Density Of Grain Boundaries Research Articles

Hot deformation mechanisms of dual phase high entropy alloys

The microstructure of high entropy alloys can finally be designed via thermomechanical treatments to tune the mechanical properties. This work investigates the modification of the microstructure after treatments at 1100 °C for three hypo-eutectic high entropy alloys. Two phases were indexed according to the BCC and FCC crystal structures using electron backscattered diffraction. Their microstructure is investigated for three hot deformation tests: at a constant strain rate of 0.001s−1, at a strain rate jumps from 0.001s−1 to 1s−1 and from 1s−1 to 0.001s−1. The BCC size and fraction strongly influence the deformation of the FCC matrix. Due to its typical semi-interconnected hypo-eutectic structure, the BCC phase carries the load at the beginning of the deformation. Progressively, the FCC phase deforms to accommodate the plastic strain due to the bending and fragmentation of the BCC phase. Fine particles of the BCC phase are formed within the FCC matrix at high temperatures, and they pin the high-angle grain boundaries formed by continuous dynamic recrystallisation. The fragmentation of the BCC phase occurs faster for thinner eutectic BCC particles, and it is a consequence of I) the formation of boundaries during plastic deformation via dynamic recovery followed by continuous dynamic recrystallisation; II) the movement of phase boundaries consuming the formed boundaries within the BCC phase, fragmenting them. A fine substructure with a high density of high-angle grain boundaries is formed at 1100 °C for the alloy with initial fine BCC eutectic particles.

Highly Efficient and Stable Perovskite Solar Cells Incorporating Chlorinated-Tin Oxide Electron Transport Layers Prepared via Coordination of Diacyl-Metal Cations

The surface passivation of metal oxide electron-transport layers (ETLs) is a powerful strategy for realizing high-performance perovskite solar cells. The surface properties of ETLs strongly influence carrier injection and transfer dynamics; therefore, control of the carrier trap density is crucial. Semiconducting small molecules are considered suitable materials for surface passivation. However, they are expensive and vulnerable to humid atmospheres. Ultrathin polymer layers can have poor surface coverage owing to their spinodal dewetting. In this study, we employ adipoyl chloride as an organic ligand for the chemically robust and efficient surface passivation of SnO2 ETLs. Through the strong coordination of diacyl-metal cations, acyl groups are adsorbed onto the SnO2 surfaces, and the density of oxygen vacancies is significantly reduced. Furthermore, the changing surface properties of the SnO2 ETLs also contribute to the improvement of perovskite morphologies. The deeper energy levels and reduced defect density of the ETLs promote electron injection and transfer at the perovskite and ETL interface. The enlarged perovskite grains are accompanied by improved electron mobility and reduced grain-boundary density. The resulting power conversion efficiency (PCE) increases from 19.36 to 21.41%. The normalized PCE is retained at 90.4% of the initial value for 720 h under 1 sun illumination without the encapsulation of the devices.

Boosting the Performance of Perovskite Solar Cells through Systematic Investigation of the Annealing Effect of E-Beam Evaporated TiO2.

Electron transport layer (ETL) plays an undeniable role in improving the performance of n-i-p planar perovskite solar cells (PSCs). Titanium dioxide (TiO2) is known as a promising ETL material for perovskite solar cell. In this work, the effect of annealing temperature on optical, electrical, and surface morphology of the electron-beam (EB)-evaporated TiO2 ETL, and consequently on the performance of perovskite solar cell, was investigated. It was found that annealing treatment at an optimized temperature of 480 °C considerably improved the surface smoothness, density of grain boundaries, and carrier mobility of TiO2 film, which resulted in nearly 10-fold improvement in power conversion efficiency (11.16%) in comparison with the unannealed device (1.08%). The improvement in performance of the optimized PSC is attributed to the acceleration of charge carrier extraction, as well as suppression of the recombination at the ETL/Perovskite interface.

