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

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    EFFECT OF PEARLITIC LAMELLA ORIENTATION ON DEFORMATION OF PEARLITE STEEL WIRE DURING COLD DRAWING
    Lichu ZHOU,Xianjun HU,Chi MA,Xuefeng ZHOU,Jianqing JIANG,Feng FANG
    Acta Metall Sin, 2015, 51 (8): 897-903.  DOI: 10.11900/0412.1961.2014.00620
    Abstract   HTML   PDF (7101KB) ( 814 )

    Cold drawing pearlitic steel wires with ultra-high strength have important applications such as the automobile tire, springs rope and bridge cables. There have been many investigations which are paid attention to the hardening mechanisms of pearlitic steel wire, covering evolution of microstructure, texture and dislocation. In this work, effects of pearlitic lamella orientation on mechanical properties and deformation of pearlite steel wire in cold drawing were investigated by combining TEM, SEM and nano-indentor. The experimental results showed that pearlitic lamellae having low angle with drawing direction would be turned to parallel the drawing direction through a combined rotating process of pearlite colonies. The <110> fiber texture in the ferrite phase formed and the distribution of dislocations in ferrite was almost uniform. The deformation between cementite and ferrite was coordinated. Pearlitic lamellae having large angle with drawing direction would be bent and turned to parallel the drawing direction. It was difficult for the bent pearlite to deform because of the fragmentation of cementite and formation of dislocation cell in ferrite. After cold drawing process, the micro-hardness of straight pearlite was higher than that of bent pearlite. The result indicated that pearlitic lamellae having low angle with drawing direction have higher working-hardening rate in drawing.

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    STRENGTH AND TOUGHNESS OF T250 MARAGING STEEL JOINT HYBRID-TREATED WITH LASER WELDING AND AGING
    Kun LI,Jiguo SHAN,Chunxu WANG,Zhiling TIAN
    Acta Metall Sin, 2015, 51 (8): 904-912.  DOI: 10.11900/0412.1961.2014.00635
    Abstract   HTML   PDF (12993KB) ( 601 )

    Maraging steels are leading members of the ultra-high strength steel family due to a combination of two solid state reactions: martensitic transformation and subsequent ageing. These steels can be hardened by the precipitation of refined Ni3(Ti, Mo) intermetallic particles. They have been widely used in the military and aerospace applications such as solid rocket motor cases and submarine shells due to their high strength and toughness. The T250 maraging steel has used Ti as one of the primary strengthening elements to replace Co, which decreases the cost of maraging steels. Its properties are comparable to the standard Co-bearing grades in the 1.4~2.1 GPa strength levels. It possesses good weldability without porosity in the weld and other weld defects. However, the combination of strength and toughness of welded joints is the main problem which has not been solved well via different welding methods so far. In this work, T250 maraging steel plate with 2 mm thickness was hybrid-treated with laser welding and aging treatment. The strength and toughness of welded joints aged before and after welding were investigated. The microstructures of parent metals and welded joints were observed with OM and SEM. Chemical compositions in parent metals and weld zones were analyzed with EPMA. The tensile strength and static toughness were acquired with the auxiliary device of Gleeble machine and could represent strength and toughness of the welded joints. The results show that the tensile strength and static toughness of the welded joint aged before welding are 62% and 28% that of the aged parent metal, respectively. However, the tensile strength and static toughness of the welded joint aged after welding reach 98% and 71% that of the aged parent metal, respectively. The weld metal is the key zone to influence the strength and toughness of the welded joints. Ni3(Ti, Mo) precipitates in the weld metal are the intrinsic reason resulting in that the strength and toughness of the welded joint aged after welding are superior to that of the welded joint aged before welding. Ni3(Ti, Mo) precipitates are beneficial to the strength and static toughness in the elastic deformation stage, and it has a dual effect on the static toughness in the plastic deformation stage of the welded joints. The reverted austenite has a negligible effect on the strength and static toughness in the elastic deformation stage, while it is detrimental to the static toughness in the plastic deformation stage of the welded joints.

