Triple Grain Junctions Research Articles

Effect of inert glass on the physical and mechanical properties of zirconia dental ceramics fabricated by digital light processing

AbstractZirconia dental ceramics incorporated with inert dental glass (IDG) were fabricated by digital light processing (DLP). The effects of the amount of added IDG on the physical properties and microstructure of sintered samples were investigated. Highly dense zirconia dental ceramic samples with a relative density of 98.9% were obtained by adding different amounts of IDG sintered at 1500°C. With the addition of 1 wt% IDG, the Vickers hardness, fracture toughness, and wear resistance increased to 13.03 GPa, 6.33 MPa m1/2, and 0.03 mg/(mm2 min), respectively. However, with the incremental addition of IDG at 3 and 5 wt%, a slight reduction in Vickers hardness, fracture toughness, and wear resistance was observed. The values recorded were 12.68 GPa, 5.90 MPa m1/2, and 0.05 mg/(mm2 min) for the former, and 11.86 GPa, 5.48 MPa m1/2, and 0.06 mg/(mm2 min) for the latter. Scanning electron microscopy observation suggested that 1 wt% IDG incorporated into zirconia dental ceramic samples resulted in an optimal microstructure, with IDG existing at triple junctions of zirconia grains to form a discrete secondary phase. The broken surface revealed transgranular fracture features. This investigation indicates that adding a proper amount of IDG is an effective method to enhance the properties of DLP‐fabricated zirconia dental ceramics.

Fabrication, oxidation and creep behavior of Ti2AlN composites

In order to enhance the limited information on elevated temperature stability of MAX phase materials, Ti2AlN MAX phase composites were fabricated in a two-step procedure and subsequently tested regarding their oxidation and creep behavior. Furthermore, commentary testing on material enhanced with different amount of Al2O3 fibers was performed. The compressive creep was investigated in the temperature range of 900–1200 °C under stresses ranging from 20 to 100 MPa, with a loading direction parallel and perpendicular to the compression direction during sintering. The oxidation behavior was characterized, demonstrating excellent oxidation resistance of the samples up to 1200 °C. Supported by microstructural investigations, the impact of the texturing of the matrix grains, the content and the orientation of Al2O3 fibers, as well as that of the interfaces between matrix grains and the Al2O3 fibers were investigated. De-cohesion and cracks formed at the triple junctions of the MAX phase grains indicate grain boundary sliding as predominant creep mechanism for the composites.

Percolation of Low‐Dimensional Water at Crystalline Interfaces Mediates Fluid Migration in Subducting Slabs

AbstractDuring subduction, metamorphic dehydration reactions in the downgoing slab release fluids, generating fluid overpressure. It has been suggested that fluid is driven to flow upward by buoyancy, but a sufficiently high permeability allowing formation of a fluid percolation network is required. Traditionally, fluid percolation has been identified based on the textural equilibrium assumption by measuring the dihedral angle at the triple junction of grains. According to this theory, grain boundaries generally cannot be infiltrated by fluid, and only the grain edge can form a fluid flow channel. We argue that this theory is insufficient because we have found that water from fluid can be adsorbed into the crystalline interface, that is, a layered mineral interlayer, a crack, or a grain boundary. The high pressure in a subducting slab drives water adsorption into the crystalline interface, forming a low‐dimensional fluidic phase, and thus fluid percolation is achieved. Because water adsorbed in the interface is fluidic, water diffusion drives fluid transport in the subducting slab. Due to water adsorption, fluid overpressure at the dehydration front may release, so that dehydration embrittlement may be excluded. Stable water adsorption in the subduction‐slab conditions is determined here by combining molecular dynamics simulations and thermodynamic calculations. Analysis based on simulations shows that water adsorption requires crystalline surfaces which do not form hydrogen bonds well.

Diffusion along a network of the triple junctions of grains

Diffusion along a network of the triple junctions of grains has been considered under the conditions of negligibly small diffusion along all other diffusion paths. The conditions have been found where such a network acts similar to a homogeneous medium. For this case, the effective diffusion coefficient in the absence of texture has been found.

