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NEW VIEW ABOUT BAINITIC TRANSFORMATION IN STEEL

C.C. Liu , Z. Liu and G.X. Chen (Department of Mechanical Engineering , Tsinghua University , Beijing 100084 , China)(Department of Mechanical Engineering , Tsinghua University , Beijing 100084 , China)(Department of Mechanical Engineering , Tsinghua University , Beijing 100084 , China)

金属学报(英文版)

ZBainitic transformation is analyzed in detail according to the factors such as morphology,crystallography,mechanical property of bainite and transformation kinetics,and conclusion is drawn that bainitic transformation is controlled by displacive mechanism and self-fempering of displacive microstructure.That is to say,bainite is the product of such procedure during which undercooled austemite first transforms into one certain displacive microstructure by displacive mechanism and then this microstructure experiences self-fempering.With this idea,several questions,which cannot be illustrated only by displacive transformation mechanism,can be explained successfully.

关键词: bainitic transformation , null , null

Long-range Correlations in Iodide Cluster Aggregates

Jizhong ZHANG , Xaodong YANG and Hengde LI(Dept. of Materials Science & Engineering , Tsinghua University , Beijing 100084 , China)Xiaoyan YE(Dept. of Chemistry , Tsinghua University , Beijing 100084 , China)

材料科学技术（英文）

The morphology of tin iodide deposits grown during gas-solid transformation demonstrated longrange correlations. These iodide deposits include basically two kinds of forms: fractal structure and dendritic-like structure. The fractal structure was a sort of tin iodide cluster aggregation.Each fractal structure was composed of numerous tin iodide clusters that connected or disconnected with each other. The average size of fractal structures is between 10 and 22 μm. The dendritic-like structure was also composed of many tin iodide clusters. These clusters aggregated and formed a dendritic pattern

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Coacervate Structures of CdI_2 Thin Film Grown during Phase Transformation

Xiaoyan YE and Lili CAO(Dept. of Chemistry , Tsinghua University , Beijing 100084 , China)Jizhong ZHANG and Hengde LI(Dept. of Materials Scieuce & Engineering , Tsinghua University , Beijing 100084 , China)

材料科学技术（英文）

The microstructure of CdI2 thin film grown during vapor-phase deposition was investigated by scanning electron microscopy (SEM). The thin film deposited on Si crystal consists of numerous sunflower-like aggregates. These aggregates display well self-assembly characteristics. The size of Sunflower-like aggregates is between 12 and 44 μm. Each sunflower-like aggregate is surrounded with many adjacent wings-"petals". The structure of central region of the "sunflower" is obviously difFerent from that of the "petal". Electron spectroscopy for chemical analysis (ESCA) was employed in determining the chemical valence of the thin film. Self-organization efFect is used to explain the coring growth process of CdI2 thin film

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γ″Precipitate in Inconel 718

Jianhong HE (Dept. of Mater. Sci. and Eng. , Tsinghua University , Beijing , 100084 , China)S.Fukuyama and K. Yokogawa (Chugoku National Industrial Research Institute , Hiro-suehiro 2-2-2 , Kure , Hiroshima , 737-01 , Japan (To whom correspondence should be addres

材料科学技术（英文）

The γ"precipitate in Inconel 718 Ni-base superalloy has been investigated using TEM. Based on a calculation of diffraction pattern. the orientation relations between precipitates and matrix are given in detail. The influence of solution temperature on growth of γ" precipitates was investigated. and a γ" precipitate free area near δ phase was found. The coarsening behaviour of γ" precipitates during short time aging is discussed

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Effects of Sintering on the Microstructure and Critical Current Density of Ag-sheathed BiPbSrCaCuO Superconducting Tapes

Junhua LIU+ , Dept. of Manufacturing Engineering , Beijing University of Aeronautics and Astronautics , Beijing 100083 , ChinaZutang WANG , Dept. of Mechanical Engineering , Tsinghua University , Beijing , 100084 , China

材料科学技术（英文）

The effects of sintering on the microstructure and the critical current density of Ag-sheathed BiPbSrCaCuO (2223) superconducting tapes prepared by the powder-in-tube technique were in- vestigated. The microstructure of oxide layer and the J_C at 77 K were strongly dependent on the sintering condition. The optimum sintering temperature, time and cooling rate to maximize J_C values were in the range about 840-850℃, 100-200 h and 50-100℃/h, respectively.

关键词: BiPbSrCaCuO , null , null , null

LOW CYCLE FATIGUE AND CREEPFATIGUE BEHAVIOROF A HIGH TEMPERATURE ALLOY ON SMOOTH ANDNOTCHED SPECIMENS

Author H.J. Shi1) , S.L. Liu2) , Y.H. He1) and Z.X. Duan2) 1) Department of Engineering Mechanics , Tsinghua University , Beijing 100084 , China2) Laboratory of Material Mechanical Properties , Beijing Institute of Aeronautical Materials , Beijing 100095 , ChinaManuscript received 18 October 1998

金属学报(英文版)

A series of low cycle fatigue and creepfatigue tests on smooth specimens and notched specimens were performed with a high temperature alloy used in the manufacture of aeroengine turbine discs. The cyclic stressstrain behavior and fracture characteristics of the notched specimens were emphatically investigated. A comparison of fatigue behavior between smooth and notched specimens was done and lifetime of notched specimens was evaluated. At the same temperature, the fatiguecreep lives of the smooth specimens are far greater than that of the notched specimens.

