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CN 21-1139/TG
Started in 1956

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    RESEARCH ON GRAIN REFINEMENT IN BULK UNDERCOOLED Fe–Co BASE ALLOYS
    MU Danning YANG Changlin WEI Xiaowei LIU Feng
    Acta Metall Sin, 2012, 48 (12): 1409-1414.  DOI: 10.3724/SP.J.1037.2012.00286
    Abstract   PDF (1901KB) ( 1222 )

    Fe–Co base alloys, owing to their excellent high tempreture soft magnetic property, have been given more and more attention. Especially, Fe–Co base bulk amorphous–nanocrystalline materials have became an important development direction, while the preparation techniques have limited the wide applications of these high performance magnetic materials. Undercooling rapid solidification technique, independent of the sample size, has unique advantages in preparing bulk microcrystalline and nanocrystalline materials. However, upon large volume of alloy melt, more heterogeneous nucleus and latent heat of crystallization will occurr, which is disadvantageous to obtain a high undercooling and to repress grain growth in the process of solidification. Obviously, regular rapid solidification technique has not met the requirements for the preparation of industrial products with a large volume. So it is important and necessary to combine other rapid solidification techniques with regular rapid solidification technique to achieve high undercooling. In present work, copper mould chilling was used for undercooled Fe–Co base alloy melts. On one hand, the latent heat of crystallization can be transmitted to outside more rapidly by copper mould, on the other hand, by increasing the cooling rate, copper mould can also make undercooled melt achieve further undercooling. In this work, applying fluxing purification and cycling superheating method, Fe44Co44Nb7B4Cu1 melts were undercooled, and microstructure evolutions of the two different kinds of Fe44Co44Nb7B4Cu1 alloy samples prepared by undercooling solidification and copper mould chilling were studied, respectively. Using SEM and EDS, the grain refinement mechanism was investigated systematically. The experimental results show that the dendrite structures chang into granular grains in both the two kinds of samples with the increase of undercooling. The critical undercooling of dendrite structures changing into granular grains is smaller in the samples prepared by copper mould chilling than that by undercooling solidification. And the smaller grains and more homogenous microstructures are found in the samples prepared by copper mould chilling under the same undercooling. In combination with the calculations and the analysis of experiment results, it indicates that the decrease of the grain size is mainly attributed to the melt supercooling, remelting and copper mold chilling which increases nucleation rate and inhibits the grain growth. While alloying element gathering at the grain boundary is not the main factor.

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    EFFECT OF CONTROLLED ROLLING PROCESSING ON NANOMETER–SIZED CARBONITRIDE OF Ti–Mo FERRITE MATRIX MICROALLOYED STEEL
    SUN Chaofan CAI Qingwu WU Huibin MAO Hongyan CHEN Hongzhen
    Acta Metall Sin, 2012, 48 (12): 1415-1421.  DOI: 10.3724/SP.J.1037.2012.00348
    Abstract   PDF (1133KB) ( 1047 )

    Single nanometer–sized particles, which are smaller than 10 nm, can significantly enhance the precipitation strengthening in microalloyed steels, thus causing their strength to be promoted greatly. In order to improve the strength of the steel, it is quite necessary to get a large amount of single nanometer–sized particles through optimizing rolling technology. In this work, the effects of two different kinds of controlled rolling technologies on the size and distribution of precipitated particles in the Ti–Mo ferritie matrix microalloyed steel have been researched using SEM, TEM and small–angle X–ray scattering. The results show that with the same total rolling reduction, the steel rolled only in γ phase crystallization zone can obtain a higher portion of single nanometer–sized particles than that rolled respectively in phase recrystallization and nonrecrystallization zones, in which those single nanometer–sized particles account for about 75% (mass fraction) of whole precipitated particles. In order to study the effect of deformation potency in γ phase zone on the amount of precipitates in phase and the micro–crystal size, nucleation rate and incubation time of following precipitates in γ→α transformation and ferritie matrix after γ→α transformation, some thermodynamics and kinetics calculations and analysis on precipitation are also conducted.

