Effect Of Microstructure Size Research Articles

Size-dependent nonlinear bending of microbeams based on a third-order shear deformation theory

In this paper, the size-dependent nonlinear bending of microbeams subjected to mechanical loading is studied using a finite element formulation. Based on the von Kármán nonlinear relationship and the third-order shear deformation theory, a size-dependent nonlinear beam element is derived by using the modified couple stress theory (MCST) to capture the microstructural size effect. The element with explicit expressions for the element vector of internal forces and tangent stiffness matrix is derived by employing the transverse shear rotation as a variable. Nonlinear bending of microbeams under different mechanical loading is predicted with the aid of Newton–Raphson iterative method. Numerical investigation shows that the derived element is efficient, and it is capable of giving accurate results by several elements. The obtained results reveal the importance of the micro-size effect on the nonlinear behavior of the microbeams, and the deflections are overestimated when the microstructural effect is ignored. The effects of the material length scale parameter, boundary conditions and loading type on the bending response of the microbeams are studied and highlighted.

Lithium Dendrite Propagation in Ta-Doped Li7La3Zr2O12 (LLZTO): Comparison of Reactively Sintered Pyrochlore-to-Garnet vs LLZTO by Solid-State Reaction and Conventional Sintering.

Ta-doped Li7La3Zr2O12 (LLZTO) garnet is a promising Li-ion-conducting ceramic electrolyte for solid-state batteries. However, it is still challenging to use LLZTO in Li metal batteries operating at high current densities because of the tendency for Li metal to nucleate and propagate along the grain boundaries. In this study, we carry out a detailed investigation to elucidate the effect of microstructure and grain size on the electrochemical properties and short circuit behavior in LLZTO. Pellets were prepared using reactive sintering from pyrochlore precursors (a method called pyrochlore-to-garnet, P2G) and compared with LLZTO synthesized using solid-state reaction (SSR) followed by conventional pressureless sintering. Both preparation methods were controlled to keep the phase and elemental composition, ionic and electronic conductivity, relative density, and area-specific resistance of the pellets constant. Reflection electron energy loss spectroscopy and X-ray photoelectron spectroscopy confirm that both types of LLZTO have similar band gaps and chemical states. Microstructure analysis shows that the P2G method results in LLZTO with an average grain size of around 3 μm, which is much smaller than the grain sizes (as large as 20 μm) seen in SSR LLZTO. Galvanostatic Li stripping/plating and linear sweep voltammetry measurements show that P2G LLZTO can withstand higher critical current densities (up to 0.4 mA/cm2 in bidirectional cycling and >1 mA/cm2 for unidirectional) than those seen in SSR LLZTO. Post-mortem examination reveals much less Li deposition along the grain boundaries of P2G LLZTO, particularly in the bulk of the pellet, compared to SSR LLZTO after cycling. The improved cycling behavior in P2G LLZTO despite the higher grain boundary area could be from more homogeneous current density at the interfaces and different grain boundary properties arising from the liquid-phase, reactive sintering method. These results suggest that the effect of grain size on Li dendrite propagation in LLZO may be highly dependent on the synthesis and sintering method employed.

Microstructural size effect on the notch fatigue behavior of a Ni-based superalloy using crystal plasticity modelling approach

Fatigue performances of critical structures are strongly affected by the microstructural (e.g. grain, defect, inclusion, etc.) size effect, and it is thus important to quantify their detrimental effect. In this work, a numerical procedure is constructed to quantify the influence of microstructure on the mechanical and fatigue behaviors of Ni-based superalloy GH4169. Specifically, by combining sub-modelling approach with crystal plasticity constitutive model, a dual-scale modelling approach is developed for studying grain-level mechanical behavior of Ni-based superalloy GH4169 notched components. In addition, the dislocation-based Tanaka-Mura-Wu model is applied for fatigue crack initiation life prediction. The Paris law is then utilized for fatigue crack propagation analysis based on the simulated short crack. To study the microstructural size effect on fatigue crack initiation and propagation behaviors, sub-models containing various grain orientation, grain size, defect size/shape are built and analysed. Finally, a series of fatigue tests on notched specimens of Ni-based superalloy GH4169 were carried out for method validation. Results indicate that the established dual-scale modelling approach and fatigue life prediction framework yields good agreement with experimental results.

