Microstructural Considerations Research Articles

Optimization of gypsum and slag contents in blended cement containing slag

Increasing the gypsum and slag contents in the blended cements containing slag is conducive to promote its green degree. The blended cement was prepared by mixing slag, gypsum and Portland clinker. Contour maps were designed to determine the optimal proportion of slag, gypsum and clinker by comprehensive considerations of strength, expansion, hydration, microstructure and crystallization stress. The strength contour map shows that there are two separate regions with high cement strength in the ternary slag-gypsum-clinker system; and the expansion deformation of these two regions are relatively low. The Rietveld XRD and pore solution analysis reveal that the occurrence of the destructive expansion results from the high crystallization stress of ettringite. It is illustrated that the gypsum content could be increased up to 25% when the clinker content is less than 5%, while the gypsum content should be limited below about 10% when the clinker content is more than 20%. The [CaO][SO3]/[Al2O3] molar ratio is suggested to direct the optimization of the slag, gypsum and clinker in blended cement. As long as the [CaO][SO3]/[Al2O3] ratio is not up to 1.5, more gypsum and slag could be added into the blended cement to obtain relatively excellent performance.

Differences in the Corrosive Spalling Behavior of Alumina-Rich Castables: Microstructural and Crystallographic Considerations of Alumina and Calcium Aluminate Matrices

The energy transition from the incineration and gasification of fossil fuels to the incineration and gasification of biomass refractory linings is being held up by a severe corrosion issue, caused by high alkali contents and the wide variety of biomass sources. Incinerators optimized for fossil fuels are commonly lined with mullite, Al2O3-Cr or SiC-based refractory products; however, those materials are not always suitable for the use of organic fuels. Hibonite (CaO·6Al2O3)-based refractory products have shown promising performance because of their high resistance against alkali attacks. Indeed, previous works have shown that the reaction between calcium hexa-aluminate and an alkali does not lead to the strong volume expansion observed with other mineral phases, such as corundum or andalusite. The present work aims to describe the reactions kinetics occurring between hibonite-based raw materials and biomass ashes. Therefore, the three main oxides contained in an average biomass, namely, CaO, SiO2 and K2O, were selected to examine the high temperature reactions with a calcium hexa-aluminate matrix. The resulting phase composition and microstructure were compared with the performance of an alumina matrix through, respectively, X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM). The post-mortem observations show a higher extent of reaction for the alumina than for the calcium hexa-aluminate. Moreover, the microstructure of the alumina matrix suffered a strong chemical spalling, while the calcium hexa-aluminate microstructure remained undamaged after the corrosion.

A model for French-press experiments of dry snow compaction

Abstract. Snow densification stores water in alpine regions and transforms snow into ice on the surface of glaciers. Despite its importance in determining snow-water equivalent and glacier-induced sea level rise, we still lack a complete understanding of the physical mechanisms underlying snow compaction. In essence, compaction is a rheological process, where the rheology evolves with depth due to variation in temperature, pressure, humidity, and meltwater. The rheology of snow compaction can be determined in a few ways, for example, through empirical investigations (e.g., Herron and Langway, 1980), by microstructural considerations (e.g., Alley, 1987), or by measuring the rheology directly, which is the approach we take here. Using a French-press or cafetière-à-piston compression stage, Wang and Baker (2013) compressed numerous snow samples of different densities. Here we derive a mixture theory for compaction and airflow through the porous snow to compare against these experimental data. We find that a plastic compaction law explains experimental results. Taking standard forms for the permeability and effective pressure as functions of the porosity, we show that this compaction mode persists for a range of densities and overburden loads. These findings suggest that measuring compaction in the lab is a promising direction for determining the rheology of snow through its many stages of densification.

Is scratch test proper to characterize microstructure and mechanical properties of cement-based materials? The effects of loading level and routine

Microstructure and mechanical properties of cement-based materials (CBMs) are vitally important for their applications in engineering fields, while the characterizing methods remain to be advanced. Cube-corner scratches were performed to explore the hardness of a hardened cement paste (HCP) with the consideration of multi-scale microstructure. Specifically, we focused on the effects of loading level and routine on the scratch results. Results show that scratch profiles are sensitive to hardness of local phases but fail to detect pores because pore profiles belong to surface roughness. Low-level and slow loads can measure more phases of the HCP while high-level loads are more likely to get the homogenized properties over a large domain. Stress intensity factor by the cube-corner scratches converges rapidly to a narrow range close to the literature. This work improves the understandings in the methodology of scratch test for characterizing the microstructure and mechanical properties of CBMs.