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    MICROSTRUCTURES AND MECHANICAL PROPERTIES OF 0Cr13 FERRITIC STAINLESS STEEL PROCESSED BY EQUAL–CHANNEL ANGULAR PRESSING AND SUBSEQUENT ANNEALING TREATMENT
    YANG Muxin YANG Gang LIU Zhengdong Du Xiqian HUANG Chongxiang
    Acta Metall Sin, 2012, 48 (12): 1422-1430.  DOI: 10.3724/SP.J.1037.2012.00291
    Abstract   PDF (2454KB) ( 1081 )

    In comparison with austenitic stainless steel, the ferritic stainless steel has obvious advantage in price due to its lower nickel content. However, the relatively poor ductility and toughness limit its applications. To overcome these shortcomings, a new thermo–mechanical approach, involving processing by severe plastic deformation and proper annealing treatment to introduce a bimodal grain size distribution, was adopted for achieving high work–hardening capability, superior strength–ductility combination and good impact toughness in metallic materials. In this work, the combined effects of severe plastic deformation and partially recrystallization on the microstructures and mechanical properties of a ferritic stainless steel were investigated and compared with the traditional forging and annealing process. An solution–treated ferritic stainless steel (0Cr13, AISI 405) was subjected to equal–channel angular pressing (ECAP, an important kind of severe plastic deformation) for two passes at room temperature and subsequent annealing treatments. Optical microscope (OM) and transmission electron microscopy (TEM) observations showed that ultrafine-grained (UFG) structure was obtained in the ECAP–processed sample. After subsequent annealing at 650—750 ℃ for 1 h, partial recrystallization occurred and the remaining island–like UFG grains (10%—35% volume fraction) distributed uniformly. Statistical measurements indicated that the microstructures of the annealed ECAP samples exhibited a bimodal grain size distribution including relatively coarse recrystallized grains (CRGs) and remaining ultrafine grains (UFGs). The average grain size for CRGs determined from OM observations was 5.1—8.3 μm and the average grain size for UFGs measured from TEM observations was 418—525 nm. By contrast, the annealed forged sample (700℃) exhibited a unimodal grain size distribution with average grain size of about 74 μm. Tensile and impact tests showed that the strength of 0Cr13 ferritic stainless steel could be improved greatly through grain refinement by ECAP process, and the strength–ductility combination could be modulated via sacrificing some strength for ductility by subsequent annealing treatment. In comparison with the conventional sample (forging+annealing at 700 ℃), the tested steel processed by the optimal processing involving ECAP deformation and annealing treatment at 700℃ showed higher yield strength, uniform ductility and static toughness (enhanced by 10%, 35% and 70% respectively), simultaneously a comparable impact toughness (212 J/cm2). The refined microstructure and higher work–hardening capacity were responsible for the improved mechanical properties of the annealed ECAP samples and the strengthening mechanisms were discussed based on the experimental results.

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    STUDY OF HUMPING TENDENCY AND AFFECTING FACTORS IN HIGH SPEED LASER WELDING OF STAINLESS STEEL SHEET
    PEI Yinglei SHAN Jiguo REN Jialie
    Acta Metall Sin, 2012, 48 (12): 1431-1436.  DOI: 10.3724/SP.J.1037.2012.00416
    Abstract   PDF (2724KB) ( 827 )

    The fiber laser was used to weld austenitic stainless steel SUS304 sheet at the speed of 24 m/min. The effects of laser power, welding speed and shielding gas on humping tendency have been investigated. The melt flow in the molten pool under different welding parameters was studied by Ti tracer method and CCD visual detection system. The results show that the humping tendency is not sensitive to the laser power. The humping is formed when welding speed exceeds 18 m/min, and goes up with the increasing of welding speed. The humping tendency is completely different as changing the direction of shielding gas under constant welding speed and laser power, and it is reduced when the shielding gas direction follows the welding direction. It can be explained that, with the increasing of welding speed, the melt flow becomes fierce, and the humping tendency goes up. The weld shape turns to the "columnar weld", when the shielding gas direction against the welding direction; the weld shape turns to the "cup weld" when the shielding gas direction follows the welding direction. The "cup weld" expanded "U area" which gentles the melt flow and reduces humping tendency. Therefore, adjusting the shielding gas direction to expand the "U area" is an effective approach to reduce the humping tendency.

