Microstructure Size Research Articles

In-situ EBSD-DIC simulation of microstructure evolution of aluminum alloy welds

A comprehensive understanding of the dynamic evolution of weld microstructure under external loads can provide key insights for high-performance laser welding. A novel in-situ EBSD-DIC simulation method is introduced to study the microstructure evolution of laser welded aluminum alloys under uniaxial tensile action. An advanced crystal plastic finite element model (CPFEM) is developed, which combines the real microstructure, grain orientation and grain size effects. The results show that the dislocation density in columnar grain zone is higher than that in equiaxed grain zone. The continuous accumulation of dislocations in the columnar region results in a high work-hardening rate. This high work hardening rate enhances the plastic deformation capacity of the columnar crystal region, resulting in local strain concentration. Columnar zones are more prone to fracture because the high-strain region is a potential fracture site. In addition, low Angle grain boundary (LAGBs) is one of the reasons that the dislocation density of the columnar grain zone is higher than that of equiaxed grain zone during tensile process, which is conducive to dislocation slip of columnar grains. This study is a fundamental innovation in simulating the microstructure evolution of laser welding. This marks a major breakthrough in simulating the evolution of crystallographic features such as grain orientation, microstress and strain and dislocation density under external loads. This work can further provide practical guidance for “microstructure characteristics - mechanical property regulation”.

Mechanism, microstructure and performance of novel electron beam freeform fabrication compared with conventional method

This paper proposes a new method: the pre-melting electron beam freeform fabrication method. This new method aims to mitigate the excessive heat input associated with directly forming a molten pool on the deposit and the poor formation caused by severe agitation of the molten pool. Consequently, it seeks to refine the internal microstructure of the part and enhance its mechanical properties. Using TC4 alloy as the research subject, this study performs a comparative analysis between the EBF and PEBF methods in terms of structural composition, deposit formation, cross-sectional morphology, microstructure, tensile strength, fracture path, and grain size. The research findings indicate that the PEBF method, due to its significantly lower heat input, results in the maximum α phase width being reduced to 1/4 to 1/5 of that observed with the EBF method. This reduction in α phase width leads to an increase in the tensile strength of the deposit from 784 to 840 MPa and an enhancement in the fracture strain from 0.22 to 0.34, which is close to the strength difference (68 MPa) calculated for the two methods using the Hall-Petch equation. The comparative results underscore the superiority of the PEBF method, demonstrating its potential to improve mechanical properties.

On the atomistic origin of internal length scale in strain-gradient plasticity models: The case of grain boundary structures and energies

The mechanical behavior of polycrystalline materials is controlled by microstructural size effects such as grain size or precipitate size. Various models of strain gradient plasticity have been proposed to capture such size effects, many of which have incorporated geometrically-necessary dislocation (GND) densities to introduce characteristic internal lengths. Recent developments have focused on models that incorporate a GND density into the internal energy functional. In such models, one needs to physically justify the functional form chosen and quantify the inherent internal length parameter. Our present study aims at probing relevant forms and internal length values in the case of grain boundary (GB) atomistic structures and core energies. We use an atomistic-to-continuum crossover approach that predicts an atomistic structure dependent GB energy by molecular static simulations, which is then recovered at the continuum-level by using a strain gradient, atomistically informed, field dislocation mechanics fast Fourier transform model. This allows (i) delineating the atomistic structure of GBs using an equivalent Nye GND density, and (ii) capturing the associated continuous elastic fields in the GB core area. We probe (i) a generalized non-quadratic GND density dependent energy functional to account for the core energy of defects, and (ii) elucidate the contributions of core versus elastic energy to the overall GB excess energy. We investigate and discuss the possible relevant choices for the energy functional form, as well as the physical origin of the inherent internal length parameter and its dependence to the types of grain boundaries, atomistic structures, and spatial resolution.