APPLICATION OF MODELS OF THE STRUCTURE OF SEPARATION SURFACES IN STEELS WHEN OBTAINING PRODUCTS WITH A GIVEN LIFE CYCLE

Background. The development of digital mechanical engineering requires the development and use of interdisciplinary concepts and models of the behavior of heterogeneous polycrystalline materials under the influence of technological and operational loads. In this regard, the development and use of approaches to modeling and analysis of the mechanical behavior of the boundary zones of separation of products, taking into account their energy, becomes important. The development of such approaches makes it possible to solve an actual scientific and technical problem – optimization of the modes of technological processing of products with a given life cycle. Objective. The use of system models of the structure of steels for the development of computational schemes and methods of managing the structural-energy state of the separation surfaces of structural components when obtaining products with increased reliability parameters, taking into account the conditions of their operation. Methods. The analysis of the structure was carried out using the concept of invariant modeling, the results of metallographic and factographic studies, electron microscopy. The assessment of the structural and energetic state of grain separation surfaces after ionplasma etching of the samples was carried out, wear and impact bending tests were carried out, and system computer simulation of the properties of polycrystalline systems was carried out using the FEniCS open finite element analysis package in Python. Results. When applying the energy description of continuous non-local environments, it was established that depending on the angles between the boundaries at the joints of the grains, the structural and energetic state of this region and its ability to form microdefects change. At angles between 140‒180°, zones are formed with a high capacity for the formation of intergranular damage and destruction under load. It is shown that the intensity of wear and intergranular fracture upon impact increases depending on the number of triple joints, in which the boundaries differ significantly in terms of energy level. Steel 40X (5135 Steel) is more sensitive to the formation of triple junctions with a high energy gradient during heat treatment than steels containing nickel, molybdenum and silicon. Conclusions. On the basis of hierarchical modeling of the structure of polycrystals, new approaches are proposed and an algorithm is created to determine the relationships between the structure of the separation surfaces of steels and their operational reliability. Diffuse damage is formed on the separation surfaces between the triple junctions of grains with a high energy gradient. Reducing the share of triple joints, which differ significantly in terms of energy when designing the technological regimes of the quenching and tempering processes, made it possible to increase the wear resistance of improved steels by 48‒53 %.

Recrystallization Mechanism and Processing Map of 18CrNiMo7-6 Alloy Steel during Hot Deformation

In this study, isothermal single-pass forming doformation of forged 18CrNiMo7-6 alloy steel was carried out by Gleeble-3500 thermal simulation testing machine. The constitutive equations and processing maps with parameters of deformation temperature and strain rate were established. The results show that the optimum hot deformation parameters are temperature 1050 °C, strain rate 0.1 s–1 with the peak power efficiency being 0.432. The mechanism of grain refinement during hot compression was also characterized by electron backscatter diffraction (EBSD). The results show that continuous dynamic recrystallization (CDRX), discontinuous dynamic recrystallization (DDRX) and grain growth are the main microstructure evolution mechanisms during hot working. The rotation of sub-grains under CDRX mechanism is the main factor for the formation of new grains. In addition, the DDRX mechanism is formed by the bulging of HAGBs at the grain boundary triple junction of the original grains, and the CDRX mechanism forms finer grains. The study also found that temperature affected the organization evolution mechanism, the DDRX mechanism plays a leading role when the temperature is low. With the increase of deformation temperature, CDRX begins to play a leading role and forms finer grains. When the deformation temperature rises to 1150 °C, the grains continue to grow at a higher temperature.

Study on static softening behavior and hot working performance of Fe-0.2C-7Mn steel

Static softening behavior and hot working performance of medium Mn steel (Fe-0.2C-7Mn) are studied by 2-step and interrupt hot compression test with a Gleeble-3500 thermal-mechanical simulator. The results show that static softening behavior of the steel exhibits obvious sensitivity to the deformation temperature, inter-pass time and strain rate. But deformation temperature has a more significant effect on the static recrystallization (SRX) grain size than strain rate. 3D hot working processing maps are established, that low temperature and medium strain rate (850 °C ∼ 925 °C and 0.02 s−1 ∼ 0.3 s−1) are the preferred thermal deformation conditions for the test steel. Microstructure observations indicate that both static recovery (SRV) and SRX occur within inter-pass time, and SRV is the predominant mechanism in determining softening fraction. The microstructure is composed of film-like α at room temperature, and most of the nucleation of SRX preferably take place at triple junctions of grains or prior grain boundaries. Besides, high-angle grain boundaries (HAGBs) migration produce twin boundaries during grain growth and the main nucleation mechanism of the SRX is strain-induced boundary migration. The kinetics model of static softening is established and the experimental values are consistent with predicted ones, which proves the reliability of the model.