Effect of Heat Treatment on the Cavitation Erosion Behavior of Nanocrystalline Surface Layer of 304 Stainless Steel

In this study, a nanocrystalline layer composed primarily of martensite phase was prepared on the surface of 304 stainless steel. Furthermore, the martensite phase content in the nanocrystalline layer was adjusted by heat treatment at 500 °C and 550 °C, respectively, and the cavitation erosion resistance of the nanocrystalline layer before and after heat treatment was investigated. The results showed that the nanocrystalline layer before and after heat treatment exhibited excellent erosion resistance, with cumulative mass loss of approximately 1/7, 1/5, and 1/3 that of the traditional 304 stainless steel, respectively. The nanocrystalline layer could significantly inhibit the growth of cavitation pits due to the high density of grain boundaries. However, due to the decrease in hardness of the nanocrystalline layer after heat treatment, the propagation speed of cavitation cracks was accelerated, and the cavitation erosion performance of the nanocrystalline layer showed a downward trend.

Damping performance of SiC nanoparticles reinforced magnesium matrix composites processed by cyclic extrusion and compression

This work dealt with the damping performance and its underlying mechanism in SiC nanoparticles reinforced AZ91D composite (SiCnp/AZ91D) processed by cyclic extrusion and compression (CEC). It was found that the CEC process significantly affects the damping performance of the composite due to alterations in the density of dislocations and grain boundaries in the matrix alloy. Although there would be dynamic precipitation of the Mg17Al12 phase during processing which increases the phase interface and limits the mobility of dislocations and grain boundaries. The results also showed that the damping capacity of 1%SiCnp/AZ91D composite continuously decreases with adding CEC pass number and it consistently increases with rising the applied temperature. Considering the first derivative of the tanδ–T curve, the dominant damping mechanism based on test temperature can be divided into three regions. These three regions are as follows (i) dislocation vibration of the weak pinning points (≤Tcr), (ii) dislocation vibration of the strong pinning points (Tcr∼TV), and (iii) grain boundary/interface sliding (≥TV).

Formation mechanism and microstructure evolution of light etched region during rolling contact fatigue in M50 steel

The microstructure evolution and formation mechanism of lighting etched region (LER) during rolling contact fatigue (RCF) in M50 steel are systematically investigated. The results indicates that LER is a kind of microstructure degeneration due to the accumulation of plastic strain. Finite element simulation indicates that, under 5 GPa contact pressure, the equivalent stress at the subsurface exceeds the yield strength of M50 steel and, thus, induces plastic deformation. The dislocation density and fraction of grain boundaries increase in LER and thus the grain size is refined and the micro-hardness increases. The microstructure evolution in LER during RCF is divided into four stages. Stage 1: dislocations and GBs increase significantly, while the micro-hardness remains almost unchanged with the increase of RCF cycles; Stage 2: GBs and micro-hardness increase gradually with the increase of RCF cycles; Stage 3: LAGBs evolve to HAGBs and thus the total length of LAGBs decreases and the total length of HAGBs increases. Dislocation density and micro-hardness both increase with the increase of RCF cycles; Stage 4: dislocations and GBs continue to increase with the increase of RCF cycles due to accumulation of plastic deformation, and the micro-hardness also increases.

Bulk CrCoNiFe alloy with high conductivity and density of grain boundaries for oxygen evolution reaction and urea oxidation reaction

Multiple-principal-element alloys (MPEAs) with maximized configurational entropy show high catalytic activities for oxygen evolution reaction (OER) and urea oxidation reaction (UOR). However, the accurate relationship between their complex components (i.e., elements, phase structure, grain boundary density) and intrinsic catalytic activity is still unclear. Herein, a series of bulk MPEAs with face-centered cubic (FCC) phase structures were fabricated by the arc-melting method under an argon atmosphere. Compared to the CrCoNi and CrCoNiFeMn, the CrCoNiFe affords a higher UOR performance with the lowest overpotential of 331 mV at 10 mA·cm−2 in 1 M KOH with 0.33 M urea, due to excellent conductivity and high density of grain boundaries. The urea electrolyzer using CrCoNiFe as anode and Pt as cathode shows a low voltage of 1.622 V at 10 mA cm−2 and long-term stability of 60 h at 20 mA cm−2 (4.08% decrease). These findings offer a facile strategy for designing bulk MPEAs electrodes for energy conversion.

