Formation Mechanism Of Defects Research Articles

A Review on Laser Powder Bed Fusion of Inconel 625 Nickel-Based Alloy

The Inconel 625 (IN625) superalloy has a high strength, excellent fatigue, and creep resistance under high-temperature and high-pressure conditions, and is one of the critical materials used for manufacturing high-temperature bearing parts of aeroengines. However, the poor workability of IN625 alloy prevents IN625 superalloy to be used in wider applications, especially in applications requiring high geometrical complexity. Laser powder bed fusion (LPBF) is a powerful additive manufacturing process which can produce metal parts with high geometrical complexity and freedom. This paper reviews the studies that have been done on LPBF of IN625 focusing on the microstructure, mechanical properties, the development of residual stresses, and the mechanism of defect formation. Mechanical properties such as microhardness, tensile properties, and fatigue properties reported by different researchers are systematically summarized and analyzed. Finally, the remaining issues and suggestions on future research on LPBF of IN625 alloy parts are put forward.

Misorientation-dependent solute enrichment at interfaces and its contribution to defect formation mechanisms during laser additive manufacturing of superalloys

A vital issue during selective laser melting of nonweldable polycrystalline nickel-base superalloys is the formation of microcracks. These are cracks occurring during the last stage of solidification and only at high angle grain boundaries (HAGBs). Solute enrichment to the remaining interdendritic liquid and its partial back-diffusion into the solid contributes to the crack nucleation mechanism. Here we use atom probe tomography coupled with transmission Kikuchi diffraction to determine the misorientation and chemical composition profiles across HAGBs (with and without cracks) and across crack-free low angle grain boundaries (LAGBs). The Gibbsian interfacial excess of solutes (mainly B, C, Si, and Zr) is at least two times higher at the HAGB compared to the LAGB. The chemical profiles show the opposite behavior to established model predictions of the last stage of solidification. Our diffusion calculations elucidate that the chemical profiles are influenced by both microsegregation (of Ti, Nb, and Si) during solidification and solid-state segregation (of B, C, and Zr) during cooling. The chemical profiles in the topmost layer indicate a negligible effect of remelting and reheating. Except for Ti-rich carbides, no secondary phases are found. Additionally, we study an alloy with a reduced content of Zr and Si (by at least 60 wt. %), relative to the standard IN738LC composition. We achieved a 99% reduction in crack length per unit area. However, the grain boundary enrichment of Zr and Si in the modified alloy was similar to the standard alloy. Based on these findings, we critically discuss the contribution of various mechanisms proposed for solidification cracking.

Nonthermal Mechanism of Defect Formation in the CdHgTe Semiconductor on Exposure to Soft X-rays

The effect of irradiation with soft X-rays produced by a laser plasma source equipped with an X-ray concentrator on the properties of the CdHgTe semiconductor are investigated. For this purpose, the mass concentration of atoms in the near-surface layer of the material is measured. It is demonstrated that the action of soft X-ray radiation leads to change in the solid-solution composition at the surface via a nonthermal mechanism and generates surface defects.

Characteristics and formation mechanisms of defects in surface layer of TC17 subjected to high energy shot peening

The microstructure and defect Characteristics in the surface layer of TC17 subjected to high energy shot peening (HESP) were investigated detailedly, in which the particular attention was to reveal the formation mechanisms of defects, the interaction among defects and the effect of the nanograins on the defects. The nanocrystalline surface layer with an average size of 54.84 nm and a depth of 20 μm was fabricated in TC17 at an air pressure of 0.35 MPa and a peening duration of 30 min. The defects in the surface layer of HESP processed TC17 consisted of the endogenous defects (including voids clusters, linear cracks and multiple cracks) and the exogenous defect (folded cracks). The high-density dislocations induced by pile-up at grain boundaries (GBs) in HESP greatly contributed to the formation of the endogenous defects, and the combined action of shot impact and plastic flow led to the generation of the exogenous defect. There existed a strong correlation among defects, in which the coalescence of voids might transform into crack whereas the regions nearby crack tips were more likely to generate new voids. Due to grain refinement in surface layer, the defects were avoided greatly at medium air pressure and longer peening duration.

