Defect Population Research Articles

Rationalization of Ring Fragmentation Data via Simple Kinetic Energy Analysis

The available data on fragmentation of alloy rings at high strain rate has been reviewed and analysed with reference to a classical statistical and energetic model. The data have also been compared to a simple calculation based on the specific kinetic energy of the ring. It is shown that fragmentation data for alloys with over a 20 times difference in strength and 7 times difference in density scatter around a single curve when plotted as a function of specific kinetic energy over a wide range of strain rates. A fit to this curve therefore provides an approximate estimate of the expected fragment number for any ductile metal without requiring any detailed knowledge about the constitutive behaviour or defect population. This fit is better than a prediction based on the classical statistical model, and similar to that for the energetic model, which also requires a knowledge of the fragmentation fracture energy. The observation that specific kinetic energy alone can explain much of the difference observed between alloys suggest that the energy in the system is more important that details of the alloy microstructure or properties in controlling the number of fragments for a material that fails by ductile fracture. The fit does not work for a case where one alloy was heat treated to a condition where brittle failure occurred.

Fatigue performance of laser powder bed fusion manufactured TiB2/AlSi10Mg composite: Experimental investigation and fracture mechanics-based life prediction model for defect tolerance analysis

The fatigue performance of AlSi10Mg composites decorated with nano-TiB2 particles fabricated through laser powder bed fusion (LPBF) was first experimentally investigated at room temperature and it exhibited superior fatigue resistance compared with other LPBF AlSi10Mg alloys and other reinforced AlSi10Mg composites. Then the fractography was examined and a fracture mechanics-based life prediction method was proposed to correlate the critical defect information (size and location) and fatigue life. To assess the crack growth driving force for critical defects, we employed Murakami's concept of the maximum stress intensity factor (SIF). The modification was made to include the effect of specimen cross-sectional geometry and initiation location since Murakami’s method would underestimate the maximum SIF in some cases. Meanwhile, the process of small crack propagation was also considered in modeling, and a unified model to describe crack arrest, small crack growth, and long crack growth was established. By inputting information about defect size and location and external loading conditions, the fatigue life could be evaluated and our results demonstrated good estimation within the [1/4,2] scatter band. Furthermore, an unbroken specimen subjected to the highest external loading was examined using X-ray computed tomography (CT) to characterize the defect population. By incorporating the CT statistical results into our life prediction model, outcomes such as non-propagating cracks or small crack growth without leading to ultimate failure could be obtained, which was consistent with CT findings. Concurrently, CT analysis revealed that the detrimental effect of large inner defects could be less significant than that of small surface defects. Therefore, we propose a novel strategy that combines defect size and location to enhance defect tolerance in additive manufacturing (AM) materials.

Evaluating the mechanical integrity and reliability of multi-channelled flat-sheet ceramic membranes for filtration applications

Ceramic membranes represent an emergent filtration technology that is receiving increasing research attention, particularly concerning their filtration behaviour (i.e., permeability, selectivity, and fouling) and industrial scalability. Given the inherent brittleness of ceramic materials, mechanical properties of ceramic membranes, especially those related to fracture, are also important pieces of information although nonetheless rarely evaluated. Therefore, a methodological approach for assessing the mechanical integrity and reliability of multi-channelled flat-sheet ceramic membranes is presented here. Specifically, a flat-sheet SiC ceramic membrane designed for submerged outside-inside vacuum-driven filtration was subjected to four-point bending, tensile, and compressive tests, and the experimentally measured failure loads were converted to failure stresses and analysed by Weibull diagrams. It was found that the ceramic membrane is ∼1.4 times stronger and ∼2.9 times less reliable in compression than in bending, and ∼3.3 times stronger and ∼5.4 times more reliable in bending than in tension. These differences in mechanical integrity and reliability are rationalized by considering the fracture modes (i.e., modes I or II) and critical flaw populations (i.e., flaws of the membrane layer or support) responsible for failure. Finally, relevant implications for ceramic “membranologists” interested in mechanical characterisation are discussed.

