Compressive Behaviors of Thin-Walled Steel Tube Stub Columns Filled with Self-Compacting Concrete Containing Recycled Aggregate.

Natural resources have been excessively consumed, and large amounts of construction wastes have been generated, owing to the fast development of civil industry, causing crucial environmental issues. Therefore, reusable construction waste fabricated into recycled concrete offers a good strategy to solve this issue. Thus, this article first develops thin-walled steel tubes stub columns filled with self-compacting concrete containing recycled coarse aggregate. Afterwards, the compressive behaviors of the columns when undergoing axial compression loading to failure are explored. Subsequently, the effect of types of self-compacting concrete and wall thickness on failure modes and the relationships between load and displacement/strain is discussed comprehensively. Moreover, models of load-displacement/strain behaviors are proposed. The results show that columns with identical wall thicknesses containing both natural and recycled coarse aggregate display similar failure modes, mainly presenting as local buckling and rupture. The shape of the load-displacement/strain curves for identical wall thicknesses are almost the same. Nevertheless, the maximum load and stiffness of columns containing recycled coarse aggregate are lower than those of columns containing natural coarse aggregate. Additionally, the maximum loads corresponding to wall thickness of 1.2 mm and 3.0 mm are decreased by 18.4% and 5.8%, respectively. Moreover, the proposed models can reasonably evaluate the relationships between load and displacement/strain. This paper demonstrates that thin-walled steel tubular columns containing recycled coarse aggregate present positive compressive behaviors and thus exhibit great potential for developing environmentally friendly and sustainable civil infrastructures.

Eccentric Compression Behaviors of Self-Compacting Concrete-Filled Thin-Walled Steel Tube Columns.

For the sake of solving sustainability issues and analyzing the complicated service force states, eccentric compression experiments on self-compacting concrete-filled thin-walled medium-length steel tube columns with a circular cross-section were carried out in the present study. Thereafter, the influence of the eccentric ratios and the wall thickness factors on the mechanical behavior and failure characteristics of both the eccentrically loaded and axially loaded columns was comprehensively analyzed. Finally, prediction formulas for the ultimate load of the columns under eccentric compression were proposed, and a comprehensive comparison of the ultimate loads between the predicted values and experimental values was also conducted. The results indicated that the typical failure characteristics of the eccentrically loaded columns presented lateral deflection together with buckling, while the axially compressed columns displayed expansion and rupture at local positions. Moreover, the ultimate loads of the eccentrically loaded columns decreased by 43.0% and 34.5% in comparison to the columns under axial compression, with the wall thickness factor decreasing from 116.7 to 46.7, respectively. Meanwhile, the ratios of the ultimate loads calculated using design codes to the tested values were in the range of 0.70~0.90, which demonstrated that the design codes could predict the ultimate loads conservatively. Additionally, the ratios of the ultimate loads calculated using the proposed formulas to the tested values were within the range of 0.99~1.08, implying that the proposed formulas were more accurate than the design codes. At the same time, the initial stiffness of the columns under eccentric compression was correspondingly lower than that of the columns undergoing axial compression. The lateral deflections along the height of the columns were almost symmetrical at different loading levels. This study could provide a meaningful approach for designing columns and facilitate their application in civil industry.

Investigation of Microstructure and Mechanical Properties of SAC105 Solders with Sb, In, Ni, and Bi Additions.

Low Ag lead-free Sn-Ag-Cu (SAC) solders have attracted great interest due to their good drop resistance, high welding reliability, and low melting point. However, low Ag may lead to the degradation of the mechanical properties. Micro-alloying is an effective approach to improving the properties of SAC alloys. In this paper, the effects of minor additions of Sb, In, Ni, and Bi on microstructure, thermal and mechanical properties of Sn-1 wt.%Ag-0.5 wt.%Cu (SAC105) were systematically investigated. It is found that the microstructure can be refined with intermetallic compounds (IMCs) distributed more evenly in the Sn matrix with additions of Sb, In, and Ni, which brings a combined strengthening mechanism, i.e., solid solution strengthening and precipitation strengthening, leading to the tensile strength improved of SAC105. When Ni is substituted by Bi, the tensile strength is further enhanced with a considerable tensile ductility higher than 25%, which still meets the practical demands. At the same time, the melting point is reduced, the wettability is improved, and the creep resistance is enhanced. Among all the investigated solders, SAC105-2Sb-4.4In-0.3Bi alloy possesses the optimized properties, i.e., the lowest melting point, the best wettability, and the highest creep resistance at room temperature, implying that element alloying plays a vital role in improving the performance of SAC105 solders.

