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ISSN 1005-0302
CN 21-1315/TG
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      28 January 2002, Volume 18 Issue 01 Previous Issue    Next Issue
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    Research Articles
    Nucleation Mechanism for Bainite
    Xi ZHANG, Xuejun JIN, T.Y.HsuXu Zuyao
    J. Mater. Sci. Technol., 2002, 18 (01): 1-3. 
    Abstract   HTML   PDF (384KB)
    Based on the experimental results that solute-depleted zone was observed in Cu-28Zn-4Al (mass fraction) at 523 K, △G is calculated as a positive according to the thermodynamic criteria for the spinodal decomposition of a ternary systems. So, the solute-depleted zone cannot be formed by spinodal decomposition. Dislocation density required by the formation of solute-depleted zone is estimated greater than 7.89×109 cm-2, according to the segregation of solute atoms around dislocations, which is not consistent with the practical situation for the alloy at 523 K. Associated with the internal friction experimental fact that internal friction peaks appear within the incubation for bainitic transformation in Cu-Zn-Al alloy, the equilibrium temperature, T0, is evaluated as 433 K for solute-depleted Cu-25Zn-3.4Al, which is lower than the experimental temperature 523 K. Thus, nucleation by shear mechanism is impossible in this circumstance. Therefore, it is concluded that, like bainite in steels and Ag-Cd, bainite in Cu-Zn-Al alloys nucleates by diffusional mechanism, just implied by the experimental existence of solute-depleted zone.
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    Stress Corrosion Cracking of Pipeline Steels
    Bingyan FANG, Enhou HAN, Ziyong ZHU, Jianqiu, WANG, Wei KE
    J. Mater. Sci. Technol., 2002, 18 (01): 3-6. 
    Abstract   HTML   PDF (1045KB)
    This paper provides a review of the differences between high pH and near-neutral pH stress corrosion cracking of pipeline steels, influencing factors, and mechanisms. The characteristics and historical information on both forms of SCC are discussed. The prospect for research in the future is also presented.
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    Oxide and Sulfide Dispersive Precipitation and Effects on Microstructure and Properties of Low Carbon Steels
    Delu LIU, Jie FU, Yonglin KANG, Xiangdong HUO, Yuanli WANG, Nanjing CHEN, Zhongbing WANG, Liejun LI
    J. Mater. Sci. Technol., 2002, 18 (01): 5-9. 
    Abstract   HTML   PDF (1040KB)
    Electron microscopic investigation on low carbon steel strips produced by the CSP process has been carried out. Large number of oxide dispersive precipitates have been observed in the ferrite matrix of the steel strips. Dimension of them is about 10~20 nm. Electron diffraction study showed that the structure of these precipitates consists with cubic system spinel structure. Their lattice parameter is about 0.83 nm. The results implied that they should be complex oxides of Fe, Al et al. Small sulfide particles with 100~300 nm in size have also been observed. Remarkable strengthening and grain refinement effects can be obtained by the precipitations. The oxygen and sulfur in steels could play beneficial role under certain conditions.
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    Investigation of Sintering Technology for Composite of Stainless Steel and Partially Stabilized Zirconia
    Wenquan ZHANG, Jianxin XIE, Changjing ZHANG, Zhiguo YANG
    J. Mater. Sci. Technol., 2002, 18 (01): 10-12. 
    Abstract   HTML   PDF (631KB)
    The sintering technology for mixed powdered extrusion rods of different proportions of stainless steel to magnesia partially stabilized zirconia (PSZ) was investigated. The effects of some sintering parameters including holding time, atmosphere and protective gas pressure on shrinkage, relative density, microstructure, micro-Vickers hardness and compression strength of sintered samples were mainly researched. The experimental results are as follows: (1) The shrinkage and the relative density of the sintered samples decrease as increasing stainless steel content in the composite, except for the case containing 90 percent of stainless steel; (2) The porosity in PSZ matrix rises as increasing the stainless steel content in the composite; (3) Longer sintering holding time, higher sintering vacuum and gas-pressure sintering process not only enhance the relative density, but also improve microstructure of composite; (4) Micro-Vickers hardness of PSZ matrix decreases as increasing stainless steel content, while that of stainless steel particles in sintered samples varies unnoticeably.
