Researchers seldom study the optimum design of a mechanical connector for subsea oil-gas pipeline based upon the sealing performance. An optimal design method of a novel subsea pipeline mechanical connector is present...Researchers seldom study the optimum design of a mechanical connector for subsea oil-gas pipeline based upon the sealing performance. An optimal design method of a novel subsea pipeline mechanical connector is presented. By analyzing the static metal sealing mechanism, the critical condition of the sealing performance is established for this connector and the formulation method of the contact pressure on the sealing surface is created. By the method the minimum mean contact pressure of the 8.625 inch connector is calculated as 361 MPa, which is the constraint condition in the optimum design of connector.The finite element model is created in ANSYS Parametric Design Language(APDL) and the structure is optimized by the zero-order method, with variance of contact pressure as the objective function, and mean contact pressures and plastic strains as constraint variables. The optimization shows that variances of contact pressure on two sealing surfaces decrease by 72.41% and 89.33%, respectively, and mean contact pressures increase by 31.18% and 52.84%, respectively. The comparison of the optimal connectors and non-optimal connectors in the water pressure experiments and bending experiments shows that the sealing ability of optimized connectors is much higher than the rated pressure of 4.5 MPa, and the optimal connectors don’t leak under the bending moment of 52.2 kN·m.This research provides the formulation to solve contact pressure on the sealing surface and a structure optimization method to design the connectors with various dimensions.展开更多
Magnesium(Mg)based alloys are promising candidates for many applications,but their untreated surfaces usually have low strength and hardness.In this study,a single point diamond turning(SPDT)technique was applied to r...Magnesium(Mg)based alloys are promising candidates for many applications,but their untreated surfaces usually have low strength and hardness.In this study,a single point diamond turning(SPDT)technique was applied to refine the grain size and improve the mechanical properties of the surface layers of Mg-Li alloys.By refining grains in the topmost layer to the nanometer scale(~60 nm),the surface hardness was found to be enhanced by approximately 60%.The atomic plastic deformation process during the SPDT was then studied by the real-time atomistic molecular dynamics(MD)simulations.A series of MD simulations with different combinations of parameters,including rake angle,cutting speed and cutting depth,were conducted to understand their influences on the microstructural evolution and associated plastic deformation mechanisms on the surface layer of the workpieces.The MD simulation results suggest that using increased rake angle,cutting speed and cutting depth can help to achieve better grain refinement.These simulation results,which provide atomic-level details of the deformation mechanism,can assist the parameter design for the SPDT techniques to achieve the high-performance heterogeneous nanostructured materials.展开更多
With the rapid development of information and multi me dia technologies, the demand for the optical plastic aspheric elements used in o pto-electronic devices, camera, optical disc and projector lens etc. has been i n...With the rapid development of information and multi me dia technologies, the demand for the optical plastic aspheric elements used in o pto-electronic devices, camera, optical disc and projector lens etc. has been i ncreased rapidly in the recent years. The key technologies of fabrication of asp heric plastic lens are the design and manufacturing moulds, selection of proper injection moulding equipment, and optimization of injection moulding parameters etc. In this paper, the effect of injection pressure, moulding temperature, cool ing time and injection speed on the surface profile of the lenses during injecti on and holding process is investigated. Surface quality of plastic lenses is mea sured by Talysurf Texture Measuring System. The experimental results showed that the injection pressure and moulding temperature are important parameters compar ing to cooling time and injection speed. A bit change of injection pressure or m oulding temperature will affect the property of the surface profile. Either incr easing injection pressure or mould temperature can achieve less shrinkage. Other wise, a lower injection pressure will produce more shrinkage, more air traps and a lower mould temperature results greater warp and higher shrinkage. The dynami c process of injection for optical plastic lenses is simulated by 3D Moldflow pl astic Insight software (MPI). The MPI will help us to optimize injection mouldin g parameters.展开更多
It has been well known that doping nano-scale catalysts can significantly improve both the kinetics and reversible hydrogen storage capacity of MgH_(2) . However, so far it is still a challenge to directly synthesize ...It has been well known that doping nano-scale catalysts can significantly improve both the kinetics and reversible hydrogen storage capacity of MgH_(2) . However, so far it is still a challenge to directly synthesize ultrafine catalysts(e.g., < 5 nm), mainly because of the complicated chemical reaction processes. Here, a facile one-step high-energy ball milling process is developed to in situ form ultrafine Ni nanoparticles from the nickel acetylacetonate precursor in the MgH_(2) matrix. With the combined action of ultrafine metallic Ni and expanded graphite(EG), the formed MgH_(2)-Ni-EG nanocomposite with the optimized doping amounts of Ni and EG can still release 7.03 wt.% H_(2) within 8.5 min at 300 ℃ after 10 cycles. At a temperature close to room temperature(50 ℃), it can also absorb 2.42 wt.% H_(2) within 1 h. It can be confirmed from the microstructural characterization analysis that the in situ formed ultrafine metallic Ni is transformed into Mg_(2)Ni/Mg_(2)NiH_4 in the subsequent hydrogen absorption and desorption cycles. It is calculated that the dehydrogenation activation energy of the MgH_(2)-Ni-EG nanocomposite is also reduced obviously in comparison with the pure MgH_(2) . Our work provides a methodology to significantly improve the hydrogen storage performance of MgH_(2) by combining the in situ formed and uniformly dispersed ultrafine metallic catalyst from the precursor and EG.展开更多
