Changes In Microstructural Characteristics Research Articles

Monte Carlo simulation of water diffusion through cardiac tissue models.

Monte Carlo diffusion simulations are commonly used to establish a reliable ground truth of tissue microstructure, including for the validation of diffusion-weighted MRI. However, selecting simulation parameters is challenging and affects validity and reproducibility. We conducted experiments to investigate critical conditions in Monte Carlo simulations, such as tissue representation complexity, simulated molecules, update duration, and compartment size. Results show significant changes in microstructure characteristics when parameters are altered, emphasizing the importance of careful control for a reliable ground truth.

Degradation response of mode I and mode III fracture resistance of sandstone under wetting–drying cycles with an acidic solution

The edge notched disk bend samples were adopted to investigate the degradation response of mode I and mode III fracture resistance of green sandstone under wetting–drying cycles in an acidic solution. Meanwhile, the degradation damage mechanism was clarified by observing the variations of the microstructures of the rock. The results indicated that the mode I and mode III fracture resistance reduced with increasing wetting–drying cycle times, which were decreased respectively by 34.3% and 36.8% after undergoing 10 cycles. Fracture toughness ratio KIIIC/KIC was slightly reduced with increasing cycle times, and the degradation damage on mode I fracture resistance was less than that of mode III. The major reason for the decrease in fracture resistance was the irreversible physical–chemical damage accumulation and the changes in microstructure characteristics during cyclic wetting–drying procedure.

Effects of different elevated temperature and long-term exposure on microstructural evolution and mechanical characteristics of IN617 Ni-based superalloy

The microstructural changes and mechanical properties of service-exposed IN617 Ni-based superalloy were studied at temperatures of 750, 800, 850 and 900 °C and different time ranging from 4250 to 105000 h. Some microscopic techniques and mechanical experiments, including Optical Microscopy (OM), Field Emission Scanning Electron Microscopy (FESEM) equipped with Energy Dispersive X-ray Spectroscopy (EDS), Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), impact, hardness and room/elevated temperatures tensile tests were employed to investigate the microstructure and mechanical performance of the superalloy. In the samples operated at 750 °C/105000 h and 800 °C/98000 h conditions, the γ′ strengthening phase was observed, leading to noticeable improvement in mechanical features of the material. At 850 °C/24000 h and 900 °C/4250 h, the γ′ weight percentage was very low to be detected, so that no significant variation in the mechanical properties of the superalloy was observed. Moreover, metallographic evaluations showed that carbide particles nucleated and grew along the grain boundaries and also within the grains with increasing exposure time during service condition, while their growth rate depended upon the operating temperature. Changes in microstructural characteristics and mechanical properties were assessed using Larson-Miller parameter. As the value of P parameter went up, the area fraction of carbides and the weight percentage of M23C6 carbide increased in return, whereas weight percentage of M6C carbide revealed a decreasing trend. This is due to the fact that a part of M6C carbide was transformed to equilibrium state of M23C6 carbide by increasing P parameter. Impact energy, ultimate tensile strength at room temperature and ultimate tensile strength at 750 °C drastically reduced with increasing P parameter. Although elongation percentage and yield strength showed a decreasing trend in terms of the P parameter, they did not follow regular changes contrary to other mechanical attributes. Measurements indicated that the average hardness value remained constant at around 215 HV with increasing P parameter up to 27500 and then decreased with a steep slope down to about 178 HV.

Effect of vacuum heat treatment on microstructure evolution and mechanical properties of 93W-4.9Ni-2.1Fe-(ZrB2) alloys

93W-4.9Ni-2.1Fe-(0.75ZrB2) alloys (wt%) were subjected to liquid phase sintering and vacuum heat treatment to optimize the microstructure and improve mechanical performance. Microstructural observations demonstrated that W–W continuity, dihedral angle and connectivity after heat-treatment remarkably decreased by 0.75, 10° and 0.7, respectively. Meanwhile, matrix phase volume fraction also increased by nearly 3%, which overcame the negative effects of non-homogeneous microstructure. However, densification and W grain size exhibited marginal changes. Mechanical properties strongly varied with the change of microstructural characteristics. Particularly, as matrix phase volume fraction increased and W–W contiguity decreased dramatically, elongation showed a distinct increasing tendency (from 18.6% to 22.4% in 93W alloys and from 17.5% to 19.5% in 93W-0.75ZrB2 alloys, respectively). Excellent W-matrix bonding strength improved the tensile property, while microstructure uniformity caused the decrease of hardness value (about 28 HV1). Furthermore, the relationship between mechanical properties and fracture mode was analyzed.

