral growth composed of facets. We then investigated the effect of the uneven surface on dislocation reduction and found that the average threading dislocation density of the grown crystal was reduced from cm−2 in the seed crystal to cm−2. We confirmed that the reduction in threading dislocation density was due to the termination of dislocations by means of inclusions, and to mergers or annihilation as they encountered one another during facet growth.

Effect of the Distribution Characteristics of TiC Phases Particles on the Strengthening in Nickel Matrix

Molecular dynamics (MD) was used to simulate the effect of TiC particles distribution on the tribological behavior of the reinforced composites. The mechanical properties, friction coefficient, number of wear atoms, stress and temperature, and microscopic deformation behavior of TiC/Ni composites during nano-friction were systematically investigated by MD to reveal the effect of TiC distribution on the friction removal mechanism of the material. It was found that the larger the radius of the TiC particles, or the shallower the depth of the TiC particles, the easier it was to generate stress concentrations around the TiC particles, forming a high dislocation density region and promoting the nucleation of dislocations. This leads to severe friction hardening, reducing the atomic number of abrasive chips and reducing the friction coefficient by approximately 6% for every 1 nm reduction in depth, thus improving the anti-wear capacity. However, when the radius of the TiC particles increases and the thickness from the surface deepens, the elastic recovery in material deformation is weakened. We also found that the presence of the TiC particles during the friction process changes the stress state inside the workpiece, putting the TiC particles and the surrounding nickel atoms into a high-temperature state and increasing the concentrated temperature by 30 K for every 1 nm increase in depth. Nevertheless, the workpiece atoms below the TiC particles invariably exist in a low-temperature state, which has a great insulation effect and improves the high-temperature performance of the material. The insight into the wear characteristics of TiC particles distribution provides the basis for a wide range of TiC/Ni applications.

Atomistic simulations of hydrogen distribution in Fe–C steels

To engineer low-cost Fe–C based steels for application in hydrogen energy technologies, an understanding of the hydrogen distribution inside the material and how it is affected by the microstructure is vital. Molecular statics and molecular dynamics simulations are used to study hydrogen distribution and transport kinetics in the ferritic and martensitic phases of Fe–C steels, with and without dislocations present. We find that hydrogen preferentially resides in martensite especially in high dislocation density regions near the martensite/ferrite boundaries, in agreement with experiments. Furthermore, the rate of hydrogen transport through ferrite is up to an order of magnitude greater than that in martensite. The fundamental mechanisms behind this phenomenon are analyzed.

Recrystallization and grain growth kinetics of IN718 manufactured by laser powder bed fusion

The recrystallization and grain growth behaviour of IN718 alloy additively manufactured by laser powder bed fusion (L-PBF) is presented herein. The effects of three different temperatures (1050, 1150 and 1250 °C) and holding times (15, 45 and 90 min) were investigated. The texture evolution of the samples was recorded via electron backscatter diffraction (EBSD). The as-built sample is composed of bowl-shaped melt pools, a chessboard-like grain pattern and has a cube texture {100}<001>. Recrystallized grains were observed in the samples treated at 1150 °C for 15 min, as well as the samples treated for longer periods and at higher temperatures. Recrystallization was observed to start from high dislocation density regions, including the overlapping melt pools and the borders of the chessboard-like pattern. The initial cube texture transforms into a first-generation cube-twin texture {122}<212> via a twinning-assisted recrystallization mechanism. Then, those recrystallization nuclei sweep through the high defect density matrix; during which almost no new twins are formed. The samples treated at 1250 °C are almost completely recrystallized, which forms a weaker cube texture and a stronger P-orientation {011}<112>. However, the growth of recrystallized grains is very limited due to the presence of non-coherent precipitates.

Observation of defect structure in ScAlMgO4 crystal using X-ray topography

We examined the defect structure in a ScAlMgO4 (SAM) crystal intended for use as a substrate of GaN using synchrotron X-ray topography. No dislocations were observed in 70% of the 40 mm SAM wafer, indicating a wide dislocation-free region. The remaining 30% of the wafer was composed of high and ultra-high dislocation density regions. Two types of dislocations, straight-type and stream-type dislocations, along the [011¯0], [11¯00], and [101¯0] directions, were predominantly present in the high dislocation density region. The dislocation colonies with only straight-type or stream-type dislocations were observed and the high dislocation density region was occupied by these colonies. The lattice strains of the wafer center region that had no dislocations and the wafer edge region that had an ultra-high dislocation density weren’t uniform. The SAM crystal had unique structures such as a wide dislocation-free region, straight-type and stream-type dislocations, formation of dislocation colonies, and lattice strains that weren’t uniform.