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    STUDY ON HEREDITARY OF PORES IN LASER REMELTING OF DIE CASTING AZ91D MAGNESIUM ALLOY
    WANG Xiangjie YOU Guoqiang ZHANG Juncheng LONG Siyuan
    Acta Metall Sin, 2012, 48 (12): 1437-1445.  DOI: 10.3724/SP.J.1037.2012.00239
    Abstract   PDF (4570KB) ( 1073 )

    Porosity has been a main problem for die casting magnesium alloy welding and casting defect repair. In order to study the formation mechanism of pores in fusion welding of die casting magnesium alloy, in this research, experiment of die casting AZ91D magnesium alloy CO2 laser re–melting was carried out. OM and SEM were employed to observe the characteristics of pores existing at both the base metal and re–melted zone, and a software for particle size analysis called nano measure 1.2 was adopted to measure the pores’ size. During analysis, the work mainly focused on the relationship of pores in re–melted zone associated with that preexisting in the base metal. The results showed that: porosity in die cast magnesium alloy base metal showed apparently hereditary characteristic in the process of re–melting. Porosity preexisting in the die casting AZ91D magnesium alloy was mainly produced at the junction region of multi–grains, with high pressure of inner gas, small size and irregularity in shape. However, pores in the re–melting zone showed diversity. Specifically, the micro–pore was small in size, nearly round in cross section and smooth in the inner wall, which was induced by hydrogen stored in the base metal. The macro–pores were vermiculate, with gas channel and metal erosion traces at the inner wall. It’s considered that the micro–pore was mainly inherited from the atomic hydrogen solution in the base metal and molecular hydrogen stored in the die casting defects. In the re–melting process, gas bubble of hydrogen was formed through nucleation and development two steps, and there was no sufficient time to grow up. As a result, the hydrogen induced pore was great in number and small in size. While the macro–pores were inherited from porosity preexisting in the base metal, the corresponded gas bubble did not require nucleation, was directly formed from the involved gas in preexisting porosity during die–casting process. Development of gas bubble for macro–pore mainly was coalescence of small gas bubbles, which made the macro–pore show channel and crude in the inner wall. Furthermore, formation mechanism of the two types of pores were analyzed, and mathematical model on relationship of macro–pore in re–melted zone associated with that in the base metal was established.

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    MULTIAXIAL FATIGUE LIFE PREDICTION FOR EXTRUDED AZ31B MAGNESIUM ALLOY
    XIONG Ying CHENG Lixia
    Acta Metall Sin, 2012, 48 (12): 1446-1452.  DOI: 10.3724/SP.J.1037.2012.00307
    Abstract   PDF (1169KB) ( 788 )

    Magnesium alloy components were widely used in automobile and aircraft industries,due to their light weight, high specific strength, stiffness, damping capacity, machinability, and recyclability. Engineering components subjected cyclic loading inevitably and led to fatigue failure. Most studies on magnesium alloy were focus on uniaxial fatigue, very limited work has been done of magnesium alloys under multiaxial loading. In this study, strain–controlled multiaxial fatigue experiments were conducted on extruded AZ31B magnesium alloy using thin–walled tubular specimens in ambient air. Four loading paths, including fully reversed tension–compression, cyclic torsion, 45? in–phase axial–torsion and 90? out–of–phase axial–torsion, were adopted in the fatigue experiments. It is observed that the strain–life curve displays a distinguishable kink under each loading path at the equivalent strain amplitude around 0.3% to 0.55%. The fatigue life under the proportional loading path is the highest when equivalent strain amplitudes higher than 0.45%, and the fatigue life under the tension–compression loading path is the highest when equivalent strain amplitudes lower than 0.45%. For the same equivalent strain amplitude, fatigue life under nonproportional loading resulted in the shortest fatigue life. Three critical plane multiaxial fatigue criteria were employed to predict fatigue life. Predictions by Smith–Waston–Topper (SWT) parameter do not agree well with the fatigue life for the tension–compression and cyclic torsion loading, and 76% predicted results are within factor–of–five boundaries. The Fatemi–Socie (FS) parameter and a modified SWT parameter are found to be able to predict fatigue lives reasonably well for all loading paths, and 95% predicted results are within factor–of–five boundaries. In addition, crack initiation of extruded AZ31B and AZ61A magnesium alloy based on experimental observation were discussed to explain prediction results vary much for the same multiaxial fatigue criterion between the two materials. It was demonstrated that AZ31B magnesium alloy and AZ61A magnesium alloy has different damage mechanism due to different microstructures. Optical microscopy observations exhibited lamellar twinning exist in a little big elongated grains with an average grain size of 50 μm in extruded AZ31B magnesium alloy at strain amplitude of 1%. In the same situation, mechanical twins were observed in almost every equiaxed grain with an average grain size of 20 μm in extruded AZ61A magnesium. Ex–situ SEM microscopic observation of the microstructure evolution showed fatigue micro–cracks were at the grain boundaries or slip bands in extruded AZ31B magnesium alloy, while at twin boundary in extruded AZ61A magnesium alloy.

