Density Of Grain Boundaries Research Articles

Enhancing thermoelectric properties of Ca3Co4O9 ceramics through oscillatory pressure sintering

Oscillatory Pressure Sintering (OPS) has been demonstrated to be a favorable manufacturing approach for the control of fine and uniform distribution of grains in various ceramic systems, which can lead to enhanced densification and thus improved physical and mechanical properties. In this study, OPS was utilized to prepare pure Ca3Co4O9 thermoelectric material. The results show that the electrical properties and thermoelectric properties of Ca3Co4O9 prepared by OPS are superior to those of samples sintered by conventional hot pressing (HP) without oscillation. This may be attributed to the better alignment of plate-like grains in the OPS samples, which increase electrical conductivity, and also the higher density of grain boundaries that enhance the phonon scattering and, thus, the reduction of thermal conductivity. This study suggests that a straightforward oscillatory pressure sintering approach can be utilized to obtain high-quality and high-performance thermoelectric materials without the need for doping, templating, or high-temperature sintering.

Excellent cryogenic mechanical properties of a novel medium-entropy alloy via vanadium-doping and hetero-grain/precipitation engineering

A novel vanadium-doped f.c.c. medium-entropy alloy (MEA) Ni42.4Co24.3Cr24.3Al3Ti3V3 was designed to achieve excellent cryogenic mechanical properties. The vanadium was added in order to increase the temperature-dependent friction stress, and aging of the MEA, which had been given a homogenization anneal at 1150 °C followed by cold-rolling to 80% reduction, at 700–900 °C was used both to produce partial recrystallization and precipitate L12 nanoparticles. Aging produced a heterogeneous microstructure composed of fine, soft recrystallized grains surrounded by harder, relatively-large non-recrystallized grains, containing L12-precipitates. Increasing the aging temperature from 700 °C to 900 °C promoted V partitioning into the f.c.c. matrix with the partitioning coefficient increasing from 0.8 to 1.9; the grain size increasing from 2.3 μm to 4.6 μm; an increase in the precipitate size from 10.5 nm to 80.0 nm; a reduction in the residual geometrically-necessary dislocation (GND) density from 9.0 × 1014 m−2 to 2.8 × 1014 m−2; and a decrease in the volume fraction of precipitates fraction from 28% to 14%. All three aged MEAs showed extraordinary cryogenic mechanical properties with an increase in the aging temperature resulting in an increase in strength but a decrease in ductility, viz, a yield strength (YS) of 1929 MPa, an ultimate tensile strength (UTS) of 2147 MPa, and an elongation to failure (ε) of 6.6% at 700 °C, YS ∼ 1735 MPa, UTS ∼ 1976 MPa, ε ∼ 9.0% at 800 °C, and YS ∼ 1274 MPa, UTS ∼ 1694 MPa, ε ∼ 24.8% at 900 °C. The dislocation back stress rose progressively with increasing strain and was invariably higher than the effective stress, accounting for 51–59% of the flow stress i.e. the back stress played a dominant role in the ultrahigh flow stress. Increasing the aging temperature also produced a greater abundance of nanoscale deformation twins, stacking faults, and dislocation networks, which are responsible for excellent strain hardening capacity and greater ductility. After deformation, the density of low-angle grain boundaries increased by 3–6 times to 0.6–1.3 μm/μm2, and the GND density increased by 4–6 times to 1.2–2.0 × 1015 m−2.

(Invited) Mixed Ionic Electronic Conductors (MIECs): The Importance of Fast and Slow Processes

