Microstructures and micromechanical behaviors of high -entropy alloys investigated by synchrotron X-ray and neutron diffraction techniques: A review

High-entropy alloys(HEAs)possess outstanding features such as corrosion resistance,irradiation resistance,and good mechan-ical properties.A few HEAs have found applications in the fields of aerospace and defense.Extensive studies on the deformation mech-anisms of HEAs can guide microstructure control and toughness design,which is vital for understanding and studying state-of-the-art structural materials.Synchrotron X-ray and neutron diffraction are necessary techniques for materials science research,especially for in situ coupling of physical/chemical fields and for resolving macro/microcrystallographic information on materials.Recently,several re-searchers have applied synchrotron X-ray and neutron diffraction methods to study the deformation mechanisms,phase transformations,stress behaviors,and in situ processes of HEAs,such as variable-temperature,high-pressure,and hydrogenation processes.In this review,the principles and development of synchrotron X-ray and neutron diffraction are presented,and their applications in the deformation mechanisms of HEAs are discussed.The factors that influence the deformation mechanisms of HEAs are also outlined.This review fo-cuses on the microstructures and micromechanical behaviors during tension/compression or creep/fatigue deformation and the application of synchrotron X-ray and neutron diffraction methods to the characterization of dislocations,stacking faults,twins,phases,and intergrain/interphase stress changes.Perspectives on future developments of synchrotron X-ray and neutron diffraction and on research directions on the deformation mechanisms of novel metals are discussed.

Microstructure Evolution of Sandstone Cemented by Microbe Cement Using X-ray Computed Tomography

The bio-sandstone, which was cemented by microbe cement, was firstly prepared, and then the microstructure evolution process was studied by X-ray computed tomography （X-CT） technique. The experimental results indicate that the microstructure of bio-sandstone becomes dense with the development of age. The evolution of inner structure at different positions is different due to the different contents of microbial induced precipitation calcite. Besides, the increase rate of microbial induced precipitation calcite gradually decreases because of the reduction of microbe absorption content with the decreasing pore size in bio-sandstone.

Application of X-ray Computed Tomography in Characterization Microstructure Changes of Cement Pastes in Carbonation Process

The microstructure characteristics and meso-defect volume changes of hardened cement paste before and after carbonation were investigated by three-dimensional （3D） X-ray computed tomograpby （XCT）, where three types water-to-cement ratio of 0.53, 0.35 and 0.23 were considered. The high-resolution 3D images of microstructure and filtered defects were reconstructed by an XCT VG Studio MAX 2.0 software, The meso- defect volume fractions and size distribution were analyzed based on 3D images through add-on modules of 3D defect analysis. The 3D meso-defects volume fractions before carbonation were 0.79%, 0.38% and 0.05% corresponding to w/c ratio=0.53, 0.35 and 0.23, respectively. The 3D meso-defects volume fractions after carbonation were 2.44%, 0.91% and 0.14% corresponding to w/c ratio=0.53, 0.35 and 0.23, respectively. The experimental results suggest that 3D meso-defects volume fractions after carbonation for above three w/c ratio increased significantly. At the same time, meso-cracks distribution of the carbonation shrinkage and gray values changes of the different w/c ratio and carbonation reactions were also investigated.

Studies of Microstructure of Kernels of Macadamia integrifolia and Its Hybrids through MRI, X-Ray Tomography and Confocal Microscopy

The aim of this study was to apply the existing techniques that enable examination ofmacadamia kernels to provide a better understanding of physico-chemical properties of kernels during postharvest processing. These techniques, such as X-ray tomography, could be applied for quality monitoring in the macadamia industry. The objectives of this study were to investigate the browning centre symptoms that usually occur in macadamia nuts-in-shell. The applied techniques included confocal microscopy, X-ray tomography and magnetic resonance imaging （MRI）. Five different varieties of macadamia nuts （A38, 246, 816, 842 and Daddow） were selected to include distinct characteristics, such as drop pattern and growing location. Analysis of the microstructure of kernels by confocal microscopy showed the distribution of possible brown pigment compounds as well as the distribution of lipids, carbohydrates and proteins inside macadamia cells. Physical properties data, including shell density and seed to volume ratio, were obtained by X-ray tomography. Magnetic resonance diffusion tensor imaging used in this study showed marked differences in microstructure which indicate that different varieties exhibit different microstructures expressed as fraction ofanisotropy and apparent diffusion coefficient that appear to be related to the occurrence of the brown centre. Hence, the findings of this study have potential to improve the existing postharvest techniques used in the macadamia processing industry. They will be of benefit to the industry in terms of improved quality control and cost reduction.

