Evaluation Of Dislocation Density Research Articles

Evaluation of Dislocation Density and Dislocation Strengthening Mechanism in Ultra Low-Carbon Martensitic Steel

Evaluation of Dislocation Density and Dislocation Strengthening Mechanism in Ultra Low-Carbon Martensitic Steel

A Comparative Study of Methods for Calculating the Dislocation Density in GaN-on-Si Epitaxial Wafers.

A series of characterization methods involving high-resolution X-ray diffraction (HR-XRD), electron channel contrast imaging (ECCI), cathodoluminescence microscopy (CL), and atomic force microscopy (AFM) were applied to calculate the dislocation density of GaN-on-Si epitaxial wafers, and their performance was analyzed and evaluated. The ECCI technique, owing to its high lateral resolution, reveals dislocation distributions on material surfaces, which can visually characterize the dislocation density. While the CL technique is effective for low-density dislocations, it is difficult to accurately identify the number of dislocation clusters in CL images as the density increases. The AFM technique analyzes surface dislocation characteristics by detecting surface pits caused by dislocations, which are easily affected by sample and probe conditions. A prevalent method for assessing the crystal quality of GaN is the rocking curve of HR-XRD (ω-scan), which calculates the dislocation density based on the FWHM value of the curves. By comparing the above four dislocation characterization methods, the advantages and limitations of each method are clarified, which also verifies the applicability of DB=β29b2 for GaN-on-Si epitaxial wafers. This provides an important reference value for dislocation characterization in GaN-on-Si materials. The accuracy evaluation of dislocation density can truly and reliably reflect crystal quality, which is conducive to further optimization. Furthermore, this study can also be applied to other heterogeneous or homogeneous epitaxial materials.

Evaluation of Dislocation Density and Dislocation Strengthening Mechanism in Ultra Low-Carbon Martensitic Steel

Evaluation of Dislocation Density and Dislocation Strengthening Mechanism in Ultra Low-Carbon Martensitic Steel

Quantitative evaluation of dislocation density in SUS316L utilizing non-contact microwave reflection method

Quantitative evaluation of dislocation density in SUS316L utilizing non-contact microwave reflection method

Evaluation of Dislocation Density of Coarse and Fine Grains in Bimodal Harmonic Structured Steel Observed by Diffraction Contrast Tomography using Ultrabright Synchrotron Radiation

Evaluation of Dislocation Density of Coarse and Fine Grains in Bimodal Harmonic Structured Steel Observed by Diffraction Contrast Tomography using Ultrabright Synchrotron Radiation

A study on micro‐mechanical deformation behaviour of heat‐treated 20MnMoNi55 steel

AbstractSeveral heat treatment procedures are designed considering critical temperatures of phase transformation evaluated through dilatometric testing of 20MnMoNi55 steel to transform low carbon bainitic as‐received material into ferrite‐martensite dual‐phase steels consisting of varied martensite fractions. A thorough metallographic study correlated with the micro‐hardness of constituent phases ensures morphological characteristics along with its fractional variations in as‐received and dual‐phase steels. The impact of fractional variation in constituent phases on the uniaxial monotonic deformation characteristics of dual‐phase steels has been observed with a correlation study between experimental tensile and finite element simulated results. Therefore, a physical‐based model with a 2‐dimensional representative volume element has been established, addressing actual morphological characteristics obtained from metallographic studies. Moreover, the constitutive flow behaviours of ferrite and martensite are also derived from the dislocation‐based hardening model to address the actual deformation phenomenon. Finally, an inhomogeneous deformation behaviour among constituent phases and localization of plastic strain in ferrite matrix has been observed with von‐Mises stress, and equivalent plastic strain distribution through finite element simulated results. This phenomenon is again confirmed with kernel average misorientation mapping and geometrically necessary dislocation density evaluation through electron backscattered diffraction of tensile samples subjected to different degrees of plastic strain.

