Defect-free Region Research Articles

Research on the influence law of geomagnetic field on magnetic signals and compensation modeling

To study the influence law and mechanism of the geomagnetic field on magnetic signals in the process of magnetic memory testing, the linear scanning detection results of defective specimens with three different initial magnetization strengths (0 A/m, −70 A/m, 105 A/m) along the horizontal and vertical directions in the geomagnetic field ambient are analyzed and discussed. The result shows that the magnetic signals in the defect-free region only change in amplitude with the change of detection angle, and have no effect on the overall shape of the magnetic signal curve; the characteristic parameters Sm and Km of the peaks of the magnetic signals in the defective region show a corresponding relationship with the measurement angle in the horizontal detection. The initial magnetization strength of the specimen itself affects the change rule of the defective characteristic peaks. The mechanism analysis reveals that the extracted magnetic signals are affected by the combination of the geomagnetic field, the induced magnetic field of the specimen itself, and the leakage magnetic field. Finally, aiming at the problem that the geomagnetic field will change the topography of the defect peaks along different detection orientations, a geomagnetic compensation model is proposed for the barrel-shaped equipment when performing circumferential scanning detection, which can reduce the interference of the geomagnetic field aberration on the detection results, to prevent the misjudgment and omission of the specimen’s defect location.

The combined effects of uniaxial pressure and defects on the mechanism of martensitic transformation in pure iron: A molecular dynamics study

The as-produced martensitic steel fabricated by selective laser melting (SLM) is generally softer and weaker than the wrought specimen due to the residual austenite in the microstructure. To avoid additional post-heat treatment, this study proposes a strategy to reduce the residual austenite by applying additional pressure after each laser scanning. Therefore, the combined effects of uniaxial pressure and stacking faults (SFs) on martensitic transformation (MT) in the parent face-centered cubic (FCC) phase of pure Fe are investigated via molecular dynamics (MD) simulation. The simulation results show that the supercooling, uniaxial pressure, and pre-existing SF defects in the parent FCC phase provide chemical driving force, and mechanical driving force and reduce the energy barrier of MT, respectively. The Ms temperatures increase gradually with the pressure from 0 to 200 MPa. The MT process consists of nucleation and growth. The heterogeneous nucleation occurs in the parent FCC phase with pre-existing defects and favoring intersecting-SF, SF, and defect-free FCC phase region in order. The homogeneous nucleation occurs in the parent FCC phase without pre-existing defect and the MT process is achieved by the growth and coalescence of the scattered crystal nucleus. The stress field generated by the dislocation lines at the intersecting region of SFs results in the lattice distortion of the atoms nearby, reducing the energy barrier of MT nucleation. The study verifies the feasibility of reducing residual austenite in SLM-ed martensitic steel by applying pressure during the cooling process of each layer.

Design a Ti6Al4V–Ti43Al9V laminated composite with high strength and fracture toughness

A novel Ti6Al4V–Ti43Al9V metal-intermetallic-laminated (MIL) composite was successfully fabricated using hot-pack rolling. The microstructure and mechanical properties of the composites were also investigated. The microstructure observation showed that the composite exhibited a low extent of dynamic recrystallization (DRX), the acicular α2 phase was precipitated in the interfacial region and the γ phase was elongated along the rolling direction. This unique microstructure enables the composite to simultaneously achieve high strength and fracture toughness. The enhanced tensile strength was attributed to the defect-free interfacial region and the low extent of DRX in the composite. The composite was toughened by crack bridging, crack deflection, the acicular α2 phase precipitated in the interfacial region, and the elongated γ phase.

