High Density Of Grain Boundaries Research Articles

Graded microstructure and mechanical properties of spark plasma sintered Fe-Cr alloys

Developing graded microstructure and mechanical properties is critical for accelerating the design and optimization of structural materials for a wide range of applications. In this study, Fe-Cr alloys were fabricated by spark plasma sintering (SPS) technique. The microstructure, microhardness, and tensile properties of the as-fabricated and thermally annealed variants were investigated. Graded grain structures were created in the as-fabricated cylinders along both axial and radial directions. Grain size was gradually reduced from the sample periphery to the center. Microhardness measurements reveal a gradual decrease of hardness towards the periphery of the samples. The as-fabricated Fe-Cr alloys show a desired combination of tensile strength and elongation, primarily due to the formation of a high density of oxides, voids, dislocations, and grain boundaries. After in-situ thermal annealing at 600 °C under 60 MPa for 2 h using SPS, the Fe-Cr alloys underwent minimal grain growth, and the graded grain structures were retained. The hardness was more uniformly distributed in the annealed variants, and the tensile strength was reduced with an increase in the total elongation, which is attributed to the dissolution of nano-sized oxide particles and the relief of the residual stress. This study demonstrates that SPS coupled with subsequent heat treatment can tailor the graded microstructure and control the mechanical properties of Fe-Cr alloys, showing potential applications in other alloy systems.

Bubble-Mediated Large-Scale Hierarchical Assembly of Ultrathin Pt Nanowire Network Monolayer at Gas/Liquid Interfaces.

Extensive macroscale two-dimensional (2-D) platinum (Pt) nanowire network (NWN) sheets are created through a hierarchical self-assembly process with the aid of biomolecular ligands. The Pt NWN sheet is assembled from the attachment growth of 1.9 nm-sized 0-D nanocrystals into 1-D nanowires featuring a high density of grain boundaries, which then interconnect to form monolayer network structures extending into centimeter-scale size. Further investigation into the formation mechanism reveals that the initial emergence of NWN sheets occurs at the gas/liquid interfaces of the bubbles produced by sodium borohydride (NaBH4) during the synthesis process. Upon the rupture of these bubbles, an exocytosis-like process releases the Pt NWN sheets at the gas/liquid surface, which subsequently merge into a continuous monolayer Pt NWN sheet. The Pt NWN sheets exhibit outstanding oxygen reduction reaction (ORR) activities, with specific and mass activities 12.0 times and 21.2 times greater, respectively, than those of current state-of-the-art commercial Pt/C electrocatalysts.

Local measurement of geometrically necessary dislocation densities and their strengthening effect in ultra-high deformed pearlite

The strength of pearlitic wires can be increased by cold-drawing to a world record level for bulk ductile materials of 7 GPa. The underlying strengthening mechanisms are not fully understood, as the application of usual characterization is challenging because of the small grain sizes and the high degree of deformation. Here we demonstrate that the microstructure of the wires can be directly probed by nano-beam diffraction (NBD) orientation mapping in a transmission electron microscope even after a drawing strain of 6.52. We observe a highly fragmented microstructure with a high density of low-angle grain boundaries (LAGBs) within the ferrite lamella. That makes it difficult to define grain sizes in the ordinary way. We thus calculate an equivalent grain size based on the density of high-angle grain boundaries (HAGBs) per measurement area and an average density of geometrically necessary dislocations (GNDs) calculated from all local misorientation gradients below 15° misorientation. Total strengths calculated from a summed Hall-Petch and Taylor effect of the latter values as well as carbon solid solution hardening are in good agreement with the strengths as measured by tensile tests. Our results show that the GNDs are similarly important as HAGBs in terms of their contributions to the total strength. On this basis, the experimental evidence of the strengthening mechanism with emphasis on GNDs, particularly in ultra-high deformed materials is highlighted. The present results also validate the application of NBD to assessing mechanical properties of other ultra-high deformed materials where mechanical tests often are not feasible.