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    Experimental Study on Non-proportional Multiaxial Strain Cyclic Characteristics and Ratcheting of U71Mn Rail Steel
    Guozheng KANG, Qing GAO, Lixun CAI, Yafang SUN, Xianjie YANG
    J. Mater. Sci. Technol., 2002, 18 (01): 13-16. 
    Abstract   HTML   PDF (720KB)
    An experimental study was carried out on the strain cyclic characteristics and ratcheting of U71Mn rail steel subjected to non-proportional multiaxial cyclic loading. The strain cyclic characteristics were researched under the strain-controlled circular load path. The ratcheting was investigated for the stress-controlled multiaxial circular, elliptical and rhombic load paths with different mean stresses, stress amplitudes and their histories. The experiment shows that U71Mn rail steel features the cyclic non-hardening/softening, and its strain cyclic characteristics depend greatly on the strain amplitude but slightly on its history. However, the ratcheting of U71Mn rail steel depends greatly not only on the values of mean stress and stress amplitude, but also on their histories. In the meantime, the shape of load path and its history also apparently influence the ratcheting. The ratcheting changes with the different loading paths.
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    Laser Surface Hardening of 9CrSi Steel
    Nunghee RYU, Sunggoon KANG
    J. Mater. Sci. Technol., 2002, 18 (01): 17-20. 
    Abstract   HTML   PDF (4452KB)
    The effects of laser hardening parameters such as beam power, beam diameter and scanning rate on microstructure and hardness of 9CrSi steel were investigated. The microstructure of the surface layer of 9CrSi steel was changed from pearlite to martensite, retained austenite and carbide by laser hardening. The depth of the hardened layer increased with increasing laser energy density and the surface hardeness increased by 3~5 times as high as the untreated steel. The laser hardened surface had good wear resistance due to martensite and carbide in the surface layer. The wear mode at low speed was abrasive, while the wear mode at high speed was adhesive.
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    Ultrasonic Analysis of Cracking Propagation Morphology in the Fusion Zone of High Strength Steel
    Yajiang LI, Zengda ZOU, Dezhen LIU, Huiqiang WU, Juan WANG
    J. Mater. Sci. Technol., 2002, 18 (01): 21-23. 
    Abstract   HTML   PDF (164KB)
    Cracking morphology in the fusion zone of HQ130 high strength steel was researched by "the y-silt test" and "three point bend test", ultrasonic test and microscope. HQ130+QJ63 high strength steel was welded by Ar+CO2 gas shielded arc welding under the condition of no-preheat. Experimental results indicated that welding crack produced in the partially melted zone of the weld root of HQ130 steel side and propagated parallel to the fusion zone. The cracking developed alternatively between the weld and the partially melted zone, and is not strictly to rupture at W/F boundary surface. Controlling weld heat input (E) about 16 kJ/cm could make the cracking rate to be lowest and satisfy the performance requirement of welded joint zone.
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    AFM Studies of Platinum Silicide Thin Films on Silicon Grown by Pulsed Laser Deposition
    Meicheng LI, Liancheng ZHAO, Wei CAI, Dage LIU
    J. Mater. Sci. Technol., 2002, 18 (01): 24-26. 
    Abstract   HTML   PDF (137KB)
    PtSi ultra-thin films were grown on Si-wafer using pulsed laser deposition (PLD). The surface structure of these films was studied by atomic force microscopy (AFM). In addition, the compositional structure of the PtSi as determined from X-ray photoelectron spectroscopy (XPS) is discussed. First report of a possible growth mechanism is presented, on studying the variation of morphological features (i.e., roughness and size of crystallites) with annealing temperatures and the film thicknesses.
