Formation Of Microstructural Defects Research Articles

Atomic-Scale Structural Properties in NiCo2O4/CuFe2O4 Bilayer Heterostructures on (001)-MgAl2O4 Substrate Regulated by Film Thickness.

Changing film thickness to manipulate microstructural properties has been considered as a potential method in practical application. Here, we report that atomic-scale structural properties are regulated by film thickness in an NiCO2O4(NCO)/CuFe2O4(CFO) bilayer heterostructure prepared on (001)-MgAl2O4 (MAO) substrate by means of aberration-corrected scanning transmission electron microscopy (STEM). The misfit dislocations at the NCO/CFO interface and antiphase boundaries (APBs) bound to dislocations within the films are both found in NCO (40 nm)/CFO (40 nm)/MAO heterostructures, contributing to the relaxation of mismatch lattice strain. In addition, the non-overlapping a/4[101]-APB is found and the structural transformation of this kind of APB is resolved at the atomic scale. In contrast, only the interfacial dislocations form at the interface without the formation of APBs within the films in NCO (10 nm)/CFO (40 nm)/MAO heterostructures. Our results provide evidence that the formation of microstructural defects can be regulated by changing film thickness to tune the magnetic properties of epitaxial bilayer spinel oxide films.

Accelerated radiation tolerance testing of Ti-based MAX phases

MAX phases have recently attracted significant attention for potential nuclear applications due to their novel properties such as unique hexagonal-compact nanolayered crystal structure, high-machinability due to lower hardness levels than conventional ceramics, and high-chemical inertness. In order for MAX phases to be used in nuclear reactors, two aspects deserve detailed investigations: (i) their phase stability at high-temperatures and (ii) microstructural defect formation and recovery induced by energetic particle irradiation. To date, degradation mechanisms of MAX phases at high-temperatures and following irradiation are largely unexplored fields of research. This work focuses on the evaluation of two Ti-based MAX phases—Ti2AlC and Ti3SiC2—within the context of extreme environments. To accomplish this, a one-of-a-kind comparison between neutron irradiations, performed over a decade of research at the high flux isotope reactor, and heavy-ion irradiations, carried out in situ in a transmission electron microscope, has been conducted. The results show Ti-based MAX phases are prone to accelerated decomposition under the conditions investigated. This questions the hypothesis that MAX phases exhibit high phase stability, especially when used in future nuclear energy systems where energetic particle irradiation is a dominating degradation mechanism.

Development of turkey feather fiber-filled thermoplastic polyurethane composites: Thermal, mechanical, water-uptake, and morphological characterizations

This present study centers sensitively on the determination of the effect of natural turkey feather fibers (TFFs) loading on fundamental features (thermal, mechanical, water-uptake, and micro-structural) of thermoplastic polyurethane (TPU). The composites with different TFFs contents (3, 6, 9, and 12 wt.%) were fabricated by the melt blending method using the twin screw extruder and micro-injection molder. The samples were characterized by means of differential scanning calorimeter (DSC), universal mechanical (tensile and hardness) tester, water-uptake, and scanning electron microscope (SEM) techniques. The thermal analysis depicted that the melting temperatures of the soft and hard segments as well as the crystallinity degree of TPU increased consistently with the increase of TFFs loading level thanks to the formation of better close-packed TPU chains in the matrices. As for the mechanical test results, when compared neat TPU, the tensile strengths were reinforced by 26.8% and 19.7%, and the modulus increased by 6.6% and 45.1% for the composite samples including 3% and 6% of TFFs, respectively. However, drastic diminishment were observed at further contents. Additionally, TFFs loadings brought about gradual increase in the water-uptake capacities of the composites due to the increasing of the number of voids and omnipresent flaws in TPU matrices. The taken SEM images also revealed that, at low contents, there existed the enrichment of interfacial adhesion between TFFs and TPU matrix, whereas the morphological appearance of the composites get worse at high contents accompanied by the formation of micro-structural defects.

