ISSN 0412-1961
CN 21-1139/TG
Started in 1956

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    INFLUENCE OF COHERENT AUSTENITE TWIN BOUNDARIES ON THE VARIANT SELECTION OF BAINITIC FERRITE IN Fe-C-Mn-Si STEELS
    WANG Xixia, GUO Hui, WANG Ding, BAI Yin, YANG Shanwu, HE Xinlai
    Acta Metall Sin, 2012, 48 (4): 385-392.  DOI: 10.3724/SP.J.1037.2011.00697
    Abstract   PDF (1160KB) ( 1012 )
    To develop higher performance steels the requirements of the control of the microstructure is increasingly enhanced. The crystallographic orientation characteristics in the microstructure attract more and more attention because the mechanical properties are usually determined by the grain boundary density, especially high angle grain boundary density. So far researchers have conducted comprehensive investigation of the variant selection during the phase transformation. To decrease the nucleation barrier, the new phase usually chooses a specific orientation (variant) to decrease the interfacial energy between the parent and new phases. The variant selection occurs on the grain boundaries has been well studied. The nucleation and variant selection on austenite twin boundaries, however, are seldom reported, especially in the low carbon steels in which austenite can hardly be retained to the room temperature. In this work, the variant selection of bainitic ferrite on coherent austenite twin boundaries was studied using electron back scattering diffraction (EBSD) in Fe-C-Mn-Si steels with 0.05%C or 0.4%C (mass fraction). The orientation relationship between bainitic ferrite and austenite is close to K-S relationship in both steels. It was observed that the variant pairs with the similar crystallographic orientation nucleated on both sides of austenite twin boundaries. The twin grain boundaries were erased after the bainitic ferrite variants grew up. Crystallographic analysis showed that no more than three pairs of variants could be formed on one austenite twin boundary, the habit planes of which were all parallel to the twin boundary. As a result, the bainitic ferrite variant nucleated first would grow and expand along the twin boundary. In 0.05C steel, only one pair of variants was observed on the austenite twin boundary because the first nucleated bainitic ferrite variant pair grew fast due to the low carbon content which covers the twin boundary very soon and leaves no chance for the other variant pairs to nucleate on this twin boundary. In 0.4C steel, all three pairs of variants could be formed on one twin boundary because the higher carbon content slowed down the growth of first nucleated bainitic ferrite and more variant pairs could be nucleated on the twin boundary.
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    HETEROGENEOUS MICROSTRUCTURE AND TEXTURE EVOLUTION DURING FABRICATION OF Zr-Sn-Nb  ZIRCONIUM ALLOY SHEETS
    CHEN Jianwei, LUAN Baifeng, CHAI Linjiang, YU Hongbing, LIU Qing,ZHOU Jun, LI Zhongkui
    Acta Metall Sin, 2012, 48 (4): 393-400.  DOI: 10.3724/SP.J.1037.2011.00809
    Abstract   PDF (1125KB) ( 917 )
    Zirconium alloys are widely used as fuel cladding and structural materials for nuclear reactors due to the low neutron absorption cross-section, good corrosion resistance and acceptable mechanical properties. These properties are greatly dependent on microstructural and textural features, such as grain morphology, grain size, crystallographic texture and distribution of precipitates. It is necessary to understand microstructure and texture evolution during fabrication in order to optimize the manufacturing process and to improve the service performance. In this work, microstructure and texture evolution during fabrication of Zr-Sn-Nb new zirconium alloy sheets are investigated using XRD, SEM-ECC, TEM and EBSD. The results show that the random texture formed by β quenching transforms into tilt basal texture after hot rolling. The basal texture keeps stable during the following  fabrication stages. The texture of the rolling sheets is mainly characterized as <1010> direction parallel to rolling direction (<1010>//RD), while the texture of the annealing sheets is <1210> direction parallel to rolling direction (<1210>//RD). The microstructure evolves from a weave Widmansatten structure of β quenching stage to heterogeneous deformation structures associated with hot and cold rolling and then to a fully recrystallized structure after final annealing. The cold rolling sheets present more heterogeneous structures in which the C axes of less deformed grains mostly concentrate in the normal direction. The larger grains in annealed structures mostly belong to the <1210>//RD basal texture while the smaller grains are in the <1010>//RD orientation. The reason for the heterogeneous deformation structures and texture evolution during annealing are discussed according to the deformation and recrystallization mechanisms.
