In this work, a new process is developed to improve the functional stability of Ni-rich NiTi alloys. Repetitive temperature- and stress-induced phase transformation is first conducted to generate ...dislocation networks in the grain interior. Dislocations serve as nucleation sites for Ni4Ti3 nanoprecipitates, which are formed after subsequent low-temperature (523 K) aging. With the presence of dislocations, a homogeneous distribution of nanoprecipitates in the grains is expected, enhancing the strength of the NiTi matrix and resisting plastic deformation during the martensitic transformation. As a result, an improved functional stability of NiTi alloys is achieved.
Schematic representation of the microstructure and corresponding superelastic behavior of coarse-grained Ni-rich Ni-Ti resulting from traditional aging treatment (a) → (b) and from the suggested combination of creation of a dislocation network within the grains due to cycling through the martensitic transformation (a) → (c) and subsequent precipitation along dislocations during low-temperature aging (b) → (c). Display omitted
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Single atom alloys (SAA) have recently drawn increased attention due to their unique structure, high atomic utilization, and fascinating catalytic performance. However, their controllable synthesis ...still presents a challenge. This study proposes an electrochemical self-catalysis (ESC) strategy to synthesize Pd@Pt/C SAA catalysts, that is, depositing Pt atoms on Pd nanocrystals through in situ decomposition of sodium formate. The relationship between composition and structure of Pd@Pt/C is distinguished through a combination of electrochemical analysis, sphere-corrected scanning transmission electron microscopy, and X-ray adsorption spectra. That relationship evolved from SAA to a sea-island structure and even a core–shell structure with composition-controllable atomic ratios, highlighting the great diversity and convenience of this method in nanostructure construction. The Pd@Pt/C SAA catalyst showed excellent catalytic activity to formic acid oxidation with a peak current density of 5.2 A/mgmetal, which is about 18.6 times that of the commercial Pd/C. density functional theory calculations revealed that the enhanced activity was due to the “passivation” of Pd sites near the Pt single atoms, which attenuated the adsorption of CO. Based on electrochemical principles, this ESC strategy was also expanded to prepare a series of Pd-based SAA, including Pd–Au, Pd–Ir, and Pd–Bi.
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Though NiTi alloys are widely utilized in biomedical and automotive industries, conventional processing methods can hardly fabricate porous NiTi alloys. Selective laser melting shows prominent ...advantages in fabricating complex structure with relatively high surface finish and geometrical accuracy. In the present paper, we investigated the compression and superelasticity behaviors of selective laser melting fabricated NiTi porous structures with tiny strut (0.6 mm, 0.4 mm and 0.2 mm). When the strut thickness decreased from 0.6 mm to 0.2 mm, the compressive strain of the NiTi structure firstly decreased and then increased, the best compressive strain (28%) was obtained when the strut thickness was 0.4 mm and the scanning speed was 400 mm/s. The recovery strain increased monotonously as the strut thickness decreased, the best recovery strain (10%) was obtained when the strut thickness was 0.2 mm and the scanning speed was 200 mm/s. The compressive strain was closely related to both the martensitic transformation temperature and the defect volume fraction, while the recovery strain was mainly affected by the martensitic transformation temperature.
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•Compressive strain decreases and then increases as strut thickness decreases. .•Both transformation temperature and defect affect compressive strain.•Recovery strain increases monotonously as strut thickness decreases. .•Recovery strain is mainly affected by transformation temperature.
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In this study, two sets of optimized laser powder bed fusion (LPBF) additive manufacturing parameters with similar energy density but different laser powers (HP: high laser power with high scanning ...speed, LP: low laser power with low scanning speed) were used to produce fully dense and crack-free NiTi samples. The microstructure, phase transformation and mechanical properties of the LPBF fabricated HP and LP NiTi were investigated using scanning electron microscopy (SEM), differential scanning calorimeter (DSC), X-ray diffraction (XRD) and dynamic mechanical analysis (DMA). The results showed that the HP and LP NiTi samples had different microstructures, phase transformation temperatures and mechanical properties. It was found that the HP NiTi samples predominantly contained austenite at room temperature and exhibited lower phase transformation temperatures. In contrast, the LP NiTi samples contained a large amount of martensite and had larger thermal memory recovery and better damping capacity. Additionally, the microstructure, phase transformation temperatures and mechanical properties were found to vary at different locations along the building direction in both HP and LP NiTi. This study implies that by manipulating the LPBF processing parameters, in particular the laser power, the phase transformation, microstructure and dynamic mechanical properties of the LPBF fabricated NiTi can be controlled to target different applications.
•High and low laser power can be used to fabricate fully dense NiTi by LPBF.•High power LPBF NiTi forms austenite with lower phase transformation temperatures.•Low power LPBF fabricated NiTi forms martensite with better damping capacity.•Different regions in NiTi have different microstructures and mechanical properties.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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•P-Pd nanoparticles were facilely prepared based on polyphosphide chemistry.•The soluble phosphorus anion was used as both P source and reducing agent.•P significantly improves ...catalytic activity through size and electronic effects.•The P-Pd exhibited surprising anti-CO poisoning ability.•DFT calculations showed the indirect pathway was inhabited by P-doping.
