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ISSN 1005-0302
CN 21-1315/TG
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      28 November 2000, Volume 16 Issue 06 Previous Issue    Next Issue
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    Research Articles
    Effects of annealing on high-magnetoresistance tunnel junctions using Co75Fe25 ferromagnetic electrodes
    Xiufeng HAN, Terunobu Miyazaki
    J. Mater. Sci. Technol., 2000, 16 (06): 549-553. 
    Abstract   HTML   PDF (1119KB)
    Tunnel junctions, Ta (5 nm)/Ni79Fe21 (3 nm)/Cu (20 nm)/Ni79Fe21 (3 nm)/lr(22)Mn(78) (10 nm)/Co75Fe25 (4 nm)/Al (d nm)-oxide/Co75Fe25 (4 nm)/Ni79Fe21 (20 nm)/Ta(5 nm) wherein d=1.3 and 0.8 nm, with small active area from 100x100 down to 3x3 mum(2) were fabricated using a micro-fabrication technique. The optical lithography combined with Ar ion-beam etching and CF4 active etching was used to pattern the junction area. A thin barrier layer and a short plasma-oxidation time for At-oxide layer were used in order to reduce the junction resistance and increase the TMR ratio. The optimum annealing temperature and time for the highest TMR obtained were investigated to be around 300℃ for an hour in this work. TMR ratio, effective barrier height and width, and breakdown voltage of the junctions can be remarkably enhanced upon annealing. High TMR ratio of 49.7% at room temperature and 69.1% at 4.2 K were achieved. TMR ratio and resistances decrease with increasing de bias voltage from 0 to 1000 mV or with increasing temperature from 4.2 K to room temperature.
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    Distortion regions near the grain boundary and their effects on nanocrystalline materials
    Chiwei LUNG, Kang LONG, Enke TIAN, Zongjun LIANG
    J. Mater. Sci. Technol., 2000, 16 (06): 554-558. 
    Abstract   HTML   PDF (1379KB)
    The distortion regions near grain boundaries in cr-Fe-C solid solution were studied by use of internal friction method. It was found that the total thickness of these regions is quite large though the thickness of real grain boundaries is usually very thin. It was also found that the smaller the grain size, the thicker the total distortion region. A model for the structure of distortion regions near grain boundaries is proposed. Their effects on nanocrystalline materials are discussed.
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    Determination of elastic and creep properties of thin-film systems from indentation experiments
    Z.F.Yue, G.Eggeler
    J. Mater. Sci. Technol., 2000, 16 (06): 559-567. 
    Abstract   HTML   PDF (5998KB)
    Based on the detailed computer simulation of the indentation testing on the thin-fi Im systems, the. present paper explores the detailed procedure of determining elastic properties (elastic modulus E-(f) and Poisson ratio nu ((f))) and creep parameters (C-CREEP ((f)) and n(CREEP) ((f))) for a simple Norton law ((epsilon) over dot =C-CREEP ((f))sigma (n)(CREEP) ((f)), where (epsilon) over dot is creep strain rate, and sigma is the stress) material for a thin film coated on a creep substrate, whose elastic properties(E-(s) and nu ((s))) and creep properties (C-CREEP((s)) and n(CREEP) ((s))) of the substrate are known, from indentation elastic and creep testing, respectively. The influences of the thickness of the thin-film and the size of the indenter on the indentation behavior have been discussed. It is shown that the boundary between the thin film and the substrate has great influence on the indentation creep behavior. The relative sizes of indentation systems are chosen so that the behavior of the indentation on the film is influenced by the substrate. The two elastic parameters E-(f) and nu ((f)) of the film are coupled on the influence of the elastic behavior of indentation. With the two different size indenters, the two elastic parameters E-(f) and v((f)) of the film can be uniquely determined by the indentation experimental slopes of depth to applied net section stress results. The procedure of determining of the two Norton law parameters C-CREEP ((f)) and n(CREEP) ((f)) includes the following steps by the steady indentation rate a. The first step to calculate the creep indentation rate on certain loads of the two different sizes of indenters on a set of assumed values of C-CREEP ((f)) and n(CREEP) ((f)) Then to build relationship between the creep indentation rate and the assumed C-CREEP((f)) and n(CREEP)((f)). With the experimental creep indentation rate to intersect two sets of C-CREEP((f)) and n(CREEP)((f)), which have the same values of a. The last step is to build the C-CREEP((f)) and n(CREEP)((f)) curves from the intersection points for the two indenters. These two curves C-CREEP((f)) and n(CREEP)((f)) will intersect at one point. The axial values of the point are what we want. The procedure presented in the paper to determine the elastic parameters and creep parameters from the indentation experiments can be applied to any material systems.
