Effect of Temperature on Material Transfer Behavior at Different Stages of Friction Stir Welded 7075-T6 Aluminum Alloy

In this work, the morphologies of weld of 7075-T6 aluminum alloy via friction stir welding （FSW） were analyzed by optical microscopy, the temperature field was attained by numerical simulation, and the effect of temperature on material transfer behavior in the thermal-mechanical affected zone （TMAZ） at different stages was mainly investigated. The FSW process consists of three stages. It is very interesting to find that the maximum transfer displacement of material appears at the final stage of welding process, then at the stable stage and at the initial stage, which results from the difference of peak temperatures at different stages. At any stage, the material in TMAZ near the surface of weld transfers downwards, the material in the middle of weld moves upwards and the material near the bottom of weld hardly moves. In any cross section of weld, the largest transfer displacement of material appears in the middle of weld. The increase of rotational velocity and the decrease of welding speed are both beneficial to the transfer displacement of material in the middle of weld.     

Achieving Large-area Bulk Ultrafne Grained Cu via Submerged Multiple-pass Friction Stir Processing

Large-area bulk ultrafine grained (UFG) pure Cu was successfully prepared by multiple-pass overlapping friction stir processing(FSP) under additional rapid cooling.Overlapping FSP did not exert a significant effect on the microstructure and mechanical properties of the FSP UFG Cu.Similar average grain size was achieved in the transitional zone(TZ) of the multiple-pass FSP sample compared to that in the nugget zone of the single-pass FSP sample,and the TZ exhibited a strong{111}〈112〉type A fiber shear texture.Very weak softening occurred in the TZ of the multiple-pass FSP UFG Cu,resulting in a relatively uniform hardness distribution throughout the whole processed zone.A high yield strength of w310 MPa and a uniform elongation of w13%were achieved in the bulk FSP UFG Cu.This study provides an effective strategy to prepare large-area bulk UFG materials.     

Microstructure and Mechanical Properties of a Dissimilar Friction Stir Weld between Austenitic Stainless Steel and Low Carbon Steel

Dissimilar fusion welding of austenitic stainless steels to carbon steels has some metallurgical and technical problems.It was suggested that the solid-state nature of friction stir welding(FSW) can overcome these problems and produce a sound weld with reliable mechanical properties.In this study,plates of 304 stainless steel and st37 steel were welded together by FSW at tool rotational speed of 600 r/min and welding speed of 50 mm/min.In the stir zone(SZ) of 304 stainless steel,the results showed a refined grain structure with some features of metadynamic recrystallization.In the SZ of st37 steel,the hot deformation of material in the austenite region produced small austenite grains.These grains transformed to fine ferrite and pearlite by cooling the material after FSW.The production of fine grains increased the hardness and tensile strength in the SZ of both sides with respect to their base metals(BMs).     

Effect of Yttrium Addition on Microstructural Characteristics and Superplastic Behavior of Friction Stir Processed ZK60 Alloy

As-extruded ZK60 and ZK60-Y magnesium alloy plates were successfully processed via friction stir processing （FSP） at a tool rotation rate of 1600 r/rain and a traverse speed of 200 mm/min. FSP resulted in the formation of equiaxed recrystallized microstructures with the average grain sizes of ,-8.5 and -4.7 μm in the ZK60 and ZK60-Y alloys, respectively. Moreover, FSP broke and dispersed the MgZn2 and W-phase （Mg3Zn3Y2） particles and dissolved MgZn2 phase in the FSP ZK60 alloy. With the addition of rare earth element yttrium （Y） into the ZK60 alloy, the ratio of the high angle grain boundaries （HAGBs） in the FSP alloys increased from 64% to 90%, and a certain amount of twins appeared in the FSP ZK60-Y alloy. The maximum elongation of 1200% and optimum strain rate of 3 X 10-3 s-1 achieved at 450 °C in the FSP ZK60-Y alloy were substantially higher than those of the FSP ZK60 alloy. This is attributed to the fine grains with high ratio of HAGBs and the distribution of a large number of dispersed second phase particles with high thermal stability in the FSP ZK60-Y alloy. Grain boundary sliding was identified as the primary deformation mechanism in the FSP ZK60 and ZK60-Y alloys from the superplastic data analyses and surficial morphology observations.     