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    INVESTIGATION OF THE TOUGHNESS OF LOW CARBON TEMPERED MARTENSITE IN THE SURFACE OF Ni-Cr-Mo-B ULTRA-HEAVY PLATE STEEL
    WANG Xiaoyong, PAN Tao, WANG Hua, SU Hang,LI Xiangyang, CAO Xingzhong
    Acta Metall Sin, 2012, 48 (4): 401-406.  DOI: 10.3724/SP.J.1037.2011.00698
    Abstract   PDF (820KB) ( 935 )
    Low carbon tempered martensite structures in the surface of Ni-Cr-Mo-B ultra-heavy plate steel was studied. The toughness at different locations in the surface zone was investigated by impact testing, and the sub-microstructures, such as packets and blocks of the martensite, at different distances (3, 15 and 25 mm) from the surface were observed by OM, SEM and electron backscattered diffraction (EBSD), and the dislocation density were tested by XRD and positron annihilation spectroscopy (PAS). The results indicate that the toughness of the tested steel in the 10 mm from the surface deteriorates sharply, which is mainly due to the bigger packet and block size and higher dislocation density in the tempered martensite.
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    STUDY ON THE PRECIPITATION OF Cu-RICH CLUSTERS IN THE RPV MODEL STEEL BY APT
    XU Gang, CAI Linling, FENG Liu, ZHOU Bangxin,LIU Wenqing, WANG Jun'an
    Acta Metall Sin, 2012, 48 (4): 407-413.  DOI: 10.3724/SP.J.1037.2011.00606
    Abstract   PDF (759KB) ( 918 )
    Reactor pressure vessel (RPV) is nonreplaceable component for the pressurized water reactor (PWR) in the nuclear power plants. RPVs are usually made of low alloy ferritic steels and A508-III steel is one type of these materials. After long-term service under the neutron irradiation, the ductile-to-brittle transition temperature (DBTT) of the RPV steel, which is the main parameter used to measure the degree of the embrittlement, will shift towards higher temperature. This phenomenon is termed irradiation-induced embrittlement, and it is a main factor to affect the operation safety and the lifetime of nuclear power plants. It is realized that the irradiation-induced embrittlement is mainly attributed to the precipitation of Cu-rich nanophases with a high number density. The precipitation process of Cu-rich nanophases can be well characterized by an atom probe tomography (APT) analysis for their size, composition and number density, and the Cu-rich nanophases obtained by the APT analysis are usually termed Cu-rich clusters. It is worthwhile to investigate the  precipitation process of Cu-rich clusters by thermal aging for better understanding the mechanism of embrittlement. In order to accelerate the precipitation of Cu-rich clusters, experiment was performed by a RPV model steel containing higher Cu content than commercially available A508-III steel. RPV model steel was prepared by vacuum induction melting with higher content of Cu (0.6%, mass fraction). The specimens of the RPV model steel were tempered at 660 ℃ for 10 h followed by air cooling after water quenching from 880 ℃, and then they were isothermally aged at 370 ℃ for different time. The precipitation process of Cu-rich clusters is investigated by APT analysis. The results show that the Cu-rich clusters are on the stage of the nucleation when the specimens were aged at 370 ℃ for 1150 h. After specimens were aged for 3000 and 13200 h, the average equivalent diameter of the Cu-rich clusters increases from 1.5 nm to 2.4 nm, and the average Cu content in the Cu-rich clusters vary from 45% to 55 % (atomic fraction). The number density of the Cu-rich clusters in both types of the specimens is at the order of 1022 m-3. The Cu concentration in the ferritic matrix is (0.15±0.02)% for the specimen aged at 370 ℃ for 13200 h, which is still higher than the limitation of Cu solubility in the ferritic matrix at 370 ℃. It means that the precipitation process of Cu-rich clusters does not reach the equilibrium state. The analysis results also show that Ni, Si, P atoms, but not Cu atoms, segregate near the interface between the cementite and the ferritic matrix, and Mn, Mo, S atoms are enriched in the cementite.
