Low Boundaries Research Articles

Sensitization of TiO2 nanotube arrays photoelectrode via homogeneous distribution of CdSe nanoparticles by electrodeposition techniques

CdSe has attracted interest as the absorber layer in photoelectrochemical cell applications. In this work, titania nanotubes thin film electrodes were prepared by the anodisation method of titanium foil were used due to their ability to produce a uniformly stable structure with high surface area, excellent charge transfer, low interfacial grain boundaries and effective absorption of light. Three electrochemical deposition techniques were used to deposit CdSe onto TiO2 NTAs, namely potentiostatic deposition, cyclic voltammetric deposition, and pulse electrodeposition techniques as a novel to compare between these methods. X-ray diffractometry (XRD), field emission scanning electron microscopy (FESEM) coupled with energy dispersive X-ray spectroscopy (EDX), high-resolution transmission electron microscopy (HRTEM), and UV–Visible diffuse reflectance spectroscopy were used for the characterization of CdSe/TiO2 NTAs nanostructures. The photoelectrochemical performance of the nanostructured CdSe/TiO2 NTAs was investigated in 0.01 M Na2S under visible light illumination. The use of pulse electrodeposition resulted in a greater uniformity in the distribution of CdSe loaded onto TiO2 nanotube arrays. Thus the performance of semiconductor heterostructures prepared by this technique shows a substantial improvement compared to the other two techniques.

Microstructure and hardness of NbTiZr and NbTaTiZr refractory medium-entropy alloy coatings on Zr alloy by laser cladding

Two refractory medium-entropy alloy (RMEA) coatings (NbTiZr and NbTaTiZr) with BCC solid-solution phase as their main constitutional phase were successfully fabricated on Zr702 alloy by utilizing pulsed laser cladding. Phase constitutions, chemical compositions, crystallographic orientations and hardnesses of the coatings were carefully characterized and analyzed. Results show that both the RMEA coatings correspond to cladding zones (CZs) that are composed of irregular-shaped bulk grains (BCC solid-solution phase) with scattered orientations and 370–380 μm in thickness. The addition of Ta is able to promote grain refinement and produce higher micro strains in the coatings. Theoretical analyses show that the phase constitution in the RMEA coatings is consistent with that predicted according to several thermodynamic or structural parameters. Below the CZs, there exist heat-affected zones (HAZs) consisting of a mixture of bulk α-Zr grains and fine martensite plates. Average thicknesses of the HAZs in both the specimens are 180–190 μm. Hardnesses of the NbTiZr and the NbTaTiZr coatings are 510 ± 22 HV and 550 ± 26 HV, respectively, significantly higher than that of the substrate (196 ± 4 HV). Such high hardnesses are attributed to combined effects of solid-solution strengthening, grain refinement strengthening and increased contents of low angle boundaries.

The effects of microstructures and radiation damage on the deformation behavior of a HT-9 alloy using microtensile testing

Understanding the influence of radiation damage on the mechanical properties of HT-9 and other tempered martensitic alloys is part of the mission in developing radiation-tolerant materials for the next generation of nuclear reactors. Although there has been extensive data on the macroscopic mechanical properties of the irradiated HT-9 alloys and microstructural changes, it is not well-understood how radiation damage and the resulting microstructural changes influence the local mechanical properties. In this study, we utilized in situ SEM microtensile testing to investigate the deformation behavior of specific martensitic boundaries in 1 dpa proton-irradiated HT-9 and answered the question of what is the weakest link. We provided the direct observation of the failure modes of microtensiles containing high angle and low angle martensitic boundaries. In the unirradiated condition, the deformation is ductile and no martensitic boundary failure is observed. In the irradiated condition, the high angle martensitic boundaries are more susceptible to radiation-induced boundary failure as compared to the low angle martensitic boundaries.