The amorphous Fe78Si9B13 ribbons were bend stress relaxed at various temperature well below the crystallization temperature （Tx） for different time. The effect of pre-annealing on the subsequent bend stress relaxati...The amorphous Fe78Si9B13 ribbons were bend stress relaxed at various temperature well below the crystallization temperature （Tx） for different time. The effect of pre-annealing on the subsequent bend stress relaxation was examined. The variation of the microstructure and microhardness during bend stress relaxation process was studied using X-ray diffraction （XRD）, atomic force microscopy （AFM） and Vickers microhardness test,respectively. Curvature radius of the amorphous Fe78Si9B13 ribbons decreased with increase bend stress relaxation temperature and time. The microhardness of the stress relaxed specimens increased with time at 300℃ due to the forming of nanocrystals during bend stress relaxation. The pre-annealing reduced the decrease rate of the curvature radius of stress relaxed specimens.展开更多
Tool condition monitoring(TCM)is a key technology for intelligent manufacturing.The objective is to monitor the tool operation status and detect tool breakage so that the tool can be changed in time to avoid significa...Tool condition monitoring(TCM)is a key technology for intelligent manufacturing.The objective is to monitor the tool operation status and detect tool breakage so that the tool can be changed in time to avoid significant damage to workpieces and reduce manufacturing costs.Recently,an innovative TCM approach based on sensor data modelling and model frequency analysis has been proposed.Different from traditional signal feature-based monitoring,the data from sensors are utilized to build a dynamic process model.Then,the nonlinear output frequency response functions,a concept which extends the linear system frequency response function to the nonlinear case,over the frequency range of the tooth passing frequency of the machining process are extracted to reveal tool health conditions.In order to extend the novel sensor data modelling and model frequency analysis to unsupervised condition monitoring of cutting tools,in the present study,a multivariate control chart is proposed for TCM based on the frequency domain properties of machining processes derived from the innovative sensor data modelling and model frequency analysis.The feature dimension is reduced by principal component analysis first.Then the moving average strategy is exploited to generate monitoring variables and overcome the effects of noises.The milling experiments of titanium alloys are conducted to verify the effectiveness of the proposed approach in detecting excessive flank wear of solid carbide end mills.The results demonstrate the advantages of the new approach over conventional TCM techniques and its potential in industrial applications.展开更多
Heterogeneous gradient nanostructured metals have been shown to achieve the strength-ductility synergy, thus potentially possessing the enhanced tribological performance in comparison with their homogeneous nanograine...Heterogeneous gradient nanostructured metals have been shown to achieve the strength-ductility synergy, thus potentially possessing the enhanced tribological performance in comparison with their homogeneous nanograined counterparts. In this work, a facile laser surface remelting-based surface treatment technique is developed to fabricate a gradient nanostructured layer on a TiZrHfTaNb refractory highentropy alloy. The characterization of the microstructural evolution along the depth direction from the matrix to the topmost surface layer shows that the average grain size in the ~100 μm-thick gradient nanostructured layer is dramatically refined from the original ~200 μm to only ~8 nm in the top surface layer. The microhardness is therefore gradually increased from ~240 HV in matrix to ~650 HV in the topmost surface layer, approximately 2.7 times. Noticeably, the original coarse-grained single-phase bodycentered-cubic TiZrHfTaNb refractory high-entropy alloy is gradually decomposed into TiNb-rich bodycentered-cubic phase, TaNb-rich body-centered-cubic phase, ZrHf-rich hexagonal-close-packed phase and TiZr Hf-rich face-centered-cubic phase with gradient distribution in grain size along the depth direction during the gradient refinement process. As a result, the novel laser surface treatment-introduced gradient nanostructured TiZrHfTaNb refractory high-entropy alloy demonstrates the significantly improved wear resistance, with the wear rate reducing markedly by an order of magnitude, as compared with the as-cast one. The decomposed multi-phases and gradient nanostructures should account for the enhanced wear resistance. Our findings provide new insights into the refinement mechanisms of the laser-treated refractory high-entropy alloys and broaden their potential applications via heterogeneous gradient nanostructure engineering.展开更多
The energy state and atomic level structure of metallic glasses(MGs)are very sensitive to their cooling rates,and a lower cooling rate generally causes a lower energy and more relaxed state of MGs.In this work,the Zr4...The energy state and atomic level structure of metallic glasses(MGs)are very sensitive to their cooling rates,and a lower cooling rate generally causes a lower energy and more relaxed state of MGs.In this work,the Zr41.2 Ti13.8 Cu12.5 Ni10 Be22.5(Vit.1)ribbons with a thickness of 40μm and 110μm and the strips with a thickness of 320μm and 490μm were produced by single-roll melt spinning and twin-roll casting,respectively.The increase in thickness of either ribbons or strips results in a lower energy state with a smaller relaxation enthalpy,a lower content of free volume,and a higher hardness.Although the cooling rate of the twin-roll produced 320μm-thick strip is almost one magnitude lower than that of the single-roll produced 110μm-thick ribbon,the former,however,possesses a rejuvenated energy state as compared to the latter.Molecular dynamics simulations reveal that the squeezing force during twin-roll casting affects the evolution of connection types of clusters,and the 2-atom and 4-atom connections are prone to be retained,which results in a higher energy state of MGs.Such a rejuvenation process during twin-roll casting can overwhelm the relaxation process caused by the lower cooling rate.Therefore,twin-roll casting is not only a method being capable for producing strips with a large thickness,but also prone to obtain a high energy state of the MG strip.展开更多