The role of a shear planar mesodefect in the nucleation of a crack at a grain junction due to athermal grain boundary sliding

Ductile fracture of polycrystalline metals is usually preceded by a long deformation stage, during which grains of polycrystal are gradually divided into mutually misoriented regions (fragments) separated by strain-induced grain boundaries. The size of these regions usually does not exceed 0.2 – 0.4 mm. At such small sizes of fragments, classical models of crack nucleation based on the concept of lattice dislocation pile-ups retarded by grain boundaries become incorrect. In recent years, models have been developed to describe the nucleation of cracks under the action of elastic stress fields of rotational and shear-type mesodefects formed at the grain junctions and grain boundaries due to inhomogeneous plastic deformation in the ensemble of polycrystal grains. In this paper we consider the possibility of the nucleation of microcracks at the grain junction due to athermal sliding along the grain boundary, containing a planar mesodefect of the shear-type induced by strain. It is assumed that a planar mesodefect, represented in the initial state by uniformly distributed glissile components of orientational misfit dislocations, loses its stability when the external stress exceeds a certain threshold value. As a result of the strain-induced grain boundary sliding and plastic shear retarding, a stress concentrator arises near the triple junction of grains, creating conditions for the appearance of a Zener crack. The dependences of the critical external stress for the nucleation of microcracks on the length of the mesodefect, its strength and the threshold stress of athermal sliding are obtained. It is shown that the presence of a mesodefect at the grain boundary can lead to a significant decrease of the crack nucleation stress in comparison with the case of pure grain boundary sliding. It is concluded that the proposed model can be considered as one of the possible mechanisms for the nucleation of microcracks in materials with a fragmented structure.

Magnetics Hysteresis Properties and Microstructure of High-Energy (Nd,Dy)\u2013Fe\u2013B Magnets with Low Oxygen Content

Magnetic properties and microstructure of high-energy (Nd,Dy)–Fe–B magnets with Dy of no more than 1 wt % prepared via a low-oxygen routine are studied. Oxygen content in magnets does not exceed 0.20 wt %. 0.5 wt %–Dy addition reliably stabilizes the coercivity MHc higher than 13 kOe; in this case, the maximum energy density product (BH)max of magnets is 48.5–49.5 MG Oe. High magnetic hysteresis properties are gained via optimization of chemical and phase compositions of magnets, as well as their microstructure. The grain size of the main Nd2Fe14B phase is approximately 3.5 μm; and according to X-ray analysis, the weight fraction of additional Nd-rich phases (NdOx and Nd2O3) does not exceed 2.5%. Scanning electron microscopy study has demonstrated that in triple junctions of Nd2Fe14B grains there are two types of inclusions (В and С) of the NdOx phase, which significantly differ by their chemical composition. С-phase inclusions with low oxygen content (х ≈ 0.03) are enriched in Fe (40–50 wt %); whereas, В-phase with high oxygen content (х ≈ 0.70) contains 3–5 times less Fe. The angular dependences of coercivity of (Nd,Dy)–Fe–B magnets are presented.

Effects of residual thermal stress on mechanical behavior of cermets with different grain sizes

The distribution of the residual thermal stress (RTS) in the as-sintered cermets and its interactions with the applied stress were studied by simulations and experiments, using the WC-Co cemented carbides as examples. The influences of grain size, grain morphology and phase configuration on the stress state and mechanical behavior were quantified in the model, which was constructed from real multi-phased microstructures. The results indicate that the volume-averaged RTS is a power function of the mean WC grain size. There is a high level of compressive stress in the WC regions near the triple junctions of adjacent WC grains and Co binder, and a high level of tensile stress inside the Co phase in the vicinity of WC/Co phase boundaries. In the cermets with RTS, the metallic binder exhibits a gradually increasing strain response rate under the external loading. It is found that a smaller WC grain size, a lower proportion of Co thin-layers, and a higher fraction of triple junctions of adjacent WC grains and Co phase, are beneficial to have less stress concentration and simultaneously high strength and fracture toughness of the cermets. This study provides a new approach to characterize stresses in the multi-phased materials and a strategy to adjust stress distribution by tailoring the microstructure for excellent comprehensive mechanical properties.

