Microscopic Defects Research Articles

Research on phase transition induced plastic deformation in nanoindentation of single crystal diamond

Nanoindentation experiments were performed on the {100} crystal plane of single crystal diamond using molecular dynamics simulation to study the mechanism of its plastic deformation. The relationship between phase transition and plastic deformation during the nanoindentation process was discussed using the load-displacement (P–h) curve and phase transition evolution process obtained from the experiment. It was observed that the initial plastic characteristic of diamond was because of the phase transition, rather than dislocation slip. The nucleation and growth of 1/2[110]{111} perfect dislocation ring during the plastic deformation of single crystal diamond were determined by observing pop-in events and dislocation evolution in P-h curves. The plastic deformation of single crystal diamond was attributed to the formation and growth of 1/2[110]{111} complete dislocation ring caused by the phase transition from disordered sp3 to ordered sp2 carbon atoms based on the further analysis of microscopic defects. It was considered that the phase transition during the nanoindentation of single crystal diamond is an important excitation source for plastic deformation at the atomic scale. Finally, it was observed through the analysis of bond length and angle during the phase transition process that the CC bond fracture of diamond was because of the transformation from sp3 to sp2 carbon atoms, and the fracture and recombination of adjacent carbon atoms, which led to graphitization. This conclusion confirmed that graphitization is the mechanism of plastic deformation of diamond and the pop-in event is caused by the stress release caused by the rupture of CC bond.

Improved hydrogen storage performance of Sm-Mg composites by adding nano-graphite

Sm5Mg41 + x wt% nano-graphite (NG) (x = 0, 2, 4, 8, 12) composites are synthesized via ball-milling. The influences of NG content on microstructure and hydrogen storage properties of the composites are researched systematically. The composites consist of Mg, Sm5Mg41, SmMg3, and NG. Nano scale Sm3H7 and MgH2 phases are observed after the hydrogen absorbing, and the newly formed Mg phase and the remaining Sm3H7 phase appear after hydrogen desorbing. NG is embedded in the surface of the composites in the form of nanosheets. Nanocrystalline Mg and some microscopic defects, including twin crystal, grain boundaries, and dislocations can be observed in the dehydrogenated sample. Such microscopic defects and nanocrystalline boundaries can reduce the total potential barrier needed to overcome during the hydrogen absorption/desorption reactions and thus the hydrogen storage kinetics of the alloy are improved. For the composites with x = 2, 4, 8 and 12, the activation energies of hydrogen desorption are 132.08, 128.06, 112.90, and 112.35 kJ/mol H2, respectively. The enthalpy changes for the hydrogen absorption are evaluated to be − 79.18, − 78.09, − 76.30, − 76.10 kJ/mol H2, indicating that the addition of NG had no obvious effect on the thermodynamic properties of the composites.

Improving high-temperature mechanical properties of laser powder bed-fused Inconel 738 alloy by hot isostatic pressing: Tailoring precipitates and healing defects

Microscopic defects and inferior high-temperature mechanical properties that occur during laser powder bed fusion (LPBF) of Inconel 738 alloys can severely hinder their industrial applications. Hot isostatic pressing (HIP) is a promising method to overcome these issues. In this study, the effects of HIP on the defects, grain structure, precipitated phase behavior, room- and high-temperature mechanical properties of an as-built Inconel 738 alloy were investigated. The results showed that, after HIP treatment, the defect densities in the XY and YZ planes of the as-built Inconel 738 alloy decreased from 0.537% and 0.292% to 0.251% and 0.123%, respectively. Moreover, the grain morphology of the as-built Inconel 738 alloy changed from elongated columnar grains to coarse equiaxed grains after HIP treatment, and the grain size became six times larger than that of the original sample, which was attributed to recrystallization and grain growth. Moreover, HIP treatment eliminated cellular dendritic segregation and significantly reduced dislocation densities in the as-built Inconel 738 alloy. Consequently, fewer M23C6 carbides were formed, but block-like M23C6 carbides were uniformly distributed at the grain boundaries after a combination of HIP and standard heat treatments. The HIP treatment reduced the room-temperature tensile properties of the as-built Inconel 738 alloy because of grain growth, low dislocation density, and coarser precipitates. However, the high-temperature (800 °C) tensile properties of the HIP sample with standard heat treatment (σuts = 827.8 MPa, σy = 744.3 MPa, εf = 2.2%) increased significantly compared to those of the as-built sample with standard heat treatment (σuts = 715.6 MPa, σy = 653.3 MPa, εf = 0.6%) and alleviated the embrittlement fracture owing to the formation of uniform carbides and lower defect densities. These results indicate that HIP treatment can significantly improve the high-temperature mechanical properties of LPBF-built nickel-based superalloys with high Al and Ti contents.