Understanding the High-Temperature Fatigue Crack Growth from Exceptional Nano-\u03b1 Phases and {10 bar{1} 2} Deformation Twins in Hot Deformed Titanium Alloy

In this study, the fatigue crack growth rate (FCGR) of Ti-6Al-4V alloy at 723 K was measured by direct current potential drop (DCPD) method, and exceptional nano-α phases and {10 bar{1} 2} deformation twins were newly found at the crack tip in Ti-6Al-4V alloy. The results showed that nano-α phases have Burgers orientation relationships (OR) (0001)α//(110)β, [ bar{2} 110]α//[ bar{1} 11]β with β phases. The terraced-structure interface consisted of (0 bar{1} 10)α//(1 bar{1} 2)β and (10 bar{1} 0)α//(1 bar{1} 0)β. Numerous dislocations accumulated in the β phase and became the diffused channels of O and V atoms. The α stabilizers (O element) diffused into the crystal lattice and β stabilizers (V element) spread out of the crystal lattice which accelerated the nano-α phases nucleation. <a> and <c + a> dislocations piled up at the primary α grain, interface and low angle grain boundaries (LAGBs), and dissociated into twinning dislocations to promote the twin nucleation. Dislocation transformation enabled nano-twins to grow through the primary α/β interface and strain transfer led that deformation twins nucleated in the adjacent primary α grains. With the effects of nano-α phases, LAGBs and twins, the resistance of crack propagation increased and the driving force decreased, and resulted in the low FCGR.Graphic

Double-sided GTAW of nuclear grade steel: Mechanical and microstructure perspectives

This article investigates the microstructural evolution and mechanical integrity of austenitic stainless steel SS321, a titanium stabilized nuclear grade mainly preferred for severe corrosive environments. The double-sided gas tungsten arc welding (DS-GTAW) technique was utilized to fabricate the butt joint having a plate thickness of 6 mm. A heat input of 1.4058 kJ/mm was used to obtain the maximum depth of penetration (DoP) of 3.3 mm with welding speed 120 mm/min and current 220A. Optical microscopy reveals the microstructure of weldment and base metal (SS321). The DS-GTA weldments were subjected to tensile, bend, impact and microhardness tests and the results are evaluated. The fusion zone consists of columnar, equiaxed dendrites, and intermetallic compounds of Titanium carbide (TiC). From EBSD examination the higher fraction of Low Angle Grain Boundaries is corroborated to the increase in tensile strength and reduction in impact toughness of the weldment. The ferrite measurement reveals the increase in ferrite content in the weldment (6.1 FN) and this attributed to the presence of retained δ-ferrite in comparison to SS321 (1.2 FN). X-Ray Diffraction (XRD) pattern reveals austenite and ferrite peaks are present in the SS321 and weldment. Ductile mode of fracture (using SEM-Scanning electron microscope) was observed in the uni-axial tensile and impact test of weldment specimens.

LPAQR-Net: Efficient Vertebra Segmentation From Biplanar Whole-Spine Radiographs.

Vertebra segmentation from biplanar whole-spine radiographs is highly demanded in the quantitative assessment of scoliosis and the resultant sagittal deformities. However, automatic vertebra segmentation from the radiographs is extremely challenging due to the low contrast, blended boundaries, and superimposition of many layers, especially in the sagittal plane. To alleviate these problems, we propose a lightweight pyramid attention quick refinement network (LPAQR-Net) for efficient and accurate vertebra segmentation. The LPAQR-Net consists of three components: (1) a lightweight backbone network (LB-Net) to prune network parameters and memory footprints to strike an optimal balance between speed and accuracy, (2) a series of global attention refinement (GAR) modules to selectively reuse low-level features to facilitate the feature refinement, and (3) an attention-based atrous spatial pyramid pooling (A-ASPP) module to extract weighted pyramid contexts to improve the segmentation of blurred vertebrae. Moreover, the multi-class training strategy is employed to alleviate the over-segmentation of adjacent vertebrae. Evaluation results on both frontal and lateral radiographs of 332 AIS patients show our method achieves accurate vertebra segmentation with significant reductions in inference time and computational demands compared to the state-of-the-art. Meanwhile, results on the public AASCE2019 dataset also demonstrate the good generalization ability of our model. It is the first attempt to explore the lightweight network for vertebra segmentation from biplanar whole-spine radiographs. It simulates radiologists gathering nearby contexts for accurate and robust vertebra boundary inference. The method can provide efficient and accurate vertebra segmentation for clinicians to perform a fast and reproducible spinal deformity evaluation.