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    LOCALIZED ELECTROCHEMICAL CHARACTERIZATION OF HIGH STRENGTH ALUMINIUM ALLOY AT THE CRACK TIP IN 3.5NaCl SOLUTION
    SHENG Hai, DONG Chaofang, XIAO Kui, LI Xiaogang
    Acta Metall Sin, 2012, 48 (4): 414-419.  DOI: 10.3724/SP.J.1037.2011.00692
    Abstract   PDF (747KB) ( 972 )
    The electrochemical microcapillary technique and scanning Kelvin probe (SKP) were applied to study the localized electrochemical characterization and corrosion behavior at the crack tip of 2024-T351 aluminium alloy in 3.5NaCl. To investigate the effect of stress on the localized corrosion process at the crack tip, numerical simulation of the stress distribution on a pre-cracked wedge-open loading (WOL) specimen was conducted using a commercial software package ABAQUS 6.10. Polarization curves revealed that corrosion potential at the crack tip was more negative than that within the region away from the crack tip. Localized electrochemical impedance spectroscope (EIS) showed that the passive film was thinner at the crack tip than within the region away from the crack tip. The results indicate that the crack tip is more electrochemically active than the region away from the crack tip. Furthermore, passive film at the crack tip was less stable than that on other region of the specimen surface. SKP measurements demonstrated that there was a non-uniform distribution of Volta potential on the pre-cracked WOL specimen surface, with a more positive Volta potential occurring at the crack tip after 24 h immersion into 3.5NaCl solution. This might be explained by the preferential anodic dissolution and the accumulation of the corrosion product at the crack tip. Numerical simulation results showed that a very high local stress concentration was developed at the crack tip, which could enhance the electrochemical activity around the crack tip and promote the dissolution process therein. Moreover, a galvanic couple could also occur between the crack tip acting as the anode and the distant region as the cathode, which results from the differences of the corrosion potentials and the electrochemical activities between them. As a result, the anodic dissolution of 2024-T351 aluminium alloy at the crack tip is enhanced.
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    ELECTROCHEMICAL STUDY ON CORROSION BEHAVIORS OF MILD STEEL IN A SIMULATED TIDAL ZONE
    MU Xin, WEI Jie, DONG Junhua, KE Wei
    Acta Metall Sin, 2012, 48 (4): 420-426.  DOI: 10.3724/SP.J.1037.2011.00666
    Abstract   PDF (3759KB) ( 1080 )
    The environment of the tidal zone is very complex. And the dry-wet alternation and interaction of sea erosion leads to serious corrosion of metal materials, making it difficult to adopt protective measures. Therefore, it is of great significance to study the corrosion and protection of metal materials in the tidal zone. Corrosion weight loss results showed that the corrosion behaviors of metal in different regions of tidal zone were altered, but the corrosion mechanism of metal materials in the tidal zone is not obvious. In order to study the corrosion mechanism of tidal corrosion, without considering the seawater splashing effect, a corrosion experimental trough was designed to simulation the tidal zone and immersion zone, and the corrosion behaviors of Q235B mild steel (designated) in it were monitored in situ by the potentiostat and electrochemical workstation. The results showed that the corrosion behaviors of Q235B steel at diverse positions are not the same. The corrosion rate of Q235B steel in the mid tide zone and low tide zone is higher than the highest tide zone and immersion zone. In a tidal range fluctuation cycle, the open-circuit potential variation law of the Q235B steel is related with the dry/wet state of the metal surface and the cathode/anode reactions. In the long term tidal corrosion process, the open-circuit potential variation of Q235B steel is related with the thickness of the rust layer.