The deformation behavior in Zr36 Cu64 metallic glasses with pre-introduced indent-notches has been studied by molecular dynamics simulation at the atomic scale.The indent-notches can trigger the formation of densely-p...The deformation behavior in Zr36 Cu64 metallic glasses with pre-introduced indent-notches has been studied by molecular dynamics simulation at the atomic scale.The indent-notches can trigger the formation of densely-packed clusters composed of solid-like atoms in the indent-notch affected zone.These denselypacked clusters are highly resistant to the nucleation of shear bands.Hence,there is more tendency for the shear bands to nucleate outside the indent-notch affected zone,which enlarges the deformation region and enhances both the strengthening effect and the plastic deformation ability.For indent-notched MGs,when determining the initial yielding level,there is a competition process occurring between the densely-packed clusters leading to the shear band formation outside the indent-notch affected zone and the stress-concentration localizing deformation around the notch roots.When the indent-notch depth is small,the stress-concentration around the notch root plays a dominant role,leading to the shear bands initiating from the notch root,reminiscence of the cut-notches.As the indent-notch depth increases,there are many densely-packed clusters with high resistance to deformation in the indent-notch affected zone,leading to the shear band formation from the interface between the indent-notch affected zone and the matrix.Current research findings provide a feasible means for improving the strength and the plasticity of metallic glasses at room temperature.展开更多
Searching for free-standing and cost-efficient hydrogen evolution reaction(HER)electrocatalysts with high efficiency and excellent durability remains a great challenge for the hydrogen-based energy industry.Here,we re...Searching for free-standing and cost-efficient hydrogen evolution reaction(HER)electrocatalysts with high efficiency and excellent durability remains a great challenge for the hydrogen-based energy industry.Here,we report fabrication of a unique hierarchically porous structure,i.e.,nanoporous Ni(NPN)/metallic glass(MG)composite,through surface dealloying of the specially designed Ni_(40)Zr_(40)Ti_(20)MG wire.This porous composite is composed of micrometer slits staggered with nanometer pores,which not only enlarges effective surface areas for the catalytic reaction,but also facilitates the release of H2 gas.As a result,the NPN/MG hybrid electrode exhibited the prominent HER performance with a low overpotential of 78 m V at 10 m A cm^(-2)and Tafel slope of 42.4 m V dec^(-1),along with outstanding stability in alkaline solutions.Outstanding catalytic properties,combining with their free-standing capability and cost efficiency,make the current composite electrode viable for HER applications.展开更多
Durability, rate capability, capacity and tap density are paramount performance metrics for promising anode materials, especially for sodium ion batteries. Herein, a carbon free mesoporous CoTiO3 micro-prism with a hi...Durability, rate capability, capacity and tap density are paramount performance metrics for promising anode materials, especially for sodium ion batteries. Herein, a carbon free mesoporous CoTiO3 micro-prism with a high tap density （1.8 gcm^-3） is newly developed by using a novel Co-Ti- bimetal organic framework （BMOF） as precursor. It is also interesting to find that the Co-Ti-BMOF derived carbon-free mesoporous CoTiO3 micro-prisms deliver a superior stable and more powerful Na^＋ storage than other similar reported titania, titanate and their carbon composites. Its achieved ca- pacity retention ratio for 2,000 cycles is up to 90.1% at 5 A g^-1.展开更多
Magnesium(Mg)-based alloys have already been widely studied as the hydrogen storage materials because of their high reversible hydrogen storage capacity,low cost,light weight,etc.However,the poor de/hydrogenation kine...Magnesium(Mg)-based alloys have already been widely studied as the hydrogen storage materials because of their high reversible hydrogen storage capacity,low cost,light weight,etc.However,the poor de/hydrogenation kinetic properties dramatically hinder the practical applications.In this work,the MgH_(2)-ANi_(5)(A=Ce,Nd,Pr,Sm,and Y) composites were prepared by a high-energy ball milling method.which can effectively refine the particle size thus improving the kinetic properties.Experimental results reveal that the MgH_(2)-ANi_(5) composites mainly consist of Mg_(2)NiH_(4),MgH_(2) and rare earth(RE) hydride,which will be dehydrogenated to form Mg_(2)Ni,Mg and stable RE hydride reversibly.Accordingly,the asmilled MgH_(2)-ANi_(5)(A=Ce,Nd,Pr,Sm,and Y) composites with various A-elements can respectively contribute to a reversible hydrogen storage capacity of 6.16 wt%,5.7 wt%,6.21 wt%,6.38 wt%,and 6.5 wt%at a temperature of 300℃,and show much better kinetic properties in comparison to the pure MgH_(2) without any additive.In-situ formed Mg_(2) Ni and stable RE hydride(such as CeH_(2.73) and YH_(2)) might act as effective catalysts to significantly improve the hydrogen storage properties of MgH_(2).The present work provides a guideline on improving the kinetic properties of the Mg-based hydrogen storage alloys.展开更多
The machining of hard-to-cut materials with a high degree of precision and high surface quality is one of the most critical considerations when fabricating various state-of-the-art engineered components.In this invest...The machining of hard-to-cut materials with a high degree of precision and high surface quality is one of the most critical considerations when fabricating various state-of-the-art engineered components.In this investigation,a comprehensive three-dimensional model was developed and numerically simulated to predict kerf profiles and material removal rates while drilling the aluminum-7075-T6 aerospace alloy.Kerf profile and material removal prediction involved three stages:jet dynamic flow modeling,abrasive particle tracking,and erosion rate pre-diction.Experimental investigations were conducted to validate the developed model.The results indicate that the jet dynamic characteristics and flow of abrasive particles alter the kerf profiles,where the top kerf diameter increases with increasing jet pressure and standoff distance.The kerf depth and hole aspect ratio increase with jet pressure,but decrease with standoff distance and machining time.Crosssectional profiles were characterized by progressive edge rounding and parabolic shapes.Defects can be minimized by utilizing high jet pressure and small standoff distance.The material removal rate increases with increasing jet pressure,abrasive particle size,and exposure time,but decreases with increasing standoff distance.展开更多