Molecular dynamics simulation of phase transformation of fused silica under nanoparticle impact: The influence of temperature and impact velocity

In high power laser facilities, the ejected nano- or microparticles from the exit surface of fused silica produced by laser irradiation can cause damage to adjacent optical components because of the high temperature and high impact velocity of particles. Classic molecular dynamics (MD) simulation was conducted to investigate the damage mechanism of optical elements under nanoparticle impact. The increasing temperature coupled with the rising compressive stress facilitated the densification of fused silica, and the over-coordinated silicon atoms were mainly affected by compressive stress until the compressive stress peaked. The influence of local temperature emerged after the compressive stress peaked. The time when the largest volume of heat affected zone appeared was closer to the time when the compressive stress reached the maximum under the higher impact velocity of the nanoparticle. The changes in microstructural characteristics of fused silica showed that the hot compression of fused silica make large ring smaller firstly and then make small ring larger. The findings are instructive in the damage mechanism of optics under the impact of nano- or micro-particle.

Changes in microstructure characteristics of cement paste on carbonation

This study investigates the influence of carbonation on the microstructure of cement paste cast with Ordinary Portland Cement (OPC), fly ash based Portland Pozzolana Cement (PPC) and Limestone Calcined Clay Cement (LC3) using X-ray diffraction (XRD), thermal analysis (TGA), scanning electron microscope (SEM) and mercury intrusion porosimeter (MIP). A comparison is made between samples exposed to 3% carbon dioxide concentration and those exposed to natural CO2 concentrations. It is observed that the products formed on carbonation are similar in both the exposure conditions. Distinct rims corresponding to decalcified C-S-H are observed around clinker grains in the carbonated samples. Coarsening of pore structure is observed on carbonation in all the cements and an increase in the total porosity is observed in blended cements. Thermodynamic modeling indicates irrespective of the type of cement the total solid volume is reduced on complete carbonation. The volume change occurring on carbonation obtained from experimental results and thermodynamic modeling are compared.

Synthesis, sintering and characterization of ceria-based solid electrolytes codoped with samaria and gadolinium using the Pechini method

Studies on fuel cell components have attracted interest due to the growing demand for sustainable energy sources. In the present study, synthesis of nanometric powders of the Ce0.8Sm0.2−xGdxO1.9 system (x = 0; 0.05; 0.1) system was carried out using the polymeric precursor method (Pechini), followed by calcination at 600°C for 1h and pressureless sintering. Characterizations were carried out with differential thermal analysis (DTA), thermogravimetric analysis (TG), infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The grain and grain boundary contributions to the ionic conductivity of the ceramic disks obtained were assessed by complex impedance spectroscopy (CIS). Microstructural characterization was conducted by SEM. Electrical characterization showed greater grain conductivity for samples that were codoped with increasing levels of gadolinium, likely due to less deformation in the crystalline lattice with the addition of an element that contains an ionic radius closer to that of the host matrix of ceria (Ce < Gd < Sm). Grain boundary conductivity was lower than grain conductivity with a gradual rise in the same codoping element. This results from changes in microstructural characteristics due to increased codoping, including a reduction in relative density.

An Experimental Study on Evolution of Grain‐Scale Stress/Strain and Geometrical Necessary Dislocations in Advanced TA15 Titanium Alloy during Uniaxial Tension Deformation

This study aims to characterize the spatial distributions of grain‐scale stress/strain and geometrical necessary dislocations (GNDs) in TA15 titanium alloy subjected to tensile deformation at room temperature. The microstructures under different strains are investigated through electron backscatter diffraction (EBSD) measurements. Evolution of intergranular stress/strain is revealed by analysis of high angular resolution EBSD patterns. The spatial distributions of GNDs are calculated using a self‐developed routine. Results indicate that many factors influence the spatial distributions of grain‐scale residual stress/strain and GNDs during plastic deformation, and the dominant factor varies with the changes in microstructure characteristics and macroscopic deformation parameters.

Stress Dependence of Switching Field during the Devitrification of Finemet-Based Magnetic Microwires

We have studied the influence of the thermal treatment on the stress dependence of the switching field during the devitrification of amorphous Fe73.5Cu1Nb3Si11.5B11 microwires. The non-destructive test (NDT) method based on the magnetization reversal of ferromagnetic materials is sensible to changes in microstructural characteristics and the stress state of the material. The stress dependence has been explained considering the magnetoelastic contribution to the switching mechanism which is modified applying the tensile stresses and changing the magnetostriction constant and strength of the internal stresses distribution through thermal treatments. We show that by properly setting a frequency during the measurement and adequate treatment of the sample, it is possible to vary the sensitivity, magnitude and stress dependence of the sample. Keywords: Magnetic bistability, glass-coated microwires, switching field, stress sensor

Etching technique for revealing work hardening regions in Mg–9Al–1Zn alloy

A method for revealing the localised work hardened region (plastic deformation zone) in Mg–9Al–1Zn alloy (AZ91) has been developed using a new etching technique. With this technique, etching with the solution (9 g picric acid, 30 mL acetic acid, 150 mL ethyl alcohol and 20 mL H2O) was executed on the samples after plastic deformation and then heating to 300°C for 3 h. Because of the high strain energy in the plastic deformation zone, the microstructural characteristic in the deformation zone changed substantially after the heating process, where severe precipitation of Mg17Al12 phase occurred in the Mg rich α phase. With the change of microstructural characteristics, the deformation region can be revealed. The plastic zone was revealed by the different degrees of etching response in the deformation and undeformed regions; the plastic zone was slightly etched, whereas the other region was deeply etched. From the different surface height level, the deformation zone was found to be observable even at low magnification.