Control of flow lines during the forging process of bearing outer rings with a deviated groove

In this paper, the evolution of flow lines during the forging of bearing rings was investigated. The formation mechanisms of flow-line defects were revealed by a finite element method (FEM) simulation of the entire forging process. To avoid the flow-line disorder caused by the bearing ring manufacturing method, two hot forging processes were employed to optimize the flow-line distribution of an M50 steel bearing ring with a deviated groove. The simulation and experimental results show that the punching die and the location of the punching recess had a significant effect on the flow-line formation. The V-shaped punching pin and punching recess setting at the bottom of the billet resolved the flow-line disorder caused by the punching process. Finally, a bearing outer ring with a reasonable flow-line distribution was obtained. Furthermore, the improved hot forging process reduced the average grain size and improved the M2C carbide-aggregation phenomenon. Given the direction of the flow lines, the bearing ring exhibited mechanical anisotropy in the tensile direction.

Electric field spectroscopy of material defects in transmon qubits

Superconducting integrated circuits have demonstrated a tremendous potential to realize integrated quantum computing processors. However, the downside of the solid-state approach is that superconducting qubits suffer strongly from energy dissipation and environmental fluctuations caused by atomic-scale defects in device materials. Further progress towards upscaled quantum processors will require improvements in device fabrication techniques, which need to be guided by novel analysis methods to understand and prevent mechanisms of defect formation. Here, we present a technique to analyse individual defects in superconducting qubits by tuning them with applied electric fields. This provides a spectroscopy method to extract the defects’ energy distribution, electric dipole moments, and coherence times. Moreover, it enables one to distinguish defects residing in Josephson junction tunnel barriers from those at circuit interfaces. We find that defects at circuit interfaces are responsible for about 60% of the dielectric loss in the investigated transmon qubit sample. About 40% of all detected defects are contained in the tunnel barriers of the large-area parasitic Josephson junctions that occur collaterally in shadow evaporation, and only approx3% are identified as strongly coupled defects, which presumably reside in the small-area qubit tunnel junctions. The demonstrated technique provides a valuable tool to assess the decoherence sources related to circuit interfaces and to tunnel junctions that is readily applicable to standard qubit samples.

Kinetic approach to defect reduction in directed self-assembly

As a potential solution to next-generation nanolithography, directed self-assembly (DSA) of block copolymers (BCPs) is still restrained in high-volume manufacturing primarily due to its defectivity issue. Though defects possess greater free energies than aligned morphologies and are highly energetically unfavorable, they can be kinetically trapped by the energy barriers and persist for a long time during annealing. Therefore, understanding the kinetics of defect annihilation is crucial in revealing the mechanism of defect formation and in further reducing defectivity in DSA. We focus on two types of predominant defects in DSA—dislocation and bridge. A kinetic model of each defect type is developed through statistical analysis of experimental data, providing insight into possible approaches of further defect reduction. We also investigate the impact of annealing temperature and film thickness on annihilation kinetics and discuss the reasons behind the observed results. By simply optimizing annealing conditions and film thickness, we have successfully reduced the total defect density by 1 order of magnitude. Though these findings are based on polystyrene- b -poly(methyl methacrylate) (PS- b -PMMA), we anticipate they could be readily applied to other BCP platforms as well.

Impact of Molecular Dynamics Simulations on Research and Development of Semiconductor Materials

ABSTRACTAtomic scale defects critically limit performance of semiconductor materials. To improve materials, defect effects and defect formation mechanisms must be understood. In this paper, we demonstrate multiple examples where molecular dynamics simulations have effectively addressed these issues that were not well addressed in prior experiments. In the first case, we report our recent progress on modelling graphene growth, where we found that defects in graphene are created around periphery of islands throughout graphene growth, not just in regions where graphene islands impinge as believed previously. In the second case, we report our recent progress on modelling TlBr, where we discovered that under an electric field, edge dislocations in TlBr migrate in both slip and climb directions. The climb motion ejects extensive vacancies that can cause the rapid aging of the material seen in experiments. In the third case, we discovered that the growth of InGaN films on (0001) surfaces suffers from a serious polymorphism problem that creates enormous amounts of defects. Growth on ($11\bar{2}0$) surfaces, on the other hand, results in single crystalline wurtzite films without any of these defects. In the fourth case, we first used simulations to derive dislocation energies that do not possess any noticeable statistical errors, and then used these error-free methods to discover possible misuse of misfit dislocation theory in past thin film studies. Finally, we highlight the significance of molecular dynamics simulations in reducing defects in the design space of nanostructures.