Reference-free infrared thermography detection with subsurface heating for deep cavity in adhesive of hidden frame glass curtain wall

Since the current infrared thermography (IRT) is not effective in detecting deep and invisible cavities in the silicone structural adhesive of hidden frame glass curtain walls (HFGCW), a reference-free IRT with subsurface heating for the deep cavity is proposed. A near-infrared linear laser with high energy density and high transmission is chosen as the subsurface heating source to directly heat the silicone structural adhesive through the glass. Temporal sequence reconstruction and image enhancement based on reference-free calibration are proposed to reduce thermal inhomogenety and thermal noise and ensure comparable results for damage detection under different environments. The effects of traditional surface heating and subsurface heating are compared and analyzed through numerical simulations. And an evaluated feature, which is the maximal temperature difference feature, derived from temperature difference is used to quantitatively analyze the thermal effect caused by different cavities. The subsurface heating simulation results showed that the highest temperature difference between the region with cavity and defect-free region is up to 88% higher than that of traditional surface heating. The experiments revealed that the deep cavities of different lengths, located at 7 mm, 9 mm, and 11 mm below the glass surface, can be successfully detected using subsurface heating and reference-free calibration. A quadratic linear model is proposed to reflect the relationship between the depths and lengths of cavities and the evaluated feature. In conclusion, the proposed method can protect the HFGCW from deep and invisible cavities which can reduce its adhesion and strength.

Directional Carrier Transport in Micrometer-Thick Gallium Oxide Films for High-Performance Deep-Ultraviolet Photodetection.

Incorporating emerging ultrawide bandgap semiconductors with a metal-semiconductor-metal (MSM) architecture is highly desired for deep-ultraviolet (DUV) photodetection. However, synthesis-induced defects in semiconductors complicate the rational design of MSM DUV photodetectors due to their dual role as carrier donors and trap centers, leading to a commonly observed trade-off between responsivity and response time. Here, we demonstrate a simultaneous improvement of these two parameters in ε-Ga2O3 MSM photodetectors by establishing a low-defect diffusion barrier for directional carrier transport. Specifically, using a micrometer thickness far exceeding its effective light absorption depth, the ε-Ga2O3 MSM photodetector achieves over 18-fold enhancement of responsivity and simultaneous reduction of the response time, which exhibits a state-of-the-art photo-to-dark current ratio near 108, a superior responsivity of >1300 A/W, an ultrahigh detectivity of >1016 Jones, and a decay time of 123 ms. Combined depth-profile spectroscopic and microscopic analysis reveals the existence of a broad defective region near the lattice-mismatched interface followed by a more defect-free dark region, while the latter one serves as a diffusion barrier to assist frontward carrier transport for substantially enhancing the photodetector performance. This work reveals the critical role of the semiconductor defect profile in tuning carrier transport for fabricating high-performance MSM DUV photodetectors.

Beta-crystallin interacts with phospholipid membrane forming semi-transmembrane defects: A study using atomic force microscopy.

Highly concentrated lens proteins, mostly crystallins, are responsible for maintaining the structure and refractivity of the eye lens. These crystallins alter the refractivity of the lens due to self-modifications and might change the mechanical properties by interacting with the membrane. However, the interaction between a crucial lens protein, β-crystallin, with the membrane is still ambiguous. In this study, we explore the interaction of native β-crystallin extracted and purified from the cortex of the bovine lens with the 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membrane using atomic force microscopy (AFM). We prepared a supported lipid membrane by incubating small unilamellar vesicles (SUVs) on top of a freshly cleaved mica surface. Incubating β-crystallin solution on a defect-free membrane introduced locally modified semi-transmembrane defects. Rather than developing in multiple locations, such defects expand in the area with increased incubation time. Interestingly, the thickness mismatch between the β-crystallin-induced defects and the unaffected membrane is about half of the bilayer membrane thickness, suggesting partial removal of the distal leaflet. Force curves were obtained using force spectroscopy to access the membrane's mechanical properties. Typical membrane force curves were obtained in a defect-free region. However, the force curves obtained inside semi-transmembrane defects deviate from the characteristic membrane force curves. Membrane elasticity in the defect-free region is not significantly different from the membrane before incubating with the β-crystallin. We speculate that the defect region consists of a combination of three structural possibilities: the presence of proximal monolayer with free acyl chain, β-crystallin combined with lipids forming lipoprotein, and β-crystallin binding with lower leaflet acyl chains monolayer. Our results confirm that the interaction of β-crystallin with the POPC membrane significantly modulates membrane structure by creating semi-transmembrane defects in the supported membrane.