Evaluation of Dislocation Density of Coarse and Fine Grains in Bimodal Harmonic Structured Steel Observed by Diffraction Contrast Tomography using Ultrabright Synchrotron Radiation

Austenitic stainless steels with a bimodal harmonic structure, in which the fine grain structure (Shell) exists around the coarse grain structure (Core), are prepared by powder metallurgy to improve both strength and ductility. Herein, X‐ray diffraction contrast tomography, a 3D grain mapping technique for polycrystalline materials using ultrabright synchrotron radiation X‐rays, is used to reconstruct the grain shape and location and to evaluate the average excess dislocation density of the Core and Shell structures. This technique allows one to evaluate the excess dislocation density not only on the surface, but also inside the sample where damage occurs in tensile tests. The results show that the excess dislocation density of the Shell structure is higher than that of the Core structure. The excess dislocation density of homogeneous austenitic stainless steels with grain sizes similar to the Core structure of the harmonic structured stainless steel is higher than that at comparable stresses, indicating that the deformation of the bimodal harmonic structured alloy is localized in the fine grain structure. This is consistent with the results obtained from electron backscatter diffraction analysis, in which the surface grains are evaluated.

Methods of dislocation structure characterization in AIIIBV semiconductor single crystals

The development pace of advanced electronics raises the demand for semiconductor single crystals and strengthens the requirements to their structural perfection. Dislocation density and distribution pattern are most important parameters of semiconductor single crystals which determine their performance as integrated circuit components. Therefore studies of the mechanisms of dislocation nucleation, slip and distribution are among the most important tasks which make researchers face the choice of suitable analytical methods. This work is an overview of advanced methods of studying and evaluating dislocation density in single crystals. Brief insight has been given on the main advantages and drawbacks of the methods overviewed and experimental data have been presented. The selective etching method (optical light microscopy) has become the most widely used one and in its conventional setup is quite efficient in the identification of scrap defects and in dislocation density evaluation by number of etch pits per vision area. Since the introduction of digital light microscopy and the related transfer from image analysis to pixel intensity matrices and measurement automation, it has become possible to implement quantitative characterization for the entire cross-section of single crystal wafers and analyze structural imperfection distribution pattern. X-ray diffraction is conventionally used for determination of crystallographic orientation but it also allows evaluating dislocation density by rocking curve broadening in double-crystal setup. Secondary electron scanning electron microscopy and atomic force microscopy allow differentiating etch patterns by origin and studying their geometry in detail. Transmission electron microscopy and induced current method allow obtaining micrographs of discrete dislocations but require labor-consuming preparation of experimental specimens. X-ray topography allows measuring bulky samples and also has high resolution but is hardly suitable for industry-wide application due to the high power consumption of measurements.Digital image processing broadens the applicability range of basic dislocation structure analytical methods in materials science and increases the authenticity of experimental results.

Methods of dislocation structure characterization in A IIIB V semiconductor single crystals

The development pace of advanced electronics raises the demand for semiconductor single crystals and strengthens the requirements to their structural perfection. Dislocation density and distribution pattern are most important parameters of semiconductor single crystals which determine their performance as integrated circuit components. Therefore studies of the mechanisms of dislocation nucleation, slip and distribution are among the most important tasks which make researchers face the choice of suitable analytical methods. This work is an overview of advanced methods of studying and evaluating dislocation density in single crystals. Brief insight has been given on the main advantages and drawbacks of the methods overviewed and experimental data have been presented. The selective etching method (optical light microscopy) has become the most widely used one and in its conventional setup is quite efficient in the identification of scrap defects and in dislocation density evaluation by number of etch pits per vision area. Since the introduction of digital light microscopy and the related transfer from image analysis to pixel intensity matrices and measurement automation, it has become possible to implement quantitative characterization for the entire cross-section of single crystal wafers and analyze structural imperfection distribution pattern. X-ray diffraction is conventionally used for determination of crystallographic orientation but it also allows evaluating dislocation density by rocking curve broadening in double-crystal setup. Secondary electron scanning electron microscopy and atomic force microscopy allow differentiating etch patterns by origin and studying their geometry in detail. Transmission electron microscopy and induced current method allow obtaining micrographs of discrete dislocations but require labor-consuming preparation of experimental specimens. X-ray topography allows measuring bulky samples and also has high resolution but is hardly suitable for industry-wide application due to the high power consumption of measurements. Digital image processing broadens the applicability range of basic dislocation structure analytical methods in materials science and increases the authenticity of experimental results.

