Showing papers on "Alloy published in 2021"

Noble-Metal Based Random Alloy and Intermetallic Nanocrystals: Syntheses and Applications.

Abstract: Precise control over the size, shape, composition, structure, and crystal phase of random alloy and intermetallic nanocrystals has been intensively explored in technologically important application...

Unveiling the In Situ Dissolution and Polymerization of Mo in Ni 4 Mo Alloy for Promoting the Hydrogen Evolution Reaction.

Abstract: NiMo alloys are efficient electrocatalysts in alkaline water electrolyzer for the hydrogen evolution reaction (HER). Metals are usually considered to be stable during the cathodic process. However, the actual behaviors of Mo in the NiMo alloys are unexplored. Here, we present the instability of Mo in the Ni4 Mo alloy as a highly efficient HER electrocatalyst in an alkaline medium. Mo in Ni4 Mo is oxidized and dissolved in the form of MoO42- first. The dissolved MoO42- will re-adsorb on the electrode surface and polymerize. Theoretical calculations indicate that the adsorption of the dimer Mo2 O72- can promote the HER activity of metal Ni. The addition of MoO42- to the electrolyte can not only repair the durability of Ni4 Mo alloy, but also facilitate the HER activity of pure metal of Ni, Fe, and Co. Our findings provide insight into the structural transformation mechanism and performance-enhanced origin of cathodic materials under the reaction conditions.

Alloying-realloying enabled high durability for Pt-Pd-3d-transition metal nanoparticle fuel cell catalysts.

Abstract: Alloying noble metals with non-noble metals enables high activity while reducing the cost of electrocatalysts in fuel cells. However, under fuel cell operating conditions, state-of-the-art oxygen reduction reaction alloy catalysts either feature high atomic percentages of noble metals (>70%) with limited durability or show poor durability when lower percentages of noble metals (<50%) are used. Here, we demonstrate a highly-durable alloy catalyst derived by alloying PtPd (<50%) with 3d-transition metals (Cu, Ni or Co) in ternary compositions. The origin of the high durability is probed by in-situ/operando high-energy synchrotron X-ray diffraction coupled with pair distribution function analysis of atomic phase structures and strains, revealing an important role of realloying in the compressively-strained single-phase alloy state despite the occurrence of dealloying. The implication of the finding, a striking departure from previous perceptions of phase-segregated noble metal skin or complete dealloying of non-noble metals, is the fulfilling of the promise of alloy catalysts for mass commercialization of fuel cells.

Bifunctional nanoprecipitates strengthen and ductilize a medium-entropy alloy.

Abstract: Single-phase high- and medium-entropy alloys with face-centred cubic (fcc) structure can exhibit high tensile ductility1,2 and excellent toughness2,3, but their room-temperature strengths are low1–3. Dislocation obstacles such as grain boundaries4, twin boundaries5, solute atoms6 and precipitates7–9 can increase strength. However, with few exceptions8–11, such obstacles tend to decrease ductility. Interestingly, precipitates can also hinder phase transformations12,13. Here, using a model, precipitate-strengthened, Fe–Ni–Al–Ti medium-entropy alloy, we demonstrate a strategy that combines these dual functions in a single alloy. The nanoprecipitates in our alloy, in addition to providing conventional strengthening of the matrix, also modulate its transformation from fcc-austenite to body-centred cubic (bcc) martensite, constraining it to remain as metastable fcc after quenching through the transformation temperature. During subsequent tensile testing, the matrix progressively transforms to bcc-martensite, enabling substantial increases in strength, work hardening and ductility. This use of nanoprecipitates exploits synergies between precipitation strengthening and transformation-induced plasticity, resulting in simultaneous enhancement of tensile strength and uniform elongation. Our findings demonstrate how synergistic deformation mechanisms can be deliberately activated, exactly when needed, by altering precipitate characteristics (such as size, spacing, and so on), along with the chemical driving force for phase transformation, to optimize strength and ductility. Increased strength and ductility in a medium-entropy alloy of Fe, Ni, Al and Ti is demonstrated using nanoprecipitates that simultaneously hinder phase transformation and block dislocation motion.

