Interior Of Grains Research Articles

Microstructure criterion for the preferential locations of helium bubble precipitation in a polycrystal

Helium bubble distribution at grain boundaries and inside grains affects materials' susceptibility to helium embrittlement. In this paper, we propose a microstructure criterion to quantitatively analyze the helium bubble preference for precipitation locations. The results show that the grain-boundary energy and the helium-to-vacancy ratio are the key factors in determining bubble formation mainly at the grain boundaries or in the grain interiors. Experimental observation based on the electron backscatter diffraction technique confirmed that helium bubbles are more likely to precipitate in the grain interiors of Inconel X750 alloy than pure nickel due to grain-boundary segregation-induced decrease in grain-boundary energy. The precipitation of intergranular bubbles can be reduced by lowering grain-boundary energy and reducing the helium-to-vacancy ratio at grain boundaries. This work provides a general framework for explaining the intrinsic relation between the formation of inter- and intra-granular helium bubbles and promising strategies for designing highly helium-enduring materials.

Microstructural Changes in 9Cr-1Mo-V-Nb Weld Metal after Aging at 1013 K

In order to understand microstructural changes in 9Cr-1Mo-V-Nb weld metal after long term use, microstructure and precipitates distribution before and after aging at 1013 K were investigated. In the weld metal, regions with coarse or fine prior austenite grains were observed due to thermal cycle during welding. In the coarse grain region, precipitate particles inferred to M23C6 were densely located on grain boundaries, however, in the fine grain regions, they were sparsely observed not only on grain boundaries but also inside grains. Post weld heat treatment (1013 K/7.7 h) followed by aging (1013 K/100 h) led to ferrite grains formation in the fine grain region. EBSD analysis implied that dislocation density in ferrite grains was low. After the aging, mean diameter of particles became coarser and interparticle spacing became sparser in the fine grain region than in the coarse grain region. On the other hand, dislocation density calculated by hardness in martensite structure was almost no deference between these regions before and after the aging. Therefore, it was suggested that ferrite grains were formed because pinning energy by precipitate particles locally reduced in the fine grain region.

Synthesis and investigation of the morphological, optical, electrical and dielectric characteristics of a disodium cobalt orthogermanate.

A sodium cobalt germanate material (Na2CoGeO4) was synthesized and studied. The X-ray powder diffraction pattern revealed a monoclinic crystal system with the Pn space group. The calculated value of the direct bandgap (Eg) was (3.89 ± 0.02) eV. Electrical measurements were undertaken in a frequency range from 40 Hz to 1 MHz and a temperature range from 403 K to 573 K. Nyquist plots revealed only a semicircle in the whole impedance spectra due to the effect of interior grains. The AC conductivity of Na2CoGeO4 was associated with the correlated barrier hopping (CBH) model. Studies of the complex electric modulus M*(ω) confirmed that the relaxation process was thermally activated.

Simultaneously enhanced strength and ductility of copper composites by tailoring the intragranular carbonized polymer dots distribution

Distribution states of reinforcement have considerable impact on the strength and ductility of metal composites. Herein, we reported a “nanodispersion-in-grains” strategy to simultaneously strengthen and stabilize Cu composites, which relied on a uniform dispersion of extremely fine carbonized polymer dots (CPD) inside grains. The intragranular CPD distribution not only elevated the strength of pure Cu by ∼45%, but also activated hardening mechanisms via dislocation-nanoparticle interactions. Meanwhile, 0.2 wt% CPD/Cu composite displayed good electrical conductivity of 96.5% IACS, exhibited a remarkable potential in applications as advanced multifunctional materials.