Quantitative evaluation of dislocation density in as-quenched martensite with tetragonality by X-ray line profile analysis in a medium-carbon steel

The crystal structure of as-quenched martensite in steel changes from body-centered cubic to body-centered tetragonal because of the presence of interstitial carbon, and the c/a ratio increases with increasing carbon content. Because the X-ray diffraction peak profiles associated with the c-axis overlap those of the a-axis, the diffraction peak observed in martensite gradually broadens as the solute carbon increases. This may cause an overestimation of the dislocation density in the X-ray line profile analysis (for example, modified Williamson-Hall/Warren-Averbach method). In this study, the effect of the tetragonality of martensite in the X-ray line profile analysis was investigated to evaluate the dislocation density in a medium-carbon martensitic steel. The apparent dislocation density estimated without considering peak broadening due to tetragonality was calculated to be 1.7 × 1016 m−2 in the as-quenched 0.55%C steel. This value is approximately six times higher than that estimated using transmission electron microscopy (TEM), indicating that the dislocation density was overestimated. To eliminate the effect of tetragonality, the specimens were tempered at temperatures below 623 K. The estimation based on the TEM method revealed that the dislocation density only slightly decreased by tempering at 573 K, indicating negligible recovery at this temperature. The dislocation density obtained in the tempered specimen was calculated to be approximately 4.5 × 1015 m−2 using the mWH/WA method. This value was regarded as the true dislocation density of the as-quenched martensite in the 0.55%C steel.

The modified Williamson-Hall plot and dislocation density evaluation from diffraction peaks

Diffraction peak profiles were calculated numerically for dislocation ensembles with different spatial arrangements and correlations between the Burgers vector signs. The latter property determines the stored elastic energy in the crystal and the width of the diffraction peaks. It is shown that within the approximation of the asymptotic line-profile theory the relationship between peak breadth and the magnitude of the diffraction vector in the modified Williamson-Hall (mWH) plot is linear. The slope of the line is proportional to the arrangement parameter M0.3 and to the square root of the dislocation density. The only rigorous way for determining M is the asymptotic Fourier method. Therefore, the evaluation of the dislocation density from the mWH-plot alone is impossible and should be avoided. The mWH plot however, is very useful in practice. Its linearity indicates a consistent asymptotic line-profile analysis.

Effect of Solute Carbon on the Evaluation of Dislocation Density in as Quenched Martensite by X-ray Diffraction

Effect of Solute Carbon on the Evaluation of Dislocation Density in as Quenched Martensite by X-ray Diffraction

Evaluation of Dislocation Densities in Various Microstructures of Additively Manufactured Ti6Al4V (Eli) by the Method of X-ray Diffraction.

Dislocations play a central role in determining strength and flow properties of metals and alloys. Diffusionless phase transformation of β→α in Ti6Al4V during the Direct Metal Laser Sintering (DMLS) process produces martensitic microstructures with high dislocation densities. However, heat treatment, such as stress relieving and annealing, can be applied to reduce the volume of these dislocations. In the present study, an analysis of the X-ray diffraction (XRD) profiles of the non-heat-treated and heat-treated microstructures of DMLS Ti6Al4V(ELI) was carried out to determine the level of defects in these microstructures. The modified Williamson–Hall and modified Warren–Averbach methods of analysis were used to evaluate the dislocation densities in these microstructures. The results obtained showed a 73% reduction of dislocation density in DMLS Ti6Al4V(ELI) upon stress relieving heat treatment. The density of dislocations further declined in microstructures that were annealed at elevated temperatures, with the microstructures that were heat-treated just below the β→α recording the lowest dislocation densities.

Evaluation of depth of dislocation visibility in SEM electron channeling contrast imaging in Ti-6Al-4V alloy using serial sectioning method

In this study, we conducted a quantitative evaluation of dislocation density by scanning electron microscopy electron channeling contrast imaging for α grains of a Ti-6Al-4V alloy deformed at room temperature. The depth of visibility of dislocations is experimentally measured as 140 to 160 nm by a serial sectioning observation. This result is compared with the theoretical value and applied to evaluate dislocation density. These factors confirm that the theoretically calculated value of the depth of visibility, at 5 to 6 times the extinction distance, is valid for the hexagonal close-packed Ti alloy.

Dislocation Density-Based Multiscale Modeling of Deformation and Subgrain Texture in Polycrystals

In this work, a viscoplastic fast Fourier transform (FFT)-based code is combined with a continuum dislocation dynamics (CDD) framework to analyze the mechanical behavior of polycrystalline MgAZ31 material under unidirectional tensile test. A crystal plasticity formulation including the size effects through a stress/strain gradient theory, dislocation density flux among neighboring grains and grain boundary back stress field is implemented into the CDD and coupled with VPFFT for this purpose. Then, an electron backscatter diffraction-based orientation image microscopy of a sample microstructure is applied as an input to the code. The model predicts, among other things, distributions of stress, strain, mobile dislocation density, geometrically necessary dislocation and stress–strain behavior. The numerical findings are compared with experimental results, and the micromechanical behavior of the polycrystal is discussed regarding dislocation density evaluation in different stages of strain hardening.