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    Modeling of solidification microstructures based on fully coupling of macro-transport phenomena with cellular automata
    Qiang DU, DianZhong LI, Yiyi LI
    J. Mater. Sci. Technol., 2000, 16 (06): 568-572. 
    Abstract   HTML   PDF (4578KB)
    This paper has attempted to simulate the microstructure formation based on fully coupling of temperature field, concentration field and velocity field with micro-kinetics. The authors presented a new way, which is the combination of FDM and cellular automata (CAFD) to visualize the microstructure formation of the thin complex superalloy turbine blades cast by the vacuum investment process. The distribution, orientation and mechanism of the heterogeneous nucleation, the growth kinetics of dendrites and the columnar to equiaxed transition (CET) are considered. Capitalizing on these simulating schemes, the comprehensive influence of key process variables on the scale and uniformity of grains has been investigated quantitatively. The simulated grain size and morphology agree well with the experimental results.
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    First-principles investigation on solution hardening of interstitial impurity elements (O, N) in gamma-TiA1
    Longguang ZHOU, Lianlong HE, Lin DONG, Caibei ZHANG
    J. Mater. Sci. Technol., 2000, 16 (06): 573-576. 
    Abstract   HTML   PDF (993KB)
    We have calculated the electronic structures of O-doped and N-doped gamma -TiAl using the first-principles discrete variational method (DVM) with the aim to understand the solution hardening effects of oxygen and nitrogen in gamma -TiAl. Our combination analysis on the electronic density, density of states (DOS) and the local environment total bond orders (LTBO) will show that, X atom (X is O or. N) can strongly bind with its six surrounding atoms via electronic hybridizations of Ti-3d/X-2p and Al-3p/X-2p. As a sequence, there forms a "hard" cohesive region around the impurity atom. A pinning model based on the calculations is proposed to explain the hardening effects. The consistent results are obtained between the present calculation and formal test experiments.
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    Influence of solid fraction on gravity segregation of Sn in A1-20Sn alloy casting
    Peng ZHANG, Yunhui DU, Daben ZENG, Jianzhong CUI, Limin BA
    J. Mater. Sci. Technol., 2000, 16 (06): 577-580. 
    Abstract   HTML   PDF (837KB)
    The influence of solid fraction of AL-20Sn alloy mushy on gravity segregation of Sn in casting was studied and, the relationship between solid fraction and the temperature of alloy mushy and that between solid fraction of alloy mushy and size of Sn particle in ingot were determined. The results show that the relationship between solid fraction and the temperature of alloy mushy was f(s)=1683-4.86t+0.0035t(2). The extent of gravity segregation of Sn in casting reduced gradually with the increasing of solid fraction of alloy mushy. When solid fraction of alloy mushy was larger than 40%, the gravity segregation of Sn in casting could be removed basically, and the relationship between solid fraction of alloy mushy and size of Sn particle in ingot was s=-0.64f(s)+70.8.
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    Preparation of Li-B alloy and study of its microstructure and discharge characteristics
    Zhijian LIU, Zhiyou LI, Wei DUAN, Xuanhui QU, Baiyun HUANG, Siqi ZHANG
    J. Mater. Sci. Technol., 2000, 16 (06): 581-584. 
    Abstract   HTML   PDF (1072KB)
    A LI-B alloy has been prepared using a pretreated amorphous B powder and pure Li ingot as starting materials by continually slow addition of B powder and intensified stirring in the process of melting. The microstructure and the discharge characteristic of the materials have been investigated. Results show that the problem of temperature control in synthesis would be modified by means of continual addition of B powder, the Li7B6 would be more finery distributed in the metal Li by means of intensified stirring. The discharge characteristic of the Li-B alloy using amorphous B as starting materials is almost the same with that of using crystalline B.
