Microstructure Formation Research Articles

Synergetic Improvement of Flexoelectric Coefficient in liquid Crystal Embedded Flexible PVDF Polymer Composite for Energy Harvesting Applications.

Flexoelectricity is the universal electric polarization of dielectrics upon exertion of a non-uniform strain gradient. With the advancement of nano-technology and miniaturization of electronic devices, flexoelectricity holds the promise to address the power requirements for such device operation. The direct flexoelectric effect in liquid crystal (LC) embedded Poly(vinylidene fluoride) (PVDF) polymer films is examined for the first time by the application of external strain on the films. Physical characterizations such as DSC, dielectric spectroscopy, XRD and FESEM are carried out to study the composite films' intrinsic and extrinsic properties like dielectric, crystallinity and morphologies. The value of the flexoelectric coefficient (μ12) increases with the concentration of LC incorporation. At 3wt%, μ12 attains a maximum value of 68 nC/m, which is more than a three-fold increase compared to that of the pure PVDF film. The role of Maxwell Wagner Sillars (MWS) polarization in determining flexoelectric polarization in polymer composites is also discussed. Moreover, the influence of the microstructure and domain size formation in determining the flexoelectric response are discussed in detail to infer the behaviour of the flexoelectric coefficients of the films. Potential device applications based on this phenomenon have been proposed for future research in sensing and actuation. This article is protected by copyright. All rights reserved.

Faulty machining and micro-segregation assisted fatigue failure of cylinder head stud of a diesel generator

A detailed failure analysis of cylinder head stud bolt of a four-stroke diesel generator has been presented. The study included optical emission analysis, tensile testing, hardness testing, optical microscopy, SEM, and EDX analyses. The investigation showed that the main mechanism of failure was corrosion-fatigue. Fatigue initiation was assisted by pitting corrosion induced by faulty machining, moisture, and presence of tempered martensite and non-metallic inclusions at the surface. Propagation of the fatigue crack was encouraged by localized corrosion within the tempered martensite, non-metallic inclusions, and Mn/Cr/Mo/Si micro-segregation. Both excessive C and Mn/Cr/Mo/Si micro-segregation caused the formation of mixed microstructure of tempered martensite and fresh martensite with retained austenite. The non-uniform microstructure led to non-uniform hardness distribution and caused localized deformation during cyclic loading and promoted fatigue crack propagation.

Prediction of the microstructure of cold-compacted Astaloy 85Mo with deep generative models

In powder metallurgy, iron-based powders are compacted to so-called green bodies, which undergo a subsequent sintering process. The microstructure of the green bodies affects the sintering process and the resulting properties of the produced component. Assessing the microstructure experimentally is time consuming and costly, while simulation approaches rely on simplifications that significantly affect relevant microstructural information. This work investigates the suitability of deep generative models to synthetically generate realistic micrographs of green bodies as an alternative to experiments and simulations. For that purpose experiments comparing Generative Adversarial Neural Networks and Variational Autoencoders were conducted. The two deep learning frameworks were used to train models which produce synthetic micrographs in dependence of two parameters, namely powder particle size and green body porosity. The data utilized to train and evaluate the models was acquired from compacted water-atomized Astaloy 85Mo via scanning electron microscopy. Besides visual inspection, the trained models were evaluated via quantitative metrics. Distributions of relevant microstructural parameters such as pore perimeter or minimum and maximum pore feret diameter of the synthetically generated micrographs were compared to those of experimentally acquired micrographs. The results indicate that Generative Adversarial Networks represent a promising approach to train models, which produce synthetic micrographs that capture relevant properties of green body micrographs in dependence of given parameters such as powder particle size. In contrast, the Variational Autoencoders trained for this study did not capture microstructural properties well.