Influence of V addition on the mechanical properties of FeCo alloys: a molecular dynamics study

Interatomic potentials for the Fe-Co and Fe-V binary systems have been developed based on an Fe interatomic potential with enhanced mechanical capabilities and merged to describe the Fe-Co-V ternary system based on the second nearest-neighbor modified embedded-atom method (2NN MEAM) formalism. Emphasis has been placed on the correct prediction of FeCo elastic properties and antiphase boundary (APB) energies to ensure proper description of the mechanical behaviors. The potentials are then used to analyze the crack nucleation and propagation mechanisms in polycrystalline FeCo and FeCo-2V alloys under different ordering degrees at grain boundaries (GB) and within the grains. The results show that disordering inside grains has a larger impact on the ductility of the binary FeCo. The mobility of the antiphase domains (APD) inside the grains delays the ultimate tensile strength (UTS) and fracture point. The observed fracture procedure shows that triple junctions of GBs serve as the focal points of stresses concentration. The poor mobility at the triple GBs promotes crack nucleation and crack propagation, leading to an intergranular fracture in ordered and disordered binary alloys. The effects of adding 2 at. % V to the FeCo alloy have also been investigated via molecular dynamics (MD) and hybrid grand canonical Monte Carlo-MD (GCMC-MD) simulations. It was shown that V prefers to migrate to (a) the GBs in an ordered state, and (b) GBs and APBs in a disordered state. The ordered FeCo-2V displays a decreased UTS and loss of ductility at room temperature. Introducing V exclusively to the APBs greatly increases the mobility of APD and migration of atoms to GBs boosting the ductility of the alloy.

Microstructure optimization and magnetic properties enhancement of hot pressed Nd-Fe-B magnets via grain boundary diffusion of NdCu powder

Microstructure modification of Nd-Fe-B magnets plays a key role in regulating magnetic properties. In this work, low melting point NdCu alloy was added to improve the coercivity of NdFeB ribbons, and then high coercivity nanocrystalline NdFeB magnets were prepared by hot pressing sintering (HPS). The result shows that the coercivity of NdFeB powders blending with 15 wt% Nd-Cu powders obtained a maximum increment, as high as 12.69 kOe. After HPS, the fully dense magnets with excellent magnetic properties are obtained. Microstructure analysis confirmed that the Nd-Cu alloy has diffused into the interior of hard magnetic grains along the grain boundary during heat treatment. The enrichment of Nd, Pr, and Cu in the grain boundary phase weakens the magnetism of the grain boundary phase and reduces the intergranular exchange coupling. This is the main reason for the coercivity enhancement. In addition, the diffusion of Nd70Cu30 can significantly improve the thermal stability of the magnets.

Flake Cu-5Ag alloy powder with enhanced oxidation resistance via aging

In this study, flake Cu-5Ag alloy powder with mean diameter-thickness ratio (D/T) higher than 110 were successively prepared by ball milling. Effect of milling time on the morphology and D/T of flake powder was investigated in detail. The subsequent aging treatment at 400 ℃ was proved to be effective in enhancing the oxidation resistance of flake Cu-5Ag powder. After oxidation at 300℃ for 10 min, the oxygen content in the surface of aged Cu-5Ag flake powder was reduced by approximately 50 % compared to the as-milled Cu-5Ag flake powder. The aging enhanced the precipitation of Ag phase along the grain boundaries and in the interior of grains, which could act as hindrances to the preferential inward diffusion of oxygen along the defects during oxidation.

High-performance bulk Bi0.4Sb1.6Te3.0 thermoelectrics prepared from nanocrystal precursor synthesized via chemical precipitation

Bi0.4Sb1.6Te3.0 is a known p-type thermoelectric material with high thermoelectric performance near room temperature. In this study, bulk Bi0.4Sb1.6Te3.0 materials are synthesized via spark plasma sintering using single nanometer-sized nanocrystals as precursors, which are prepared in the solution phase by chemical precipitation from metallic ions. The obtained bulk material consists of submicron-to-micrometer-sized disk-shaped grains with isotropic distribution. Additionally, structural distortion such as strain and/or defects can be directly introduced into grains upon sintering, presumably due to the incomplete coalescence of the small nanocrystal precursors to form larger grains during the sintering process. The resultant bulk material exhibits high electrical conductivity and considerably low lattice thermal conductivity, due to the utilization of impurity-free nanocrystal precursors and local lattice distortions included inside grains acting as scattering nodes of phonons, respectively. The maximum dimensionless figure of merit (ZT) is achieved to be 1.42 ​at 355 ​K, and, therefore, this work provides effective methodology to fabricate high-performance chalcogenide-based thermoelectric materials.