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    THE COMBINED EFFECT OF HOT DEFORMATION PLUS QUENCHING AND PARTITIONING TREATMENT ON MARTENSITE TRANSFORMATION OF LOW CARBON ALLOYED STEEL
    Cunyu WANG,Ying CHANG,Jie YANG,Kunmin ZHAO,Han DONG
    Acta Metall Sin, 2015, 51 (8): 913-919.  DOI: 10.11900/0412.1961.2014.00709
    Abstract   HTML   PDF (6986KB) ( 995 )

    A combined process of hot deformation with different deformation temperatures plus two step quenching and partitioning (Q&P) treatment was applied to low carbon alloyed steel. The effect of stress (30% plastic deformation) on the start temperature of martensite transformation (Ms), volume fraction of retained austenite and mechanical properties was analyzed. It found that comparing with specimen treated by conventional two-step Q&P process, the microstructure of steel treated by combined process was finer and finer with the decreasing hot deformation temperature, and the typical curved micromorphology of martensite exists. Moreover, the Ms of specimen treated by combined process is increased with the increasing of deformation temperature. The effect of stress on the Ms can be attributed to the effect of stress on the grain boundaries of austenitic parent phase, where a large amount of dislocation induced by the stress is prior to occur so as to promote formation of martensite. However, the stability of untransformed austenite was improved by the plastic deformation when matensite transformed so as to get the more retained austenite (the highest volume fraction of retained austenite obtained by combined process of hot deformation at 750 ℃ is 17.2%). Moreover, the mechanical properties were improved by the combined process, namely, the highest hardness of specimen were obtained when hot deformation at 650 ℃ and the highest plasticity were obtained when hot deformed at 750 ℃.

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    STUDY ON σ PHASE PRECIPITATION OF HR3C STEEL USED IN ULTRA-SUPERCRITICAL BOILER
    Hui WANG,Congqian CHENG,Jie ZHAO,Zhi YANG
    Acta Metall Sin, 2015, 51 (8): 920-924.  DOI: 10.11900/0412.1961.2015.00028
    Abstract   HTML   PDF (2667KB) ( 1463 )

    HR3C steel is a new type of austenitic heat-resistant steel which has been widely used for super-heater and re-heater tubes in the ultra-supercritical (USC) boiler. The mechanical properties of HR3C steel were dependent on the stability of the microstructure, particularly the large amount of precipitates formed during service. The precipitation of s phase in HR3C steel during long-term aging for 2000 h at temperature of 700 and 750 ℃ was investigated by OM, SEM and TEM. The phase calculation method was applied to understand the factors influencing the precipitation. After 1000 h of the aging duration, irregular mass second phase was found to precipitate at the grain boundary, followed by the subsequent increasing and coarsening with time. The constituent elements of the phases were determined as Fe and Cr through SEM equipped with EDS. Further SAED analysis results led to the confirmation that these phases were basically s-FeCr compound. Combined with the prediction made through New PHACOMP method, the microstructure or phase structure in initial state may affect the subsequent precipitation behavior.

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    EFFECTS OF TEMPERING TEMPERATURE ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF GRANULAR BAINITE IN 2.25Cr-1Mo-0.25V STEEL
    Zhonghua JIANG,Pei WANG,Dianzhong LI,Yiyi LI
    Acta Metall Sin, 2015, 51 (8): 925-934.  DOI: 10.11900/0412.1961.2014.00719
    Abstract   HTML   PDF (11117KB) ( 1441 )

    2.25Cr-1Mo-0.25V steel is the most popular material used for pressure-vessel applied at elevated-temperature in hydrogen environment. For higher process efficiencies in future coal-conversion plants, chemical processing plants, and petrochemical-refining plants, much thicker cross-section component are necessary for constructing much larger pressure-vessel for these plants. Because of the thick cross-section, the cooling rate in the central region of the component is insufficient to obtain low bainite during quenching treatment, and a large amount of granular bainite appears in the central region. Previous studies have shown that good impact toughness can be achieved by appropriate tempering for 2.25Cr-1Mo-0.25V steel with low bainite microstructure. However, the impact toughness of 2.25Cr-1Mo-0.25V steel with granular bainite after tempering always cannot satisfy the demanding requirement due to the unclear understanding of the evolution of microstructure and mechanical properties during tempering. In this work, the influence of tempering on the microstructure and mechanical properties of 2.25Cr-1Mo-0.25V steel with granular bainite microstructure was investigated by OM, XRD, SEM, TEM and EPMA. The results show that the normalized 2.25Cr-1Mo-0.25V steel with granular bainite microstructure is composed of bainite ferrite and island of martensite and austenite (M-A island). Nanoindentation test indicates that M-A island is much harder than that of metrix bainite ferrite, because of the high concentration of carbon in M-A islands. The synergistic effect of the decomposition of M-A islands and softening of bainite ferrite determined that Charpy absorbed energy at -18 ℃ increases first and then decreases with the increasement of tempering temperature. The degree of decomposition of M-A islands and the morphology, size and distribution of carbides in granular bainite, coupled with the softening effect of bainite ferrite recrystallization are the key factors determining low-temperature impact toughness of 2.25Cr-1Mo-0.25V steel.