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    TYPE IV CREEP RUPTURE CHARACTERISTICS OF P92 STEEL WELDMENT
    WANG Xue1, PAN Qiangang2, TAO Yongshun2, ZHANG Yinglin1,ZENG Huiqiang2,LIU Hong2
    Acta Metall Sin, 2012, 48 (4): 427-434.  DOI: 10.3724/SP.J.1037.2011.00646
    Abstract   PDF (1096KB) ( 1161 )
    Creep tests at 600~650℃ with applied stresses in the range 100~240MPa and microstructural observations by means of OM, SEM, TEM were conducted on weld joints of P92 steel prepared by SAW process to investigated its characteristics of Type Ⅳ creep rupture. The results showed that Type Ⅳ failure took place at higher temperature and lower stress and tend to have a critical condition expressed by Larson-Miller parameter(L.M.P.)or stress level which values are 35.5 and 120MPa respectively. Type Ⅳ failure showed a lack of ductility and located in the fine grained HAZ(heated to just above AC3)close to intercritical HAZ, where microstructural changes are obviously different from those in the base metal, including formation of equiaxed sub-grain structure, mass precipitation and rapid growth of Laves phases on the grain boundaries during creep exposure, which lead to the Type Ⅳ failure. The size of M23C6 carbide in the AC3 FGHAZ was almost the same as that in the base metal, which has little effect on the failure. Type Ⅳ rupture is a brittle intergranular fracture due to cavity coalescence, which were nucleated at coarse precipitates of Laves phase. The void area ratio of f or AA is employed to quantify grain boundary damage and evaluate Type Ⅳ failure of P92 steel weld joints, and when their values were above 1~1.2% or 0.5%, Type Ⅳ failure would occur.
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    EFFECT OF COOLING PATH ON THE HOLE-EXPANSION PROPERTY OF MEDIUM CARBON STEEL
    WANG Bin, LIU Zhenyu, ZHOU Xiaoguang, WANG Guodong
    Acta Metall Sin, 2012, 48 (4): 435-440.  DOI: 10.3724/SP.J.1037.2011.00738
    Abstract   PDF (809KB) ( 734 )
    In the present paper, ultra fast cooling (UFC) technology was applied in the cooling process to treat medium carbon steels after hot strip rolling, and the finish rolling temperature and UFC stop temperature are controlled as the important parameters. The effects of cooling path on the microstructure and hole-expansion property of annealing medium carbon steels were investigated. The results show that finely dispersed spherical cementite could be formed in ferrite matrix after annealing treatment. With decreasing the fininsh rolling temperature and UFC stop temperature, the spheroidized cementites after annealing were more homogeneous and dispersed finely. During hole-expansion test, cracks were observed to form usually in the edge region around the punched hole when the tangential elongation exceeded the forming limit, and cracks are mainly formed in the way of micro-voids coalescence. Fine and homogeneous microstructure comprised of ferrite and spheroidized cementite could improve the elongation of the experimental sheets, suppressing the coalescence of micro-voids, and improving the hole-expansion property.
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    EFFECTS OF COOLING PROCESS ON MICROSTRUCTURE, MECHANICAL PROPERTIES AND PRECIPITATION BEHAVIORS OF NIOBIUM-TITANIUM MICRO-ALLOYED STEEL
    CHEN Jun, TANG Shuai, LIU Zhenyu, WANG Guodong
    Acta Metall Sin, 2012, 48 (4): 441-449.  DOI: 10.3724/SP.J.1037.2011.00633
    Abstract   PDF (1115KB) ( 813 )
    The tested steels were cooled to room temperature using different cooling paths after two-stage rolling, and effects of cooling paths on microstructure, mechanical properties and precipitation behaviors of Nb-Ti micro-alloyed steels were investigated. The results show the hot rolled plates with fine grain were produced at the cooling path of ultra fast cooling + air cooling, and the average grain size, lower yield strength and ultimate tensile strength are about 7.76 μm, 425 MPa and 500 MPa, respectively. The fine precipitation particles ranging from 2 nm to 7 nm were observed in the samples cooling with ultra fast cooling + furnace cooling, but are only a few globular precipitates in the samples cooling with ultra fast cooling + air cooling. The inter-phase precipitation was observed in samples cooling with air cooling after finish rolling. These plates with different cooling paths were annealed at 700 ℃ for 300 s. The precipitation particles were obviously coarsened during annealing. It can be found that the average grain size of the samples with cooling path of ultra fast cooling + furnace cooling is 6.47 μm and the increments of lower yield strength and ultimate tensile strength are about 50 and 30 MPa, respectively. The strength increment mainly depends on fine grain strengthening.For niobium-titanium micro-alloyed steels containing 0.03%Nb (mass fraction), because the volume fraction of precipitates is limited, grain boundaries strengthening is higher than precipitation hardening, making changes of strength be in good agreement with that of grain size. In addition, the strain hardening exponent is mainly related to average grain size, and strain hardening exponent increases with average grain size increasing.