Microstructural Characteristics of Asphalt Concrete with Different Gradations by X-ray CT

The main objective of this paper is to evaluate the effects of asphalt concrete types on the microstructural characteristics at high-temperature. Suspend-dense structure and Skeleton-dense structure were selected to investigate the deformation of pavement at meso-scale. The internal microstructures of typical asphalt concretes, AC, SUP and SMA, were scanned by X-ray CT device, and microstructural changes before and after high-temperature damage were researched by digital image processing. Adaptive threshold segmentation algorithm（ATSA） based on image radius was developed and utilized to obtain the binary images of aggregates, air-voids and asphalt mastic. Then the shape and distribution of air-voids and aggregates were analyzed. The results show that the ATSA can distinguish the target and background effectively. Gradation and coarse aggregate size of asphalt mixtures have an obvious influence on the distribution of air-voids. The movements of aggregate particles are complex and aggregates with elliptic sharp show great rotation. The effect of gradation on microstructure during high-temperature damage promotes the research about the failure mechanism of asphalt concrete pavement.

Synthesis, X-ray Structure and Aggregation Effect of Tetramethoxy Substituted Dibenzo[ fg, op]naphthacene

Tetramethoxy substituted dibenzo[fg,op]naphthacene was synthesised by using Friedel-Crafts acylation, tandem Aldol-Michael solvent-free reaction and palladium catalyzed dehydrohalogenation cyclization as key steps. The X-ray structure of the product and its aggregation effect in solvent were also reported.

Mechanical Behavior Based on Aggregates Microstructure of Ultra-high Performance Concrete

We developed ultra-high performance concrete(UHPC)incorporating mullite sand and brown corundum sand(BCS),and the quartz sand UHPC was utilized to prepare for comparison.The properties of compressive strength,elastic modulus,ultrasonic pulse velocity,flexural strength,and toughness were investigated.Scanning electron microscopy and nanoindentation were also conducted to reveal the underlying mechanisms affecting macroscopic performance.Due to the superior interface bonding properties between mullite sand and matrix,the compressive strength and flexural toughness of UHPC have been significantly improved.Mullite sand and BCS aggregates have higher stiffness than quartz sand,contributing to the excellent elastic modulus exhibited by UHPC.The stiffness and volume of aggregates have a more significant impact on the elastic modulus of UHPC than interface performance,and the latter contributes more to the strength of UHPC.This study will provide a reference for developing UHPC with superior elastic modulus for structural engineering.

Effect of Modification Treatment on Chloride Ions Permeability and Microstructure of Recycled Brick-mixed Aggregate Concrete

The modification methods of pozzolan slurry combined with sodium silicate and silicon-based additive were respectively adopted to treat recycled coarse brick-mixed aggregate(RCBA)in this study.The compressive strength and chloride permeability resistance of recycled aggregate concrete(RAC)before and after modification treatment were tested,and the microstructure of RAC was analyzed by mercury intrusion porosimetry(MIP)and scanning electron microscopy(SEM).The results show that the physical properties of RCBA strengthened by modification treatment are improved,and the compressive strength and chloride permeability resistance of treated RAC are also significantly improved.The modification treatment optimizes the pore size distribution of RAC,which increases the number of gel pores and transition pores,and decreases the number of capillary pores and macro pores.The surface fractal dimension shows a significant correlation with chloride diffusion coefficient,indicating that the variation of chloride permeability of treated RAC is consistent with the microstructure evolution.

Kernel crack characteristics for X-ray computed microtomography(μCT)and their relationship with the breakage rate of maize varieties