Microgeometry of the surface of electron beam additive manufacturing products. Selective electron beam melting

The results of the analysis of works related to the formation of surface microgeometry in the process of selective electron-beam melting are presented, and the physical model of this process is refined. The developing additive technology of selective electron-beam alloying and the directions in which its research is carried out, in particular, the analysis of the metallurgical process, the formation of the microstructure of the material, the formation of microstructure defects, are described. The roughness of the surface of products obtained by the SEBM technology, as well as the microgeometry of surfaces and the mechanisms of its formation, depending on various parameters of the process, are considered.

Prediction of microstructural defects in additive manufacturing from powder bed quality using digital image correlation

Additive manufacturing (AM) of metal components offers tremendous promise to create complex parts with limited supervision. However, process quality and widespread acceptance are currently stymied by the formation of microstructural defects and residual stresses. In particular, lack-of-fusion (LoF) and keyhole defects frequently occur during AM production dependent on process conditions and can severely limit mechanical performance if not corrected. The effects of powder layer quality, where variations from the ideal have the potential to significantly alter defect formation, have not been thoroughly investigated in the literature. In this work, we employ a novel method incorporating in situ three-dimensional digital image correlation (3D-DIC) imaging of the powder bed in-process to identify and quantify the severity of powder bed irregularities during processing. Anomalies in the powder bed were detected, and their geometries quantified, layer by layer using the 3D-DIC analysis for parts produced at multiple energy density levels. Ex situ characterization via scanning electron microscopy identified the locations of physical defects for comparison to DIC data. The quantified powder bed 3D-DIC data, alongside ex situ identification of physical defect locations, was then fed into a Naïve-Bayes classification algorithm to predict the likelihood of physical defect formation based on the severity of in-process powder bed errors. This methodology has the potential to be used to predict physical defect formation based on detected powder spreading errors in-process, prior to the formation of many microstructural defects.

Shock-induced plasticity in nanocrystalline iron: Large-scale molecular dynamics simulations

Large-scale nonequilibrium molecular dynamics (MD) simulations of shock waves in nanocrystalline iron show evidence of plasticity before the polymorphic transformation takes place. The atomistic structure in the shock direction shows an elastic precursor, plastic deformation, and shock-induced phase transformation from bcc to hcp iron. In this Rapid Communication, large-scale MD models show that the shock response of iron is highly related to the ramp time of the applied shocks. For long ramp times we observe significant plastic relaxation and formation of microstructure defects. Pressure-induced phase transformations in iron are accompanied by stress relaxation achieving almost fully relaxed three-dimensional hydrostatic final states. The evolution of the stress relaxation is in agreement with theory and experiments. Analysis of the x-ray diffraction patterns calculated from the atomistic structure using the Debye equation revealed pronounced anisotropy of the line broadening that is caused by stacking faults in hcp Fe and by dislocations in bcc Fe.

Real-Time Observation of Stacking Faults in Gold Shock Compressed to 150 GPa

A new experimental approach reveals, for the first time, the formation of microstructural defects in gold when shock compressed to high pressure, a critical insight to understanding shocked states of materials.

Investigation of melting phenomena in solid-state welding processes

High-velocity impact welding has usually been presumed as a solid-state manufacturing process. In this study, by integrating meshfree numerical simulations, advanced interfacial characterizations, and diffusion calculations, the relationships between the formation of microstructural defects within the bonding area and melting phenomena were determined. It was also inferred that the emergence of significant grain refinement in the vicinity of the weld interface could be associated to melting and subsequent recrystallization due to the ultra-fast heating and cooling rate of the high pressure impact process. The new results challenge the prevailing belief regarding the solid-state nature of the impact-based welding processes.

