Showing papers on "Grain growth published in 2020"

Grain-Boundary-Rich Copper for Efficient Solar-Driven Electrochemical CO2 Reduction to Ethylene and Ethanol.

Abstract: The grain boundary in copper-based electrocatalysts has been demonstrated to improve the selectivity of solar-driven electrochemical CO2 reduction toward multicarbon products. However, the approach to form grain boundaries in copper is still limited. This paper describes a controllable grain growth of copper electrodeposition via poly(vinylpyrrolidone) used as an additive. A grain-boundary-rich metallic copper could be obtained to convert CO2 into ethylene and ethanol with a high selectivity of 70% over a wide potential range. In situ attenuated total reflection surface-enhanced infrared absorption spectroscopy unveils that the existence of grain boundaries enhances the adsorption of the key intermediate (*CO) on the copper surface to boost the further CO2 reduction. When coupling with a commercially available Si solar cell, the device achieves a remarkable solar-to-C2-products conversion efficiency of 3.88% at a large current density of 52 mA·cm-2. This low-cost and efficient device is promising for large-scale application of solar-driven CO2 reduction.

The Hall–Petch and inverse Hall–Petch relations and the hardness of nanocrystalline metals

Abstract: We review some of the factors that influence the hardness of polycrystalline materials with grain sizes less than 1 µm. The fundamental physical mechanisms that govern the hardness of nanocrystalline materials are discussed. The recently proposed dislocation curvature model for grain size-dependent strengthening and the 60-year-old Hall–Petch relationship are compared. For grains less than 30 nm in size, there is evidence for a transition from dislocation-based plasticity to grain boundary sliding, rotation, or diffusion as the main mechanism responsible for hardness. The evidence surrounding the inverse Hall–Petch phenomenon is found to be inconclusive due to processing artefacts, grain growth effects, and errors associated with the conversion of hardness to yield strength in nanocrystalline materials.

A Polymerization-Assisted Grain Growth Strategy for Efficient and Stable Perovskite Solar Cells

Abstract: Intrinsically, detrimental defects accumulating at the surface and grain boundaries limit both the performance and stability of perovskite solar cells. Small molecules and bulkier polymers with functional groups are utilized to passivate these ionic defects but usually suffer from volatility and precipitation issues, respectively. Here, starting from the addition of small monomers in the PbI2 precursor, a polymerization-assisted grain growth strategy is introduced in the sequential deposition method. With a polymerization process triggered during the PbI2 film annealing, the bulkier polymers formed will be adhered to the grain boundaries, retaining the previously established interactions with PbI2 . After perovskite formation, the polymers anchored on the boundaries can effectively passivate undercoordinated lead ions and reduce the defect density. As a result, a champion power conversion efficiency (PCE) of 23.0% is obtained, together with a prolonged lifetime where 85.7% and 91.8% of the initial PCE remain after 504 h continuous illumination and 2208 h shelf storage, respectively.

Ultrahigh high-strain-rate superplasticity in a nanostructured high-entropy alloy

Abstract: Superplasticity describes a material’s ability to sustain large plastic deformation in the form of a tensile elongation to over 400% of its original length, but is generally observed only at a low strain rate (~10−4 s−1), which results in long processing times that are economically undesirable for mass production. Superplasticity at high strain rates in excess of 10−2 s−1, required for viable industry-scale application, has usually only been achieved in low-strength aluminium and magnesium alloys. Here, we present a superplastic elongation to 2000% of the original length at a high strain rate of 5 × 10−2 s−1 in an Al9(CoCrFeMnNi)91 (at%) high-entropy alloy nanostructured using high-pressure torsion. The high-pressure torsion induced grain refinement in the multi-phase alloy combined with limited grain growth during hot plastic deformation enables high strain rate superplasticity through grain boundary sliding accommodated by dislocation activity. Superplasticity at high strain rates is challenging to achieve in high strength materials. Here, the authors show superplastic elongation in excess of 2000% in a high entropy alloy nanostructured by high-pressure torsion.

The role of texturing and microstructure evolution on the tensile behavior of heat-treated Inconel 625 produced via laser powder bed fusion

Abstract: Inconel 625 (IN625) alloy has high-temperature strength coupled with high oxidation and corrosion resistance. Additionally, due to its excellent weldability, IN625 can be processed by laser powder bed fusion (LPBF) additive manufacturing (AM) process allowing the production of complex shapes. However, post-AM heat treatment is necessary to develop the desired microstructure and mechanical properties to meet industrial needs. This work is focused on the influence of different heat treatment processes, namely stress relieving, recrystallization annealing and solution annealing on the microstructure and tensile properties of LPBF IN625 alloy. Investigation of the crystallographic texture by electron backscattered diffraction indicated that heat treatments at 1080 °C and 1150 °C tend to eliminate anisotropy in the material by the recrystallization and grain growth resulting in the formation of equiaxed grains. Tensile properties of heat-treated LPBF IN625 alloy built along different orientations revealed higher tensile properties than the minimum recommended values of wrought IN625 alloy in the annealed and solution annealed states.