Void growth yield criteria for intergranular ductile fracture

Ductile fracture through growth and coalescence of intergranular cavities is a failure mode observed experimentally in many metallic alloys used in industrial applications. Simulation of this fracture process in polycrystalline aggregates requires modeling of the plastic yielding of porous boundaries. However, classical yield criteria for porous materials such as the Gurson–Tvergaard–Needleman model and its current extensions cannot account for the complex coupling between loading state, crystallographic orientations, void shape and material behavior at grain boundaries. In order to bridge this modeling gap, two yield criteria for intergranular ductile void growth are proposed. The first one is a GTN-like model derived from limit-analysis which, once calibrated, accounts for spherical voids at the interface of rate-independent crystals. The second one is developed from a variational approach and predicts yielding in viscoplastic crystals containing intergranular ellipsoidal cavities. Both models are validated against a wide database of numerical limit-analysis of porous bi-crystals using a FFT solver. Satisfying agreements are obtained, paving the way to microstructure-informed intergranular ductile fracture simulation. The interplay between plastic yielding inside grains and along grain boundaries is finally studied based on the proposed yield criteria.

Study on dynamic recrystallization mechanism and model of TB15 titanium alloy

TB15 titanium alloy has shown a good application prospect in aviation large-scale structural parts. In this paper, the dynamic recrystallization (DRX) mechanism and model of TB15 titanium alloy were studied by thermal simulation compression experiment in temperature range of 810-930°C and strain rate range of 0.001s−1-10s−1. The microstructures under different deformation conditions were analyzed by optical microscopy (OM) and electron back scatter diffraction (EBSD). The results show that the degree of dynamic recrystallization decreased significantly with the increase of strain rate, and the nucleation position of dynamic recrystallization tended to migrate from the interior of grain to grain boundary. At low strain rate, the continuous dynamic recrystallization (CDRX) mechanism of subgrain merging and rotation was dominant. When the strain rate was 10s−1, the rare geometric dynamic recrystallization (GDRX) occurred due to strong strain concentration effect, and the dynamic recrystallization grains showed fine chain distribution. Finally, the DRX critical strain, volume fraction and grain size models of the studied alloy were established, and the DRX volume fraction model was modified by quantitative microstructure.

Effects of Zn Addition on Microstructural Evolution, Mechanical Properties, and Corrosion Resistance of Al‐6Mg Alloy Subjected to Equal Channel Angular Pressing Processing at 200 °C

The evolution of microstructure, mechanical properties, and corrosion performance of Al‐6Mg and Al‐6Mg–1Zn alloys processed using equal channel angular pressing (ECAP) at 200 °C is systematically investigated. Microstructural analysis reveals that different T and β phases are generated in the two studied alloys, resulting in different grain refinement processes. The average size of T phase in Al‐6Mg–1Zn alloy increases first and then decreases, and the distribution of T phase inside grains becomes denser with increasing ECAP deformation. The continuity of T phase at the grain boundary increases from 0 to 2 passes and decreases from 2 to 4 passes. In contrast, the β phase in Al‐6Mg alloy mainly locates at grain boundaries and becomes coarser and more continuous from 0 to 4 passes. The fracture strength of Al‐6Mg–1Zn alloy after 4‐pass ECAP reaches 391 MPa with a fracture elongation of 28%, which is superior to that of 4‐pass ECAP Al‐6Mg alloy. The 4‐pass ECAP Al‐6Mg–1Zn alloy has a higher corrosion resistance compared to the Al‐6Mg alloy under the same deformation condition. The low corrosion susceptibility in 4‐pass ECAP Al‐6Mg–1Zn alloy is attributed to sparse distribution of the T phase along the grain boundary.

