Terms Of Dislocation Density Research Articles

Influence of domain walls thickness, density and alignment on Barkhausen noise emission in low alloyed steels

This study deals with the characterization of low alloyed steels of different yield strengths (varying in the range of 235–1100 MPa) via Barkhausen noise emission. The study investigates the potential of this technique to distinguish among the low alloyed steels and all significant aspects contributing to Barkhausen noise, such as the residual stress state, microstructure expressed in terms of dislocation density, grain size, prevailing phase, as well as associated aspects of the domain wall substructure (domain wall thickness, energy, their spacing and density in the matrix). Barkhausen noise in the rolling as well as transversal direction grows along with the yield strength (up to 500 MPa) and the corresponding grain refinement of ferrite. As soon as the martensite transformation occurs in a high strength matrix, this evolution saturates, and remarkable magnetic anisotropy is developed when Barkhausen noise in the transversal direction grows at the expense of the rolling direction. The contribution of residual stresses as well as the domain wall thickness is only minor, and the evolution of Barkhausen noise is driven by the density of the domain walls and their realignment.

Effect of dislocation density distribution in work-hardened layer on cutting characteristics in the multi-cutting of 49Fe49Co2V alloy

In the multi-cutting of metallic materials, the machined surface modification generated in the preceding cutting process, could further influence the cutting characteristic in the subsequent cutting. This paper investigates the effect of the work-hardened layer (WHL) on the subsequent cutting characteristics in the multi-cutting process of 49Fe49Co2V alloy. Firstly, the WHL features in terms of dislocation density, micro-hardness, and plastic deformation in the subsurface are studied experimentally. A modified Johnson-Cook (MJC) constitutive model with the consideration of dislocation density is developed to describe the dynamic behavior for the WHL. The FE model of an orthogonal cutting process is developed with the implementation of MJC model. In this model, the preceding cutting-induced dislocation density distribution is defined in the WHL. The cutting force is measured experimentally and used to verify the FE model. The cutting force, cutting temperature and stress distribution for the WHL and the bulk material are compared. It is found that in the multi-cutting with small cutting depths, the effect of dislocation density distribution in WHL on the cutting characteristics should be taken into consideration. The study could provide guidance to determination of the machining parameters in the multi-cutting.

Influence of proton irradiation on the microstructure and mechanical properties of Nb-1Zr-0.1C alloy

• Effect of proton irradiation on Nb-1Zr-0.1C alloy proposed for CHTR. • XRD line profile analyses of the data collected using synchrotron source. • Prediction of dislocation loop formation with XRDLPA and supported by TEM analysis. • Microstructure-mechanical property correlation by tensile and nanoindentation test. • Comparison of microstructure and mechanical properties with proton irradiated pure Nb . The microstructural evolution and corresponding changes in mechanical properties of proton irradiated Nb-1Zr-0.1C alloy have been studied as a function of irradiation dose. Different XRD line profile analyses (XRDLPA) have been carried out using synchrotron XRD data to evaluate the microstructural parameters. It is observed that coherent domain size decreases with an increasing microstrain within the domain as a function of dose. The dislocation density also increases and shows saturation at the highest dose. The Wilkens arrangement parameter decreases as a function of dose indicating the formation of correlated dislocations in the alloy matrix. Transmission electron microscopy (TEM) analysis confirms the presence of dislocation loops in the highest dose sample supporting the findings of XRDLPA. The strength and ductility of unirradiated and all irradiated samples have been evaluated as a function of dose. It is observed that the ductility reduces continuously along with increasing yield strength (YS) and ultimate tensile strength (UTS) as a function of dose. Nanoindentation was also carried out to measure the change in hardness of the alloy with dose. It is observed that the value of the nanohardness increases continuously from 1.74 GPa for the unirradiated sample to 2.43 GPa in the highest dose irradiated sample. Systematic changes are observed on the morphology of fracture surface as a function of dose. Signature of ductile failure is observed in the unirradiated sample whereas brittle failure is prominent in the irradiated samples with higher dose. The change in microstructure in terms of dislocation density is corroborated with the change in mechanical properties in terms of YS of the irradiated alloy. Contradistinction of the findings of Nb-1Zr-0.1C with those of pure Nb contemplated the role of the alloying additions, especially carbon, in governing the irradiation behaviour of the alloy.