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    Oxidation and Hot Corrosion of Gradient Thermal Barrier Coatings Prepared by EB-PVD
    Hongbo GUO, Shengkai GONG, Huibin XU
    J. Mater. Sci. Technol., 2002, 18 (01): 27-30. 
    Abstract   HTML   PDF (221KB)
    The performances of gradient thermal barrier coatings (GTBCs) produced by EB-PVD were evaluated by isothermal oxidation and cyclic hot corrosion (HTHC) tests. Compared with conventional two-layered TBCs, the GTBCs exhibite better resistance to not only oxidation but also hot-corrosion. A dense Al2O3 layer in the GTBCs effectively prohibites inward diffusion of O and S and outward diffusion of Al and Cr during the tests. On the other hand, an "inlaid" interface, resulting from oxidation of the Al along the columnar grains of the bond coat, enhances the adherence of Al2O3 layer. Failure of the GTBC finally occurred by cracking at the interface between the bond coat and Al2O3 layer, due to the combined effect of sulfidation of the bond coat and thermal cycling.
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    Nanostructure Study of TiO2 Films Prepared by Dip Coating Process
    S.M.Attia, Jue WANG, Guangming WU, Jun SHEN, Jianhua MA
    J. Mater. Sci. Technol., 2002, 18 (01): 31-33. 
    Abstract   HTML   PDF (298KB)
    The microstructure properties of the sol-gel derived TiO2 films were studied by the atomic force microscopy (AFM). The films were prepared by dip coating process. The optical properties of the films were explained on the basis of the microstructure of the films.
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    Preparation and Properties of Sintering Additives Coated Si3N4 from Heterogeneous Nucleation Processing
    Juxian ZHANG, Jinjun FENG, Longqiao GAO
    J. Mater. Sci. Technol., 2002, 18 (01): 34-36. 
    Abstract   HTML   PDF (535KB)
    The sintering additives such as Al2O3 and/or Y2O3 were coated on the surfaces of Si3N4 particles via heterogeneous nucleation processing using a buffered pH solution as the precipitation reagent. They nucleated and grew only on the surfaces of Si3N4 and did not form sol particles in solution by TEM observation. The isoelectric point (IEP) of coated Si3N4 was different from that of as-received Si3N4. The IEP of Al(OH)3-coated Si3N4 occurred at pH8.4, which is close to that of alumina. When Al(OH)3-coated Si3N4 particles were coated with Y(OH)3, the IEP of coated Si3N4 powder shifted from pH8.4 to pH9.2, similar to that of yttria. In addition, the rheological data showed that Al2O3 and/or Y2O3 coated Si3N4 suspension is nearly Newtonian and that added Si3N4 suspension shows a shear rate thinning behavior.
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    Influence of Electrolytes on the Microstructure of Microarc Oxidation Coatings on Ti-6Al-4V Alloy
    Wenbin XUE, Chao WANG, Zhiwei DENG, Ruyi CHEN, Tonghe ZHANG
    J. Mater. Sci. Technol., 2002, 18 (01): 37-39. 
    Abstract   HTML   PDF (837KB)
    Dense ceramic oxide coatings of about 50 μm may be prepared on Ti-6Al-4V alloy surface by alternating-current microarc oxidation in aluminate and silicate solutions, respectively. Their phase constituent and microstructure were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that the electrolytic ions have incorporated into the interior of the coatings. The coating formed in aluminate solution is composed of the rutile TiO2 and TiAl2O5 phases rather than the rutile, anatase and amorphous SiO2 phases for the coating formed in silicate solution. However, TiAl¬2O5 content in the outer layer of the two coatings is much higher than in the inner layer. It is suggested that all these oxides may result from a rapid solidification of the molten coating in the microarc discharge zone.
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    Prediction of Austenitization and Homogenization of Q235 Plain Carbon Steel during Reheating Process
    Chunli MO, Dianzhong LI, Bainian QIAN, Yiyi LI
    J. Mater. Sci. Technol., 2002, 18 (01): 39-42. 
    Abstract   HTML   PDF (1289KB)
    In this paper, the austenitization and homogenization process of Q235 plain carbon steel during reheating is predicted using a two-dimensional model which has been developed for the prediction of diffusive phase transformation (e.g. α to γ). The diffusion equations are solved within each phase (α and γ) and an explicit finite volume technique formulated for a regular hexagonal grid are used. The discrete interface is represented by special volume elements α/γ, an volume element α undergoes a transition to an interface state before it becomes γ. The procedure allows us to handle the displacement of the interface while respecting the flux condition at the interface. The simulated microstructure shows the dissolution of ferrite particles in the austenite matrix is presented at different stages of the phase transformation. Specifically, the influence of the microstructure scale and the heating rate on the phase transformation kinetics has been investigated. The experimental results agree well with the simulated ones.