关键词: low cycle fatigue , null , null , null

DESIGN AND PREPARATION OF SILICON NITRIDE COMPOSITE WITH HIGH FRACTURE TOUGHNESS AND NACRE STRUCTURE

Y Huang , H.N Hao , Y.L. Chen and B.L.Zhou (1)Department of Materials Science and Engineering , Tsinghua University , Beijing 100084 , China 2)Institute of Metal Research , Chinese Academy of Sciences , Shenyang 110015 , China Manuscript received 26 August 1996)

金属学报(英文版)

A new way of designing and preparing silicon nitride ceramic composite with high fracture toughness and nacre structure has been proposed. To mimic the laminated structure of nacre, Si_3N_4 matrix ceramic layer can be obtained through compacting rolling method. To mimic the secondary toughening of nacre structure, SiC whisker is added into Si_3N_4 and acts as the secondary toughening phase. Boron nitride (BN)is selected to mimic the organic layer in nacre so as to form the weak interfaces between Si_3N_4 layers. Alumina is added into BN to adjust the bonding strength of the interface.The Si_3N_4 sheets are stacked into the die after coating with BN. After the removal of the organic matter in them, the green body is hot pressed at 1820℃for 1.5 hours under N_2 atmosphere. The fracture toughness of the so-made Si_3N_4 composite at room temperature is 20.36MPa m~(1/2), the three-point bending strength at room temperature is 651.47MPa. The crack spreads and deflects along the interface between BN and Si_3N_4 layer and extends through the BN layer into Si_3N_4 layer. The improvement of the fracture toughness may be due to the staircase-shape-like crack which provides the long crack path, the fracture and deformation of Si_3N_4 layer, and the pullout of SiC whiskers from the Si_3N_4 layer.

关键词: :secondary toughening , null , null , null

ORIENTATION VARIANT SELECTION AND ITS EFFECTING FACTORS IN TITANIUM SHEET

Z.S. Zhu , N.P.Chen , J.L. Gu and M.G. Yan(1)Institute of Aeronautical Materials , P.O. Box 8115 , Beijing 100095 , China 2)Department of Materials Science and Engineering , Tsinghua University , Beijing 100084 , China Manuscript received 26 August 1996)

金属学报(英文版)

By means of crystallite orientation distribution function (ODF) analysis, the influence on orientation variant selection of cold rolling reductions, recrystallization annealing and cooling rates during α→β→α cyclic phase transformation treatments in cold rolled titanium sheet has been investigated. The results show that, cold rolling reduction and annealing treatment both have a great effect on the formation of phase transformation textures.No variant selection occurs in cold rolled and recrystallization annealed titanium sheet which resulte in a more randomly distributed texture.The phase transformation texture formed α→β→α transformation is inherited after 3 cycles of α→β→α phase transformation. The cooling rates during phase transformation have little effect on orientation variant selection.

关键词: :texture , null , null , null , null

STRENGTHENING CAST IRON MATRIX BY ALLOYING ELEMENTS WITH POSITIVE AND NEGATIVE SEGREGATION

Z.K. Fan , Y.Q. Wang , S.H. Liangand HS. Huang(School of Materials Science and Engineering , Xi'an University of Technology , Xi'an 710048 , China)(Mechanical Engineering Department , Tsinghua University , Beijing 100084 , China)

金属学报(英文版)

The combinational use of alloying elements with positive and negative segmpation hasbeen shown to be very thective in strengthening the matrix of cast iron. Negativesegmpation alloying elements, Cu and Si, make the inner parts of dendritic austenitesand eutectic cells get good handenability. On the contrury, positive segmpation alloyingelements Cr and Mn make the outer parts of dendrites have higher microhaofness.Appropriate combination of the above two tyPes Of alloying elements can produce auniform microharkness and high strength in grny cast irons.

关键词: cast iron , null , null

STM Study on the Surface Relief and Mechanism of Bainite Transformation

Zhigang YANG , Hongsheng FANG , Jiajun WANG , Yankang ZHENG and Chunming LI(Dept. of Materials Science and Engineering , Tsinghua University , Beijing 100084 , China)

材料科学技术（英文）

Scanning tunneling microscopy (STM ) has been first employed to study the surface relief accompanying bainite transformation in a Fe-2.17C-12.18Cr-0.31Si-0.26Mn (wt pct) steel. With the exclusive vertical resolution of STM, we observed that the surface relief associated with bainite is a group of surface reliefs related to subplates, subunits and sub-subunits. As a whole, the relief group is in a tent shape, not of invariant plane strain (IPS) type, which is obviously different from that of martensite, and implying that bainite is not formed by shear mechanism. The fine structure of bainite in Fe-1.0C-4Cr-2.0Si (wt pct) alloy has also been studied with STM and TEM. It is found that the bainite plate is composed of subplates, subunits and sub-subunits. On the basis of the fine structure inside a bainitic ferrite plate observed under STM, a sympathetic-ledgewise mechanism of bainite formation is proposed

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