MIECs are ideal materials for use as electrodes in solid state ionic devices. They have found application in SOFCs, SOECs, and solid state battery devices. In this contribution we will focus on MIEC used in the SOFC/SOEC where oxygen is the mobile ion. Most of these materials are complex perovskite materials, for example La1-xSrxCoyFe1-yO3-δ (LSCF). The focus of experimental observation in these materials has been on the fast process of transport of the highly mobile oxygen ion and much is known about the effect of non-stoichiometry in determining the level of oxygen ion transport. Most of these materials are very good mixed conductors at temperatures above 600°C but the lattice oxygen transport drops off markedly at lower temperatures. Recently there have been attempts to improve the lower temperature behaviour by producing nanomaterials, usually in thin film form [1-3], with a high density of grain boundaries. These have been shown to introduce fast diffusion paths along the grain boundaries and have been shown to improve the electrochemical performance. What has been neglected in the study of these materials is the evolution of the cation distribution in these complex oxides. The cation distribution has a marked effect upon the performance of an MIEC electrode, for example, the segregation of Sr to the surface which is detrimental to the oxygen exchange [4, 5], the development of defect clusters which can trap the oxygen vacancies, and the local chemistry of the grain boundaries [6], affecting transport in polycrystalline materials. Cation transport is a slow process but has importance during the short times and high temperatures during processing, and even more relevantly for the low temperatures but very long times associated with operation. This has implications for the durability of these devices. The contrast between these fast and slow processes will be discussed in this contribution and their relevance to different processing regimes and operational durability.

NbN thin films grown on silicon by molecular beam epitaxy for superconducting detectors

Superconducting nanowire single photon detectors (SNSPDs) made with thin NbN films can reach high performances. While sputtering has been the deposition method of choice, here, we show that ammonia-molecular beam epitaxy (NH3-MBE) can produce pertinent epitaxial cubic NbN thin films on silicon substrates using an AlN buffer. Despite granular morphology and a high density of grain boundaries as well as the presence of rotational twins, Tc = 12.7 K for a 5.6 nm thick film and saturation of internal detection efficiency up to 850 nm are achieved. Morphology and stoichiometry as well as strain have a strong impact on the detector properties, highlighting the importance of a precise control of the growth parameters. These results pave the way for high fabrication yield of SNSPDs on large-scale silicon wafers using epitaxial NbN thin films grown by MBE.

Crystallography of transformation products with different cooling rates in low-carbon alloy steel and its effect on low-temperature toughness uniformity of heavy plates

The crystallography of transformation products with different cooling rates has been investigated in a typical low-carbon alloy steel, and its effect on low-temperature impact toughness uniformity in the thickness direction of heavy steel plates has been elucidated. The results show that both the full lath martenite (LM) microstructure transformed at the cooling rate of 45 K s−1 and granular bainite (GB) + lath bainite (LB) + LM microstructure transformed at the cooling rates of 3∼6 K s−1 have higher density of high-angle grain boundaries (HAGBs). The density of HAGBs in the full GB microstructure transformed at the cooling rate of 0.5 K s−1 is the lowest, and that in the LB + LM microstructure transformed at the cooling rates of 15∼22 K s−1 is the middle. In the GB + LB + LM microstructure, the division of austenite by multi-stage transformation reduces the LB and LM transformation temperatures, contributing to a weakening of variant selection and a tendency to close-packed plane grouping. Therefore, the GB + LB + LM microstructure achieves the refinement degree similar to the full LM microstructure, but slightly higher than that of the LB + LM microstructure. For the 60 mm thick heavy steel plate with ultra-fast cooling, the cooling rate decreases from 41 K s−1 at the subsurface to 14 K s−1 at the 1/4-thickness and then to 4 K s−1 at the 1/2-thickness. The full LM microstructure, LB + LM microstructure, and GB + LB + LM microstructure are obtained in sequence, resulting in a more uniform density of HAGBs. Consequently, the relatively uniform impact toughness at 143 K in the thickness direction is achieved with a small difference of ∼30 J.

Crystallographic characteristics of acicular ferrite nucleated on inclusions in a HSLA steel