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    MICROSTRUCTURE AND MECHANICAL PROPERTIES OF HEAT TREATMENT LASER SOLID FORMING SUPERALLOY INCONEL 718
    Kan SONG,Kai YU,Xin LIN,Jing CHEN,Haiou YANG,Weidong HUANG
    Acta Metall Sin, 2015, 51 (8): 935-942.  DOI: 10.11900/0412.1961.2014.00648
    Abstract   HTML   PDF (6183KB) ( 1563 )

    With the development of additive manufacturing technology of metal, laser solid forming (LSF) has become an important fabricating method for high performance and complex Inconel 718 alloy components. However, there still exist a certain microsegregation and a large uneven distribution of residual stress in as-deposited Inconel 718 alloy due to rapid heating and cooling in LSF. Heat treatment is a necessary method for further improving the microstructure and mechanical properties. In this work, the microstructure and mechanical properties of LSFed Inconel 718 alloy heat treated with high temperature solution, d phase aging and double aging treatment was investigated, the dislocation configuration of heat treated LSFed Inconel 718 alloy was characterized. It is found that the recrystallization occurs after the heat treatment, which leads to the transition from the columnar grain in the as-deposited to the equiaxed grain. Laves phase is dissolved completely after the heat treatment, and the needle d phase and the g phase precipitate along the grain boundary and in the g phase matrix, respectively. The strength, elongation and reduction of area of the heat treated Inconel 718 alloy satisfy the wrought standards. There are two kinds of interactions between the dislocation and the g phase, the shearing mechanism and the Orowan bypass mechanism, which play the dominant role corresponding to the lower and the higher distribution density of g phase, respectively. Additionally, the dislocations pile up at the d phase owing to the larger size of the d phase in the heat treated Inconel 718 alloy compared with that in the wrought. The dislocation glide can be also hindered by carbide due to the pinning and drag effect.

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    INFLUENCE OF TEMPERATURE ON TENSILE BEHAVIORS OF K416B Ni-BASED SUPERALLOY WITH HIGH W CONTENT
    Jun XIE,Jinjiang YU,Xiaofeng SUN,Tao JIN,Yanhong YANG
    Acta Metall Sin, 2015, 51 (8): 943-950.  DOI: 10.11900/0412.1961.2014.00622
    Abstract   HTML   PDF (10840KB) ( 465 )

    Ni-based superalloys with high content of W are often used to manufacture gas turbine vanes and high temperature forging dies due to high temperature capability and low cost. The microstructure of Ni-based superalloys consists of g matrix, g′ phase and carbides generally. The deformation mechanisms of alloy mainly include dislocation loops formation, shearing of dislocation into g′ phase and formation of anti-phase boundary (APB) and stacking fault. Although the deformation mechanism of Ni-based superalloys has been studied widely, the relationship between tensile property and deformation mechanism of K416B superalloy at different temperature is still unclear up to now. Therefore, the influence of temperature on tensile behaviors of K416B Ni-based superalloy with high W content was investigated in the present work by means of tensile test at different temperatures. It has been found that the yield and tensile strengths of K416B alloy increase with rising temperature at 20~800 ℃. When the temperature exceeds 800 ℃, the tensile property of the alloy decreases gradually. The deformation feature of the alloy during tensile test at room temperature is that the dislocations shear into g′ phase or cross g′phase by Orowan mechanism. As the dislocations shear into g′ phase, they decompose to form the stacking fault. The dislocation density in the matrix of the alloy increases with the rising temperature and the dislocation tangles in the matrix play the role of strengthening in the alloy during tensile test at 800 ℃. As the temperature further enhancing, the amount of dislocations shearing into g′ phase increases and then the tensile strength of the alloy decreases. Under the condition of middle-low temperature, the brittle fracture occurs in the alloy due to the fact that the cracks are initiated and propagated along M6C carbide with large size. During tensile test at high temperatures, the tensile fracture mode of the alloy is micro-porous aggregation along the g +g′ eutectic interface, which is the main reason for the alloy exhibiting the ductile fracture.