Failure location effects on slow crack growth parameter estimates

AbstractAs part of Roman Space Telescope component development, the slow crack growth (SCG) parameters of a glass were measured using constant stress rate testing of ASTM C1161 beams. Failures occurred on the faces, bevels and sides of the test specimens thereby providing an opportunity to examine the effects of failure location on SCG parameter estimates. Censoring of the data by location did not have a statistically significant effect on the slow crack growth parameter estimates. Linear and nonlinear fits were also made to various strength estimators. Nonlinear fits gave slightly lower estimates of the parameters. The main influence on parameter estimates was a low strength, infrequent flaw population. Exponential model parameters were estimated via swarm optimization.

Robust water diffusion modeling in a structural polymer joint based on experimental X-ray tomographic data at the micrometer scale

Structural bonding is a technique increasingly used in the industrial field. For applications in aggressive environments such as seawater, predicting the effect of moisture on the mechanical behavior of bonded assemblies is of paramount importance. The objective of this work is to analyze water diffusion in an epoxy adhesive material and, more specifically, to propose a robust method for choosing the most appropriate diffusion model. Experimental studies of the water absorption in a two-component epoxy structural adhesive, using gravimetry and X-ray tomography, were first performed. The presence of a population of pore-type defects in the polymeric joint helped to characterize the evolution of water diffusion kinetics. Thus, two diffusion mechanisms were identified: a first one related to the migration of water molecules within the adhesive matrix, and a second one related to the penetration of water into the pores. Then, Dual-Fick and Langmuir models were retained, as the two diffusion models most likely to capture the above mechanisms. Although it was shown that both models could give similar results in terms of global diffusion behavior, the results arising from these two models differ at the local scale, especially for extended periods of time. Therefore, special attention was paid to the second absorption mechanism, and a comparison of waterfronts between theoretical predictions and experimental tomographic data was achieved, leading to the final choice of a Dual-Fick diffusion model.

Defect analysis and microstructural characterization of zinc-substituted cadmium oxide nanocrystallites by positron annihilation and supplementary methods

Pure and zinc-doped cadmium oxide nanocrystallites of sizes in the range 25 nm to 16 nm are synthesized by adopting a chemical precipitation method and by varying the doping concentration from 0.0 to 0.25. The decrease in nanocrystallite sizes with increasing substitution is expected from the smaller ionic radii of Zn2+. But more revealing is the interfacial defects formation at higher concentration of doping, which is attributed to the dissimilar crystalline structure of ZnO and CdO. X-ray diffraction patterns show well defined peaks and additional characterisation is done through transmission electron microscopy. The optical band gap measurements indicate the dominance of substitution-induced disorder over the confinement of excitons, leading to a decrease in the band gap energies. The results of positron annihilation studies confirm the cancellation of the existing vacancy type defects in the initial stage, followed by the substitution. Photoluminescence spectra reveal the distinct peaks of optical plasmonic excitations and the defect population in the bandgap and the intensity variations agreed with that of the defect related positron annihilation lifetime intensity. The segregation of ZnO phase leading to the formation of interfacial boundaries is found as a strong deterrent against the success of continued substitution.

Size effects in 3D‐printed polymer‐derived, zirconium diboride‐reinforced ceramic composites