Dislocation microstructure and its influence on corrosion behavior in laser additively manufactured 316L stainless steel

The hierarchical nature of additively manufactured materials necessitates a multimodal approach for quantifying microstructural features and corresponding chemical heterogeneities that ultimately impact their properties and performance. In laser powder-bed fusion (L-PBF) 316L stainless steel, corrosion behavior has been discussed in the context of chemical heterogeneities formed in the presence of these hierarchical microstructures. Here, we employ a suite of advanced synchrotron x-ray techniques and correlative transmission electron microscopy for the analysis of microstructure and chemical heterogeneities in L-PBF 316L as a function of printing speed. Our findings reveal an appreciable dislocation density consistent with the formation of a cellular dislocation microstructure in L-PBF 316L, which is correlated to spatial variations in the local Cr concentration and the formation of complex Mn 7 C 3 nanoinclusions. Cyclic voltammetry experiments reveal that relative to wrought 316L, the printed samples exhibit either a comparable or marginally reduced susceptibility to uniform corrosion but with an increased affinity for pitting particularly in the samples printed at the highest speed with the largest dislocation density. Given the spatial correlations between regions of high dislocation density and the formation of chemical heterogeneities known to degrade corrosion performance, our findings demonstrate the impact of the microstructural defect state and its variation with printing speed on the resistance of L-PBF 316L to uniform and localized corrosion.

Improved strength and ductility of AZ31B Mg alloy sheets processed by accumulated extrusion bonding with artificial cooling

In this study, accumulated extrusion bonding (AEB) process with application of artificial water cooling was successfully performed to fabricate fined-grains AZ31B Mg alloy sheets at 150°C, 200°C and 250°C. The resultant microstructure and mechanical properties are systematically investigated. It reveals that the processing temperature has an important effect on the microstructural evolution during extrusion. During AEB process at 150°C and 200°C, {10–12} tensile twinning was activated at early stage of extrusion, and subsequently continuous dynamic recrystallization (CDRX) occurred and dominated the further deformation. However, for the sample extruded at 250°C, hardly any twins can be observed, and new fined dynamic recrystallized grains were found along grain boundaries. Artificial cooling was utilized to reduce the rate of grain growth out of the extrusion die, resulting the grains significantly refined from 11 µm to 2.5 µm. Local high dislocation density region was also observed in the microstructure of sample processed at 150°C in artificial cooling condition, and the degree decreased with the processing temperature increase. The results summarized from tensile tests indicated that due to the grain refinement the strength and ductility was significantly enhanced (YS of 186 MPa vs. 145 MPa, UTS of 391 MPa vs. 336 MPa and FE of 31.5% vs. 24.5% compared with the as-received sample). Subsequently, annealing treatment at different temperatures was applied to eliminate the high dislocation density. The sample annealed at 200°C exhibited the best comprehensive mechanical property with YS of 179 MPa, UTS of 390 MPa and FE of 33.0%. As the annealing temperature increasing, the dislocation density was reduced by static recrystallization (SRX) and grain growth, leading to a decreased strength and ductility.

Nested size effects in the nanoindentation response of additively manufactured 316L stainless steel

Recent work has shown that the presence of cellular substructures in selectively laser melted (SLM) 316L stainless steel coincides with significant increases in yield stress and ultimate strength. These substructures have been associated with dislocation cell formation, with cell walls and cell interiors corresponding to regions of high and low dislocation density, respectively. Motivated by this finding, the present effort seeks a more comprehensive understanding of this phenomenon through spatially resolved hardness mapping via nanoindentation. Initially it was assumed that a spatially varying “cellular” hardness pattern would emerge, corresponding to the geometry and the characteristic features (cell walls and cell interior) of the cell structure. However, due to multiple, simultaneously operating hierarchical size effects, the apparent hardness deviates from this anticipated behavior. Direct evidence is provided in the present work indicating that these size effects are due to the indenter, characteristic sub-structural length, and grain size. Analysis of the load/displacement behavior enabled observation of each of these size effects, independent of the local anisotropy due to crystal orientation.