Acicular ferrite (AF) has been regarded as the optimal microstructure in steel welds and heat affect zones (HAZs), because of its superior combination of strength and toughness. Although various aspects of AF have been investigated, the crystallographic characteristics of AF, including orientation relationship, variant selection and variant pairing, are still not fully studied. In this research, a Ti-deoxidized high strength low alloy steel was prepared and subjected to HAZ thermal cycle simulation tests to attain AF microstructure. Optical micrographs were obtained and EBSD mappings of AF microstructures were conducted. Through crystallographic analysis, the orientation relationship between AF and austenite was found close to that of bainite transformed at a high temperature (580 °C). In the AF microstructure, all 24 variants are formed and variant selection is nearly random. Compared with bainite, the variant pairing of AF is more random despite the slight favour of certain variant pairs. Most of the adjacent AF laths are from different close-packed plane (CP) groups, leading to the classic interlocking morphology. Meanwhile, a large number of neighbouring AF laths belong to different Bain groups, resulting in a high density of high angle grain boundaries (HAGBs). Intragranular nucleation of primary AF laths on inclusions contributes both to the high HAGB density and to the interlocking morphology of AF, while the sympathetic nucleation of secondary AF laths only contributes to the latter. These results suggest that to increase the HAGB density in AF, promoting the intragranular nucleation on inclusions is more important than enhancing the sympathetic nucleation.

Controlled Stepwise Wet Etching of Polycrystalline Mo Nanowires

AbstractWith the persistent downscaling of integrated circuits, molybdenum (Mo) is currently considered a potential replacement for copper (Cu) as a material for metal interconnects. However, fabricating metal nanostructures with critical dimensions of the order of 10 nm and below is challenging. This is because the very high density of grain boundaries (GBs) results in highly non‐uniform surface profiles during direct wet etching. Moreover, wet etching of Mo with conventional aqueous solutions is problematic, as products of Mo oxidation have different solubility in water, which leads to increased surface roughness. Here, a process is shown for achieving a stable and uniform soluble surface layer of Mo oxide by wet oxidation with H2O2 dissolved in IPA at −20 °C. The oxide layer is then selectively dissolved, and by repeating the oxidation and dissolution multiple times, a uniform etch profile is demonstrated with a fine control over the metal recess. Ultimately, this presents a method of precise and uniform wet etching for Mo and other metals needed to fabricate complex nanostructures that are critical in developing next‐generation electronic devices.

Microstructure and mechanical property comparison of Inconel alloys fabricated using micro and conventional laser powder bed fusion

Compared to conventional laser powder bed fusion (c-LPBF), micro laser powder bed fusion (μ-LPBF) offers enhanced capabilities for fabricating complex components with exceptional accuracy. This is achieved through the utilization of a finer laser beam and smaller metal powder, leading to distinct differences in microstructure when compared to c-LPBF. Despite promising outcomes, a comprehensive understanding of these distinctions remains incomplete. One notable observation in the μ-LPBF samples in this study was the presence of a weaker 〈100〉 texture along the build direction. Additionally, these samples exhibited smaller grain sizes, a higher density of low-angle grain boundaries, and the presence of geometrically necessary dislocations. Significantly, μ-LPBF demonstrates the potential to produce IN718 with exceptional isotropic mechanical properties, primarily attributed to its capacity to generate refined microstructures and facilitate distinct solidification behaviors. Of particular significance was the remarkable 39.1 % increase in yield strength observed along the building direction by μ-LPBF. Furthermore, the differences in yield strength and ultimate tensile strength between various directions are notably reduced to 7 and 41 MPa, respectively, with a minimal total elongation variation of only 1.8 %. To gain deeper insights into the solidification behavior, Computational fluid dynamics were conducted in conjunction with the discrete element method.

Multiple cycles of combined cold rolling and short-time annealing on the microstructure and properties evolution of NiAl bronze

In this study, three cycles of combined cold rolling and annealing (CRA) were applied to study the mechanical behavior of NiAl bronze (NAB) alloy. The underlying mechanisms for microstructure and mechanical properties were elucidated. Microstructure characterization and quantitative analysis demonstrated significant morphology changes by CRA. The results showed that large α-Cu phase is compressed, lamellar κIII phase is broken and sphericalized, and κII phase accumulates and forms a hard phase band skeleton. Finally, the microstructure with spheroidized κIII phase embedded in the skeleton of the κII phase is obtained. The optimal ultimate tensile strength of 903 MPa and yield strength of 596 MPa are obtained after one cycle of CRA, which exhibits an improvement of about 40 % and 130 % compared to the as-cast samples, respectively. The best ductility of 18.0 % is obtained after three cycles of CRA, with an improvement of about 75 % over the as-cast samples. The high strength is mainly attributed to the grain refinement and high density of dislocations. Additionally, the higher density of low-angle grain boundaries can promote the formation and growth of twins. The saturation fraction of twin boundaries in experimental alloys of about 66 % is achieved after two cycles of CRA.