Selective Laser Melting Additive Manufactured Tantalum: Effect of Microstructure and Impurities on the Strengthening-Toughing Mechanism.

The balance between the strength and the toughness of pure tantalum (Ta) fabricated with selective laser melting (SLM) additive manufacturing is a major challenge due to the defect generation and affinity for oxygen and nitrogen. This study investigated the effects of energy density and post-vacuum annealing on the relative density and microstructure of SLMed tantalum. The influences of microstructure and impurities on strength and toughness were mainly analyzed. The results indicated that the toughness of SLMed tantalum significantly increased due to a reduction in pore defects and oxygen-nitrogen impurities, with energy density decreasing from 342 J/mm3 to 190 J/mm3. The oxygen impurities mainly stemmed from the gas inclusions of tantalum powders, while nitrogen impurities were mainly from the chemical reaction between the molten liquid tantalum and nitrogen in the atmosphere. The proportion of <110> texture decreased after vacuum-annealing at 1200 °C, while that of the <100> texture increased. Concurrently, the density of dislocations and small-angle grain boundaries significantly decreased while the resistance of the deformation dislocation slip was significantly reduced, enhancing the fractured elongation up to 28% at the expense of 14% tensile strength.

New insights into the effects of solidification process on the thermodynamic parameters and mechanical properties of CuAlBeNbNi shape memory alloy

In this work, the influence of the processes of unidirectional solidification and squeeze casting in microstructure, thermodynamic parameters and mechanical properties of CuAlBeNbNi shape memory alloy (SMA) was investigated. The results show that the unidirectional solidification process causes an increase in phase transformation temperatures and ductility, improves fracture toughness and reduces stress-induced martensite, residual stress and dissipated energy after load-unload tensile test of the CuAlBeNbNi alloy. The squeeze casting process, on the other hand, causes an increase in the density of grain boundaries, increasing the stress-induced martensite and the elastic energy of the direct martensitic transformation, while decreasing the impact toughness, increasing the residual deformation, the residual stress and the dissipated energy after the tensile cycle tests. These results indicate that the processes of unidirectional solidification and squeeze casting offer an alternative method to modify the thermodynamic parameters, mechanical properties, and microstructural properties of the CuAlBeNbNi SMA while keeping the composition constant. This study aims to fill the knowledge gap regarding the impact of solidification processes on the properties of Cu-based SMAs, specifically investigating CuAlBeNbNi SMAs.

Fabrication of Li anode metal via bulk mechanical property analysis

Two types of lithium (Li) metals with a different average grain size, i.e., ∼10 μm in orders and above 100 μm, are successfully produced by extrusion process and rapid molding process. The results obtained from nano-indentation tests show that hardness is influenced by grain size, and it decreases with a finer grained structure, which is not the general trend as those of other light-weight pure metals. It is remarkable to observe that these Li metals have a large strain rate dependence (a large m-value) even at room-temperature, irrespective of the grain size. Compared with the Frost-Ashby deformation mechanism map, the dominant deformation mechanism is the dislocation climb. On the contrary, this unusual behavior in the fine-grained Li metal is due to the partial contribution of grain boundary sliding to deformation. This behavior is associated with the enhancement of grain boundary diffusivity and high density of grain boundaries. Concerning the effect of grain size on electrochemical characteristics, similar to the mechanical property, the results obtained from electrodeposition and dissolution tests display that the fine-grained Li metal has much better over-potential of charging and discharging operation as well as voltage response.

Enhanced nanocrystalline stability of BCC iron via copper segregation

Nanocrystalline alloys are known for their high density of grain boundaries (GBs), which results in low thermal stability and a tendency for coarsening. In this work, we investigated Fe–Cu alloys to explore the effects of Cu concentration on the thermal stability of nanocrystalline samples. Using hybrid molecular dynamics (MD) and Monte Carlo (MC) simulations, annealing treatments of nanocrystalline samples with varied Cu concentrations were conducted. The simulation results revealed that Cu atoms tended to accumulate at GBs. Subsequently, the model with Cu segregation was subjected to mechanical creep loading at various temperatures. Through the analysis of atomic structure evolution during creep deformation, it was found that Cu segregation efficiently stabilizes GBs and restricts their movement. The present findings highlight that the thermal stability of nanocrystalline Fe–Cu alloys can be effectively improved by introducing suitable Cu segregation at GBs.