A phenomenological understanding of the novel design of hierarchical structure for 1 GPa ultrahigh strength and high toughness combination low alloy steel

In the present study we propose a novel design of hierarchical structure with the aim to obtain high strength of 1 GPa and low temperature toughness in a heavy low alloy steel plate by intercritical quenching and tempering (IQ&T). The hierarchical microstructure consisted of bimodal prior austenite grains with complex microstructure obtained via intercritical quenching from 810 °C and tempering at 500 °C. The large size grains with average diameter of ∼22 μm consisted of tempered martensite and fine intercritical ferrite. While the surrounding fine grains with average diameter of ∼12 μm consisted of tempered lower bainite and tempered lath martensite. Ultra-high yield strength of 998 MPa and tensile strength of 1059 MPa were obtained for the novel hierarchical microstructure steel, which was similar to the conventional quenched and tempered (Q&T) steel. Whereas, the low temperature impact energies at both −40 and −80 °C were ∼100% higher for the hierarchical microstructure steel than the conventional Q&T steel. The significant improvement of low temperature toughness is attributed to the synergistical effect of hierarchical microstructure. The fine intercritical ferrite in large grains increased the plastic deformation ability during impact and was beneficial for formation of dimples, and the surrounding fine grains offered high density of large mis-orientation grain boundaries to deflect crack propagation, leading to high impact energy at low temperature.

The Evolution of Structural Defects under Irradiation in W by Molecular Dynamics Simulation.

Tungsten (W) can be used in plasma-facing components in a fusion reactor because of its excellent radiation resistance. Some studies have found that nanocrystalline metals with a high density of grain boundary show a higher ability to resist radiation damage compared to conventional coarse-grained materials. However, the interaction mechanism between grain boundary and defect is still unclear. In the present study, molecular dynamics simulations were carried out to explore the difference of defect evolution in single-crystal and bicrystal W, while the effects of temperature and the energy of the primary knocked atom (PKA) were taken into account. The irradiation process was simulated at the temperature range of 300 to 1500 K, and the PKA energy varied from 1 to 15 keV. The results show that the generation of defects is more sensitive to the energy of PKA than temperature; the number of defects increases at the thermal spike stage with the increase of the PKA energy, but the correlation with temperature is not strong. The presence of the grain boundary prevented the recombination of interstitial atoms and vacancies during the collision cascades, and the vacancies were more likely to form large clusters than interstitial atoms in the bicrystal models. This can be ascribed to the strong segregation tendency of the interstitial atoms to grain boundaries. The simulations provide useful information for understanding the role of grain boundaries in the evolution of irradiated structural defects.

Shear-induced directional grain growth in Ag nanocrystalline films under nanoindentation

Nanocrystalline metallic materials exhibit different mechanical properties compared with coarse-grained counterparts. In essence, the high density of grain boundaries contributes to significant portion of plasticity, which is more pronounced in thin films. Due to intrinsic geometric limitation, the responses of nanocrystalline films under shear-dominant loading are seldom understood. Here, we take Ag film as an example, with a larger indentation depth than film thickness, the deformation beneath the pyramidal surfaces of the indenter is considered to be dominated by in-plane shear stress with gradient distribution. The shear-dominant loading leads to directional grain elongation within indentation region, accompanied with frequent formation of nanotwins across the grain boundaries. These findings enrich our understanding of stress induced grain growth under nanoindentation, and provide a novel way to study shear-dominated plastic deformation in metallic nanocrystalline films.

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.

Accelerating Industrial-Level NO3 - Electroreduction to Ammonia on Cu Grain Boundary Sites via Heteroatom Doping Strategy.

Although the electrocatalytic nitrate reduction reaction (NO3 - RR) is an attractive NH3 synthesis route, it suffers from low yield due to the lack of efficient catalysts. Here, this work reports a novel grain boundary (GB)-rich Sn-Cu catalyst, derived from in situ electroreduction of Sn-doped CuO nanoflower, for effectively electrochemical converting NO3 - to NH3 . The optimized Sn1% -Cu electrode achieves a high NH3 yield rate of 1.98mmol h-1 cm-2 with an industrial-level current density of -425mA cm-2 at -0.55V versus a reversible hydrogen electrode (RHE) and a maximum Faradaic efficiency of 98.2% at -0.51V versus RHE, outperforming the pure Cu electrode. In situ Raman and attenuated total reflection Fourier transform infrared spectroscopies reveal the reaction pathway of NO3 - RR to NH3 by monitoring the adsorption property of reaction intermediates. Density functional theory calculations clarify that the high-density GB active sites and the competitive hydrogen evolution reaction (HER) suppression induced by Sn doping synergistically promote highly active and selective NH3 synthesis from NO3 - RR. This work paves an avenue for efficient NH3 synthesis over Cu catalyst by in situ reconstruction of GB sites with heteroatom doping.

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.

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.

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.

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.

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.