A multi-scale model of film/substrate interface damage due to the evolution of vacancy concentration inside the film

Experimental studies show that the fatigue of polymer-supported film starts from the interface and is due to the evolution of vacancy concentration. Based on the experimental results, a multi-scale damage model is established to bridge the evolution of microscopic defects and the degradation of macroscopic mechanical properties. The length-scale-effect investigation found that the fatigue life of the polymer-supported film will gradually increase with the decrease of grain size to film thickness ratio (d/h). It concludes that the proposed model is valid, and d/h is an essential factor affecting the polymer-supported film’s fatigue life.

Avoiding cracks in additively manufactured non-weldable directionally solidified Ni-based superalloys

Additive manufacturing of directionally solidified Ni-based superalloys faces at least two critical obstacles, namely, the formation of stray equiaxed grains and the susceptibility to cracking; circumventing both of these simultaneously is considered difficult. In this study, a comparative study of a non-weldable superalloy IN738 fabricated through the laser directed energy deposition (DED) without preheating the base plate and the electron beam powder bed fusion (EB-PBF) with preheating up to the upper bound of ductility dip temperature range was performed. With appropriate process parameters, a steep and unidirectional temperature gradient, a sufficiently high cooling rate at the liquid/solid interface, and a relatively low cooling rate at the γ′ solvus are obtained simultaneously in the EB-PBF process. The prevalence of these conditions results in the growth of well-aligned columnar dendrites, mitigates the elemental segregation, reduces the built-in microscopic defects, and lowers the stored deformation energy. Consequently, cracking is successfully prevented and reasonable room temperature tensile properties are achieved in the as-printed EB-PBF product. Moreover, recrystallization is not triggered during the post-printing heat treatment, and thus the <001> fiber texture is preserved. This study provides a detailed understanding of the critical factors that need to overcome for producing directionally solidified superalloys through additive manufacturing. • Non-weldable IN738 superalloys was successfully fabricated by EB-PBF with preheating up to the upper bound of DTR. • A steep and unidirectional temperature gradient permits the directional growth of columnar dendrite. • High cooling rates during solidification suppress element segregation and low cooling rates at γ′ solvus help avoid cracks. • The as-printed EB-PBF product with a low stored deformation energy exhibits an excellent resistance to recrystallization.

Microstructural optimization and mechanical enhancement of SLM Ti6Al4V TPMS scaffolds through vacuum annealing treatment

Ti6Al4V triply periodic minimal surfaces (TPMS) scaffolds having a porosity of 70 % and pore size of 500–600 µm were deposited by selective laser melting (SLM), and the effect of vacuum annealing treatment (VAT) on its microstructural optimization and mechanical enhancement were explored. The results showed that only microscopic regular pore defects formed within SLM scaffolds, and VAT had no positive effect on reducing or eliminating porosity. However, VAT induced narrowly weak β(110) and β(200) diffraction peaks and transformed the fully acicular α`/α-martensites into long-rod-shaped α martensites within β matrix. Nanoindentation properties of VAT samples were slightly deteriorated, which is mainly caused by coarsened α and soft β phase formation in absence of acicular α`/α. In contrast with the negligible modified compression elastic modulus and maximum compression strength after VAT, the compression energy absorption capacity and tensile strength were enhanced by 14.5 % and 17.3 %, respectively. Tensile fracture position shifted from the bulk-lattice boundary with quasi-cleavage failure mechanism to the mid-position of lattice region with ductile fracture mode, exhibiting excellent tensile ductility with an increment of about 200 % in elongation. Conclusively, VAT is an effective method to alter the brittle α`/α-martensites into the size-moderate α + β mixture, simultaneously improving the mechanical strength and ductility of Ti6Al4V TPMS scaffolds.

