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

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    Original Article
    DEVELOPMENT OF CELLULAR AUTOMATON MODELS AND SIMULATION METHODS FOR SOLIDIFICATION OF ALLOYS
    ZHAO Jiuzhou , LI Lu , ZHANG Xianfei
    Acta Metall Sin, 2014, 50 (6): 641-651.  DOI: 10.3724/SP.J.1037.2013.00567
    Abstract   HTML   PDF (3006KB) ( 881 )
    Dendritic structure is the most frequently observed solidification microstructure of alloys. It has a dominant effect on the mechanical properties of alloys. The formation of the dendritic microstructure has attracted extensive attentions. It has been demonstrated that numerical simulation is a powerful tool for studying the microstructure formation during the solidification of alloys. Various models, such as the front-tracking (FT) model, the phase-field (PF) model and the cellular automaton (CA) model have been proposed to simulate the formation process of dendrite. Compared with other methods, CA is an effective numerical simulation method with high calculation efficiency and clear physical meaning. It is more suitable to be applied to simulate the formation kinetics of the dendritic microstructure of alloys. It has been widely applied in the investigation of the solidification of alloys. This paper makes a detailed introduction to the common process of CA modeling and simulation, the constructing method of CA model and the calculation method for some key parameters such as nucleation rate of nuclei, growth velocity of dendrite, etc. A review of the development of the CA models for the solidification of alloys is carried out. The features and applications of the existing CA models are critically assessed. The applications of the CA models in the investigations of the practical solidification process are summarized. The problems to be solved and the future development of CA models are also pointed out.
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    PRECIPITATION BEHAVIOR AND PRECIPITATION STRENGTHENING OF NANOSCALE CEMENTITE IN CARBON STEELS DURING ULTRA FAST COOLING
    WANG Bin, LIU Zhenyu, Feng Jie, ZHOU Xiaoguang, WANG Guodong
    Acta Metall Sin, 2014, 50 (6): 652-658.  DOI: 10.3724/SP.J.1037.2013.00584
    Abstract   HTML   PDF (3406KB) ( 950 )
    In recent years, the precipitation strengthening by cementite, which is a common and economical second phase constituent in steels, has drawn renewed attention in the context of precipitation strengthening, because if cementites can be effectively refined to the scale of a few nanometers, they can induce significant precipitation strengthening effect. Therefore, nanoscale cementite is viewed as a viable option to replace precipitates of microalloying elements for reducing alloy costs in steel products. Given that cementites are usually to form lamellar pearlite structure in a traditional cooling process and generally tend to coarsen at relatively high temperatures, the thermodynamic feasibility for the formation of nanoscale cementite precipitates during cooling has been determined in the previous study, and the non-equilibrium precipitation of nanoscale cementite can be realized by increasing the cooling rate after hot rolling. Thus, the ultra fast cooling (UFC) technology was applied after the hot strip rolling for the research of precipitation behavior and precipitation strengthening of nanoscale cementite in carbon steels. The experimental results demonstrated that the UFC technology shows the unique effects on strengthening in carbon steels and a large number of dispersed nanoscale cementite precipitates with the size of 10~100 nm have been formed in 0.17%C and 0.33%C steels. The nanoscale precipitation of cementite was realized in the microstructure by UFC without the microalloy elements addition. Both the yield strength and tensile strength of the steels increased gradually with the stop temperature of UFC decreasing, and the yield strength increments of 0.17%C and 0.33%C steel were more than 100 MPa, when the stop temperature of UFC decreased from 890 ℃ to 600 ℃. Besides, thermomechanical treatment (TMT) process was introduced after UFC to explore uniform nucleation of cementite in hot-rolled carbon steels, and it is a feasible way to realize the uniform precipitation of nanoscale cementite in the entire miscrostructure for the further strengthening improvement. This was accomplished by subjecting the UFC cooled steel to a small degree of plastic deformation, with the aim to increase the dislocation density evidently. By combining UFC and TMT processing, the yield strength of 0.17%C steel is greater than 600 MPa, leading to a superior strengthening effect.