Exploring mechanochemical reactions at the nanoscale: theory versus experiment.

Mechanochemical reaction pathways are conventionally obtained from force-displaced stationary points on the potential energy surface of the reaction. This work tests a postulate that the steepest-descent pathway (SDP) from the transition state to reactants can be reasonably accurately used instead to investigate mechanochemical reaction kinetics. This method is much simpler because the SDP and the associated reactant and transition-state structures can be obtained relatively routinely. Experiment and theory are compared for the normal-stress-induced decomposition of methyl thiolate species on Cu(100). The mechanochemical reaction rate was calculated by compressing the initial- and transition-state structures by a stiff copper counter-slab to obtain the plots of energy versus slab displacement for both structures. The reaction rate was also measured experimentally under compression using a nanomechanochemical reactor comprising an atomic-force-microscopy (AFM) instrument tip compressing a methyl thiolate overlayer on Cu(100) (the same system for which the calculations were carried out). The rate was measured from the indent created on a defect-free region of the methyl thiolate overlayer, which also enabled the contact area to be measured. Knowing the force applied by the AFM tip yields the reaction rate as a function of the contact stress. The result agrees well with the theoretical prediction without the use of adjustable parameters. This confirms that the postulate is correct and will facilitate the calculation of the rates of more complex mechanochemical reactions. An advantage of this approach, in addition to the results agreeing with the experiment, is that it provides insights into the effects that control mechanochemical reactivity that will assist in the targeted design of new mechanochemical syntheses.

Transport and Breakdown Mechanisms of Gate Leakage Current in Lattice-Matched In₀.₁₇Al₀.₈₃N/GaN HEMTs

Charge transport and breakdown mecha- nisms of gate leakage current in lattice-matched In0.17Al0.83N/GaN high electron mobility transistors (HEMTs) were investigated by combining the current–voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}-{V}$ </tex-math></inline-formula> ) measurement, the bias-step stress method, and the emission microscopy (EMMI) technique. Based on a refined dislocation model, the detailed dislocation-related charge transport processes were depicted, suggesting an excessive Fowler–Nordheim (FN) tunneling current at high reverse biases. It is further proposed that: 1) at the heterojunction interface, these tunneling electrons will lose the obtained energy by releasing a large number of photons as “hot spots” and phonons as heat and 2) due to a reduced effective bandgap, the breakdown electric field at the dislocation site is significantly decreased compared with the defect-free region, which triggers a premature current breakdown, once the surface electric field is increased to the critical value.

The Differences in Spatial Luminescence Characteristics between Blue and Green Quantum Wells in Monolithic Semipolar (20-21) LEDs Using SNOM.

The differences in spatially optical properties between blue and green quantum wells (QWs) in a monolithic dual-wavelength semipolar (20-21) structure were investigated by scanning near-field optical microscopy (SNOM). The shortest wavelength for green QWs and the longest wavelength for blue QWs were both discovered in the region with the largest stress. It demonstrated that In composition, compared to stress, plays a negligible role in defining the peak wavelength for blue QWs, while for green QWs, In composition strongly affects the peak wavelength. For green QWs, significant photoluminescence enhancement was observed in the defect-free region, which was not found for blue QWs. Furthermore, the efficiency droop was aggravated in the defect-free region for green QWs but reduced for blue QWs. It indicates that carrier delocalization plays a more important role in the efficiency droop for QWs of good crystalline quality, which is experimentally pointed out for the first time.