Effect of Microstructure and Macro Size of Walnut Shell Chars on Their Electromagnetic Induction Heating Behavior

With increasing global demand for renewable energy, biomass and renewable electricity will be more and more widely applied in chemical processes. A promising direction is heating biochar directly by electromagnetic induction to support their high-temperature reactions, such as calcium carbide synthesis. To clearly understand the fundamental principles in biochar’s induction heating, this study investigates the relations between the physicochemical properties of walnut shell chars prepared at pyrolysis temperatures of 600–1400 °C and their induction heating behavior. The properties of chars include their electrical conductivity and structure parameters characterized by X-ray diffraction (XRD), Raman spectroscopy, electron spin resonance (ESR), ultimate analysis, as well as chars’ geometrical sizes. It is found that the chars prepared at pyrolysis temperatures equal to and higher than 800 °C are semiconductors with electric conductivities of 44–510 S/m. The threshold parameters for XRD, Raman, ESR, and ultimate analysis as well as geometrical size for induction heating are determined.

Analysis of burr formation in finish machining of nickel-based superalloy with worn tools using micro-scale in-situ techniques

The formation of burrs is among the most significant factors affecting quality and productivity in machining. Burrs are a negative byproduct of machining processes that are difficult to avoid because of a limited understanding of the complex burr formation mechanisms in relation to cutting conditions, including both process parameters and tool condition. Thus, the objective of this work was to characterize burr formation under finish machining conditions via a high-speed, high-resolution in-situ experimental method. Various parameters pertaining to burr geometry such as height, thickness, and initial negative shear angle were measured both during and after cutting. Results showed that varying the conditions of uncut chip thickness, tool-wear, and cutting speed all have a significant effect on burr formation, although certain burr metrics were found to be insensitive with respect to different process conditions because the difference was statistically insignificant. This study provides new insights into the relationships between the workpiece material's microstructure, machining parameters, and tool condition on both crack formation and propagation/plasticity during burr formation. Using digital image correlation (DIC) and a physics-based process model not previously utilized for burr formation analysis, the displacement and corresponding flow stress were calculated at the exit burr root location. This novel semi-analytical approach revealed that the normalized stress at the exit burr root was approximately equal to the flow stress for a variety of different conditions, indicating the potential for model-based prediction of burr formation mechanics. Finally, this study investigates factors that influence fracture evolution during exit burr formation. It was found that negative exit burrs are a direct result of high strain rate and high uncut chip thickness, which was expected, but also a microstructural size effect and a tool-wear effect, neither of which have been previously reported. By harnessing ultra-high-speed imaging and advanced optical microscopy techniques, this manuscript deals with the fundamentals of burr formation, including new insights into material response at the grain-scale to the loads imposed with both sharp and worn tools.

Geometrical and microstructural size effects in progressive forming using wires

With the increasing trend toward product miniaturization, progressive forming has been widely used in fabricating meso‑/micro-scaled parts using sheet metals. However, there are difficulties for such processes and sheet metals to be employed to manufacture the parts and components with local rotating features, and their rotating axis is not in the extruding/forming direction. In this work, a progressive forming process directly using metal wires was first developed. The constitutive model using the brass CuZn35 wires of different diameters (0.4–1.2 mm) as a case study material with different grain sizes (30.9–159.2 μm) was developed. By using the multi-scaled fork-shaped parts with flat tines and cylindrical heads as case micro-parts to be micro-formed, the dimensional errors induced by springback increased with the grain size while decreasing as the specimen size increased. The surface quality deteriorates with the increase in grain size, and the micro-cracks form more easily on the side surface of micro-parts with larger grains. The undesired geometry issue of the fabricated parts was overcome by optimizing the tool design. This work further expands the promising application of the progressive forming process in the high-efficiency fabrication of meso‑/micro-scaled parts with irregular features.

The Effect of Microstructure Size on Microstructure and Properties of High Boron Alloy During Quenching and Partitioning (Q&P) Process

The Effect of Microstructure Size on Microstructure and Properties of High Boron Alloy During Quenching and Partitioning (Q&P) Process

Opportunities and Challenges in Modelling of Machining Performance with Various Coatings and Lubricants in Inconel 718 Machining

Decades of research and industrial applications have shown the significant impact of tool coatings and lubricants on machining performance, particularly in difficult-to-cut materials such as nickel-based super alloy Inconel 718. However, the ability to quantify and optimize the beneficial effects of coatings and lubricants continues to be primarily driven by empirical approaches. In this paper, a comparison between experimentally measured tool wear and numerically modelled process metrics, such as temperatures and stresses, is presented. Based on the results and observed deviations, future directions for more industrially relevant cutting tool development are proposed. Major challenges include the need for physics-based tribological models that consider different process interactions as well as microstructural and geometric size effects.