Experimental investigation on the cutting mechanism and surface generation in orthogonal cutting of UD-CFRP laminates

As carbon fiber reinforced polymer (CFRP) have been widely used as the main material of aeronautic load-carrying parts, there are more and more demands of relevant machining work. Due to the significant anisotropy of CFRPs, defects may easily occur in the machining process. This paper investigated the stress state of carbon fiber and resin microelements and the formation mechanism of machined surface defects at the micro level, and analyzed the fracture mechanism of carbon fibers and its effects on machined surface morphology at various fiber orientation angles θ in the process of CFRP cutting. The results revealed that the cutting mechanism under parallel fiber cutting conditions is the interface layering separation dominated by in-plane shear stress and the machined surface is relatively flat and smooth; the cutting mechanism under perpendicular fiber cutting conditions is shear-fracture separation dominated by shear stress and the machined surface is mainly characterized by regular fiber cross fractures; the cutting mechanism includes the interface layering separation and shear-fracture separation, and the machined surface is characterized by a regular fluctuating surface composed of fiber cross fractures with fiber orientation angle 0° < θ < 90°; the cutting mechanism is the bending fracture separation dominated by in-plane bending stress and the machined surface is mainly characterized by pits with fiber orientation angle 90° < θ < 180°.

Rearrangement of interstitial defects in alpha-Fe under extreme condition

In this study, by theoretical means, we reveal the main mechanisms that underpin the microstructure evolution driven by the formation of self-interstitial atoms (SIAs) clusters in body centered cubic iron under extreme conditions. Using Frenkel pairs accumulation simulations we point the complex interplay between the two families of interstitial defects, the dislocation loops with Burgers vectors <100> and ½<111> and the tridimensional C15 clusters. We reconcile the previous sparse understanding of microstructure evolution that put in opposition various mechanisms of defects formation by showing that both ½ <111> loops self-interactions and C15 clusters transformations produce <100> loops. Moreover, we exhibit the fact that these tri-dimensional clusters can form under irradiations with only the Frenkel pair accumulation that mimics electron irradiation and not only in high-energy cascades as it was previously stated. Finally, we show that the tridimensional C15 clusters even precede production of loops under irradiation.

Al-DTPA microfiber assisted formwork construction technology for high-performance SiC membrane preparation

High gas permeance and particle interception efficiency are critical for the performance of dust filtration membrane, which requires the membrane to have higher porosity support and ultrathin separation layer. In this work, Al-DTPA (aluminium-diethylenetriamine pentaacetic acid) microfibers were prepared as the sacrificial intermediate layer to bridge the gaps between the large pore support for ameliorating the infiltration of the suspension in the membrane preparation process. The temporary Al-DTPA constructed layer was subsequently removed via calcination, which has few effects on decreasing the combine force of support and separation layer. An ultrathin SiC membrane with its thick of ~100 μm and pore size of 2–5 μm was successfully prepared on the SiC support with a pore size of 30 μm. The preparation and defect formation mechanism were analyzed by investigating the formula and the preparation process. The resulting asymmetric, transition layer-free, defect-free SiC membrane exhibited excellent gas permeance (240–277 m3 m−2 h−1·kPa−1) and dust interception efficiency (99.8%) for particles less than 0.5 μm. The successful fabrication of this transition layer-free and defect-free asymmetric membrane enable the use of mismatched particle size between the support and separation layer, which may provide a new route for the preparation of high-performance gas-solid separation ceramic membranes.

Correlation between the formation of particle defects on sputtered Cu seed layers and Cu targets

For advanced copper (Cu) interconnection applications, this work investigated the influence of impurity particles in the Cu target on the formation of particle defects on the sputtered Cu film. By using a liquid particle counter detection system and X-ray photoelectron spectroscopy, the properties of the impurity particles in the Cu targets were characterised. It is revealed that the impurity particles in the Cu target contain 98.5% carbon, and around 75% impurity particles have a size <1 μm and about 10% ones have a size larger than 2 μm. Subsequently, the correlation between carbon particles in the Cu target and particle defects on the sputtered Cu film is explored. For the carbon particles larger than 0.15 μm in the Cu target, the particle defects formed on the Cu seed layer exhibit a pronounced linear increase as a function of the carbon particle number. However, such a linear relationship becomes unclear for the bigger carbon particles; meanwhile, a small quantity of bigger carbon particles in the Cu target also can generate more particle defects on the Cu seed layer in comparison with smaller carbon particles. Moreover, the formation mechanism of particle defects on the Cu seed layer was also discussed.