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    CHARACTERISTICS OF THE FRICTION WELDING INTERFACE BETWEEN SINGLE CRYSTAL SUPERALLOY DD3 AND FINE GRAINED SUPERALLOY GH4169
    Suigeng DU,Xifeng WANG,Man GAO
    Acta Metall Sin, 2015, 51 (8): 951-956.  DOI: 10.11900/0412.1961.2014.00686
    Abstract   HTML   PDF (6110KB) ( 954 )

    Due to high demand of welded turbine blisk, a fine grain superalloy, GH4169, has been widely used to make the disk and a single crystal superalloy, DD3, has been used for blades. In this work, the joint micro-structure and the mechanism of friction welding between the GH4169 and DD3 have been investigated by using the SEM and TEM equipped with EDS. The research results show that there is a friction deformed band of the GH4169 in the weld zone. The heat and mechanical affected zones of the two alloys form dynamic recrystallization grains. The bonding interface is between their dynamic recrystallization grains. The two alloys bond with the common grains and the common grain boundaries. Their compositional change mainly occurs within the common grains and the common grain boundaries at bonding interface. The common grain (C2) at the viewpoint of TEM analysis and adjacent dynamic recrystallization grain (C3) of the GH4169 has a specific orientation relationship, [1ˉ14]C2 ∥ [1ˉ10]C3 , (220)C2 ∥ (220)C3. With the friction welding thermal cycles and post weld heat treatment, the g' phase precipitates with tiny spherical distribution at the common grain and the two sides of dynamic recrystallization grains, and coherent with g matrix. But no g" phase precipitates.

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    MICROSTRUCTURE EVOLUTION AND SOLUTE SEGREGATION IN DIRECTIONALLY SOLIDIFIED TiAl ALLOYS WITH HIGH Nb CONTENT
    Yong LI,Guohuai LIU,Zhaodong WANG,Tianliang FU,Xinzhong LI,Yanqing SU,Jingjie GUO,Hengzhi FU
    Acta Metall Sin, 2015, 51 (8): 957-966.  DOI: 10.11900/0412.1961.2014.00703
    Abstract   HTML   PDF (10575KB) ( 522 )

    TiAl-Nb alloys have been determined as the advanced direction for the development of the high temperature TiAl alloys, so being one of the advanced materials for turbines of aircraft engines and gas-burning power-generation plants. However, highly-Nb addition can lead to the complex solidification behavior of TiAl-Nb alloy and multi-phase microstructure, which is important for the mechanical properties during the alloy design. Bridgman type directional solidification experiments were conducted for Ti-46Al-(8, 9, 10)Nb alloy. The effect of the growth rate and Nb content on the microstructure, phase transition and microsegregation was investigated, and finally the selection diagram of the phase transition and the microstructure of the directional solidified TiAl-Nb alloy were obtained. The results show that the planar-cellular-dendritic evolution of solid-liquid interface can be observed with the increase of the growth rate. Meanwhile the fully b phase solidification changes to the peritectic solidification with the increase of the growth rate, and correspondingly the final microstructure is composed of the a2/g lamellar structure and a multiphase microstructure (B2 phase, a2/g lamellar structure) respectively. The increase of the b-stabilizer Nb content can promote the fully b phase solidification and the formation of the multiphase microstructure (B2 phase, a2/g lamellar structure). The contribution of the growth rate and the Nb content to the phase transition and the microstructure is connected with the solute segregation (S-segregation, b-segregation) closely. The increase of the S-segregation amplitude can easily promote the peritectic reaction, which always leads to the highly solute segregation and the concentrated distribution of plenty of B2 phase in the core of the dendrite. b-segregation is the mainly origin of the B2 phase formation, in which the Nb enrichment in the retained b phase directly determines the morphology and the dimension of the B2 phase. Finally according to the selection diagram of the solidification process and the microstructure of the Ti-46Al alloy with the growth rate and the Nb content, the high Nb content and the low growth rate during fully b solidification should be selected for the prefer microstructure with the homogeneous distribution and the low solute segregation.