Severe plastic deformation(SPD)-induced gradient nanostructured(GNS)metallic materials exhibit superior mechanical performance,especially the high strength and good ductility.In this study,a novel high-speed machining...Severe plastic deformation(SPD)-induced gradient nanostructured(GNS)metallic materials exhibit superior mechanical performance,especially the high strength and good ductility.In this study,a novel high-speed machining SPD technique,namely single point diamond turning(SPDT),was developed to produce effectively the GNS layer on the hexagonal close-packed(HCP)structural Mg alloy.The high-resolution transmission electron microscopy observations and atomistic molecular dynamics simulations were mainly performed to atomic-scale dissect the grain refinement process and corresponding plastic deformation mechanisms of the GNS layer.It was found that the grain refinement process for the formation of the GNS Mg alloy layer consists of elongated coarse grains,lamellar fine grains with deformation-induced-tension twins and contraction twins,ultrafine grains,and nanograins with the grain size of~70 nm along the direction from the inner matrix to surface.Specifically,experiment results and atomistic simulations reveal that these deformation twins are formed by gliding twinning partial dislocations that are dissociated from the lattice dislocations piled up at grain boundaries.The corresponding deformation mechanisms were evidenced to transit from the deformation twinning to dislocation slip when the grain size was below 2.45μm.Moreover,the Hall-Petch relationship plot and the surface equivalent stress along the gradient direction estimated by finite element analysis for the SPDT process were incorporated to quantitatively elucidate the transition of defo rmation mechanisms during the grain refinement process.Our findings have implications for the development of the facile SPD technique to construct high strength-ductility heterogeneous GNS metals,especially for the HCP metals.展开更多
We have investigated the low cycle fatigue(LCF)properties and the extent of strengthening in a dense additively manufactured stainless steel containing different volume fractions of cell structures but having all othe...We have investigated the low cycle fatigue(LCF)properties and the extent of strengthening in a dense additively manufactured stainless steel containing different volume fractions of cell structures but having all other microstructure characteristics the same.The samples were produced by laser powder bed fusion(L-PBF),and the concentration of cell structures was varied systematically by varying the annealing treatments.Load-controlled fatigue experiments performed on samples with a high fraction of cell structures reveal an up to 23 times increase in fatigue life compared to an essentially cell-free sample of the same grain configuration.Multiscale electron microscopy characterizations reveal that the cell structures serve as the soft barriers to the dislocation propagation and the partials are the main carrier for cyclic loading.The cell structures,stabilized by the segregated atoms and misorientation between the adjacent cells,are retained during the entire plastic deformation,hence,can continuously interact with dislocations,promote the formation of nanotwins,and provide massive 3D network obstacles to the dislocation motion.The compositional micro-segregation caused by the cellular solidification features serves as another nonnegligible strengthening mechanism to dislocation motion.Specifically,the cell structures with a high density of dislocation debris also appear to act as dislocation nucleation sites,very much like coherent twin boundaries.This work indicates the potential of additive manufacturing to design energy absorbent alloys with high performance by tailoring the microstructure through the printing process.展开更多
Purpose To develop an understanding of the role of shoulder padding in rugby union by investigating player perceptions and attitudes towards shoulder padding and extending research into shoulder injuries in rugby.Meth...Purpose To develop an understanding of the role of shoulder padding in rugby union by investigating player perceptions and attitudes towards shoulder padding and extending research into shoulder injuries in rugby.Methods An online survey was distributed to past and current rugby players over 13 years old in 2018.Questions related to the participants’demographic,attitudes to shoulder padding and shoulder injury history.Results Six hundred and sixteen rugby players responded to the survey;66.1%of respondents had worn shoulder padding at some point.The age group 24-29 years old(ΔR^(2)=0.03,B=−0.53,P=0.015)had an inverse association with padding effectiveness while playing experience groups 1-2 years(ΔR^(2)=0.03,B=0.8,P=0.032),3-5 years(ΔR^(2)=0.03,B=0.70,P=0.002)and 6-9 years(ΔR^(2)=0.03,B=0.41,P=0)had a positive association.There are 37.1%of respondents consider-ing shoulder padding to be effective at preventing cuts and abrasions with 21.9%finding it very effective;50.3%considered it to be effective or very effective(9.7%)at preventing contusion;45.5%wore padding for injury prevention,while 19.2%wore padding to protect from reoccurring injury.Sprain/ligament damage(57.5%)and bruising(55.5%)were the most com-monly reported injuries.Conclusions The primary reason for wearing shoulder padding was as a means of injury prevention.Research should focus on quantifying the injury preventive capabilities of shoulder padding.Bruising,cuts and abrasion injuries to the shoulder are prevalent presenting new findings that these injuries are underreported.展开更多
基金Supported by National Natural Science Foundation of China(Grant Nos.51279042,51105088)
文摘Researchers seldom study the optimum design of a mechanical connector for subsea oil-gas pipeline based upon the sealing performance. An optimal design method of a novel subsea pipeline mechanical connector is presented. By analyzing the static metal sealing mechanism, the critical condition of the sealing performance is established for this connector and the formulation method of the contact pressure on the sealing surface is created. By the method the minimum mean contact pressure of the 8.625 inch connector is calculated as 361 MPa, which is the constraint condition in the optimum design of connector.The finite element model is created in ANSYS Parametric Design Language(APDL) and the structure is optimized by the zero-order method, with variance of contact pressure as the objective function, and mean contact pressures and plastic strains as constraint variables. The optimization shows that variances of contact pressure on two sealing surfaces decrease by 72.41% and 89.33%, respectively, and mean contact pressures increase by 31.18% and 52.84%, respectively. The comparison of the optimal connectors and non-optimal connectors in the water pressure experiments and bending experiments shows that the sealing ability of optimized connectors is much higher than the rated pressure of 4.5 MPa, and the optimal connectors don’t leak under the bending moment of 52.2 kN·m.This research provides the formulation to solve contact pressure on the sealing surface and a structure optimization method to design the connectors with various dimensions.