Enhanced mechanical properties of Zr65Cu15Ni10Al10 bulk metallic glass by simultaneously introducing surface grooves and multiple shear bands

Recent investigations have demonstrated that the plasticity of bulk metallic glasses (BMGs) at room temperature can be improved by artificially introducing both macroscopic and microscopic defects. In this work, both surface grooves and multiple shear bands (MLSBs) are concurrently introduced into the surface of a Zr65Cu15Ni10Al10 BMG employing an ultrasonic bonding machine, whose sizes are also tailored by tuning gas pressures and times. After surface treatment, MLSBs appear around grooves due to the pre-plastic deformation, while numerous nano-scale crystals resulting from thermal-induced nanocrystallization appear within the grooves, leading to the emergence of hierarchical structural heterogeneities. As a result, the strength and plasticity are effectively enhanced compared to as-cast samples when suitable experimental parameters are adopted. The plastic deformation mechanisms change from single main shear banding to multiple shear banding. Therefore, the serrated plastic behaviors exhibit a transition from chaotic to self-organized critical dynamics after surface treatment, confirming the modification of the shear banding instability. The present study provides a viable method to tailor the mechanical properties of BMGs and sheds light on their dynamic plastic deformation behaviors.

PLEURITIS AND EMPYEMA AFTER TRANSARTERIAL CHEMOEMBOLIZATION OF HEPATOCELLULAR CARCINOMA

PLEURITIS AND EMPYEMA AFTER TRANSARTERIAL CHEMOEMBOLIZATION OF HEPATOCELLULAR CARCINOMA

Mechanical behaviors regulation of triply periodic minimal surface structures with crystal twinning

Triply periodic minimal surface (TPMS) structures have been realized as excellent mechanical materials with high specific strength, energy absorption, and unique layered deformation mechanism due to their saddle shape-surface with non-positive Gaussian curvature. However, the performance of TPMS structures is limited by anisotropic mechanical behavior owing to the oblique shear band with stress concentration when the load beyond the yield stress, resulting in structural catastrophic failure. Herein, a strategy is proposed to manipulating the path of stress transfer by introducing crystal twinning to achieve a designable deformation behavior of the TPMS structures. Various contact reflection twin boundaries for the gyroid (G) and diamond (D) surface structures have been introduced by connecting the ideal structures by mirror-symmetry while maintaining the structural integrity. Compression tests were carried out from various directions of perfect and twinned- G and D surface scaffolds after 3D printing. It was found that the twin boundary can effectively protect the structure from catastrophic failure by deflecting the cracks under compressive loads, and regulate the deformation behavior by various structural design. This study provides new insights in applying the microscopic crystal defects to macroscopic architectural materials, which also contributes to the understanding of these unique microscopic structures. Graphical abstract • Inspired by crystal twinning, deformation programmable TPMS structures were constructed. • Effects of the number, location and orientation of twin boundaries on TPMS structures were investigated. • FE simulations reveal that twin boundaries effectively manipulate the direction and path of stress transfer. • This method can be extended to other type of crystal defects and may yield different phenomena.

Development of the human pancreas and its exocrine function.

The pancreas has both endocrine and exocrine function and plays an important role in digestion and glucose control. Understanding the development of the pancreas, grossly and microscopically, and the genetic factors regulating it provides further insight into clinical problems that arise when these processes fail. Animal models of development are known to have inherent issues when understanding human development. Therefore, in this review, we focus on human studies that have reported gross and microscopic development including acinar-, ductal-, and endocrine cells and the neural network. We review the genes and transcription factors involved in organ formation using data from animal models to bridge current understanding where necessary. We describe the development of exocrine function in the fetus and postnatally. A deeper review of the genes involved in pancreatic formation allows us to describe the development of the different groups (proteases, lipids, and amylase) of enzymes during fetal life and postnatally and describe the genetic defects. We discuss the constellation of gross anatomical, as well as microscopic defects that with genetic mutations lead to pancreatic insufficiency and disease states.