Microstructure and Nanoindentation Behavior of FeCoNiAlTi High-Entropy Alloy-Reinforced 316L Stainless Steel Composite Fabricated by Selective Laser Melting.

Selective laser melting (SLM) is one of the metal additive manufactured technologies with the highest forming precision, which prepares metal components through melting powders layer by layer with a high-energy laser beam. The 316L stainless steel is widely used due to its excellent formability and corrosion resistance. However, its low hardness limits its further application. Therefore, researchers are committed to improving the hardness of stainless steel by adding reinforcement to stainless steel matrix to fabricate composites. Traditional reinforcement comprises rigid ceramic particles, such as carbides and oxides, while the research on high entropy alloys as reinforcement is limited. In this study, characterisation by appropriate methods, inductively coupled plasma, microscopy and nanointendation assay, showed that we successfully prepared the FeCoNiAlTi high entropy alloy (HEA)-reinforced 316L stainless steel composites using SLM. When the reinforcement ratio is 2 wt.%, the composite samples show higher density. The SLM-fabricated 316L stainless steel displays columnar grains and it varies to equiaxed grains in composites reinforced with 2 wt.% FeCoNiAlTi HEA. The grain size decreases drastically, and the percentage of the low angle grain boundary in the composite is much higher than in the 316L stainless steel matrix. The nanohardness of the composite reinforced with 2 wt.% FeCoNiAlTi HEA is twice as high as the 316L stainless steel matrix. This work demonstrates the feasibility of using a high-entropy alloy as potential reinforcement in stainless steel systems.

Efficient Coarse-Grained Superplasticity of a Gigapascal Lightweight Refractory Medium Entropy Alloy.

Superplastic metals that exhibit exceptional ductility (>300%) are appealing for use in high-quality engineering components with complex shapes. However, the wide application of most superplastic alloys has been constrained due to their poor strength, the relatively long superplastic deformation period, and the complex and high-cost grain refinement processes. Here these issues are addressed by the coarse-grained superplasticity of high-strength lightweight medium entropy alloy (Ti43.3 V28 Zr14 Nb14 Mo0.7 , at.%) with a microstructure of ultrafine particles embedded in the body-centered-cubic matrix. The results demonstrate that the alloy reached a high coarse-grained superplasticity greater than ≈440% at a high strain rate of 10-2 s-1 at 1173 K and with a gigapascal residual strength. A consecutively triggered deformation mechanism that sequences of dislocation slip, dynamic recrystallization, and grain boundary sliding in such alloy differs from conventional grain-boundary sliding in fine-grained materials. The present results open a pathway for highly efficient superplastic forming, broaden superplastic materials to the high-strength field, and guide the development of new alloys.

Effect of Bi, Sb, and Ti on Microstructure and Mechanical Properties of SAC105 Alloys.