A new etching technique for revealing the plastic deformation zone in an Al–Cu–Mg alloy

A new etching technique for revealing the plastic deformation zone in an Al–Cu–Mg alloy has been developed. The etching with the proposed etching agent was conducted on the deformed sample after being heated to 673 K for 3 h. With this etching technique, the plastic deformation zone was clearly observed even under low magnification. This was due to the change of microstructural characteristics in the plastic deformation zone after the heating process, in which there is significant precipitation of Al2Cu and Mg2Si, caused by the high energy arising from the severe deformation.

Critical behavior of transport in sea ice

Geophysical materials such as sea ice, rocks, soils, snow, and glacial ice are composite media with complex, random microstructures. The effective fluid, gas, thermal, and electromagnetic transport properties of these materials play an important role in the large-scale dynamics and behavior of many geophysical systems. A striking feature of such media is that subtle changes in microstructural characteristics can induce changes over many orders of magnitude in the transport properties of the materials, which in turn can have significant large-scale geophysical effects. For example, sea ice, which mediates energy transfer between the ocean and atmosphere, plays a key role in global climate, and serves as an indicator of climatic change, is a porous composite of ice, brine and gases. Relevant length scales range from microns and millimeters for individual brine structures, to centimeters and meters for connected brine channels across floes, to hundreds of kilometers across an ice pack. Sea ice is distinguished from many other porous composites, such as sandstones or bone, in that its microstructure and bulk material properties can vary dramatically over a relatively small temperature range. The fluid permeability of sea ice ranges over six orders of magnitude for temperatures between 0°C and −25°C. Moreover, small changes in brine volume fraction around a threshold value of about 5%, corresponding to variations in temperature around a critical point of about −5°C, control an important transition between low and high fluid permeability regimes. Below this critical temperature, the sea ice is effectively impermeable, while for higher temperatures the brine phase becomes connected over macroscopic scales, allowing fluid transport through the ice. This transition has been observed to impact a wide range of phenomena such as surface flooding and snow–ice formation, enhancement of heat transfer due to fluid motion, mixing in the upper ocean, melt pool persistence, surface albedo (ratio of reflected to incident radiation) and other optical properties, growth and nutrient replenishment of algal and bacterial communities living in sea ice, and remote sensing of the sea ice pack from space. Recently, we have shown how continuum percolation theory can be used to understand the critical behavior of fluid transport in sea ice. Here we review this application of percolation theory to sea ice, and briefly discuss electromagnetic transport in sea ice, in particular how the geometry and connectivity of the brine microstructure determine its effective complex permittivity.

On the local microstructural characteristics observed in sand cast Al–Si alloys

Abstract The aim of this work is to explore the phenomenology that describes the local solid formation and heat transfer occurring during sand cast alloys solidification in order to propose an explanation to the observed changes of local microstructural characteristic lengths in hypoeutectic and eutectic Al–Si based alloys. Microstructural observations are made in different radial positions of solidified rod castings. Also, solidification kinetics information is obtained using the Fourier thermal analysis method. A coupled heat transfer-solidification kinetics model is employed to predict the thermal history, the solidification kinetics and some microstructural parameters in order to compare the predictions with experimental results. The model and the experimental outcome suggest that there is a strong dependence of the local solidification kinetics on the local heat transfer. The analysis of this dependence is used to propose an explanation to the observed changes in microstructural characteristics at different locations within sand castings.

The Effect of Heat Transfer on Local Solidification Kinetics of Eutectic Al-Si Cast Alloy

Recently, Fourier thermal analysis (FTA) has been proposed as a suitable technique to obtain information about local solidification kinetics in casting alloys. In this work, FTA was applied to a near-eutectic aluminum-silicon cast alloy in order to seek experimental evidence supporting the solidification kinetics obtained from this method. Also, a heat-transfer/solidification-kinetics model was used to compare predictions with experimental results. The metallographic findings, focused on interlamellar eutectic spacings in different locations within a cylindrical casting, support the solidification kinetics obtained from FTA. The model and experimental outcome including FTA results and metallographic observations suggest that local solidification kinetics depend strongly on local heat transfer, and the analysis of this dependence may be used to explain the observed changes in microstructural characteristics at different locations within castings.