Atomistic and Theoretical Insights on Nanoprecipitates-Controlled Strengthening Behavior for Optimum Mechanical Performance in Nanotwinned NiCoCr Alloys

Abstract The nanoprecipitates and nanotwins enable to improve the mechanical performance of NiCo-based alloys. In this work, the molecular dynamics (MD) simulations are performed to investigate the strengthening mechanisms of nanotwinned medium-entropy NiCoCr alloys with various distributions and volume fractions of nanoprecipitates. MD simulations reveal that mechanical performance for the precipitates located in twin boundaries is better than that located in the twin lamellae. The precipitate-induced strengthening makes the nanotwinned NiCoCr alloys to achieve the maximum flow stress during increasing the precipitate volume fraction. The influences of volume fraction and distribution of the precipitate on winding and cutting mechanisms are analyzed comprehensively. The dislocation winding behavior, hindered twin boundaries deformation, and the adjacent precipitates connection control the precipitate strengthening mechanisms. A dislocation-based theoretical model is developed to forecast the size-dependent flow stress of nanotwinned metals with nanoprecipitates, in which the Orowan bypass mechanism and the dislocation pile-up behaviors are involved. The relationship between the microstructural size and the flow stress of nanotwinned metallic materials with nanoprecipitates is explored. The predictions for the flow stresses varied with the precipitate volume fraction are agreeable well with the results of MD simulation. The predicted maximum flow stresses and the corresponding critical volume fractions of nanoprecipitates are sensitive to the microstructural sizes.

Microstructural evolution, crystallographic texture, grain morphology, and mechanical integrity of wire arc additively manufactured Inconel 625 alloy

The material in wire arc additive manufacturing (WAAM) undergoes complex material flow and multiple thermal heating and cooling cycles, forming highly heterogeneous microstructures in terms of size, crystallographic orientations, and mechanical properties. The inhomogeneity also depends on the dislocation density and phases, which are influenced by the thermal history of the process. In this study, the Cold Metal Transfer (CMT) process was used to deposit a 60-layer build of Inconel 625 alloy. Detailed variations in the microstructural size, orientations, and phases along the building direction were studied using optical microscopy, electron backscatter diffraction (EBSD), and X-ray diffraction (XRD). Microstructural observations reveal dendrites, equiaxed crystals, cellular, and columnar structures with primary and secondary dendrites. Dynamic recrystallization (DRX) followed by abnormal grain growth was found in the build. The average grain size varies with deposited height, with a grain size of around 13 ± 1 μm near the substrate, 45 ± 1 μm in the middle region, and 18 ± 1 μm at the top. The top region exhibited a strong intensity of recrystallized Cube, Cube-ND, and Cube-RD textures, with weaker intensities of copper and brass textures. The middle and bottom regions show strong intensities of Goss, copper, F, S, and E textures, respectively. The highest dislocation density of 5.122 × 10−4 nm−2 was found in the top region, while the lowest (4.14 × 10−4 nm−2) was observed in the bottom region. The ultimate tensile strength of the build ranged from 603 ± 05 MPa to 699 ± 10 MPa, while the yield strength varied from 313 ± 07 MPa to 365 ± 08 MPa along different orientations. Vickers hardness results showed a slight variation, from 240 ± 5 to 260 ± 2 HV, from bottom to the top of the deposited build. The findings from this study provide valuable insights into the microstructural evolution mechanism and mechanical behavior of WAAM-fabricated Inconel 625, which can guide other researchers in optimizing process parameters, enhancing material properties, and understanding the effects of thermal history on additive manufacturing of high-performance alloys.

Failure Analysis of Burst Tube Caused by Corrosion Thinning of the Tube Wall of Boiler Economizer

Abstract A boiler economizer is in continuous operation during several burst pipes. Burst tubes are located near the elbow below the weld, and the weld on both sides of the tube wall is thinning seriously. There is a flue gas formation vortex in the furnace chamber; the outer wall of the tubes is thicker ash, and fins and tubes are corroded to varying degrees. This study employed meticulous macroscopic observation, precise chemical composition analysis, detailed metallography, advanced scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) to determine the causes of the coal economizer tube burst and tube wall thinning, ensuring the accuracy of the findings. The chemical composition, microstructure, and grain size of the base material met the requirements of the relevant standards. The heat-affected zone and weld seam show the Widmannstatten structure, a superheated tissue caused by excessive heat input during welding. The outer wall of the pipe corrosion is more serious, resulting in serious wall thinning, and ultimately, the pipe bursts, unable to withstand the pressure inside the pipe. It was eventually determined that the cause of the pipe burst was that the operating temperature of the economizer was close to the dew point, and the flue gas contained sulfur dioxide, sulfur trioxide, and hydrogen chloride that met water vapor at that temperature to form acid. Acid on the outer wall of the pipe was caused by flue gas dew point corrosion, seriously causing the pipe wall to thin. When the pipe wall is thinned to a certain extent due to corrosion, it cannot withstand the pressure inside the pipe, and rupture will occur.