Grain Boundary Precipitation Control of GCP Phase Using TCP or A2 Phase in Ni-Based Alloys

To cover the grain boundary (GB) of the Ni phase with precipitates, the GB precipitation behavior of both topologically close-packed (TCP) or A2 and geometrically close-packed (GCP) phases was investigated in two Ni–Nb–(Co, Cr) ternary systems. The Ni/TCP or A2/GCP three-phase region existed in both systems. In the Ni-Nb-Co ternary system, Nb was approximately equally partitioned into both Co7Nb2 (mC18 structure, TCP) and (Ni, Co)3Nb (D019 structure, GCP) phases. In the Ni–Nb-Cr ternary system, Nb and Cr were mainly partitioned into the Ni3Nb (D0a structure, GCP) and Cr (A2 structure) phases, respectively. In the Ni–Nb–Co ternary system, the Co7Nb2 phase grew along the GB, whereas the (Ni, Co)3Nb phase grew toward the grain interior (GI). However, the growth of the Ni3Nb phase toward the GI was suppressed in the Ni–Nb–Cr ternary system. The suppression of growth of the GCP phase and covering the GB using both the TCP or A2 and GCP phases might be possible in a system where the precipitation of the GCP phase nucleating on the GB prior to the TCP or A2 phase increases supersaturation for precipitation of the TCP or A2 phase.

The role of Ti in cavitation erosion and corrosion behaviours of NAB alloy in 3.5 % NaCl solution

The influence of titanium on the cavitation erosion and corrosion behaviors of a NAB alloy in 3.5 % NaCl solution is investigated using tensile test, mass loss, SEM, Tafel plots, electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS). The results show that the NAB alloy consists of the α phase, β phase and κ phase, in which titanium will enter the κ phase to form the titanium-rich κ phase. With the increment of titanium, the titanium-rich κ phase changes from mainly distributed at grain boundaries to more dispersed at grain boundaries and inside grains. In the case of high titanium content, the κ phase rich in titanium grows and produces an obvious segregation phenomenon, resulting in a decrease in the mechanical property and corrosion resistance of the substrate. The mass loss experiment shows that the mass loss caused by cavitation erosion decreases with increasing titanium content, and it increases with increasing titanium content when titanium content is over 0.478 %. The electrochemical experiments show that the corrosion resistance of NAB alloy in 3.5 % NaCl solution increases with increasing titanium content and reaches a maximum when titanium content is 0.478 %. The reason for the increased corrosion resistance of a NAB alloy containing titanium in 3.5 % NaCl solution is related to the appearance of TiO2 in the passive film automatically formed on a NAB alloy in air.

Corrosion behavior of severely plastically deformed Mg and Mg alloys

Magnesium (Mg) alloys have several advantages, such as low density, high specific strength and biocompatibility. However, they also suffer weak points, such as high corrosion, low formability and easy ignition, which makes their applications limited. Many studies have been conducted to overcome these disadvantages and further improve the advantages of Mg alloys. Severe plastic deformation (SPD) is one of the most important techniques and has great effects on the microstructure refinement of Mg alloys and improvements in their strength and formability. Several researchers have studied the corrosion behavior of SPD-processed Mg alloys in recent decades. However, these studies have reported some controversial effects of SPD on the corrosion of Mg alloys, which makes the research roadmap ambiguous. Therefore, it is important to review the literature related to the corrosion properties of Mg alloys prepared by SPD and understand the mechanisms controlling their corrosion behavior. Effective grain refinement by SPD improves the corrosion properties of pure Mg and Mg alloys, but control of the processing conditions is a key factor for achieving this goal because texture, dislocation density, size and morphology of secondary phase also importantly affects the corrosion properties of Mg alloys. Reduced grain size in the fine grain-size range can decrease the corrosion rate due to the increased barrier effect of grain boundaries against corrosion and the formation of a stable passivation layer on the surface of fine grains. Basal texture reduces the corrosion rate because basal planes with the highest atomic planar density are more corrosion resistant than other planes. Increased dislocation density after SPD deteriorates the corrosion resistance of the interior grains and thus proper annealing after SPD is important. The fine and uniform distribution of secondary phase particles during SPD is important to minimize the micro-galvanic corrosion effect and retain small grains during annealing treatment for removing dislocations.