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    Numerical Simulation of the TIG Welding Arc Behavior
    Chuansong WU, Jinqiang GAO
    J. Mater. Sci. Technol., 2002, 18 (01): 43-46. 
    Abstract   HTML   PDF (622KB)
    Achieving an effective utilization and exploitation of TIG welding arcs require a thorough understanding of the plasma properties and its physical processes. Through simultaneous solutions of the set of conservation equations for mass, momentum, energy and current, a mathematical model has been developed to predict the velocity, temperature, and current density distributions in argon welding arcs. The predicted temperature fields in arc regions, and the distribution of current density and heat flux at the anode agree well with measurements reported in literatures. This work could lay the foundation for developing a comprehensive model of the TIG welding process where a dynamic, two-way coupling between the welding arc and the weld pool surface is properly represented.
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    Optimization of Machining Characteristics for Al/SiCp Composites using ANN/GA
    R.Karthikeyan, R.Adalarasan, B.C.Pai
    J. Mater. Sci. Technol., 2002, 18 (01): 47-50. 
    Abstract   HTML   PDF (676KB)
    The present work is focused on optimization of machining characteristics of Al/SiCp composites. The machining characteristics such as specific energy, tool wear and surface roughness were studied. The parameters such as volume fraction of SiC, cutting speed and feed rate were considered. Artificial neural networks (ANN) were used to train and simulate the experimental data. Genetic algorithms (GA) was interfaced with ANN to optimize the machining conditions for the desired machining characteristics. Validation of optimized results was also performed by confirmation experiments.
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    Numerical Simulations of Equiaxed Dendrite Growth Using Phase Field Method
    Yutuo ZHANG, Dianzhong LI, Yiyi LI, Weicheng PANG
    J. Mater. Sci. Technol., 2002, 18 (01): 51-53. 
    Abstract   HTML   PDF (489KB)
    Phase field method offers the prospect of being able to perform realistic numerical experiments on dendrite growth in a metallic system. In this paper, the equiaxed dendrite evolution during the solidification of a pure material was numerically simulated using the phase field model. The equiaxed dendrite growth in a two-dimensional square domain of undercooled melt (nickel) with four-fold anisotropy was simulated. The phase field model equations were solved using the explicit finite difference method on a uniform mesh. The formation of various equiaxed dendrite patterns was shown by a series of simulations, and the effect of anisotropy on equiaxed dendrite morphology was investigated.
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    Transient Liquid Phase Bonding of Ni-base Single Crystal Superalloy
    Wen LI, Tao JIN, Xiaofeng SUN, Yi GUO, Hengrong GUAN, Zhuangqi HU
    J. Mater. Sci. Technol., 2002, 18 (01): 54-56. 
    Abstract   HTML   PDF (801KB)
    The Ni-base single crystal superalloy was bonded by the transient liquid phase (TLP) bonding, using a Ni-base flexible metal cloth as an insert alloy. TLP bonding of superalloy was carried out at 1473~1523 K for 0.5~24 h in vacuum. The [001] orientation of each test specimen was aligned perpendicular to the joint interface. The bonded region was observed by optical microscopy, and the microstructural and compositional analyses across the bonded interlayer were performed by using a scanning electron microscopy (SEM). The electron back scattering diffraction (EBSD) method was applied to determine the crystallographic orientation. The results indicated that the chemical homogeneity across the bonded region can be achieved, and γ′ phase both in the bonded interlayer and in the superalloy substrate is almost identical, while the bonded interlayer had almost matched the crystallographic orientation of the bonded substrates.
