Grain Boundary Microstructure Research Articles

Grain boundary microstructure engineering of Li1.3Al0.3Ti1.7(PO4)3 electrolytes with a low-temperature-prepared nanopowder and Bi2O3 additive

All-solid-state lithium batteries have great potential applying to the fields of transportation and portable devices because of their high safety and energy density as the power source. As a critical component of device, solid-state electrolyte has been paid much attention. Given the excellent physical and chemical stability, Li1+xAlxTi2-x(PO4)3 (LATP)-based electrolyte has been considered as one of the most promising electrolytes for the next generation batteries. However, a further promotion of its ionic conductivity becomes extremely difficult, originating mainly from the poor connection of grains and a low relative density of LATP. Here, the nanosized and homogenous LATP (x = 0.3) powders were synthesized through reducing the agglomeration of precursor powders and preparing at low temperatures. It is revealed that uniformly superfine LATP powders are able to improve the microstructure of grain boundaries during the pellet sintering process, thereby reducing the grain boundary resistance. Furthermore, as-synthesized nanosized powders mixed with Bi2O3 additive present a better enhancement on the connection of grains and the relative density in LATP. On the basis of this understanding, we prepared LATP electrolytes with 0.5 wt% Bi2O3 exhibiting a high ionic conductivity of 8.56 × 10−4 S/cm, a high relative density of 94.4 % and a low activation energy of 0.27 eV, which is demonstrated as a prospective electrolyte in all-solid-state lithium battery LiFePO4/PEO/LATP/PEO/Li.

Effect of post-sinter annealing on structure and coercivity of (Zr, Ti)-doped Nd-Ce-Fe-B magnets

Post-sinter annealing effects on the structure and coercivity have been investigated for the (Zr, Ti)-doped Nd-Ce-Fe-B sintered magnets. In them, an in-situ formed Zr-rich Zr-Ti phase with the strip shape is found to be beneficial for the coercivity Hcj, because its distribution along grain boundaries (GBs) after annealing at 700℃ can block the magnetic domain within matrix phases passing through the adjacent RE2Fe14B grains. It, coupled with the optimized distribution of RE-rich phase, accounts for the 21.1 % increase of Hcj before and after annealing at 700 ℃. This temperature much higher than the melting-point (465 ℃) of RE-rich phase is the optimum annealing temperature, at which the RE-rich liquid phase is good for the distribution of Zr-Ti phase along GBs rather than at triple junction regions, optimizing GB microstructure. Such new findings deepen the understanding on microstructure evolutions of the GB modified Nd-Fe-B magnet during annealing and further provide a theoretical basis for the Hcj improvement.

Function of cleavage strength for symmetrical tilt grain boundaries

The cleavage strength for grain boundaries (GBs) plays a crucial role in understanding the GBs' behavior and microstructure evolution during deformation. However, its universal discipline remains uncertain, and a quantitative description is currently unavailable. Here we propose the theoretical models to systematically quantify the strength of symmetrical tilt GBs in the discrete atomic perspectives. Specifically, based on the interaction of metallic atomic pairs, the interaction between the arbitrary parallel planes is expressed by considering the atomic pair distribution on both sides of the separated planes. On this basis, the function of GBs’ cleavage strength is proposed by considering that the interaction at GBs is composed of the interaction of the interfaces that make up GBs and the interaction between mirror atomic pairs at both sides of the symmetry plane. Furthermore, the theoretical prediction of the critical energy release rate Gc for the sharp crack extension is proposed based on the present cleavage strength function. All the theoretical models are validated by the ab initio simulations or citations. This work quantitatively proposes the cleavage strength function of symmetrical tilt GBs, predicts the critical energy release rate for cleavage, and provides a new methodology for studying mechanical properties of materials in the discrete atomic perspectives.

Modelling of the hydrogen embrittlement in austenitic stainless steels

The paper shows the 3D model of hydrogen diffusion in austenitic stainless steel. To model the behaviour of the material a real microstructure of grain boundaries, of dislocation density, of number of vacancy and the states of precipitates is taking in account and implemented in the software (ANSYS). The effect of each single hydrogen trap was physically determined. To improve the affordability of the results a high number of elements and nodes were carried out in the simulation. The purpose of the model is to identify the hydrogen diffusion in different conditions evaluating the weight of each trap in the process. The model allowed to define the hydrogen saturation of the microstructure in different temperature conditions. The results showed that the trapping effect of vacances is negligible at room temperature and their effect increases as the diffusion temperature increases. The main trapping effect is experienced by the grain boundary then by dislocations and then by the lattice. The trapping effect of precipitates has a remarkable dependence on their dimensions.