Microstructure evolution of wire-arc additively manufactured 2319 aluminum alloy with interlayer hammering

Abstract: The cold metal transfer (CMT) based wire-arc additive manufacturing (WAAM) technology has been widely recognized as a suitable method for fabricating large-sized aluminum alloy components. However, the poor mechanical properties of the as-deposited aluminum alloys prevent their wide application in the aerospace industry. In this paper, three categories of samples with different interlayer deformation strains were fabricated by WAAM. These samples were further investigated to evaluate the effects of interlayer deformation on the mechanical properties, microstructural evolution, and the underlying strengthening mechanism. The grain size distribution and internal sub-microstructure were characterized by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). As compared to the as-deposited samples, the yield strength and ultimate tensile strength of the 50.8% deformed sample increased from 148.4 to 240.9 MPa and from 288.6 to 334.6 MPa, respectively. The microstructure of the samples with interlayer hammering exhibited highly refined grain, which is a combined result of deformation and subsequent intrinsic in-situ heat treatment induced by the next deposition layer. The recrystallized grains can be further deformed with subsequent hammering, which leads to an increase in dislocation density and contributes to an increase in ultimate tensile strength of the additively manufactured 2319 aluminum alloys with interlayer hammering.

A novel crack-free Ti-modified Al-Cu-Mg alloy designed for selective laser melting

Abstract: A novel crack-free Ti-modified Al-Cu-Mg alloy for SLM was developed here, based on the thermodynamic calculations of the crack susceptibility index and growth-restriction factor. We found that the introduction of Ti into the Al-Cu-Mg alloy effectively promoted the grain refinement and columnar-to-equiaxed grain transition as a result of the heterogeneous nucleation provided by Al3Ti precipitates. The hot tearing cracks were eliminated after Ti modification due to the formation of the homogeneous and fine equiaxed microstructure. We created a new high-strength Al-Cu-Mg-Ti alloy with a tensile strength of 426.4 MPa, yield strength of 293.2 MPa and ductility of 9.1%. This novel Ti-modified Al alloy with fine equiaxed grains and highly-enhanced mechanical properties offers a new compositional space for the printable lightweight material categories specifically for the SLM technique.

Passive Layers and Corrosion Resistance of Biomedical Ti-6Al-4V and β-Ti Alloys

Abstract: The high specific strength, good corrosion resistance, and great biocompatibility make titanium and its alloys the ideal materials for biomedical metallic implants. Ti-6Al-4V alloy is the most employed in practical biomedical applications because of the excellent combination of strength, fracture toughness, and corrosion resistance. However, recent studies have demonstrated some limits in biocompatibility due to the presence of toxic Al and V. Consequently, scientific literature has reported novel biomedical β-Ti alloys containing biocompatible β-stabilizers (such as Mo, Ta, and Zr) studying the possibility to obtain similar performances to the Ti-6Al-4V alloys. The aim of this review is to highlight the corrosion resistance of the passive layers on biomedical Ti-6Al-4V and β-type Ti alloys in the human body environment by reviewing relevant literature research contributions. The discussion is focused on all those factors that influence the performance of the passive layer at the surface of the alloy subjected to electrochemical corrosion, among which the alloy composition, the method selected to grow the oxide coating, and the physicochemical conditions of the body fluid are the most significant.

Unique surface patterns emerging during solidification of liquid metal alloys

Abstract: It is well-understood that during the liquid-to-solid phase transition of alloys, elements segregate in the bulk phase with the formation of microstructures. In contrast, we show here that in a Bi–Ga alloy system, highly ordered nanopatterns emerge preferentially at the alloy surfaces during solidification. We observed a variety of transition, hybrid and crystal-defect-like patterns, in addition to lamellar and rod-like structures. Combining experiments and molecular dynamics simulations, we investigated the influence of the superficial Bi and Ga2O3 layers during surface solidification and elucidated the pattern-formation mechanisms, which involve surface-catalysed heterogeneous nucleation. We further demonstrated the dynamic nature and robustness of the phenomenon under different solidification conditions and for various alloy systems. The surface patterns we observed enable high-spatial-resolution nanoscale-infrared and surface-enhanced Raman mapping, which reveal promising potential for surface- and nanoscale-based applications. During a liquid-to-solid phase transition, a Bi–Ga alloy forms ordered nanostructured patterns on its surface.