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    Role of ordering energy in formation of grain structure and special boundaries spectrum in ordered alloys with L1(2) superstructure
    O.B.Perevalova, E.V.Konovalova, N.A.Koneva, E.V.Kozlov
    J. Mater. Sci. Technol., 2000, 16 (06): 585-590. 
    Abstract   HTML   PDF (1785KB)
    It was revealed that an average energy of special boundaries is proportional to APE energy in the alloys with the tla superstructure. This fact proves the appearance of the GAPBs in the planes of location of special boundaries in coincidence sites of ordered alloys. It was determined that the more energy of special boundaries in ordered alloys, the more energy of complex stacking fault. There is a correlation between the distribution of special boundaries as a function its relative energy and ordering energy: the more ordering energy, the more degree of washed away of distribution. The correlation between average relative energy of special boundaries and ordering energy was detected: the more ordering energy, the more average energy of special boundaries. The reverse dependence between ordering energy and average number of special boundaries in grains limited by boundaries of general type was discovered.
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    Australian coral as a biomaterial: Characteristics
    J.Hu, R.Fraser, J.J.Russell, B.Ben-Nissan, R.Vago
    J. Mater. Sci. Technol., 2000, 16 (06): 591-595. 
    Abstract   HTML   PDF (1078KB)
    In order to produce effective implants, the materials used must be biocompatible. Hydroxyapatite (HAp) is a bioactive material similar to the mineral component of teeth and bone which is often used for orbital implants and bone graft applications. HAp can be manufactured from corals via hydrothermal conversion. Coral is particularly useful as a starting material for hydroxyapatite production because of its porous nature. When a porous structure is used tissue ingrowth can occur readily and hence an excellent mechanical bond can be achieved. A large pore size and a high degree of pore interconnections are desirable implant properties. In the present paper a comparison of the properties of four different species of Australian coral has been made to determine the most favourable species to use as a starting material for hydrothermal conversion.
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    EPR and magnetic susceptibility investigations of Fe containing TeO2-B2O3-SrF2 glasses
    I. Ardelean, M.Peteanut, V.Simon, F.Ciorcas
    J. Mater. Sci. Technol., 2000, 16 (06): 596-600. 
    Abstract   HTML   PDF (889KB)
    Iron ions were used as probes to explore the structural and magnetic properties of 70TeO(2). 25B(2)O(3). 5SrF(2) vitreous matrix. The distribution of Fe3+ ions on different structural aggregates was revealed by means of EPR, as depending on Fe2O3 concentration. Strongly distorted octahedral sites were detected for the isolated paramagnetic ions, and also clusters of Fe ions especially at high Fe2O3 content of samples. Magnetic susceptibility measurements evidenced both dipole-dipole and superexchange type interactions involving iron ions. Mixed valence states of iron ions were also detected.
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    Microstructural characteristics of La0.7Ca0.3MnO3 films and multilayers
    Mingguang WANG, DouXing LI, Lianlong HE, Hengqiang YE, Guangcheng XIONG
    J. Mater. Sci. Technol., 2000, 16 (06): 601-604. 
    Abstract   HTML   PDF (1034KB)
    La0.7Ca0.3MnO3 (LCMO) films and La0.7Ca0.3MnO3/Gd0.7Ca0.3MnO3 (LCMO/GCMO) multilayers have been prepared by pulsed laser deposition. The microstructures of both systems were investigated by transmission electron microscopy (TEM). The main structure of the films and the multilayers was monoclinic with a unit cell of size 2a(p) x root 2a(p) x root 2a(p), where a(p) is the lattice constant of single perovskite crystal. The LCMO films were composed of three-dimension multitwinning domains, while the LCMO/GCMO multilayers showed two-domain structure. In LCMO/GCMO multilayers, LCMO layers were coherent with GCMO layers and the interfaces between LCMO and GCMO layers were free from mismatch dislocation, which resulted in highly strained multilayerd structures.