Microstructural Size Effect on Strain-Hardening of As-Quenched Low-Alloyed Martensitic Steels

Microstructural Size Effect on Strain-Hardening of As-Quenched Low-Alloyed Martensitic Steels

High-temperature oxidation behavior of additively manufactured IN625: Effect of microstructure and grain size

High-temperature oxidation of additively manufactured (AM) Ni-base alloy IN625 has been studied in air and Ar-5%H2-3%H2O at 900–1000 °C. AM material is found to oxidize faster than the conventionally manufactured (CM) IN625 due to severe intergranular oxidation observed in the former. The AM IN625 was heat treated at 1100–1250 °C and hot rolled at 980 °C in order to modify the AM microstructure, primarily grain size, and analyze its role in alloy oxidation behavior. Grain size is shown to affect overall oxidation kinetics but not the intergranular oxidation morphology.

Buckling mode transition in nonlinear strain gradient-based stability behavior of axial-thermal-electrical loaded FG piezoelectric cylindrical panels at microscale

In the current investigation, the microstructural-dependent nonlinear stability characteristics of functionally graded (FG) piezoelectric cylindrical micropanels under combinations of axial mechanical load with external electric actuation and temperature are studied. To this purpose, an efficient numerical strategy based upon the moving Kriging meshfree (MKM) technique is employed within the framework of the modified strain gradient continuum elasticity. The established unconventional formulations take the buckling mode transition phenomenon into consideration in the presence of microstructural size effects including rotation gradient, dilatation gradient, and deviatoric stretch gradient tensors, The derived microsize-dependent panel model has the capability to satisfy the function property of Kronecker delta via imposing essential boundary conditions with no use of predefined mesh and directly at the associated nodes. The unconventional nonlinear equilibrium curves are traced including the modal transition corresponding to different parametric change values. It is displayed that the microsize dependency leads to shift the minimum nonlinear stability loads associated with the first and second buckling modes to a lower panel deflection and a higher panel shortening. Also, it is revealed that the effects of all three microstructural gradient tensors on the second nonlinear stability load are higher than that on the first one, and the both cases are more prominent than the microsize dependencies on the critical buckling load. Also, it is observed that the value of property gradient index has a negligible role on the first and second critical shortenings as well as shell deflections at the first and second minimum nonlinear stability points.

Investigation on the Microstructure Hardness for the Hot Compressed Duplex Steel

Effect of microstructure size and type on the hardness for the duplex steel were disclosed by using of optical microscope (OM), scanning electron microscope (SEM) and nanoindenter for the samples hot compressed under different temperature with reduction of 10%, 30%, 50% and 70%. OM and SEM were used to measure the average martensite lamellar width, space and indenter morphology. nanoindenter test characterized the microstructure hardness for the samples under different process. Experiment results show that martensite hardness for the sample hot compressed at 950°C has larger diversity than that of sample hot compressed at 1200°C. The martensite hardness fluctuation range for the sample compressed at 950°C is almost from about 7GPa to 12GPa, while, for the sample compressed at 1200°C, the fluctuation range is basically from about 9GPa to 12GPa. However, the average hardness for the samples hot compressed at 950°C is comparably smaller, which is related with lower quench temperature. The larger martensite hardness fluctuation is mainly related with induced ferrite formation and finer martensite lamellar width. For the ferrite phase, the hardness fluctuation range is lower.

A strain gradient brittle fracture model based on two-scale asymptotic analysis

Based on a two-scale asymptotic analysis procedure, we rigorously derive a new macroscopic brittle fracture model for materials with micro-cracks inside. Strain gradient effects are naturally involved in the model in the absence of any phenomenological assumptions. Coefficients of the fracture criterion are expressed in form of derivatives of the fourth- and sixth-order effective elastic tensors that are explicitly related to solutions of the first order unit cell problems. The model is numerically implemented into a finite element code. Abilities of the model to predict microstructure size effects and strain gradient effects on macro fracture behaviors are confirmed by numerical simulations. The model is also validated against experimental measurements.