Characterization and quantification of numerical errors in threshold displacement energy calculated by molecular dynamics in bcc-Fe

The accumulation of radiation defects in nuclear fusion/fission materials often causes adverse changes in material properties. The threshold displacement energy (TDE) is one of the fundamental quantities used to evaluate the number of formed radiation defects. In the present study, to understand the error sources in TDE calculation using the molecular dynamics method and to find an optimal strategy for accurate and precise calculation, we analyze how the calculated TDE is affected by the calculation settings, including the system shape and size, the recoil energy increment in searching the threshold value, the sampling numbers of recoil directions and recoil event time samples, and the potential model. Bcc-Fe is used as a test material, and three potential models are employed for comparison. As a result, the calculation errors caused by these settings are successfully classified into accuracy error and precision error and then quantified. The type and trend of the error caused by each setting are not qualitatively different between the tested potential models, although they are quantitatively different. This potential model dependence can often be reasonably explained from the recoil direction dependence of the TDE and the defect formation mechanisms. Finally, using the equations derived to quantify the accuracy and precision of the calculated TDE and the calculation cost, we suggest some strategies to minimize the error with a given limited computational resource.

Analysis of inner fold and bulge defects on J55 steel for oil casing pipe

Typical macroscopic and microscopic morphologies of inner fold and bulge defects of J55 oil casing pipe are observed, and inclusions and the metallographic structure near these defects are detected to obtain an understanding of the defect formation mechanisms. The results show that the main inclusions near the origin of bulge defects are CaO·Al2O3 and MgO·Al2O3, with a similar inclusion composition being found in the inner fold layer. Brittle nonmetallic inclusions are the main causes of bulge and inner fold defects. Microcracks are formed owing to relative sliding between the inclusions and the metal matrix during the rolling process, and further extension of these microcracks then leads to the creation of bulge defects under sizing stress. Inner fold defects are formed from bulge defects owing to recompression and breaking of the latter as the plug exits from the pipe. It can therefore be concluded that the fundamental measures to avoid bulge and inner fold defects in oil casing pipe should be optimization of the deoxidization process and improvements in steel cleanliness.

The Problem of Formation of Mixed Crystals and High-Efficiency K2(Co, Ni)(SO4)2 • 6H2O Optical Filters

This review, for the first time, summarizes the results of studies of the defect formation mechanisms in mixed crystals grown from aqueous solutions. The general mechanism of interaction of a crystal with a foreign solution is described (reaction of isomorphous replacement). As a result of this reaction, the crystal surface turns into a mosaic of local areas where multidirectional processes (dissolution and growth) occur simultaneously. Data on mosaic microinhomogeneity, which is a new type of composition inhomogeneity inherent solely to multicomponent crystals, is presented. A new mechanism for the mismatch stress relaxation in heterocompositions of brittle crystals grown from low-temperature solutions is described; in this case, the formation of misfit dislocations is impossible and stress relaxation occurs due to the formation of numerous inclusions at the interface. The general concept of growing high-quality mixed crystals from solutions is described, using the example of K2(Co, Ni)(SO)2 · 6H2O (KCNSH) mixed crystals.

Defect-Assisted Permeation Through a Phospholipid Membrane: Experimental and Computational Study of the Peptide WKW.

We investigate membrane permeation by the peptide WKW that is amidated at its C-terminus and therefore carries a positive charge of +2. To facilitate an efficient calculation, we introduce a novel set of simple coarse variables that measure permeation depth and membrane distortion. The phospholipid head groups shift toward the center of the membrane, following the permeating peptide, and create a defect that assists permeation. The Milestoning algorithm was used in the new coarse space to compute the free-energy profile and the mean first passage time. The barrier was lower than expected from a simple continuum estimate. This behavior is consistent with the known behavior of positively charged cell-penetrating peptides, and is explained by a detailed mechanism of defect formation and propagation revealed by the simulations.