Potential thermoelectric materials of indium and tellurium co-doped bismuth selenide single crystals grown by melt growth technique

In the present work, the thermoelectric properties of potential thermoelectric materials (Bi1−xInx)2Se2.7Te0.3 grown as high-quality single crystals by the melt growth technique were investigated between 10 and 350 K. Powder X-ray diffraction confirms the hexagonal crystal structure of all studied crystals. The high-resolution X-ray diffraction study reveals the direction of growth, single-crystal quality, dislocation density, and the influence of dopants on the inner plane structure of the crystals. A clean surface with very low angle grain boundaries is observed by the field emission scanning electron microscopy. Energy-dispersive X-ray analysis confirms the elemental composition of the crystals. Electrical resistivity has shown degenerate semiconducting behavior with low activation energy. The Seebeck coefficient confirms p-type for the pristine and n-type conducting behavior for the doped samples, with the correlation to the carrier concentration and carrier mobility in the order of 1025/m3 and 10−4 m2/V s, respectively. Thermal conductivity has shown the dominant behavior of phonon scattering. A significant reduction in the electrical resistivity was found for the co-doped (Bi0.96In0.04)2Se2.7Te0.3 sample, leading to an enhancement of the power factor (PF) and thermoelectric figure of merit (ZT) by a factor of about 8.0 and 4.1, respectively, as compared to the pristine Bi2Se3 sample at 350 K. The highest ZT value of about 0.285 is achieved for (Bi0.96In0.04)2Se2.7Te0.3 at 350 K.

Improved fracture behavior and microstructural characterization of heterogeneous-structured tungsten

Tungsten and its alloys are known to be brittle at room temperature and have high ductile-to-brittle transition temperatures. Improving the ductility of tungsten has significant impact on the range of applications. Here, we report a kind of heterogeneous-structured pure tungsten, which shows an improved ductility. The heterogeneous-structured tungsten is produced by hot-rolling and partial recrystallization of pure tungsten. The structural heterogeneity is composed by elongated grains with a high density of low angle grain boundaries and blocky regions of recrystallized grains. The fracture behavior is systematically studied on the heterogeneous-structured tungsten, which shows that the path of crack propagation plays an important role on improving the ductility. Trangranular fracture is the preferred crack path in the banded regions with elongated grains and intergranular fracture dominates the crack propagation when the crack extends to the blocky recrystallized grains. The cooperation of trangranular and intergranular fracture modes increases the total work to fracture and thus improves the ductility. This study is instructive for the development of advanced high-performance pure tungsten with good mechanical properties.

Effects of Laser-Powder Bed Fusion Process Parameters on the Microstructure and Corrosion Properties of AlSi10Mg Alloy

In this study, the effects of microstructural modifications induced by tuning the laser-powder bed fusion (L-PBD) process parameters on electrochemical stability of the L-PBF-AlSi10Mg alloy are investigated. Three groups of L-PBF-AlSi10Mg samples were fabricated utilizing combinations of L-PBF process parameters for their Upskin layers. The implemented process parameters modifications were found to be not only effective in reducing the as-printed surface roughness of the components, but also led to the formation of cyclic small-large melt pools (MPs) in Upskin layers of the fabricated samples. Such consecutive modification in the size of MPs led to the increased inhomogeneity of the microstructure, contributing to the formation of a coarser intercellular eutectic-Si network, larger grain size, and lower density of low angle grain boundaries. Among all fabricated samples, the sample that experienced the fastest cooling during solidification was found to reveal the highest corrosion resistance and the best passive film stability on its Upskin surface both in naturally-aerated and deaerated 3.5 wt.% NaCl electrolyte, owing to the finer Al-Si eutectic structure that forms along its large MP-boundaries. The as-printed microstructure of the L-PBF-AlSi10Mg was found a dominant factor in determining the necessity of applying post-printing surface polishing procedures to attain better corrosion properties.