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    STUDY ON TWO CRITICAL MECHANISMS OF PLC EFFECT OF 5456 Al–BASED ALLOY
    FU Shihua CHENG Teng ZHANG Qingchuan CAO Pengtao HU Qi
    Acta Metall Sin, 2012, 48 (12): 1453-1458.  DOI: 10.3724/SP.J.1037.2012.00270
    Abstract   PDF (1287KB) ( 489 )

    The normal critical behavior at low temperature and inverse critical behavior at high temperature of 5456 Al–based alloy were observed via tension tests at different temperatures. By comparing the stress–strain curves at different temperatures, the lower and upper envelope curves were identified. Before the critical strain, the stress followed the lower envelope curve at low temperature while followed the upper envelope curve at high temperature. The subsequent serrations, which were upward at low temperature but downward at high temperature, waved between the two envelope curves. Furthermore, in relation with stress and dislocation motion, two types of dislocation motion for stable plasticity corresponding to the upper and lower envelope curves were presented, respectively. The lower envelope curve implied few dislocations were pinned by solute, while the upper envelope curve implied some dislocations were pinned by solute prior to escape. Finally, two critical mechanisms were proposed that the critical strain depended on the first pinning process in normal behavior and on the first unpinning process in inverse behavior.

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    EFFECT OF HEAVILY DRAWING ON THE MICROSTRUCTURE AND PROPERTIES OF Cu–Cr ALLOYS
    SONG Lunan LIU Jiabin HUANG Liuyi ZENG Yuewu MENG Liang
    Acta Metall Sin, 2012, 48 (12): 1459-1466.  DOI: 10.3724/SP.J.1037.2012.00263
    Abstract   PDF (4207KB) ( 950 )

    Cu–Cr alloy is a kind of promising materials used as conductor due to its good strength and high conductivity. Heavy cold deformation could increase the strength effectively. Most of Cu and Cr phase are elongated into filaments during cold drawing. There exits plenty of Cu/Cr interface and the structure of Cu/Cr interface is thought to play an essential role in the properties of the Cu–Cr alloy. In this work, Cu–Cr wires were prepared by cold drawing method. The evolution of the microstructure and the structural change of phase interface during cold drawing have been investigated and the relationship between properties and microstructure also established. The microstructure consists of Cu matrix and Cr particles before cold drawing. As the drawing strain increases, both of Cu and Cr phases evolve into filamentary structure. Some residual Cr particles are still found in the alloy even at high drawing ratio. There is a relationship of (111)Cu//(110)Cr between Cu fibers and Cr fibers at high strain levels. The Cu/Cr interface is non–coherent before cold drawing and gradually evolves into the coherent interface at high drawing strains. The inter–solution ability of Cu and Cr elements across the Cu/Cr interface is enhanced with the increase in the drawing strain. The coherent Cu/Cr interface and the increasing of interface density should be responsible for the strength rising to an almost constant value and the increase in electrical resistivity of Cu–Cr alloys at high strain levels.

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    CRYSTALLIZATION BEHAVIOR OF Zr55Al10Ni5Cu30 AMORPHOUS ALLOYS WITH DIFFERENT MORPHOLOGIES AND THERMAL HISTORY CONDITIONS
    HU Qiao LIN Xin YANG Gaolin HUANG Weidong LI Jinfu
    Acta Metall Sin, 2012, 48 (12): 1467-1473.  DOI: 10.3724/SP.J.1037.2012.00433
    Abstract   PDF (2129KB) ( 961 )

    This work investigated the crystallization behavior of Zr55Al10Ni5Cu30  amorphous alloy (bulk and powder with different morphologies) under different heating and cooling rate conditions, and has found that the crystallization behavior of Zr55Al10Ni5Cu30  under different thermal history conditions was quite different. Under conventional heat treatment condition, the heating and cooling rates of the alloy were lower, the Zr55Al10Ni5Cu30  bulk amorphous alloy began to crystallization above the outset crystallization temperature Tx, the CuZr2 nanocrystalline was formed. With increasing temperature, its grain size became bigger, when exceeding the melting point, the thick CuZr2 lath+eutectic organizations of lath (CuZr2)+(Zr, Cu, Al, Ni) were formed under furnace cooling condition; under laser melting condition, however, the heating and cooling rates of alloy were higher, with increasing number of pulse frequency, the crystallization effect of Zr55Al10Ni5Cu30  bulk amorphous alloy accumulated gradually, in heat affected zone spherulites were precipitated and grew up, subsequently became unstable and finally a belt of equiax crystals were formed along the weld pool boundary. For amorphous alloy powder, due to its poorer the cooling performance than bulk amorphous, the cooling rate was lower, causing the powder particles crystallized into equiaxed dendrite during laser melting process, and with increasing laser energy, the equiaxed dendrite size increased.