Reducing stress corrosion cracking susceptibility of high-strength aluminum alloy and its fastener by a novel electromagnetic shocking treatment

A novel electromagnetic shocking treatment (EST) is put forward to reduce the stress corrosion cracking (SCC) susceptibility of high-strength aluminum alloys and its fasteners by selectively modifying grain boundary (GB) microstructure. The effect of EST on the microstructure, mechanical properties and SCC susceptibility of T73-Al7075 alloy and its high-lock nuts has been investigated. Mechanical properties, electrochemical performance and constant strain SCC test have been carried out for T73-Al7075 alloy, microhardness test and SCC test under wet-dry cyclic condition have been carried out for its high-lock nuts, as well as microstructure characterizations via SEM, EBSD and TEM have been carried out for T73-Al7075 alloy. Comparing to the received sample, the strength of EST sample basically keep consistent while the elongation increases slightly, besides, the electrical conductivity and electrochemical corrosion performance improves. The SCC susceptibility of T73-Al7075 alloy and its high-lock nuts have been reduced after EST. Microstructure characterization shows that the grain morphology and matrix intragranular precipitate distribution of the EST sample keep almost the same as those of the received sample, which is consistent with the almost unchanged mechanical properties of alloy sample. However, EST induces the transformation of grain boundary precipitates (GBPs) and the formation of stripy/wavy GB as observed experimentally, which indicates that EST promotes nonlinear GB wetting or pre-melting and even grain bridging. The phase transformation and GB complexion transition induced by EST are main factors attributing to the obvious improvement of SCC resistance of T73-Al7075 alloy and its high-lock nuts. It can be deduced that anodic dissolution cracking will be relieved because of the dissolution and coarsening of GBPs. Hydrogen induced cracking and crack propagation can be relieved with the occurrence of grain bridging. They are both beneficial to improve SCC resistance of Al7075 alloys. This work provides a novel EST method to targetedly embellish GB microstructure, optimize GB complexion and connectivity, and then enhance the service performance of solid alloys and even finished parts.

Microstructure and magnetic properties of grain boundary diffused Nd-Fe-B magnets with Tb-Al-Ga alloy

The coercivity of Nd-Fe-B magnets can be effectively improved by grain boundary diffusion using low melting point alloy as diffusion source. In this work, Tb-Al-Ga alloy was used as diffusion source for grain boundary diffused Nd-Fe-B magnets. Tb-rich shell structure, potential synergistic effect of Al-Ga, interface between diffusion source and the magnets were analyzed. When the magnets were processed at 875 °C for 10 h, the coercivity increased by 123 % from 10.38 kOe to 23.15 kOe and the remanence and the maximum magnetic energy product only decreased by 3 %. Tb element mainly formed shell structure with high magneto-crystalline anisotropy field surround the main phase grains. Non-magnetic Al element was distributing around the main phase grains as network structure, which could well isolate the main phase grains. The Tb-rich shell with high magneto-crystalline anisotropy field and the non-magnetic phase with magnetic exchange decoupling between the adjacent grains contributed to substantial increase in the coercivity of the grain boundary diffused magnets. The grain boundary phase was wetted by Al element, which promoted Tb element diffused into the magnets more deeply.

Metavalent bonding impacts charge carrier transport across grain boundaries

Understanding the mechanisms underpinning the charge carrier scattering at grain boundaries is crucial to design thermoelectrics and other electronic materials. Yet, this is a very challenging task due to the complex characteristics of grain boundaries and the resulting difficulties in correlating grain boundary structures to local properties. Recent advances in characterizing charge transport across grain boundaries are reviewed, demonstrating how the microstructure, composition, chemical bonding and electrical properties of the same individual grain boundary can be correlated. A much higher potential barrier height is observed in high-angle grain boundaries. This can be ascribed to the larger number density of deep trapping states caused by the local collapse of metavalent bonding. A novel approach to study the influence of the local chemical bonding mechanism around defects on the resulting local properties is thus developed. The results provide insights into the tailoring of electronic properties of metavalently bonded compounds by engineering the characteristics of grain boundaries.