Deformation mechanism and in-situ TEM compression behavior of TB8 β titanium alloy with gradient structure

Severe plastic deformation is known to induce grain refinement and gradient structure on metals’ surfaces and improve their mechanical properties. However, the fundamental mechanisms behind the grain refinement and micromechanical properties of materials subjected to severe plastic deformation are not still well studied. Here, ultrasonic surface rolling process (USRP) was used to create a gradient microstructure, consisting of amorphous, equiaxed nano-grained, nano-laminated, ultrafine laminated and ultrafine grained structure on the surface of TB8 β titanium alloy. High energy and strain drove element co-segregation on sample surface leading to an amorphous structure during USRP processing. In situ transmission electron microscope compression tests were performed in the submicron sized pillar extracted from gradient structure and coarse grain, in order to reveal the micromechanics behavior of different grain morphologies. The ultrafine grained layer exhibited the lowest yield stress in comparison with single crystal and amorphous-nanocrystalline layers; the ultrafine grained layer and single crystal had an excellent strain hardening rate. The discrepancy among the grain sizes and activated dislocation sources led to the above mentioned different properties. Dislocation activities were observed in both compression test and microstructure evolution of USRP-treated TB8 alloy. An evolution of dislocation tangles and dislocation walls into low angle grain boundaries and subsequent high angle grain boundaries caused the grain refinement, where twinning could not be found and no phase transformation occurred.

Microstructure characterization of gradient structured surface layer in pure zirconium

A gradient nanostructure with thickness of 150 μm at surface layer was produced on pure zirconium (Zr) by ultrasonic shot peening (USSP). The microstructure of the gradient layer was characterized by electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM). The results indicate that multiple gradient features that consist of grain size gradient, strain gradient (low angle grain boundaries (LAGBs) gradient and geometrically necessary dislocation (GND) density gradient) and deformation twinning gradient are observed. By high resolution EBSD analysis, it is found that 101¯2<101¯1>/112¯1<1¯1¯26> twinning and dislocation slip are observed in the highly deformed surface layer. The deformation twins are parallel or intersected with each other and subdivide the coarse grains into small blocks. The movement and accumulation of dislocations gradually transform into LAGBs which finally forms finely distributed equiaxed grains. The mutual promotion between slip and twinning accelerates the process of grain refinement. The combination of twin-twin intersections and twin-LAGBs interactions ultimately leads to the formation of nano grains in pure Zr during USSP process

Effect of nitrogen on grain boundary character distribution in 316 stainless steel

Influence of nitrogen content and percentage of prior rolling deformation on evolution of grain boundary character distribution in 316L(N) stainless steel, during grain boundary engineering type thermo-mechanical processing, have been studied in the present work. 10% rolling deformation followed by annealing produces highest fraction of low energy Σ3 boundaries and high fraction of low energy grain boundary network in high nitrogen SS, leading to improved corrosion resistance.

Carleson perturbations of elliptic operators on domains with low dimensional boundaries

We prove an analogue of a perturbation result for the Dirichlet problem of divergence form elliptic operators by Fefferman, Kenig and Pipher, for the degenerate elliptic operators of David, Feneuil and Mayboroda, which were developed to study geometric and analytic properties of sets with boundaries whose co-dimension is higher than 1. These operators are of the form −divA∇, where A is a weighted elliptic matrix crafted to weigh the distance to the high co-dimension boundary in a way that allows for the nourishment of an elliptic PDE theory. When this boundary is a d-Alhfors-David regular set in Rn with d∈[1,n−1) and n≥3, we prove that the membership of the harmonic measure in A∞ is preserved under Carleson measure perturbations of the matrix of coefficients, yielding in turn that the Lp-solvability of the Dirichlet problem is also stable under these perturbations (with possibly different p). If the Carleson measure perturbations are suitably small, we establish solvability of the Dirichlet problem in the same Lp space. One of the corollaries of our results together with a previous result of David, Engelstein and Mayboroda, is that, given any d-ADR boundary Γ with d∈[1,n−2), n≥3, there is a family of degenerate operators of the form described above whose harmonic measure is absolutely continuous with respect to the d-dimensional Hausdorff measure on Γ.