基金the National Key Research and Development Program of China(2018YFE0124900)the National Natural Science Foundation of China(51861165204/51778370/51701171/51971187)+2 种基金the Natural Science Foundation of Guangdong(2017B030311004)the Shenzhen Science and Technology Project(GJHZ20180928155819738)the Partner State Key Laboratories in Hong Kong from the Innovation and Technology Commission(ITC)(Project Code:1-BBXA)。
文摘Magnesium(Mg)based alloys are promising candidates for many applications,but their untreated surfaces usually have low strength and hardness.In this study,a single point diamond turning(SPDT)technique was applied to refine the grain size and improve the mechanical properties of the surface layers of Mg-Li alloys.By refining grains in the topmost layer to the nanometer scale(~60 nm),the surface hardness was found to be enhanced by approximately 60%.The atomic plastic deformation process during the SPDT was then studied by the real-time atomistic molecular dynamics(MD)simulations.A series of MD simulations with different combinations of parameters,including rake angle,cutting speed and cutting depth,were conducted to understand their influences on the microstructural evolution and associated plastic deformation mechanisms on the surface layer of the workpieces.The MD simulation results suggest that using increased rake angle,cutting speed and cutting depth can help to achieve better grain refinement.These simulation results,which provide atomic-level details of the deformation mechanism,can assist the parameter design for the SPDT techniques to achieve the high-performance heterogeneous nanostructured materials.
文摘With the rapid development of information and multi me dia technologies, the demand for the optical plastic aspheric elements used in o pto-electronic devices, camera, optical disc and projector lens etc. has been i ncreased rapidly in the recent years. The key technologies of fabrication of asp heric plastic lens are the design and manufacturing moulds, selection of proper injection moulding equipment, and optimization of injection moulding parameters etc. In this paper, the effect of injection pressure, moulding temperature, cool ing time and injection speed on the surface profile of the lenses during injecti on and holding process is investigated. Surface quality of plastic lenses is mea sured by Talysurf Texture Measuring System. The experimental results showed that the injection pressure and moulding temperature are important parameters compar ing to cooling time and injection speed. A bit change of injection pressure or m oulding temperature will affect the property of the surface profile. Either incr easing injection pressure or mould temperature can achieve less shrinkage. Other wise, a lower injection pressure will produce more shrinkage, more air traps and a lower mould temperature results greater warp and higher shrinkage. The dynami c process of injection for optical plastic lenses is simulated by 3D Moldflow pl astic Insight software (MPI). The MPI will help us to optimize injection mouldin g parameters.
基金financial support from the National Basic Research Program of China (2018YFB1502100)the support from the PolyU grant (No.G-YW5N)。
文摘It has been well known that doping nano-scale catalysts can significantly improve both the kinetics and reversible hydrogen storage capacity of MgH_(2) . However, so far it is still a challenge to directly synthesize ultrafine catalysts(e.g., < 5 nm), mainly because of the complicated chemical reaction processes. Here, a facile one-step high-energy ball milling process is developed to in situ form ultrafine Ni nanoparticles from the nickel acetylacetonate precursor in the MgH_(2) matrix. With the combined action of ultrafine metallic Ni and expanded graphite(EG), the formed MgH_(2)-Ni-EG nanocomposite with the optimized doping amounts of Ni and EG can still release 7.03 wt.% H_(2) within 8.5 min at 300 ℃ after 10 cycles. At a temperature close to room temperature(50 ℃), it can also absorb 2.42 wt.% H_(2) within 1 h. It can be confirmed from the microstructural characterization analysis that the in situ formed ultrafine metallic Ni is transformed into Mg_(2)Ni/Mg_(2)NiH_4 in the subsequent hydrogen absorption and desorption cycles. It is calculated that the dehydrogenation activation energy of the MgH_(2)-Ni-EG nanocomposite is also reduced obviously in comparison with the pure MgH_(2) . Our work provides a methodology to significantly improve the hydrogen storage performance of MgH_(2) by combining the in situ formed and uniformly dispersed ultrafine metallic catalyst from the precursor and EG.
文摘The amorphous Fe78Si9B13 ribbons were bend stress relaxed at various temperature well below the crystallization temperature （Tx） for different time. The effect of pre-annealing on the subsequent bend stress relaxation was examined. The variation of the microstructure and microhardness during bend stress relaxation process was studied using X-ray diffraction （XRD）, atomic force microscopy （AFM） and Vickers microhardness test,respectively. Curvature radius of the amorphous Fe78Si9B13 ribbons decreased with increase bend stress relaxation temperature and time. The microhardness of the stress relaxed specimens increased with time at 300℃ due to the forming of nanocrystals during bend stress relaxation. The pre-annealing reduced the decrease rate of the curvature radius of stress relaxed specimens.