AbstractPreceramic polymers are of interest for use in many manufacturing techniques such as injection molding, ceramic fiber infiltration, and additive manufacturing. However, off‐gassing of low molecular weight oligomers occurs when these polymers cure, potentially leading to porosity in the cured part. To study how porosity and strength are affected by the size of the printed part, and the presence of a high surface area nano‐scale filler, polycarbosilane (PCS) microrods of varying diameter were fabricated via direct ink writing (DIW), an additive manufacturing technique, with two ink formulations containing either zirconium diboride (ZrB2) alone, or ZrB2 and fumed alumina (FA). Sets of microrods were printed in a range of sizes by using print nozzles of 450, 634, 979, 1 346, or 1 702 μm in diameter, which were thermally cured, pyrolyzed to form ceramic composite microrods, and tested in 3‐pt flexure. Porosity increased with increasing diameter, while failure strength decreased. For a given nozzle size, the microrods containing FA displayed lower porosity and higher strength (up to ∼500 MPa) compared to the microrods containing only ZrB2. Weibull strength analysis was performed on each group of microrods and shows that the addition of FA increased Weibull modulus from 4.63 ± 1.56 to 9.35 ± 0.601. In conjunction with optical microscopy, this analysis indicates two distinct flaw populations in the printed materials, porosity which arises during the curing step and cracking which arises during pyrolysis of the larger specimens.

A Crystallographic Basis for Critically Evaluating the Mechanisms for Secondary Grain Formation During Directional Solidification

AbstractA crystallography-based method is presented for the critical appraisal of possible mechanisms that trigger the formation of secondary grains during directional solidification. The method permits an analysis of a large population of defects, while avoiding the pitfalls of the metallographic sectioning approach that is affected by dendrite stereology. Here, the nickel-base superalloy CMSX-4, an alloy commonly used for single crystal turbine blade applications, is studied. All secondary grains originate exclusively at the external surface and when the off-axial primary $$\langle 0\,0\,1\rangle$$ ⟨ 0 0 1 ⟩ crystal orientations are measured, are evident at both the converging and diverging dispositions of the single crystal primary dendrites without a noticeable bias. Almost all of the secondary grains have low misorientations, with an average misorientation between 5 to 15 deg. No systematic deviation between the individual $$\langle 0\,0\,1\rangle$$ ⟨ 0 0 1 ⟩ orientations of the secondary grain and the single crystal is observed. A significant twist contribution about an axis within ~ 30 deg from one of the secondary arms occurs when primary arms converge on the external surface, but both twist and tilt prevail for the diverging case. Both nucleation and buoyancy driven thermo-solutal convection can be eliminated as potential mechanisms. Thermo-mechanical deformation is deduced to be the most likely mechanism; deformation must originate in the vicinity of the primary dendrite tips. It is proposed that dendrite deflection arises primarily from the resistance encountered by the primary tips with the external surface during axial contraction in the presence of a dominant vertical thermal gradient.

Fluorination-mitigated high-current degradation of amorphous InGaZnO thin-film transistors

As growing applications demand higher driving currents of oxide semiconductor thin-film transistors (TFTs), severe instabilities and even hard breakdown under high-current stress (HCS) become critical challenges. In this work, the triggering voltage of HCS-induced self-heating (SH) degradation is defined in the output characteristics of amorphous indium-gallium-zinc oxide (a-IGZO) TFTs, and used to quantitatively evaluate the thermal generation process of channel donor defects. The fluorinated a-IGZO (a-IGZO:F) was adopted to effectively retard the triggering of the self-heating (SH) effect, and was supposed to originate from the less population of initial deep-state defects and a slower rate of thermal defect transition in a-IGZO:F. The proposed scheme noticeably enhances the high-current applications of oxide TFTs.

Vacancy segregation and intrinsic coordination defects at (1 1 1) twist grain boundaries in diamond

High radiation damage resistance has become increasingly important for applications of functional materials in harsh environments. The high radiation damage resistance diamond cubic carbon is a unique material which is considered for radiation environments. Beyond the intrinsic radiation hardness of crystal, the interaction between point defects and interfaces (e.g., grain boundaries) increases resistance to radiation damage. In effect, grain boundaries act as sinks for point defects and promote their mutual annihilation. Furthermore, interfaces can be exploited to obtain controlled segregation of point defects, thus avoiding the degradation of material properties due to uncontrolled point defect segregation and coalescence.In the present work, we investigate the interaction between vacancies – as a radiation defect in the bulk crystal – and (1 1 1) twist grain boundaries in crystalline diamond using atomistic simulation with analytical bond order potential. Based on previous observations on structure–property relationships characterising the (1 1 1) twist grain boundaries, this work analyses the emergence of periodic segregation patterns, which are rationalised by the underlying interface structure and their periodic intrinsic coordination defect populations. This observation suggests that randomly distributed vacancies in bulk single crystals can form self-organised defect arrays in the grain boundary. The coordination defect content correlates with the interface sink efficiency, providing guidelines for designing interfaces with high sink efficiency.