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    Cu–Y2O3 COMPOSITES PREPARED BY LIQUID PHASE IN SITU REACTION
    ZHUO Haiou TANG Jiancheng YE Nan
    Acta Metall Sin, 2012, 48 (12): 1474-1478.  DOI: 10.3724/SP.J.1037.2012.00362
    Abstract   PDF (890KB) ( 657 )

    A growing trend to use new oxide dispersion strengthened (ODS) copper–based composites is observed recently world–wide. Yttria (Y2O3 ) and most rare earth oxides are potentially attractive as dispersiods for copper–based composites owing to their thermodynamic stability. Cu–0.9Y2O3  (volume fraction, %) composites were prepared with Cu–0.4Y (mass fraction, %) alloy by in situ reaction at liquidus temperature. The objective of the work was to investigatechanges in structure and strengthening mechanism of Cu–Y2O composites. TEM observation and SAD analysis of the composites indicate that the obtained Y2O3  nano–particles are uniformly distributed in copper matrix, their mean size and space between particles are 5.0 nm and 20 nm, respectively, and the cubic Y2O phase is coherent with copper matrix, which indicated (422)Y2O3//(111)Cu and [011]Y2O3//[112]Cu orientation relationship. The strengthening mechanism of the composites is analyzed and explained in three aspects: matrix strengthening, fine particles strengthening according to the Orowan model and shear model. The tensile strength of Cu–0.9Y2O3 composites is 568 MPa, which is strengthened by both Orowan mechanism and shear mechanism, the strength value added by Orowan mechanism and shear mechanism can be calculated to be 185 and 195 MPa, respectively.

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    MICROSTRUCTURES AND MECHANICAL PROPERTIES OF Al2O3–BASIC EUTECTIC IN SITU COMPOSITES DIRECTIONALLY SOLIDIFIED BY LASER FLOATING ZONE REMELTING
    JIA Xiaojiao ZHANG Jun SU Haijun SONG Kan LIU Lin FU Hengzhi
    Acta Metall Sin, 2012, 48 (12): 1479-1486.  DOI: 10.3724/SP.J.1037.2012.00419
    Abstract   PDF (2713KB) ( 628 )

    Directionally solidified oxide eutectic in situ composites have been attracting increasing interest in recent years for use as the next generation of ultra–high–temperature structural materials because of their excellent high–temperature strength, oxidation and creep resistance, as well as outstanding microstructural stability. Al2O3/YAG/ZrO2 ternary eutectic in situ composites with high density are prepared by laser floating zone remelting technique. The microstructure evolution of Al2O3/YAG/ZrO2 ternary eutectic under high temperature gradient and different growth rates is investigated. The relationship between solidification rate and eutectic spacing for the ternary oxideeutectic is quantificationally characterized. On this basis, the mechanical properties and relationship between microstructure and fracture toughness are analysed. The results show that the directionally solidified Al2O3/YAG/ZrO2 ternary eutectic in situ composite belongs to typical irregular lamellar eutectic structure. The microstructure is rapidly refined with the increase of the solidification rate V . The minimal eutectic spacing observed is as fine as 0.46 μm when the solidification rate is 200 μm/s. The relationship between the average eutectic spacing (λav) and V is determined to be λavV 0.5=12.4 μm1.5·s−0.5. Moreover, the ternary eutectic lamellar spacing is much smaller than the binary one at the same solidification condition. The average hardness and room–temperature fracture toughness of the ternary eutectic are (19.0±1.0) GPa and (3.31±0.2) MPa·m1/2, respectively. As compared with the binary eutectic, the crack arrest, deflection and mismatch of thermal expansion coefficient of eutectic phases are the predominant toughening mechanisms of ternary eutectic composite.