Surface texture and hardness of dental alloys processed by alternative technologies

Technological developments have led to the implementation of novel digitalized manufacturing methods for the production of metallic structures in prosthetic dentistry. These technologies can be classified as based on subtractive manufacturing, assisted by computer-aided design/computer-aided manufacturing (CAD/CAM) systems, or on additive manufacturing (AM), such as the recently developed laser-based methods. The aim of the study was to assess the surface texture and hardness of metallic structures for dental restorations obtained by alternative technologies: conventional casting (CST), computerized milling (MIL), AM power bed fusion methods, respective selective laser melting (SLM) and selective laser sintering (SLS). For the experimental analyses metallic specimens made of Co-Cr dental alloys were prepared as indicated by the manufacturers. The specimen structure at the macro level was observed by an optical microscope and micro-hardness was measured in all substrates. Metallic frameworks obtained by AM are characterized by increased hardness, depending also on the surface processing. The formation of microstructural defects can be better controlled and avoided during SLM and MIL process. Application of power bed fusion techniques, like SLS and SLM, is currently a challenge in dental alloys processing.

Effect of the Ammonia Flow on the Formation of Microstructure Defects in GaN Layers Grown by High-Temperature Vapor Phase Epitaxy

High-temperature vapor phase epitaxy (HTVPE) is a physical vapor transport technology for a deposition of gallium nitride (GaN) layers. However, little is known about the influence of the deposition parameters on the microstructure of the layers. In order to fill this gap, the influence of the ammonia (NH3) flow applied during the HTVPE growth on the microstructure of the deposited GaN layers is investigated in this work. Although the HTVPE technology is intended to grow GaN layers on foreign substrates, the GaN layers under study were grown on GaN templates produced by metal organic vapor phase epitaxy in order to be able to separate the growth defects from the defects induced by the lattice misfit between the foreign substrate and the GaN layer. The microstructure of the layers is characterized by means of high-resolution x-ray diffraction (XRD), transmission electron microscopy and photoluminescence. In samples deposited at low ammonia flow, planar defects were detected, along which the nitrogen atoms are found to be substituted by impurity atoms. The interplay between these planar defects and the threading dislocations is discussed. A combination of XRD and micro-Raman spectroscopy reveals the presence of compressive residual stress in the samples.

The Study of Morphological Structure and Raman Spectra of 3C-SiC Membranes

In this paper, membranes of 3C-SiC with dimensions up to 10 mm x 15 mm2 have been fabricated in epitaxial 3C-SiC/ Si wafers by the means of photolithography, reactive ion etching of 3C-SiC and wet etching of Si. Scanning electron microscope (SEM) micrographs were used to observe the structure of the membrane and the wall formed by the Si wet etching. The quality of the 3C-SiC membranes were observed using Raman Spectroscopy. The remains of <111> Si substrate which was unetched during the Si wet etching were presented with the formation of microstructure defects which showed distinct peaks in comparison to the high quality 3C-SiC membranes at different position. Here, the effect of the membrane fabrication procedures to the 3C-SiC membrane properties especially the morphological structure and its Raman characteristics is discussed in detail.

Role of nanofillers in low speed impact enhancement of composites

The current study investigates the impact characteristics of polymeric-based nanocomposites strengthened with carbon nanotubes, nanoclay, aluminum oxide and silicon carbide particles nanofillers. Different weight percentages of each nanofiller were prepared. A dead weight drop mechanism was utilized to compare the impact characteristics of different nanocomposite materials. An X-ray technique was utilized to characterize the formation of microstructural defects of the laminated composites post impact. The results showed that the nanoclay fillers were the best to enhance the impact and mechanical properties of the composite materials with 4.3 wt% of nanoclay being the optimum percentage.

