Additive manufacturing (AM) of metals, also known as metal 3D printing, typically leads to the formation of columnar grain structures along the build direction in most as-built metals and alloys. ...These long columnar grains can cause property anisotropy, which is usually detrimental to component qualification or targeted applications. Here, without changing alloy chemistry, we demonstrate an AM solidification-control solution to printing metallic alloys with an equiaxed grain structure and improved mechanical properties. Using the titanium alloy Ti-6Al-4V as a model alloy, we employ high-intensity ultrasound to achieve full transition from columnar grains to fine (~100 µm) equiaxed grains in AM Ti-6Al-4V samples by laser powder deposition. This results in a 12% improvement in both the yield stress and tensile strength compared with the conventional AM columnar Ti-6Al-4V. We further demonstrate the generality of our technique by achieving similar grain structure control results in the nickel-based superalloy Inconel 625, and expect that this method may be applicable to other metallic materials that exhibit columnar grain structures during AM.
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•Doping Co2+/3+ to NiFe-LDH reduces the OER overpotential on (001) and (110) planes.•Fe3+ in NiCo2+Fe-LDH and Co3+ in NiCo3+Fe-LDH are identified as OER active sites.•Co3+-doped ...NiFe-LDH has a lower OER overpotential than Co2+-doped NiFe-LDH.•The improved activity is attributed to the tailored electron density of metal site.
NiFe-layered double hydroxide (NiFe-LDH) has been widely accepted as promising catalyst candidates for the oxygen evolution reaction (OER). Recently, incorporating Co atoms in NiFe laminates has been recognized as an effective way to enhance the OER activity, but their roles have been rarely studied. Herein, density functional theory plus U (DFT + U) calculations are employed to evaluate the OER thermodynamics after introducing bivalent Co2+ or trivalent Co3+ and consequent sites sensitivity in NiFe-LDH catalyst. Generally, based on computational results, incorporation of Co2+/Co3+ into NiFe-LDH could modulate the electronic structure of metal sites and thus reducing their OER overpotential. Moreover, Co3+-doped NiFe-LDH has the lowest overpotential of η = 0.413 eV among the above proposed structure. This point was fully demonstrated by Co2+ and Co3+-incorporated NiFe-LDH nanosheets made by a co-precipitation method, by showing OER onset overpotential of 249 mV for Co3+-doped NiFe-LDH, 264 mV for Co2+-doped NiFe-LDH, which are 33 mV and 18 mV lower than that of pristine NiFe-LDH (282 mV), respectively. Such improved OER activity were attributed to the lowered overpotential at the *OOH formation step for Co2+-doped NiFe-LDH and the deprotonation step for Co3+-doped NiFe-LDH, which was the potential limiting step for their OER process. This work should be inspiring for future designing of more efficient NiFe-based oxygen evolution electrocatalysts.
•An in-depth critical assessment of mechanical properties of SEBM Ti-6Al-4V lattice.•An exhaustive list of mechanical properties of human bone for lattice design.•A comprehensive mechanical ...comparison of SEBM Ti-6Al-4V lattice with human bone.•A clear identification of opportunities for SEBM Ti-6Al-4V lattice as bone implant.
Additively manufactured Ti-6Al-4V lattices display unique mechanical and biological properties by virtue of their engineered structure. These attributes enable the innovative design of patient-specific medical implants that (i) are conformal to the intended surgical geometry, (ii) mimic the mechanical properties of natural bone, and (iii) provide superior biological interaction to traditional implants. Selective electron beam melting (SEBM) is an established metal additive manufacturing (AM) process that has enabled the design and fabrication of a variety of novel intricate lattices for implant applications over the last 15 years. This article reviews the technical and clinical characteristics of SEBM Ti-6Al-4V lattices, including (i) the SEBM process and its capabilities, (ii) the structures of human bones with an exhaustive list of corresponding mechanical properties from literature, (iii) the mechanical properties of SEBM Ti-6Al-4V lattices of various designs and their shortcomings when compared to human bones, (iv) microstructural control of SEBM Ti-6Al-4V lattices for improved performance, (v) the lattice manufacturability and associated geometric errors, and (vi) clinical cases. Existing literature on the mechanical response of SEBM Ti-6Al-4V lattice structures is exhaustively evaluated for documentation quality using established theoretical models. This extensive data-set allows novel insights into the effect of lattice design on mechanical response that is not possible with the individual data; and provides a comprehensive database for those who are actively involved in patient-specific SEBM implant design. On this basis, outstanding challenges and research opportunities for SEBM Ti-6Al-4V lattices in the biomedical domain are identified and discussed.