Effect of a novel semi-solid middle-layer structure formed during twin-roll casting on the mechanical properties of AA6022 alloy

During a twin-roll casting (TRC) process with low-rolling force and near-liquidus melt treatment, a novel semi-solid middle structure is produced. The formation of this structure and its influence on the alloy mechanical properties were investigated using polarized light microscopy, electron backscattered diffraction, scanning electron microscopy, and transmission electron microscopy. The middle structure was generated by rolling a semi-solid melt with high thixotropic strength. The size of its sedimentary grains strongly affected the thixotropic strength by influencing the grain boundary density, which promoted different grain movements in the rolling process. The large grains slid along each other, whereas the small grains were deformed, inhibiting their macrosegregation. Compared with the conventional TRC structure, the semi-solid middle layer containing small sedimentary grains increased the slab elongation from 17.5% to 25.4% without reducing the tensile strength; however, the slab with large sedimentary grains exhibited low plasticity. The effect of the middle structure on the alloy mechanical properties is elucidated in terms of dislocation density, grain size, texture type, and resistance to crack propagation. The obtained results can help develop new approaches to designing TRC slab structures with desired characteristics.

Effect of Nb addition and Pre-strain on Hydrogen Embrittlement of Low-carbon Steels with Ferrite-pearlite Structure

The effect of pre-strain on the hydrogen embrittlement of Nb-free and Nb-added low-carbon steels with ferrite-pearlite structure was investigated in this study. After the steels were electrochemically charged with hydrogen, slow-strain rate tensile (SSRT) tests were conducted on them to examine hydrogen embrittlement behavior. The SSRT test results revealed that the Nb-added steel had a lesser decrease of elongation and reduction of area than the Nb-free steel. The formation of NbC carbide and grain refinement caused by the Nb addition improved resistance to hydrogen embrittlement. The loss of elongation and the reduction of area after hydrogen charging occurs when pre-strain is increased. The pre-strain increases dislocation density and thus increases the amount of reversible hydrogen trap sites associated with hydrogen embrittlement. 10% pre-strained specimens exhibited a significant loss in elongation and reduction of area, regardless of Nb addition. Based on the results of electron back-scatter diffraction, fractographic, and silver decoration analyses for Nb-free and Nb-added steels, the hydrogen embrittlement mechanism in low-carbon steels with different amounts of pre-strain is discussed in terms of dislocation density and hydrogen distribution.

Enhanced Strain Relaxation in AlGaN Layers Grown on Sputter‐Based AlN Templates

Strain plays a crucial role in the performance of ultraviolet (UV) optoelectronic devices. The strain states of AlGaN layers grown on different AlN templates are investigated. Two types of AlN templates are prepared: one is grown solely by metal‐organic chemical vapor deposition (MOCVD), and the other is fabricated with the combination of direct current (DC) sputtering, high‐temperature annealing, and MOCVD growth. The qualities of 4 μm‐thick MOCVD AlN and 1.2 μm‐thick sputter‐based AlN are almost equivalent in terms of dislocation density and surface morphology. However, the strain relaxation ratio of the AlGaN layer is higher for the 1.2 μm sputter‐based AlN than the 4 μm MOCVD AlN, indicating that the strain state depends on the total thickness of the epilayers on the sapphire substrate. In addition, it is found that the curvature of the sample at room temperature is governed mainly by the thickness of the AlGaN layer. As a result, sputter‐based AlN templates allow for the enhanced strain relaxation of the AlGaN layer with a small sample bowing.

A framework and finite element formulation for incompatibility tensor‐based gradient plasticity

AbstractThe present contribution deals with a gradient plasticity formulation that accounts for energetic contributions of plastic incompatibilities that are interpreted in terms of dislocation densities. At the outset of the theory, an extended form of the dissipation inequality in terms of a nonlocality residual is assumed in accordance with [1]. Taking into account dislocation density tensors as additional arguments of the free energy density function, this form naturally gives rise to the balance equation of a generalised (quasi) nonlocal stress field that is energetically conjugated to the plastic deformation. Against this background one furthermore observes that the energetic dual to the dislocation density tensor is closely related to a back stress‐type part of the generalised stress field. For the solution of the resulting system of coupled partial differential equations, a multi‐field finite element approach is taken, with the plastic consistency conditions being accounted for on global level by means of Fischer‐Burmeister complementarity functions.