MAIN STAGES OF DESIGNING RESOURCE-SAVING TECHNOLOGIES FOR INGOT DEFORMATION ON HYDRAULIC PRESSES

Objective. To study the technological process of forging large ingots on hydraulic presses in order to identify and reduce resource consumption. Research methods. A finite element method that makes it possible to assess the stress-strain state of a workpiece, the possibility of levelling it, and homogenising it by controlling the factors that form the optimal forging method for a given workpiece. Results. A resource-saving technological process based on the optimal forging method has been developed, which allows to bring the quality of the designed products to a new level and leads to an increase in technical and economic indicators of production. By controlling the stress-strain state of the metal, high quality forged products can be achieved and resource-saving technologies for forging forgings of high-alloy steel grades and alloys can be created. Scientific novelty. The factors that form the rational resource-saving technological process of plastic deformation and the method of forging large forgings from alloyed, stainless steels and alloys on hydraulic presses, as well as the directions of their optimisation, have been formed. The finite element method allows us to predict the distribution fields of the workpiece’s stress-strain parameters, metal microstructure, and grain size. Practical value. Practically grounded recommendations for optimal modes of forging ingots from tool steel grades were developed. This will reduce energy consumption, save time in the production of forged products and generally intensify the process of plastic deformation. The proposed recommendations can be applied not only in the processes of forging tool steel grades but also in other types of hot plastic deformation of metals of a wide range.

Optimization of Magnetic Field-Assisted Laser Cladding Based on Hierarchical Analysis and Gray Correlation Method

Process parameters directly affect the quality of laser cladding. In this study, magnetic field-assisted laser cladding experiments were carried out on the surface of 300 M ultra-high-strength steel by setting laser energy density, magnetic field strength, and frequency as processing parameters. The optimization of laser cladding process parameters was investigated based on evaluating the quality of the laser cladding layer through hierarchical analysis and gray correlation analysis. Based on orthogonal test data, the correlation coefficients of the process parameters with the single objective function and the correlation degree of the multi-objective function were calculated by using the gray theory. Then the comprehensive objective optimization was carried out according to the gray correlation degree. The optimization problem with multiple process objectives was transformed into a single gray correlation degree optimization method to realize the optimization of process objectives and obtain the optimal combination of process parameters. The validation experiments indicate that the quality of the laser cladding layer can be greatly improved by employing optimal process parameters. The optimized laser cladding layer shows a reduced microstructure size and enhanced wear resistance, indicating the effectiveness of the optimization approach.

Numerical simulation of GTAW for ZW61 magnesium alloy thin plates: Coupling the finite element method with the cellular automata method

ZW61 magnesium alloy has a wide range of application prospects as a lightweight green engineering material. In this paper, the temperature field and microstructure of gas tungsten arc welding (GTAW) for ZW61 magnesium alloy are simulated by the finite element method and the cellular automata (CA) method. The results show that the microstructure in the center of the fusion zone (FZ) is all equiaxed grains affected by compositional supercooling. While at the edge of the molten pool, the crystals produced by associative crystallization evolve into columnar grains after undergoing competitive growth. Furthermore, the temperature field of the molten pool alters as the welding heat input increases. Especially, the temperature gradient behind the molten pool slows down. Thus, the cooling rate during solidification of the molten pool decreases, increasing the size of the weld microstructure. Meanwhile, solute concentration plays an essential role in the weld microstructure evolution. The rise in Zn content both refines the size of the equiaxed grains and inhibits the growth of the columnar crystals. Moreover, the experimental results of the thermal cycling curves and the FZ microstructure exhibit minimal error with the simulation results, verifying the reliability of the model.