Preparation of Cemented Carbide Insert Cutting Edge by Flexible Fiber-Assisted Shear Thickening Polishing Method.

Reasonable cutting edge preparation can eliminate microscopic defects and improve the performance of a cutting tool. The flexible fiber-assisted shear thickening polishing method was used for the preparation of cemented carbide insert cutting edge. The influences of the polishing angle and polishing speed on the cutting edge preparation process were investigated, and the cutting edge radius and K-factor were employed as evaluation indexes to evaluate the edge shape. A prediction model of the cutting edge radius was also established using the mathematical regression method. The results show that the polishing angle has a more significant effect on the cutting edge radius. The cutting edge preparation efficiency is the highest under the polishing angle of 10°, and the cutting edge radius increased from the 15 ± 2 μm to 110 ± 5 μm in 5 min. The cutting edge shape can be controlled by adjusting the polishing angle, and the K-factor varies from 0.14 ± 0.03 to 0.56 ± 0.05 under the polishing angle (from −20° to 20°). The polishing speed has a less effect on the cutting edge radius and shape, but increasing the polishing speed within a certain range can improve the efficiency of cutting edge preparation. The flank face roughness decreased from the initial Ra 163.1 ± 10 nm to Ra 5.2 ± 2 nm at the polishing angle of −20°, which is the best polishing angle for the flank face surface roughness. The ANOVA method was employed to evaluate the effective weight of the polishing angle and polishing speed on preparation efficiency. The polishing angle (86.79%) has the more significant influence than polishing speed (13.21%) on the cutting edge preparation efficiency. The mathematical regression method was used to establish the model of the prediction of the cutting edge radius with polishing angle and speed, and the models were proved rationally. The results indicate that the FF-STP is an effective method for the high consistency preparation of cemented carbide insert cutting edge.

Very low levels of n-butyl acrylate comonomer strongly affect residual stress relaxation in styrene/acrylic random copolymer films

Processing-induced residual stresses in polymer films can cause microscopic defects and macroscopic dimensional changes. Although stress relaxation behavior of polymer films has been studied for several decades, little work has addressed random copolymers. We probed the residual stress relaxation in non-equilibrated random copolymer films of styrene (S) and n-butyl acrylate (nBA) using two optical techniques, ellipsometry and fluorescence. Both techniques show that incorporation of very low levels of nBA (2 mol% to 5 mol%) in S/nBA random copolymers can lead to dramatic increases in stress relaxation time of copolymer films relative to neat polystyrene films. For example, relative to polystyrene, the bulk-film stress relaxation time at 130 °C of 95/5 mol% S/nBA copolymer increases by factors of at least six as determined by ellipsometry and three as determined by fluorescence. Nanoconfined, 25-nm-thick films exhibit similar differences. This work provides implications for how low comonomer levels may deleteriously affect stress relaxation in polymer films, and we highlight that these findings are important for applications in which thin polymer films are required.

Synchronously modulated the morphology and crystal defects of α-MnO2 for high-performance mono-component electromagnetic wave absorber

Developing mono-component electromagnetic wave (EMW) absorbing materials is desirable owing to their simple preparation methods; thus, it is crucial to investigate EMW absorbing mechanisms. However, owing to the requirements of excellent impedance matching and attenuation capability, EMW material design is considerably challenging. To address these challenges, this study reports an Ag+-doped α-MnO2 with a hierarchical structure for EMW absorption. The microscopic morphology and crystal defects are synchronously modulated, resulting in considerable improvement in EMW absorption performance; an optimal reflection loss of −68.6 dB and an effective absorption bandwidth (EAB) of 4.2 GHz are achieved at a thin thickness of 1.43 mm. Thus, thin thickness, enhanced absorption intensity, and widened absorption bandwidth are simultaneously achieved. The regulating effect of Ag+ doping on the morphology of MnO2 and the corresponding EMW absorption mechanism are also discussed. The findings of this study provide new insights for designing mono-component EMW absorbing materials with higher absorption performance.