Effect of grain boundary engineering on corrosion behavior of 316 L stainless steel bipolar plate in proton exchange membrane fuel cells environment

This study illustrates the effect of grain boundary engineering (GBE) on the corrosion behavior of 316 L stainless steel (SS) in a proton exchange membrane fuel cells (PEMFCs) environment. It shows that the GBE increased the density of grain boundary and the coincidence site lattice (CSL) performes a better corrosion resistance, a preferential corrosion phase that the impurity elements are more likely biased at during the cast, the Random grain boundaries(RGBs) decreases. Furthermore, the texture on GBE2's closed-packed (111) plane reduces the surface atoms' dissolution velocity, thereby, decreasing the thermodynamic tendency to form dense corrosion films and nucleation kinetics. The above combined reasons lead to the lowest corrosion resistance of GBE2.

Ultralow dark current and high on–off ratio of perovskite photodetectors enabled by 3D/2D/3D heterostructures unipolar barrier

Lead halide perovskite photodetectors exhibit outstanding photoelectric performance in multiple parameters and have become a widely studied hotspot. The perovskite with single crystal structures significantly reduces the density of grain boundaries and defect states, thereby improving the carrier lifetime and mobility. However, the suppression of dark current is one of the most important concerns for the research of perovskite photodetectors. Here, we constructed a pBp unipolar barrier photodetector by inserting two-dimensional perovskite into three-dimensional perovskite through a pressure bonding process. The dark current of the photodetector decreased by more than two orders of magnitude, while the photocurrent remained unchanged. The perovskite unipolar barrier photodetectors exhibit excellent photoelectric performance, with an on–off ratio of 106, a specific detectivity of 4.11 × 1012 Jones, a −3 dB cutoff frequency of 344 kHz, and a linear dynamic range of 160 dB, providing a prospective path for weak light detection.

Dependence of resistivity on temperature and the mechanism of strengthening in a single-phase Cu–12Mn–3Ni alloy

The microstructural evolution of a single-phase Cu–12Mn–3Ni alloy during rolling and annealing has been investigated using electron backscatter diffraction and X-ray diffraction techniques. We have established the effects of microstructural characteristics on material strength and the temperature coefficient of resistance (TCR). The yield strength increased to 682 MPa after cold rolling with a strain of 0.87 due to an increase in the density of dislocation and low-angle grain boundary. A significant annealing softening is observed for the cold-rolled Cu–Mn–Ni alloy at temperatures above 400 °C, caused by recrystallization. The difference in yield strength for fully recrystallized alloys is mainly attributed to a difference in grain size, consistent with the Hall–Petch relationship. The addition of Ni and Mn enhanced the degree of difficulty in dislocation passing through the grain boundary, resulting in a higher Hall-Petch constant (274 MPa μm1/2) for Cu–12Mn–3Ni alloys when compared with pure copper. The TCR is related to grain size, dislocation and Cu–12Mn–3Ni alloy texture. Grain coarsening and reduced dislocation density can effectively enhance the resistance temperature stability. In addition, the strong copper texture formed during cold rolling results in a difference in TCR associated with the rolling direction (RD) and transverse direction (TD) of the cold-rolled sheet. The Cu–Mn–Ni alloy crystal exhibits a lower resistance sensitivity to temperature when the [110]Cu direction is parallel to the current direction compared with the [111]Cu direction parallel to the current direction. The absolute value of the TCR in the TD of the cold-rolled sheet is 10.2 × 10−6 °C−1, lower than that in the RD (19.3 × 10−6 °C−1).