Grain boundary modification and properties enhancement of sintered Nd-Fe-B magnets by ZnO solid diffusion

A grain boundary diffusion (GBD) process using rare earth-free compound as the diffusion source is employed to enhance coercivity of sintered Nd-Fe-B magnets. Typically, a ZnO layer was coated on the Dy-free sintered Nd-Fe-B magnets by sputtering deposition. After a solid diffusion heat treatment, the coercivity and corrosion resistance have been significantly improved. For a 4 mm-thick magnet, the diffusion of ZnO led to a maximum increase of intrinsic coercivity from 1085 to 1290 kA/m. This coercivity enhancement is much higher than those obtained in the previous investigations by GBD of non-RE compounds or alloys. Based on the microstructure analysis, the coercivity improvement is induced by the modifications of the composition and structure of grain boundary phase, which is slightly different from the conventional rare earth-based GBD. The formation of oxides at the triple junction of grains is the main reason for the enhanced corrosion resistance of diffused magnets. The present work thus demonstrated that ZnO can be employed as the GBD source for both coercivity enhancement and corrosion resistance improvement, which are important for preparing low-cost and high-performance Nd-Fe-B permanent magnets.

Analysis of Corrosion of Hastelloy-N, Alloy X750, SS316 and SS304 in Molten Salt High-Temperature Environment

Nickel superalloy Hastelloy-N, alloy X-750, stainless steel 316 (SS316), and stainless steel 304 (SS304) are among the alloys used in the construction of molten salt reactor (MSR). These alloys were analyzed for their corrosion resistance behavior in molten fluoride salt, a coolant used in MSR reactors with 46.5% LiF+ 11.5% NaF+ 42% KF. The corrosion tests were run at 700 °C for 100 h under the Ar cover gas. After corrosion, significant weight loss was observed in the alloy X750. Weight loss registered in SS316 and SS304 was also high. However, Hastelloy-N gained weight after exposure to molten salt corrosion. This could be attributed to electrochemical plating of corrosion products from other alloys on Hastelloy-N surface. SEM–energy-dispersive X-ray spectroscopy (EDXS) scans of cross-section of alloys revealed maximum corrosion damage to the depth of 250 µm in X750, in contrast to only 20 µm on Hastelloy-N. XPS wide survey scans revealed the presence of Fe, Cr, and Ni elements on the surface of all corroded alloys. In addition, Cr clusters were formed at the triple junctions of grains, as confirmed by SEM–EBSD (Electron Back Scattered Diffraction) analysis. The order of corrosion resistance in FLiNaK environment was X750 < SS316 < SS304 < Hastelloy-N.

ДИФФУЗИОННОЕ ЗАРОЖДЕНИЕ ПОР В СТЫКАХ ЗЕРЕН СУБМИКРОКРИСТАЛЛИЧЕСКИХ МАТЕРИАЛОВ

The conditions of diffusional cavity nucleation in submicrocrystalline materials processed by the methods of intensive plastic deformation (equal-channel angular pressing, multiaxial forging, high pressure torsion, etc.) are analyzed. To date, the question of the mechanism of nucleation of cavities in such materials remains debatable due to the fact that the processing of materials by the methods of intensive plastic deformation is carried out at high hydrostatic pressures that prevent the appearance of pores. The possibility of diffusive nucleation of nanopores in the region of triple junctions of grains containing negative strain-induced wedge disclinations, generating high tensile stresses in the vicinity of triple junctions, comparable in magnitude to external hydrostatic pressure, is shown. Such junction disclinations inevitably occur at the grain junctions due to the heterogeneity of the plastic deformation through the ensemble of polycrystal grains. It is shown that an important condition for the nucleation of cavities is not only the presence of high internal tensile stresses from junction disclinations, but also an extremely high concentration of nonequilibrium strain-induced vacancies characteristic of submicrocrystalline metals, comparable in values to the vacancy concentration, at temperatures close to solidus. The influence of the strength of junction disclinations, the value of external hydrostatic pressure and the degree of supersaturation of the material by nonequilibriumstrain-induced vacancies on the rate of diffusional nucleation and the volume of critical pore nuclei is analyzed. It is established that in order to effectively suppress the process of pore formation in the grain boundary triple junctions, it is necessary to apply an external hydrostatic pressure that compensates for internal elastic fields from junction disclinations.