Microstructure mechanism, cyclic deformation behavior of an Fe-Ni-Cr alloy considering non-Masing behavior

The non-Masing behavior of Sanicro 25 alloy was studied during high-temperature low-cycle fatigue (700 °C). The results showed that Sanicro 25 alloy exhibits the non-Masing behavior when total strain amplitude (εa) increased. The analysis of stable hysteresis loops showed that the stress amplitude (σa) increased with increasing εa. At the stable cyclic stage, the rate of increase in σa with number of cycles increased when εa increased. Based on the EBSD observation, the Geometrically Necessary Dislocation (GND) density increased as εa increased. The region of high dislocation density was distributed along the grain boundaries, which led to the increasing flow stress. The fraction of twin boundaries decreased and the grain size increased with increasing εa, which led to a more obvious cyclic yielding stage. Therefore, the non-Masing behavior occurred at a relatively high εa. Considering the influence of non-Masing behavior on the probability density function relative to the cyclic yield level, the relationship of cyclic stress and plastic strain with the increment of the proportional stress limit δσ0 and Bauschinger strain β was established. σa increases linearly with increasing δσ0. σa decreases with increasing β.

Factors affecting preservation of coesite in ultrahigh‐pressure metamorphic rocks: Insights from TEM observations of dislocations within kyanite

AbstractTo understand the preservation of coesite inclusions in ultrahigh‐pressure (UHP) metamorphic rocks, an integrated petrological, Raman spectroscopic and focussed ion beam (FIB) system–transmission electron microscope (TEM) study was performed on a UHP kyanite eclogite from the Sulu belt in eastern China. Coesite grains have been observed only as rare inclusions in kyanite from the outer segment of garnet and in the matrix. Raman mapping analysis shows that a coesite inclusion in kyanite from the garnet rim records an anisotropic residual stress and retains a maximum residual pressure of ~0.35 GPa. TEM observations show quartz is absent from the coesite inclusion–host kyanite grain boundaries. Numerous dislocations and sub‐grain boundaries are present in the kyanite, but dislocations are not confirmed in the coesite. In particular, dislocations concentrate in the kyanite adjacent to the boundary with the coesite inclusion, and they form a dislocation concentration zone with a dislocation density of ~109 cm−2. A high‐resolution TEM image and a fast Fourier transform‐filtered image reveal that a tiny dislocation in the dislocation concentration zone is composed of multiple edge dislocations. The estimated dislocation density in most of the kyanite away from the coesite inclusion–host kyanite grain boundaries is ~108 cm−2, being lower than that in kyanite adjacent to the coesite. In the case of a coesite inclusion in a matrix kyanite, using Raman and TEM analyses, we could not identify any quartz at the grain boundaries. Dislocations are not observed in the coesite, but numerous dislocations and stacking faults are developed in the kyanite. The estimated overall dislocation density in the coesite‐bearing matrix kyanite is ~108 cm−2, but a high dislocation density region of ~109 cm−2 is also present near the coesite inclusion–host kyanite grain boundaries. Inclusion and matrix kyanite grains with no coesite have dislocation densities of ≤108 cm−2. Dislocation density is generally reduced during an annealing process, but our results show that not all dislocations in the kyanite have recovered uniformly during exhumation of the UHP rocks. Hence, one of the key factors acting as a buffer to inhibit the coesite to quartz transformation is the mechanical interaction between the host and the inclusion that lead to the formation of dislocations in the kyanite. The kyanite acts as an excellent pressure container that can preserve coesite during the decompression of rocks from UHP conditions. The search for and study of inclusions in kyanite may be a more suitable approach for tracing the spatial distribution of UHP metamorphic rocks.

Role of Al2O3 inclusions on the localized corrosion of Q460NH weathering steel in marine environment

The influence of Al2O3 inclusions on the localized corrosion behaviour of Q460NH steel was investigated in a simulated marine environment. According to the current sensing atomic force microscopy (CS-AFM) result, there was no galvanic couple between Al2O3 inclusions and the adjacent steel. Therefore, a different mechanism from that traditionally used (considering the galvanic coupling between inclusions and matrix) to explain the selective dissolution of the matrix around inclusions, dominated the pit initiation and propagation process in this system. Electron backscattered diffraction (EBSD) analysis showed a region of high dislocation density around the inclusions. Both microcrevices and the high dislocation density region around inclusions can easily induce localized corrosion. The catalytic-occluded cells and oxygen-concentration cell could accelerate the propagation of the pits. Furthermore, inclusion clusters had a larger negative effect than single inclusions on the corrosion resistance of the steel. Scanning vibrating electrode technique (SVET) measurements confirmed that inclusions play a key role in the corrosion process.