Facile route to bulk ultrafine-grain steels for high strength and ductility

Abstract: Steels with sub-micrometre grain sizes usually possess high toughness and strength, which makes them promising for lightweighting technologies and energy-saving strategies. So far, the industrial fabrication of ultrafine-grained (UFG) alloys, which generally relies on the manipulation of diffusional phase transformation, has been limited to steels with austenite-to-ferrite transformation1–3. Moreover, the limited work hardening and uniform elongation of these UFG steels1,4,5 hinder their widespread application. Here we report the facile mass production of UFG structures in a typical Fe–22Mn–0.6C twinning-induced plasticity steel by minor Cu alloying and manipulation of the recrystallization process through the intragranular nanoprecipitation (within 30 seconds) of a coherent disordered Cu-rich phase. The rapid and copious nanoprecipitation not only prevents the growth of the freshly recrystallized sub-micrometre grains but also enhances the thermal stability of the obtained UFG structure through the Zener pinning mechanism6. Moreover, owing to their full coherency and disordered nature, the precipitates exhibit weak interactions with dislocations under loading. This approach enables the preparation of a fully recrystallized UFG structure with a grain size of 800 ± 400 nanometres without the introduction of detrimental lattice defects such as brittle particles and segregated boundaries. Compared with the steel to which no Cu was added, the yield strength of the UFG structure was doubled to around 710 megapascals, with a uniform ductility of 45 per cent and a tensile strength of around 2,000 megapascals. This grain-refinement concept should be extendable to other alloy systems, and the manufacturing processes can be readily applied to existing industrial production lines. Bulk ultrafine-grained steel is prepared by an approach that involves the rapid production of coherent, disordered nanoprecipitates, which restrict grain growth but do not interfere with twinning or dislocation motion, resulting in high strength and ductility.

Investigation on mechanical, tribological and microstructural properties of Al–Mg–Si–T6/SiC/muscovite-hybrid metal-matrix composites for high strength applications

Abstract: The wide range of aluminium variants (alloys and composites) has made it an important material for aviation, automotive components, auto-transmission locomotive section units, SCUBA tanks, ship, vessels, submarines fabrication and design etc regardless of the fact that the aluminium alloys were being utilized in myriads of sectors owing to its exceptional superior and versatile functional characteristics, the property such as wear-resistant ought to be enhanced in order to further prolong diverse spectrum of applications An aluminium alloy having lower hardness and tensile strength has been incorporated with silicon carbide that drastically strengthens the properties This study involves fabrication of aluminium silicon carbide with muscovite/hydrated aluminium potassium silicate/aluminosilicate in stir casting method to obtain a hybrid metal matrix composite Maintaining a constant amount of aluminium and silicon carbide, muscovite or hydrated aluminium potassium silicate is varied to obtain three distinctive compositions of (Al/SiC/muscovite) composites The mechanical characteristics like tensile-strength, flexural-strength, toughness, hardness, scratch adhesion, percent-porosity and density were studied The dispersion of muscovite and silicon carbide particles were observed by viewing the microstructure photographs obtained using optical microscopy and Scanning Electron Microscope (SEM) EDAX analysis affirms the presence of reinforcing constituents in Al–Mg–Si–T6 alloy matrix A drum type wear apparatus was utilized to evaluate the percentage of wear-loss in different compositions using different loads and it was found that the wear-loss decreases linearly as the muscovite percentage was increased

The carrier transition from Li atoms to Li vacancies in solid-state lithium alloy anodes

Abstract: The stable cycling of energy-dense solid-state batteries is highly relied on the kinetically stable solid-state Li alloying reactions. The Li metal precipitation at solid-solid interfaces is the pr...

Phase-Separated Mo–Ni Alloy for Hydrogen Oxidation and Evolution Reactions with High Activity and Enhanced Stability