ИССЛЕДОВАНИЕ ВЛИЯНИЯ СТЫКОВЫХ ДИСКЛИНАЦИЙ НА ДЕФОРМИРУЮЩЕЕ НАПРЯЖЕНИЕ ПОЛИКРИСТАЛЛОВ

Plastic deformation of polycrystalline solids is accompanied by the appearance of linear rotational-type mesodefects at grain boundary ledges and triple junction of grains, such as starin induced junction disclinations. Junction disclinations generate long-range spatially inhomogeneous fields of elastic stresses, which significantly influence on the structure formation, strain hardening and fracture of materials. In present work a comparative analysis of the contributions of junction disclinations of different sign and strength to the plastic flow stress of a polycrystal is performed. The results of calculationsshow, that when a pile-up of lattice dislocations passes through the elastic field of disclinations, general regularities are observed.Regardless of the sign of disclination, it has a retarding effect on the plastic shear. The equilibrium distributions of the linear density and the density of the Burgers vector of dislocations pile-upretarded by the elastic field of disclination are calculated.It is shown that the largest number of dislocations is concentrated not in the pile-up head, as in classical dislocation pile-upsstoped near impenetrable barriers, but in its central part. The dependences of the critical stress of the passage of the head dislocation of the pile-up through the force barrier of disclination are calculated depending on the strength and sign of disclination, the number of dislocations in the pile-up, and the distance between the disclination and the slip plane of lattice dislocations.It is shown that the change in the sign of disclination significantly influences on the form of the equilibrium distribution of dislocations along the length of the pile-up, but practically does not affect the value of the critical shear stress. It is shown that for a fixed number of dislocations in the pile-up, the critical shear stress increases with the distance between the slip plane and disclination. Thus, when plastic deformation is localized, the greatest strengthening effect from the elastic field of junction disclination is achieved not near the boundary, but far from it.

Magnetic Structure and Nanomechanical Properties of Sintered Permanent Magnets Nd–Dy–Fe–B USC-20L

Using scanning electron microscopy (EDS analysis), magnetic force microscopy, and nanoindentation, a metallographic study of the magnetic structure and nanomechanical properties of sintered rare-earth magnets Nd–Dy–Fe–B of the USC-20L brand has been carried out (Ural Strip Casting Technology). The microstructure of the sintered Nd–Dy–Fe–B magnet of the USC-20L grade includes the following components: grains of the Nd2Fe14B phase are separated by lamellas of the phases enriched in neodymium. Inclusions Nd–29.1% Fe–6.2% C–2.2% O–1.4% Dy are located at the triple junctions of Nd2Fe14B grains. Inclusions Nd–4.5% Fe–9.1% O–6.7% C–4.5% Fe–2% Dy are along grain boundaries containing Nd and Dy oxides. The chemical composition of grains is Fe–25% Nd–6.9% C–1.6% Dy–1.4% B. It is found that, due to irregular grain growth, the interlayers of the enriched Nd phase are connected to each other in the region of grain junctions, causing a concentration of internal stresses and the appearance of a crack. The crack propagates along the grain boundary from one wetted joint of the grains to another, due to mechanical stresses. The phenomenon of intergranular wetting by a phase of Nd-enriched Nd2Fe14B/Nd2Fe14B grain boundaries is observed. It has been established that phases enriched with Nd can pseudo-incompletely (or pseudo-partially) wet such grain boundaries, i.e., form a nonzero contact angle along the grain boundaries and in triple joints. Based on the results of Magnetic Force Microscopy (MFM), a conclusion is made about the presence of a one-dimensional domain structure; domains cross grain boundaries. The presence of pores and inclusions of Nd and Dy oxides localized along grain boundaries is noteworthy. The average transverse domain size of the banded structure is ~1 μm, the energy of the domain wall γ ~ 14 kJ/m2, and the width of the domain wall δ ~ 0.6 × 10–9 m. The nanoindentation method is used to measure the values of nanohardness (H, GPa), elastic modulus (E, GPa), contact stiffness (S, N/m), elastic work (We, nJ) and plastic strain (Wp, nJ) in submicrovolumes of Nd2Fe14B grains. According to the measurement results, the minimum value of Nd2Fe14B grain adhesion Kint = 0.539 MPa m0.5 is estimated.