EBSD, XRD and SRS characterization of a casting Al-7wt%Si alloy processed by equal channel angular extrusion: Dislocation density evaluation

Aluminum‑silicon (AlSi) alloys of high silicon contents are composite materials; they are used whenever high casting properties are required. They are slightly ductile below 8wt%Si. An increase in ductility can be obtained by refining Si-crystals in elaboration or by a further hot working. In the present work, an Al-7wt%Si alloy was processed by Equal Channel Angular Extrusion (ECAE) at temperatures 20 °C and 160 °C up to three passes. The die was formed by two cylindrical channels with characteristic angles Φ = 110° and Ψ = 0. EBSD, X ray diffraction (XRD) and Strain Rate Sensitivity (SRS) were used to characterize the microstructure and the mechanical properties. High levels of strain were introduced at both temperatures. The activation volume was lowered to 125b3 and 210b3 at 20 °C and 160 °C respectively and was considered to be dislocation density dependent. The remaining dislocation densities calculated from EBSD, XRD and SRS experiments are quite different. This was explained by the scale difference and by the sensitivities of the methods to the free surface effect.

Dislocation density evaluation of three commercial SA508Gr.3 steels for reactor pressure vessel

Reactor pressure vessel (RPV) plays a crucial role in the safe operation of nuclear power plants (NPPs). However, RPV steels are generally subjected to high-energy neutron irradiation during the entire in-service lifetime, which causes the hardening and embrittlement of RPV steels. The dislocation density (ρ) is known to affect strongly the mechanical properties of RPV steels. The dislocation density of three commercial SA508 Gr.3 steels manufactured by different countries was thoroughly investigated by X-ray diffraction (XRD), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). The dislocation densities of three SA508 Gr.3 steels calculated by XRD are as high as 1013 m−2 and consistent with results of tempered steels reported in prior studies. The results calculated by XRD match qualitatively with those obtained from TEM and kernel average misorientation (KAM) analysis of EBSD (EBSD-KAM).

Processing of fine grained AISI 304L austenitic stainless steel by cold rolling and high-temperature short-term annealing

An advanced thermomechanical process based on the formation and reversion of deformation-induced martensite was used to refine the grain size and enhance the hardness of an AISI 304L austenitic stainless steel. Both low and high reversion annealing temperatures and also the repetition of the whole thermomechanical cycle were considered. While a microstructure with average austenite grain size of a few micrometers was achieved based on cold rolling and high-temperature short-term annealing, an extreme grain refinement up to submicrometer regime was obtained by cold rolling followed by low-temperature long-term annealing. However, the required annealing time was found to be much longer, which negates its appropriateness for industrial production. While a magnificent grain refinement was achieved by one pass of the high-temperature thermomechanical process, the reduction in grain size was negligible by the repetition of the whole cycle. It was found that the hardness of the thermomechanically processed material is much higher than that of the as-received material. The results of the present work were shown to be compatible with the general trend of grain size dependence of hardness for AISI 304L stainless steel based on the Hall-Petch relationship. The results were also discussed based on the X-ray evaluation of dislocation density by modified Williamson-Hall plots.

On the evaluation of dislocation densities in pure tantalum from EBSD orientation data

We analyze measurements of dislocation densities carried out independently by several teams using three different methods on orientation maps obtained by Electron Back Scattered Diffraction on commercially pure tantalum samples in three different microstructural states. The characteristic aspects of these three methods: the Kernel average method, the Dillamore method and the determination of the lattice curvature-induced Nye’s tensor component fields are reviewed and their results are compared. One of the main features of the uncovered dislocation density distributions is their strong heterogeneity over the analyzed samples. Fluctuations in the dislocation densities, amounting to several times their base level and scaling as power-laws of their spatial frequency are observed along grain boundaries, and to a lesser degree along sub-grain boundaries. As a result of such scale invariance, defining an average dislocation density over a representative volume element is hardly possible, which leads to questioning the pertinence of such a notion. Field methods allowing to map the dislocation density distributions over the samples therefore appear to be mandatory.

Evaluation of Dislocation Density in Cold Worked Low Carbon Ferritic Steel

Evaluation of Dislocation Density in Cold Worked Low Carbon Ferritic Steel