A phenomenological model of twinning-mediated strain hardening

We present a model of strain-hardening in materials with twinning-mediated plasticity. While the model is largely phenomenological, it was motivated by microstructural considerations and can be claimed to be physically sound. Using simple, yet sensible, assumptions regarding the stress-driven twinning kinetics and dislocation-twin interactions, constitutive equations governing the strain hardening behaviour were obtained and solved numerically. The solutions were compared with the experimental data on uniaxial deformation of pure magnesium and Mg alloy ZK60, and this comparison substantiated the validity and practicality of the model proposed.

Microstructures evolution, formation mechanisms and properties of Sicp/ Al composite coatings on Ti-6Al-4v substrate via mechanical alloying method

In this present work, high-energy mechanical alloying method was successfully applied to fabricated Al-SiC coatings, which possessed a composite structure. The coatings fabricated by different Al-SiC weight ratio were studied. The results of scanning electron microscope(SEM) and Energy Dispersive X-ray spectrometer(EDS) revealed the microstructures and phase compositions of as-synthesized coatings, respectively. To test the mechanical properties of coated samples prepared at different Al content, the tests of microhardness value were carried out from the outermost coating to the inner substrate. Then, the effect of milling duration on coatings was also studied. Adhesion behaviors of the coatings at different milling time were investigated through scratch tests. And friction and wear resistance were also tested. Which further confirmed the surface strengthening effect of the coating. The formation mechanisms of the Al–SiC composite coating and Al–SiC composite powder were elaborated. Overall consideration of microstructure, phase composition and mechanical properties, the coating prepared with Al–SiC raw powder ratio of 65:35 at 7 h milling time was the optimal processing parameters on fabrication of Al–SiC composite coating. The Al–SiC coating shows high performance in adhesion behaviors and wear resistance, this method is capable to be applied to fabricate coatings on the inner walls of tubular specimens .

Effects of Cu addition on formability and surface delamination phenomenon in high-strength high-Mn steels

The formability of austenitic high-Mn steels is a critical issue in automotive applications under non-uniformly-deformed environments caused by dynamic strain aging. Among austenite stabilizing alloying elements in those steels, Cu has been known as an effective element to enhance tensile properties via controlling the stacking fault energy and stability of austenite. The effects of Cu addition on formability, however, have not been sufficiently reported yet. In this study, the Cu addition effects on formability and surface characteristics in the austenitic high-Mn TRIP steels were analyzed in consideration of inhomogeneous microstructures containing the segregation of Mn and Cu. To reveal determining factors, various mechanical parameters such as total elongation, post elongation, strain hardening rate, normal anisotropy, and planar anisotropy were correlated to the hole-expansion and cup-drawing test results. With respect to microstructural parameters, roles of (Mn,Cu)-segregation bands and resultant Cu-rich FCC precipitates on the formability and surface delamination were also discussed.

A unified explicit analytical expression for considering anomalous eddy-current losses of ME laminate

Based on the equivalent circuit approach, a unified explicit analytical expression for RME field and voltage coefficients, taking into account the anomalous eddy-current losses of domain magnetization rotation (DMR) and domain wall displacements (DWD) in the magnetic phase, is established for ME laminate. For slabs with bar domains, the combination of classical eddy-current model and Polivanov’s eddy-current model in derivation achieved the consideration of microstructure of magnetic domains influencing the anomalous eddy-current losses. The proposed expressions are capable of reduction to some existing models, in which only the classical eddy-current losses or anomalous eddy-current losses of DWD are considered. The results prove that (1) the proposed novel expressions unified the previous analytical models on investigations of the anomalous eddy-current losses of ME laminate. (2) The prediction accuracy of the resonant ME (RME) coefficients corresponding to ME coupling effect has been increased due to considering the anomalous eddy-current losses. (3) The present investigations offer analytical expressions for obtaining the lower and upper boundary values of anomalous eddy-current losses for the specific ME laminate under intermediate values of and appropriate The numerical results provide theoretical guidance in design, manufacturing and application of smart structures with ME laminate.