Control over Crystal Size in Vapor Deposited Metal-Halide Perovskite Films.

Abstract: Understanding and controlling grain growth in metal halide perovskite polycrystalline thin films is an important step in improving the performance of perovskite solar cells. We demonstrate accurate control of crystallite size in CH3NH3PbI3 thin films by regulating substrate temperature during vacuum co-deposition of inorganic (PbI2) and organic (CH3NH3I) precursors. Films co-deposited onto a cold (-2 °C) substrate exhibited large, micrometer-sized crystal grains, while films that formed at room temperature (23 °C) only produced grains of 100 nm extent. We isolated the effects of substrate temperature on crystal growth by developing a new method to control sublimation of the organic precursor, and CH3NH3PbI3 solar cells deposited in this way yielded a power conversion efficiency of up to 18.2%. Furthermore, we found substrate temperature directly affects the adsorption rate of CH3NH3I, thus impacting crystal formation and hence solar cell device performance via changes to the conversion rate of PbI2 to CH3NH3PbI3 and stoichiometry. These findings offer new routes to developing efficient solar cells through reproducible control of crystal morphology and composition.

Hot cracking, crystal orientation and compressive strength of an equimolar CoCrFeMnNi high-entropy alloy printed by selective laser melting

Abstract: Hot cracking, grains size, crystal orientation and compressive strength of an equimolar CoCrFeMnNi high-entropy alloy (HEA) printed by selective laser melting (SLM) were investigated. The CoCrFeMnNi HEA printed by SLM suffered from hot cracking no matter of the employed printing parameters. The preferred orientation of the printed sample was transformed in the order: 〈2 3 3〉→〈0 0 1〉→〈2 0 3〉→〈1 0 1〉, as the volumetric energy density (VED) rises. Also, the increased VED favors the grain growth and the increase of the grain orientation spread, owing to the presence of high temperature gradient and larger residual stress. The fracture strength increases with the VED, and the maximum compressive strength rises to 2447.7 MPa, but the elongation rate is still 77.6% due to the ultrafine microstructure. The results of this study have reference value for the preparation of HEA materials with controllable grain characteristics, crystalline texture and complex structures by SLM.

High breakdown strength and energy storage performance in (Nb, Zn) modified SrTiO3 ceramics via synergy manipulation

Abstract: Dielectric capacitors with high energy storage properties are key enablers for potential applications. We report SrTi0.985(Zn1/3Nb2/3)0.015O3–x wt%ZnNb2O6 ceramics with high breakdown strength and energy storage performance by synergy manipulation. The structures, energy storage and dielectric properties of the ceramics are systematically investigated. Introduction of ZnNb2O6 mediates the contradiction between permittivity and breakdown strength. As a result, a high breakdown strength of 422 kV cm−1 and an excellent energy storage density of 2.35 J cm−3 are achieved in x = 4.5 ceramics, which also exhibit fast discharge features (τ0.9 < 1.5 μs), good thermal stability (25–150 °C) and outstanding cyclic characteristics (up to 5 × 105 times). Further results indicate that ZnNb2O6 additives partly dissolve into perovskite lattices to promote the formation of superlattice structures, improving the local polarization, and partly disperse around grain boundary regions to inhibit grain growth and relieve the direct damage from a strong electric field to grains, increasing the breakdown strength. This strategy should be generalizable for designing high performance dielectrics and other novel composite materials that benefit from synergistic effect manipulation.

Microstructural characteristics and crack formation in additively manufactured bimetal material of 316L stainless steel and Inconel 625

Abstract: This research illustrates the rationale of adopting a preferred printing sequence by examining crack generation predominated by resultant interfaces and microstructural inhomogeneity, through underlying governing mechanisms in directed energy deposition of 316L stainless steel/Inconel 625 (SS316L/IN625) bimetals. For this purpose, microstructural and crystallographic characterizations augmented by numerical simulations were employed on additively manufactured two distinct interfaces, i.e. Type-I (IN625 deposition on SS316L) and Type-II (SS316L deposition on IN625). Changing the printing sequence generated these two types of interfaces with unique morphologies, which was found attributable to the compositional variations and mismatch in thermal properties. Type-I interface was typified by gradual-change composition in the transition zone, causing the IN625 grains to grow epitaxially on the grains of SS316L. Type-II interface was characterized as a compositional sudden-change zone (CSCZ) adjacent to SS316L, leading to merging bidirectional nucleation and grain growth from the bottom IN625 and upper CSCZ, and lack of epitaxial growth. Additionally, high cracking susceptibility occurred near the Type-II interface rather than the Type-I interface, which was related to solidification and liquidation cracking, and further promoted ductility dip cracking. This research will provide a guideline for the additive manufacturing of bimetals with the consideration of printing sequence to control interface formation for a crack-free structure.