文摘Tool condition monitoring(TCM)is a key technology for intelligent manufacturing.The objective is to monitor the tool operation status and detect tool breakage so that the tool can be changed in time to avoid significant damage to workpieces and reduce manufacturing costs.Recently,an innovative TCM approach based on sensor data modelling and model frequency analysis has been proposed.Different from traditional signal feature-based monitoring,the data from sensors are utilized to build a dynamic process model.Then,the nonlinear output frequency response functions,a concept which extends the linear system frequency response function to the nonlinear case,over the frequency range of the tooth passing frequency of the machining process are extracted to reveal tool health conditions.In order to extend the novel sensor data modelling and model frequency analysis to unsupervised condition monitoring of cutting tools,in the present study,a multivariate control chart is proposed for TCM based on the frequency domain properties of machining processes derived from the innovative sensor data modelling and model frequency analysis.The feature dimension is reduced by principal component analysis first.Then the moving average strategy is exploited to generate monitoring variables and overcome the effects of noises.The milling experiments of titanium alloys are conducted to verify the effectiveness of the proposed approach in detecting excessive flank wear of solid carbide end mills.The results demonstrate the advantages of the new approach over conventional TCM techniques and its potential in industrial applications.
基金supported by the joint Ph D project between the Hong Kong Polytechnic University and Southern University of Science and Technologythe grant from the Research Committee of Poly U under student account code RK2N+1 种基金supported by the National Natural Science Foundation of China Projects (Nos. 51701171 and 51971187)the Fundamental Research Program of Shenzhen (Grant No. JCYJ20170412153039309)。
文摘Heterogeneous gradient nanostructured metals have been shown to achieve the strength-ductility synergy, thus potentially possessing the enhanced tribological performance in comparison with their homogeneous nanograined counterparts. In this work, a facile laser surface remelting-based surface treatment technique is developed to fabricate a gradient nanostructured layer on a TiZrHfTaNb refractory highentropy alloy. The characterization of the microstructural evolution along the depth direction from the matrix to the topmost surface layer shows that the average grain size in the ~100 μm-thick gradient nanostructured layer is dramatically refined from the original ~200 μm to only ~8 nm in the top surface layer. The microhardness is therefore gradually increased from ~240 HV in matrix to ~650 HV in the topmost surface layer, approximately 2.7 times. Noticeably, the original coarse-grained single-phase bodycentered-cubic TiZrHfTaNb refractory high-entropy alloy is gradually decomposed into TiNb-rich bodycentered-cubic phase, TaNb-rich body-centered-cubic phase, ZrHf-rich hexagonal-close-packed phase and TiZr Hf-rich face-centered-cubic phase with gradient distribution in grain size along the depth direction during the gradient refinement process. As a result, the novel laser surface treatment-introduced gradient nanostructured TiZrHfTaNb refractory high-entropy alloy demonstrates the significantly improved wear resistance, with the wear rate reducing markedly by an order of magnitude, as compared with the as-cast one. The decomposed multi-phases and gradient nanostructures should account for the enhanced wear resistance. Our findings provide new insights into the refinement mechanisms of the laser-treated refractory high-entropy alloys and broaden their potential applications via heterogeneous gradient nanostructure engineering.
基金supported financially by the National Natural Science Foundation of China(Nos.51790484,51701213 and 51801174)the National Key Research and Development Program of China(No.2018YFB0703402)+2 种基金the Liaoning Revitalization Talents Program(Nos.XLYC1802078,XLYC1807062)the China Postdoctoral Science Foundation(No.2018M633005)the Dongguan Innovative Research Team Program(No.2014607134).
文摘The energy state and atomic level structure of metallic glasses(MGs)are very sensitive to their cooling rates,and a lower cooling rate generally causes a lower energy and more relaxed state of MGs.In this work,the Zr41.2 Ti13.8 Cu12.5 Ni10 Be22.5(Vit.1)ribbons with a thickness of 40μm and 110μm and the strips with a thickness of 320μm and 490μm were produced by single-roll melt spinning and twin-roll casting,respectively.The increase in thickness of either ribbons or strips results in a lower energy state with a smaller relaxation enthalpy,a lower content of free volume,and a higher hardness.Although the cooling rate of the twin-roll produced 320μm-thick strip is almost one magnitude lower than that of the single-roll produced 110μm-thick ribbon,the former,however,possesses a rejuvenated energy state as compared to the latter.Molecular dynamics simulations reveal that the squeezing force during twin-roll casting affects the evolution of connection types of clusters,and the 2-atom and 4-atom connections are prone to be retained,which results in a higher energy state of MGs.Such a rejuvenation process during twin-roll casting can overwhelm the relaxation process caused by the lower cooling rate.Therefore,twin-roll casting is not only a method being capable for producing strips with a large thickness,but also prone to obtain a high energy state of the MG strip.
基金supported by the National Natural Science Foundation of China under Grant 51801174the Program for the Top Young Talents of Higher Learning Institutions of Hebei under Grant BJ2018021+2 种基金the National Key R&D Program of China under Grant 2018YFA0703602the Hong Kong Scholars Program under Grant XJ2017049support from the National Science Foundation under grant number CMMI 17-026。
文摘The deformation behavior in Zr36 Cu64 metallic glasses with pre-introduced indent-notches has been studied by molecular dynamics simulation at the atomic scale.The indent-notches can trigger the formation of densely-packed clusters composed of solid-like atoms in the indent-notch affected zone.These denselypacked clusters are highly resistant to the nucleation of shear bands.Hence,there is more tendency for the shear bands to nucleate outside the indent-notch affected zone,which enlarges the deformation region and enhances both the strengthening effect and the plastic deformation ability.For indent-notched MGs,when determining the initial yielding level,there is a competition process occurring between the densely-packed clusters leading to the shear band formation outside the indent-notch affected zone and the stress-concentration localizing deformation around the notch roots.When the indent-notch depth is small,the stress-concentration around the notch root plays a dominant role,leading to the shear bands initiating from the notch root,reminiscence of the cut-notches.As the indent-notch depth increases,there are many densely-packed clusters with high resistance to deformation in the indent-notch affected zone,leading to the shear band formation from the interface between the indent-notch affected zone and the matrix.Current research findings provide a feasible means for improving the strength and the plasticity of metallic glasses at room temperature.