Physics-Informed Machine Learning of Argon Gas-Driven Melt Pool Dynamics

Abstract Melt pool dynamics in metal additive manufacturing (AM) is critical to process stability, microstructure formation, and final properties of the printed materials. Physics-based simulation including computational fluid dynamics (CFD) is the dominant approach to predict melt pool dynamics. However, the physics-based simulation approaches suffer from the inherent issue of very high computational cost. This paper provides a physics-informed machine learning method by integrating the conventional neural networks with the governing physical laws to predict the melt pool dynamics such as temperature, velocity, and pressure without using any training data on velocity. This approach avoids solving numerically the non-linear Navier-Stokes equation, which significantly reduces the computational cost (if including the cost of velocity data generation). The difficult-to-determine parameters' values of the governing equations can also be inferred through data-driven discovery. In addition, the physics-informed neural network (PINN) architecture has been optimized for efficient model training. The data-efficient PINN model is attributed to the extra penalty by incorporating governing PDEs, initial conditions, and boundary conditions in the PINN model.

Study of Nanostructured Layer Growth on Plasma-Facing Materials Irradiated with Helium Plasma

Tungsten and stainless steel samples were irradiated with stationary helium plasma in the plasma linear multicusp plasma device. The surface of the material is modified under the influence of helium plasma with the formation of nanostructures and microstructures on the surface. The fluence of helium ions equal to 8 × 1027 ions/m2 was achieved on the tungsten sample. Depending on the helium ion fluence, fuzzlike layers, loops, and bubbles of 20- to 500-nm scale were formed on the tungsten surface. The fuzz layer thickness depends on the duration of plasma irradiation in a wide range of fluence. Saturation of the growth of the thickness of the tungsten fuzz layer was observed at a fluence of more than 8 × 1026 ions/m2. The growth of microstructures and nanostructures on the surface of stainless steel irradiated with helium plasma was observed. The growth of nanostructured layers is explained by a theoretical model considering the dynamics of adatoms under the influence of plasma.

Tailoring Microstructure and Mechanical Properties of Additively Manufactured Inconel 625 by Remelting Strategy in Laser Powder Bed Fusion

Tailoring Microstructure and Mechanical Properties of Additively Manufactured Inconel 625 by Remelting Strategy in Laser Powder Bed Fusion

Effect of Si concentration and substrate temperature on the microstructure and magnetic properties of FeCoNiCu0.5Six high entropy alloy films

It has been known that Si has the effect to increase the resistivity, permeability, and to decrease the magnetostriction coefficient in Si-containing steel as soft magnetic material. On the other hand, high-entropy alloys are intensively studied as soft magnetic materials in recent years, especially as thin films. In the present work, Si-containing (FeCoNiCu0.5Six) high entropy alloy films (HEAFs) have been prepared by magnetron co-sputtering under different deposition conditions. The microstructure and magnetic properties of FeCoNiCu0.5Six HEAFs have been investigated to clarify the effects of Si concentration on microstructure formation and magnetic properties. It has been found that Si addition strongly affects the magnetic properties of FeCoNiCu0.5Six HEAFs of FCC phase by decreasing the coercivity of the films, but hardly affects the phase formation in the films. On the other hand, it has been found that deposition of the films at moderate high substrate temperatures greatly improve the magnetic properties by increasing the saturation magnetization while keeping low coercivity.

A multiscale bifurcation analysis using micromechanical-based constitutive tensor for granular material

The current study presents a multiscale approach that investigates material instability and localization phenomena in plastic granular materials and the discrete-continuum duality. A bifurcation and stability analysis in continuum mechanics usually requires the material’s tangent (stiffness) operator, the computation of which in a micromechanical approach such as Discrete Element Modeling (DEM) requires specific treatment. To bridge the discrete and continuum worlds, a new computational approach incorporating strain probing is proposed to reconstruct the elastoplastic constitutive tensor and its spectral characteristics from DEM simulations. The probing technique permits the computation of the tangent operator that inherits microstructural information from the discrete world to analyze bifurcation in elastoplasticity at the macro level. An incrementally linear constitutive tensor is computed, distinct for each of the ensemble of probing directions belonging to a particular tensorial zone or sector of incremental stress or strain space, thus making it directionally non-linear. Following such an approach, material instability can be evaluated from the spectral characteristics of the tangent constitutive tensor deduced from DEM probing calculations belonging to an identified tensorial zone. A meso-scale analysis is finally offered to detect shear band localization through the well-known Rice’s criterion as a continuum-based concept extended to a micromechanical discrete modeling framework. These new numerical results show that the multiscale proposed approach, which allows access to microstructural information, is consistent with a continuum one such as when predicting the localization angle during shear banding in a granular specimen in DEM.