Microstructure and particle size effects on selected mechanical properties of waste glass-reinforced aluminium matrix composites

The development of new lightweight engineering materials with highly improved mechanical properties has been a thought-provoking issue of materials engineering lately. The attempt being made by the industrial and academic bodies has engendered the shift to composite materials over conventional ones. Aluminium reinforced composites are now being used in the transportation industries such as automobile and aerospace. Waste glass is a hard ceramic material which is majorly made up of silica, it is cheap, available and can serve as a replacement for costly reinforcement like silica, silicon carbide and titanium carbide. As a result, this research studied the effect of stir casting parameters (weight fraction of reinforcement and particle size) on the physical and mechanical properties of aluminium Al 6061/waste glass composites. The results obtained showed that density and fracture toughness of the samples reduced with an increase in weight fraction and particle size of waste glass. Brinell hardness improved progressively with increase in the weight fraction of waste glass and a reduction in particle size. Tensile strength improved from 0 to 8 wt% after which a reduction was noted at 10 wt% for samples reinforced with −6 µm particle size while 6 wt% reinforcement had the best tensile strength for samples reinforced with −13 µm and –23 µm particles sizes. The morphology examined showed the interfacial interaction between matrix and reinforcement in relation to mechanical properties. The study established the effectiveness of the composites as compared to the pure matrix which made them better candidate materials for various engineering applications like automobiles.

Flow characteristics and microstructural evolution in pulsed current assisted micro-scaled compression of stainless steel sheet

Electrically assisted (EA) microforming has pervasive benefits for the fabrication of high-performance microproducts. In this research, the coupled effect between the microstructural size effect and pulsed current on the flow characteristics of 304 stainless steel sheets along thickness direction was explored by using EA micro-compression tests. The results revealed that the strain hardening rate and the deformation stress of 304 stainless steel decreases during the EA deformation, especially when the current density exceeds 57.16 A/mm2. Besides, the effect of grain size on the micro-scaled compression is gradually weakened with the augment of current density and plastic strain. The electron backscatter diffraction (EBSD) results showed in the perspective of microstructure that the decrease of flow stress and strain hardening rate with the rise of pulsed current density is related to the reduction of dislocation density and texture strength due to the dynamic recrystallization during EA deformation. The pulsed current suppresses the grain refinement of fine-grained material and promotes that of coarse-grained material, and the recrystallization rate of EA deformation increases with the decrease of initial grain size. In addition, the recrystallization texture induced by the pulsed current can promote grain deflection and weakens the effect of twins in coarse-grained material. That can explain the reason for the weakening of the grain size effect. The findings in the research have important guiding significance for understanding the coupled mechanism of electroplastic and grain size effect on the flow characteristics of difficult-to-deform material, and further promoting the application of EA micro-rolling of high-strength materials.

Inverse Domain-Size Dependence of Piezoelectricity in Ferroelectric Crystals.

Piezoelectricity of ferroelectric crystals is widely utilized in electromechanical devices such as sensors and actuators. It is broadly believed that the smaller the ferroelectric domain size, the higher the piezoelectricity, arising from the commonly assumed larger contributions from the domain walls. Herein, the domain-size dependence of piezoelectric coefficients of prototypical ferroelectric crystals is theoretically studied based on thermodynamic analysis and phase-field simulations. It is revealed that the inverse domain-size effect, i.e., the larger the domain size, the higher the piezoelectricity, is entirely possible and can be just as common. The nature of the domain-size dependence of piezoelectricity is shown to be determined by the propensity of polarization rotation inside the domains instead of the domain wall contributions. A simple, unified, analytical model for predicting the domain-size dependence of piezoelectricity is established, which is valid regardless of the crystalline symmetry, the materials chemistry, and the domain structures of a ferroelectric crystal, and thus can serve as a guiding tool for optimizing piezoelectricity of ferroelectric materials beyond the "nanodomain" engineering. In addition, the theoretical approach can be extended to understand the microstructural size effect of multifunctional properties in ferroic and multiferroic materials.

Thermoelectric Properties of PbSe Nanocomposites from Solution-Processed Building Blocks

High thermoelectric performance and low material cost are essential to the commercialization of thermoelectric modules. Microstructural design, size effects, and rational band engineering are among the keys to high thermoelectric performance, especially for nanocomposites. Here, we demonstrate a facile and versatile bottom-up approach to produce nanocomposites by heat consolidating solution-processed PbSe nanoparticles with a proper amount of Ag. In this PbSe-Ag system, a certain amount of Ag diffuses into the PbSe matrix and partially substituted Pb while other forms Ag2Se at the interphases. This increased the electrical conductivity and inversed the n-type behavior of PbSe to p-type, which is observed from the positive Seebeck coefficient for PbSe-Ag nanocomposites over the temperature range. Nonetheless, the decrease of total lattice thermal conductivity at high temperature is ascribed to the effective phonon at grain boundaries, Ag2Se nanodomain, and point defect. Overall, we obtained an optimized figure of merit, ZT = 0.97 at 723 K for 5 wt % Ag, a significant improvement of 8.8 times than the pure PbSe. Thus, this partially substituting host atom and the presence of a secondary phase provide another potential strategy to enhance the thermoelectric performance of PbSe, promoting its commercialization.