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    Preparation of a Cation Exchanger from Cork Waste: Thermodynamic Study of the Ion Exchange Processes
    M.Gómez Corzo, A.Macías-García, M.A. Díaz-Díez, M.Gómez Corzo, M.J.Bernalte Garcí
    J. Mater. Sci. Technol., 2002, 18 (01): 57-59. 
    Abstract   HTML   PDF (653KB)
    An ion exchanger was prepared by sulfonation of cork-waste chars. The exchange properties of the resultant material were characterized using Na+, Ca2+ or Fe3+ aqueous solutions. The content of metal ions in the solutions were determined by atomic absorption spectrometry. On the basis of the results obtained, the chemical equilibrium and its thermodynamic aspects related to the ion exchange process were studied. It was found that equilibrium constant K varies by the order: Na+0 and ΔSο>0, with -ΔGο following the sequence: Ca2+>Na+>Fe3+
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    Reactive Milling and Mechanical Properties of NiAl Composite with HfC DispersoidsFubao
    YANG, Jianting GUO, Jiyang ZHOU
    J. Mater. Sci. Technol., 2002, 18 (01): 59-62. 
    Abstract   HTML   PDF (1010KB)
    A NiAl-based composite with HfB2 dispersed particles has been synthesized by mechanical alloying of Ni, Al, Hf and C powders. The formation mechanism of NiAl-HfC during milling can be attributed to two chemical reactions: Ni+AlNiAl+ΔH; Hf+C→HfC+ΔH, induced by mechanical collision in a certain period of time, which results in an abrupt exothemic reaction. Hot pressing (HP) and hot isostatic pressing (HIP) have been used to make the NiAl-10HfC compacts near fully dense. Compressive testing from room temperature to 1000℃ indicated that the yield stress of NiAl-10HfC composite is 3~4 times higher than that of cast NiAl and correspond to the MA NiAl-10TiB2 composite. In the meantime, yield strength at high temperature is dependent on strain rate, and deformation is controlled by diffusion mechanism.
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    Study on the Thermal Expansion and Thermal Cycling of AlNp/Al Composites
    Qiang ZHANG, Gaohui WU, Dongli SUN, Bofeng LUAN
    J. Mater. Sci. Technol., 2002, 18 (01): 63-65. 
    Abstract   HTML   PDF (665KB)
    The AlN particle reinforced aluminum matrix composites with 50% volume fraction were fabricated by squeeze-casting technology. The thermal expansion behavior and its response to thermal cycling were studied between 20℃ and 400℃. Compared with four theoretical models, the measured CTEs of the composite lie within the elastic bounds derived by Schapery's analysis. Schapery's model and Kerner's model agree well with the CTEs of the composites at lower temperature and elevated temperature, respectively. Strain hysteresis was observed between heating and cooling curves during cycling. This was attributed primarily to the anelastic behavior of the matrix induced by matrix residual stresses.
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    Dislocation Mechanism of Diamond Crystal Growth from Fe-Ni-C System at High Temperature-High Pressure
    Longwei YIN, Musen LI, Dongsheng SUN, Fengzhao LI, Zhaoyin HAO, Xiumei DONG
    J. Mater. Sci. Technol., 2002, 18 (01): 66-68. 
    Abstract   HTML   PDF (933KB)
    Some dislocations, which are generated in the diamond single crystal during the diamond crystal growth from Fe-Ni-C system, may affect diamond crystal growth mode at high temperature-high pressure (HPHT). The concentric dislocation loops were successfully examined by Moiré images. The surface morphologies of growing and as-grown diamond single crystals were observed by scanning electron microscopy (SEM). The concentric dislocation loops formation process and their effect on the diamond crystal growth mode were analyzed. It should be noted that whatever the nature of the dislocation is, should the Burgers vector of dislocation has a component at the direction normal to the growth interface, the dislocation will make the face parallel to the growth interface grow into spiral face. The presence of consecutive spiral steps on the diamond crystal surface also provides a direct evidence of the dislocation mechanism of diamond crystal growth.