Effectively improving the hardness-strength-toughness of carburized bearing steel via nanoprecipitates and fine grain structure

Cryogenic treatments are fast and efficient methods used to improve the mechanical properties of metal components by promoting retained austenite (RA) transformation. However, the relationship between the evolution of RA with the gradient changes in content and hardness-strength-toughness improvements during this period are unclear. The present study investigated the beneficial effects of a cryogenic treatment on the microstructural evolution and mechanical properties of a gradient-structured carburized M50NiL bearing steel. The results revealed that the RA average sizes, the densities of the high-angle grain boundaries (DHAGBs) and low-angle grain boundaries (DLAGBs), and the hardness of the carburized layer changed with increasing distance from the surface. Compared with the untreated samples, the cryogenically treated samples demonstrated a 30% greater effective hardened layer thickness as well as 15.6% and 22.9% higher strength and toughness, respectively. The synergistic enhancements of the hardness, strength, and toughness provided by the cryogenic treatment were attributed to the increased nanoprecipitate content and finer grain structure generated in situ during transformation and further decomposition of the RA, which resulted in reductions of up to 83.6% in the RA content. This work provided new ideas for innovative enhancements of gradient-structured materials with superior mechanical properties.

Significance of Melt Pool Structure on the Hydrogen Embrittlement Behavior of a Selective Laser-Melted 316L Austenitic Stainless Steel.

The hydrogen embrittlement (HE) behavior of a selective laser-melted (SLM) 316L austenitic stainless steel has been investigated by hydrogen charging experiments and slow strain rate tensile tests (SSRTs) at room temperature. The results revealed that compared to the samples without H, the ultimate tensile strength (UTS) and elongation (EL) of specimens were decreased from 572 MPa to 552 MPa and from 60% to 36%, respectively, after 4 h of electrochemical hydrogenation with a current density of 100 mA/cm2. The negative effects of hydrogen charging were more pronounced on the samples' ductility than on their strength. A quasi in situ EBSD observation proved that there was little phase transformation in the samples but an increased density of low angle grain boundaries, after 4 h H charging. After strain was applied, the surface of the H-sample displayed many hydrogen-induced cracks along the melt pool boundaries (MPBs) showing that these MPBs were the preferred areas for the gathering and transferring of hydrogen.

Epitaxial Grown VO2 with Suppressed Hysteresis and Low Room Temperature Resistivity for High-Performance Thermal Sensor Applications

The ability of VO2 to undergo semiconductor-to-metal phase transition (SMT) upon heating makes it a very attractive material for uncooled bolometers. The SMT of VO2 represents a large temperature coefficient of resistance, which is an important parameter for the development of highly responsive microbolometers. However, other characteristics of the SMT of VO2 such as its high transition temperature (341.2 K), the sharpness of the transition, its hysteresis, and the high room temperature resistivity limit the performance of this material in microbolometers. In this work, we grow a high-quality epitaxial ultrathin film VO2 on c-plane Al2O3 by pulsed laser deposition. The low deposition temperature and tuning the oxygen partial pressure during the growth process enable control over the grain size and oxygen vacancy concentration. This allowed controlling the SMT parameters of the samples. In particular, we show that the high density of grain boundaries associated with nanosized grains suppresses the thermal hysteresis of the SMT. Simultaneous control over the density of oxygen vacancies and the size of grains enables the adjustment of the temperature coefficient of resistance, room temperature resistivity, SMT temperature, sharpness, and thermal hysteresis toward suitable values for the fabrication of efficient VO2-based uncooled bolometers. Compared with other VO2 fabrication methods, this approach can be viewed as a simpler alternative for VO2 fabrication with favorable properties for practical bolometer applications.