The most significant problem of maize grain mechanical harvesting quality in China at present is the high grain breakage rate(BR).BR is often the key characteristic that is measured to select hybrids desirable for mechanical grain harvesting.However,conventional BR evaluation and measurement methods have challenges and limitations.Microstructural crack parameters evaluation of maize kernel is of great importance to BR.In this connection,X-ray computed microtomography(μ-CT)has proven to be a quite useful method for the assessment of microstructure,as it provides important microstructural parameters,such as object volume,surface,surface/volume ratio,number of closed pores,and others.X-ray computed microtomography is a non-destructive technique that enables the reuse of samples already measured and also yields bidimensional(2D)cross-sectional images of the sample as well as volume rendering.In this paper,six different maize hybrid genotypes are used as materials,and the BR of the maize kernels of each variety is tested in the field mechanical grain harvesting,and the BR is used as an index for evaluating the breakage resistance of the variety.The crack characteristic parameters of kernel were detected by X-ray micro-computed tomography,and the relationship between the BR and the kernel crack characteristics was analyzed by stepwise regression analysis.Establishing a relationship between crack characteristic parameters and BR of maize is vital for judging breakage resistance.The results of stepwise multiple linear regression(MLR)showed that the crack characteristics of the object surface,number of closed pores,surface of closed pores,and closed porosity percent were significantly correlated to the BR of field mechanical grain harvesting,with the standard partial regression coefficients of–0.998,–0.988,–0.999,and–0.998,respectively.The R2 of this model was 0.999.Results validation showed that the Stepwise MLR Model could well predict the BR of maize based on these four variables.

Small Angle Neutron Scattering and X-Ray Diffraction Studies of Nanocrystalline Titanium Dioxide

Nanocrystalline titanium dioxide powder is characterized for phase analysis as well as particle size and its distribution by x-ray diffraction and small angle neutron scattering measurements. Analysis of the SANS data in the momentum transfer range q = 0.1 - 1.8 nm–1 reveals an average particle size of 24.82 nm in good agreement with the particle size determined earlier by transmission electron microscopy. XRD measurement proves co-existence of rutile and anatase phases in this commercial TiO2 nanocrystalline powder.

Study on NiO/Fe interface with X-ray photoelectron spectroscopy

Different monolayers （ML） of Fe atoms were deposited on NiO （001） substrates or NiO underlayers using molecular beam epitaxy （MBE）, pulse laser deposition （PLD）, and magnetron sputtering （MS）. The magnetic properties and microstructure of the films were studied. The apparent magnetic dead layer （MDL） is found to exist at the NiO/Fe interfaces of the MBE sample （about 2 ML MDL）, the PLD sample （about 3 ML MDL）, and the MS sample （about 4 ML MDL）. X-ray photoelectron spectroscopy indicates the presence of ionic Fe （Fe2＋ or Fe3＋） and metallic Ni at the NiO/Fe interfaces, which may be due to the chemical reactions between Fe and NiO layers. This also leads to the formation of MDL. The thickness of the MDL and the reaction products are related with the deposition energy of the atoms on the substrates. The interfacial reactions are effectively suppressed by inserting a thin Pt layer at the NiO/Fe interface.

Investigation of Changes in the Fine Structure of Graphitizing Carbon Materials during Heat Treatment by X-Ray Diffraction Analysis

The properties of synthetic graphite materials, widely used in advanced fields, are determined by their structure, which is formed in the process of high-temperature (~2500°C) heat treatment. The fine structure of a graphitizing carbon material based on petroleum coke containing 1.3 wt% S at various stages of graphitization was studied by X-ray diffraction analysis. Some of the samples contained the addition of dispersed Fe2O3. It is shown that the heat-treated material in the range 1200°C - 2600°C is heterogeneous, its component composition is determined by the processing temperature and the presence of Fe2O3 additive. The observed dependence of the component composition on the heat treatment temperature suggests that the process of graphitization of the carbon material, apparently, develops through a number of metastable states.

Effect of Pre-wetting Lightweight Aggregates on the Mechanical Performances and Microstructure of Cement Pastes

The water absorption and desorption processes of different types of lightweight aggregates were studied.Subsequently,the influences of pre-wetting lightweight aggregates on compressive strength,microhardness,phase composition,hydration parameters and micromorphology of the cement pastes were investigated.The results showed that the water absorption and desorption capacities of the lightweight aggregates increased with the decrease of the densification degree.With the addition of pre-wetting lightweight aggregates,the compressive strength of the cement pastes would increase.Moreover,the enhancement effect was more obviously with the desorption capacity of pre-wetting lightweight aggregates increasing.Especially,sample S1 with pre-wetting red-mud ceramisites had the highest compressive strength,of which increased to 49.4 MPa after 28 d curing age.The reason is that mainly because the addition of pre-wetting lightweight aggregates can promote the generation of C–S–H gels in the interfacial zone,and the hydration degree of the interfacial zone increases with the water desorption of pre-wetting lightweight aggregates increasing.It is contributed to optimize the microstructure to enhance microhardness of the interfacial zone,resulting in the compressive strength of the cement-based materials improving.Therefore,the pre-wetting lightweight aggregates with high porosity and strength are the potential internal curing agents for high-strength lightweight concretes.