Mechanisms of microstructural evolution in high-volume TiC reinforced steel composites: Insights into particle coarsening, morphology, and thermochemical reactions

Titanium carbides reinforced steel matrix (TiC-steel) composites exhibit a range of excellent properties, including increased strength-to-weight ratio, high-temperature strength, and outstanding wear resistance. Prior research has demonstrated that the mechanical properties of TiC-steel composites are significantly affected by particle coarsening and the associated core-rim structure. Nevertheless, the fundamental mechanisms of thermochemical reactions that govern the formation of the final microstructure in these composites, including the core-rim structure, remain incompletely explored. This study elucidates the microstructure evolution mechanism of high-volume TiC-steel composites, particularly particle coarsening, through thermodynamic calculations and chemical analysis. The investigation unveils the coarsening process, shedding light on the formation of more stable phases and associated compositional changes, including the incorporation of substitutional elements and carbon depletion (resulting in low stoichiometry, x<1) within the newly developed (Ti,M)Cx. Additionally, the increased carbon content contributes to pearlite emergence within the steel matrix. Notably, key factors influencing the distinct morphologies observed in growing TiC particles are identified, attributed to variations in Mo, V, and W concentrations, as elucidated by rigorous thermodynamic and first-principles calculations. This study represents significant progress in understanding the intricate microstructure development of high-volume TiC-steel composites.

The influence of varying Cu doping concentrations on the microstructure, phase evolution and surface wettability of ceramic glazes modified with nano Cu-ZnO

This study intended to develop nano Cu-doped nano ZnO-modified ceramic glazes. In this scope, nano-sized ZnO and nano Cu powders were incorporated into a commercial glaze composition and then the modified glaze was applied to the surface using the spraying method. The glazed tiles were sintered in a laboratory furnace at 1210oC for 10 min. The surfaces of the sintered samples were analyzed using scanning electron microscopy. X-Ray diffractometer, surface profilometer and contact angle goniometer. The study examined the influence of varying levels of Cu doping on the microstructural evolution and formation of phases, surface roughness, as well as the contact angle. Additionally, wettability conversion experiments were conducted under UV light. As a result, the addition of Cu increased the presence of Willemite crystals and also increasing the quantity and size of Plagioclase crystals. The contact angle of the sintered commercial glaze coated surface was measured to be 51.8o. However, when nano ZnO was added, the contact angle reduced to 44.4o. Likewise, the contact angle of the Cu doped nano ZnO coated surfaces decreased even further to approximately 33o. The inclusion of nano ZnO in commercial glaze resulted in a decrease in the Ra value, while doping Cu led to a modest increase in the Ra value. UV irradiation resulted in a reduction in the contact angles of all surfaces.

Higher laser power improves strength but reduces corrosion resistance of Mg WE43 processed by powder bed fusion

Powder bed fusion – laser beam (PBF-LB) of Mg alloys provides new possibilities for the production of complex structures with optimized designs, both for weight reduction in aerospace applications, as well as for patient-specific implants in orthopedic applications. However, even though numerous studies have been carried out on the topic, the influence of the individual PBF-LB process parameters on the microstructure and resulting material properties of Mg alloys remains ambiguous. Thus, this study aims to investigate the influence of laser power on the surface roughness, microstructure and resulting key material properties, namely corrosion resistance and mechanical performance. Samples were produced by PBF-LB from gas atomized Mg-4%Y-3%Nd-0.5%Zr (WE43) alloy powder, using three different laser powers: 60 W, 80 W, and 90 W. Contrary to expectation, the 90 W samples exhibited the highest degradation rate, while 60 W samples had the lowest, despite the latter having highest surface roughness and large internal pores. The higher degradation rate for the 90 W samples was instead found to stem from the near-surface microstructure. The higher energy input and subsequently reduced grain size, resulted in an increased amount of second phase precipitates than for the 60 W samples, thereby increasing the tendency for pitting via microgalvanic corrosion. For the tensile strength and elongation at break, the opposite trend was observed. Here, a reduction in grain size and an increase in precipitates for the 90 W samples were found to be beneficial. In conclusion, a definite influence of laser power on the formation of microstructure was observed, ultimately impacting the resulting corrosion and tensile properties of WE43. Future work should investigate the influence of other PBF-LB process parameters, with the aim of establishing an optimum balance between corrosion resistance and mechanical properties.

