Mechanical properties of Mg-8Gd-3Y-0.5Zr alloy with bimodal grain size distributions

The Mg-Gd-Y-Zn-Zr alloys are representational and potential age-hardening systems as reported in the past ten years,but their mechanical properties are still dependent on the grain size and its distribution.The effect of bimodal structure on mechanical properties of Mg-8Gd-3Y-0.5Zr alloy with bimodal grain size distributions was investigated.The results suggested that the volume fraction of fine grain(FG)and coarse grain(CG)could be controlled by combined processes of hot forging,extrusion and annealing.And for the present alloys with bimodal grain size distribution,the improvement of strength is still attributed to the grain refinement.The morphology of bimodal grain size distribution has a marked impact on the ductility of the alloy,i.e.with the increase of coarse grain volume fraction,the elongation to failure increases at the beginning and then decreases.The mechanism of the toughening effect of bimodal grain size distribution on the Mg-Gd-Y-Zn-Zr alloys with bimodal grain size structure has been discussed.

Influence of Particle Size Distribution of Bimodal Reactive Alumina on Properties of LCC Refractory Castables

The effect of particle size distribution of alumina has been investigated for silica-free tabular alumina low cement castables（ LCC）. Three different combinations of alumina have been included in the matrix formulation of the castables. All the three combinations are composed of a bimodal reactive alumina and a fine ground monomodal reactive alumina. The first A1 and second A2 combinations are respectively composed of bimodal and monomodal aluminas from Alteo,with a different fine /coarse particles ratio for the bimodal alumina. The two Alteo combinations have been compared with a third combination C composed of a bimodal commercially available grade and a monomodal commercially available grade. Optimization of particle size packing has been performed for the three different formulations using the Dinger and Funk model. With this optimization,the two formulations based on Alteo material（ PFR,PBR and PFR40） achieve the same level of performance in applicative tests（ flowability,cold physical properties,mechanical resistance,crystalline phases,thermal shocks and corrosion） as reference solutions on the market.

Dependence of bimodal size distribution on temperature and optical properties of InAs quantum dots grown on vicinal GaAs （100） substrates by using MOCVD

Self-assembled lnAs quantum dots （QDs） are grown on vicinal GaAs （100） substrates by using metal-organic chemical vapour deposition （MOCVD）. An abnormal temperature dependence of bimodal size distribution of InAs quantum dots is found. As the temperature increases, the density of the small dots grows larger while the density of the large dots turns smaller, which is contrary to the evolution of QDs on exact GaAs （100） substrates. This trend is explained by taking into account the presence of multiatoinic steps on the substrates. The optical properties of InAs QDs on vicinal GaAs（100） substrates are also studied by photoluminescence （PL） . It is found that dots on a vicinal substrate have a longer emission wavelength, a narrower PL line width and a much larger PL intensity.

Synthesis and Characterization of Bimodal Mesoporous Silica

Mesoporous silica with controllable bimodal pore size distribution was synthesized with cetyltrimethylammonium bromide （CTAB） as chemical template for small mesopores and silica gel as physical template for large mesopores. The structure of synthesized samples were characterized by Fourier transform infrared （FT-IR） spectroscopy, X-ray diffraction （XRD）, scanning electron microscopy （SEM）, transmission electron microscopy （TEM） and N2 adsorption-desorption measurements. The experimental results show that bimodal mesoporous silica consists of small mesopores of about 3 nm and large mesopores of about 45 nm. The small mesopores which were formed on the external surface and pore walls of the silica gel had similar characters with those of MCM-41, while large mesopores were inherited from parent silica gel material. The pore size distribution of the synthesized silica can be adjusted by changing the relative content of TEOS and silica gel or the feeding sequence of silica gel and NH4OH.

Preparation of Bimodal Mesoporous Silica Molecular Sieves from Organotrialkoxysilane and Tetraethoxysilane

The synthesis of silica with bimodal, disordered mesopores structure, by template-mediated sol-gel processing and subsequent heat-treatment process, was investigated. In the presence of nonionic surfactant Tx-15, the organically modified mesostructured nano-particles were synthesized by co-condensation of organosiloxane （RTES） and tetraethoxysilane （TEOS） in the first step, and then the bimodal porous silica was formed by the removal of the incorporated organic groups using programmed calcination. Here, RTES used was methyltriethoxysilane （MTES）, vinyltriethoxysilane （VTES） and phenyltriethoxysilane （PhTES）, respectively. Organotrialkoxysilane, TEOS and Tx-15 formed MSU-2-1iked assemblies, around which the nano-particles aggregated and cross-linked, considerably influencing the pore size distribution of both primary pores and secondary pores. By changing the type and amount of the organotrialkoxysilane added, a broad variety of silica with a controlled bimodal mesopore distribution could be easily synthesized. Textural and structural properties were determined by X-ray diffraction, high-resolution transmission electron microscopy, N2 adsorption/desorption, and thermogravimetric analysis.