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    Influence of Grain Boundary on Fatigue Behavior of Ni-base Bicrystals
    Jiansong WAN, Zhufeng YUE, Xiaoliang GENG, Zhixian LU
    J. Mater. Sci. Technol., 2002, 18 (01): 69-72. 
    Abstract   HTML   PDF (663KB)
    The influence of the grain boundary on the fatigue behavior was studied by two three-point-bending (TPB) specimens. One TPB specimen was named Bicrystal 1, whose pre-crack was along the grain boundary and the applied load paralleled to pre-crack direction, while the other TPB specimen was named Bicrystal 2, whose the pre-crack was perpendicular to the grain boundary and the applied load paralleled also to the pre-crack. It was found that the rate of the fatigue crack growth of Bicrystal 1 was about a tenfold higher than that of Bicrystal 2. The fatigue behavior of Bicrystal 2 specimens was dependent on the distance between the crack tip and grain boundary. The crack growth rate was highest when the crack tip was at a critical distance to the grain boundary, while the rate was the lowest when the crack tip reached grain boundary. After the crack was over the grain boundary, the crack growth rate increased. The crystallographic finite element method was applied to analyze the stress and strain structure ahead of the crack, in order to reveal the above characteristics of the fatigue behavior. It is the grain boundary-induced redistribution of stresses near the crack tip that induces the difference of fatigue behavior.
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    Microstructure Evolution of Cu-Pb Monotectic Alloys Processed in Drop Tube
    Chongde CAO, Bingbo WEI
    J. Mater. Sci. Technol., 2002, 18 (01): 73-76. 
    Abstract   HTML   PDF (1288KB)
    Rapid solidification of Cu-Pb monotectic alloys has been accomplished during free fall in a 3 m drop tube. Both macrosegregated and uniformly dispersed structures are observed in Cu-40 wt pct Pb alloy droplets, whereas droplets of composition Cu-64 wt pct Pb exhibit only macrosegregation morphologies. The microstructures are strongly dependent on droplet size. The higher undercooling tends to facilitate liquid phase separation and results in more extensive macrosegregation in smaller droplets. There exists a pronounced tendency for the Pb-rich liquid to occupy the surface of the droplets of both compositions, resulting from the quite lower surface tension of the Pb-rich phase and causing a Pb-rich layer at the surface of the solidified droplet. The nucleation of monotectic cells in the Cu-40 wt pct Pb droplets with dispersed structures preferentially occurs at the droplet surface. A single nucleation event takes place more frequently as droplet size is reduced.
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    Precipitation in Ni-35 at. pct Cr Alloy
    A.A.Al-Aql, M.S.Al-Salhi, M.I.Ansari
    J. Mater. Sci. Technol., 2002, 18 (01): 77-79. 
    Abstract   HTML   PDF (630KB)
    Precipitation study was carried out in Ni-35 at. pct Cr alloy by measuring electrical resistivity at different ageing temperatures and long ageing times. During ageing, it was observed that electrical resistivity initially rose with ageing time and then fell after very long ageing at a constant temperature. The initial increase in resistivity during ageing may be attributed to the formation of GP zones/Cr-rich precipitates at early stages by nucleation process and decrease in resistivity, and after attaining a maximum, is due to the growth of precipitate particles and subsequently by coarsening of these precipitates. The results of electrical resistivity measurements during ageing were confirmed by X-ray diffraction analysis indicating the formation of a Cr-rich phase and a Cr3Ni2 phase in the Ni matrix.
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    Failure Mechanism of Laminated Damping Steel Sheet during Tensile-Shearing
    Chengguo WANG, Xitai SUN, Yujun BAI
    J. Mater. Sci. Technol., 2002, 18 (01): 80-82. 
    Abstract   HTML   PDF (748KB)
    The tensile-shear failure zone of the laminated damping steel sheet was investigated by scanning electron microscopy and X-ray photoelectron spectroscopy. It is found that there exists cohesive failure in polymer sandwich and sub-boundary failure between the steel sheet and the polymer. The sub-boundary layer is dominantly polymer material. The tensile-shear failure of the laminated damping steel sheet is a process during which the crazes form, grow up and merge into cracks.