Abstract: DOI: 10.1002/aenm.202003511 advantage of alkaline polymer electrolyte fuel cells (APEFCs) and alkaline water electrolysis (AWE) is the use of nonprecious metal catalysts, which is expected to significantly reduce the cost and promote the practical application of hydrogen energy.[6–8] At present, major obstacles to the application of APEFC and AWE are the slow kinetics of hydrogen electrode reactions and oxygen electrode reactions.[9,10] Hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) belong to hydrogen electrode reactions, and Mo–Ni alloy is one of the most promising electrocatalysts for hydrogen electrode reactions.[11–13] Intermetallic compound Mo–Ni alloy (IC-MoNi), such as MoNi4, MoNi3, and MoNi, will produce a synergistic effect between Mo and Ni for enhanced hydrogen electrode reactions.[14–17] However, since the Mo element in IC-MoNi is unstable during the electrode reaction process, the breakdown potential of IC-MoNi is low, resulting in poor electrochemical stability.[14–17] Moreover, Ni also suffers from low stability at potentials about 0.1 V versus reversible hydrogen electrode (RHE) due to its relatively strong binding affinity toward oxygen species.[18] How to improve the stability of IC-MoNi has become an important challenge. The phase-separated alloy is different from the intermetallic compound alloy. For example, the phase-separated Mo–Ni alloy (PS-MoNi) is composed of phase-separated Mo metal phase and phase-separated Ni metal phase. Because Mo metal phase is stable and has a high breakdown potential,[17] the PS-MoNi may have better structural and electrochemical stability than IC-MoNi. For PS-MoNi, the electron density of Mo metal can also be adjusted by Ni metal, which is similar to IC-MoNi.[14–22] In addition, amorphous materials are more resistant to corrosion due to the absence of grain boundaries. Therefore, amorphous PS-MoNi may be a good material for HOR and HER with high activity and enhanced stability. Herein, we synthesized the PS-MoNi composed of phaseseparated Mo and Ni metal phases. The X-ray absorption spectroscopy (XAS) and the energy dispersive X-ray spectroscopy (EDS) mappings illustrate that we have synthesized PS-MoNi composed of Mo metal and embedded Ni metal nanoparticles. Excitingly, PS-MoNi shows excellent hydrogen electrode activity with the high exchange current density (−4.883 mA cm−2), which is comparable to the reported highest value for non-noble The development of alkaline polymer electrolyte fuel cells and alkaline water electrolysis requires nonprecious metal catalysts for the hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER). Herein, it is reported a phase-separated Mo–Ni alloy (PS-MoNi) that is composed of Mo metal and embedded Ni metal nanoparticles. The PS-MoNi shows excellent hydrogen electrode activity with a high exchange current density (−4.883 mA cm−2), which is comparable to the reported highest value for non-noble catalysts. Moreover, the amorphous phase-separated Mo–Ni alloy has better structural and electrochemical stability than the intermetallic compound Mo–Ni alloy (IC-MoNi). The breakdown potential of PS-MoNi is as high as 0.32 V, which is much higher than that of reported IC-MoNi. The X-ray absorption near edge structure (XANES) and density functional theory (DFT) calculations indicate the electrons transfer from Mo to Ni for PS-MoNi, leading to suitable adsorption free energies of H* (ΔGH*) on the surface of Mo. This means that the electron density modulation of Mo metal by embedded Ni metal nanoparticles can produce excellent HOR and HER performance.

Effect of Si addition on microstructure and wear behavior of AlCoCrFeNi high-entropy alloy coatings prepared by laser cladding

Abstract: AlCoCrFeNiSix (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) high-entropy alloy (HEA) coatings were deposited on the surface of AISI 304 stainless steel via laser cladding technology with different content of Si element. The evolution in microstructure, microhardness, and wear behavior of the coatings was investigated comprehensively to explore the effect of Si element. The results showed that the HEA coatings consisted of disordered Fe-Cr and ordered Al-Ni solid solution phases with a body-centered cubic (BCC) structure. Si element dissolved into the solid solutions, leading to lattice distortion. As Si content increased, the microstructure was refined, and a small quantity of Cr23C6 phase was precipitated along the grain boundaries. The microhardness of the coatings was linearly proportionate to Si content. The quantitative analysis revealed that the improvement of microhardness was dominated by the effect of dislocation strengthening, rather than solution strengthening and fine-grain strengthening. With the increase in Si content, the average friction coefficient and wear rate of the HEA coatings were all reduced remarkably, and the main wear mechanism evolved from adhesive wear, abrasive wear and delamination wear to oxidation wear. These phenomena were believed to be associated with the formation of the oxide film on the worn surface, which was identified as Fe2O3, Fe3O4, Cr2O3, SiO2, and SiO.

In-Sn alloy core-shell nanoparticles: In-doped SnOx shell enables high stability and activity towards selective formate production from electrochemical reduction of CO2

Abstract: SnO2 has been recognized as excellent catalyst towards formate production from electrochemical CO2 reduction (CO2R). However, it is a great challenge to prepare SnO2 that is stable under the working condition of CO2R and the active center of the SnO2-based catalyst towards CO2R has been illusive. In this work, Sn-In alloy nanoparticles display an InSn4 intermetallic core and an amorphous In-doped tin oxide shell and a CO2-to-formate faradaic efficiency of 94% and a current density of 236 mA cm−2 was achieved at −0.98 V. Operando X-ray absorption and Raman spectroscopy reveal that the In-doped tin oxide shell remains stable under CO2R condition. Density functional theory calculations indicate that the In-doped tin oxide results in the formation of oxygen vacancy, stabilized tin oxide shell and exergonic pathway for CO2-to-formate, which might account for the high performance of the catalysts towards CO2R.