Self-reference Lock-in Thermography for Detecting Defects in Metal Bridge Spans

Introduction. Incipient fatigue damage in the metal superstructures of bridges creates certain threats to the safety of operation. Various methods of non-destructive testing are used for their timely detection and diagnosis. A modern and popular on-the-day solution is the method of infrared (IR) thermography. Due to the specifics of the operation of IR cameras, additional processing of recordings received from these cameras is required to obtain an accurate result. This work aims at presenting a method for processing thermofilms and describing the possibilities of its application under real conditions.Materials and Methods. A method for processing thermographic films was described. It provided detecting temperature anomalies using only information from the camera. The results of its application on the elements of existing metal bridge spans are presented.Results. It is shown that there are temperature anomalies for existing defects. This means that the defects continue to develop, which was confirmed by subsequent observations of their condition. In addition, a case of temperature anomaly in the defect-free external region was identified. This might be a sign of an incipient defect that could not be diagnosed by other methods. If the presence of this defect is confirmed during repeated examinations, it will be possible to diagnose hidden defects that have not yet come to the surface, and/or detect potentially collapsing places.Discussion and Conclusions. The IR thermography performance as a method of non-contact non-destructive testing is shown, as well as its operability on real objects under random load.

Molecular dynamics simulations of displacement cascade near precipitate in zirconium alloys

Precipitates play an important role in the evolution of irradiation-induced defects and mechanical property of irradiated metals. In this work, the effects of a Zr2Cu precipitate on the production and subsequent evolution of cascade-induced point defects (vacancies and interstitials) in ZrCu alloy were investigated by molecular dynamics simulations at room temperature. The simulation results show that the precipitate increases the number of residual point defects at the end of cascade. However, most of the residual defects reside in the precipitate and near precipitate boundary. In the matrix, more interstitials survive than vacancies. In addition, a defect-free region is seen in matrix and near precipitate boundary, indicating that the precipitate boundary traps vacancies/interstitials in matrix and retains them in the precipitate. To explain these observations, the formation energy and migration energy of point defects are calculated. The formation energies of both vacancy and interstitial decrease as they are moving to the precipitate boundary from matrix and precipitate, indicating that they can be absorbed by precipitate boundary and result in the defect-free region. However, the migration barrier of vacancy is lower than that of interstitial. Therefore, more vacancies in matrix are absorbed. The current work provides new insights into understanding the irradiation effects in zirconium alloys.

Oxygen concentration dependence of as-grown defect formation in nitrogen-doped Czochralski silicon single crystals

The interstitial oxygen concentration ([OI]) dependence of the as-grown defect (voids and dislocation clusters) behavior in nitrogen-doped silicon crystals fabricated by Czochralski technique (Cz-Si) have been investigated. Nitrogen-doped Cz-Si crystals with [OI] values of 7–8 and 1–3 × 1017 atoms cm−3 are evaluated, and it was found that the range of v/G in which as-grown defect-free (AGDF) region can be obtained become wide with decreasing [O I], where v is the crystal growth rate and G is the axial temperature gradient at the solid–liquid interface. This result indicates that as-grown defect suppression effect by nitrogen doping is enhanced at low [OI].By the analysis based on a thermodynamic model, it is shown that nitrogen–vacancy (NV) and nitrogen–oxygen (NO) complexes compete in the nitrogen-doped Cz-Si crystals and their concentration fraction changes with [OI] value. At low [OI], the concentration of the NO complex becomes low; thus, the concentration of nitrogen that can form NV complexes increases, thereby enhancing the as-grown defect suppression effect. Silicon wafers for insulated gate bipolar transistors (IGBTs) are required AGDF and oxide precipitate-free, therefore nitrogen-doped Cz-Si crystals with low [OI] values are suitable for the fabrication of silicon wafers for IGBTs.