Highly two-photon and X-ray excited long-persistent luminescence in a crystalline host-guest aggregate

As a unique type of supramolecular self-assemblies,crystalline host-guest aggregates have attracted extensive interests in multiple application fields.Herein,a crystalline host-guest aggregate LIFM-HG1 was obtained with curcubit[8]uril as the host and carboxypyridinium salt as the guest.Single-crystal structural analysis indicates that the presence of abundant weak interactions in LIFM-HG1 provides a rigid environment for the guest molecule and effectively blocks the external quenchers.Spectral analysis and theoretical calculations confirm the presence of robust triplet energy levels in LIFM-HG1.Even more impressively,the intersystem crossing channels of the guest molecules are greatly opened up after the formation of the crystalline host-guest aggregate,resulting in a large k_(isc)of 6.70×10^(7)s^(−1)at room temperature for LIFM-HG1(which is∼0 for pure guest),leading to fascinatingmultichannel(including one-photon,two-photon,and X-ray)excited LPL properties.In addition,the crystalline LIFM-HG1 has a much higher triplet state luminescence efficiency under X-ray and two-photon excitation than that under single-photon excitation(A_(P)/A_(F)=86.8,44.8,10.7 under the three circumstances,respectively).And the phosphorescent emission intensity of LIFM-HG1 is 27.6 times higher than that of the crystalline guest under X-ray excitation.As a result,LIFM-HG1 shows a long afterglow retention time under both single-and two-photon excitation,and an impressive afterglow retention time of 1 s under X-ray excitation.Furthermore,the excellent lysosomal targeting and low cytotoxicity by the formation of host-guest aggregate makes LIFM-HG1 promising to be used as a novel lysosomal-targeted two-photon excited phosphorescent tracer.

Strength and Microstructure of Mortar Containing Glass Powder and/or Glass Aggregate

The compressive strength of mortar containing glass powder（GP） and/or glass aggregate（GA） was tested, and its microstructure was also studied by thermogravimetric and differential thermal analysis（TG-DTA）, scanning electron microscopy（SEM）, energy dispersive spectroscopic analysis（EDX）, and X-ray diffraction（XRD） techniques. The incorporation of GA would decrease the compressive strength of the mortar in the absence of GP. Incorporating both GA and GP could change the hydration environment, promote pozzolanic reaction of GP and improve the compressive strength. GP does not lead to but can effectively control ASR（Alkali Silica Reaction）. GP and GA do not transform the type of hydrates, but have a great influence on the amounts of hydration products, and generate more calcium silicate hydrate（C-S-H gel） with lower Ca/Si ratio. GP and GA with good gradation will make the microstructure denser.

MICROSTRUCTURE OF AGGREGATES IN DISPERSIONS DESCRIBED IN TERMS OF RADIAL DISTRIBUTION FUNCTION

Following a sticky particle model and its computer simulation scheme proposed in the previous papers, the motions of particles in both 2-and 3-dimensional shear flow fields are simulated. At a steady state of clustering process, the radial distribution functions are calculated to precisely describe the microstructure of aggregates in dispersions, and the configuration of particles is displayed,which gives a direct view of microstructure. It is found that (1) the kind of the microstructure transforms from compact clusters to a loose network as the concentration of particles increases; (2) the microstructure is independent of shear rate which only dominates the size of clusters formed at steady state.

Influence of Kyanite Content on Microstructures and Properties of Castables Containing Spherical Lightweight Mullite Aggregates

Five lightweight mullite castables were prepared using the lightweight mullite aggregate（M45A） with narrow particle size distribution and spherical shape,mullite powder（M45P）,kyanite powder,microsilica and Secar 71 cement as raw materials. After the specimens fired at 1100 ℃,1200 ℃ and 1300 ℃ for 3h,respectively,the effects of kyanite contents（ 8%,10%,12%,14%,16% by mass） on the physical properties,phase composition and microstructures were investigated through an X-ray diffractometer（ XRD）,a scanning election microscopy（ SEM） and a microscopy measurement method,etc. The results show that,for the specimens fired at 1300 ℃ with the kyanite content from 8%to16%, the bulk densities and apparent porosities change little,but the cold modulus of rupture and the crushing strengths decrease significantly,and the linear change rates increase from-0.87% to 0.64%. The most apposite mode is the specimen containing 12% kyanite fired at 1300 ℃, which has a high crushing strength of 29.3 MPa,a low linear change of 0.18%,an apparent porosity of 36.8% and a bulk density of 1.73 g/cm^3.