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    Processing TiAl-based alloy by elemental powder metallurgy
    Yong LIU, Baiyun HUANG, Yuehui HE, Kechao ZHOU
    J. Mater. Sci. Technol., 2000, 16 (06): 605-610. 
    Abstract   HTML   PDF (1961KB)
    TiAl-based alloys with various compositions (includingTi-48Al, Ti-47Al-2Cr-2Nb, Ti-47Al-2Cr-2Nb-0.2B and Ti-47Al-3Cr, in mole fraction) had been prepared by elemental powder metallurgy (EPM). The results have shown that the density of the prepared Ti-48Al alloy increases with increasing hot pressing temperature up to 1300 degreesC. The Ti-48Al alloy microstructure mainly consisted of island-like Ti3Al phase and TiAl matrix at hot pressing temperature below 1300 degreesC, however, coarse alpha (2)/gamma lamellar colonies and gamma grains appeared at 1400 degreesC. It has also indicated that the additions of elemental Cr and B can refine the alloy microstructure. The main microstructural inhomogeneity in EPM TiAl-based alloys was the island-like aa phase or the aggregate of alpha (2)/gamma lame[[ar colony, and such island-like structure will be inherited during subsequent heat treatment in (alpha+gamma) field. Only after heat treatment in a field would this structure be eliminated. The mechanical properties of EPM TiAl-based alloys with various compositions were tested, and the effect of alloy elements on the mechanical properties was closely related to that of alloy elements on the alloy microstructures. Based on the above results, TiAl-based alloy exhaust valves were fabricated by elemental powder metallurgy and diffusion joining. The automobile engine test had demonstrated that the performance of the manufactured valves was very promising for engine service.
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    Effects of predeformation on the reverse martensitic transformation of TiNi shape memory alloy
    Yanjun ZHENG, Lishan CUI, Fan ZHANG, Dazhi YANG
    J. Mater. Sci. Technol., 2000, 16 (06): 611-614. 
    Abstract   HTML   PDF (770KB)
    DSC was used to study the effects of predeformation on the reverse martensitic transformation of near-equiatomic TiNi alloy. Both the start temperature A(s) and the finish temperature A(f) of the reverse transformation increased with increasing degree of predeformation, but the algebraic difference between A(s) and A(f) decreased with increasing predeformation until it reached a minimum value, then remained unchanged with further deformation. Transformation heat also increased with increasing predeformation until it reached a maximum value, then decreased with further predeformation. All the phenomena above were considered to be closely related with the release of elastic strain energy during predeformation.
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    Review on fabrication methods of in situ metal matrix composites
    ChunXiang CUI, Yutian SHEN, Fanbin MENG, Seuk Bong Kang
    J. Mater. Sci. Technol., 2000, 16 (06): 619-626. 
    Abstract   HTML   PDF (1641KB)
    This paper deals with a series of novel processing techniques based on the in situ production of metal matrix composites (MMCs). In situ techniques involve a chemical reaction resulting in the formation of a very fine and thermodynamically stable reinforcing ceramic phase within a metal matrix. As a result, this provides thermodynamic compatibility at the matrix-reinforcement interface. The reinforcement surfaces are also likely to be free of contamination and, therefore, a stronger matrix-dispersion bond can be achieved. Some of these technologies including DIMOX(TM), XD, PRIMEX(TM), reactive gas infiltration, high-temperature self-propagating synthesis (SHS), and liquid-solid, or solid-gas-liquid reactions as well as plasma in situ MMCs are expressed in this paper.
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    Electron beam curing of advanced composites
    Gang SUI, Weihong ZHONG, Zuogang ZHANG
    J. Mater. Sci. Technol., 2000, 16 (06): 627-630. 
    Abstract   HTML   PDF (1037KB)
    The fundamental concept of electron beam method and the application in cure of composites are elaborated in this paper. The components of electron beam curing system are introduced. The mechanisms of interaction between electron beam and polymer matrix composites are presented. Recent studies reported including work of authors themselves on electron beam curing of composites are also discussed. Moreover, the authors believe that it is necessary to do the basic research about understanding how electron beam affects cured network and the mechanical/physical properties of the composites, for establishing a quantitative or semi-quantitative formulation.
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    Stress corrosion cracking and blue brittleness of rotor steels in high temperature aqueous solutions
    Yanbin WANG, Wuyang CHU, Lijie QIAO
    J. Mater. Sci. Technol., 2000, 16 (06): 631-633. 
    Abstract   HTML   PDF (730KB)
    The tensile behaviors of two bolt steels 20Cr12NiMoWV and 25Cr2MoV, and rotor steel 30Cr2Ni4MoV used in a nuclear power plant were studied at different temperatures up to 280 degreesC in air and aqueous solution. The results showed that blue brittleness occurred near 230 degreesC and 180 degreesC for steels 30Cr2Ni4MoV and 25Cr2MoV, respectively. No blue brittleness was found for the steel containing higher Cr content. Susceptibility to stress corrosion cracking.(SCC) for the steels in aqueous solution at the test temperatures was very low except at the blue brittleness temperature. At the blue brittleness temperatures, however, the susceptibility to SCC was very high.