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    EFFECT OF CARBON AND NIOBIUM CONTENTS ON PHASE PARAMETERS AND CREEP RUPTURE TIME AT 650 ℃ FOR TP347HFG STEEL
    PENG Zhifang, DANG Yingying,PENG Fangfang
    Acta Metall Sin, 2012, 48 (4): 450-454.  DOI: 10.3724/SP.J.1037.2011.00677
    Abstract   PDF (555KB) ( 882 )
    TP347HFG is an austenitic stainless steel which is considered to be among the grades with the highest potential for use in super critical boilers. It has been reported that micro-addition of Nb and a relatively low level of carbon content can obviously enhance creep resistance in this class of materials. Thus, C and Nb content optimization is of significant interest within the composition range from ASME standard. In this study, the effect of carbon and niobium contents on phase parameters (phase composition, volume fraction and size) and creep rupture time and the related mechanism were investigated for TP347HFG steel. Creep rupture tests were carried out under applied stresses of 230 and 150 MPa, respectively, at 650 ℃. The rupture times of two samples with different C and Nb contents were 199 and 420 h at 230 MPa, 2426 and 8837 h at 150 MPa, respectively. The sample with lower C content and higher Nb content corresponded to longer rupture time at each of the stress level. The EPMA-EDS+MPSM (multiphase separation method) and TEM-EDS results show that the creep rupture times were remarkably increased for the sample with relatively lower C and higher Nb contents, which correspond to increasing the volume fraction of nano-scale MX and decreasing that of M23C6 as well as retarding the coarsening of M23C6. On the other hand, more Cr maintained in matrix could also benefit the creep rupture lives. In addition, Thermo-Calc software was used to calculate the mole fraction and the concentration of precipitated phases with various combinations of C and Nb contents over the range of 500-1300℃ and the results were in good agreement with the experimental ones.
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    EFFECT OF QUENCHING TEMPERATURE ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF 550 MPa GRADE THICK STEEL PLATE
    WAN Decheng, YU Wei, LI Xiaolin, ZHANG Jie, WU Huibin, CAI Qingwu
    Acta Metall Sin, 2012, 48 (4): 455-460.  DOI: 10.3724/SP.J.1037.2011.00722
    Abstract   PDF (909KB) ( 1263 )
    The heat treatments of 730-910 ℃ quenching and 600 ℃ tempering were applied to enhance the low temperature toughness of 550 MPa grade thick steel plate. Moreover, the effect of quenching temperature on the microstructure and mechanical properties was studied. The results showed that strength and toughness of the specimen decreased at first and then increased as quenching temperature increased within intercritical region. When the quenching temperature was raised up to austenite region, the strength increased further, but the toughness decreased. Mechanical properties of the steel subjected to intercritical quenching at 760 ℃ and tempering were the worst of all of the specimen, due to coarsened polygonal ferrite and the lath, acicular M/A constituent along grain boundaries and inside the grains, which deteriorated the toughness seriously. On the other hand, the steel treated by intercritical quenching at 850 ℃ and tempering showed the optimum combination of strength and toughness compared with the steel treated by quenching after austenization and tempering. This is attributed to microstructure refinement, higher fraction of high angle grain boundaries caused by the formation of ferrite, abundant homogeneous dislocation cell substructure and stable thin film retained austenite.