Phase-field simulation framework for modeling martensite and bainite formation in steel

In this study, we present a combination of phase evolution, chemical diffusion, temperature evolution, and finite strain elasto-plasticity to simulate the martensitic and bainitic transformation using the phase-field software library OpenPhase (OpenPhase, 2023). It is demonstrated how the carbon concentration significantly influences the martensite start temperature and the resulting microstructure. Furthermore, the kinetics of the transformation is strongly influenced by plasticity. For bainitic transformation, it is demonstrated how the holding temperature significantly influences carbon partitioning and the resulting microstructure: higher holding temperatures allow increased carbon diffusion and partitioning, stabilizing retained austenite, which is in good agreement with experimental observations. The present study offers new insights into the microstructure formation mechanisms during martensitic and bainitic transformations in low-carbon steel and offers a consistent modeling approach to model complex phase transformation scenarios in steel and other construction materials.

Additive manufacturing of NiTi architected metamaterials

Additive manufacturing has revolutionized the creation of complex and intrinsic structures, offering tailored designs for enhanced product performance across various applications. Architected cellular or lattice structures exemplify this innovation, customizable for specific mechanical or functional requirements, boasting advantages such as reduced mass, heightened load-bearing capabilities, and superior energy absorption. Nonetheless, their single-use limitation arises from plastic deformation resulting from localized yield damage or plastic buckling. Incorporating NiTi shape memory alloys (SMAs) presents a solution, enabling structures to recover their original shape post-unloading. In this study, an NiTi architected metastructure, featuring auxetic behavior and a negative Poisson's ratio, was designed and fabricated via laser powder bed fusion (LPBF). The samples exhibit promising superelastic performance with recoverable deformation strains at room temperature. Comprehensive characterization processes evaluated the functional performance of the fabricated metastructures. The metastructure geometry promoted microstructure formation primarily along the wall thickness. Cycling compression tests, conducted at three applied force levels, demonstrated stable cyclic behavior with up to 3.8 % reversible deformation strain, devoid of plastic buckling or yielding damage. Furthermore, the NiTi metastructures displayed robust energy absorption capacity and damping behavior, underscoring their potential for reusable energy dissipators in various industries including aerospace, automotive, construction, and etc.

In Situ Analysis of Stress and Microstructure Evolution during Welding of High-Alloy Steels Using Energy-Dispersive X-Ray Diffraction

AbstractConstrained thermal expansion and contraction during welding cause a compression-tension cycle and plastic deformation in the heat-affected zone, leading to work hardening. The nature of this hardening effect—isotropic or kinematic—determines the final local yield stress and thus affects the residual stress state. Therefore, mechanical hardening must be modeled correctly in welding simulations for accurately predicting welding residual stresses. Previous studies, relying on comparisons with experimental ex situ results, led to different recommendations regarding the choice of the hardening model and thus require clarification. In this work, the stress evolution in the heat-affected zone of a tungsten inert gas weld is studied in situ using energy-dispersive x-ray diffraction and a novel method of stress analysis based on crystallite anisotropy. Additionally, microstructural information is gathered through line profile analysis. Results are shown for both austenitic and ferritic high-alloy steels and compared to ex-situ results including a validation of the new method of stress analysis. Finally, conclusions on the nature of work hardening are drawn.