The behaviour of micro-injection moulding inserts produced with material jetting technology

The increasing interest in additive manufacturing and advancements in precision and production quality have sparked attention to its use in industrial prototyping. High-performance polymer resins enhance flexibility in mass production processes like micro-injection moulding. This flexibility allows for reconfigurable moulds using resin inserts, enabling the transition of devices, such as microfluidics, from labs to large-scale production. Despite progress, limitations exist in additive manufacturing, including the minimum size of microstructures and the brittle behaviour of resin inserts during moulding cycles, impacting overall production capacity. This study focuses on using a micro-injection moulding machine to reproduce PMMA thin plates with single straight microchannels (250 μm) and with different side wall angles (0°, 15°, and 30°), investigating the best compromise between capability and insert resistance to moulding cycles. A careful study of the dynamics leading to the insert failure and the resin limits was carried out. The results showed that it is possible to produce about 15 micro-structured thin plates with good accuracy using a resin insert realised with Material Jetting technology.

Synthesis of Ag Nanowires with High Aspect Ratio for Highly Sensitive Flexible Strain Sensor

AbstractIn recent years, the research and development of various flexible wearable devices have rapidly advanced due to the continuous introduction of flexible electronic product. These devices have found applications in health monitoring, cardiovascularcare, internal and external workload, and more.Flexible sensors, being a vital component of flexible devices, determine the functionalities and performance capabilities of the devices. Metal nanomaterials, especially silver nanowires, are widely used in flexible sensors in past research due to their great advantages in flexibility and sensitivity. In this work, Silver nanowires with aspect ratios of 600, 1000, and 1400 were synthesized by adjusting the synergy of Br‐ and Cl‐, along with other process parameters, leading to an improved production efficiency of silver nanowires.The stability of the conductive network is enhanced when the aspect ratio of silver nanowires is 1000 and 1400.The sensor demonstrated high sensitivity at high strain, along with an extended strain range.Moreover, it was observed that increasing the aspect ratio of silver nanowires led to a more stable conductive network, thus enhancing the sensor's stability with over 10000 stretching cycles. under the appropriate deposition density, silver nanowires with aspect ratio of 600 have high sensitivity to low strain, and silver nanowires with aspect ratio of 1400 have high sensitivity to high strain, up to 247.3. Introducing microstructures on the surface of PDMS resulted in an increased maximum sensitivity of the sensor with decreasing microstructure size, reaching a maximum sensitivity of 322.2.

Strain energy density maximization principle for material design and the reflection in trans-scale continuum theory

Traditional efforts in the design of damage-tolerant structural materials were largely exercises in optimizing the combination of strength and ductility. However, the simultaneous consideration of these two conflicting mechanical indices, improving one inevitably sacrifices the other, makes the design extremely complex and difficult, due to the dilemma of choosing between them. Here, physically guided by the energy variational expression in trans-scale continuum mechanics theory, we propose a general mechanics principle for material design that involving only one index: towards strong and tough material the strain energy density limit (w) should be maximized, i.e., strain energy density maximization principle, referred to as wmax principle. It aims to guide the attainment of exceptional comprehensive mechanical properties, while circumventing the dual-index dilemma by employing a singular index w. Extensive experimental data analyses prove that (i) the maximum wmax always exists, at a critical dimension of characteristic microstructure dc,micro, and (ii) w can effectively index strength-ductility synergy and the wmax is conjugated with both high strength and high ductility, verifying the validity of wmax principle. The universality, practicality and downward compatibility are also examined. The dc,micro approaches twice the span of strain gradient region around internal boundary, suggesting that the microstructure state with wmax is the critical state with strongest strain gradient. Importantly, the w improvement as a function of the characteristic size of either microstructure or deformation field can be well captured by strain gradient theory, confirming the consistence between the wmax principle, experimental results and trans-scale continuum theories. This principle opens up a new design concept for advanced structural materials from the perspective of microstructure-w-mechanical properties relationship.