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    CREVICE CORROSION OF LOW ALLOY STEEL ANDCARBON STEEL IN THE SIMULATED GROUNDWATER AT 90 ℃
    XU Qiufa, PANG Xiaolu, LIU Quanlin, GAO Kewei
    Acta Metall Sin, 2014, 50 (6): 659-666.  DOI: 10.3724/SP.J.1037.2014.00027
    Abstract   HTML   PDF (7553KB) ( 1050 )
    Carbon steel and low alloy steel as candidates of high-level radioactive waste packaging materials,will undergo groundwater corrosion during long term disposal in underground repository, so it is necessary to study the corrosion behaviors of carbon steel and low alloy steel in the specific environment. Crevice corrosion behaviors of carbon steel Q235 and low alloy weathering steel Corten A were studied in a simulated groundwater at 90 ℃ through immersion tests and electrochemical measurements. SEM, EDS and Raman spectra were employed to analyze the corrosion product scales. The results show that both steels occured crevice corrosion and the crevice corrosion depth increased with corrosion time. Corten A underwent more serious crevice corrosion than Q235. In neutral or acidic solution, the corrosion resistance of Corten A was superior to Q235, but when the pH value of the solution was lower than 1, Corten A exhibited lower corrosion resistance than Q235. The alloying elements Cr, Cu and Si in Corten A were harmful to the resistance to crevice corrosion in the solution.
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    THE TEXTURE EVOLUTION AT THE CENTER OF PEARLITIC STEEL WIRE DURING DRAWING AND ITS INFLUENCE ON THE MECHANICAL PROPERTIES
    ZHAO Tianzhang , SONG Hongwu , ZHANG Guangliang , CHENG Ming , ZHANG Shihong
    Acta Metall Sin, 2014, 50 (6): 667-673.  DOI: 10.3724/SP.J.1037.2013.00799
    Abstract   HTML   PDF (2461KB) ( 1002 )
    The cold drawing pearlitic steel wires are widely used in industry such as the automobile tire and ropes. And it possesses an ultra high strength, almost the highest in all the steel products. So many investigations are focused on the hardening mechanisms of pearlitic steel wire during cold drawing, including the microstructure fining, texture evolution and cementite dissolution. In this study, the electron backscatter diffraction (EBSD) and the visco-plastic self-consistent (VPSC) model are used to investigate the texture evolution law at the center of wire during the drawing, as well as its influences on the mechanical behaviors. The EBSD results show that the as-received wires after dry drawing and quenching have a little <110> fiber texture along the drawing direction. And with wet drawing strain increasing, the intense of <110> fiber texture increases apparently. The calculations using VPSC have a good agreement with the EBSD results, which indicate that VPSC can successfully predict the texture category and its evolution law in pearlitic steel wire during drawing. The predictions show that the <110> fiber texture is gradually generated at the center of wire with strain increasing and exhibit the path of individual orientation in the inverse pole figures during the drawing. The orientations at the line linking <113> and <012> seem stable. The orientations located at the line linking <001> and <111> prefer to turn to the stable orientations and then turn to <110>. The other orientations turn to <110> directly. The volume of <110> orientations within 15 degrees of drawing direction increases with strain increasing and get saturation finally. The tensile yield stress of the wire center increases with the initial volume of <110> fiber texture increasing.