The Sn-Ag-Cu (SAC) solder alloys with a low Ag (Ag < 3 wt.%) content have attracted great attention owing to their low cost, increased ability in bulk compliance, and plastic energy dissipation. However, some of their mechanical properties are generally lower than the SAC alloys with a higher Ag content. Adding alloying elements is an effective approach for improving the mechanical properties of the SAC alloys. In this study, the effect of Bi, Sb, and Ti on Sn-1 wt.%Ag-0.5 wt.%Cu (SAC105) solder alloys was investigated. The SAC solders with four compositions: SAC105-1 wt.%Bi, SAC105-1 wt.%Sb, SAC105-1 wt.%Bi-1 wt.%Sb, SAC105-1 wt.%Bi-1 wt.%Sb-0.4 wt.%Ti were prepared. The microstructure and phase compositions were characterized using electron scanning microscopy, and X-ray diffraction. The thermal properties and wettability were also examined. Uniaxial tensile tests and nano-indentation tests were conducted to evaluate the mechanical properties. The results show that adding Bi or Sb could increase the strength of SAC105 alloys mainly due to the solid solution strengthening effect. The creep resistance of SAC105 alloys was also improved with the additions of Bi and Sb. The co-additions of Bi and Sb into SAC105 alloys exhibit an enhanced creep resistance than that calculated by the theoretical calculation. The further addition of Ti into SAC105-1Bi-1Sb alloys demonstrated a much-improved creep resistance, which could be attributed to the synergistic effects of both solid solution strengthening and the precipitation hardening effects.

Fabrication of Ni-Cu-W Graded Coatings by Plasma Spray Deposition and Laser Remelting.

In this study, Ni-Cu-W graded coatings are produced by atmospheric plasma spraying and subsequently remelted by laser. The surface morphology, hardness, compositional fluctuations and corrosion resistance of the Ni-Cu-W coating are investigated. The coatings after laser remelting are densified and become more homogenous with an excellent corrosion resistance and high hardness, which can be used to explore the new materials.

Anisotropic Mechanical Response and Strain Localization of a Metallic Glassy-Fiber-Reinforced Polyethylene Terephthalate Fabric.

Optimizing the mechanical properties of composites through microstructural design has been a long-standing issue in materials science. In this study, we reinforced a typical polymer, i.e., polyethylene-terephthalate-woven fabric, with a type of Fe-based metallic glassy fiber (MGF) with an extremely large Young's moduli. The MGF-reinforced fabrics, with three different fiber bundle orientations (0°, 45°, and 90°), were investigated by in situ electron-microscopy mechanical testing techniques in conjunction with a digital image correlation (DIC) technique. The fabrics exhibited a pronounced anisotropic mechanical response, and the associated characteristics were verified to depend on the fiber bundle orientation relative to the external load. Furthermore, localized strains near the intersections of the fiber bundles were found to be much higher than the global strain. It is confirmed that the restriction from warp to weft is the dominant factor influencing strain localization during deformation. Our results are enlightening for understanding the fracture mechanisms of composites.

Annealing of Al-Zn-Mg-Cu Alloy at High Pressures: Evolution of Microstructure and the Corrosion Behavior.

Extruded Al-Zn-Mg-Cu alloy samples with grains aligned parallel to the extrusion direction were subjected to high-pressure annealing. The effects of annealing pressure on the microstructure, hardness, and corrosion properties (evaluated using potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS)) were investigated. Phase analysis showed the presence of MgZn2 and α-Al phases, the MgZn2 phase dissolved into the matrix, and its amount decreased with the increasing annealing pressure. The recrystallization was inhibited, and the grains were refined, leading to an increase in the Vickers hardness with increasing the annealing pressure. The corrosion resistance was improved after high-pressure treatment, and a stable passivation layer was observed. Meanwhile, the number of corrosion pits and the width of corrosion cracks decreased in the high-pressure annealed samples.

Effect of Substrate Plate Heating on the Microstructure and Properties of Selective Laser Melted Al-20Si-5Fe-3Cu-1Mg Alloy.

The Al-20Si-5Fe-3Cu-1Mg alloy was fabricated using selective laser melting (SLM). The microstructure and properties of the as-prepared SLM, post-treated SLM, and SLM with substrate plate heating are studied. The as-prepared SLM sample shows a non-uniform microstructure with four different phases: fcc-αAl, eutectic Al-Si, Al2MgSi, and δ-Al4FeSi2. With thermal treatment, the phases become coarser and the δ-Al4FeSi2 phase transforms partially to ß-Al5FeSi. The sample produced with SLM substrate plate heating shows a relatively uniform microstructure without a distinct difference between hatch overlaps and track cores. Room temperature compression test results show that an as-prepared SLM sample reaches a maximum strength (862 MPa) compared to the heat-treated (524 MPa) and substrate plate heated samples (474 MPa) due to the presence of fine microstructure and the internal stresses. The reduction in strength of the sample produced with substrate plate heating is due to the coarsening of the microstructure, but the plastic deformation shows an improvement (20%). The present observations suggest that substrate plate heating can be effectively employed not only to minimize the internal stresses (by impacting the cooling rate of the process) but can also be used to modulate the mechanical properties in a controlled fashion.