Effect of scanning strategies on the microstructure and mechanical behavior of 316L stainless steel fabricated by selective laser melting

Abstract: Five scanning strategies were applied for relating the microstructural and crystallographic morphology to the mechanical properties of 316L stainless steel fabricated by selective laser melting (SLM). The results indicate that scanning strategies have a significant effect on the microstructure, grain growth, grain size and therefore mechanical properties of SLM-built 316L stainless steel. Applying the scanning strategy with the rotation angle between successive layers breaks the columnar growth of grains and fine equiaxial grains are formed, leading to an improvement of tensile strength and good ductility of SLM-built samples. Moreover, the microhardness is improved by applying the scanning strategy with a rectangular pattern. This work demonstrates the scanning strategy during SLM process acts as an essential factor affecting energy input, and therefore tailors the meso-/micro-scale structure and mechanical behavior of metals.

Processing of dense high-entropy boride ceramics

Abstract: Dense (Hf0.2,Zr0.2,Ti0.2,Ta0.2,Nb0.2)B2 high-entropy ceramics with high phase purity were produced by two-step spark plasma sintering of precursor powders synthesized by boro/carbothermal reduction of oxides. The reacted powders had low oxygen (0.404 wt%) and carbon (0.034 wt%) contents and a sub-micron average particle size (∼0.3 μm). Powders were synthesized by optimizing the excess B4C content of the reaction mixture and densified by a two-step spark plasma sintering process. The relative density increased from 98.9% to 99.9% as the final sintering temperature increased from 2000 °C to 2200 °C. The resulting ceramics were nominally single-phase (Hf,Zr,Ti,Ta,Nb)B2 with oxygen contents as low as 0.004 wt% and carbon as low as 0.018 wt%. The average grain size increased from 2.3 ± 1.2 μm after densification at 2000 °C to 4.7 ± 1.8 μm after densification at 2100 °C, while significant grain growth occurred during sintering at 2200 °C. The high relative densities, low oxygen and carbon contents, and fine grain sizes achieved in the present study were attributed to the use of synthesized precursor powders with high purity and fine particle size, and the two-step synthesis-densification process. These are the first reported results for dense high-entropy boride ceramics with high purity and fine grain size.

Improvement of strength-ductility balance by Mn addition in Mg–Ca extruded alloy

Abstract: Mg and its alloys have been extensively used in many fields given their low density and extensive resources. However, their strength and plasticity cannot satisfy the requirements of large-scale application. This investigation aims to provide a deep understanding of the influence of Mn on microstructure evolution to develop high strength and ductility of Mg–Ca–Mn extruded alloys. Results revealed that the mechanical properties containing tensile, compress strengths, and yield asymmetry of as-extruded alloys were significantly enhanced with the increase in Mn. The grains of Mg–Ca alloy were significantly refined because a mass of Mn particles efficaciously blocked newborn grain growth during extrusion. The enhanced strengths were mainly ascribed to the fine dynamic recrystallized grains and abundant fine Mn particles. Furthermore, the microstructure evolution and strengthening mechanism were discussed.

Microstructural characterization and thermomechanical behavior of additively manufactured AlSi10Mg sheet cellular materials

Abstract: There has been an increasing interest in designing new types of architected metallic cellular structures (metallic meta-structures) for various engineering applications, such as thermal management devices, due to the advancements in metallic additive manufacturing technologies. In this work, the microstructure and mechanical properties of as-built and heat-treated additively manufactured AlSi10Mg triply periodic minimal surface (TPMS) sheet-based Schoen's I-graph - Wrapped Package (IWP) cellular structures are studied. Tensile coupons of the base material and IWP cellular structures are fabricated using laser powder bed fusion 3D printing technique, and thermomechanical tests are carried out. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), micro-computed tomography (micro-CT), and optical microscopy were utilized to visualize the surface morphology, fracture surfaces of the tensile coupons, grain orientation maps, and internal morphology of the samples. Micro-CT and experimentally measured relative densities of the fabricated cellular structures were found to be less than designed mainly due to the lack of proper fusion of metallic powder on complex surfaces leading to voids, especially at low relative densities. As-built samples undergo compression tests at 25 °C and exhibited brittle fracture while heat-treated samples undergo compression tests at 25 °C and 150 °C and exhibit more ductile behavior which is attributed to the grain growth during heat treatment, which was determined through the study of the EBSD maps. The tensile strength of the base material decreases with the increase of testing temperature, which is associated with a significant increase in elongation at fracture.