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    CORROSION BEHAVIOR OF 20SiMn STEEL REBAR IN CARBONATE/BICARBONATE SOLUTIONS WITH THE SAME pH VALUE
    CAO Fengting, WEI Jie, DONG Junhua, KE Wei
    Acta Metall Sin, 2014, 50 (6): 674-684.  DOI: 10.3724/SP.J.1037.2014.00041
    Abstract   HTML   PDF (2247KB) ( 771 )
    The aggressive ions, such as Cl- and SO42-, as well as the carbonation caused by CO2 from the air are two main reasons for the depassivation of steel rebar in reinforcement concrete. Under normal conditions, the pH value of concrete pore solution is taken as the criterion for determining whether the corrosion of steel occurs or not. However, carbonation process results not only in the decrease of the pH value of concrete pore solution, but also in the accession of HCO3- and CO32-. It is demonstrated that these two ions are able to influence the corrosion behaviors of steel rebar. Additionally, the failure of reinforcement concrete is a time consuming process, so the corrosion evolution laws of steel at the presence of HCO3- and CO32- is necessary to study systemically. Nevertheless, little relative work has been done so far. In this work, the electrochemical behavior of 20SiMn steel in three different content carbonate buffer solutions (0.01, 0.05 and 0.5 mol/L) was studied using electrochemical techniques (polarization curves, free corrosion potential measurements, EIS, Mott-Schottcky (MS) curves and cycle voltage curves) and surface analysis techniques (SEM and in situ Raman spectroscopy), compared with that in NaOH solution (0.437×10-3 mol/L ). These four solutions are of the same pH value 10.64. The results indicated that 20SiMn steel was in active corrosion state in NaOH solution and low content carbonate solution, while it was in passive state in high content carbonate solutions. In NaOH solution, 20SiMn steel was destroyed by uniform corrosion and the corrosion products were a-Fe2O3 and g-FeOOH, transformed from Fe(OH)2. In 0.01 mol/L carbonate solution, 20SiMn steel was destroyed by localized corrosion, and the final products were a-Fe2O3 and b-FeOOH, developed from the intermediate products GRs (green rusts). The passive film formed on 20SiMn steel was more resistive in 0.05 mol/L carbonate solution than that in 0.5 mol/L due to the formation of soluble complex anion Fe(CO3)22- in latter solution. There was a maximum corrosion resistance of the passive film with the increase of carbonate content.
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    STRENGTHENING EFFECTS OF MICROSTRUCTURE EVOLUTION DURING EARLY AGEING PROCESS IN Al-Mg-Si ALLOY
    WANG Bo, WANG Xiaojiao, SONG Hui, YAN Jujie, QIU Tao, LIU Wenqing, LI Hui
    Acta Metall Sin, 2014, 50 (6): 685-690.  DOI: 10.3724/SP.J.1037.2013.00733
    Abstract   HTML   PDF (2453KB) ( 911 )
    The microstructure evolution, atomic ratio (r) of Mg and Si in different precipitates, and precipitation strengthening effects during the ageing process were investigated by HRTEM, atom probe tomography (APT) and hardness testing in LT24 aluminum alloy used for nuclear fuel cladding alternative materials. The results show that the early stage of ageing at 180 ℃ led to a significantly increasing of hardness and the formation of high density of solute clusters and Guinier-Preston (GP) zones in the alloy. The alloy reaches peak hardness after ageing at 180 ℃ for 4 h due to a significant increasing density of the precipitates. After the peak hardness, a hardness plateau is maintained for longer ageing time, because of the precipitate is still the main strengthening phase in the specimens. The precipitates grow larger and the r increases with the increasing of ageing time. The r in needles changes from 1.23 to 1.35. needles are the main precipitation strengthening phase of the alloy. The precipitation sequence during the early ageing treatment in alloy can be described as follows: supersaturated solid solution →solute atom clusters→solute atom clusters+GP zone→solute atom clusters+GP zone+.
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    DYNAMIC SOFTENING MECHANISM OF 2099 ALLOY DURING HOT DEFORMATION PROCESS
    ZHANG Fei , SHEN Jian , YAN Xiaodong , SUN Jianlin , JIANG Na , ZHOU Hua
    Acta Metall Sin, 2014, 50 (6): 691-699.  DOI: 10.3724/SP.J.1037.2013.00718
    Abstract   HTML   PDF (9891KB) ( 782 )
    Dynamic softening mechanism of 2099 alloy was investigated by isothermal compression tests, thermal activation parameters calculation and comparison, EBSD and TEM techniques. On the basis of Zener-Hollomon parameter (Z) and deformation temperature (T), combining thermal activation parameters and microstructures analysis, the softening mechanism of 2099 alloy during hot deformation has been proposed. Dislocation cross slip plays a dominant role under the conditions of lnZ≥35.5 and T≤380 ℃. While, deformation mechanisms such as cross slip, climb of dislocation and unzipping of attractive junction play a joint role when lnZ≤37.4 and T≥340 ℃. Particularly, dynamic recrystallization occurred in the range of lnZ≤35.1 and T≥420 ℃, cross slip, climb and unzipping of dislocation and dynamic recrystallization are the main softening mechanisms in this condition. Dynamic recrystallization nucleation mechanisms are constituted of grain boundaries bulging nucleation and subgrain rotated induced nucleation, and the latter becomes more significant with increasing temperature and decreasing strain rate.