Room temperature control of grain orientation via directionally modulated current pulses

Traditional approaches to control the microstructure of materials, such as annealing, require high temperature treatment for long periods of time. In this study, we present a room temperature microstructure manipulation method by using the mechanical momentum of electrical current pulses. In particular, a short burst of high-density current pulses with low duty cycle is applied to an annealed FeCrAl alloy, and the corresponding response of microstructure is captured by using Electron Backscattered Diffraction (EBSD) analysis. We show evidence of controllable changes in grain orientation at specimen temperature around 28 °C. To demonstrate such microstructural control, we apply the current pulses in two perpendicular directions and observe the corresponding grain rotation. Up to 18° of grain rotation was observed, which could be reversed by varying the electropulsing direction. Detailed analysis at the grain level reveals that electropulsing in a specific direction induces clockwise rotation from their pristine state, while subsequent cross-perpendicular electropulsing results in an anticlockwise rotation. In addition, our proposed room temperature processing yields notable grain refinement, while the average misorientation and density of low-angle grain boundaries (LAGBs) remain unaltered. The findings of this study highlight the potentials of ‘convective diffusion’ in electrical current based materials processing science towards microstructural control at room temperature.

Towards the understanding of fracture resistance of an ultrahigh-strength martensitic press-hardened steel

Identifying the fracture resistance capabilities and gaining a deeper understanding of the controlling damage mechanisms are important to the development of press-hardened steels (PHS) for automotive applications. In this study, the fracture properties of a novel PHS alloyed with Cr and Si (CrSi-PHS) were assessed by using uniaxial tensile and double-edge notched tensile (DENT) tests. Under uniaxial tension, the fracture resistance is characterized by the fracture strain and work of fracture, and the CrSi-PHS presents a desirable compromise with strength when comparing with other grades of advance high-strength steels (AHSS). The large fracture strain is attributed to the high resistance to damage nucleation even with a high density of grain boundaries. Fracture toughness is characterized by the DENT tests. The fracture toughness parameters of CrSi-PHS are lower than that of the commercial PHS 22MnB5 but comparable to the multiphase AHSSs with lower strength levels. The stress triaxiality near the fatigue pre-crack in DENT tests leads to a change of damage mechanism in CrSi-PHS. The crack propagates discontinuously by joining the micro-cracks and subsequently by the ductile fracture of micro-ligaments. The local plasticity at the crack tip region is suggested to contribute significantly to the energy dissipation. The observations demonstrate a strong dependence of the fracture mode and fracture resistance on the loading condition in the ultrahigh-strength martensitic steels.

Study on the oxidation behavior of a novel thermal barrier coating system using the nanocrystalline coating as bonding coating on the single-crystal superalloy

A novel thermal barrier coating system with a double columnar structure was developed on a single-crystal superalloy N5 to investigate its cyclic oxidation behavior. The bond coating is a columnar nanocrystalline coating deposited by magnetron sputtering, with the same chemical composition as the superalloy N5. Compared to the NiCrAlY/YSZ thermal barrier coating, this novel system does not experience elemental interdiffusion during high-temperature exposure. Additionally, the high density of grain boundaries in the bond coating promotes the selective oxidation of Al, which significantly improves its oxidation resistance and the rumpling resistance of the thermal barrier coating.

The mechanism of anisotropic micro-milling properties in additively manufactured Ti-6Al-4V alloy

Selective Laser Melting (SLM) technology offers great flexibility in manufacturing complex parts. However, secondary processing of selective laser melted (SLMed) parts is essential due to the poor surface morphology. This study investigated the influence of microstructural anisotropy in SLMed Ti-6Al-4V alloy on micro-milling performance. The orthogonal milling tests were conducted on the top, front, and side surfaces of specimens. Parameters including cutting force, chip morphology, burr morphology, surface quality, microhardness, and microstructure were evaluated to represent the anisotropy. The experimental findings demonstrated that the tool's orientation relative to the columnar grains influenced the slip strain modes and the density of grain boundaries in the shear band, thereby determining the correlation of the cutting force with the machined surface and feed direction. The variations in grain boundary density in different machining regions caused plastic anisotropy, which affects chip morphology, burr formation, and machined surface roughness. This research could provide valuable insights for manufacturers in selecting appropriate machining methods for precision processing.