Novel quantification of porosity defects on fatigue behavior for cast aluminum-silicon alloys by X-ray micro-tomography

Casting pores, as inevitable microscopic defects in castings, usually have a significant influence on the mechanical properties of materials, especially the fatigue properties. In this study, high cycle fatigue (HCF) tests for smooth specimens made of cast AlSi7Mg1 alloy (close to ASTM A357) with casting pores are performed. X-ray micro-tomography is used to obtain the 3D information of pores and cracks. The experimental logarithmic fatigue life is found to have a linear relation with the equivalent size and relative location of the critical pore. In addition, locally concave or convex areas on crack surfaces are discovered and the formation mechanism is identified. Based on a continuum damage mechanics (CDM) based HCF damage evolution equation, a pore-sensitive fatigue model is proposed by considering the size and location of pores in 3D. The test results and calculation results show that the developed pore-sensitive fatigue model can accurately predict the fatigue life.

Discussion on the Causes of Cracking and Failure of Anticorrosion Layer of a Pipeline

The cause of a rare cracking failure accident of the polyethylene (PE) anti-corrosion layers of a pipeline is analyzed and determined. The properties of the PE raw materials and the anti-corrosion layers were detected and analyzed according to the standard, and the micro-morphology of the anti-corrosion layers was characterized by the scanning electron microscope (SEM). The storage modulus and loss of the anti-corrosion layers at the range of -100~40 °C were tested by the dynamic mechanical analysis (DMA), and the stress of the anti-corrosion layers at low temperature (-40 °C) was calculated. The average thermal expansion coefficient of the anti-corrosion layers in the range of -40~60 °C was tested by the thermomechanical analysis (TMA), and the value at room temperature was obtained, which was compared with the value of the steel materials. The failure accidents caused by raw materials and anti-corrosion layers due to the quality problems, as well as aging and damage of anti-corrosion layers are excluded. The SEM results confirmed that the anti-corrosion layers with no obvious large-scale defects, there were microscopic defects less than 1 μm. At the low temperature (-40 °C), the storage modulus of the anti-corrosion layers is 2.58 GPa, and the corresponding stress is 23.99 MPa, which is higher than the tensile strength test value (22.21 MPa). The average thermal expansion coefficient of the anti-corrosion layers is 14.3×10-5/°C, which is higher than that of the steel material (1.2×10-5/°C). The high stress at low temperature (-40 °C) is one of the reasons for induced defects to cause cracks, and the huge difference in thermal expansion coefficient between the anti-corrosion layers and the steel material is another reason for that.

Simulation and experimental study of tool wear and damage in milling SA 508III steel

SA 508III steel is a typical difficult-to-machine material with high strength and high-temperature mechanical properties. In the milling process, the carbide tool is seriously damaged and their service life is short under the thermal-mechanical load of low cycle and high impact. In this paper, firstly, the finite element simulation of milling SA 508III steel is carried out to study the characteristics of thermal-mechanical load and tool wear during milling. Secondly, the wear experiment of cemented carbide tools is carried out. The morphology changes of the rake face and flank face of cemented carbide tools are observed and analyzed. The nature of tool wear and damage is discussed. Finally, the rule of crack development from microscopic defects to macroscopic cracks until tool fracture is described, and the tool failure mechanism is explored. The research results can provide theoretical basis and technical support for the optimal design of cutting tools of high-efficiency milling SA 508III steel and the improvement of cutting performance and service life of tools.

Spectral properties and β-ray scintillation of GdVO4:Eu single crystals grown by the floating zone method

GdVO4:Eu single crystals were successfully grown along the [110] direction by the floating zone method. The grown crystals had no cracks and inclusions for any Eu-concentration. The polarizing microscopy proved that the grown crystal contained no microscopic defects such as low-angle grain boundaries and inclusions. The crystal showed red fluorescence with the peak wavelength of 620 nm. The emission showed strong polarization dependence; the emission of π-polarization is more intensive than that of strongly the σ-polarization. Under irradiation of X-ray or electron beam, the crystals also showed strong red fluorescence based on the host excitation. The fluorescence lifetime was measured to be about 785 μs. Scintillation the GdVO4:Eu single crystals for β-ray was successfully demonstrated using nuclear medicine 18F-fludeoxyglucose.