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    INFLUENCE OF HEAT TREATMENT ON LOW-CYCLE FATIGUE BEHAVIOR OF EXTRUDED Mg-7%Zn-0.6%Zr-0.5%Y ALLOY
    ZHANG Siqian , WU Wei , CHEN Lili , CHE Xin , CHEN Lijia
    Acta Metall Sin, 2014, 50 (6): 700-706.  DOI: 10.3724/SP.J.1037.2013.00781
    Abstract   HTML   PDF (3684KB) ( 751 )
    The low-cycle fatigue tests have been conducted for the Mg-7%Zn-0.6%Zr-0.5%Y alloys (mass fraction) subjected to extrusion, aging (T5) and solution plus aging (T6) treatment, respectively. The influence of heat treatment on the fatigue deformation behavior of the alloy has also been systematically investigated. The results show that T5 and T6 treatment can improve the cyclic deformation resistance of Mg-7%Zn-0.6%Zr-0.5%Y alloys. T5 treatment can reduce the fatigue life of the alloy. However, T6 treatment can improve the fatigue life at high total strain amplitudes, and reduce the fatigue life at low total strain amplitudes. The relationship between elastic strain amplitude, plastic strain amplitude and reversals to failure of the alloys can be described by Basquin and Coffin-Manson equations, respectively. For the alloys subjected to both T5 and T6 treatments, the increase in the cyclic deformation resistance is mainly due to the formation of long period stacking ordered (LPSO) phase. The twins formed during the fatigue deformation may be responsible for the decrease in the fatigue life of the alloy subjected to T5 treatment.
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    INVESTIGATION OF MICROSTRUCTURE ANDTEXTURE OF b PHASE IN A FORGED TC18TITANIUM ALLOY BAR
    LI Kai , YANG Ping , SHA Aixue , YAN Mengqi
    Acta Metall Sin, 2014, 50 (6): 707-714.  DOI: 10.3724/SP.J.1037.2014.00003
    Abstract   HTML   PDF (9449KB) ( 1076 )
    To understand the differences in the mechanical properties between the center and surface regions of large size forged TC18 titanium alloy bar, electron backscatter diffraction (EBSD) technique is applied to reveal the differences in textures and strains in b phase in addition to the microstructures observation. The influence of the states of b phase on the deviation to the Burgers orientation relationship (OR) between a /b phases is also analyzed according to the acquired EBSD information. It is found that the b phases in different positions of forged bar are in different states of strain, textures and grain sizes. The b phase in the center shows strong <100> texture and coarse and inhomogeneous grain sizes which all contribute to the difference in properties. The spheroidization of ap phase proceeded mainly within b grains at subgrain boundaries and the OR between two phases changed slightly as the lamella ap phase transforms into block-like ap phase, but changed strongly during transforming to globular morphology. The misorientation in b phase is an effective parameter to evaluate the contribution of work-hardening state for the enhancement of strength, whereas the level of the deviation to Burgers OR is an effective parameter to evaluate the spheroidizing rate and the recovery extent.