Linear friction welding of dissimilar Ni-based superalloys: microstructure evolution and thermo-mechanical interaction

Linear friction welding (LFW) technology, as an advanced manufacturing approach to fabricate the bimetallic turbine blisks, is receiving increasing attention from aerospace industries. Current work experimentally investigated the microstructural evolution and microhardness in LFW of FGH4096 and GH4169 superalloys. A complete dynamic recrystallization (DRX) was obtained in friction interface zone (FIZ) with no obvious secondary phases including δ and carbide phases on GH4169 side, and primary and secondary γ′ phases on FGH4096 side. The thermal-mechanically affected zone (TMAZ) was characterized by incomplete DRX where a significantly lower amount of low angle grain boundaries existed together with partially dissolved secondary phases depending upon the distance from weld line. The above-mentioned distinct microstructural behavior, manifesting different thermo-mechanical fields at different areas in the joint, necessitated the further analysis of numerical simulations of LFW of two dissimilar alloys. A thermo-mechanical coupling model was developed to predict the temperature, plastic strain and stress distributions. Satisfactory comparisons between numerical and experimental results were observed. The findings in the work are believed to be one of few currently available to focus on the study of metallurgical and thermo-mechanical coupling behavior of the dissimilar nickel-based superalloy joint made by the LFW process.

The impact of macro/microstructure features on the mechanical properties of refill friction stir spot–welded joints of AA2219 alloy with a large thickness ratio

AA2219 aluminium alloy plates of large thickness ratios were successfully produced by refill friction stir spot welding (RFSSW) method. In addition to common defects, such as holes and hook, insufficient refill and weak bonding also appeared in the joints with large thickness ratios. Besides, the stir zone of the typical RFSSWed joint exhibited refined and equiaxed grains because of the dynamic recrystallization, while the elongated grains with a large proportion of the low angle grain boundaries were the main microstructure features in the thermomechanically affected zone due to insufficient heat input and deformation. Moreover, in all cases of large thickness ratio, the micro-hardness distribution of the joints presented an M-shaped profile. The maximum tensile-shear strength reached 7.2 kN, corresponding to the rotation speed of 2000 rpm, welding time of 3.6 s and plunge depth of 2.2 mm, which displayed a close correlation with the defect morphology, such as the weak bonding length and hook height. Also, two types of fracture modes were observed in the tensile-shear specimens, plug fracture mode and mixed fracture mode.

Constitutive Characteristics, Microstructure, and Texture Evolution of As‐Cast 42CrMo Alloy in Nonisothermal Multipass Compression

The nonisothermal multipass deformation behavior of as‐cast 42CrMo alloy was studied with declining temperature, constant pass strain, varying strain rate, and interval time. The stresses are used to develop the constitutive model. As the finishing temperature increases from 990°C to 1070°C, the stress decreases gradually and the softening effect increases, which results in a large grain size and inhomogeneous microstructure. The low angle grain boundaries transform into high angle grain boundaries through absorbing dislocations. The noticeable stress softening in a high strain rate is attributed to the thermal softening, dynamic recovery, and dynamic recrystallization. The thermal softening is no longer considered to be the main interpass softening mechanism at a low strain rate. The interval time has a negligible effect on the stress, but the significant changes in grain size and texture component are caused by the interpass softening. The average grain size is approximately 40 μm, and the distorted grain boundaries and small fine grains are found in the interval times of 0.5–5 s, implying the dynamic recovery and grain growth. The near {001}&lt;110&gt; and {110}&lt;112&gt; orientation exerts an important influence on the grain refinement.