Microstructure and mechanical properties of WC-Co-based cemented carbide with bimodal WC grain size distribution

In order to improve the wear resistance of coarse-grained WC-Co cemented carbides,the fine WC powder were used to reinforce the metallic binder.These WC-Co-based cemented carbides having bimodal WC grain size distributions were synthesized by liquid phase sintering.For comparison,the cemented carbides having unimodal WC grain size distributions were synthesized.The microstructure,hardness,fracture toughness and wear resistance of these cemented carbides were investigated.The results show that adding fine WC powder is an effective method to improve the wear resistance of coarsegrained WC-Co cemented carbides.The WC size,mean free path and fracture toughness decrease with the addition of fine WC powder,while the hardness exhibits an opposite trend.The impact-wear coefficient of bimodal distribution cemented carbides is noticeably lower than that of the unimodal one with the same hardness,which means that the cemented carbides with bimodal grain structure have better combination of hardness and impact-abrasive wear resistance.The impact-abrasive wear mechanism of the bimodal cemented carbides is that the fine WC grains prevent abrasive wear and the coarse WC grains prevent impact wear.

Detail fatigue rating method based on bimodal Weibull distribution for DED Ti-6.5Al-2Zr-1Mo-1V titanium alloy

Previous studies have shown that the fatigue life distribution of metal materials fabricated with Additive Manufacturing(AM) methods, such as Direct Energy Deposited(DED) Ti-6.5Al-2Zr-1Mo-1V alloys, exhibits two peaks. To promote the application of AM in aerospace and other engineering fields, developing a fatigue strength evaluation method suitable for AM materials based on their unique fatigue behaviours and fatigue life distributions is necessary. In this paper, a novel Detail Fatigue Rating(DFR) method was developed to evaluate the fatigue strength of DED Ti-6.5Al-2Zr-1Mo-1V based on a bimodal Weibull distribution and the excessive restriction on the allowable stress of potential was improved. Meanwhile, a Bimodal Weibull distribution model for fatigue life and its parameter estimation method were established based on a twoparameter Weibull distribution. The fatigue life at a specific reliability level and confidence level was calculated by using the bootstrap method. The calculation results showed that fatigue life estimated by using the bimodal Weibull distribution at the high reliability level and high confidence level is higher than that estimated by using the two-parameter Weibull distribution. Furthermore,The S-N curve at the specified confidence level and reliability level was fitted.

Fabrication and Dealloying Behavior of Monolithic Nanoporous Copper Ribbons with Bimodal Channel Size Distributions

Monolithic nanoporous copper （NPC） ribbons with bimodal channel size distributions can be fabricated through chemical dealloying of Mg-32 Cu alloy in an acidic solution at room temperature. The microstructure of the as- dealloyed samples was characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray analysis. These NPC ribbons are composed of interconnected large-sized channels （hundreds of nm） with highly porous channel walls （tens of nm）. Both large- and small-sized channels are open, bicontinuous, and interpenetrating. Additionally, it is the first time to find that the evolution process of porous structure along the thickness direction of samples during the dealloying is from the interior to exterior, which is just contrary to the coarsening process along the thickness direction during the post-dealloying. Meanwhile, the corresponding mechanism is discussed in detail.

EFFECTS OF AMINO GROUPS AND MICROSTRUCTURE OF ORGANIC MESOPOROUS SILICA SUPPORTED METALLOCENE CATALYSTS ON ETHYLENE POLYMERIZATION

Mesoporous silica （MS）, 3-aminopropyltriethoxysilane （APTES） modified mesoporous silica （AMS）, bis（3- trimethoxysilylpropyl）amine modified mesoporous silica （BAMS） and APTES modified solid spherical silica （AS） were prepared and used to immobilize metallocene catalysts for ethylene polymerization. Gel permeation chromatography results showed that polyethylenes （PEs） catalyzed by AMS （or BAMS） supported metallocene catalysts at the molar ratios of Al/Zr = 100, 300 and 500 were of bimodal molecular weight distribution （BMWD）; while PEs catalyzed by the above catalysts at the molar ratios of Al/Zr 〉 800 were of monomodal molecular weight distribution （MMWD）. However, MS （or AS） supported metallocene catalysts could only produce PEs with MMWD in spite of the molar ratio of Al/Zr. It was because that AMS （or BAMS） supported catalysts possessed two active sites for ethylene polymerization at low molar ratios of Al/Zr due to the combination effects of mesopore geometrical constraint and amino groups of the supports, which was confirmed by X-ray photoelectron spectroscopy. This brings forward a novel and easy method for the synthesis of polyolefin with BMWD.