Effects of rare-earth elements on twinning deformation of Al alloys from first-principles calculations

Effects of rare-earth (RE = Ce, Dy, Er, Ho, La, Tb, and Yb) elements on twinning deformation of Al alloys are studied using first-principles calculations. The influence rules of RE atoms doped at the first nearest neighbor layer on the generalized planar-fault energy curves are analyzed, and the interaction energies of RE atoms in various atomic layers of stacking fault configurations are studied. On the basis of the improved twinning criterion formula and uniform distribution model, the influence rules of RE solute concentration on twinnability at the crack tip, at the grain boundary, and inside the grain of Al alloys are further analyzed. Results show that RE elements can evidently decrease the minimum energy barriers of dislocation nucleation and motion, and La and Ce atoms have significant effects on promoting dislocation nucleation and motion. Ce and La atoms can promote twinning deformation at the crack tip. Except for the La atom, the remaining RE atoms inhibit twinning deformation at the grain boundaries, and La atom at high concentrations is beneficial to promote twinning deformation at the grain boundaries. RE atoms (Dy, Er, Ho, La, Tb, and Yb) for twinning deformation inside Al alloy grains exhibit slight effect. Nevertheless, the Ce atom can greatly promote twinning deformation inside the grains when the solute concentration is in the range of 5%–7.1%, and the twinning ability decreases with the increase in solute concentration again. This work takes the lead in revealing the mechanism of twinning deformation by considering the interactions between RE atoms and generalized faults, which provides valuable guidance for exploiting high-performance Al–RE alloys.

On the fundamental possibility of using the bacterial and catagenetic hypotheses of the origin of oil in assessing its resources

Research subject. The estimations of oil resources in Western Siberia obtained using a basin modeling of the Bazhenov formation as an oil source stratum indicated a significant deficit of its potential, calculated based on the pyrolytic kinetic parameters of catagenesis. Aim. To search for possible additional oil generating mechanisms parallel to catagenesis, thus clarifying the conceptual foundations of the modern theory of the organic origin of oil. Key points. Based on a review of literature publications, a hypothesis was formulated about the evolution of organic matter from sedimentary flow floccules to the injection of proto-oil into the microcracks of the autofluid fracturing of source rocks. In the upper unconsolidated bottom sediment layers, biofilms appear whose microbial matrix forms a basis for future kerogen. Proto-oil bitumoids appear as a by-product of biofilm microbial metabolism under the anaerobic conditions at diagenesis stage. Рroto-oil bitumoids are encapsulated and absorbed on the surfaces and inside the chambers and conducting channels of the biofilm polymer matrix (future kerogen). Local ultra-high pressure (LUHP) inside kerogen grains appears at the final “smectite – illite” transition stage of host clays, acting as a driving force for encapsulated bitumoid expulsion from kerogen grains. On the walls of microcracks, proto-oil bitumoids undergo hydropyrolysis, consuming cohesive film water and changing the surface philicity. Microphotographs and SEM images of thin sections are provided in order to illustrate LUHP-expulsion traces. Two types of oil source rocks are differentiated based on the possibility of proto-oil LUHP expulsion: those with a plastic mineral framework and a rigid mineral framework. Conclusion. The presence of porous spaces filled with bitumoids was established in the kerogen of immature potentially oil source rocks. This mass of encapsulated bitumoids is an additional resource of oil generation within the “oil window”. Their expulsion volume can be estimated and attributed to a certain temperature range. Directions for further research are outlined.

Eutectoid decomposition of βNb5Si3 → αNb5Si3 + Nbss in Nbss/Nb5Si3 in-situ composites revealed by electron microscopy

Eutectoid decomposition of βNb5Si3 → αNb5Si3 + Nbss in Nbss/Nb5Si3 in-situ composites has been systematically investigated by electron microscopy. βNb5Si3 would transform into different αNb5Si3 orientation variants around 1075 °C. There is no inherent orientation relationship (OR) between βNb5Si3 and αNb5Si3, which might be regulated by the diffusive nature of the transformation. Numerous Nbss precipitates would form within αNb5Si3 with crystallographic OR of <001>Nb// < 001>α. This transformation is promoted by formation of dislocations and the diffusion of Nb at elevated temperatures, which will propagate from Nbss/βNb5Si3 interfaces to the interior of βNb5Si3 grains.