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    FUNCTION OF MICROELEMENT Hf IN POWDER METALLURGY NICKEL-BASED SUPERALLOYS
    Yiwen ZHANG,Benfu HU
    Acta Metall Sin, 2015, 51 (8): 967-975.  DOI: 10.11900/0412.1961.2014.00704
    Abstract   HTML   PDF (10823KB) ( 559 )

    Hafnium (Hf) is one of the most important microelements in powder metallurgy (P/M) superalloy. Hf modifies the microstructure and drastically improves mechanical properties in P/M superalloy. The effect of Hf in a nickel-based P/M superalloy was systematically studied by means of FEG-SEM, TEM, AES, EDS and physical and chemical phase analysis. Hf mainly distributes at interdendritic region of the solidification powder in form of solid solution, which is helpful to reduce prior particle boundary (PPB). Hf facilitates morphology of g′ phase to be unstable and enhances the large cubic g phase to split into smaller ones, so the g′ phase turns into a stable state with a lower energy faster. Hf is mainly distributed in g′ phase and MC carbides, which changes the distribution of element between the g′ phase, MC and g solid solution, which is beneficial to eliminate notch sensitivity and improves overall mechanical properties of the alloy.

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    RESEARCH ON THE INTERFACIAL HEAT TRANSFER COEFFECIENT BETWEEN CASTING AND CERAMIC SHELL IN INVESTMENT CASTING PROCESS OF Ti6Al4V ALLOY
    Heng SHAO,Yan LI,Hai NAN,Qingyan XU
    Acta Metall Sin, 2015, 51 (8): 976-984.  DOI: 10.11900/0412.1961.2015.00037
    Abstract   HTML   PDF (1438KB) ( 931 )

    Investment casting process is an important way to get complex parts of titanium alloy. However there are few research on the interfacial heat transfer coefficient (h) between casting and shell thus the temperature simulation of investment casting process of titanium alloy is often inaccurate. In order to get a relatively accurate h, a one-dimensional mathematical model for the reverse calculation of h between casting and shell in investment casting process of Ti6Al4V alloy was built and the analytic relationship between temperature and time/heat flux was established. Considering the calculated h is significantly affected by the error of parameters such as the specific heat capacity and thermal diffusivity of shell and position of thermocouples, research on the error of these parameters is essential. The relationship between the error of these parameters and the temperatures in the casting and shell was studied and it was found that the effect of different kind of error on the temperature field was obviously different. An experiment based on the one-dimensional mathematical model was done and temperatures of different positions were measured. Based on the effect of different kind of error and the difference between the calculate temperature field and the measured temperatures, the proportion of effect of each kind of error was assessed. These errors were revised on the basis of the assessment, thus a relatively accurate h between the casting and shell was obtained. The relationships between h and thickness of the solidified layer on the casting/temperature at the surface of casting can be divided into 4 stages: (1) Metal was liquid and h kept about 440 W/(m2K); (2) Solid layer appeared on the surface, and h declined nearly 60%; (3) Solid layer grew up before metal became completely solid and h declined nearly 20% of its maximum; (4) After metal solidified, h declined slowly as temperature on the surface of casting dropped. These relationships were applied in a three-dimensional model for numerical simulation of the temperature field. Temperatures of different positions in casting and shell were calculated and calculated temperatures agreed with measured ones well. Thus the accuracy of h was identified and it can help solve problems in the production in investment casting process of Ti6Al4V alloy.