Wide-field probing of silica laser-induced damage precursors by photoluminescence photochemical quenching.

We describe a wide-field approach to probe transient changes in photoluminescence (PL) of defects on silica surfaces. This technique allows simultaneous capture of spatially resolved PL with spontaneous quenching behavior. We attribute the quenching of PL intensity to photochemical reactions of surface defects and/or subsurface fractures with ambient molecules. Such quenching curves can be accurately reproduced by our theoretical model using two quenchable defect populations with different reaction rates. The fitting parameters of our model are spatially correlated to fractures in silica where point defects and mechanical stresses are known to be present, potentially indicating regions prone to laser-induced damage growth. We believe that our approach allows rapid spatial resolved identification of damage prone morphology, providing a new pathway to fast, non-destructive predictions of laser-induced damage growth.

In situ monitoring the effects of Ti6Al4V powder oxidation during laser powder bed fusion additive manufacturing

Making laser powder bed fusion (L-PBF) additive manufacturing process sustainable requires effective powder recycling. Recycling of Ti6Al4V powder in L-PBF can lead to powder oxidation, however, such impact on laser-matter interactions, process, and defect dynamics during L-PBF are not well understood. This study reveals and quantifies the effects of processing Ti6Al4V powders with low (0.12 wt%) and high (0.40 wt%) oxygen content during multilayer thin-wall L-PBF using in situ high speed synchrotron X-ray imaging. Our results reveal that high oxygen content Ti6Al4V powder can reduce melt ejections, surface roughness, and defect population in the built parts. With increasing oxygen content in the part, there is an increase in microhardness due to solid solution strengthening and no significant change in the microstructure is evident.

A Survey on Zeolite Synthesis and the Crystallization Process: Mechanism of Nucleation and Growth Steps

Zeolites, as a class of crystalline minerals, find a wide range of applications in various fields, such as catalysis, separation, and adsorption. More recently, these materials have also been developed for advanced applications, such as gas storage, medical applications, magnetic adsorption, and zeolitic-polymeric membranes. To effectively design zeolites for such intriguing applications, it is crucial to intelligently adjust their crystal size, morphology, and defect population in relation to crystal perfection. Optimizing these fundamental parameters necessitates a deep understanding of zeolite formation mechanisms, encompassing the thermodynamics and kinetics of nucleation steps as well as crystallite growth. In this review, we discuss the formation of zeolites from this perspective, drawing on recent studies that highlight new achievements in remodeling and modifying zeolite synthesis routes. The ultimate aim is to provide better comprehension and optimize the functionality of zeolites for the aforementioned applications.

Double perovskite structured Ca2MgWO6:Sm3+ nanophosphor: Tailored for future-generation WLEDs and dosimetry applications

In fast-growing technologies, the development of innovative materials which can be used in multiple applications to fulfill the present necessary needs is highly motivated. Hence, in the present work, amber red-emitting double perovskite structured Sm3+ ions-doped Ca2MgWO6 nanophosphors were synthesized via the solution combustion route. The structural studies confirm the monoclinic crystal phase of the prepared nanophosphors. The energy band gap of the prepared nanophosphors was estimated and found to be ∼ 3.86 – 3.97 eV. The photoluminescence emission spectra showcase three intense and sharp peaks centered at ∼ 570, 603, and 645 nm, which ascribed to the 4G5/2 → 6H5/2, 4G5/2 → 6H7/2 and 4G5/2 → 6H9/2 transitions of Sm3+ ions, respectively. The photometric properties of the nanophosphors revealed amber-red region emission with ∼ 99.9% of chromatic purity. Thermoluminescence glow curves of the synthesized nanophosphors after irradiation of 10 KGy doses of γ-rays exhibit clear distinctive peaks at ∼ 140, 232, and 264 °C, which ascribed to increased luminescent trapping levels and defect population. The thermoluminescence linear response of the Ca2MgWO6:Sm3+ (3 mol%) nanophosphors within a wide range of 0.1–10 KGy of γ- ray doses are noticed. The various kinetic and trapping parameters were estimated via the computerized glow curve deconvolution method. The aforementioned results revealed that the prepared nanophosphors can be used for both future-generation white light-emitting diodes and dosimetry applications.

