Grain Boundary Microstructure Research Articles

Grain boundary crystallography and segregation in Ni-based superalloy INC738 manufactured by electron-beam powder bed fusion in as-built and annealed conditions

The excellent high-temperature properties of Ni-based superalloy INC738 are due to its hierarchical microstructure, making it an ideal engineering material for high-temperature applications. Engineering parts are now increasingly made via electron beam powder bed fusion (EPBF), an additive manufacturing technique suitable for such hard-to-weld Ni-based superalloys, due to lower thermal gradients and unmatched scan path control. The thermal cycles induced by EPBF impact characteristics of the γ-matrix, γ’ precipitates, secondary phases such as carbides, grain boundary (GB) solute segregation and, in turn, properties including GB cohesion and strength. However, a more thorough understanding of the GB microstructure evolution with focus on GB chemistry and character is required to optimise properties. We systematically investigate texture, grain structure, GB habit planes, and GB segregation in INC738 fabricated with linear versus random EPBF scanning strategies. We show that random scanning is a suitable strategy to inhibit cracking, refine grains, and decrease segregation of Cr, Mo, C, and B at GBs. For both scanning strategies, γ/γ GBs predominantly terminate on {100} planes and are decorated with C, B, Mo, and W. Upon 2 h annealing at 1180 °C and 1250 °C, the GB character and texture are shown to remain stable despite a reduction in GB interfacial excess. After 24 h annealing at 1250 °C, GB segregation and depletion are nearly eradicated, while static recrystallisation is observed with a predominant formation of annealing twins and GBs terminating on {111} planes. These findings are critical for defect-free additive manufacturing of INC738 and similar grades for superior high-temperature performance.

Influences of symmetric tilt grain boundary on the stretching deformation behaviors of Cu bicrystals

The microstructure of grain boundary (GB) becomes an important factor affecting the strength of nanomaterials during plastic deformation. In this work, Cu bicrystals with different coincidence site lattice (CSL) interfaces are constructed, and the influences of the symmetric tilt interface on the mechanical properties and plastic deformation behaviors during uniaxial tension are studied by molecular dynamics simulation. The results indicate that the peak stress of tensile stress-strain curves for most bicrystals is lower than its monocrystalline samples, except CSL3, CSL11 and CSL57 bicrystals with a continuous and single structural unit. The CSL3 bicrystal has the highest strength, while CSL33 bicrystal has the lowest strength among all the tested samples. The CSL11 and CSL57 bicrystals with a lower GB energy have a higher stability of interface, which delays the yielding. Moreover, the generation of twin and stair-rod dislocation at the interfaces strengthens the CSL11 and CSL57 bicrystals. The CSL3 almost has the same parameters as its monocrystalline sample S3, massive research has confirmed that the sample with a higher density twin (CSL3 interface) exhibits higher combined properties of strength and plasticity. This work may provide a guideline for GB engineering to improve material properties by controlling the GB structure.

New estimates of sulfate diffusion rates in the EPICA Dome C ice core

Abstract. To extract climatically relevant chemical signals from the deepest, oldest Antarctic ice, we must first investigate the degree to which chemical ions diffuse within solid ice. Volcanic sulfate peaks are an ideal target for such an investigation because they are high-amplitude, short-duration (∼3 years) events with a quasi-uniform structure. Here we present an analysis of the EPICA Dome C sulfate record over the last 450 kyr. We identify volcanic peaks and isolate them from the non-sea-salt sulfate background to reveal the effects of diffusion: amplitude damping and broadening of peaks in the time domain with increasing depth and age. Sulfate peak shape is also altered by the thinning of ice layers with depth that results from ice flow. Both processes must be simulated to derive effective diffusion rates. This is achieved by running a forward model to diffuse idealised sulfate peaks at different rates while also accounting for ice thinning. Our simulations suggest a median effective diffusion rate of sulfate ions of 2.4±1.7×10-7 m2 yr−1 in Holocene ice, slightly faster than suggested by previous work. The effective diffusion rate observed in deeper ice is significantly lower, and Holocene ice shows the highest rate of the last 450 kyr. Beyond the Holocene, there is no systematic difference between the effective diffusion rates of glacial and interglacial periods despite variations in soluble ion concentrations, dust loading, and ice grain radii. Effective diffusion rates for 40 to 200 ka are relatively constant and of the order 1×10-8 m2 yr−1. Our results suggest that the diffusion of sulfate ions within volcanic peaks is relatively fast initially, perhaps through an interconnected vein network, but slows significantly after 40 kyr. In the absence of clear evidence for a controlling influence of temperature on sulfate diffusivity with depth and age, we hypothesise that the rapid decrease in effective diffusion rate from the time of deposition to ice of 50 ka age may be due to a switch in the mechanism of diffusion resulting from the changing location of sulfate ions within the ice microstructure and/or interconnectedness of veins and grain boundaries.

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.

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.

Retracted] 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 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.