An improved preparation method of core-shell Al@Sn-Bi microspheres and its microstructure formation mechanism

This paper presents an improved method for achieving controllable preparation of the composition and size of the monotectic alloy core-shell structure microspheres. This advancement is expected to enhance the commercial application of monotectic alloy core-shell structure microspheres. By installing a heating device at the top of the drop tube in the pulsated orifice ejection method setup, we achieved in-situ heat treatment of falling droplets, resulting in the successful preparation of Al45(Sn42Bi58)55 core-shell structure microspheres. Furthermore, we investigated the formation mechanism of the Al@Sn-Bi core-shell structure. Nine sets of Al-Sn-Bi microspheres with varying in-situ heat treatment temperatures were prepared. Analysis via X-ray diffraction and transmission electron microscopy revealed that the microspheres consisted of Al-rich, Sn-rich, and Bi-rich phases, without any intermetallic compounds. Scanning electron microscopy results indicated that as the in-situ heat treatment temperature increased, the structure of the Al-Sn-Bi microspheres transitioned from irregular to regular core-shell configurations. This suggests that higher heat treatment temperatures prolong liquid phase separation time, leading to increased repetition of Marangoni motion within the droplet, Ostwald ripening of the minor phase droplets, and Soret effect of the matrix phase. However, at temperatures exceeding 700 °C, the solidification rate of alloy droplets decreased, causing the Al-rich phase core to deviate from the center and form a crescent structure due to Stokes motion.

Complex Spatial Illumination Scheme Optimization of Backscattering Mueller Matrix Polarimetry for Tissue Imaging and Biosensing.

Polarization imaging and sensing techniques have shown great potential for biomedical and clinical applications. As a novel optical biosensing technology, Mueller matrix polarimetry can provide abundant microstructural information of tissue samples. However, polarimetric aberrations, which lead to inaccurate characterization of polarization properties, can be induced by uneven biomedical sample surfaces while measuring Mueller matrices with complex spatial illuminations. In this study, we analyze the detailed features of complex spatial illumination-induced aberrations by measuring the backscattering Mueller matrices of experimental phantom and tissue samples. We obtain the aberrations under different spatial illumination schemes in Mueller matrix imaging. Furthermore, we give the corresponding suggestions for selecting appropriate illumination schemes to extract specific polarization properties, and then provide strategies to alleviate polarimetric aberrations by adjusting the incident and detection angles in Mueller matrix imaging. The optimized scheme gives critical criteria for the spatial illumination scheme selection of non-collinear backscattering Mueller matrix measurements, which can be helpful for the further development of quantitative tissue polarimetric imaging and biosensing.

Towards higher-quality low salt surimi gels: Significance of the combinatorial effects of chickpea protein with transglutaminase on their micro-structures

The textural properties of low-salt hybrid gel made from silver carp (Hypophthalmichthys molitrix) and chickpea protein (CPP) with or without external transglutaminase (TGase) were investigated. A decrease in gel strength (P﹤0.01), hardness (P﹤0.01), storage modulus (G′) and loss modulus (G″) of the hybrid gel containing CPP appeared regardless of the addition of TGase. Furthermore, the addition of CPP combining with TGase significantly induced a significantly higher resilience (from 43.5% to 50.3%), cohesiveness (from 0.72 to 0.79), proportion of immobilized water (from 95.7% to 97.4%), number of pores (from 3181 to 4868) and apparent porosity (from 30.55% to 40.26%) than those of the group containing TGase alone, which indicated the combining effect of CPP and TGase on the textural properties. The SDS-PAGE profiles showed that the covalent complex between CPP and myofibrillar proteins was not observed. During the gelation of myofibrillar protein filaments, CPP wrapped the fibres shown by the microstructure morphology and increased the friction between fibres, leading to an increase in relaxation time. TGase promoted this effect and induced the formation of a denser microstructure. The present study provided a feasible approach for the preparation of high-quality low salt surimi products with more favorable texture and acceptability.