Characterisation of Photocathodes Based on Pb Thin Film Deposited by UV Pulsed Laser Ablation简

This work deals with the deposition of lead(Pb) thin films by the U V pulsed laser ablation technique,for their further use as photocathode devices in superconducting radio frequency guns.Scanning electron microscopy and atomic force microscopy analyses were performed to study the morphological features of Pb thin films deposited on Si(100) and Nb substrates.The films showed a granular structure with a nearly fully covered surface only for that one deposited on Nb substrata X-ray diffraction measurements indicate the growth of polycrystalline Pb thin films with a preferential orientation along(111) planes.Results of the photoemission performance of Pb thin film deposited on Nb substrate showed a very encouraging average value of quantum efficiency of 6×10~(-5) through a single-photon absorption process,promoting further studies in the realisation of Pb photocathodes by this techniqua     

Evolution of the Microstructure and Strength in the Nugget Zone of Friction Stir Welded SiCp/Al-Cu-Mg Composite简

A 6 mm-thick SiCp/2009AI composite plate was successfully joined by friction stir welding （FSW） using an ultrahard material tool to investigate the evolution of the microstructure and the strength in the nugget zone （NZ）. While some SiC particles were broken up during FSW, most of them rotated in the matrix. Large compound particles on the interfaces were broken off during FSW, whereas the amorphous layer and small compound particles remained on the interfaces. The dynamically recrystallized AI grains nucleated on the surface of fractured SiC particles during FSW, forming nano-sized grains around the SiC particles. The yield strength of the NZ decreased slightly due to the variation in the size, shape, and distribution of the SiC particles. The clean interfaces were beneficial to the load transfer between SiC particles and AI matrix and then increased the ultimate tensile strength of the NZ.     

Fabrication of (Ti,Hf)-rich NiTiHf Alloy Using Graphitic Mold and Crucible

In this research, fabrication of a(Ti,Hf)-rich NiTiHf alloy by using vacuum induction melting(VIM) process and a graphitic crucible was investigated. For this purpose, casts with the nominal composition of Ni49Ti36Hf15 were prepared in graphitic crucible and mold. Optical microscopy(OM), scanning electron microscopy(SEM), energy dispersive X-ray spectroscopy(EDS), X-ray diffraction(XRD), and differential scanning calorimetry(DSC) tests were employed to characterize the samples. Results demonstrated that microstructure of the first cast was composted of a B2 austenite phase as well as a great amount of two differently formed(Ti,Hf)C carbides.Moreover, no austenite 4 martensite transformation peak was detected in the DSC curve of this sample,indicating a drastic decline in the transformation temperatures. In the succeeding cast, however, owing to the formation of carbide layers on the inner surfaces of the graphitic crucible and mold during the initial casting process, the amounts of carbides decreased remarkably. This cast exhibited transformation temperatures above100° C, while XRD pattern denoted the presence of B190 monoclinic martensite phase at room temperature.All in all, results confirmed that VIM process using graphitic mold and crucible can be considered as an appropriate method for the fabrication of(Ti,Hf)-rich NiTiHf high temperature shape memory alloys.     

Development and Characterization of Al2O3 Dispersed Al/Mg/Cu/Ti Matrix Composite

The effect of dispersion with different weight fractions of Al2O3 particles in metallic matrices （AI/Mg/Ti/Cu） fabricated by powder metallurgy was investigated. In the case of 15 wt% Al2O3 reinforced composites, peak hardness was attained which subsequently decreased with increasing the content of Al2O3. A correlation between the microhardness and nanomechanical properties at submicron scale was examined for all the composites. Specific strength and specific modulus were measured in order to figure out the performance of the composites.     