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    HOT CORROSION BEHAVIOR OF A (NiCoCrAlYSiB+AlSiY) COMPOSITE COATING
    LU Xuyang, YU Daqian, JIANG Sumeng, LIU Shanchuan, GONG Jun, SUN Chao
    Acta Metall Sin, 2012, 48 (4): 461-468.  DOI: 10.3724/SP.J.1037.2012.00031
    Abstract   PDF (1408KB) ( 697 )
    Two kinds of the coatings, the conventional NiCoCrAlYSiB coating and the composite coating (NiCoCrAlYSiB+AlSiY), were prepared using arc ion plating (AIP) on the Ni base single crystal superalloy substrate. Comparative studies of the hot corrosion behaviors of the substrate and two different coatings in Na2SO4 + K2SO4 and Na2SO4 + NaCl at 900 and 700 ℃ were investigated. The results showed that, during the hot corrosion at 900 ℃, NiO was formed on the surface of the substrate while Cr2O3 was formed on the surface of the conventional coating with the formation of internal oxidation and sulfidation, and Al2O3 was formed on the surface of the composite coating. Slighter internal oxidation appeared in the outer layer of the composite coating. The composite coating contained sufficient Al reservoirs to supply the formation and repair of Al2O3. During the hot corrosion at 700 ℃, NiO was formed on the surface of the substrate and Cr2O3 was formed on the surface of the conventional coating with severe internal oxidation, while internal oxidation appeared in the outer layer of the composite coating. The Cr-enriched inner layer did not suffer from corrosion damage, which can improve the corrosion resistance of the composite coating.
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    INFLUENCE OF C CONTENT ON STRUCTURE AND MECHANICAL PROPERTIES OF ZrCN COMPOSITE FILMS
    YU Lihua, MA Bingyang, XU Junhua
    Acta Metall Sin, 2012, 48 (4): 469-474.  DOI: 10.3724/SP.J.1037.2011.00714
    Abstract   PDF (800KB) ( 1375 )
    ZrCN thin films with different C contents were deposited by reactive unbalanced magnetron sputtering. Their chemical composition, microstructure, surface morphology, mechanical and tribological properties were investigated by XPS, XRD, SEM, AFM, nanoindentation and tribometer. The results indicated that the atomic ratios of (C+N)/Zr played an important role in phase configuration, microstructure, mechanical and tribological properties. When the ratio was less than 1, a Zr(C, N) solid solution was formed due to the dissolution of C into the ZrN lattice. When the ratio was larger than 1, the amorphous phase CN and C appeared and the ZrCN thin films had a fcc crystal structure. As the C contents increased, the diffraction peak decreased and widen, and the hardness of ZrCN thin films increased first and then decreased. As the C contents increased, the coefficient of friction of ZrCN thin films decreased and the wear scar became more shallower and narrower. The incorporation of C changed the wear mode and improved the friction and wear behaviors. The hardness of ZrCN film reached 31 GPa and friction coefficient was 0.26 when C content was 13.2%.
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    PREPARATION, STRUCTURE AND MAGNETIC PROPERTIES OF SmCo5 NANOPARTICLES AND NANOFLAKES
    LIU Rongming, YUE Ming, ZHANG Dongtao,Liu Weiqiang, ZHANG Jiuxing
    Acta Metall Sin, 2012, 48 (4): 475-479.  DOI: 10.3724/SP.J.1037.2011.00643
    Abstract   PDF (662KB) ( 868 )
    Surfactant-assisted high energy ball milling technique is a new method of producing magnetic nanoparticles. In this study, permanent magnetic SmCo5 nanoparticles and nanoflakes with high room-temperature coercivity values and narrow particle size distributions were produced by this technology and a subsequent size-selection process. The SmCo5 nanoparticles with average particle sizes of 9.8 and 47.5 nm, exhibited room-temperature coercivity values of 6.8×104 and 7.3×105 A/m, respectively, while the SmCo5 nanoflakes, with the mean particle size of about 1.4 μm and average thickness of 75 nm, showed excellent permanent magnetic properties with an obvious c-axis crystal texture, a strong magnetic anisotropy and high coercivity values of 5.5×105 and 1.6×106 A/m in their easy-axis and hard-axis directions, respectively. The coercivity values of SmCo5 nanoparticles and nanoflakes exhibited a significant particle size dependance effect.