Microstructure Ramifications on Electrodeposition for Solid-State Electrolytes

Safe Li-ion batteries with enhanced energy and power density rely on utilization of lithium metal anodes in conjunction with non-flammable solid electrolytes that inhibit dendritic growth through mechanical rigidity. The kinetics, transport, mechanics complexations are strongly dependent on the microstructure of the solid electrolyte. In particular, garnet type electrolytes exhibit crystalline structure consisting of grain, grain boundaries and voids while glassy sulfide type electrolytes exhibit an amorphous structure with voids; with prominent dependence of the transport properties (ionic conductivity) and mechanical properties (shear modulus, fracture toughness) on the structural arrangement. The Li metal-solid electrolyte interface also shows a wide degree of stochasticity due to the imperfect solid-solid contact delineating the importance of surface roughness effects on kinetics and interlayer design to promote surface uniformity. In this work, we investigate these coupled interactions and delineate electrodeposition stability for aforementioned class of inorganic solid electrolytes cognizant of microstructural considerations.

Micro-grinding Temperature Prediction Considering the Effects of Crystallographic Orientation and the Strain Induced by Phase Transformation

This paper proposes a physical-based model to predict the temperature in the micro-grinding of maraging steel 3J33b with the consideration of material microstructure and process parameters. In micro-grinding, the effects of crystallography on the grinding machinability become significant, since the depth of cut is of the same order as the grain size. In this research, the Taylor factor model for multi-phase materials is proposed to quantify the crystallographic orientation (CO) with respect to the cutting direction by examining the number and type of activated slip systems. Then, the flow stress model is developed, in which both the athermal stress resulted from the COs and the strain induced by the phase transformation are taken into account. On the basis of the flow stress model, the grinding forces are predicted followed by the calculation of the grinding heat. In the investigation, the triangular heat flux distribution and the reported energy partition model are applied in the calculation of workpiece temperature. Furthermore, the temperature model is validated by conducting an orthogonal-designed experiment, with the predictions of the maximum temperature in good agreement with the experimental data. Moreover, the predictive data is compared with the predictions resulted from the two other previously reported models. The results indicate that the proposed temperature model with considering the effect of CO and the phase transformation improved the prediction accuracy of the micro-grinding temperature.

A theoretical strain relationship for identifying the failure of laboratory-scale rocks under triaxial compression

ABSTRACT A theoretical strain relationship between peak strain (ε peak) and crack damage strain (ε cd) is established for identifying the failure of the rock under triaxial compression. The relationship is derived from the two-dimensional renormalisation group theory with consideration of the rock heterogeneous microstructures. It is afunction of five parameters, including the confining stress (σ 3), Young’s modulus (E), friction angle (φ), Poisson’s ratio (γ) and homogeneity index of Weibull distribution (m). The σ 3, E, φ and γ values can be readily obtained from the stress–strain data, and amethod for obtainingmvalue using σ 3, σ cd, ε cd, E, φ and γ values is proposed. Comparison with the triaxial compression test results of 48 shale specimens with four bedding dip angles shows the failure identifications using the established relationship well agree with the experimental measurements, demonstrating the ability of the established relationship to identify the failure of the laboratory-scale rock specimens under triaxial compression. Single-parameter sensitivity analysis shows that the established relationship between the ε cd and ε peak is affected by the parameters with different sensitivity. The highest to lowest sensitivity of the parameters are φ, σ 3, E, γ andm, respectively.

Designing and transforming yield-stress fluids

We review progress in designing and transforming multi-functional yield-stress fluids and give a perspective on the current state of knowledge that supports each step in the design process. We focus mainly on the rheological properties that make yield-stress fluids so useful and the trade-offs which need to be considered when working with these materials. Thinking in terms of “design with” and “design of” yield-stress fluids motivates how we can organize our scientific understanding of this field. “Design with” involves identification of rheological property requirements independent of the chemical formulation, e.g. for 3D direct-write printing which needs to accommodate a wide range of chemistry and material structures. “Design of” includes microstructural considerations: conceptual models relating formulation to properties, quantitative models of formulation-structure-property relations, and chemical transformation strategies for converting effective yield-stress fluids to be more useful solid engineering materials. Future research directions are suggested at the intersection of chemistry, soft-matter physics, and material science in the context of our desire to design useful rheologically-complex functional materials.