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    EFFECT OF ANODIZING TREATMENT ON BENDING FATIGUE PROPERTIES OF 2014-T6 ALUMINIUM ALLOY
    ZHANG Yanbin, ZHANG Limin, ZHANG Jiwang, ZENG Jing
    Acta Metall Sin, 2014, 50 (6): 715-721.  DOI: 10.3724/SP.J.1037.2014.00021
    Abstract   HTML   PDF (6035KB) ( 951 )
    Anodizing treatment was performed on the notched portion of fatigue specimens of the 2014-T6 aluminium alloy. The thicknesses of the oxide layer induced by the treatment were 5, 10 and 20 μm, respectively. Fatigue tests were carried out on the specimens with and without anodizing treatment using a rotating bending fatigue machine. According to experimental results, the effect of anodizing treatment on the bending fatigue properties of the aluminium alloy was analyzed. In addition, the dependence of the bending fatigue properties of the material upon the thickness of the oxide layer was discussed. The results indicate that the anodizing treatment decreases the bending fatigue life of the aluminium alloy under low and medium stress. With increasing thickness of the oxide layer, the crack initiation site transfers from the oxide layer surface to the substrate surface and the bending fatigue life of the alloy decreases; however, when the oxide layer thickness increases to a certain extent, the S-N curve above the fatigue limit of the alloy does not change even if the thickness continues to increase.
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    THE STRESS-STRAIN RELATIONSHIP OF TSV-Cu DETERMINED BY NANOINDENTATION
    QIN Fei, XIANG Min, WU Wei
    Acta Metall Sin, 2014, 50 (6): 722-726.  DOI: 10.3724/SP.J.1037.2013.00782
    Abstract   HTML   PDF (1495KB) ( 1162 )
    In 3D electronic package technologies, through silicon via (TSV) plays a critical important role. TSVs are usually fully filled by electroplating copper, namely TSV-Cu, which has very different mechanical properties from bulk copper. To obtain the mechanical properties of the TSV-Cu, the Berkovich nanoindentation tests were conducted, and the Oliver-Pharr algorithm and the continuous stiffness measurement method were used to acquire the elastic modulus and hardness. Then finite element modeling (FEM) simulations are adopted for reverse analysis of the nanoindentation loading process to determine the representative stress and strain of the TSV-Cu by comparing the maximum value of simulated load to that of experimental load. The strain hardening exponent of the TSV-Cu is determined by dimension functions. The yield strength of the TSV-Cu is acquired by substituting the representative stress, the representative strain and the strain hardening exponent into a power law stress-strain constitution. Finally, a power law elastic-plastic stress-strain relationship of TSV-Cu is built. The obtained elastic modulus and hardness of the TSV-Cu are 155.47 GPa and 2.47 GPa, respectively; the strain hardening exponent is 0.4892 and the yield strength is 47.91 MPa.
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    DEVELOPMENT OF MASTER SINTERING CURVE FOR SPARK PLASMA SINTERING OF 93W-5.6Ni-1.4Fe HEAVY ALLOY
    HU Ke, LI Xiaoqiang, QU Shengguan, YANG Chao, LI Yuanyuan
    Acta Metall Sin, 2014, 50 (6): 727-736.  DOI: 10.3724/SP.J.1037.2013.00712
    Abstract   HTML   PDF (1386KB) ( 597 )
    Tungsten heavy alloys are used for a number of applications, including radiation shields, counter weights, electrical contacts, vibration dampeners and kinetic energy penetrators. The most common compositions consist of W along with some combination of Ni, Fe, or Cu. The alloys are usually fabricated by the conventional powder metallurgy technique, in which the elemental blended powders are first compacted and then followed by a high temperature sintering. An important processing goal for this alloy is to obtain a high density with fine grain size. It is therefore desirable to predict its densification behavior and final density. Recently, the master sintering curve (MSC) theory provides a better understanding of whole sintering process. In previous work, the densification and grain growth mechanisms during spark plasma sintering (SPS) of 93W-5.6Ni-1.4Fe heavy alloy were investigated. In this investigation, the master sintering curve approach was first extended theoretically to spark plasma sintering of 93W-5.6Ni-1.4Fe heavy alloy. Two master sintering curves of 93W-5.6Ni-1.4Fe heavy alloy in different heating rate stages (with heating rate of 100 ℃/min as division point) during SPS process were developed. Both of the master sintering curves can effectively predict the densification behavior of 93W-5.6Ni-1.4Fe heavy alloy during SPS process, as well as the shrinkage and final density. The calculated densification function c quantitatively shows that the densification process increases with temperature when heating rate is higher than 100 ℃/min. In addition, the apparent densification activation energies calculated by MSC are roughly identical to those obtained by Arrhenius method.