Creep lifetime prediction of 9Cr-1Mo (grade T91) steel via small punch creep tests and hierarchical multiscale analysis

ABSTRACT In this study, the creep deformation of ferritic/martensitic 9Cr-1Mo (Grade T91) steel boiler tubes that occurs during the operation of thermal power plants is studied using crystal elasto-viscoplasticity finite element analysis (CEV-FEA). The material properties and creep behaviour of Grade T91 were obtained through experimentation and computational analysis at different scales, e.g., molecular dynamics simulation, scanning electron microscopy, small punch (SP) tests. A triple junction of grains with various combinations of crystallographic orientations of polycrystalline Grade T91 was also simulated by CEV-FEA to investigate the stress concentration evolution during creep deformation. The stress concentration at the triple junction was then used as an input for the creep cavitation model, i.e., as the representative stress driving creep cavitation. The combination of CEV-FEA and SP test experiments can improve the evaluation of creep lifetime for as-received boiler tubes with minimum requirements for the amount of material and testing time.

Microstructural investigations in binderless tungsten carbide with grain growth inhibitors

The microstructure of binderless tungsten carbide, with small additions of Cr3C2 or VC, was investigated using transmission electron microscopy associated with X-ray energy dispersive spectrometry. The distribution of Cr and V elements was determined. In the material sintered with Cr3C2, numerous (Cr,W)2C grains were found, some of them displaying an epitaxy orientation relationship with the basal facet of adjacent WC grains, and Cr segregation was observed in all examined grain boundaries. In the carbide with VC additive, small V-rich carbides were found at triple junctions of WC grains. Unlike Cr, no V segregation was detected in grain boundaries. The grain growth inhibiting effect of Cr3C2 and VC is very likely different. For Cr3C2, it is supposed that both (Cr,W)2C grains and Cr segregation reduce the mobility of grain boundaries. For VC, probably the grain boundary triple junctions are pinned by small V-rich carbide grains.

Origin of corundum within anorthite megacrysts from anorthositic amphibolites, Granulite Terrane, Southern India

Abstract Growth of corundum in metamorphosed anorthosites and related basic-ultra-basic rocks is an exceptional feature, and its origin remains elusive. We describe the occurrence of and offer an explanation for the genesis of corundum in anorthositic amphibolites from ~2.5 Ga old basement of the Granulite Terrane of Southern India (GTSI). The studied amphibolites from two localities, Manavadi (MvAm) and Ayyarmalai (AyAm), contain anorthite lenses (An90–99) with euhedral to elliptical outline set in a finer-grained matrix of calcic plagioclase (An85–90) and aluminous amphibole (pargasite-magnesiohastingsite). The lenses, interpreted as primary magmatic megacrysts, and the matrix are both recrystallized under static condition presumably during the regional high pressure (HP) metamorphism (~800 °C, 8–11 kbar) at ~2.45 Ga. Corundum occurs in the core of some of the recrystallized anorthite lenses (An95–99) in two modes: (1) Dominantly, it forms aggregates with magnetite (with rare inclusion of hercynite; in MvAm) or spinel (and occasionally hematite-ilmenite; in AyAm). The aggregates cut across the polygonal grain boundaries of the anorthite and contain inclusions of anorthite. (2) Corundum also occurs along the grain boundaries or at the triple junctions of the polygonal anorthite grains, where it forms euhedral tabular grains, sieved with inclusions of anorthite or forms skeletal rims around the recrystallized anorthite, such that it seems to be intergrown with anorthite. Combined petrological data and computed phase relations are consistent with growth of corundum in an open system during regional metamorphism in the presence of intergranular fluids. Two mechanisms are proposed to explain the formation of the corundum in the amphibolites: (1) corundum + magnetite/spinel aggregates formed dominantly by oxy-exsolution of pre-existing Al-Fe-Mg-(Ti)-spinel. This pre-existing spinel may be primary magmatic inclusions within the anorthite phenocrysts or could have formed due to reaction of primary magmatic inclusions of olivine with the host anorthite. Pseudosections of fO2-nH2O-T-P in the CaO–FeO–MgO–Al2O3–SiO2–H2O (CFMASH) system indicate that fO2 and H2O strongly influence the formation of corundum + amphibole from the initial magmatic assemblage of anorthite (phenocrysts) + spinel ± olivine (inclusions). (2) The corundum with anorthite presumably formed through desilification and decalcification of anorthite, as is indicated by computed phase relations in isobaric-isothermal chemical potential diagrams (µSiO2-µCaO) in parts of the CASH system. Growth of corundum in this mode is augmented by high activity of anorthite in plagioclase, high pressure, and low-to-medium temperature of metamorphism. This study thus presents a new viable mechanism for the origin of corundum in anorthositic amphibolites, and basic-ultra-basic rocks in general, which should provide new insight into lower crustal processes like high-pressure metamorphism.