Elimination of room-temperature brittleness of Fe–Ni–Co–Al–Nb–V alloys by modulating the distribution of Nb through the addition of V

Compared with Ti–Ni-based and Cu–Al-based alloys, Fe–Ni–Co–Al-based superelastic alloys have received great attention in recent years because of their low cost and excellent performance. However, the precipitation of brittle precipitates on grain boundaries in Fe–Ni–Co–Al-based polycrystalline alloys significantly reduces the mechanical properties. In this paper, the effects of V on the formation of brittle precipitates on both grain boundaries and inside grains and mechanical properties in Fe–Ni–Co–Al–Nb alloy were investigated systematically. The results show that γ′(Ni3Al) precipitate with L12 structure precipitate out on grain boundaries and inside grains in the Fe–Ni–Co–Al–Nb alloy during aging without V addition. The γ′ precipitate with L12 structure on the grain boundaries grew to form a heavily-coarsened L12 precipitate (HCLP γ′ precipitate) due to the accumulation of Nb on the grain boundaries, resulting in a significant decrease in elongation. With the addition of V, intragranular Nb and V can combine together with γ′ to form Ni3(Al, Nb, V) (nano-sized γ′ precipitate) composite precipitates, which improves the strength. More important, since the diffusion coefficient of V was larger than that of Nb, V preferentially occupied the lattice positions on grain boundaries, repelling Nb from the grain boundaries to the matrix. Thus, the coarsening of HCLP γ′ precipitate on the grain boundaries was reduced, and the elongation of Fe–Ni–Co–Al–Nb–V alloy was significantly improved.

Antibacterial Property and Metagenomic Analysis of Milk Kefir.

Milk kefir fermentation has been used in households for generations. Consumption of milk kefir has been associated with various health benefits, presumably from the probiotics of yeast and bacteria that make up the kefir grains. In addition, many of the microbes are known to produce novel antimicrobial compounds that can be used for other applications. The microbes living inside kefir grains differ significantly depending on geographical location and production methods. In this study, we aimed to use metagenomic analysis of fermented milk by using three different kefir grains (kefir 1, kefir 2, and kefir 3) from different US sources. We analyzed the microbial compositions of the three milk fermentation samples. This study revealed that each sample contains unique and distinct groups of microbes, kefir 1 showed the least diversity, and kefir 3 showed the highest diversity. Kefir 3 is rich in Proteobacteria while kefir 2 is dominated by the Firmicutes. Using bacterial indicator growth analyses carried out by continuous readings from microplate-based bioreactor assays suggested that kefir 2 fermentation filtrate has higher antibacterial property. We have screened 30 purified cultures of kefir 2 sample and isolated two lactic acid bacteria strains with higher antibacterial activities; the two strains were identified as Leuconostoc mesenteroides 28-1 and Lentilactobacillus kefiri 25-2 by 16S genomic PCR with confirmed antibacterial activities of fermentation filtrate after growing under both aerobic and anaerobic conditions.

Studies on tuning surface electronic properties of hydrogenated diamond by oxygen functionalization

Ultra-wide bandgap and the absence of shallow dopants are the major challenges in realizing diamond based electronics. However, the surface functionalization offers an excellent alternative to tune electronic structure of diamonds. Herein, we report on tuning the surface electronic properties of hydrogenated polycrystalline diamond films through oxygen functionalization. The hydrogenated diamond (HD) surface transforms from hydrophobic to hydrophilic nature and the sheet resistance increases from ~8 kΩ/sq. to over 10 GΩ/sq. with progressive ozonation. The conductive atomic force microscopic (c-AFM) studies reveal preferential higher current conduction on selective grain interiors (GIs) than that of grain boundaries confirming the surface charge transfer doping on these HDs. In addition, the local current conduction is also found to be much higher on (111) planes as compared to (100) planes on pristine and marginally O-terminated HD. However, there is no current flow on the fully O-terminated diamond (OD) surface. Further, X-ray photoelectron spectroscopic (XPS) studies reveal a redshift in binding energy (BE) of C1s on pristine and marginally O-terminated HD surfaces indicating surface band bending whilst the BE shifts to higher energy for OD. Moreover, XPS analysis also corroborate c-AFM study for the possible charge transfer doping mechanism on the diamond films which results in high current conduction on GIs of pristine and partially O-terminated HDs.