The role of Cr, P, and N solutes on the irradiated microstructure of bcc Fe

The objective of this study is to understand irradiation-induced and assisted defect evolution in binary body center cubic (bcc) Fe-based alloys. The broader class of bcc ferritic alloys are leading candidates for advanced nuclear fission and fusion applications, in part due to their exceptional void swelling resistance. However, their irradiated microstructure evolution is sensitive to solute species present, since these solutes can act as traps for irradiation-induced defects due to the surrounding tensile or compressive stress fields. Here, three alloys (Fe-9.5%Cr, Fe-4.5%P, and Fe-2.3%N) are selected for study because they systematically exhibit varying solute sizes and solute positions (i.e., substitutional or interstitial). Ex situ and in situ ion irradiations reveal that Fe-P has a considerably finer and denser population of irradiation-induced defects than Fe-Cr and Fe-N at the same irradiation conditions, which is attributed to strong defect trapping at undersized substitutional P, consequently hindering the development of extended defects. Meanwhile, oversized substitutional solutes (e.g., Cr) and interstitial solutes (e.g., N) may also suppress dislocation loop development due to weak solute-defect trapping.

Deformation and failure of the CrCoNi medium-entropy alloy subjected to extreme shock loading.

The extraordinary work hardening ability and fracture toughness of the face-centered cubic (fcc) high-entropy alloys render them ideal candidates for many structural applications. Here, the deformation and failure mechanisms of an equiatomic CrCoNi medium-entropyalloy (MEA) were investigated by powerful laser-driven shock experiments. Multiscale characterization demonstrates that profuse planar defects including stacking faults, nanotwins, and hexagonal nanolamella were generated during shock compression, forming a three-dimensional network. During shock release, the MEA fractured by strong tensile deformation and numerous voids was observed in the vicinity of the fracture plane. High defect populations, nanorecrystallization, and amorphization were found adjacent to these areas of localized deformation. Molecular dynamics simulations corroborate the experimental results and suggest that deformation-induced defects formed before void nucleation govern the geometry of void growth and delay their coalescence. Our results indicate that the CrCoNi-based alloys are impact resistant, damage tolerant, and potentially suitable in applications under extreme conditions.

Defect annealing in heavy-ion irradiated tungsten: Long-time thermal evolution of saturated displacement damage at different temperatures

Understanding the time-dependence of thermal evolution and recovery of displacement damage is of great significance for evaluations of material performance in harsh fusion environments. In this study, the long-time thermal evolution of irradiation-induced defects has been investigated in heavy-ion irradiated tungsten at doses above the saturation limit. Polycrystalline tungsten samples were irradiated with 6 MeV copper ions up to 0.6 dpa at room temperature, followed by a series of isothermal annealing experiments at 653 and 1123 K for 5000, 10000 and 20000 s, respectively. Annealing at both temperatures triggered the coevolution of interstitial and vacancy defects, causing changes in defect morphology, size and population statistics. The damage microstructure remained stable after 5000 s at 653 K. Subtle variations in defect density and size were found, indicating the quasi-thermal-equilibrium state of the damage microstructure. In this temperature regime, small defect clusters were depleted via annihilation and absorption, while large defect clusters and defect-impurity complexes were pinned by traps requiring further thermal activation. Pronounced time-dependence in defect evolution was found at 1123 K. Defect recovery and defect coarsening became more prominent with increasing duration. Almost all dislocation networks dissociated into individual dislocation lines after 20000 s, while nano-sized voids and loops became fewer but gained an increase in size. These findings can be attributed to the collective impact of the increasing mobility of small vacancy clusters, the continual dissociation of large vacancy clusters and small voids, and the thermally activated detrapping of defect clusters from impurities and/or other traps.