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    STUDY ON THE CORROSION RESISTANCE OF Zr–0.7Sn–0.35Nb–0.3Fe–xGe ALLOY IN LITHIATEDWATER AT HIGH TEMPERATURE UNDER HIGH PRESSURE
    XIE Xingfei ZHANG Jinlong ZHU Li YAO Meiyi ZHOU Bangxin PENG Jianchao
    Acta Metall Sin, 2012, 48 (12): 1487-1494.  DOI: 10.3724/SP.J.1037.2012.00434
    Abstract   PDF (3674KB) ( 709 )

    The corrosion resistance of Zr–0.7Sn–0.35Nb–0.3Fe–xGe (x=0.05, 0.1, 0.2, mass fraction, %) alloys was investigated in lithiated water with 0.01 mol/L LiOH at 360 /18.6 MPa by autoclave tests. The microstructures of the alloys and oxide films on the corroded specimens were observed by TEM and SEM. The results show that the corrosion resistance of the Zr–0.7Sn–0.35Nb–0.3Fe alloys in lithiated water at high temperature under high pressure is markedly improved by Ge addition. The alloy with 0.1%Ge shows the best corrosion resistance. In Zr–0.7Sn–0.35Nb–0.3Fe–xGe alloys, there exists fine Zr(Fe, Cr, Nb)2 and Zr(Fe, Cr, Nb, Ge)2 second phase particles (SPPs) with a close–packed hexagonal crystal structure (hcp) and coarse Zr3Ge SPPs with a tetragonal crystal structure (TET). The oxide films formed on the Zr–0.7Sn–0.35Nb–0.3Fe–0.1Ge alloys corroded for 220 d are compact and thin. The micro–pores and micro–cracks are hardly detected and many ZrO2 columnar grains exist in the oxide films formed on the Zr–0.7Sn–0.35Nb–0.3Fe–0.1Ge alloys. This indicates that the suitable amount of Ge could not only delay the process that the vacancies diffuse to form micro–pores and micro–pores develop to form micro–cracks, but also could retard the evolution from ZrO2 columnar grains to ZrO2 equiaxed grains.

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    EFFECTS OF SHEWANELLA ALGAE ON CORROSION OF Zn–Al–Cd ANODE
    ZHANG Jie SONG Xiuxia LUAN Xin SUN Caixia DUAN Jizhou HOU Baorong
    Acta Metall Sin, 2012, 48 (12): 1495-1502.  DOI: 10.3724/SP.J.1037.2012.00309
    Abstract   PDF (1630KB) ( 780 )

    Shewanella is a typical iron–reducing bacteria which can reduce insoluble ferric iron to soluble ferrous iron and consume oxygen, being considered as the reasons of corrosion inhibition. The main study on Shewanella algae (SA) is the degradation of environmentally harmful organic compounds and heavy metals, and there are very few reports on the interaction of metal corrosion and SA. In this paper, the bacteria isolated from yellow rust layer were identified as SA by using molecular biology techniques. The growth curve of SA was determined with spectrophotography. The results showed that the growth curve was divided into three phases: exponential growth phase, steady phase and decay phase. The effects of SA on corrosion of Zn–Al–Cd anode were investigated by using electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) and fluorescence microscopy (FM).The results showed that corrosion potential for samples exposed to the culture medium containing SA was higher than that for samples exposed to the sterile culture medium during the whole experiment. Rct value in the culture medium containing SA was much greater than that of anode in the sterile culture medium. The bacteria could inhibit the corrosion of the specimen. The reason was that a biofilm layer was formed on the sample surface and the oxygen was consumed through the metabolic activities of bacteria in the culture medium containing bacteria. The biofilm layer was formed on the 5th day in the culture medium containing bacteria. While in bacteria–free system, obvious corrosion pits and white corrosion products were seen on the sample surface. The complete biofilm was formed on 7th day, and it detached from the sample on 11th day because of the exhaustion of nutrients and oxygen, showing that the biofilm formation had a great relationship with the presence of nutrients and oxygen.