Contouring strategies to improve the tensile properties and quality of EBM printed Inconel 625 parts

Abstract This work systematically analyzes the influence of rough surfaces and porous subsurfaces in electron beam melting (EBM) printed components. Consequently, it applies various contouring strategies to improve the tensile properties of EBM printed Inconel 625 alloy parts. It is shown that no contouring (i.e., only hatching) creates a rough surface with numerous surface voids (as translated to surface notches). Although the commercially used multi-spot contouring can smoothen the surface to some extent (∼34 %), it fails to create a defect-free superficial region by leaving ∼25 % surface voids (translated to large surface notches) and ∼4 % subsurface porosity. These superficial defects form due to an interrupted shrinkage, occurring on the surface and in the contouring region. In contrast, optimal post-hatching high energy continuous contouring creates a thick and consistent post-hatching track that can successfully reconsolidate surface voids remaining from the hatching step. In comparison with the multi-spot contouring, this reduces the surface and subsurface porosity down to ∼10 % and ∼0.4 %, respectively, and hence increases the apparent stiffness by ∼140 %, tensile strength by ∼105 % and elongation by ∼260 %. This nearly reaches the mechanical properties of the conventionally machined parts (UTS ∼635 ± 20 MPa and elongation ∼50 ± 2 %).

Global Fabric Defect Detection Based on Unsupervised Characterization

Fabric texture intelligent analysis comprises the following characteristics: objective detection results, high detection efficiency, and accuracy. It is significantly vital to replace manual inspection for smart green manufacturing in the textile industry, such as quality control and rating, and online testing. For detecting the global image, an unsupervised method is proposed to characterize the woven fabric texture image, which is the combination of principal component analysis (PCA) and dictionary learning. First of all, the PCA approach is used to reduce the dimension of fabric samples, the obtained eigenvector is used as the initial dictionary, and then the dictionary learning method is operated on the defect-free region to get the standard templates. Secondly, the standard templates are optimized by choosing the appropriate dictionary size to construct a fabric texture representation model that can effectively characterize the defect-free texture region, while ineffectively representing the defective sector. That is to say, through the mechanism of identifying normal texture from imperfect texture, a learned dictionary with robustness and discrimination is obtained to adapt the fabric texture. Thirdly, after matching the detected image with the standard templates, the average filter is used to remove the noise and suppress the background texture, while retaining and enhancing the defect region. In the final part, the image segmentation is operated to identify the defect. The experimental results show that the proposed algorithm can adequately inspect fabrics with defects such as holes, oil stains, skipping, other defective types, and non-defective materials, while the detection results are good and the algorithm can be operated flexibly.

Defect Formation in Titanium Alloy during Non-stationary Process of Local Metallurgy

The paper studies Ti-6Al-4V alloy specimens obtained by wire-based electron-beam additive manufacturing (EBAM) and characterized by the structure with a defective region produced by uncontrolled wire fluctuations during 3D printing. Metallographic analysis and tensile strength tests and microhardness measurements are conducted to determine the defect influence on the structure and properties of the alloy. It is shown that the defective region represents the primary β-phase grains that are finer than those of the EBAM specimens commonly exhibiting a columnar structure. The specimen microhardness both in the defective and defect-free regions, is identical. The deformation behavior of the alloy is mostly subject to the influence of such defects, since the plasticity and ultimate tensile strength of the defective region are respectively higher and lower than that of the defect-free region.

Fabric defect detection via low-rank decomposition with gradient information and structured graph algorithm

The low rank decomposition model shows the potential of fabric defect detection, in which a matrix is decomposed into a sparse matrix representing the defect free region (background) and identifying the defect area (foreground). However, there are still two shortcomings. Firstly, when the texture of defect image has high gradient feature, the sparse matrix obtained by the existing model still retains a large number of edges of undetected regions. Secondly, due to the imprecision of prior information, most models will misjudge the defect free points around the defect block when dealing with the small defect area or the defect area containing multiple cycles. In order to solve these problems, this paper proposes a fabric defect detection method based on low rank decomposition of gradient information and structured graph algorithm: 1) structured graphics algorithm, according to the characteristics of fabric defect image, fabric defect image is divided into defect free block with local feature and defect damage period. 2) In the merging process, an adaptive threshold is set according to the number of cycles contained in the current block to encourage intra lattice merging and prevent the merging of defective blocks and surrounding non defective blocks. 3) The defect prior information calculated from the segmentation results is used to guide matrix decomposition to weaken the defect free region and highlight the defect area under the sparse term. We evaluated our model on a standard database and compared it with the four latest methods. The total TPR and fpr of this method are 87.3% and 1.21% respectively on box, star and point databases, which is the best performance.