Effects of Strontium,Magnesium Addition,Temperature Gradient,and Growth Velocity on AI-Si Eutectic Growth in a UnidirectionaUy-solidified Al-13 wt% Si Alloy

Al-Si eutectic growth mechanism was investigated in a directionally-solidified AI-1 3 wt% Si alloy with different strontium （Sr） and magnesium （Mg） additions, growth velocities and temperature gradients. Macro- and micro- scale metallographic analyses revealed that addition level of Sr and Mg, temperature gradient and growth velocity are important factors affecting stability of solidifying AI-Si eutectic front and the final morphology of eutectic grains in the solidified A1-13 wt% Si alloys. By varying （tailoring） these factors, a variety of eutectic grain structures and morphologies such as planar front, cellular structure, a mix of cellular and columnar, or equiaxed dendrites, can be obtained. Increasing temperature gradient, reducing growth velocity, or decreasing Sr and Mg contents is beneficial to stabilizing planar growth front of eutectic grains, which is qualitatively in accordance with constitutional supercooling criterion for binary eutectic growth. In contrast, adding more Sr and Mg, increasing growth velocity, or decreasing temperature gradient produces large constitutional supercooling, leading to columnar-equiaxed transition （CET） of eutectic structure, which can be interpreted on the basis of Hunt＇s Model. It is also found that both solute concentration and solidification variables have significant impact not only on eutectic growth, but also on gas porosity formation.     

Microstructure,Phase Transformation,Precipitation Behavior and Mechanical Properties of P/M Cu40Zn-1.0 wt% Ti Brass Alloy via Spark Plasma Sintering and Hot Extrusion

The effect of titanium addition on the microstructure and mechanical properties of brass Cu4OZn has been studied via the powder metallurgy （P/M） route. The water-atomized Cu4OZn-1.0 wt% Ti alloy powder was consolidated at different temperatures in the range of 400-600℃ using spark plasma sintering （SPS） and hot extrusion subsequently. Results show that the super-saturated solid solution titanium element in rapidly cooled brass Cu4OZn powder created high chemical potential for a precipitate reaction, showing significant grain refinement effects on the consolidated Cu4OZn matrix. Consequently, excellent mechanical properties were obtained by precipitation hardening and work hardening after sintering and extrusion, with yield strength of 390 MPa, ultimate tensile strength of 617 MPa, and Vickers micro-hardness of 192 HV, which are 28.7%, 23.4%, and 23.9% higher values than those of extruded Cu4OZn brass, respectively.     

Role of Double Oxide Film Defects in the Formal ion ol’ Gas Porosity in Commercial Purity and Sr-containing A1 Alloys

The role of double oxide film （bifilm） defects in the formation of gas porosity in commercial purity and Srcontaining AI alloys was investigated by means of a reduced pressure test （RPT） technique. The liquid metal was poured from a height into a crucible to introduce oxide defects into the melt. The melt was then subjected to different ＂hydrogen addition＂ and ＂holding in liquid state＂ regimes before RPT samples were taken. The RPT samples were then characterized by determining their porosity parameters and examining the internal surfaces of the pores formed in them by scanning electron microscopy. The results indicated oxide defects as the initiation sites for the growth of gas porosity, both in commercial purity and Sr-containing AI alloys. The results also rejected reduction of the surface tension of the melt, increase in the volumetric shrinkage and reduction in interdendritic feeding as the possible causes of an increase in the porosity content of the AI castings modified with strontium. The change in the composition of the oxide layers of double oxide film defects was suggested to be responsible for this behaviour.     

A Comparative in vitro Study on Biomedical Zr-2.5X(X=Nb,Sn) Alloys

Nb and Sn are major alloying elements in Zr alloys.In this study,the microstructure,mechanical properties,corrosion behavior,cytocompatibility and magnetic resonance imaging(MRI) compatibility of Zr-2.5X(X = Nb,Sn) alloys for biomedical application are comparatively investigated.It is found that Zr—2.5Nb alloy has a duplex structure of α and β phase and Zr—2.5Sn alloy is composed of 7.phase.Both separate addition of Nb and Sn can strengthen Zr but Nb is more effective in strengthening Zr than Sn.The studied Zr—2.5X(X = Nb,Sn) alloys show improved corrosion resistance compared to pure Zr as indicted by the decreased corrosion current density.The alloying addition of Nb enhances the pitting resistance of Zr,whereas the addition of Sn decreases the pitting resistance of Zr.The extracts of Zr—2.5X alloys produce no significant deleterious effect on fibroblast cells(L-929) and osteoblast-like cells(MG 63),indicating good in vitro cytocompatibility.The Zr—2.5X(X = Nb,Sn) alloys show decreased magnetic susceptibility compared to pure Zr and their magnetic susceptibility is far lower than that of pure Ti and Ti—6AI—4V alloy.Based on these facts,Zr-2.5Nb alloy is more suitable for implant material than Zr-2.5Sn alloy.Sn is not suitable as individual alloying addition for Zr because Sn addition decreases the pitting resistance in physiological solution.     