Improved trade-off between strength and plasticity in titanium based metastable beta type Ti-Zr-Fe-Sn alloys

An impressive strengthening ability of Laves phases is favorable to develop titanium alloys with an improved trade-off between strength and plasticity. Therefore, the Ti-xZr-7Fe-ySn (x = 25, 30, 35 wt% and y = 1, 2 wt%) alloys were first designed in such a manner that a Laves phase would precipitate in these alloys and then the investigated alloys were produced by cold crucible levitation melting. A hexagonal close-packed C14 type Laves phase along with a dominant fraction of body-centered cubic β phase are formed in all the as-cast Ti-xZr-7Fe-ySn alloys except in Ti-25Zr-7Fe-2Sn. The volume fraction of the Laves-C14 phase is found to be sensitive to the quantities of Zr and Sn. Amongst all the investigated alloys, Ti-35Zr-7Fe-2Sn shows a better dislocation-pinning ability in terms of dislocation density (3.96 × 1015 m−2), yield strength (1359 MPa) and hardness (437 HV), whereas Ti-25Zr-7Fe-1Sn shows a better deformation ability in terms of compressive strain at failure (36.2%) and plastic strain (31.9%). Crack propagation, regions of dimples and deformation bands are examined in the fracture analyses. Moreover, in this work, Ti-25Zr-7Fe-1Sn exhibits the best strength and plasticity trade-off in terms of a product of ultimate strength and compressive strain at failure (77.4 GPa %).

Stabilization of coupled convection–diffusion-reaction equations for continuum dislocation transport

The plasticity of crystalline materials can be described at the meso-scale by dislocations transport models, typically formulated in terms of dislocation densities. This leads to sets of coupled nonlinear partial differential equations involving diffusive and convective transport mechanisms. Since exact solutions for these systems are not available, numerical approximations are needed to efficiently solve them. The properties of these systems of equations cause most traditional numerical methods to fail, even for the case of a single equation. For systems of equations the problem is even more challenging due to the lack of fundamental principles guiding numerical discretization strategies. Special strategies must be developed and carefully applied to obtain physically meaningful and numerically stable approximations. The objective of this paper is to construct a coefficient perturbation-based stabilization technique for general systems of equations and to apply it to the modelling of one-dimensional dislocation transport. A detailed numerical study is carried out in order to demonstrate its ability to render well-behaved and physically admissible numerical approximations.

A comparison study of dislocation density, recrystallization and grain growth among nickel, FeNiCo ternary alloy and FeNiCoCrMn high entropy alloy

The microstructural evolutions in terms of dislocation density, annealing twin density as well as with respect to microstructural changes due to recrystallization and grain growth were investigated in pure Ni, equiatomic FeNiCo alloy, and FeNiCoCrMn high entropy alloy (HEA) during the thermomechanical process. All samples were single phase and showed a face-centered cubic (FCC) lattice structure. This was maintained during thermomechanical processing comprising of cold swaging by 85% reduction of cross-sectional area and subsequent annealing at 800 °C. The level of dislocation accumulation during cold swaging increased with the number of constituent elements. The FeNiCoCrMn HEA obtained the highest dislocation density, followed by the FeNiCo and Ni, respectively. After the annealing at 800 °C for 0.5 h, all samples achieved the large fraction of recrystallized grains with minor fraction of substructured grains and no deformed grain. The FeNiCoCrMn HEA obtained the smallest recrystallized grain size (∼5 μm) after the annealing at 800 °C for 0.5 h. This could be a result of the highest dislocation density generated during cold swaging prior to the annealing. The prolonged annealing at 800 °C for up to 24 h led to a grain growth for all the samples, however, at different growth rates. The FeNiCoCrMn HEA revealed the lowest rate of grain growth, but the microstructural changes during the annealing were not significantly different between the FeNiCo and Ni samples. Besides the effect of the number of constituent elements, the type and the combination of constituent elements have an effect on the microstructural evolution during the annealing.

A synchrotron X-ray and electron backscatter diffraction based investigation on deformation and failure micro-mechanisms of monotonic and cyclic loading in titanium

Synchrotron X-ray diffraction technique has been used to estimate defect structure in terms of dislocation density, crystallite size and micro-strain in commercially pure titanium subjected to tension and cyclic deformation in stress and strain control mode. Statistical analysis of micro-texture data collected from electron backscatter diffraction approximately from the same region as that of synchrotron X-ray has been used to correlate orientation dependent micro-strain and dislocation density with deformation microstructure and micro-texture. Two different orientations, namely, A with prismatic-pyramidal and B with basal orientation along the loading axis has been considered. Weak initial texture yet significant anisotropy in hardening/softening response and failure mode for monotonic tension and cyclic loading paths has been observed. Higher strain hardening response of orientation A during monotonic tensile deformation can be attributed to the evolution of lower micro-strain on basal orientation grains i.e, 〈0002〉ǁND along with extensive multi-variant twinning that also restricts crack propagation and delays failure in stress control mode. On the other hand, in strain control mode, orientation B shows higher fatigue life due to the generation of lower micro-strain in the basal orientation grains and single variant twinning that can undergo detwinning easily is responsible for delayed crack nucleation and subsequent failure.