Synergistic effects between potassium bubbles and high-energy-rate forging enabled ultrahigh strength and ductility of tungsten alloy

The intrinsic brittleness of tungsten (W) significantly restricts its application as the primary material for plasma-facing components. In this study, we propose a novel approach to improve both the strength and ductility of W-based alloys based on the synergistic effects between potassium (K) bubbles and high-energy-rate forging. K doping was introduced into W, followed by a two-step high-energy-rate forging (HERF) process. This leads to laminated matrix grains with refined sizes and deformation textures, as well as an increase in the density of low-angle grain boundaries and smaller size K bubbles along the grain boundaries. As a result, the prepared W alloys exhibit a substantially reduced ductile-to-brittle transition temperature within the range of room temperature to 50 °C, along with a notable ultimate tensile strength of up to 1393 MPa. Impressively, these alloys also demonstrate remarkable elongation at 200 °C, reaching a value as high as 33.12%. The remarkable mechanical strength and exceptional ductility exhibited by these alloys highlight their considerable potential for commercial application in the field of nuclear fusion engineering.

Mass transfer in Yb3Al5O12 films at high temperatures under oxygen potential gradients

Mass transfer along grain boundaries (GBs) in two kinds of polycrystalline Yb3Al5O12 (YbAG) wafers with chemical compositions that deviated slightly in both directions from the stoichiometric composition was investigated at high temperatures by the oxygen permeation technique with 18O2. Oxygen permeation through both wafers proceeded with mutual diffusion of oxide ions from the higher oxygen partial pressure PO2(hi) surface side to the lower PO2(lo) surface side and with the diffusion of ytterbium ions in the opposite direction. The apparent activation energies for oxygen permeation due to GB diffusion of oxide ions and ytterbium ions were 420–440 and 260–300 kJ mol−1, respectively, and almost no difference in the activation energies due to the different chemical compositions was observed. However, the oxygen permeability constants, which were divided by the GB density, for the aluminum-rich YbAG were clearly lower than those for the ytterbium-rich YbAG.

Grain boundary density matters in lithiation performances of aluminum foil anode for lithium ion batteries

Alloy foils are highly promising anode materials for high energy density lithium-ion batteries (LIBs). However, limited lithiation sites and inhomogeneous lithiation processes often lead to stress concentration and severe volume changes, resulting in rapid degradation of the electrode structure. In this work, the lithiation behavior and the corresponding structural evolution of aluminum (Al) foil anode are investigated by modulating the grain boundaries (GBs) density. The results show that abundant GBs network endows Al foil with uniform and distributed lithiation sites, and improved lithiation kinetics. The derived uniform volume change can effectively delocalize stress concentration, and hinder the formation of penetrated cracking network, giving rising to good structural stability and prolonged cycle life of Al foil anode. This work can enrich feasible strategies to alleviate the pulverization and serial issues of the metal foil anode by GBs density regulation for high specific energy LIBs.

Single-crystallization of electrolytic copper foils

Depending on the production process, copper (Cu) foils can be classified into two types, i.e., rolled copper (r-Cu) foils and electrolytic copper (e-Cu) foils. Owing to their high electrical conductivity and ductility at low cost, e-Cu foils are employed extensively in modern industries and account for more than 98% of the Cu foil market share. However, industrial e-Cu foils have never been single-crystallized due to their high density of grain boundaries, various grain orientations and vast impurities originating from the electrochemical deposition process. Here, we report a methodology of transforming industrial e-Cu foils into single crystals by facet copy from a single-crystal template. Different facets of both low and high indices are successfully produced, and the thickness of the single crystal can reach 500 µm. Crystallographic characterizations directly recognized the single-crystal copy process, confirming the complete assimilation impact from the template. The obtained single-crystal e-Cu foils exhibit remarkably improved ductility (elongation-to-fracture of 105% vs. 25%), fatigue performance (the average numbers of cycles to failure of 1600 vs. 200) and electrical property (electrical conductivity of 102.6% of the international annealed copper standard (IACS) vs. 98.5%) than original ones. This work opens up a new avenue for the preparation of single-crystal e-Cu foils and may expand their applications in high-speed, flexible, and wearable devices.