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    Microstructure and mechanical properties of TiB2/(Ni+Mo) composites fabricated by hot pressing
    Jinyong ZHANG, Zhengyi FU, Weiming WANG, Hao WANG
    J. Mater. Sci. Technol., 2000, 16 (06): 634-636. 
    Abstract   HTML   PDF (1145KB)
    The fabrication of TiB2 ceramics with high performance at a relatively low temperature requires a second phase to assist the densification and control the microstructure. Moreover the interface characteristics of composites must also be considered. Nickel was chosen as a good metallic-bearing additive in this study. In order to optimize the interface, Mo was added. The results show that Mo can affect the interreaction between TiB2 and Ni, and metallic liquid plays an important role in limiting the TiB2 grain growth in sintering.
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    A novel technology for preparation of high performance fiber by radio frequency heating CVD
    Nanlin SHI, Kun LUO, Yapei ZU, Yading DUAN
    J. Mater. Sci. Technol., 2000, 16 (06): 637-638. 
    Abstract   HTML   PDF (574KB)
    In this paper, radio frequency (r.f.) heating CVD technology for preparation of high performance fiber has been introduced. SiC, B fibers have been produced by this technology.
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    Calculation of the surface segregation of Cu-0.3 at. pct Au alloy
    Liping ZHENG, Risheng LI, Yuanxun ZHANG, Shi QIU, Bingyao JI
    J. Mater. Sci. Technol., 2000, 16 (06): 639-640. 
    Abstract   HTML   PDF (414KB)
    Monte Carte simulation technique with the energetics described by the embedded atom method has been used to calculate the surface segregation of Cu-0.3 at. pet Au alloy at T = 800 K. It is found that the component Au in the first surface layer (i.e.(100), (110) and (111) faces) is about 50 similar to 60 times as large as that in the bulk.
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    Magnetostriction of pseudobinary compounds Pr0.15TbxDY0.85-xFe2 (x=0 to 0.85)
    Yangxian LI, Jingping QU, Chengchun TANG, Guangheng WU, Wenshan ZHAN
    J. Mater. Sci. Technol., 2000, 16 (06): 641-642. 
    Abstract   HTML   PDF (403KB)
    The compound ingots of Pr0.15TbxDy0.85-xFe2 (x=0 to 0.85) were prepared by are melting in a water Cu boat using are furnace under a purified Ar atmosphere. Appropriate annealing (850 degreesC, 100 h) can obtain single Laves phase compound. The magnetostriction for these systems will rise obviously when partially substituted Tb or Dy by Pr.
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    Effect of the partial substitution of V for Nb on the magnetic properties of Fe-based nanocrystalline alloy
    Guiqin ZHOU, Fazeng LIAN, Guozhen LI
    J. Mater. Sci. Technol., 2000, 16 (06): 643-644. 
    Abstract   HTML   PDF (433KB)
    The variation of the magnetic properties of the nanocrystalline alloys for the partial substitution of V for Nb with crystallizing treatment temperature and time was investigated. The variation law of the magnetic properties with the annealing temperature and time is essentially the same. The magnetic properties of the Fe-based nanocrystalline alloys for the partial substitution of V for Nb reduce, and the crystallizing treatment temperature of the alloys increases. The optimum properties of Fe74Cu1Nb3Si13B9 nanocrystalline alloys crystallized at 550 degreesC x 60 min are mu (0) = 9.2x10(4), mu (m)=54.8x10(4), H-c=1.14 A/m and B-s=1.26 T. The best properties for Fe74Cu1Nb2.5V0.5Si13B9 alloys annealed at 560 degreesC x 60 min are mu (0)=8.79x10(4), mu (m)=50.18x10(4), H-c=1.26 A/m and B-s=1.24 T.
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ISSN: 1005-0302
CN: 21-1315/TG
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