Experimental study on structural, optoelectronic and room temperature sensing performance of Nickel doped ZnO based ethanol sensors

Nano crystalline undoped (1Z) Zinc Oxide (ZnO) and 5, 10 and 15 Wt. % (1ZN, 2ZN and 3ZN) of Nickel doped ZnO based sensors were fabricated using the hydrothermal approach on Fluorine doped Tin Oxide (FTO) glass substrates. X-ray diffraction (XRD) analysis proved the hexagonal Wurtzite structure of ZnO. Parametric variations in terms of dislocation density, bond length, lattice parameters and micro strain with respect to dopant concentration were analysed. The prominent variations in the crystallite size, optical band gap and Photoluminescence peak ratio of devices fabricated was observed. The Field Emission Scanning Electron Microscope (FESEM) images showed a change in diameter and density of the nanorods. The effect of the operating temperature, concentration of ethanol and the different doping levels of sensitivity, response and recovery time were investigated. It was inferred that 376% of sensitivity with a very quick response and recovery time of <5 s and 10 s respectively at 150 °C of 3ZN sensor has better performance compared to other three sensors. Also 3ZN sensor showed improved sensitivity of 114%, even at room temperature with response and recovery time of 35 s and 45 s respectively.

A continuum approach to combined γ/γ′ evolution and dislocation plasticity in Nickel-based superalloys

In single crystal Nickel-based superalloys subject to creep loading, the γ/γ′ phase microstructure co-evolves with the system of dislocations under load. Computational modeling thus requires multiphysics approaches capable of describing and simulating both phase and defect microstructures within a common conceptual framework. To do so, we formulate a coupled continuum model of the evolution of phase and dislocation microstructures. The simulated γ/γ′ phase microstructure accounts for concentration as well as crystallographic order parameters. Dislocation microstructure evolution is described in terms of dislocation densities and associated stress-driven dislocation fluxes. The creep strain curve is obtained as a natural by-product of the microstructure evolution equations. We perform simulations of γ/γ′ evolution for different dislocation densities and establish the driving forces for microstructure evolution by analyzing in detail the changes in different contributions to the elastic energy and chemical free energy density, as well as the evolution of stress concentrations that may trigger the transition from dislocation flow in the γ channels towards shearing of the γ′ precipitates. Our investigation reveals the mechanisms controlling the process of directional coarsening (rafting) and demonstrates that the kinetics of rafting significantly depends on characteristics of the dislocation microstructure. In addition to rafting under constant load, we investigate the effect of changes in loading conditions and explore the possibility of improving creep properties by pre-rafting along a different loading path.

Halogen-free vapor phase epitaxy for high-rate growth of GaN bulk crystals

Here, we propose a halogen-free vapor phase epitaxy (HF-VPE) technique to grow bulk GaN single crystals. This technique employs the simplest reaction for GaN synthesis (reaction of Ga vapor with NH3) and can potentially achieve a high growth rate, a prolonged growth duration, a high crystal quality, and a low cost. The analyses of thick HF-VPE-GaN layers grown under optimized growth conditions revealed that high-quality crystals, both in terms of dislocation density and impurity concentration, are obtained at high growth rates of over 100 µm/h.

Evolution of real structure in Ge‐Si mosaic crystals

The structure and properties of Czochralski (Cz)‐grown Ge1‐xSix mosaic crystals were investigated using optical microscopy, atomic force microscopy, X‐ray diffraction analysis, microprobe analysis, FTIR and transmission electron microscopy. The role of segregation, form of solid‐liquid interface and dislocation generation in the development of mosaic structure were analyzed and used for optimization of growth parameters such as Si concentration and growth rate. The dislocation density estimated experimentally was compared with the calculated data. Composition fluctuations caused by formation of cellular structure at the interface lead to a local lattice misorientation that is one of the reasons for crystal mosaicity. Model of mosaic structure generation in terms of dislocation density and composition variations is presented.