Force Spectroscopy of Complex Biopolymers with Heterogeneous Elasticity

Cellular biopolymers can exhibit significant compositional heterogeneities as a result of the non-uniform binding of associated proteins, the formation of microstructural defects during filament assembly, or the imperfect bundling of filaments into composite structures of variable diameter. These can lead to significant variations in the local mechanical properties of biopolymers along their length. Existing spectral analysis methods assume filament homogeneity and therefore report only a single average stiffness for the entire filament. However, understanding how local effects modulate biopolymer mechanics in a spatially resolved manner is essential to understanding how binding and bundling proteins regulate biopolymer stiffness and function in cellular contexts. Here, we present a new method to determine the spatially varying material properties of complex biopolymers from the observation of passive thermal fluctuations of the filament conformation. We develop new statistical mechanics-based approaches for heterogeneous filaments that estimate local bending elasticities as a function of the filament arc-length. We validate this methodology using simulated polymers with known stiffness distributions, and find excellent agreement between derived and expected values. We then determine the bending elasticity of microtubule filaments of variable composition generated by repeated rounds of tubulin polymerization using either GTP or GMPCPP, a nonhydrolyzable GTP analog. Again, we find excellent agreement between mechanical and compositional heterogeneities.

Force spectroscopy of complex biopolymers with heterogeneous elasticity

Cellular biopolymers can exhibit significant compositional heterogeneities as a result of the non-uniform binding of associated proteins, the formation of microstructural defects during filament assembly, or the imperfect bundling of filaments into composite structures of variable diameter. These can lead to significant variations in the local mechanical properties of biopolymers along their length. Existing spectral analysis methods assume filament homogeneity and therefore report only a single average stiffness for the entire filament. However, understanding how local effects modulate biopolymer mechanics in a spatially resolved manner is essential to understanding how binding and bundling proteins regulate biopolymer stiffness and function in cellular contexts. Here, we present a new method to determine the spatially varying material properties of individual complex biopolymers from the observation of passive thermal fluctuations of the filament conformation. We develop new statistical mechanics-based approaches for heterogeneous filaments that estimate local bending elasticities as a function of the filament arc-length. We validate this methodology using simulated polymers with known stiffness distributions, and find excellent agreement between derived and expected values. We then determine the bending elasticity of microtubule filaments of variable composition generated by repeated rounds of tubulin polymerization using either GTP or GMPCPP, a nonhydrolyzable GTP analog. Again, we find excellent agreement between mechanical and compositional heterogeneities.

Microstructure Investigations of the Phase Boundaries in the Bridgman TRIP Steel Crystal

Formation of microstructure defects at the phase boundaries in TRIP steels was investigated with the aid of microstructure analysis on a TRIP steel crystal, which was grown by the Bridgman technique. The microstructure studies comprised scanning electron microscopy (SEM), X-ray diffraction (XRD), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and transmission electron microscopy with high resolution (HRTEM). Initial XRD measurements revealed that the crystals under study consist of austenite and ferrite with extremely strong preferred orientations. Subsequent XRD pole figure measurements and EBSD scans have shown that the orientation relationship between austenite and ferrite can be described by the Nishiyama-Wassermann model. For a detailed description of the microstructure of the Bridgman crystal, the orientation distribution of crystallites within the individual phases was investigated using the XRD reciprocal space mapping and the rocking curve measurements. These experiments have shown that the density of microstructure defects is much lower in ferrite than in austenite. The direct information about the defect structures at the phase boundaries between austenite and ferrite was obtained from the TEM micrographs, which revealed complicated micro-twin structures at the boundaries between the neighbouring phases. HRTEM discovered very narrow stripes of ferrite embedded in austenite that were regarded as a source of the microstructure defects in austenite.

Formation of microstructural defects in electrodeposited Co/Cu multilayers

The effect of the thickness of the copper layers on the microstructure and magnetoresistance of electrodeposited Co/Cu multilayers was investigated by using the combination of wide-angle and small-angle X-ray scattering, transmission electron microscopy with high resolution and magnetoresistance measurement. The main tasks of the study were the classification of the microstructural defects, which form during the electrodeposition of Co/Cu multilayers, the description of mechanisms that are involved in the formation of these defects and the correlation of the kind and density of the microstructural defects as obtained using the microstructural analysis with the magnitude of the magnetoresistance effect, which was concluded from the magnetoresistance curves.