Making high-strength bimodal Ti–5Al–5V–5Mo–3Cr alloy more ductile by grain-boundary shearing

The conflict between strength and ductility of bimodal Ti–5Al–5V–5Mo–3Cr (Ti-5553) becomes more evident as yield strength attains 1400 MPa, which limits the further application of the alloy in the ultra-high-strength condition. In this paper, we found that the conventional bimodal Ti-5553 alloy with 1400 MPa yield strength processes four typical kinds of grain boundary microstructures. However, two kinds of grain boundaries with the short α side-plate are almost un-deformable in tension, contributing to the strength-versus-ductility conflict. Fortunately, short α side-plate might transform to α laminate that is parallel to the grain boundary of prior β grain after the dual-ageing consisting of the high-temperature ageing and the low-temperature ageing. It is found that α laminate is capable of shearing, which is beneficial to the ductility of bimodal Ti-5553 alloy. Base on the dual-ageing strategy, a novel bimodal Ti-5553 alloy with excellent combination of yield strength (1391 MPa), ultimate tensile strength (1484 MPa) and tensile elongation (12%) has been fabricated. The strength versus ductility data of the novel alloy might extend beyond the benchmark range established by the strongest and most ductile Ti-alloys known. The result of the present work is instructive for designing the grain-boundary microstructure of high-strength bimodal near-β titanium alloys.

Evolution of grain boundary regions in Sm-Co-Fe-Cu-Zr magnets

Grain boundary regions (GBs) with incomplete cellular nanostructure will deteriorate the squareness and coercivity in iron-rich SmCo magnets. In order to reduce the deteriorations, the morphology and evolution of GBs in Sm-Co-Fe-Cu-Zr magnets are systematically investigated. It clearly reveals that less precipitated particles (Zr-rich and Cu-rich particle) disperse in GBs, and more 1:5H cell boundary phase exists from GBs to grain interiors with Cu content increase, which leads to an improvement of squareness. On this basis, the Cu content in 1:5H cell boundary phase is gradually decreases, which partially weakens the coercivity. Our findings suggest that careful controlling Cu content in magnets can effectively optimize the microstructure of grain boundary and promote the cellular nanostructure from GBs to grain interiors. That may provide important considerations for the preparation of magnet with high squareness and high coercivity.

A novel electromagnetic shock treatment to selectively modify grain boundary and improve the corrosion resistance of aluminium alloy

A novel electromagnetic shock treatment (EST) is put forward to selectively modify grain boundary (GB) microstructures and further improve corrosion resistance of T73-Al7075 alloys. Electrochemical tests and GB microstructure characterization by TEM (HRTEM) are conducted. The results show that EST with appropriate electromagnetic pulse energy being utilized can modify GB microstructure and enhance the corrosion resistance of T73-Al7075 alloy. In EST sample, GB precipitates (GBPs) dissolve and coarsen simultaneously, and stripy/wavy GBs with nanometer scale are more commonly observed which indicates that GBs relax within nanometer scale and even promote nonlinear grain bridging. The transformation of GBPs as well as the transition of GB complexions improve the corrosion resistance of EST sample. This provides a promising method to selectively modify GB microstructure instantaneously and improve related properties of solid alloys.

Structural properties of grain boundary in graphene grown on germanium substrates with different orientations

The direct synthesis of graphene with high-quality on semiconducting germanium (Ge) substrates has been developed recently, which has provided a promising way to integrate graphene with semiconductors for the application of electronic devices. However, the defects such as grain boundaries (GBs) introduced during the growth process have a significant influence on the crystalline quality of graphene and the performance of related electronic devices. Therefore, the investigation of the formation of GBs in graphene grown on a Ge substrate is essential for optimizing the crystalline quality of graphene. Herein, the formation mechanism and microstructure of GBs in graphene grown on Ge (110), Ge (001), and Ge (111) substrates via a chemical vapor deposition method are revealed. Ex situ atomic force microscopy is utilized to monitor the evolution of graphene domains. It is found that a single crystalline graphene film without GBs is formed on Ge (110), while polycrystalline graphene films with GBs are grown on Ge (001) and Ge (111) substrates, as suggested by transmission electron microscopy and x-ray photoelectron spectroscopy measurements. Our work may motivate the future exploration in improving the crystalline quality of graphene grown on a semiconducting substrate and the performance of associated electronic devices.

Effect of Microstructure on Fatigue-Crack Propagation of 18CrNiMo7-6 High-Strength Steel

The effect of microstructure on fatigue crack propagation of 18CrNiMo7-6 high-strength steel was studied by scanning electron microscopy and electron backscatter diffraction techniques. The crack propagation path was related to the microstructure of the original austenite grain boundary, lath martensite and crystal orientation. The crack path was tortuous and many small cracks or extrusions appeared on both sides of the main crack, with most of the extrusions parallel to the martensite lath. When the crack propagation encountered a high-angle boundary or passed through different crystal orientations, the crack was deflected, and when it encountered a low-angle boundary, the crack expanded.