Frictional Wear and Corrosion Behavior of AlCoCrFeNi High-Entropy Alloy Coatings Synthesized by Atmospheric Plasma Spraying.

High-entropy alloy coatings (HEAC) exhibit good frictional wear and corrosion resistances, which are of importance for structure materials. In this study, the microstructure, surface morphology, hardness, frictional wear and corrosion resistance of an AlCoCrFeNi high-entropy alloy coating synthesized by atmospheric plasma spraying (APS) were investigated. The frictional wear and corrosion resistance of the coating are simultaneously improved with an increase of the power of APS. The influence of the APS process on the microstructure and mechanical behavior is elucidated. The mechanisms of frictional wear and corrosion behavior of the AlCoCrFeNi HEAC are discussed in detail.

Microstructure and Mechanical Properties of Al-(12-20)Si Bi-Material Fabricated by Selective Laser Melting.

In this study, a combination of Al-12Si and Al-20Si (Al-(12-20)Si) alloys was fabricated by selective laser melting (SLM) as a result of increased component requirements such as geometrical complexity and high dimensional accuracy. The microstructure and mechanical properties of the SLM Al-(12-20)Si in as-produced as well as in heat-treated conditions were investigated. The Al-(12-20)Si interface was in the as-built condition and it gradually became blurry until it disappeared after heat treatment at 673 K for 6 h. This Al-(12-20)Si bi-material displayed excellent mechanical properties. The hardness of the Al-20Si alloy side was significantly higher than that of the Al-12Si alloy side and the disparity between both sides gradually decreased and tended to be consistent after heat treatment at 673 K for 6 h. The tensile strength and elongation of the Al-(12-20Si) bi-material lies in between the Al-12Si and Al-20Si alloys and fracture occurs in the Al-20Si side. The present results provide new insights into the fabrication of bi-materials using SLM.

Strong and Ductile Electroplated Heterogeneous Bulk Nanostructured Nickel.

Porosity-free bulk nanostructured nickel cannot be fabricated by conventional electroplating due to hydrogen bubbling at the cathode. Here, we developed a cathode-rotating electroplating technique to remove the bubbles in order to obtain millimeter-scale nanostructured nickel rods with low porosity. The grain sizes ranged from 20 to 300 nm. The range produced by the new technique was broader than those that have been reported. The heterogeneous microstructure contributed to high work hardening rate, yield strength, and ductility of the rods in tension. The ductility was larger than electroplated thin nickel film with comparable ultimate strength in the literature. Dislocations accumulated at pre-existing twins, grain boundaries, and at the grain interior mediated the plastic deformation of the rods.

Novel Ultralight-Weight Complex Concentrated Alloys with High Strength.

To explore a novel high strength and low modulus ultralight-weight complex concentrated alloys (ULW-CCAs), a series of light alloys are designed and explored based on some low-density and low modulus elements, such as Al, Li, Mg, Ca, Si, and Y. An Al19.9Li30Mg35Si10Ca5Y0.1 (at %) CCA with a high specific strength of 327 KPa·m-3 is successfully developed. After adjusting the composition, the Al15Li35Mg48Ca1Si1 CCA with the good compressive plasticity is successfully developed. The Al15Li38Mg45Ca0.5Si1.5 and Al15Li39Mg45Ca0.5Si0.5 CCAs exhibit good plasticity of >45%, and >60%, respectively. These ULW-CCAs show the high specific strength, good ductility, and low Young's modulus, as compared with the previously reported CCAs.