Zr-Ti getter film integration in MEMS vacuum packaging

Titanium, zirconium, and Zr-Ti alloy thin films were evaporated under UHV. AFM, TEM and SEM observations showed that grain size at the surface of the films varies with their thickness and composition. The single-metal films exhibit larger grain than alloy films. The highest density of grain boundaries is found for Zr-Ti film with a balanced content in Zr and Ti. For the three types of films, the grain boundary density at the surface decreases when increasing the film thickness. The gettering properties of the films were studied by activation annealing in the range of 200 °C–400 °C by in situ sheet resistance monitoring and by integrating them into MEMS packages sealed at 300 °C. In both cases, the sorption of gases was found to increase with the surface density of grain boundaries. In the specific activation conditions of MEMS packaging, it is shown that reducing the film thickness from 400 nm to 50 nm enhances gas sorption by the Zr-Ti getter and for the first time, the activation of a getter film thinner than 100 nm integrated in a MEMS package was demonstrated.

Microstructure characteristics and mechanical properties of deposited metals with different types of bainite

In this work, three types of bainite microstructure have been gained in the deposited metals by adjusting the Ni content (1 wt %, 2.5 wt %, and 4 wt %). The microstructure feature of different types of bainite has been characterized by optical microscope, scanning electron microscope, transmission electron microscope, and electron backscattering diffraction. Microstructure changed from granular bainite with different morphology to lath bainite with the increase of Ni content. The number of elongated Martensite-Austenite constituents in granular bainite increased with the increasing Ni content from 1 wt % to 2.5 wt % due to the increase in hardenability. The complex orientation in the deposited metal with 4 wt % Ni content attributed to the numerous autocatalytic nucleation occurrence during the bainite transformation. After crystallographic characterization, the bainite variant selection tendency decreased with the bainite type change from granular bainite to lath bainite with autocatalytic nucleation, and the density of high-angle grain boundaries increases obviously. The deposited metal with numerous autocatalytic nucleation has an outstanding match of high strength and excellent low-temperature toughness due to the effective grain-refinement strengthening. The large-sized elongated Martensite-Austenite constituents in the deposited metals with 1 wt % and 2.5 wt % Ni contents deteriorate low-temperature toughness in two aspects, promoting crack nucleation and reducing the crack propagation resistance. Interlaced bainite is the most optimized among different bainite types. This work guides the microstructure design in high-strength deposited metals.

Role of Carbon Content on Microstructure Evolution and Impact Toughness in Coarse-Grained Heat-Affected Zone of High-Strength Steel

The effect of carbon content in the base metals of high-strength steel on the microstructure and impact toughness of simulated welding focusing on a coarse-grained heat-affected zone (CGHAZ) at different heat inputs was systematically investigated by using scanning electron microscopy (SEM) and electron back-scattering diffraction (EBSD). The Charpy impact test confirmed that there was an optimal heat input, which caused the CGHAZ to obtain the highest impact toughness. The optimal heat input is ~20 kJ/cm and remains unchanged with an increase in carbon content from 0.04 to 0.12 wt.%. However, the impact toughness of the CGHAZ decreases with the increase in carbon content at each heat input. Microstructure characterization showed that a CGHAZ with 0.04 wt.% carbon gradually changed from lath bainite (LB) to granular bainite (GB) with an increase in heat input, while it changed from lath martensite (LM) to LB and then to GB for a CGHAZ with 0.12 wt.% carbon. Although the density of high-angle grain boundaries (HAGBs) obtained at 20 kJ/cm in the high-carbon sample is higher than that of the low-carbon sample, its impact toughness is lower, which is related to the parallel structure of the lath bundles and the morphology the austenite penetration.

Cu-doped mesoporous SnO2 nanoparticles with rich grain boundaries and oxygen vacancies for photocatalytic CO2-to-CO conversion.