In this work, we reported a novel polyphosphide strategy for the synthesis of phosphorus doped Pd (P-Pd) using red phosphorus as the starting material at quasi-ambient conditions. Polyphophide anions, as the key reaction intermediates, served as the reducing agent and phosphorus source to modulate the surface electronic structure of Pd. The P-Pd obtained exhibited topmost CO tolerance and electrocatalytic activity to formic acid oxidation among the state-of-arts reports. The mass activity and turnover frequency of P-Pd reached 4413 mA mg−1Pd and 16.04 s−1 at 0.8 V, which were 23.7 and 6.4 times that of commercial Pd/C respectively. After 1000 repeated cycles, 82% initial activity was reserved. Combined with the electrochemical analysis and the density functional theory calculation, the boosted electrochemical performances can be attributed to the size and electronic effects induced by the P doping, which increase the surface actives sites, inhibit the adsorption of CO and change the reaction pathway to favorable CO2 route. A full cell was also assembled to demonstrate the practical potential of the P-Pd, which showed a maximum power density of 21.56 mW cm−2. This polyphophide-based reaction route provides a new strategy for the preparation of efficient and durable phosphorus doped alloys for electrocatalysis.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Laser powder-bed fusion has been widely utilized to produce complex NiTi devices because of relatively better surface finish and geometrical accuracy. However, NiTi alloys fabricated by laser ...powder-bed fusion suffer from strength degradation and unstable shape recovery behavior induced by the formation of pore defects and the variation of martensitic transformation, respectively. In this work, the mechanical and functional properties of HfH2-decorated NiTi shape memory alloy fabricated by laser powder-bed fusion have been investigated. Through careful clarification of the effects of microstructure characteristics on mechanical and functional properties, it is concluded that the addition of Hf (<5 at%) efficiently strengthens the NiTi matrix and thus eliminates strength degradation induced by pore defect formation. Moreover, the addition of Hf (<5 at%) into the NiTi matrix weakens martensitic transformation variation through improvement of the critical stress to induce martensitic transformation, resulting in stability enhancement of shape recovery behavior.
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•Microstructure characteristics vary as fabricating parameters change.•Hf addition eliminates strength degradation induced by pore defect.•Hf addition increases recovery stability by weakening transformation variation.
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Both uniform and graded gyroid cellular structures (GCSs) of NiTi have been successfully fabricated by laser powder bed fusion (LPBF) additive manufacturing. NiTi GCSs were produced with gradients ...along two different directions, and the surface morphology and mechanical properties of different GCSs were systematically studied using micro-x-ray computed tomography, monotonic compression, and compression–compression fatigue testing. The results showed that all the LPBF NiTi structures exhibited similar nominal compressive elastic modulus (5–7 GPa) to human bones. By altering the density distribution of the novel GCSs, different deformation and fatigue properties were achieved. GCSs with graded density parallel to each layer (Y-GCSs) exhibited a similar shear compression failure behavior and slightly better fatigue behavior compared to the uniform structure. In contrast, GCSs with graded density parallel to the building direction (Z-GCSs) exhibited a layer-by-layer deformation and collapse behavior under compression, while the fatigue life was worse than the uniform structure.
Direct formic acid fuel cells (DFAFCs) are considered promising sustainable power sources due to their high energy density, nonflammability, and low fuel crossover. However, serious CO poisoning and ...activity attenuation of the anodic formic acid oxidation reaction (FAOR) greatly restrict the output and durability of DFAFCs. Inspired by the specific relationship between the composition, type, and property of alloys, in this work, we synthesize a series of hybrid substitutional/interstitial quaternary alloys P-PdAuAg by means of a novel polyphosphide route to address these issues. Due to the simultaneous interstitial P-doping and metal (Au, Ag, Pd) co-reduction, the P-PdAuAg quaternary alloy obtained is only 3 nm in diameter with abundant defects. It not only achieves a new high mass activity of 8.08 A mgPd –1 (6.78 A mgcatalyst –1) but also maintains high stability in the high potential range and harsh reaction conditions. Both the activity and anti-poisoning ability are far exceeding those of the currently reported FAOR catalysts. Detailed density functional theory (DFT) calculations reveal that the superb electrochemical performances originate from the shift of the d-band center of Pd as a result of the synergistic electronic/ligand effects between Pd, Au, Ag, and P. The introduction of interstitial P inhibits the occurrence of an indirect reaction pathway on Pd, while Au and Ag suppress the adsorption of CO and optimize the sequential dehydrogenation steps, leading to boosted reaction kinetics and CO tolerance. This work pioneered a facile way for the synthesis of Pd-based substitutional/interstitial hybrid alloys, providing a promising means of further improving the performance of alloying catalysts.
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Wrought aluminium alloys popular for automotive and aerospace applications are susceptible to solidification cracking when fabricated via laser powder bed fusion (LPBF). Another long-standing and ...common issue for these alloys is microstructure coarsening and corresponding strength loss caused by elevated temperature exposure. To tackle these challenges, this study designs and develops a class of 1–4 wt% Ce modified Al6061 alloys. The best alloy, with 3 wt% Ce, achieves crack-free fabrication via LPBF due to a reduction in the solidification temperature range and a new solidification pathway that achieved 0.9 solid mass fraction at just 14 °C below the solidification onset. Furthermore, a fine microstructure consisting of coarsening-resistant τ1-CeAlSi eutectic forms, and after hot isostatic pressing, the tensile strength and elongation of the 3 wt% Ce alloy can reach 153 ± 6 MPa and 18.3% at room temperature and 89 ± 6 MPa and 32.5% at 200 °C, respectively. The observed ductility is attributed to nanoscale dispersion of discrete, coarsening resistant τ1-CeAlSi particles within grains and to the presence of large columnar α-Al grains. Meanwhile, solidification cracking was inhibited by continuous grain boundary τ1-CeAlSi eutectic accumulation, which converted to discrete nanoscale τ1-CeAlSi after hot isostatic pressing. This research uncovers a simple and effective approach of designing Al-alloys for LPBF with great potential for both room temperature and high temperature applications in automotive and aerospace industries.
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