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    EFFECTS OF ELECTRICAL CURRENT AND ITS POLARITY ON THE PROPERTIES OF FRICTION AND WEAR OF COPPER-IMPREGNATED METALLIZED CARBON
    WANG Yian, LI Jinxu, QIAO Lijie
    Acta Metall Sin, 2012, 48 (4): 480-484.  DOI: 10.3724/SP.J.1037.2011.00674
    Abstract   PDF (779KB) ( 606 )
    Copper-impregnated metallized carbon has been widely used in maglev vehicles and high-speed railway trains due to its excellent electrical conductivity and high mechanical strength. The wear of copper-impregnated metallized carbon has aroused wide concern. To decrease the cost of maintenance and keep trains running safely, a better understanding of the wear mechanisms is needed. In this work, the effects of electrical current and its polarity on sliding friction and wear of copper-impregnated metallized carbon against Cr-Zr-Cu alloy rings were studied on UMT-2 tribometer with a brush-on-ring configuration. SEM and EDS were used to observe the morphologies of the worn surfaces and analyze the compositions of worn surfaces. The results showed that the wear mass loss increased with the rising of electrical current, the friction coefficient with electrical current was lower than that without electrical current. The wear mass losses of positive brush specimens were higher than those of negative brush specimens. It was found that the surface damage of the worn surface of brush specimens became more serious with greater electrical current, the positive brush specimen suffered a heavier oxidation than that of negative brush specimen. Abrasive wear, adhesive wear and arc erosion were the dominant mechanisms during the electrical sliding process.
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    PHASE-FIELD METHOD SIMULATION OF THE EFFECT OF ELASTIC STRAIN ENERGY ON COARSENING DYNAMICS DURING THE α2O PHASE TRANSFORMATION IN Ti-Al-Nb ALLOYS
    ZHOU Guangzhao, WANG Yongxin, CHEN Zheng
    Acta Metall Sin, 2012, 48 (4): 485-491.  DOI: 10.3724/SP.J.1037.2011.00704
    Abstract   PDF (778KB) ( 817 )
    The Ti-Al-Nb alloys have received significant attention due to its excellent properties for high-temperature applications. The α2O phase transformation taking place in these alloys leads to complex multi-variant and multi-domain microstructure, which has been extensive researched by experimental studies. The morphology, size, spatial arrangement of multi-variant and the volume fraction of precipitated phase, which are determined by the elastic strain energy, affect the important physical and mechanical properties of these alloys. So it is important to examine the effect of elastic strain energy on coarsening dynamics during the α2O phase transformation. In this study, phase-field method has been used to simulate the α2O phase transformation, and the effect of elastic strain energy on coarsening dynamics especially the morphology, orientation, number and the volume fraction of precipitated phase particles have been discussed. The results show that elastic strain energy has great impact on the morphology and orientation of precipitated phase particles. As the result of elastic strain energy, particles transformed into block and aligned along the elastic soft directions. The greater elastic strain energy in system without applying any external stress, the easier nucleation and the smaller volume fraction and mean size of particles when system was steady. An applied stress can result in the selective growth of precipitated phase variants, which promotes the precipitation of favored variants and retards the precipitation of other variants, finally changes the morphology. When system applied small pressure stress, the number of particles increased which eventually led to the reduction of mean size. Volume fraction of precipitated phase increased with increasing external stress when it is over a certain extent.
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    VALENCE ELECTRON STRUCTURE ANALYSIS OF EQUILIBRIUM AND METASTABLE PHASES OF Al3M(M=Ti, Zr, Hf)
    HUANG Lian, GAO Kunyuan, WEN Shengping, HUANG Hui, WANG Wei, NIE Zuoren
    Acta Metall Sin, 2012, 48 (4): 492-501.  DOI: 10.3724/SP.J.1037.2011.00742
    Abstract   PDF (633KB) ( 1503 )
    The valence electron structure of Al3M(M=Ti, Zr, Hf) with three crystal structures (L12, D022, D023) and the corresponding strongest bond energy (EA) values have been calculated from the empirical electron theory (EET) of solids and molecules. Based on the calculated EA, the stability of the phases with different structures and the sequence of phase transition have been analyzed semi--quantitatively. The results showed that, the EA of the equilibrium phases, i.e., D022-Al3Ti, D023-Al3Zr and D022-Al3Hf, were 57.7, 71.6 and 75.6 kJ/mol, respectively, which showed the same trend in magnitude with the corresponding melting point. This consistence supports the reliability of EET--based calculation results. Similarly, the EAof Al3Ti, Al3Zr and Al3Hf with three structures have been calculated and the calculated phase transition sequences are the same as the experimental results and those from first--principles calculation. The L12-type metastable phases of three intermetallic compounds exhibit many excellent characteristics, whereas their phase stability is crucial for application. The EA is supposed to be a measure for the stability of metastable phase. The calculated EA of L12 structure implied the phase stability in the order of Al3Ti3Zr3Hf, which was the same as that from the transition temperatures experimentally. The EA calculated by EET, therefore, could be a good measure for the stability of metastable phase.