High-Voltage Cycling Induced Thermal Vulnerability in LiCoO2 Cathode: Cation Loss and Oxygen Release Driven by Oxygen Vacancy Migration.

The release of the lattice oxygen due to the thermal degradation of layered lithium transition metal oxides is one of the major safety concerns in Li-ion batteries. The oxygen release is generally attributed to the phase transitions from the layered structure to spinel and rocksalt structures that contain less lattice oxygen. Here, a different degradation pathway in LiCoO2 is found, through oxygen vacancy facilitated cation migration and reduction. This process leaves undercoordinated oxygen that gives rise to oxygen release while the structure integrity of the defect-free region is mostly preserved. This oxygen release mechanism can be called surface degradation due to the kinetic control of the cation migration but has a slow surface to bulk propagation with continuous loss of the surface cation ions. It is also strongly correlated with the high-voltage cycling defects that end up with a significant local oxygen release at low temperatures. This work unveils the thermal vulnerability of high-voltage Li-ion batteries and the critical role of the surface fraction as a general mitigating approach.

Analysis of Acoustic Signals for Diagnostics of Composite Materials

This article presents a method for the analysis of diagnostic acoustic signals for defect detection in composite materials. For this purpose, a multiple-scale decomposition of the acoustic signal and the determination of informational entropy for its components are used. It is established that, in the presence of small defects, the distribution of the power spectral density is little different from the power spectral density for a defect-free region. As a criterion for defect detection, a linear correlation coefficient is selected for vectors composed of informational entropies of multiple-scale components of the acoustic signal.

Defect Detection in Composite Products Based on Sparse Moving Window Principal Component Thermography

As a nondestructive testing (NDT) technology, pulsed thermography (PT) has been widely used in the defect detection of the composite products due to its efficiency and large detection range. To enhance the distinction between defective and defect-free region and eliminate the influence of the measurement noise and nonuniform background of the thermal image generated by PT, a number of thermographic data analysis approaches have been proposed. However, these traditional methods only consider the correlations among the pixel while leave the time series correlations unmodeled. In this paper, a sparse moving window principal component thermography (SMWPCT) method is proposed to incorporate several thermal images using the moving window strategy. Also, the sparse trick is used to provide clearer and more interpretable results because of the structure sparsity. The effectiveness of the method is verified by the defect detection experiment of carbon fiber-reinforced plastic specimens.

Fracture mechanism of a high tensile strength and fracture toughness Ti6Al4V–TiAl laminated composite

Ti6Al4V (wt.%)-Ti43Al9V0·3Y (at.%) laminated composites were fabricated from as-forged TiAl and commercial Ti6Al4V sheet by hot-pack rolling, and the microstructure of composites were investigated. The results showed that the target laminated composites were achieved by reaction and diffusion bonding. A defect free interfacial region was observed indicating that The Kirkendall effect was successfully avoided by substituting Al metal foils with Ti43Al9V alloy sheets. The microstructure, from the Ti6Al4V side to the Ti43Al9V side, had four different microstructure regions which are α+β, α2, α2+γ+B2 and B2+γ, respectively. The tensile properties and fracture toughness of composites were examined. It was noteworthy that the composites not only retained high tensile properties in both room temperature and 700 °C, but also possessed high fracture toughness. It was also proved that the unique mix-phase interfacial region obtained by hot-pack rolling has a great contribution to the improvement of the properties of composites and strengthening-toughening mechanism were clarified profoundly.