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    MICROSTRUCTURE OF DIRECTLY EXTRUDED Mg-1Zn-1Ca ALLOY AND ITS CORROSION BEHAVIOR IN SBF SOLUTION
    Zhongming ZHANG,Kai YU,Weiwei REN,Ying MA,Chunjie XU,Zengzhe XI
    Acta Metall Sin, 2015, 51 (8): 985-992.  DOI: 10.11900/0412.1961.2014.00652
    Abstract   HTML   PDF (4083KB) ( 602 )

    The as-extruded Mg-1Zn-1Ca (mass fraction, %) alloys was fabricated successively by alloying, homogenization treatment and hot extrusion. The corrosion behavior of the alloy in simulated body fluid (SBF) solution was evaluated by electrochemical test and immersion test. The microstructure and morphology of corrosion product were observed by OM and SEM. Compositions of corrosion layer and different phases were investigated by EDS analysis. Fourier Transform infrared spectroscopy was also conducted to identify the functional groups in the corrosion products and XRD was also used to determine the phase constitutes of the corrosion products. The results show that Mg-1Zn-1Ca alloy consists of three phases, i.e. a-Mg, Mg2Ca and Ca2Mg6Zn3. After immersion in SBF solution for 72 h, the corrosion products is composed of HA (Ca10(OH)2(PO4)6), CaCO3, MgCl2 and Mg(OH)2. During the stage of immersion, the high active Mg2Ca phases act as the anode and corrode first, so they protect the around a-Mg substrate; the Ca2Mg6Zn3 phases are the lowest active, so they accelerate the corrosion of around α-Mg substrate. The corrosion resistance of as-cast Mg-1Zn-1Ca alloy are better than as-extruded alloy.

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    INVESTIGATION OF THE ANISOTROPIC GROWTH OF OXIDE LAYERS FORMED ON Zr-4 ALLOYS CORRODED IN LiOH AQUEOUS SOLUTION
    Shaoqiu GOU,Bangxin ZHOU,Shijing XIE,Long XU,Meiyi YAO,Qiang LI
    Acta Metall Sin, 2015, 51 (8): 993-1000.  DOI: 10.11900/0412.1961.2015.00049
    Abstract   HTML   PDF (7947KB) ( 531 )

    Zr-4 coarse-grained specimens were corroded in static autoclave at 360 ℃, 18.6 MPa in 0.01 mol/L LiOH aqueous solution for 70 and 160 d exposure. EBSD, SEM and HRTEM were used to investigate the microstructures and crystal structures of oxide layers, and the relationships between the oxide thickness and the grain orientation of the metal matrix. The results showed that the oxide layers formed on the grain surfaces with the orientations nearby basal plane (0001) were thicker, and exhibited a prominent anisotropic for the oxide growth when Zr-4 specimens were corroded in LiOH aqueous solution for 160 d, but this was not the case for 70 d. The grains with the surface orientation nearby (0001), (1010) and (1120) were selected from the specimens corroded for 70 d to investigate the effect of metal grain orientation on the microstructure of oxide layers. The results showed that the crystal structure and microstructure of oxide layers formed on different metal grains were obviously different, and the scattering of m-ZrO2 columnar grain orientations in the oxide layers formed on near basal plane (0001) was wider than that on near prismatic plane (1010) and (1120). Besides the majority of m-ZrO2, c-ZrO2, t-ZrO2 and sub-oxide phase Zr3O were also detected at the oxide/metal interface, and it showed that the microstructure and crystal structure of oxide layers were very complex. The microstructural evolution of oxide layers will affect the diffusion of oxygen and subsequently the growth of oxide. Therefore, the microstructural evolution of oxide layers, which was affected by the different microstructure of oxide layers formed initially on grains and the water chemistry of corrosion tests, resulted in the anisotropic growth of oxide layers when Zr-4 specimens were corroded in LiOH aqueous solution in subsequent corrosion tests.

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    ELECTROCHEMICAL BEHAVIOR OF Ni2+ IN SiO2-CaO-MgO-Al2O3 MOLTEN SLAG AT 1673 K
    Chuan HONG,Yunming GAO,Chuanghuang YANG,Zhibo TONG
    Acta Metall Sin, 2015, 51 (8): 1001-1009.  DOI: 10.11900/0412.1961.2015.00065
    Abstract   HTML   PDF (2950KB) ( 688 )