Morphological Failure Study of Low Nickel Austenitic Grade SS: Metal Spinning

The formability of the austenitic grade of low Ni-treated steels (SSLNA) was examined by metal turning measure. The treated steel items are considered for choice as prepared with consumption safe properties. In monetary terms, they can contend with greater expense designing metals and amalgams dependent on nickel or titanium, while offering a scope of consumption opposing properties reasonable for a wide scope of utilizations. In this investigation, the austenitic grade of low nickel-tempered steel material was spun and met with the crack. The deformation of spinning is done on the various angles with change in percentage of thickness from 14 to 80%. The characterization of the fracture has been concentrated by the field emission scanning electron microscope to notice the morphological states of the material. The 12.5 mm WD of sample provides clear rupture in the grain boundaries of microstructure. The essential investigation was inspected by energy-dispersive spectroscopy. A solid connection was seen with both examinations approving the presence of voids in the material that causes the remaining anxieties which prompts the break on the SSLNA material.

Microstructure evolution and electrochemical corrosion behavior of Al–Zn–Mg aluminum alloy

Electrochemical corrosion behavior of Sn-containing Al–Zn–Mg aluminum alloy has been studied in detail. The localized corrosion behaviors were studied by electrochemical impedance spectroscopy (EIS) analysis, and the potentiodynamic polarization measurements. The grain structure, grain-boundary microstructure, grain-boundary microchemistry, pitting and intergranular corrosion morphology were characterized and observed using SEM, EDS, TEM, SAED and HRTEM analyses. Based on these tests, the effects of grain-boundary on the corrosion resistance in our Sn-containing Al–Zn–Mg alloys before/after bake hardening were analyzed systematically. Finally, the relationship between chemical composition, microstructure evolution and corrosion behaviour was revealed. The results indicate that the bake hardening process improves the corrosion resistance compared to the pre-aging state. The grain size has little effect on the electrochemical corrosion bahavior.

Microstructure and Magnetic Properties of Grain Boundary Insulated Fe/Mn0.5Zn0.5Fe2O4 Soft Magnetic Composites.

Mn0.5Zn0.5Fe2O4 nano-powder was coated on Fe microparticles by mechanical ball milling combined with high-temperature annealing. The effects of milling time on the particle size, phase structure and magnetic properties of core–shell powder were studied. Scanning electron microscopy (SEM), energy-dispersive spectroscopy and X-ray diffraction showed that the surface of the milled composite powder was composed of thin layers of uniform Mn0.5Zn0.5Fe2O4 insulating powder. SEM also revealed a cell structure of Fe particles, indicating that the Fe particles were well separated and isolated by the thin Mn0.5Zn0.5Fe2O4 layers. Then, Fe/Mn0.5Zn0.5Fe2O4 soft magnetic composites were prepared by spark plasma sintering. The amplitude permeability of Fe/Mn0.5Zn0.5Fe2O4 SMCs in the Fe/Mn0.5Zn0.5Fe2O4 soft magnetic composites was stable. The resistivity decreased with the increase in sintering temperature. The loss of the composite core was obviously less than that of the iron powder core. Hence, the preparation method of Mn0.5Zn0.5Fe2O4 insulating iron powder is promising for reducing core loss and improving the magnetic properties of soft magnetic composites.

Microstructure and Magnetic Properties of Grain Boundary Diffused Nd-Fe-B Magnets with Tb-Al-Ga Alloy

Microstructure and Magnetic Properties of Grain Boundary Diffused Nd-Fe-B Magnets with Tb-Al-Ga Alloy

Molecular dynamics investigation on tilt grain boundary energies of beta-titanium and tungsten at high temperature

The grain boundary energies (GBEs) of symmetric tilt grain boundaries (STGBs) and asymmetric tilt grain boundaries (ATGBs) for W at 0 and 2400 K and β-Ti at 1300 K were calculated by means of molecular statics and dynamics simulations to investigate the effects of temperature on GBE and the relationships between GBEs and grain boundary (GB) planes. Generally, the variation trends of GBE with the tilt angle are similar for the three cases, when the tilt axis is specified. It is of course that these similarities result from their similar GB microstructures in most cases. However, the variation trends of GBE with tilt angle are somewhat different between β-Ti at 1300 K and W at 2400 K for STGBs with <100> and <110> tilt axes. This difference mainly stems from the following two reasons: firstly, the GB microstructures of W at 2400 K and β-Ti at 1300 K are different for some STGBs; secondly, the atoms at the STGB of β-Ti at 1300 K tend to evolve into the local ω- or α-like structures distributed at the STGB for some STGBs with <110> tilt axis, which makes the corresponding STGBs more stable, thereby decreasing the GBEs. Furthermore, a geometric parameter θ, the angle between the misorientation axis and the GB plane, was defined to explore the relationships between GBEs and GB planes. It was found that the relationships between GBEs and GB planes can be described by some simple functions of sin(θ) for the GBs with definite lattice misorientation, which can well explain and predict the preferred GB planes for the GBs having the same lattice misorientation. Our calculations not only extend the investigation of GBs to higher temperature, but also deepen the understanding on the temperature contributions to the microstructure evolution at GBs and on the relationships between GBEs and possible geometric parameters.