Transmission scanning electron microscopy: Defect observations and image simulations

The new capabilities of a FEG scanning electron microscope (SEM) equipped with a scanning transmission electron microscopy (STEM) detector for defect characterization have been studied in parallel with transmission electron microscopy (TEM) imaging. Stacking faults and dislocations have been characterized in strontium titanate, a polycrystalline nickel-base superalloy and a single crystal cobalt-base material. Imaging modes that are similar to conventional TEM (CTEM) bright field (BF) and dark field (DF) and STEM are explored, and some of the differences due to the different accelerating voltages highlighted. Defect images have been simulated for the transmission scanning electron microscopy (TSEM) configuration using a scattering matrix formulation, and diffraction contrast in the SEM is discussed in comparison to TEM. Interference effects associated with conventional TEM, such as thickness fringes and bending contours are significantly reduced in TSEM by using a convergent probe, similar to a STEM imaging modality, enabling individual defects to be imaged clearly even in high dislocation density regions. Beyond this, TSEM provides significant advantages for high throughput and dynamic in-situ characterization.

Prediction of corrosion fatigue crack initiation behavior of A7N01P-T4 aluminum alloy welded joints

Through investigating the corrosion fatigue crack initiation behavior of A7N01P-T4 aluminum alloy welded joints in 3.5 wt.% NaCl solution, corrosion fatigue crack initiation life is formulated as [Formula: see text] and the mechanism of corrosion fatigue crack initiation is proposed. SEM and TEM tests revealed that several corrosion fatigue cracks formed asynchronously and the first crack does not necessarily develop into the leading crack. The uneven reticular dislocations produced by fatigue loading are prone to piling up and tangling near the grain boundaries or the second phases and form the “high dislocation-density region” (HDDR), which acts as an anode in microbatteries and dissolved to form small crack. Thus the etching pits, HDDR near the grain boundaries and second phases are confirmed as the main causes inducing the initiation of fatigue crack.

Enhanced Hot Workability and Post-Hot Deformation Microstructure of the As-Cast Al-Zn-Cu-Mg Alloy Fabricated by Use of a High-Frequency Electromagnetic Casting with Electromagnetic Stirring

The feasibility of producing an Al-Zn-Cu-Mg (7075) aluminum (Al) alloy using high-frequency electromagnetic casting (HFEMC) and electromagnetic stirring (EMS) was explored, and the microstructure, hot compressive deformation characteristics, and processing maps of the as-cast and homogenized EMS 7075 alloys were examined. The obtained results were compared with those of an alloy of the same composition, produced by direct chill casting (DCC). Application of the HFEMC/EMS technology resulted in grain refinement and suppression of dendritic growth. The grain size of the as-cast EMS 7075 alloy was smaller than that of the as-cast DCC 7075 alloy by more than half. This grain-size reduction increased the strain rate sensitivity and decreased the flow stress. The grain refinement also resulted in enhanced hot workability. Hot workability of the EMS 7075 alloy, however, considerably deteriorated after homogenization treatment. This resulted from the disappearance of the solute-segregated phases that play a role of accelerating dynamic recovery and continuous dynamic recrystallization during compressive deformation and the occurrence of considerable grain coarsening during homogenization treatment. The as-cast EMS 7075 alloy also showed a higher quality of post-hot working microstructure (after T6 heat treatment) compared to the homogenized EMS 7075 alloy. This resulted because the segregated phases in the as-cast microstructure served as the nucleation sites for new grains during static recrystallization by having created localized regions of high dislocation density around them during compressive deformation, especially at high strain rates. The current study showing that the as-cast EMS microstructure can yield a high hot workability as well as a high quality of post-hot working microstructure encourages the direct use of as-cast EMS 7075 alloy billets as feedstock for hot extrusion or forging.