Study on macro-meso mechanical properties of cemented tailings backfill with high fly ash content.

The use of cement and fly ash (FA) to prepare cemented tailings backfill (CTB) lowers backfill mining costs while also reducing pollution caused by the accumulation of waste materials like tailings and FA, making it a green backfill mining process. While adding FA to CTB may reduce costs, too much FA might weaken CTB's strength property. Mechanical tests were used to explore the effects of FA content and curing time on the uniaxial compressive strength (UCS) and deformation modulus of CTB in this research. The effects of FA content on failure modes, strain energy, and crack evolution of CTB were studied using a numerical model that considered FA content and particle contact mode. The influence mechanism of different FA contents on CTB was also revealed at the microscopic level. The results demonstrate that the UCS of CTB has a quadratic polynomial and a linear relationship with FA content and curing time respectively, and that the elasticity modulus and secant modulus of CTB increase and then decrease with FA content under different curing times. The peak strain energy of CTB increases and subsequently declines with the FA content, and crack propagation inside CTB may be limited by regulating the FA content. A reasonable content of FA can optimize the size and distribution of CTB microscopic defects, enabling them to exhibit superior strength property. This study systematically explores the mechanism of different FA contents on the strength property of CTB from a macro-micro perspective, providing an essential reference value for improving the recycling of FA and waste residues such as tailings.

Excimer Laser Patterned Holey Graphene Oxide Films for Nonenzymatic Electrochemical Sensing.

The existence of point defects, holes, and corrugations (macroscopic defects) induces high catalytic potential in graphene and its derivatives. We report a systematic approach for microscopic and macroscopic defect density optimization in excimer laser-induced reduced graphene oxide by varying the laser energy density and pulse number to achieve a record detection limit of 7.15 nM for peroxide sensing. A quantitative estimation of point defect densities was obtained using Raman spectroscopy and confirmed with electrochemical sensing measurements. Laser annealing (LA) at 0.6 J cm-2 led to the formation of highly reduced graphene oxide (GO) by liquid-phase regrowth of molten carbon with the presence of dangling bonds, making it catalytically active. Hall-effect measurements yielded a mobility of ∼200 cm2 V-1 s-1. An additional increase in the number of pulses at 0.6 J cm-2 resulted in deoxygenation through the solid-state route, leading to the formation of holey graphene structure. The average hole size showed a hierarchical increase, with the number of pulses characterized with multiple microscopy techniques, including scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. The exposure of edge sites due to high hole density after 10 pulses supported the formation of proximal diffusion layers, which led to facile mass transfer and improvement in the detection limit from 25.4 mM to 7.15 nM for peroxide sensing. However, LA at 1 J cm-2 with 1 pulse resulted in a high melt lifetime of molten carbon and the formation of GO characterized by a high resistivity of 3 × 10-2 Ω-cm, which was not ideal for sensing applications. The rapid thermal annealing technique using a batch furnace to generate holey graphene results in structure with uneven hole sizes. However, holey graphene formation using the LA technique is scalable with better control over hole size and density. This study will pave the path for cost-efficient and high-performance holey graphene sensors for advanced sensing applications.

Mechanism of Surface Defects in Ultra-Precision Machining of Sesquioxide Laser Crystal Tm: GdScO3.

It is well-known that the surface quality of laser gain crystal elements is very high in order to ensure the stability of laser system and laser output quality. In the ultra-precision machining process of a new sesquioxide laser crystal Tm: GdScO3, it is required to achieve very high surface shape and very low surface defects. In this paper, the molecular dynamics simulation model of single particle grinding was established. It was found that the normal load and tangential friction imposed by abrasive particles on the surface of components cause the spalling of atoms on the substrate surface, which constitutes the removal of materials at the macro-level. At the same time, it causes the displacement of the sub surface atoms, which constitutes the microscopic defects in the structure. Through the structural characterization of macro defects, it was confirmed that the essence of micro defects is the amorphous and distortion of surface structure, and the depth can reach 100 nm. The results of lapping and polishing experiments show that the adjustment of pressure has a limited effect on the improvement of surface defects in the process of machining crystal elements with granular abrasive.