Increasing minority carrier lifetime in as-grown multicrystalline silicon by low temperature internal gettering

We report a systematic study into the effects of long low temperature (≤500 °C) annealing on the lifetime and interstitial iron distributions in as-grown multicrystalline silicon (mc-Si) from different ingot height positions. Samples are characterised in terms of dislocation density, and lifetime and interstitial iron concentration measurements are made at every stage using a temporary room temperature iodine-ethanol surface passivation scheme. Our measurement procedure allows these properties to be monitored during processing in a pseudo in situ way. Sufficient annealing at 300 °C and 400 °C increases lifetime in all cases studied, and annealing at 500 °C was only found to improve relatively poor wafers from the top and bottom of the block. We demonstrate that lifetime in poor as-grown wafers can be improved substantially by a low cost process in the absence of any bulk passivation which might result from a dielectric surface film. Substantial improvements are found in bottom wafers, for which annealing at 400 °C for 35 h increases lifetime from 5.5 μs to 38.7 μs. The lifetime of top wafers is improved from 12.1 μs to 23.8 μs under the same conditions. A correlation between interstitial iron concentration reduction and lifetime improvement is found in these cases. Surprisingly, although the interstitial iron concentration exceeds the expected solubility values, low temperature annealing seems to result in an initial increase in interstitial iron concentration, and any subsequent decay is a complex process driven not only by diffusion of interstitial iron.

High leaded tin bronze processing during multi-directional forging: Effect on microstructure and mechanical properties

The high leaded tin bronze alloy has been studied during multidirectional forging (MDF) process with differing crystallite size (D) in the scale of 240–90nm. The effect of MDF strain (εMDF∼ 0.25, 0.50, 0.75) after 3, 6 and 9 passes respectively on the development of homogeneity and refinement in terms of dislocation density, twin spacing, twin lamellae thickness and their division have been studied using high-resolution transmission electron microscopy (HRTEM) and micro-hardness measurements using Vickers micro-hardness. X-ray diffraction peak broadening investigation has revealed that the crystallite size decreases down to 90nm after 9 pass (εMDF∼ 0.75). The tensile fracture surfaces have been studied using scanning electron microscope (SEM) of received (AR) and MDF specimens. The results of crystallite size and dislocation density on the strength of refined microstructure and nanostructure have been correlated with mechanical flow stress.

Internal stresses in a homogenized representation of dislocation microstructures

To develop a continuum theory based on the evolution of dislocation microstructures, two challenges have to be resolved: the correct representation of the kinematics of dislocation motion in terms of dislocation density and the formulation of a mobility law reflecting an effective description of the physical behavior of the discrete many-body problem.Kröner's classical continuum theory has inspired different approaches to model plasticity based on the motion of dislocations. Amongst them, the Continuum Dislocation Dynamics (CDD) theory was formulated as a generalization of the classical theory. The CDD theory allows for a continuous representation of the evolution of dislocation microstructures and is found to be kinematically complete.Here, a numerical formulation of the CDD theory is presented and constitutive laws for the incorporation of dislocation interactions are derived based on the representation of the dislocation microstructure in two dimensions. An error measure is introduced to analyze the constitutive law and the results are compared to discrete dislocation dynamics simulations. Important aspects for the implementation of a 3D theory are discussed.

&lt;i&gt;In Situ&lt;/i&gt; X-Ray Diffraction Study of Thermal Stability of Cu and Cu-Zr Samples Processed by ECAP

X-ray diffraction (XRD) studies of ECAP (equal-channel angular pressing) materials were performed after annealing and by in-situ measurements in XRD high-temperature chamber for samples prepared by different number of passes and number of revolutions, respectively. Main attention was given to Cu and Cu-Zr samples. Significant dependence on number of passes was found for ECAP samples. In-situ measurements were focused not only on temperature dependence but also on time evolution of the diffraction line profiles. Evaluation in terms of dislocation densities, correlation and crystallite size and its distribution was performed by our own software MSTRUCT developed for total powder diffraction pattern fitting. Abnormal growth of some grains with annealing is well-known for copper and leads to the creation of bimodal microstructure. Therefore a special care must be given to the evaluation and a model of two Cu components (larger and smaller crystallites) was fitted to the data if an indication of some crystallite growth appears either in the XRD line profile shape or in two-dimensional diffraction patterns.

Correlation between the sub-structure parameters and the manufacturing technologies of metal threads in historical textiles using X-ray line profile analysis

Micro-structure can talk when documentation is missing. In ancient Roman or medieval periods, kings, queens, or just rich people decorated their clothes or even their horse covers richly with miniature jewels or metal threads. The origin or the fabrication techniques of these ancient threads is often unknown. Thirteen thread samples made of gold or gilt silver manufactured during the last sixteen hundred years are investigated for the micro-structure in terms of dislocation density, crystallite size, and planar defects. In a few cases, these features are compared with sub-structure of similar metallic threads prepared in modern, twentieth century workshops. The sub-structure is determined by X-ray line profile analysis, using high resolution diffractograms with negligible instrumental broadening. On the basis of the sub-structure parameters, we attempt to assess the metal-threads manufacturing procedures on samples stemming from the fourth century A.D. until now.