Achieving high strength and high ductility in nanostructured metals: Experiment and modelling

Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Kowloon, Hong Kong, Chinagrains to a^1-martensite nanograins with bimodal grain size distribution for lower strain rates to nanotwins in the ultrafine/coarse grained austenite phase for higher strain rates. Meanwhile, we will further address the mechanism-based plastic models to describe the yield strength, strain hardening and ductility in nanostructured metals with bimodal grain size distribution and nanotwinned polycrystalline metals. The proposed theoretical models can comprehensively describe the plastic deformation in these two kinds of nanostructured metals and excellent agreement is achieved between the numerical and experimental results. These models can be utilized to optimize the strength and ductility in nanostructured metals by controlling the size and distribution of nanostructures.

Fatigue behavior of direct laser deposited Ti-6.5Al-2Zr-1Mo-1V titanium alloy and its life distribution model

The present work aims to investigate the fatigue behavior of Direct Laser Deposition(DLD) Ti-6.5 Al-2 Zr-1 Mo-1 V titanium alloy under constant amplitude stress. 22 pieces of DLD Ti-6.5 Al-2 Zr-1 Mo-1 V titanium alloy standard cylinder specimens were tested under a stress level of 800 MPa with a stress ratio of 0.06. Fatigue fractography and fatigue life data were obtained.Through the fracture surface analysis, the specimens were divided into two categories in accordance with the location of crack initiation and defect types. Comparison of fatigue life and behavior between two specimen types was given, which was followed by a discussion about the impact of defect type, size and position on the fatigue life of the specimen. The fatigue test results also show a large variation of fatigue life. To illustrate the statistical characteristics of the fatigue life, probabilistic analysis was performed, and a novel bimodal lognormal model was established. The model has a good fit with the experimental data and can reduce the scatter of the fatigue life significantly.

SYNTHESIS OF BIMODAL POLYETHYLENE WITH UNSYMMETRICAL α-DIIMINE NICKEL COMPLEXES: INFLUENCE OF LIGAND BACKBONE AND UNSYM-SUBSTITUTED ANILINE MOIETY

A series of unsymmetrical a-diimine nickel complexes with various backbones and substituted aniline moieties were synthesized and characterized. The crystallographic analysis of the nickel complexes confirmed the existence of meso-and rac-configuration in solid structure. Nickel complexes after activation by MAO were screened for ethylene polymerization to evaluate backbone substituent effect on synthesis of bimodal PE. Acenaphthyl nickel complex with planar backbone afforded a bimodal PE with a broad polydispersity, whereas camphyl nickel complex with rigid and bulky backbone afforded a monomodal PE with a narrow polydispersity. Steric effect of aniline moiety for acenaphthyl nickel complex was also examined, and bimodal PE with dominant high-molecular-weight fraction was obtained by modifying substituents on aniline moiety.

Synthesis of Bimodal Distributed Cyclic Olefin Copolymers with Improved Tensile Properties

Ethylene/norbornene(NBE)copolymers are the most representative and widely-used cyclic olefin copolymers(COCs).Higher NBE content in COCs leads to higher transparency,glass transition temperature and mechanical strength,but also makes the COC more brittle and therefore hampers its end uses.In this study,to improve the brittleness of COCs while maintaining their high strength,heat-resistant and transparency,bimodal distributed COCs have been prepared via two selective catalysts reactive blending in one pot.The shape of the molecular weight distribution curves of the obtained COCs could be easily controlled by adjusting the proportion of the two catalysts.These COCs maintained high glass transition temperature,balanced tensile performance,high transparency.

Boosting xenon adsorption with record capacity in microporous carbon molecular sieves

Xenon/krypton(Xe/Kr)separation is an important task in industry,yet it remains challenging to develop adsorbents with high Xe/Kr selectivity and adsorption capacity of Xe,especially at low partial pressures.Herein,we report a series of microporous carbon molecular sieves(CMSs)for Xe/Kr separation.Those materials have ideal bimodal pore size distributions that not only provide substantial space for the accommodation of gas molecules,but also allow selective diffusion of gas molecules.Additionally,the carbon frameworks decorated with polar oxygen-containing functional groups afford higher affinity for Xe than Kr,which is proven by density functional theory(DFT)calculations and charge density difference analysis.The optimal CPVDC-700 exhibits a high selectivity of Xe/Kr and,more importantly,a record-high uptake of Xe(2.93 mmol g^(-1))at 0.2 bar and298 K,which is the highest among all the reported carbon adsorbents.Breakthrough experiments confirm the excellent performance of such CMSs for Xe/Kr separation,and the dynamic adsorption uptake of Xe and productivity of high-purity Kr are calculated to be 2.91 mmol g^(-1)and 208 m L g^(-1)(9.29 mmol g^(-1)),respectively,which also set up a new benchmark for Xe/Kr separation of carbon adsorbents.