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    Influence of Silica Fume on Corrosion Behaviour of Reinforced Steel in Different Media
    Z.A.El-Hadi, N.S.Tantawi, I.Z.Selim
    J. Mater. Sci. Technol., 2002, 18 (01): 83-88. 
    Abstract   HTML   PDF (1160KB)
    Electrochemical and corrosion behaviour of reinforced steel embedded in cement pastes incorporating different amounts of silica fume as a partial replacement of cement has been studied in chloride and sulphate solutions by using different electrochemical techniques. The results indicate that, while steel passivity degree is low in the control samples upon soaking in the corrosive media, it has been high in samples incorporating silica fume and increased with increasing silica fume content. The improvement effect of silica fume may be attributed to the pore solution structure of the cement paste, which limits the mobility of aggressive ions near the surface of the steel. The mechanism of steel corrosion due to chloride and sulphate attack and passivation effect of silica fume are discussed.
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    An Innovative Approach for Improving the Reliability of Reticulated Porous Ceramics
    Xinwen ZHU, Dongliang JIANG, Shouhong TAN
    J. Mater. Sci. Technol., 2002, 18 (01): 89-92. 
    Abstract   HTML   PDF (981KB)
    An innovative approach has been developed to fabricate reticulated porous ceramics (RPCs) with uniform macrostructure by using the polymeric sponge as the templates. In this approach, the coating process comprises of two stages. In the first stage, the thicker slurry was used to coat uniformly the sponge substrate. The green body was preheated to produce a reticulated perform with enough handling strength after the sponge was burned out. In the second stage, the thinner slurry was used to coat uniformly the preform. The population of the microscopic and macroscopic flaws in the structure is reduced significantly by recoating process. A few filled cells and cell faces occur in the fabrication and the struts were thickened. A statistical evaluation by means of Weibull statistics was carried out on the bend strength data of RPCs, which were prepared by the traditional approach and innovative approach, respectively. The result shows that the mechanical reliability of RPCs is improved by the innovative approach. This innovative approach is very simple and controlled easily, and will open up new technological applications for RPCs.
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    Oxygen Behavior in Bulk Amorphous Zr-base Alloy
    Keqiang QIU, Qingsheng ZHANG, Aimin WANG, Haifeng ZHANG, Bingzhe DING, Zhuangqi HU
    J. Mater. Sci. Technol., 2002, 18 (01): 92-94. 
    Abstract   HTML   PDF (723KB)
    Bulk Zr55Al10Ni5Cu30 metallic glass plates with a dimension of 85mm×35mm×4mm and a complicated plate were fabricated by injecting casting method using spongy zirconium and industrial purity aluminum, nickel and copper as raw materials. It was shown that the holding time of liquid metals at elevated temperatures had a great influence on the oxygen content of the plates due to the contamination resulting from the atmosphere. Increasing holding time resulted in the increase of oxygen content in the injected alloy. The glass transition temperatures of the bulk metallic glass plates are higher than that reported in the literature and crystallization temperature is lower for the one with higher oxygen content at the same heating rate. The extension of the undercooled liquid region ΔTX reaching about 87 K is 3 K higher than that previously reported and 26 K higher than that with oxygen content of 0.076 wt pct for the one with oxygen content as high as 0.065 wt pct. Therefore the oxygen content of the alloy has a significant influence on the glass forming ability and thermal stability of bulk metal glass. It is suggested that direct correlation between high glass forming ability and large ΔTX is only valid for a well-defined low oxygen concentration or has to be reconsidered by incorporating oxygen as an additional alloying element.
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    Evaluation of Morphology-Stable Critical Size of KNbO3 Crystals
    Lixia CAI, Weiqing JIN, Zhilei PAN, Xin'an LIANG, Shinichi Yoda
    J. Mater. Sci. Technol., 2002, 18 (01): 95-96. 
    Abstract   HTML   PDF (455KB)
    Skeletal form of KNbO3 crystals growing in Li2B4O7 solvent was in-situ observed at 900℃ and it was found that shallow depression started to develop on the surface of KNbO3 crystals when the crystal size exceeded several micron, typically 7 micron. Based on the quantitative criterion derived by Chernov, the estimated critical size of KNbO3 crystals was 1 micron, which was consistent with the experimental measurement. The kinetic coefficients, kcorner and kcr, in the criterion were experimentally obtained in the diffusive-convective and diffusive-advective flow states respectively.
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