Enhancing load-bearing capacity of martensitic stainless steel resistance spot welds: Microstructural vs. geometrical approaches

The load-bearing capacity of resistance spot welds is determined by a combination of weld geometrical attributes, such as the size of the weld nugget (WN), and weld metallurgical features. When resistance spot welds are made on martensitic stainless steels (MSSs), their load-bearing capacity is reduced due to the formation of a brittle martensitic microstructure within the WN. In this study, we investigated two distinct approaches to enhance the mechanical properties of AISI420 MSS spot welds. The results indicated that manipulating the WN size did not effectively produce strong welds in both tensile-shear (TS) and cross-tension (CT) tests. Therefore, we focused on examining the impact of the weld microstructure by employing two different interlayers: low carbon steel (LCS) and pure nickel. The analysis using electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA), along with equilibrium and non-equilibrium phase diagrams, revealed that the microstructure of the WN with low carbon steel interlayer of various thicknesses exhibited similarities with spot welds made without an interlayer, and the solidification mode was still δ-ferrite. This microstructure primarily consisted of martensite (M) and likely residual delta ferrite (δ) phases, resulting in no significant improvements in mechanical properties. In contrast, the utilization of the nickel interlayer led to remarkable enhancements in mechanical properties. This improvement can be attributed to a shift in the solidification mode from primary delta (δ) to austenite (γ) within an optimal interlayer thickness. This transition facilitated the development of a predominantly austenitic (γ) microstructure, contributing to the formation of an exceptionally tough microstructure within the weld nugget. Consequently, the load-bearing capacity was significantly improved.

Bifidobacterium animalis subsp. lactis BB12-Derived Postbiotic Powders Enhance Antioxidant and Physicochemical Properties of Low-Fat Yoghurt

AbstractFortifying conventional foods with postbiotic powders (PP) is an effective approach for the production of functional products. Preserving both functional properties of PP and the physical characteristics of fortified products is essential during this process. This study aimed to investigate the antioxidant activity and consumer testing of low-fat yoghurt fortified with PP and changes in their physical properties, including rheology, water holding capacity, and microstructure over a 21-day storage period. PP, derived from Bifidobacterium animalis subsp. lactis BB12 grown in cheese whey (CW) and skim milk (SM), was added to the product at 1% individually (B12-CW and B12-SM) and in a mixture (BB12-CW-SM). The results indicated that the antioxidant activity of the samples fortified with PP was 4.6 − 6.3%, almost double the values in samples without PP, despite their similar viability of starters (Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus) (&gt; 8.5 log cfu/g). Adding BB12-CW significantly reduced mouthfeel, flavor, and overall acceptability of the product, while adding BB12-SM and BB12-CW-SM did not alter any consumer acceptability. Similarly, regardless of PP types, their fortification had negligible effects on viscosity, viscoelastic properties, shear-thinning behaviors, water-holding capacity, and microstructure formation, which remained nearly unchanged during storage. Overall, the addition of PP, particularly BB12-SM and BB12-CW-SM, significantly increased antioxidant activity while preserving important physical and consumer acceptance of yoghurt. These findings underscore the potential of postbiotics as functional ingredients, enhancing both the nutritional values and consumer appeal of yoghurt, thereby promoting its health benefits.

Advanced control of microstructure in ice cream by agitation and mixing during freezing process

Agitation and mixing play an important role in ensuring the appropriate dispersion of ice crystals, bubbles, and fat globules during the freezing process in ice cream manufacturing. The dispersion determines the internal structure of ice creams and hence, the ice cream quality. To achieve advanced control of the internal structure of ice creams, in this study, the effect of agitation on air entrainment, micronisation of air bubbles, and fat globule agglomeration was investigated using a batch-type freezer with an anchor impeller. Besides, rheological properties of an ice cream sample that was stored in the refrigerator for 24 h after freezing were analysed. In addition to steady agitation, unsteady agitation with a periodical change in the rotation direction was employed. Temperature measurement and flow observation suggested more effective scraping of frozen phase from the cooling surface for unsteady agitation than for steady agitation. Furthermore, air entrainment and fat globule agglomeration during freezing were promoted under unsteady agitation. Consequently, the sample manufactured under unsteady agitation exhibited enhanced thermal robustness compared to that manufactured under steady agitation. Thus, unsteady agitation can be an effective strategy to control the formation of microstructures in ice cream.