基金supported by National Natural Science Foundation of China(Nos.11790293,51871016,51671018,51671021,and 51961160729)the Funds for Creative Research Groups of China(No.51921001)+3 种基金111 Project(B07003)the Program for Changjiang Scholars and Innovative Research Team in University of China(IRT 14R05)the Fundamental Research Funds for the Central Universities(Nos.FRF-GF-19-011A,FRF-TP-18-004C1,FRF-BD-19-002B,and FRF-TP19-054A2)partially supported by State Key Laboratory for Advanced Metals and Materials(2018Z-19)。
文摘Searching for free-standing and cost-efficient hydrogen evolution reaction(HER)electrocatalysts with high efficiency and excellent durability remains a great challenge for the hydrogen-based energy industry.Here,we report fabrication of a unique hierarchically porous structure,i.e.,nanoporous Ni(NPN)/metallic glass(MG)composite,through surface dealloying of the specially designed Ni_(40)Zr_(40)Ti_(20)MG wire.This porous composite is composed of micrometer slits staggered with nanometer pores,which not only enlarges effective surface areas for the catalytic reaction,but also facilitates the release of H2 gas.As a result,the NPN/MG hybrid electrode exhibited the prominent HER performance with a low overpotential of 78 m V at 10 m A cm^(-2)and Tafel slope of 42.4 m V dec^(-1),along with outstanding stability in alkaline solutions.Outstanding catalytic properties,combining with their free-standing capability and cost efficiency,make the current composite electrode viable for HER applications.
基金supported by the National Natural Science Foundation of China(51402155 and 21373107)Priority Academic Program Development of Jiangsu Higher Education Institutions(PAPD)(YX03002)+2 种基金Jiangsu National Synergistic Innovation Center for Advanced Materials(SICAM)Foundation of NJUPT(NY217077)PolyU Start-up Fund for New Recruits(No.1-ZE8R)
文摘Durability, rate capability, capacity and tap density are paramount performance metrics for promising anode materials, especially for sodium ion batteries. Herein, a carbon free mesoporous CoTiO3 micro-prism with a high tap density （1.8 gcm^-3） is newly developed by using a novel Co-Ti- bimetal organic framework （BMOF） as precursor. It is also interesting to find that the Co-Ti-BMOF derived carbon-free mesoporous CoTiO3 micro-prisms deliver a superior stable and more powerful Na^＋ storage than other similar reported titania, titanate and their carbon composites. Its achieved ca- pacity retention ratio for 2,000 cycles is up to 90.1% at 5 A g^-1.
基金the National Key R&D Program of China(2018YFB1502101,2019YFB1505101)the Foundation for Innovative Research Groups of the National Natural Science Foundation of China(NSFC51621001)National Natural Science Foundation of China(51771075,51701171)。
文摘Magnesium(Mg)-based alloys have already been widely studied as the hydrogen storage materials because of their high reversible hydrogen storage capacity,low cost,light weight,etc.However,the poor de/hydrogenation kinetic properties dramatically hinder the practical applications.In this work,the MgH_(2)-ANi_(5)(A=Ce,Nd,Pr,Sm,and Y) composites were prepared by a high-energy ball milling method.which can effectively refine the particle size thus improving the kinetic properties.Experimental results reveal that the MgH_(2)-ANi_(5) composites mainly consist of Mg_(2)NiH_(4),MgH_(2) and rare earth(RE) hydride,which will be dehydrogenated to form Mg_(2)Ni,Mg and stable RE hydride reversibly.Accordingly,the asmilled MgH_(2)-ANi_(5)(A=Ce,Nd,Pr,Sm,and Y) composites with various A-elements can respectively contribute to a reversible hydrogen storage capacity of 6.16 wt%,5.7 wt%,6.21 wt%,6.38 wt%,and 6.5 wt%at a temperature of 300℃,and show much better kinetic properties in comparison to the pure MgH_(2) without any additive.In-situ formed Mg_(2) Ni and stable RE hydride(such as CeH_(2.73) and YH_(2)) might act as effective catalysts to significantly improve the hydrogen storage properties of MgH_(2).The present work provides a guideline on improving the kinetic properties of the Mg-based hydrogen storage alloys.
基金supported by the Japan International Cooperation Agency(JICA)in the scope of the Egypt-Japan University of Science and Technology(E-JUST)and special thanks to Alexstone Co.,Ltd.for allowing us to use their machining center for experiments.