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    INFLUENCE OF TEMPERATURE ON PITTING CORROSION RESISTANCE OF Cr26Mo1 ULTRA PURE HIGH CHROMIUM FERRITE STAINLESS STEEL IN 3.5%NaCl SOLUTION
    WEI Xin, DONG Junhua, TONG Jian, ZHENG Zhi,KE Wei
    Acta Metall Sin, 2012, 48 (4): 502-507.  DOI: 10.3724/SP.J.1037.2011.00489
    Abstract   PDF (665KB) ( 792 )
    The remarkable uniform corrosion resistance of ultra pure high chromium ferrite stainless steel in various rigorous corrosive environments is based on the formation of a passive film on its surface. However, it is suspected that this stainless steel was easy to suffer from pitting corrosion. In this work, the influences of temperature on the pitting corrosion resistance of Cr26Mo1 ultra pure high chromium ferrite stainless steel were studied by electrochemical methods such as cyclic polarization curves, Mott-Schottky curves and electrochemical impedance spectroscopy in 3.5%NaCl solution. The results showed that with the increase of the temperature, open-circuit corrosion potential and pitting corrosion potential decreased, corrosion current density increased, the impedance of passive film decreased. The semi-conductive styles and properties of passive film changed at different temperatures. Also, the pregnancy time of pitting shortened and the sensitivity of pitting increased remarkably as the temperature increased. In addition, the cyclic polarization curves indicated that the repassivation of existing pits was more difficult when the potential was swept toward the negative direction.
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    ELECTROCHEMICAL PREPARATION OF PANI/CeO2 COMPOSITE WITH SPECIAL MORPHOLOGY
    SANG Xiaoguang, ZENG Fanwu, LIU Xiaoxia
    Acta Metall Sin, 2012, 48 (4): 508-512.  DOI: 10.3724/SP.J.1037.2011.00483
    Abstract   PDF (626KB) ( 825 )
    Composite materials consisting of conducting polymers and inorganic particles have attracted considerable attention as they combine the advantages of both components and have potential applications in many fields. Electrodeposition is an attractive method for the preparation of thin films since it offers the advantages of low processing temperature, normal handling pressure, high purity of deposition, and controlled thickness of the film. Polyaniline/CeO2 (PANI/CeO2) composite with “horn--like” morphology was prepared by potentiostatic electrodeposition method in 0.05 mol/L Ce(NO3)3 containing 0.05 mol/L CH3COOH and 0.025 mol/L aniline (An) at 1.1 V (vs SCE) (pH=5.5). The morphology and composition of the PANI/CeO2composite were characterized by SEM and TEM. The results indicated that the PANI/CeO2 composite had “horn--like” morphology. Characteristic vibrations of PANI were observed on the Fourier transform infrared spectrum (FTIR) of PANI/CeO2 composite. The PANI/CeO2 composite displayed characteristic electrochemical behaviors of PANI in phosphate buffer (pH=6.9), based on cyclic voltammetric experiments. The influences of electrodeposition conditions on the morphology of the composite were investigated by changing the reaction time and the potentiostatic potential. The results showed that potential is the key factor for the formation of “horn--like” PANI/CeO2 composite. PANI/CeO2 composite obtained at 0.8 V existed in nanoparticles. No “horn-like” composite was observed at this lower potential, even after prolonged reaction time. More protons were released during the electrochemical polymerization of aniline and the electrochemical deposition of CeO2 at higher potential. One--dimensional growth of PANI was promoted due to the effect control of secondary growth of PANI at lower pH media, resulting in nanofibrous composite. More PANI and CeO2 were formed along with the electrodeposition and so more protons were released. Thus the nanofiber changed thinner and “horn-like” PANI/CeO2 was formed.
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