An advantage of the powder-bed-based metal additive manufacturing (AM) processes is that the powder can be reused. The powder reuse or recycling times directly affect the affordability of the ...additively manufactured parts, especially for the AM of titanium parts. This study examines the influence of powder reuse times on the characteristics of Ti-6Al-4V powder, including powder composition, particle size distribution (PSD), apparent density, tap density, flowability, and particle morphology. In addition, tensile samples were manufactured and evaluated with respect to powder reuse times and sample locations in the powder bed. The following findings were made from reusing the same batch of powder 21 times for AM by selective electron beam melting: (i) the oxygen (O) content increased progressively with increasing reuse times but both the Al content and the V content remained generally stable (a small decrease only); (ii) the powder became less spherical with increasing reuse times and some particles showed noticeable distortion and rough surfaces after being reused 16 times; (iii) the PSD became narrower and few satellite particles were observed after 11 times of reuse; (iv) reused powder showed improved flowability; and (v) reused powder showed no measurable undesired influence on the AM process and the samples exhibited highly consistent tensile properties, irrespective of their locations in the powder bed. The implications of these findings were discussed.
This study investigates the effect of annealing treatment on the phase transformation and mechanical properties of the equiatomic TiZrNbTa MEA from room temperature to 1200 °C. After annealing at ...1200 °C for 24h, the single solid-solution body-centred cubic (BCC) phase in the as-cast Ta25Zr25Nb25Ti25 transformed into an extremely high number density (~103/μm2) of Ta–Nb-rich BCC nanocuboidal phase (28 ± 10 nm square, BCC1) and a nanostrip-like Zr-rich BCC phase (3 ± 2 nm thick, BCC2). The phase separation from BCC to BCC1 and BCC2 arises from two primary reasons: (i) the high positive mixing enthalpy of both Ta–Zr and Nb–Zr (strong tendency to separation between each pair), and (ii) the 3–4 orders of magnitude higher mobility of Zr than Ta, Nb and Ti in these MEAs (kinetically driven). Detailed CALPHAD simulations of phase formation in this MEA agreed with experiments and provided insightful phase transformation details. The calculated diffusion distance of Zr (~4.1 nm) from the CALPHAD data corresponds to the measured Zr-rich nanostrip thickness (3 ± 2 nm). The nanocuboidal BCC1-BCC2 structure exhibited 112 -type of twinning deformation under compression at room temperature. The Ta25Zr25Nb25Ti25 MEA retained yield strength of ~410 MPa at 1000 °C and ~210 MPa at 1200 °C. The phase transformation during cooling after annealing and the microstructural evolution during compression at temperatures from 600 °C to 1200 °C were characterized and discussed in detail.
T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematologic malignancy, and T-ALL patients are prone to early disease relapse and suffer from poor outcomes. The PTEN, PI3K/AKT and Notch ...pathways are frequently altered in T-ALL. PTEN is a tumor suppressor that inactivates the PI3K pathway. We profiled miRNAs in Pten-deficient mouse T-ALL and identified miR-26b as a potentially dysregulated gene. We validated decreased expression levels of miR-26b in mouse and human T-ALL cells. In addition, expression of exogenous miR-26b reduced proliferation and promoted apoptosis of T-ALL cells in vitro, and hindered progression of T-ALL in vivo. Furthermore, miR-26b inhibited the PI3K/AKT pathway by directly targeting PIK3CD, the gene encoding PI3Kδ, in human T-ALL cell lines. ShRNA for PIK3CD and CAL-101, a PIK3CD inhibitor, reduced the growth and increased apoptosis of T-ALL cells. Finally, we showed that PTEN induced miR-26b expression by regulating the differential expression of Ikaros isoforms that are transcriptional regulators of miR-26b. These results suggest that miR-26b functions as a tumor suppressor in the development of T-ALL. Further characterization of targets and regulators of miR-26b may be promising for the development of novel therapies.
In this study, the origin of the high tensile ductility (>10% strain to fracture) of a new class of high-strength (yield strength >500 MPa) aluminium (Al) metal matrix composites (Al MMCs) ...co-reinforced by ex-situ carbon nanotubes (CNTs) and in-situ γ−Al2O3 nanoparticles was investigated. The microstructures of these unique Al MMCs prior to and post tensile testing were characterized in detail using high resolution transmission electron microscopy, in order to understand their response to tensile deformation. The pathways to the in-situ formation of γ−Al2O3 nanoparticles were identified and attributed to the high energy ball milling process, and the subsequent spark plasma sintering, which converted the amorphous Al-oxide into γ−Al2O3 nanoparticles. The in-situ γ−Al2O3 nanoparticles and Al matrix exhibited a semi-coherent interface, which led to a markedly increased dislocation density in the matrix around the nanoparticles during tensile deformation. These dislocations were precursors to microvoid formation and consequent dimples on further deformation. The co-operation of CNTs and γ−Al2O3 nanoparticles resulted in a long strain-softening stage and therefore excellent tensile ductility. The results suggested that the exploitation of hybrid reinforcement by ex-situ CNTs and in-situ nanoparticles can enable the fabrication of high-performance metal matrix composites.