Low-cost preparation of highly-efficient thermoelectric BixSb2-xTe3 nanostructured powders via mechanical alloying

In this paper, a low-cost and facile method for preparing highly-efficient Bi2Te3-based thermoelectric materials is demonstrated. P-type BixSb2-xTe3 (x ​= ​0.2, 0.3, 0.4, 0.5) nanostructured powders were fabricated via mechanical alloying, and consequently consolidated into pellets by hot-press sintering. The effect of Bi/Sb ratio on the thermoelectric properties was investigated. The higher hole density and mobility in samples with lower Bi concentration resulted in higher electrical conductivity and power factor values. The absence of preferred grain orientation and the high density of grain boundaries in these nanostructured materials led to significantly low lattice thermal conductivity. As a result, a peak ZT of 1.13 ​at 355 ​K and an average ZT of 1.01 over the investigated temperature range was achieved for Bi0.3Sb1.7Te3, suggesting that mechanical alloying can be considered as an advantageous technique for single-step synthesis of highly-efficient thermoelectric BixSb2-xTe3 alloys.

Microstructure, mechanical properties and thermal stability of friction-stir-processed Al–Cu–Mg–Ag alloy

The mechanical properties and thermal stability of Al–Cu–Mg–Ag alloy prepared by friction stir processing and subsequent aging were investigated by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. After peak aging treatment, the mechanical properties in the stir zone are significantly higher than those of the base metal while plasticity stays the same. With increasing rotational speed, the mechanical properties of the stir zone improve and have a stronger age-hardening response, which is mainly attributed to the fine equiaxed grains and the high number density of plate-like Ω plates. After the heat exposure, the mechanical properties of the stir zone decreased more drastically compared to the base material. This is because the high density of dislocations and grain boundaries promotes elemental diffusion and accelerates the coarsening of the precipitates in the stir zone.

Oxidation resistance of the 316L manufactured by cold-rolled and selective laser melting in CO2 and air

The high-temperature oxidation behavior of 316 L manufactured by cold-rolled and selective laser melting in CO 2 and air was investigated. Scanning electron microscopy, X-ray diffraction, and energy disperse spectroscopy were employed to characterize the oxidation products on investigated 316 L. Results showed that oxidation kinetics of all samples followed a parabolic law from 600 ℃ to 900 ℃. At 700 ℃, cold-rolled 316 L exposed a better oxidation resistance, while selective laser melting 316 L was more resistant to oxidation at 600 ℃, 800 ℃ and 900 ℃. The above oxidation behavior may be caused by the recrystallization of cold-rolled 316 L and high density of grain boundaries in selective laser melting 316 L.

Inhibiting creep in nanograined alloys with stable grain boundary networks.

Creep, the time-dependent deformation of materials stressed below the yield strength, is responsible for a great number of component failures at high temperatures. Because grain boundaries (GBs) in materials usually facilitate diffusional processes in creep, eliminating GBs is a primary approach to resisting high-temperature creep in metals, such as in single-crystal superalloy turbo blades. We report a different strategy to inhibiting creep by use of stable GB networks. Plastic deformation triggered structural relaxation of high-density GBs in nanograined single-phased nickel-cobalt-chromium alloys, forming networks of stable GBs interlocked with abundant twin boundaries. The stable GB networks effectively inhibit diffusional creep processes at high temperatures. We obtained an unprecedented creep resistance, with creep rates of ~10^-7 per second under gigapascal stress at 700°C (~61% melting point), outperforming that of conventional superalloys.

Preparing Thick Gradient Surface Layer in Cu-Zn Alloy via Ultrasonic Severe Surface Rolling for Strength-Ductility Balance.

A gradient structure (GS) design is a prominent strategy for strength-ductility balance in metallic materials, including Cu alloys. However, producing a thick GS surface layer without surface damage is still a challenging task limited by the available processing technology. In this work, a gradient structure (GS) surface layer with a thickness at the millimeter scale is produced in the Cu-38 wt.% Zn alloy using ultrasonic severe surface rolling technology at room temperature. The GS surface layer is as thick as 1.1 mm and involves the gradient distribution of grain size and dislocation density. The grain size is refined to 153.5 nm in the topmost surface layer and gradually increases with increasing depth. Tensile tests indicate that the single-sided USSR processed alloy exhibits balanced strength (467.5 MPa in yield strength) and ductility (10.7% in uniform elongation). Tailoring the volume fraction of the GS surface layer can tune the combination of strength and ductility in a certain range. The high strength of GS surface layer mainly stems from the high density of grain boundaries, dislocations and dislocation structures, deformation twins, and GS-induced synergistic strengthening effect. Our study elucidates the effect of the thick GS surface layer on strength and ductility, and provides a novel pathway for optimizing the strength-ductility combination of Cu alloys.