High-strength ultrafine-grained CoCrFeNiNb high-entropy alloy prepared by mechanical alloying: Properties and strengthening mechanism

Abstract An equiatomic CoCrFeNiNb alloy was prepared by conventional induction melting and by 8 h of mechanical alloying and compaction via spark plasma sintering. The alloy prepared via mechanical alloying showed a uniform ultrafine-grained microstructure composed of an FCC solid solution strengthened by HCP Laves phases. A detailed TEM inspection revealed the presence of nanocrystalline Cr2O3 particles at the triple junctions of the present grains as well as stacking faults and nanotwins found exclusively in the interior of the FCC solid solution grains. The as-cast alloy had a high initial hardness of 648 ± 18 HV 30 and ultimate compressive strength of 1374 MPa. On the other hand, the mechanically alloyed alloy compacted at 1000 °C showed even higher hardness of 798 ± 9 HV 30 as well as an ultra-high strengths that reached 2412 MPa. Based on the TEM quantitative analyses considering the contributions of different structural constituents and lattice defects, the aforementioned strength value was found to be in good agreement with results from microstructure strengthening modelling, which indicated a calculated mean value of 2300 ± 300 MPa. Moreover, the mechanically alloyed alloy also showed exceptional thermal stability even after long-term annealing/testing at 600 °C because it maintained a hardness of 777 ± 5 HV 30, and strength of 2284 MPa.

Comparison of xenon and gallium sources on the detection and mapping of lithium in Li-containing materials by using ToF-SIMS combined FIB-SEM.

Li can find itself a wide range of applications since it is the lightest metal. However, Li detection by microscopy-based techniques is problematic because of the highly susceptible nature during electron beam irradiation. ToF-SIMS is a versatile technique to detect Li but the detection of light materials is also problematic due to the large ion contaminated zone and low sputtering yield. By combining ToF-SIMS with a recently launched Xe ion source FIB-SEM, which has small ion contamination and high sputtering yield features, can produce more realistic data at near surface and below the surface region especially for the detection of lightweight materials such as Li. In this study, Li detection and mapping capabilities of ToF-SIMS attached to the FIB-SEM with Ga and Xe ion sources were discussed for Al incorporated Li7 La3 Zr2 O12 solid electrolyte sample that contains Li and Al rich regions at triple junctions. In spite of smoother milling from Ga source, Xe performs more precisely in Li mapping. Low ion contaminated zone, high sputtering yield and low straggling obtained from Monte Carlo simulations are the main advantages of Xe ion sources. The Li detection efficiency for Xe is higher than Ga source discriminating the LiAlO2 phase placed at the triple junctions of grains and La2 Zr2 O7 regions placed at the outer side of LLZO neighbouring the LiAlO2 phase. LAY DESCRIPTION: Li can find itself a wide range of applications since it is the lightest metal. However, Li detection by microscopy-based techniques is problematic because of the highly susceptible nature during electron beam irradiation. ToF-SIMS is a versatile technique to detect Li but the detection of light materials is also problematic due to the large ion contaminated zone and low sputtering yield. By combining ToF-SIMS with a recently launched Xe ion source FIB-SEM, which has small ion contamination and high sputtering yield features, can produce more realistic data at near surface and below the surface region especially for the detection of lightweight materials such as Li. In this study, Li detection and mapping capabilities of ToF-SIMS attached to the FIB-SEM with Ga and Xe ion sources were discussed for Al incorporated Li7 La3 Zr2 O12 solid electrolyte sample that contains Li and Al rich regions at triple junctions. In spite of smoother milling from Ga source, Xe performs more precisely in Li mapping. Results were also supported from Monte Carlo simulations of ion-atom interactions. The Li detection resolution of xenon is much higher than gallium source discriminating the LiAlO2 phase placed at the triple junctions of grains and La2 Zr2 O7 regions placed at the outer side of LLZO neighbouring the LiAlO2 phase.