Abrasion Resistance Enhancement of Ultrafine-structured WC-Co Coating Fabricated by using in situ Synthesized Composite Powder

The ultrafine WC-Co composite powder was synthesized by a newly developed rapid route based on in situ reactions. By using the as-synthesized composite powder, the granulation processing was then carried out to prepare the ultrafine-structured thermal spraying feedstock. The influences of the heat-treatment process on density of the feedstock powder, phase constitution and wear resistance of the resultant WC-Co coatings fabricated by high velocity oxy-fuel （HVOF） were investigated. The results showed that increasing the heating temperature and extending the holding time leaded to remarkable increase in the density and flowability of the feedstock powder. As a result, the decarburization of the in-flight particles could be decreased and the wear resistance of coating was significantly enhanced. The present study demonstrated that the developed techniques for the ultrafine powder and its thermal-sprayed coatings had very promising applications in scaling up to produce ultrafine-structured cermet coatings with excellent performance.     

Role of β Phase during Microarc Oxidation of Mg Alloy AZ91D and Corrosion Resistance of the Oxidation Coating

Selective growth of oxidation coating was observed on Mg alloy AZ91 D when this alloy was treated by microarc oxidation （MAO） technique, and then the role of intermetallic phase Mg17AI12 （β phase） during MAO was investigated. Corrosion resistance and anti-corrosion mechanism of the MAO coating were also studied. Optical microscopy and scanning electron microscopy （SEM） were used to characterize β phase and coating microstructure. Corrosion properties of the coated alloy were studied by potentiodynamic polarization test and electrochemical impedance spectroscopy （EIS） in 3.5% NaCI solution. Results showed that sparking discharge preferentially occurred on ~-Mg phase rather than on β phase at the early stage of MAO; however selective growth of the coating disappeared gradually with the increasing oxidation time and β phase would not further inhibit coating growth at the prolonged stage of MAO. MglTAI12 phase ultimately was unable to destroy the integrity and continuity of MAO coating. The MAO coating could restrain charge transfer process and then greatly enhance corrosion resistance of AZ91D alloy. Sealing treatment of MAO coating by stearic acid could further improve the corrosion resistance of AZ91 D alloy.     

Fabrication of Shape-Controlled Au Nanoparticles in a TiO2-Containing Mesoporous Template Using UV Irradiation and Their Shape-Dependent Photocatalysis简

A SiO_2-TiO_2 template with ordered tubular mesochannels has been prepared by the sol—gel method.Au nanorods are deposited in the tubular mesochannels of the SiO_2—TiO_2 template,and the shape of Au is changed from nanorods to nanospheres by ultraviolet irradiation during thermal deposition.The photocatalytic activity of mesoporous SiO_2—TiO_2 with/without Au nanorods/nanospheres is evaluated.Deposition of Au in the mesoporous SiO_2—TiO_2 template enhances the photocatalysis of TiO_2.Interestingly,the sample containing Au nanorods exhibits higher photocatalytic activity than that with Au nanospheres.Photocatalysis by exciting surface plasmon resonance is not detected in the composite samples regardless of the shape of the deposited Au nanoparticles.     

Isothermal Oxidation Behavior of Dysprosium/S-Doped β-NiAl Alloys at 1200°C

A β-NiAl alloy with normal purity, a S-doped and a Dy and S co-doped b-NiAl alloys were prepared by arc-melting and their corresponding S contents were less than 20×10 6, 33×10-6 and 22×10-6, respectively. The isothermal oxidation behavior of the alloys at 1200° C was investigated and the extent of S segregation at the scalee alloy interface was determined by scanning Auger microscopy. S-doping had no significant effect on the phase transformation rate from q- to a-Al2O3, while the addition of Dy retarded this process. For the Sdoped alloy, scale rumpling occurred only after 2 h thermal exposure and numerous large voids were observed at the scalee alloy interface where S segregated. In contrast to this, the oxide scale formed on the Dy and S co-doped alloy still remained flat even after 50 h isothermal oxidation and only small voids existed at the interface where S segregation was not detected.     