Photocatalytic conversion of carbon dioxide into fuels provides an effective approach to realize carbon resource utilization. However, the photocatalytic efficiency is still relatively low due to the recombination of photogenerated charges. Herein, we have designed Cu-doped SnO2 nanoparticles (Cu-SnO2) using a glucose-involved hydrothermal crystallization method for the photocatalytic reduction of CO2. The rich oxygen vacancies facilitated the separation and transfer of photogenerated charges, and the confined effect of the typical mesoporous structure promoted the adsorption of CO2, especially a high density of grain boundaries (GBs) and the doping of atomic Cu would introduce new active sites to activate CO2 molecules. This elaborately designed catalyst exhibited super and stable photocatalytic conversion activity of CO2-into-CO, with a CO optimal yield of 107 µmol g-1 in 4 h, which was 2.75 times that over pure SnO2. In situ Raman results indicated that the CO2 reduction reaction followed a *COOH pathway on Cu-SnO2. This work provides implications for the construction of a catalyst with rich defects in the field of energy and environmental catalysis.

Antibacterial copper-bearing titanium alloy prepared by laser powder bed fusion for superior mechanical performance

• A boundary engineering strategy is proposed to improve the strength and ductility of materials simultaneously. • The addition of Cu successfully verified the feasibility of the boundary engineering strategy, and improved the comprehensive mechanical properties of LPBF martensitic titanium alloy. • The role of copper addition in microstructure development are clarified. • The strengthening mechanism of LPBF Ti, Ti-5Cu and Ti-6Al-4V is explained by calculation and fitting. Copper element was added in pure titanium to produce a new biomedical titanium-copper alloy by laser powder bed fusion (LPBF). Addition of copper can eliminate the mismatch of high strength but poor ductility problem caused by lath α' martensite, which is the usual microstructure of near α titanium alloy fabricated by LPBF. Instead of by the usual trade-off relationship between strength and ductility, which is a long-standing challenge for martensitic titanium alloys, in this study, we proposed a boundary engineering strategy and aim to synergistically enhance the strength and ductility of martensitic titanium alloy fabricated by LPBF. It is hypothesized that whilst both low-angle and high-angle grain boundaries are beneficial to the strength, high-angle grain boundary can simultaneously improve the ductility of materials. To test this strategy, a Ti-5Cu (wt.%) alloy is selected to compare against pure titanium and Ti-6Al-4V at the same laser processing conditions. EBSD, TEM and XRD analysis show that the as-fabricated LPBF Ti-5Cu alloy is comprised of partially tempered martensite with extraordinarily high number density of both high-angle and low-angle grain boundaries as well as low dislocation density. Such microstructure enables a high tensile strength of 940–1020 MPa, which is at a similar level as LPBF Ti-6Al-4V, and an excellent elongation of 13%–16%, twice as much as that of LPBF Ti-6Al-4V. The mechanism of microstructure refinement in LPBF Ti-5Cu at different levels from prior-β grains, martensitic packets, blocks to laths is also discussed.

Effect of Double-Quenching on the Hardness and Toughness of a Wear-Resistant Steel

Martensitic/bainitic wear-resistant steels are widely used in civilian industry, where a good combination of strength and toughness is required. In the present study, a double-quenching process was applied and compared to the conventional single-quenching process. The microhardness and ductile–brittle transition temperature were measured, and the microstructure was characterized with scanning electron microscopy and electron backscatter diffraction (EBSD) technique. It was found that the double-quenching process refined the prior austenite grain size by 43% and simultaneously improved the toughness and hardness. The ductile-to-brittle transition temperature was decreased from −77 °C to −90 °C, and the hardness was increased by 8%. Based on the EBSD data, a detailed analysis of the grain boundary distribution was performed using a recently developed machine learning model. Unlike what was found in previous studies, for the studied wear-resistant steel, the refinement of the prior austenite grain did not increase the block boundary density while increasing the high-angle packet boundary density. As a result, the total density of the high-angle grain boundaries in the double-quenched specimen was not improved compared to the single-quenched specimen. Further inspection suggested that it is the prior austenite grain boundaries and high-angle packet boundaries that contribute to the hardness and toughness, and the key factors that determine their effectiveness are the high misorientation angle between the {110} slip planes and the high slip transmission factor.