Polarized light emission from grain boundaries in photovoltaic silicon

Some crystalline defects in photovoltaic silicon have deleterious effects on the energy conversion efficiency of the material. Distinguishing the harmful defects from the benign defects is a critical problem in the mechanics of materials for solar energy conversion. Interestingly, the visible light absorbed by silicon in the same part of the solar spectrum that is used to generate photocurrent, can also excite photoluminescence, which may be used to generate images of the microstructure. Slightly longer wavelengths in the near infrared (IR) may be used to measure strain in the material via photoelastic (PE) imaging. These two imaging modalities have recently been combined in a single instrument, and we show here the additional capability to identify and categorize defects directly by capturing the narrow band of photoluminescence emitted by regions of high dislocation density. We use this method to show that dislocations arranged in low angle grain boundaries emit polarized light, while dislocation structures in neighboring high angle grain boundaries do not emit polarized light. This capability may form the basis for next-generation, full-field optomechanics-based characterization of materials for solar energy conversion.

Effect of Gas Tungsten Arc Welding Parameters on Hydrogen-Assisted Cracking of Type 321 Stainless Steel

The susceptibility of AISI type 321 stainless steel welded by the gas tungsten arc welding (GTAW) process to hydrogen-assisted cracking (HAC) was studied in a tensile test combined with in situ cathodic charging. Specimen charging causes a decrease in ductility of both the as-received and welded specimens. The mechanical properties of welds depend on welding parameters. For example, the ultimate tensile strength and ductility increase with growing shielding gas (argon) rate. More severe decrease in the ductility was obtained after post-weld heat treatment (PWHT). In welded steels, in addition to discontinuous grain boundary carbides (M23C6) and dense distribution of metal carbides MC ((Ti, Nb)C) precipitated in the matrix, the appearance of delta-ferrite phase was observed. The fracture of sensitized specimens was predominantly intergranular, whereas the as-welded specimens exhibited mainly transgranular regions. High-dislocation density regions and stacking faults were found in delta-ferrite formed after welding. Besides, thin stacking fault plates and epsilon-martensite were found in the austenitic matrix after the cathodic charging.

EBSD analysis of plastic deformation of copper foils by flexible pad laser shock forming

Flexible pad laser shock forming (FPLSF) is a new mold-free microforming process that induces high-strain-rate plastic deformation in thin metallic foils using laser-induced shock pressure and a hyperelastic flexible pad. This paper studies the plastic deformation behavior of copper foils formed through FPLSF by investigating surface hardness and microstructure. The microstructure of the foil surface before and after FPLSF is analyzed by electron backscatter diffraction technique using grain size distribution and grain boundary misorientation angle as analysis parameters. The surface hardness of the craters experienced a significant improvement after FPLSF; the top crater surface being harder than the bottom surface. The microstructure of the copper foil surface after FPLSF was found to be dominated by grain elongation, along with minor occurrences of subgrain formation, grain refinement, and high dislocation density regions. The results indicate that the prominent plastic deformation mechanism in FPLSF is strain hardening behavior rather than the typical adiabatic softening effect known to be occurring at high-strain-rates for processes such as electromagnetic forming, explosive forming, and laser shock forming. This significant difference in FPLSF is attributed to the concurrent reduction in plastic strain, strain rate, and the inertia effects, resulting from the FPLSF process configuration. Correspondingly, different deformation behaviors are experienced at top and bottom surfaces of the deformation craters, inducing the change in surface hardness and microstructure profiles.

Characteristics of Au Migration and Concentration Distributions in Au-Doped HgCdTe LPE Materials

Annealing techniques and secondary ion mass spectrometry have been used to study the characteristics of Au migration and concentration distributions in HgCdTe materials grown by liquid phase epitaxy. Secondary ion mass spectrometry measurements showed that Au concentrations had obvious positive correlations with Hg-vacancy concentration and dislocation density of the materials. Au atoms migrate toward regions of high Hg-vacancy concentration or move away from these regions when the Hg-vacancy concentration decreases during annealing. The phenomenon can be explained by defect chemical equilibrium theory if Au atoms have a very large migration velocity compared with Hg vacancies. Au atoms will also migrate toward regions of high dislocation density, leading to a peak concentration in the inter-diffusion region of HgCdTe materials near the substrate. By use of an Hg and Te-rich annealing technique, different concentration distributions of both Au atoms and Hg vacancies in HgCdTe materials were obtained, indicating that Au-doped HgCdTe materials can be designed and prepared to satisfy the requirements of HgCdTe devices.