One Step Synthesis and Growth Mechanism of Carbon Nanotubes

A simple one step,reproducible,synthesis route for carbon nanotubes was proposed.No external catalyst was used for the synthesis.These nanotubes were obtained after decomposition of acetone at 650 ℃ in a specially designed autoclave.The pressure generated due to decomposition of acetone played a vital role in the synthesis.The X-ray diffraction pattern and transmission electron microscopy of the sample showed that the diameter of nanotubes is in the range of 3—14 nm.The thermo gravimetric analysis showed 3%weight loss below500 ℃;the content of amorphous carbon is very less.The growth mechanism of CNTs was also proposed in the present paper.     

Optical and Microstructural Characterisations of Pulsed rf Magnetron Sputtered Alumina Thin Film

Alumina thin films were deposited on fused quartz and SS304 substrate by pulsed rf magnetron sputtering with both direct and reactive methods. The films were characterised by energy dispersive X-ray spectroscopy, X-ray diffraction, scanning electron microscopy, field emission scanning electron microscopy and atomic force microscopy to reveal the microstructure, surface morphology and topography of thin films. Transmittance and reflectance of alumina thin film were evaluated after deposition on the quartz substrate. Transmittance of the quartz remains almost un-altered when alumina was deposited by the reactive sputtering. A marginal decrease of ~4% in the transmittance of quartz was, however, observed after deposition of alumina by direct sputtering. Infrared emittance of the substrate also remains almost constant after deposition of thin alumina film. Further, as-deposited alumina on SS304 obtained by both direct and reactive sputtering process was amorphous in nature. However, after annealing crystalline peaks were observed.     

Enhancement of Electric Conductivity in Transparent Glasse—Ceramic Nanocomposites of Bi2O3—BaTiO3 Glasses

The frequency and temperature dependent electrical conductivity measurements for heat-treated binary glass system with composition of （lO0-x）Bi203-xBaTi03 （x = 20, 30, 40 and 50, in mol%） were carried out. The glass was prepared by melt quenching technique and their corresponding glass-ceramic nanocomposites were obtained by suitable heat treatment. Nanostructured behavior and electrical properties of these glasses and their corresponding glass-ceramic nanocomposites were studied. X-ray diffraction （XRD） and differential scanning calorimetry confirmed the amorphous nature of the glasses. Moreover, XRD patterns of the samples indicate nanocrystallites embedded in the glass matrix. The Fourier transform infrared spectroscopy （FT-IR） spectral analysis showed that the band positions of glass system are within the wave number range of Bi06, Bi03 and Ti06 structural units. It is observed that the electrical conductivity is enhanced by 102-103 times in the transparent glass-ceramic nanocomposite phase. With further heat treatment, the conductivity decreased considerably in the stage of glass-ceramic nanocomposite phase as compared with the glassy phase sample. Therefore, partially devitrified phase is more suitable as cathode material in secondary batteries compared to its vitreous or fully crystalline counterpart. The conduction mechanism was confirmed to obey the adiabatic small polaron hopping （SPH）. AC conductivity measurements were performed as a function of temperature and frequency, showing a very slow increasing rate at low temperatures and then a fast rate at higher temperatures.     

Influence of Features of Interphase Boundaries on Mechanical Properties and Fracture Pattern in Metal--Ceramic Composites

The results of a theoretical study on the influence of strength of interphase boundaries in metal-ceramic composite on macroscopical characteristics of composite response such as strength, deformation capacity, fracture energy and fracture pattern are presented. The study was conducted by means of computer-aided simulation by means of movable cellular automaton method taking account of a developed ＂mesoscopical＂ structural model of particle-reinforced composite. The strength of interphase boundaries is found to be a key structural factor determining not only the strength properties of metal-ceramic composite, but also the pattern and rate of fracture. The principles for achievement of the high-strength values of particle/binder interfaces in the metal-ceramic composition due to the formation of the wide transition zones （areas of variable chemical composition） at the interphase boundaries are discussed. Simulation results confirm that such transition zones provide a change in fracture mechanism and make the achievement of a high-strength and a high deformation capacity of metal-ceramic composite possible.