Microstructural and mechanical characterisation of Al-Zn-Mg-Cu alloy processed by multi-directional cryo-forging

Aim of the present investigation is to study the microstructural and mechanical properties of Al-Zn-Mg-Cu alloy before and after multi-direction forging (MDF) at cryogenic condition up to 3 cycles. Microstructure evolution of specimen was examined using optical microscope and orientation imaging microscopy as well as X-ray diffraction. Mechanical properties were measured by tensile test and Vickers micro hardness. Microstructural investigation shows that after 3 cycle of MDF average grain size was reduced to 8 µm with low angle grain boundaries (LAGBs) and high dislocation density. Mechanical examination displays an improvement in hardness, yield strength and ultimate tensile strength is due to increases in grain boundaries and strain hardening effect. After 3 cycles of MDF process with cumulative strain ΣΔε = 3.64 led to the formation of fine grain structure, and microhardness were observed to be 168 HV.

EBSD studies on cryorolled LM6 Al alloy

In present work, EBSD and mechanical properties were studied for cryorolled LM6 Al alloy. Solution treated (ST) alloy was cryorolled for dissimilar thickness reduction upto 50%, 75% and 90%. To analyze the effect of CR on microstructure of LM6 alloy, EBSD and TEM microscopy techniques were made in practiced and its mechanical properties are analyzed by using tensile test and hardness. The CR LM6 Al alloy manifest an enhancement in mechanical properties on comparison with ST LM6 Al alloy. Results revealed that fraction of low angle grain boundaries and kernel average misorientation increases with increase in percentage thickness reduction. CR processed LM6 Al alloy shows the improved mechanical properties among all sample conditions substantiating the effect of grain refinement.

Microstructure Characteristics and Corrosion Resistance of Friction Stir Welded 2205 Duplex Stainless Steel

In this study, 2205 duplex stainless steel was friction‐stir‐welded at different welding speeds. The microstructural characteristics such as grain sizes, grain boundary misorientation angles, and phase contents in the stir zones of the joints were detected. The potentiodynamic polarization and electrochemical impedance spectroscopy were also measured to evaluate the corrosion property of the stir zones. The effects of the microstructures on the corrosion property for friction‐stir‐welded 2205 duplex stainless steel were analyzed. The results indicated that the FSW process refined the grains and increased the ferrite contents in the stir zones. With increasing the welding speed from 30 mm/min to 50 mm/min, the grain size in the stir zone decreased from 1.64 μm to 0.96 μm, the ferrite content decreased from 59% to 54.4%, and the fractions of low angle grain boundaries for austenite and ferrite phases decreased from 63.9% and 92.6% to 18.0% and 41.1%, respectively. In this study, the effect of grain size on the corrosion resistance was the most significant and it was followed by the phase composition and the grain boundary misorientation angle. The stir zone obtained at 50 mm/min has the highest corrosion resistance, and it is followed by the base material and the stir zone obtained at 30 mm/min.

Semi-Skipping Layered Gated Unit and Efficient Network: Hybrid Deep Feature Selection Method for Edge Computing in EEG-Based Emotion Classification

In this article, we propose a novel feature selection method based on hybrid neural networks for emotional classification avoiding expensive computation. With the development of neural networks for machine learning tools, the Electroencephalogram (EEG)-based classification of human emotions is increasingly important in providing health-care for Edge Computing in support of multiple Internet of Things (IoT) applications. However, classifying emotions through EEG signals is very challenging due to the low temporal boundaries and non-linear nature of EEG signals. Regarding non-linearity, we propose a hierarchical Semi-skipping Layered Gated Unit (SLGU) besides Efficient Network (ENet) for feature extraction. For faster processing, we have introduced a semi-skipping layer for Gated Recurrent Units (GRU) in Recurrent Neural Networks (RNN). The entered layer automatically skips the divergent factor during network training. Preprocessed EEG signals are sent to the hybrid SLGU-ENet model for deep feature extraction. To overcome the computational cost, an optimal function reduction method called the Bag of Visualized Characteristics (BoVC) is used. The entire facility is verified against two publicly available datasets. The results show that the classification performance of the proposed model achieves superior classification precision in a short processing time compared to the state-of-the-art models. The proposed algorithm required around 1.2-5 seconds, suitable for real-time IoT applications.