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    MICROSTRUCTURE OF RECRYSTALLIZATION AND THEIR EFFECTS ON STRESS RUPTURE PROPERTY OF SINGLE CRYSTAL SUPERALLOY DD6
    XIONG Jichun , LI Jiarong , SUN Fengli , LIU Shizhong , HAN Mei
    Acta Metall Sin, 2014, 50 (6): 737-743.  DOI: 10.3724/SP.J.1037.2013.00561
    Abstract   HTML   PDF (8579KB) ( 643 )
    The specimens of single crystal superalloy DD6 were grit blasted and heat treated at 1100, 1200, and 1300 ℃ for 4 h at vacuum atmosphere respectively, then the microstructure of recrystallized DD6 alloy and their effects on the stress rupture performance were investigated. The results showed that cellular recrystallization nucleated in grit blasted samples heat treated at 1100 ℃ for 4 h, the dislocation tangles were found in the front of cellular recrystallization grain boundary in DD6 alloy, equiaxed recrystallization grains nucleated in grit blasted samples heat treated at 1300 ℃ for 4 h, and the carbides precipitate at the equiaxed recrystallization grain boundary, while the coexistence of equiaxed recrystallization grains and cellular recrystallization, defined as mixed recrystallization, occurred in the grit blasted samples heat treated at 1200 ℃ for 4 h. The cellular recrystallization reduced the stress rupture lives of DD6 alloy slightly, and the equiaxed recrystallization reduced stress rupture lives seriously, while the reduction degree of the stress rupture lives of the mixed recrystallization was between cellular recrystallization and equiaxed recrystallization. Besides this, with increase of depth of recrystallization and stress, the stresses rupture life decreased. It was also found that the fracture surface configuration was belonging to intergranular fracture with equiaxed recrystallization samples. The characteristic of the fracture surface changed to dimple fracture with cellular recrystallization samples, at all these condition the crack nucleated on the recrystallization grain boundaries of specimens during stress rupture process.
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    EFFECTS OF Nb/Ti RATIOS ON THE MICROSTRUCTURAL EVOLUTIONS OF CAST Ni-BASED SUPERALLOYS DURING LONG-TERM THERMAL EXPOSURE
    SUN Wen, QIN Xuezhi, GUO Yongan, GUO Jianting, LOU Langhong, ZHOU Lanzhang
    Acta Metall Sin, 2014, 50 (6): 744-752.  DOI: 10.3724/SP.J.1037.2014.00040
    Abstract   HTML   PDF (11214KB) ( 3007 )
    Effects of Nb/Ti ratios on the microstructural evolutions of cast Ni-based suerpalloys during long-term thermal exposure are investigated by OM, SEM and TEM. The results show that Nb/Ti ratios have no influence on the evolution of g ' morphology and size during long-term thermal exposure. However, with decrease of Nb/Ti ratios in alloys, the volume fraction of g ' phase increases. Both parameters Nb/Ti and (Nb+Ti)/(W+Mo) of primary MC have a good linear relationship with Nb/Ti ratios in alloys. With decrease of Nb/Ti ratios in alloys, both parameters for primary MC linearly decrease and sequentially thermal stability of primary MC is weakened. However, the results also show that Nb/Ti and (Nb+Ti)/(W+Mo) ratios of primary MC are not the principle factors determining the thermal stability of primary MC. The degeneration degree of primary MC can be calculated by the volume fraction of primary MC before and after degeneration, while the thermal stability of primary MC can be quantitatively characterized by degeneration degree of primary MC. Furthermore, with decreased Nb/Ti ratios in alloys, the grain boundaries coarsen more severely during long-term thermal exposure. Meanwhile, precipitation tendency of M23C6 carbide on grain boundaries increases and that of M6C carbide on grain boundaries decreases. However, the precipitation and evolution of m phase during long-term thermal exposure is not affected by Nb/Ti ratios obviously.