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    THE INFLUENCE OF SENSITIVE TEMPERATURE ON THE LOCALIZED CORROSION RESISTANCE OF DUPLEX STAINLESS STEEL SAF2304
    GUO Lifang LI Xuyan SUN Tao XU Juliang LI Jin JIANG Yiming
    Acta Metall Sin, 2012, 48 (12): 1503-1509.  DOI: 10.3724/SP.J.1037.2012.00381
    Abstract   PDF (2103KB) ( 695 )

    Duplex stainless steels (DSS), characterized by a two–phase microstructure of ferrite (α) and austenite (γ), have an attractive combination of mechanical strength and corrosion resistance in various aggressive environment. DSS SAF2304 shows wide application potential due to its lower cost compared with conventional DSS and better corrosion performance than austenite steel. However, precipitations of detrimental phases inevitably occur when DSS is heated to temperatures ranging from 300 ℃ to 1000 ℃ during manufacturing and welding procedures. These precipitations will lead to the reduction of corrosion resistance of DSS due to the presence of chromium–depleted zones around them. This work investigates the influence of sensitive temperature on the localized corrosion resistance of DSS SAF2304. The resistances to intergranular corrosion and pitting corrosion of DSS SAF2304 annealed at various temperatures ranging from 600 ℃ to 950 ℃ for 2 h were investigated by means of double loop electrochemical potentiodynamic reactivation (DL–EPR) technique in a solution of 1 mol/L H2SO4+1 mol/L HCl+0.2 mol/L NaCl at 30 ℃ with a scanning rate of 1.667 mV/s and critical pitting corrosion temperature (CPT) technique in a solution of 1 mol/L NaCl with a rising rate of 1 ℃/min, respectively. The morphologies and microstructures of the specimens after electrolytic etching in 30%KOH, oxalic acid and potassium metabisulfite were characterized by OM and SEM techniques. A same trend was observed by the different evaluating techniques, which suggested that both of the resistances of intergranular corrosion and pitting corrosion of DSS SAF2304 decreased with the annealing temperature increased from 600 ℃ to 700 ℃, while a contrary trend was found from 750 ℃ to 950 ℃. In particular, the samples annealed at 700 and 750 ℃ suffered the severest corrosion. The relationship between microstructure and localized corrosion resistance was revealed by the evolution of the microstructure, and it was found that the deterioration of the resistance to localized corrosion was due to the formation of chromium–depleted zones around the precipitation of Cr2N.

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    A NEW ONE–PARAMETER KINETICS MODEL OF DYNAMIC RECRYSTALLIZATION AND GRAIN SIZE PREDICATION
    LIU Juan LI Juqiang CUI Zhenshan RUAN Liqun
    Acta Metall Sin, 2012, 48 (12): 1510-1519.  DOI: 10.3724/SP.J.1037.2012.00486
    Abstract   PDF (2927KB) ( 593 )

    Dynamic recrystallization (DRX) is considered as one of the most important microstructural evolution mechanisms to obtain fine metallurgical structures, eliminate defects and improve mechanical properties of products. Although the DRX kinetics models proposed by researchers have some differences in parameters and forms, they are all based on the Avrami function describing the relationship between dynamically recrystallized volume fraction and strain or time. Avrami equation is in the form of exponential function and the kinetics curve of DRX exhibits different when the exponent is assumed to be different (between 1 and 2). Under these conditions, however, the exponential function cannot exactly describe the "slow–rapid–slow"property of the development speed of DRX process. By introducing the velocity of development of DRX process, which is referred to as the variation of the dynamically recrystallized volume fraction with strain, namely, the first partial derivative of the dynamically recrystallized volume fraction to strain, a new kinetics model of DRX was proposed in comparison with the classical kinetics model of DRX. The new model represents the characteristics of DRX: the dynamically recrystallized volume fraction equals zero when the strain is smaller than the critical strain, and the maximum of the dynamically recrystallized volume fraction equals 1; once the strain exceeds the critical strain, the dynamically recrystallized volume fraction slowly increases first, and then rapidly increases, at last slowly increases. Consequently, the new kinetics model is in agreement with the development law of DRX process and includes few parameters which have clear physical meaning and are easy to determine. By conducting Gleeble–1500 thermomechanical simulation compression tests at the temperatures ranging from 523 to 673 K and at the strain rates 0.001, 0.01, 0.1 and 1 s−1, the kinetics model for Mg alloy AZ31B characterized by DRX for instance was built and parameters were determined. Microscopic examination shows that the experimental results are in good agreement with the predicted values, which validates the accuracy of the new kinetics model.Then combined with grain size of DRX model, the kinetic model built under steady state conditions was rewritten as superimposed step form to apply in the prediction of grain size under unsteady state conditions. The simulated data accord with the experimental results by means of quantitative metallography, which verified the rationality of the superimposed prediction method. 