Ab initio study of diffusion of hydrogen, silver and lithium in PbS and Ag2S

Lead and silver sulfides are materials with diverse technological applications, such as photovoltaic dispositives, electrodes and catalysts. More recent applications in the formation of quantum dots and catalysts synthesis for hydrogen evolution reactions involves these sulfides and some similar compounds. It is well known that defects, and consequently the diffusion process, can change some properties and performances of such materials. Herein, a study of defect formation and ionic diffusion process were performed in PbS and Ag2S lattice structure. The activation energies for diffusion of Ag+, Li+, and H+ in PbS and Ag2S lattices were calculated at the DFT level and diffusion coefficients were estimated within the Arrhenius approximation. Pb-doped Ag2S exhibited a very small activation energy and an anomalous high diffusion coefficient for Ag+ diffusion, which is in good agreement with experimental data. Less evident is the fact that Li+ present lower diffusion coefficients than Ag+. For H+ diffusion in PbS lattice, the process is completely different. The hydrogen moves over the lattice from a sulfur site to the other, apparently cleaving and forming H-S bonds. The present work can provide more understanding of the features of defect formation and solid state ionic diffusion mechanism in sulfides from an atomic perspective. Hopefully, this results can help experimentalists in the development of more efficient materials for some important technological applications.

Ion implantation of graphene with keV carbon ions: Defect types, evolution and substrate effects

We study defect formation in chemical vapor deposition grown single layered graphene (SLG) through ion implantation. The SLG is transferred onto SiO2/Si or GaN/sapphire substrate prior to the ion implantation process. Carbon ions with precisely controlled doses and energies are implanted into the SLG by an ion implanter. Micro-Raman, X-ray photoelectron spectroscopies, and Hall measurement are employed to characterize the defects in the implanted SLG. It is found that the defect density and resistance increase, while the defect type exhibits a transition from individual vacancy to extended edge type, as implant doses increase. Through detailed analysis on the peak shift of Raman bands, we found indication of strains formation in the implanted SLG due to the defect formation. Slight structural recovery and relaxation of strain can be achieved through post-annealing in high vacuum. However, unintentional doping through formation of C–O/C–N related bonds is found for the heavily implanted SLG on SiO2/GaN substrates. We discuss the probable defect formation mechanism and evolution in light of the interplay between SLG and the underneath substrates.

Numerical analysis of the effect of welding positions on formation quality during laser welding of TC4 titanium alloy parts in aerospace industry

A large number of titanium alloy components have huge volume with a complicated curved surface in aerospace industry; hence, it is necessary to develop an all-position welding process. Laser welding is regarded as a high quality method to join those components flexibly and efficiently. However, titanium alloys are very sensitive to laser processing parameters, and the formation quality can be a severe problem under different welding positions. In this work, the tendency of porosity and burn through holes was evaluated under different welding positions: vertical up (VU), vertical down (VD), and flat, with different welding parameters by using 3 mm-thick titanium alloy plates experimentally and numerically. The VU position showed a high tendency of bigger keyholes and resulted in the formation of burn through holes, which was not a problem for other positions with the same parameters. Low heat input was beneficial to eliminate burn through holes. The VD position increased frequency of keyhole collapse and led to higher porosity compared with other positions. Welding with higher laser power at higher speed could reduce this kind of porosity. Numerical simulation results well exhibited the change of the keyhole size and the molten pool flow behavior, which is a powerful tool to understand the formation mechanism of different weld defects and help to find the optimized processing parameters to eliminate the relative defects for different weld positions.

High-density tungsten fabricated by selective laser melting: Densification, microstructure, mechanical and thermal performance

High-density pure tungsten (W) fabricated by selective laser melting (SLM) has been considered as a substantial challenge due to its high melting point of 3410 °C. In this study, near fully dense W samples with a relative density of 98.71% were obtained for the first time through a series of optimization experiments during the SLM process. The characteristics of the surface and the formation mechanism of the micro defects were systematically elucidated. Additionally, it was found that the typical microstructures of horizontal and vertical planes experienced successive changes, where coarser columnar grains changed to uniform finer grains when increasing the laser scan speed from 50 mm/s to 400 mm/s. The compressive strength, micro hardness and thermal conductivity of the optimal SLM sample was improved to 1523 MPa, 428 HV3 and 148 W/m·K, which were superior to the sample produced by the conventional methods. The relationship of processing parameters to the surface morphology and microstructure evolution and material properties associated with fusion reactors was established in order to optimize the performance of SLM pure W and explore the possibility of further application in fusion reactors.