A dislocation density tensor-based crystal plasticity framework

The present contribution addresses a crystal plasticity formulation which incorporates hardening effects that are related to the presence of geometrically necessary dislocations. To this end, higher gradient contributions are introduced as additional arguments of the energy function based on microstructural considerations. Extending the derivations presented in Kaiser and Menzel (2019) for a purely phenomenological, associated type plasticity model to crystal plasticity, it is shown that the higher gradient contributions in terms of dislocation density tensors give rise to the balance equation of a generalised stress field together with non-ambiguous constitutive boundary conditions. This stress field can be shown to be energetically conjugated to the plastic flow and is additively composed of two parts: the classic stress field and a back-stress type stress field which is closely related to incompatibilities in the plastic deformation field and hence interpretable in terms of geometrically necessary dislocations. For a specific model which features twelve slip systems the constitutive response on material point level is studied in a first step before finite element based simulations, which are motivated by experimental findings on copper micro wires, are analysed in two- and three-dimensional settings.

Effects of local-brittle-zone (LBZ) microstructures on crack initiation and propagation in three Mo-added high-strength low-alloy (HSLA) steels

Fracture toughness of intercritically-reheated coarse-grained HAZ (ICRCGHAZ), i.e., local brittle zone, is severely reduced at low temperatures by the presence of undesirable microstructural features such as coarsened prior austenite grains, bainitic microstructures including bainitic ferrite (BF), granular bainite (GB), and acicular ferrite (AF), and martensite-austenite constituent (MA). In this study, effects of the microstructural features on critical CTOD and crack initiation and propagation process were investigated in the ICRCGHAZ of three Mo-added high-strength low-alloy (HSLA) steels. The Mo addition caused the increased volume fractions of GB, BF, and MA and the resultant deterioration of critical CTOD. According to the observation of cracking process, most of hard MAs, particularly MAs located along prior austenite grain boundaries (PAGBs), provided initiation sites of {100} cleavage cracks. In addition, a large hardness difference between PAGB-MA and matrix led to an easy MA/matrix interfacial separation and an increase in crack initiation sites, thereby resulting in very low critical CTOD in the high-Mo-containing steel having the highest fraction of MA. The detailed microscopic observation of cracking process at MAs and macroscopic MA/matrix hardness difference provide plausible ideas for reliably evaluating critical CTOD and for designing HSLA steels in consideration of ICRCGHAZ microstructures.

Crystal plasticity analysis of deformation anisotropy of lamellar TiAl alloy: 3D microstructure-based modelling and in-situ micro-compression

Detailed microstructure characterisation and in-situ micropillar compression were coupled with crystal plasticity-based finite element modelling (CP-FEM) to study the micro-mechanisms of plastic anisotropy in lamellar TiAl alloys. The consideration of microstructure in both simulation and in-situ experiments enables in-depth understanding of micro-mechanisms responsible for the highly anisotropic deformation response of TiAl on the intra-lamella and inter-lamella scales. This study focuses on two specific configurations of γ/α2 lamellar microstructure with the γ/α2 interfaces being aligned 25o and 55o to the loading direction. Microstructure-based CP-FEM shows that longituginal slip of super and ordinary dislocations are most responsible for the plastic anisotropy in the 25o micropillar while the anisotropy of the 55o micropillar is due to longitudinal superdislocations and longitudinal twins. In addition, transversal superdislocations were more active, making the deformation in the 25o micropillar less localised than that in the 55o micropillar. Moreover, the CP-FEM model successfully predicted substantial build-up of internal stresses at γ/α2 interfaces, which is believed to be detrimental to the ductility in TiAl. However, as evidenced by the model, the detrimental internal stresses can be significantly relieved by the activation of transverse deformation twinning, suggesting that the ductility of TiAl can be improved by promoting transverse twins.

Processing raspberry-flavored whey drink using ohmic heating: Physical, thermal and microstructural considerations.

The effect of ohmic heating processing (10, 100, 1000 Hz - 25 V;45, 60, 80 V - 60 Hz until 65 °C-30 min) on physical (color, rheology, particle size diameter), thermal (differential scanning calorimetry, DSC), physicochemical (time domain nuclear magnetic resonance, TD-NMR) and microstructural (optical microscopy) properties of raspberry-flavored whey drink was investigated. The samples were submitted to an ohmic system and conventional pasteurization (65 °C-30 min). Both processing led to increases in the color parameters (C*, h* and ∆E*) in the first 30 min. For the treatments, 10 Hz-25 V and 1000 Hz-25 V, more viscous, and consequently increased D[4,3] and D[3,2] values were observed, and similar results were obtained for the DSC behavior. The microstructure confirmed aggregation of cell structure in those configurations (10 Hz and 1000 Hz-25 V). OH, parameters for both ohmic configurations have an impact in all the evaluated parameters for raspberry-flavored whey drink, which can be interesting for the dairy industry.