The Effect of Mineral Composition, Microstructure on Physical Properties of Aggregate from River Punjkora and Kunai, Dir (Lower), Khyber Pakhtunkhwa, Pakistan

Aggregate is a worldwide raw material used for various construction purposes which are obtained from geological material through drilling, blasting, mining, and river derived. This study has been focused on river Panjkora and Kunai-derived aggregates which are natural sources of aggregate used for various construction purposes in Dir (Lower). Field observation and detailed petrographic studies of representative samples reveal that rivers Panjkora and Kunai pass from different lithological units of granite, gabbroic, and amphibolite. Petrographically, the granitic rocks of the study area are consisting of alkali feldspar, quartz, and plagioclase feldspar. Minor accessory minerals in granite were hornblende, biotite, epidote, rutile, zircon, and ore minerals. The gabbroic rocks are consisting of pyroxene, plagioclase, and minor to accessory minerals including biotite, sphene, rutile, epidote, and ore minerals. The main mineral phases in amphibolites are hornblende, plagioclase, and a minor amount of quartz, sphene, and ore minerals. Gneissose fabric has developed in amphibolite refers to its banded nature. The minor mineral phases of amphibolite include epidote, ore minerals, and garnet. The feldspar alteration in granite is in the form of sericitization and normal zoning. The physical properties of river Panjkora and Kunai float i.e., water absorption, specific gravity, fine modulus, loss Angeles and soundness were within the range. Both petrographic and geotechnical properties suggested that these aggregates are suitable to be used as coarse aggregate in concrete. These must be present in the recommendation section at the end.

Mechanism of Effects of Rare Earths on Microstructure and Properties at Elevated Temperatures of AZ91 Magnesium Alloy

By using real-space recursion method,the energetics of the undoped and Al and/or RE atoms doped 7（1450）〈0001〉 symmetric tilt grain boundaries（GBs）in AZ91 alloys were investigated.Similar calculations were performed on undoped and doped bulk α Mg for comparison.The results showed that Al atoms segregated at GBs in AZ91 alloys.When RE atoms were added,they also segregated at GBs,and their segregation is stronger than Al atoms＇.Therefore,RE atoms retard the segregation of Al atoms.Calculations of interaction energy indicated that Al atoms repelled each other,and could form ordered phase with host Mg atoms.On the contrary to the case of Al,RE atoms attracted each other,they could not form ordered phase with Mg,but could form clusters.Between RE and Al,there existed attractive interaction,and this attractive interaction was the origin of Al11RE3 precipitation.Precipitation of Al11RE3 particles with high melting point and high thermal stability along GB improves high temperature properties of AZ91 alloys.

Microstructure and anisotropic swelling behaviour of compacted bentonite/sand mixture

Pre-compacted elements （disks, tutus） of bentonite/sand mixture are candidate materials for sealing plugs of radioactive waste disposal. Choice of this material is mainly based on its swelling capacity allowing all gaps in the system to be sealed, and on its low permeability. When emplaced in the gallery, these elements will start to absorb water from the host rock and swell. Thereby, a swelling pressure will develop in the radial direction against the host rock and in the axial direction against the support structure. In this work, the swelling pressure of a small scale compacted disk of bentonite and sand was experimentally studied in both radial and axial directions. Different swelling kinetics were identified for different dry densities and along different directions. As a rule, the swelling pressure starts increasing quickly, reaches a peak value, decreases a little and finally stabilises. For some dry densities, higher peaks were observed in the radial direction than in the axial direction. The presence of peaks is related to the microstructure change and to the collapse of macro- pores. In parallel to the mechanical tests, microstructure investigation at the sample scale was conducted using microfocus X-ray computed tomography （BCT）. Image observation showed a denser structure in the centre and a looser one in the border, which was also confirmed by image analysis. This structure heterogeneity in the radial direction and the occurrence of macro-pores close to the radial boundary of the sample can explain the large peaks observed in the radial swelling pressure evolution. Another interesting result is the higher anisotropy found at lower bentonite dry densities, which was also analysed by means ofμCT observation of a sample at low bentonite dry density after the end of test. It was found that the macro-pores, especially those between sand grains, were not filled by swelled bentonite, which preserved the anisotropic microstructure caused by uniaxial compression due to the absence of microstructure collapse.