Efeito da condensação e queima na formação de defeitos microestruturais em cerâmicas feldspáticas dentárias

As cerâmicas feldspáticas dentárias são rotineiramente empregadas para reconstruções protéticas dentárias unitárias ou extensas, associadas a uma subestrutura metálica. Tendo em vista a sensibilidade técnica na confecção deste tipo restauração dentária, o objetivo deste estudo foi analisar a influência de múltiplas queimas e técnicas de condensação na presença de defeitos na microestrutura de cerâmicas feldspáticas dentárias. Os pós de duas diferentes marcas comerciais de cerâmicas feldspáticas convencionais dentina (A e B) foram caracterizados por difração de raios X (DRX) e, em seguida, por granulometria com um granulômetro associado a um programa analisador de tamanho de partículas. Para a caracterização da microestrutura foram confeccionadas amostras das cerâmicas A e B por meio de três técnicas: 1- condensada sem vibração ultrasônica e queima única; 2- condensada sob vibração ultrasônica e queima única; 3- condensada sob vibração ultrasônica e quatro queimas. As amostras foram lixadas e polidas, caracterizadas microestruturalmente por microscopia eletrônica de varredura (MEV-EDS) e microscopia ótica, e compararadas a uma cerâmica feldspática dentária termoprensável (C). A análise por DRX mostra a leucita como a única fase cristalina. A distribuição do tamanho de partículas das cerâmicas A e B apresentou aspecto trimodal, com presença de partículas um pouco maiores para a cerâmica B. A análise microestrutural da superfície revelou a presença de poros em todas as cerâmicas. No entanto, a quantidade de poros foi maior nas amostras que foram confeccionadas sem vibração e absorção de água e sinterizadas por queima única. As amostras que foram submetidas a quatro queimas apresentaram um menor volume de poros, porém com o aparecimento de trincas geradas durante o resfriamento. A técnica manual de condensação adequada sob vibração ultrasônica e posterior tratamento térmico em forno proporcionou uma quantidade de poros semelhantes à cerâmica termoprensável.

Blue and green emission using In(Ga)N/GaN quantum wells with InN well layers grown by metalorganic chemical vapor deposition

In(Ga)N/GaN multiple quantum well structures with high indium composition were grown using pure InN in well layers by low-pressure metalorganic chemical vapor deposition. Blue emission using a thin In(Ga)N well width grown with only trimethylindium as a III-source showed strong carrier localization originating from a large fluctuation in indium composition and a reduced quantum-confined Stark effect due to the use of a thin well, both of which enhanced quantum efficiency. In contrast, an increase in InN growth time to achieve green emission exhibited a large blueshift of electroluminescent emission under forward-bias current and microstructural defect formation, which were responsible for the observed degradation in emission properties.

Modeling of carbon segregation and accompanying processes during HTSC manufacture

The formation of microstructure defects and weak links, which have considerable influence on the structure-sensitive properties of high-temperature superconductors (HTSC) is discussed, taking into account the carbon segregation, which embrittles intergranular boundaries and constructs the weak links. The carbon segregation processes are associated with slow, fast and steady states of the dislocation-screened crack growth. The solutions obtained can be applied to the finite element formulations and other numerical codes by which the stress–strain states, distributions, kinetics and parameters of intergranular defects during manufacture of the HTSC systems can be predicted.

The gamma-irradiation of nuclear waste glasses

Abstract Gamma irradiation of a simulated nuclear waste storage glass causes the formation of small bubbles in the glass by a process of network decomposition. This behavior is consistent with models previously proposed to account for electron damage in these same glasses. Comparison of the γ-irradiation effects with results of electron irradiations underscores the importance of ionization damage in the formation of microstructural defects and the importance of dose rate on the defect formation efficiency.