Graphene has been widely used as a reinforcement phase in composites due to its excellent electrical, thermal and mechanical properties. In this study, graphene was modified by electroless nickel ...plating to obtain nickel-coated graphene (Ni-Gr), which was then introduced into spherical Inconel 625 powder by ball milling. Selective laser melting (SLM) was subsequently used to fabricate Inconel 625/Ni-Gr metal matrix composites containing different levels of Ni-Gr (0, 0.4 vol%, 1.0 vol%). The effects of Ni-Gr content on the microstructure, mechanical, wear and corrosion properties of Inconel 625/Ni-Gr composites were investigated. Noticeable grain refinement was observed at the addition of 0.4 vol% Ni-Gr. The micro-hardness of Inconel 625/Ni-Gr composites increased with increasing Ni-Gr content and registered 348.2 HV0.2 at the addition of 1.0 vol% Ni-Gr. However, the Inconel 625/0.4Ni-Gr composite attained the maximum tensile strength (927 MPa), yield strength (797 MPa) and elastic modulus (227 GPa), aside from exhibiting lower friction coefficients than Inconel 625 at room temperature. In addition, it displayed higher corrosion potential than Inconel 625 and lower corrosion current density than Inconel 625/1.0Ni-Gr. The addition of Ni-Gr clearly improves the micro-hardness, tensile strength, yield strength, wear and corrosion properties of Inconel 625 due to grain refinement, solid-solution strengthening and impediment to dislocation motion. And, this study has verified the feasibility of preparing Inconel 625/Ni-Gr composites by SLM.
•Nickel-coated graphene (Ni-Gr) is prepared by electroless plating and introduced into Inconel 625 powder for SLM.•The addition of Ni-Gr plays the roles of grain refinement, solid solution strengthening and impediment to dislocation motion.•The addition of Ni-Gr improves the hardness, tensile strength, yield strength, wear and corrosion properties of SLM-fabricated Inconel 625.
Ambient fine particulate matter (PM2.5) pollution has been associated with mortality from various diseases, however, its association with under-five mortality rate (U5MR) has remained largely ...unknown.
Based on the U5MR data across 2851 counties in Mainland China from 1999 to 2012, we employed approximate Bayesian latent Gaussian models to assess the association between ambient PM2.5 and U5MR at the county level for the whole nation and sub-regions. GDP growth rate, normalized difference vegetation index (NDVI), temperature, and night-time light were included as covariates using a smoothing function. We further implemented an empirical dynamic model (EDM) to explore the potential causal relationship between PM2.5 and U5MR.
We observed a declining trend in U5MR in most counties throughout the study period. Spatial heterogeneity in U5MR was observed. Nationwide analysis suggested that each 10 µg/m3 increase in annual concentration of PM2.5 was associated with an increase of 1.2 (95% CI: 1.0 – 1.3) per 1000 live births in U5MR. Regional analyses showed that the strongest positive association was located in the Northeastern part of China 1.8 (95% CI: 1.4 – 2.1). The EDM showed a significant causal association between PM2.5 and U5MR, with an embedding dimension of 5 and 7, and nonlinear values θ of 4 and 6, respectively.
China exhibited a downward trend in U5MR from 1999 to 2012, with spatial heterogeneity observed across the country. Our analysis reveals a positive association between PM2.5 and U5MR, which may support a causal relationship.
•Ambient PM2.5 was associated with an increase of 0.11 per 1000 live births in U5MR.•Empirical dynamic model suggests a causal association between PM2.5 and U5MR.•Bayesian spatiotemporal interaction model is well-fitted to estimate ecological factors impact on mortality.
In this paper, we propose a goodness-of-fit test, named pseudo “in-and-out-of-likelihood” (PIOL) ratio test, to check for misspecification in semi-parametric copula models for univariate time series. ...The proposed test extends the idea of the IOS test by Presnell and Boos (J Am Stat Assoc 99:216–227, 2004) and PIOS test by Zhang et al. (J Econom, 193:215–233, 2016), which are problematic for direct application to univariate time series. The PIOL test provides an integrated framework for both independent data and time series data. In addition, an approximation method is implemented to alleviate the computational burden of calculating the test statistics. Asymptotic properties of the proposed test statistics are discussed. The finite-sample performance is examined through simulation studies. We also demonstrate the proposed method through the analysis of a time series of daily transactions of Apple trade.
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