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    3D FINITE ELEMENT SIMULATION OF PULL–OUT FORCE OF TiNiFe SHAPE MEMORY PIPE COUPLING WITH INNER CONVEX
    ZHANG Huibo JIN Wei YANG Rui
    Acta Metall Sin, 2012, 48 (12): 1520-1524.  DOI: 10.3724/SP.J.1037.2012.00493
    Abstract   PDF (1132KB) ( 562 )

    Dimension design is a key aspect for shape memory pipe coupling, and determines the connecting strength of the connecting unit. Numerical simulation provides a theoretical basis for dimension design. In this paper, a three–dimensional constitutive model was developed to simulate the deformation and recovery process of TiNiFe shape memory pipe coupling. Finite element method is applied to simulate the stress distribution and pull–out force of the connecting units. Properties of TiNiFe alloy used in the simulation are measured from experiments. The influences of the inner convex and its height on the pull–out force are investigated. The simulated results reveal that the inner convex increases the pull–out force of the connecting units, which increases linearly with the height of the inner convex within the scope of simulations. The experimental results are in good agreement with the simulated pull–out forces with a deviation of less than 4%.

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    A STUDY OF RESIDUAL STRESS IN THE REPAIR WELD OF STAINLESS STEEL CLAD PLATE BY NEUTRON DIFFRACTION MEASUREMENT AND FINITE ELEMENT METHOD
    JIANG Wenchun WOO Wanchuck WANG Bingying TU Shan–Tung
    Acta Metall Sin, 2012, 48 (12): 1525-1529.  DOI: 10.3724/SP.J.1037.2012.00341
    Abstract   PDF (1109KB) ( 1012 )

    Stainless steel clad plate is widely used in petrochemical engineering because of its high strength and good corrosion resistance, but there are cracks to be always generated in clad metal during the fabrication and service. Welding is often used to repair the cracked zone in stainless steel clad plates, but the residual stress is generated inevitably during welding, which has a great effect on their structure integrity. Chinese code, safety assessment for in–service pressure vessels containing defects, requires through–thickness stress distribution, and assumes that the secondary membrane stress induced by welding is equal to the yield stress, and the bending stress is zero. This work uses a combination of neutron diffraction method and finite element method (FEM) to determine the residual stress in the repair weld of a stainless steel clad plate. It is found that there is a good agreement between FEM and experimental results. The residual stress is concentrated in the heat affected zone (HAZ) of the weld metal, and decreases gradually far away from this zone. Due to the local heating input difference through thickness of the clad plate, a bending stress is generated in it. In the clad metal, the maximum stress is located at HAZ, which has exceeded the yield strength because of work hardening. Therefore, the assessment result based on the code assumption would lead to a big difference from the actual one. The combined use of FEM and neutron diffraction can obtain both the stress values and through–thickness stress distribution, which meet the requirement how to treat the welding residual stress in the structure integrity assessment code.

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    CORROSION BEHAVIOR OF X80 STEEL IN YINGTAN SOIL SIMULATED SOLUTION UNDER DISBONDED COATING
    ZHAO Bo DU Cuiwei LIU Zhiyong LI Xiaogang YANG Jike LI Yueqiang
    Acta Metall Sin, 2012, 48 (12): 1530-1536.  DOI: 10.3724/SP.J.1037.2012.00374
    Abstract   PDF (4478KB) ( 1003 )

    In this work, a rectangular crevice disbanded coating model of buried steel pipeline in Yingtan soil simulated solution was made, and the in situ electrochemical measurement of electrochemical impedance spectroscopy (EIS) was used for characterization of X80 steel under the disbonded coating, at the same time, the corrosion behavior was also investigated. The results showed that the electrochemical characteristics in each position of disbonded area was almost same in the early corrosion time, and the EIS was composed of high–frequency capacitance and low–frequency inductance. After corrosion occurred, the high–frequency capacitance radius increased, and the low–frequency inductance disappeared. The extent of corrosion of X80 steel specimen surface was rather distinct with distance from holiday. The corrosion at holiday and the bottom of disbonded area was most serious because of oxygen corrosion and anodic dissolution. However, in the central, it was weaker. After removing the corrosion product, pits appeared obviously on the specimen surface in the central of disbonded area,indicating that the tendency of pitting occurring increased, and the type of corrosion changed from general corrosion to localized corrosion. According to the EIS and experimental results, the corrosion process under disbanded area could be divided into three steps: oxygen depletion, anion migration and corrosion expansion.

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