Revealing Site Occupancy in a Complex Oxide: Terbium Iron Garnet.

Complex oxide films stabilized by epitaxial growth can exhibit large populations of point defects which have important effects on their properties. The site occupancy of pulsed laser-deposited epitaxial terbium iron garnet (TbIG) films with excess terbium (Tb) is analyzed, in which the terbium:iron (Tb:Fe)ratio is 0.86 compared to the stoichiometric value of 0.6. The magnetic properties of the TbIG are sensitive to site occupancy, exhibiting a higher compensation temperature (by 90 K) and a lower Curie temperature (by 40 K) than the bulk Tb3 Fe5 O12 garnet. Data derived from X-ray core-level spectroscopy, magnetometry, and molecular field coefficient modeling are consistent with occupancy of the dodecahedral sites by Tb3+ , the octahedral sites by Fe3+ , Tb3+ and vacancies, and the tetrahedral sites by Fe3+ and vacancies. Energy dispersive X-ray spectroscopy in a scanning transmission electron microscope provides direct evidence of TbFe antisites. A small fraction of Fe2+ is present, and oxygen vacancies are inferred to be present to maintain charge neutrality. Variation of the site occupancies provides a path to considerable manipulation of the magnetic properties of epitaxial iron garnet films and other complex oxides, which readily accommodate stoichiometries not found in their bulk counterparts.

Influence of powder size on defect generation in laser powder bed fusion of AlSi10Mg alloy

The particle size distribution (PSD) often changes as powders are cycled for recycled and different powder batches are used, challenging a critical aspect of quality assurance in the Laser Powder Bed Fusion (LPBF) process. It is therefore important to understand how powder PSD affects the processability of powder in the context of LPBF. In this work, a batch of AlSi10Mg powder was sieved into four sets of powder with controlled PSD (D50 ranging from 16 μm to 76 μm). Single-track and cuboid samples were made from powders with variations in laser power whiling fixing other laser processing parameters constantly. It is shown that coarser powders are much more sensitive to changes in laser power in terms of melt pool characteristics, defect population and size, showing significant improvements in relative density upon increasing laser power. The simulations reveal with the combination of low laser power and coarse powders, pores mainly originate from insufficient melting of powders at the sides of melt pools due to insufficient and inhomogeneous absorption of laser energy. It is possible to achieve nearly fully-dense parts when processing coarse powders given that the incident laser beam energy is sufficient.

Assessment of the Critical Defect in Additive Manufacturing Components through Machine Learning Algorithms

The design against fatigue failures of Additively Manufactured (AM) components is a fundamental research topic for industries and universities. The fatigue response of AM parts is driven by manufacturing defects, which contribute to the experimental scatter and are strongly dependent on the process parameters, making the design process rather complex. The most effective design procedure would involve the assessment of the defect population and the defect size distribution directly from the process parameters. However, the number of process parameters is wide and the assessment of a direct relationship between them and the defect population would require an unfeasible number of expensive experimental tests. These multivariate problems can be effectively managed by Machine Learning (ML) algorithms. In this paper, two ML algorithms for assessing the most critical defect in parts produced by means of the Selective Laser Melting (SLM) process are developed. The probability of a defect with a specific size and the location and scale parameters of the statistical distribution of the defect size, assumed to follow a Largest Extreme Value Distribution, are estimated directly from the SLM process parameters. Both approaches have been validated using literature data obtained by testing the AlSi10Mg and the Ti6Al4V alloy, proving their effectiveness and predicting capability.