Experimental simulation of the ancient production of gold granules

Granulation is an ancient and sophisticated decorative technique. The production of granules is a crucial part of this process but has rarely been studied. The present study employed three techniques—pouring method, heating method, and crucible method—to produce gold granules. The success ratio of granule formation, granule surface morphology, microstructure, and formation were analyzed to identify the techniques used in archaeological objects. The cooling medium significantly influenced small granule formation, microstructure, and grain size. Both heating and crucible methods could control the granule formation, but these methods produced distinct microstructures. Based on these experimental granules, the probable production methods of ancient gold balls were identified. The present study provided microscopic information for determining ancient gold granule production techniques.

Temperature gradient mechanism in the heterogeneous nucleation of inoculated alloy: A quantitative phase-field study

The grain size of solidification microstructure has a significant influence on the mechanical properties of castings. Generally, smaller grain size makes the strength of castings higher. Therefore, inoculation is an effective strategy to reduce grain size and improve casting performance. In this work, the phase-field method combined with the free growth model is employed to simulate the solidification of inoculated aluminum alloy with temperature gradient. The simulations indicate that a higher temperature gradient slows the progression of already nucleated grains into impingement growth and enhances the undercooling of the melt ahead of the solid-liquid interface. Hence, more inoculants have chance to get the corresponding nucleation undercooling, promoting grain nucleated sequentially along the gradient direction. These results are consistent with previous real-time observed experiments. This work may provide guidance on improvement of the theoretical models for predicting grain size of inoculated alloys and the optimization of casting process to obtain refined grains in inoculated ingots.

Microstructure, thermal, and mechanical properties of hierarchical porous silicon carbide made by direct ink writing

AbstractHierarchical porous silicon carbide (SiC) ceramics were fabricated by combining particle‐stabilized emulsions and three‐dimensional (3D) printing. Direct ink writing (DIW) was used as the 3D printing technique. The formulation for successful printing is discussed in relation to the rheology of the emulsions. The SiC emulsions were able to be printed with a lower storage modulus (G′) and apparent yield shear stress (τy) than previously reported SiC ink pastes. The printed and sintered porous SiC ceramics possess a total porosity of 73.7% with an average pore size within the filaments of 2.2 µm in diameter. A hierarchical pore structure that contains pore sizes of about 250 µm, around 1–10 µm and smaller than 0.5 µm can be observed in the microstructure and pore size distribution. The mechanical properties showed a good strength‐to‐density ratio, and the thermal conductivity was reduced to 4.9 W/m·K. This study provides a new reliable approach for fabricating hierarchical porous SiC ceramics with low thermal conductivity.

Enhancing the performance of PNN-PHT perovskite ferroelectric via configurational entropy strategy: Component design, electric properties, and mechanism analysis

The traditional approach to designing ferroelectrics through phase boundaries has limitations in regulating polarization configuration and achieving significant performance enhancements. In contrast, the concept of high-entropy materials has emerged as a promising strategy for designing new materials with enhanced properties. Herein, by incorporating the high configurational entropy (Sconfig) strategy, the investigation explored the influence of Sconfig on the electrical properties of PNN-PHT piezoceramics. The results demonstrated that high Sconfig values were highly beneficial in improving the piezoelectric properties and significantly influenced the microstructure and grain size. The increase in Sconfig value was found to be positively correlated with the Rayleigh coefficient (α), dielectric loss (tanδ), and frequency dispersion (γ), while inversely correlated with the coercive electric field (Ec). By exploring the mechanism via Raman and simulation analyses, it was discovered that high Sconfig induces enhanced disorder, increased lattice distortion, and sharp volume changes. This ultimately resulted in a richer phase structure, reducing the potential energy barriers and leading to a lower activation energy (Ea) required for polarization switching. This finding provides novel strategies and directions for the development of high-voltage and high-coercive-field materials, holding promise for designing piezoelectric devices with enhanced power density and precision.

Influence of microstructure size on temperature distribution in biperiodic composite

In this work, the problem of heat conduction in a biperiodic composite consisting of two constituent materials was analysed. To average the discontinuous coefficients in the heat conduction equation, which arise from the heterogeneous structure, the technique of tolerance modelling was applied. The finite difference method was then used to solve the resulting system of equations and to determine the distribution of the unknowns. The finite difference method algorithm was implemented in Maple 2019 software. The main objective of the analysis was to investigate the influence of the composite’s microstructure size on the distribution of the unknowns, facilitated by the tolerance model equations derived through the tolerance modelling technique. The study demonstrated how the number of composite cells, and consequently the cell size, affects temperature values and their fluctuations under selected boundary conditions.