Multiscale modeling of coupling mechanisms in electrically assisted deformation of ultrathin sheets: An example on a nickel-based superalloy

Electrically assisted (EA) forming has ubiquitous merits over room-temperature (RT) forming and thermally aided forming for the fabrication of difficult-to-form microscale products. However, it is difficult to predict the material deformation behavior in the EA microforming process owing to the coupling between the electric current and microstructural size effect. To develop a robust constitutive model that considers the interplay between the electric and grain size effects, RT and EA quasi-static uniaxial tensile tests were performed on ultrathin nickel-based superalloy sheets with a thickness of 0.2 mm and grain sizes ranging from 27.2 to 79.4 μm. The experimental results demonstrated that the Joule heating effect and the normalized flow stress reduction were non-monotonically related to the grain size of the superalloy. The grain size effect in the polycrystalline superalloy was suppressed by the enhanced current density. A multiscale constitutive model that considered multiple strengthening mechanisms was proposed to describe the EA deformation behavior of ultrathin superalloy sheets. The multiscale model was proven to have a desirable predictive ability for the EA drawing force of thin-walled superalloy capillaries. Furthermore, the model revealed that the weakening of the grain size effect with increasing current density in the polycrystalline superalloy was caused by the combined variations in dislocation interaction, shear modulus, and strengthening mechanisms. The abnormal evolution of the surface effect with the current density was captured by the proposed constitutive model, which demonstrates that the electric current can promote the grain size effect in the multicrystalline superalloy.

Size-Dependent Free Vibration of Microbeams Submerged in Fluid

The size-dependent free vibration of microbeams submerged in fluid is presented in this paper based on the modified couple stress theory. Two different cross-section shapes of microbeams are considered, i.e. the circular cross-section and rectangular cross-section. This nonclassical microbeam model is introduced for capturing the size effect of microstructures. In this fluid and structure coupled system, the effect of hydrodynamic loading on microbeams can be expressed by the added mass method. By using Hamilton’s principle and differential quadrature (DQ) method, we can derive governing equations of microbeams in fluid, and then rewrite them in the discretized form. The frequencies and mode shapes for microbeams are determined by proposing an iterative method. Numerical examples are given to show the effect of fluid depth, fluid density, length scale parameter, slenderness ratio, boundary condition and cross-section shape on the vibration characteristics.

A Newton solver for micromorphic computational homogenization enabling multiscale buckling analysis of pattern-transforming metamaterials

Mechanical metamaterials feature engineered microstructures designed to exhibit exotic, and often counter-intuitive, effective behaviour such as negative Poisson’s ratio or negative compressibility. Such a specific response is often achieved through instability-induced transformations of the underlying periodic microstructure into one or multiple patterning modes. Due to a strong kinematic coupling of individual repeating microstructural cells, non-local behaviour and size effects emerge, which cannot easily be captured by classical homogenization schemes. In addition, the individual patterning modes can mutually interact in space as well as in time, while at the engineering scale the entire structure can buckle globally. For efficient numerical predictions of macroscale engineering applications, a micromorphic computational homogenization scheme has recently been developed (Rokoš et al., J. Mech. Phys. Solids 123, 119–137, 2019). Although this framework is in principle capable of accounting for spatial and temporal interactions between individual patterning modes, its implementation relied on a gradient-based quasi-Newton solution technique. This solver is suboptimal because (i) it has sub-quadratic convergence, and (ii) the absence of Hessians does not allow for proper bifurcation analyses. Given that mechanical metamaterials often rely on controlled instabilities, these limitations are serious. Addressing them will reduce the dependency of the solution on the initial guess by perturbing the system towards the correct deformation when a bifurcation point is encountered. Eventually, this enables more accurate and reliable modelling and design of metamaterials. To achieve this goal, a full Newton method, entailing all derivations and definitions of the tangent operators, is provided in detail in this paper. The construction of the macroscopic tangent operator is not straightforward due to specific model assumptions on the decomposition of the underlying displacement field pertinent to the micromorphic framework, involving orthogonality constraints. Analytical expressions for the first and second variation of the total potential energy are given, and the complete algorithm is listed. The developed methodology is demonstrated with two examples in which a competition between local and global buckling exists and where multiple patterning modes emerge. The numerical results indicate that local to global buckling transition can be predicted within a relative error of 6% in terms of the applied strains. The expected pattern combinations are triggered even for the case of multiple patterns.