Grain size sensitive–MTS model for Ti-6Al-4V machining force and residual stress prediction

Material properties are significantly influenced by the parameters of the machining process. The accurate prediction of machining force and residual stress reduces power consumption, enhances material properties, and improves dimensional accuracy of the finished product. Traditional method using the finite element analysis (FEA) costs a significant amount of time, and the archived mechanical threshold stress (MTS) model without consideration of microstructure of the material yields inaccurate result. In this paper, a grain size–sensitive MTS model is introduced for the machining process of Ti-6Al-4V. A grain size–sensitive term is introduced to the modified MTS model to account for evolution of the grain size. The grain size–sensitive MTS model takes the microstructure of the Ti-6Al-4V into consideration for the calculation of machining force and residual stress. The grain size–sensitive term is introduced into the athermal stress component using the initial yield stress, strain hardening coefficient, and the Hall-Petch coefficient. The analytical result is compared with those of experimental studies and the traditional Johnson-Cook model to prove the validity in the prediction of machining force and residual stress. The proposed model explores a new area for calculating cutting forces and residual stress.

Mathematical Models of Homogenization for a Rammed Earth Blend Made of Crushed Limestone and Cement in Linear Micro-poro-elasticity

Rammed earth (RE) designates natural building materials that can contain stabilized or non-stabilized soils or crushed stone blends. RE materials are non-homogenous and their performance depends on their compositions, grain size distributions, porosity, and mode of preparation. This category of building materials has recently been subject of extensive studies, due to the increasing interest in sustainable construction, which have highlighted the potential and limits of its large uptake. Although it has been demonstrated that RE can be treated as multi-component materials [1] a full characterization based on micro-structural considerations is still lacking. Full understanding of the role of porosity in the overall material performance has not been addressed. However, extensive literature [2, 3, 4, 5] is available that can describe the overall mechanical behaviour of similar materials, based on the principle continuum micromechanics. This study applies the Mori-Tanaka homogenization scheme to characterize the linear elastic behaviour of cement-stabilized crushed-limestone-based material by treating it as a porous composite. The composite comprises a solid phase consisting of limestone grains bonded together with hydrated cement bridges and a fluid phases consisting of different families of pores. The microstructure of rammed earth samples has been observed using an optical microscope. The obtained images have been collected and processed to obtain the pore sizes and pore size distribution. Finally, these statistics along with the Mori-Tanaka homogenization scheme have been used to determine the overall elastic properties that depend on a damage parameter enclosing the pore sizes, density, and texture

Perspective—Mesoscale Physics in the Catalyst Layer of Proton Exchange Membrane Fuel Cells

Proton-exchange membranes fuel-cells (PEMFC) electrochemical performance insights are predicated on a detailed understanding of species transport in the cathode catalyst layer (CCL). Traditionally, CCL microstructure considerations were approached through approximations with unresolved pore-scale features. Such simplifications cause the loss of predictability for improving the economic feasibility via lower Pt-loading or non-noble metal catalysts. With advances in visualization, microstructure resolved mesoscale models become possible. A judicious combination of lattice Boltzmann (LBM) and finite volume (FVM) is an appropriate strategy for direct numerical simulation (DNS) of the physicochemical fields that remain unresolved due to spatiotemporal limitations.

Microstructural considerations on SH-wave propagation in a piezoelectric layered structure

Shear wave based acoustic devices are being used in gaseous and liquid environments because of their high-sensitivity. The theoretical study of horizontally polarized shear (SH) waves in a layered structure consisting of a piezoelectric ceramic of PZT − 5H or BaT iO3 material overlying a couple stress substrate is presented in this paper. The considered substrate is supposed to be exhibiting microstructural properties. The closed form expression of dispersion relations are derived analytically for electrically open and short conditions. The effects of internal microstructures of the couple stress substrate, thickness of PZT − 5H or BaT iO3 ceramic, piezoelectric and dielectric constants are illustrated graphically on the phase velocity of the piezoelectric layer under electrically open and short conditions.