Effect of Uniaxial Strain on Oxygen Diffusion in Grain Boundaries of Polycrystalline YBa2Cu3O7-Coated Conductors

For polycrystalline YBa2Cu3O7 (YBCO)-coated conductors, the diffusion behavior of oxygen ions during the preparation process, especially in grain boundaries (GBs), results in the motion of oxygen vacancies, and finally has a strong impact on the critical current density (Jc) of YBCO conductors. Such the oxygen diffusion in GBs has been found to be very sensitive to applied strain. In order to deeply understand the strain effect, in this article, we have studied the oxygen diffusion in polycrystalline YBCO conductors under uniaxial strain by molecular dynamic simulation. On the two different bicrystal structural models, the calculated results display that the oxygen diffusion in the grain boundary region shows a downward trend with the loading of the uniaxial tensile or compressive strain. And also, we found that applied strain leads to the variation of the microstructure of GBs, and thus results in reducing the oxygen vacancy concentration. As our previous study concluded, the high diffusion of oxygen can be regarded as a medium for analyzing the supercurrent through grain boundary. This paper further illustrates that the relation between the activation energy of oxygen diffusion and the strain loading has an almost similar form as that between the critical current density and the strain. This indicates that the activation energy of oxygen diffusion in the grain boundary region could be closely related to the critical current density, which arouses our interest to further explore whether or not it can be used as an index of predicting the current-carrying capability of YBCO conductors.

CALPHAD-informed phase-field modeling of grain boundary microchemistry and precipitation in Al-Zn-Mg-Cu alloys

The grain boundary (GB) microchemistry and precipitation behaviour in high-strength Al-Zn-Mg-Cu alloys has an important influence on their mechanical and electrochemical properties. Simulation of the GB segregation, precipitation, and solute distribution in these alloys requires an accurate description of the thermodynamics and kinetics of this multi-component system. CALPHAD databases have been successfully developed for equilibrium thermodynamic calculations in complex multi-component systems, and in recent years have been combined with diffusion simulations. In this work, we have directly incorporated a CALPHAD database into a phase-field framework, to simulate, with high fidelity, the complex kinetics of the non-equilibrium GB microstructures that develop in these important commercial alloys during heat treatment. In particular, the influence of GB solute segregation, GB diffusion, precipitate number density, and far-field matrix composition, on the growth of a population of GB η-precipitates, was systematically investigated in a model Al-Zn-Mg-Cu alloy of near AA7050 composition. It is shown that the GB solute distribution in the early stages of ageing was highly heterogeneous and strongly affected by the distribution of GB η-precipitates. Significant Mg and Cu GB segregation was predicted to remain during overageing, while Zn was rapidly depleted. This non-trivial GB segregation behaviour markedly influenced the resulting precipitate morphologies, but the overall precipitate transformation kinetics on a GB were relatively unaffected. Furthermore, solute depletion adjacent to the GB was largely determined by Zn and Mg diffusion, which will affect the development of precipitate free zones during the early stages of ageing. The simulation results were compared with scanning transmission electron microscopy and atom probe tomography characterisation of alloys of the similar composition, with good agreement.

Evolution of microstructure, mechanical properties and corrosion behavior of Al-4Mg-2Zn-0.3Ag (wt.%) alloy processed by T6 or thermomechanical treatment

The evolution of microstructure, mechanical properties and intergranular corrosion (IGC) resistance of the heat-treatable Al-Mg-Zn-Ag alloy processed by T6 or thermomechanical treatment (TMT) technologies are systematically investigated. The mechanical properties and IGC resistance are both improved by the TMT technology. The combination of pre-deformation with aging promotes the dynamic precipitation of nanoparticles and the formation of substructure. Consequently, the hardening effect of the alloy is improved. The intergranular microstructure of grain boundary precipitates and precipitate free zone can be ameliorated by optimizing of processing parameter of TMT technology, which greatly improve the resistance of IGC of the alloy.