Thermal-induced crystallographic transformation in shells of Mytilus galloprovincialis Lamarck, 1819

Biominerals in hard parts are widely used as paleoenvironmental archives, employing proxies such as elemental and isotopic composition, microstructure, and crystallography. However, the effective selection and application of these proxies in fossil materials depends on their preservational potential. Here we evaluated the preservational potential of these proxies through controlled experiments on bivalves, one of the commonly used fossils in paleoenvironmental reconstructions. We utilized cultured Mytilus galloprovincialis specimens at five controlled temperatures to investigate the impact of elevated temperatures on various proxies extracted from both aragonitic and calcitic components of the same shell, including isotopic composition, microstructure and crystallography. Elevated temperatures first induced changes in the oxygen isotope composition (in both aragonite and calcite), followed by modification in aragonitic parts, such as changes in the aragonite phase, textures, nacre microstructure, and grain size of aragonite crystals. Despite aragonite's susceptibility to temperature, the microstructure and texture of calcite remained largely unaltered, demonstrating significantly higher resilience. Our findings emphasize the preservational potential of different shell proxies under heat exposure, ranging from oxygen isotopes to microstructure and texture. A ranking of preservation potential provides a practical guide for selecting well-preserved specimens, as textures and microstructures are not always reliable indicators. In distinguishing aragonite from calcite in fossils, our study pioneers a new avenue by proposing the analysis of high-angle boundary (HAB) content, including twin boundaries, as a reference for altered aragonite.

Internal hydrophobic treatment for mortar containing supplementary cementitious materials: thermal analysis of kinetics and products of hydration

AbstractThe presented research investigates the application of the organosilicon admixtures based on triethoxyoctylsilane (OTES) on the hydration of the cement-based material with addition of supplementary cementitious materials (SCM) such as blast furnace slag and microsilica. The influence of silane-based admixtures on the kinetics of hydration was investigated by isothermal calorimetry. The calorimetric results disclosed that applied admixtures affect the hydration process of cement paste with SCM. The DTA/TG analysis provided the information about impact of triethoxyoctylsilane on the composition and formation of the mineral phases. The DTA/TG measurements showed noticeable changes in the thermal decomposition of the tested materials and amount of bounded water. The impact of OTES on the microstructure and pore size distribution of pastes was examined by mercury intrusion porosimetry. The result showed significant changes in the range of pore diameters. The influence of organosilicon admixtures on the compressive strength of mortars after 2, 7, 28, 56 and 90 days was also investigated. The effect depended on the mineral additive used. In case of blast furnace slag, the development of compressive strength was only delayed, however, in the case of microsilica, it was stopped.

Effect of Initial Intergranular Ferrite Size on Induction Hardening Microstructure of Microalloyed Steel 38MnVS6

In this study, we investigated the effect of grain size of an initial microstructure (pearlite + ferrite) on a resulting microstructure of induction-hardened microalloyed steel 38MnVS6, which is one topical medium carbon vanadium microalloyed non-quenched and tempered steel used in manufacturing crankshafts for high-power engines. The results show that a coarse initial microstructure could contribute to the incomplete transformation of pearlite + ferrite into austenite in reaustenitization transformation by rapid heating, and the undissolved ferrite remains and locates between the neighboring prior austenite grains after the induction-hardening process. As the coarseness level of the initial microstructure increases from 102 μm to 156 μm, the morphology of undissolved ferrite varies as granule, film, semi-network, and network, in sequence. The undissolved ferrite structures have a thickness of 250–500 nm and appear dark under an optical metallographic view field. To achieve better engineering applications, it is not recommended to eliminate the undissolved ferrite by increasing much heating time for samples with coarser initial microstructures. It is better to achieve a fine original microstructure before the induction-hardening process. For example, microalloying addition of vanadium and titanium plays a role of metallurgical grain refinement via intragranular ferrite nucleation on more sites, and the heating temperature and time of the forging process should be strictly controlled to ensure the existence of fine prior austenite grains before subsequent isothermal phase transformation to pearlite + ferrite.