文摘The machining of hard-to-cut materials with a high degree of precision and high surface quality is one of the most critical considerations when fabricating various state-of-the-art engineered components.In this investigation,a comprehensive three-dimensional model was developed and numerically simulated to predict kerf profiles and material removal rates while drilling the aluminum-7075-T6 aerospace alloy.Kerf profile and material removal prediction involved three stages:jet dynamic flow modeling,abrasive particle tracking,and erosion rate pre-diction.Experimental investigations were conducted to validate the developed model.The results indicate that the jet dynamic characteristics and flow of abrasive particles alter the kerf profiles,where the top kerf diameter increases with increasing jet pressure and standoff distance.The kerf depth and hole aspect ratio increase with jet pressure,but decrease with standoff distance and machining time.Crosssectional profiles were characterized by progressive edge rounding and parabolic shapes.Defects can be minimized by utilizing high jet pressure and small standoff distance.The material removal rate increases with increasing jet pressure,abrasive particle size,and exposure time,but decreases with increasing standoff distance.
基金financially supported by the National Natural Science Foundation of China(Nos.51701171 and 51971187)the Partner State Key Laboratories in Hong Kong from the Innovation and Technology Commission(ITC)of the Government of the Hong Kong Special Administration Region(HKASR),Chinafinancial support from the PolyU Research Office(Project Code:1-BBXA)。
文摘Severe plastic deformation(SPD)-induced gradient nanostructured(GNS)metallic materials exhibit superior mechanical performance,especially the high strength and good ductility.In this study,a novel high-speed machining SPD technique,namely single point diamond turning(SPDT),was developed to produce effectively the GNS layer on the hexagonal close-packed(HCP)structural Mg alloy.The high-resolution transmission electron microscopy observations and atomistic molecular dynamics simulations were mainly performed to atomic-scale dissect the grain refinement process and corresponding plastic deformation mechanisms of the GNS layer.It was found that the grain refinement process for the formation of the GNS Mg alloy layer consists of elongated coarse grains,lamellar fine grains with deformation-induced-tension twins and contraction twins,ultrafine grains,and nanograins with the grain size of~70 nm along the direction from the inner matrix to surface.Specifically,experiment results and atomistic simulations reveal that these deformation twins are formed by gliding twinning partial dislocations that are dissociated from the lattice dislocations piled up at grain boundaries.The corresponding deformation mechanisms were evidenced to transit from the deformation twinning to dislocation slip when the grain size was below 2.45μm.Moreover,the Hall-Petch relationship plot and the surface equivalent stress along the gradient direction estimated by finite element analysis for the SPDT process were incorporated to quantitatively elucidate the transition of defo rmation mechanisms during the grain refinement process.Our findings have implications for the development of the facile SPD technique to construct high strength-ductility heterogeneous GNS metals,especially for the HCP metals.
基金financially supported by the Swedish Governmental Agency for Innovation Systems(Vinnova grant No.2016–05175)the Center for Additive Manufacturingmetal(CAM2)+2 种基金financially supported in part by a research grant from Science Foundation Ireland(SFI)under Grant No.16/RC/3872cofounded under the European Regional Development Fund and by the I-Form industry partnersfinancial support(Grant No.X210141TL210)。
文摘We have investigated the low cycle fatigue(LCF)properties and the extent of strengthening in a dense additively manufactured stainless steel containing different volume fractions of cell structures but having all other microstructure characteristics the same.The samples were produced by laser powder bed fusion(L-PBF),and the concentration of cell structures was varied systematically by varying the annealing treatments.Load-controlled fatigue experiments performed on samples with a high fraction of cell structures reveal an up to 23 times increase in fatigue life compared to an essentially cell-free sample of the same grain configuration.Multiscale electron microscopy characterizations reveal that the cell structures serve as the soft barriers to the dislocation propagation and the partials are the main carrier for cyclic loading.The cell structures,stabilized by the segregated atoms and misorientation between the adjacent cells,are retained during the entire plastic deformation,hence,can continuously interact with dislocations,promote the formation of nanotwins,and provide massive 3D network obstacles to the dislocation motion.The compositional micro-segregation caused by the cellular solidification features serves as another nonnegligible strengthening mechanism to dislocation motion.Specifically,the cell structures with a high density of dislocation debris also appear to act as dislocation nucleation sites,very much like coherent twin boundaries.This work indicates the potential of additive manufacturing to design energy absorbent alloys with high performance by tailoring the microstructure through the printing process.
基金This project is funded by World Rugby and the Engineering and Physical Sciences Research Council(EPSRC).
文摘Purpose To develop an understanding of the role of shoulder padding in rugby union by investigating player perceptions and attitudes towards shoulder padding and extending research into shoulder injuries in rugby.Methods An online survey was distributed to past and current rugby players over 13 years old in 2018.Questions related to the participants’demographic,attitudes to shoulder padding and shoulder injury history.Results Six hundred and sixteen rugby players responded to the survey;66.1%of respondents had worn shoulder padding at some point.The age group 24-29 years old(ΔR^(2)=0.03,B=−0.53,P=0.015)had an inverse association with padding effectiveness while playing experience groups 1-2 years(ΔR^(2)=0.03,B=0.8,P=0.032),3-5 years(ΔR^(2)=0.03,B=0.70,P=0.002)and 6-9 years(ΔR^(2)=0.03,B=0.41,P=0)had a positive association.There are 37.1%of respondents consider-ing shoulder padding to be effective at preventing cuts and abrasions with 21.9%finding it very effective;50.3%considered it to be effective or very effective(9.7%)at preventing contusion;45.5%wore padding for injury prevention,while 19.2%wore padding to protect from reoccurring injury.Sprain/ligament damage(57.5%)and bruising(55.5%)were the most com-monly reported injuries.Conclusions The primary reason for wearing shoulder padding was as a means of injury prevention.Research should focus on quantifying the injury preventive capabilities of shoulder padding.Bruising,cuts and abrasion injuries to the shoulder are prevalent presenting new findings that these injuries are underreported.