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    MICROSTRUCTURE EVOLUTION AND WEAR RESISTANCE OF IN SITU Mg2Si/Al COMPOSITES UNDER DIFFERENT PREPARATION CONDITIONS
    LIU Xiaobo , ZHAO Yuguang
    Acta Metall Sin, 2014, 50 (6): 753-761.  DOI: 10.3724/SP.J.1037.2013.00726
    Abstract   HTML   PDF (8224KB) ( 438 )
    Hypereutectic Al-Si alloys with high Mg content are in fact an in situ aluminium matrix composites containing a large amount of hard particles of Mg2Si, and the Mg2Si/Al composite has a potential as automobile brake disc material because the intermetallic compound Mg2Si exhibits high melting temperature, low density, high hardness, low thermal expansion coefficient (TEC) and reasonably high elastic modulus. However, the primary Mg2Si particles in normal Mg2Si/Al composites are usually very coarse and thus lead to room temperature brittleness and deficient wear resistance. Therefore, the composite with coarse primary Mg2Si particles need to be modified to obtain adequate mechanical strength and wear resistance. Numerous experiments have shown that development of a semi-solid microstructure in which dendritic characteristic is absent can lead to significant enhancement of the mechanical properties in the composite. The semi-solid forming has been recognized as a technique offering several potential advantages over casting or solid state forming, such as producing high quality components capable of full heat treatment to maximize properties, and reducing macrosegregation, solidification shrinkage and forming temperature. The key feature that permits the shaping of alloys in the semi-solid state is the absence of dendritic characteristics from the morphology of the solid phase. In the present work, in situ Mg2Si/Al composites were fabricated by using gravity casting, squeeze casting and semi-solid extrusion. The microstructure evolution and wear resistance of Mg2Si/Al composites were investigated. Mg2Si/Al semi-solid composites were fabricated by isothermal heat treatment technology, forming spherical reinforced phase and matrix structure. The effects of holding time on the microstructure and grain sizes of the composite were investigated. The results show that with P modification, Mg2Si particle in the as-cast microstructure of the composites is evolved from coarse dendrite into fine block structure with grain size of 35 μm. Furthermore, reinforcement Mg2Si with fine size and uniformly distribution exhibits regular spherical structure and a-Al grains exhibit spherical or ellipsoidal structure. The size of a-Al changes from 60 to 115 μm with increasing the holding time from 50 to 160 min. It is calculated that the cubic coarsening rate constants K of a-Al is 1.78×10-16 m3/s according to the statistical data. In addition, the hardness of squeeze casting and semi-solid extrusion composites enhanced 23.5% and 39% in comparison with casting composite, respectively. The wear test results show that, the wear resistance of Mg2Si/Al composite fabricated by using semi-solid extrusion is higher than those of composites fabricated by using gravity casting and squeeze casting under same load and wear particle size.
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    PREPARATION AND CATALYTIC PROPERTIES OF SiC FOAM SUPPORTED Co-BASED STRUCTURED CATALYSTS
    YANG Xiaodan, JIANG Chunhai, YANG Zhenming, ZHANG Jinsong
    Acta Metall Sin, 2014, 50 (6): 762-768.  DOI: 10.3724/SP.J.1037.2013.00662
    Abstract   HTML   PDF (2462KB) ( 655 )
    Co-based structured catalysts were prepared by coating Al-doped Co3O4 powder on SiC foam support using phenolic resin as the adhesion agent, followed by H2 reduction at moderate temperatures. XRD, SEM, N2 adsorption-desorption and H2 adsorption measurement were used to characterize the evolution of crystalline size of Co particles, morphology and phase assemblage of the coatings and the H2 adsorption capacities of the structured catalysts obtained at different reduction temperatures. The catalytic properties of the prepared Co-based structured catalysts were evaluated by taking the hydrolysis of NaBH4 as the model hydrogen generation reaction. Growing of the active Co particles in the coating as accompanied by the increase of crystallinity was observed with the increase of reduction temperature, which resulted in decreased chemical adsorption capacity of H2 at 120 ℃ and catalytic activity towards hydrogen generation rate. The highest hydrogen generation rate of 48.38 mL/(g·min) was achieved at the reduction temperature of 400 ℃.
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