    The modern iron and steel industry produces large emissions of CO2 annually. Electrolytic reduction of molten slag containing iron oxide at high temperature using an inert oxygen evolving anode is an alternative process to reduce or eliminate the formation of CO2. In order to establish reasonable process parameters of electrolytic method for steel containing Ni, it is necessary to master the electrochemical behavior of Ni2+ in molten slag. However, investigations on the electrochemical behavior of Ni2+ in molten slag at higher temperatures were very limited, which can probably be attributed to the experimental difficulties associated with the operation of high-temperature electrochemical cells. An electrolytic cell with a controlled oxygen flow and Pt, O2(air)|ZrO2 used as reference electrode was constructed integrally through a one-end-closed magnesia partially stabilized ZrO2 solid electrolyte tube. Electrochemical behavior of Ni2+ on Ir electrode was investigated in SiO2-CaO-MgO-Al2O3 molten slag at 1673 K by means of electrochemical techniques such as cyclic voltammetry (CV), square wave voltammetry (SWV), chronopotentiometry (CP) and potentiostatic electrolysis. The results show that both diffusion in the molten slag and electromigration in the ZrO2 solid electrolyte for the O2- are not rate-determining steps of electrochemical reduction reaction process of electroactive ions. It is feasible to study electrochemical behavior of Ni2+ in the molten slag with the aid of the electrolytic cell with a controlled oxygen flow under the present experimental conditions. The reduction of Ni2+ on the Ir electrode in the molten slag is found to be a reversible reaction with a single step, and the rate of the process is diffusion controlled. Two diffusion coefficients of Ni2+ in the molten slag containing 3%NiO derived respectively from CV and CP are (3.50±0.18)×10-6 and (2.80±0.22)×10-6 cm2/s, which are consistent with records in the relevant literatures.

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    STUDY OF MARTENSITIC TRANSFORMATION AND STRAIN BEHAVIOR IN Ni50-xCoxMn39Sn11 (x=0, 2, 4, 6) HEUSLER ALLOYS
    Zhe LI,Chen XU,Kun XU,Hao WANG,Yuanlei ZHANG,Chao JING
    Acta Metall Sin, 2015, 51 (8): 1010-1016.  DOI: 10.11900/0412.1961.2015.00071
    Abstract   HTML   PDF (1686KB) ( 557 )

    The crystal structure, phase transformations and magnetic properties for Ni50-xCoxMn39Sn11 (x=0, 2, 4, 6) Heusler alloys have been systematically studied by means of structure and magnetism measurements. The results show that with increase of Co concentration, the martensitic transformation temperatures are obviously decreased, while the Curie temperatures of austenite are gradually increased, and they present different structures at room temperature. At the same time, with increasing Co content, the austenitic magnetism rapidly enhances, while the martensitic magnetism almost keeps unchanged. This leads to a significant improvement of difference magnetization (ΔM) between two phases. For Co content added to x=4, the value of ΔM between two phases achieves about 40 Am2/kg and exhibits magnetic field-induced martensitic transformation. Using strain measurement, the strain behavior related to martensitic transition in Ni50-xCoxMn39Sn11 (x=0, 2, 4) samples was studied. It is found that the phase transition strain reaches 0.17% in Ni46Co4Mn39Sn11 sample. Within the magnetic cycles of 3 T, this sample displays a reproducible magnetostrain in temperature range of 215~235 K. Such a reproducible strain could be ascribed to the fact that a partial martensitic transformation of this sample can be driven by isothermal magnetic field.

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    COMPOSITION DESIGN OF Fe-B-Si-Ta BULK AMORPHOUS ALLOYS BASED ON CLUSTER+ GLUE ATOM MODEL
    Yaoxiang GENG,Kaiming HAN,Yingmin WANG,Jianbing QIANG,Qing WANG,Chuang DONG,Guifeng ZHANG,O TEGUS,Peter HAÜSSLER
    Acta Metall Sin, 2015, 51 (8): 1017-1024.  DOI: 10.11900/0412.1961.2014.00615
    Abstract   HTML   PDF (2552KB) ( 533 )

    The structural and compositional features of amorphous alloys can be described by cluster-plus-glue atom model, which is an effective method for the composition design of amorphous alloys. In the Fe-B binary system, Fe2B phase is an intermetallic phase related to Fe83B17 eutectic point. Under the framework of the highest radial number density and isolation principle, the local structure of Fe2B phase is characterized by a B-centered Archimedean octahedral antiprism [B-B2Fe8] atomic cluster. Combined with the electron consistence criterion, the [B-B2Fe8]Fe (here the center and shell atoms are separated by a hyphen, a cluster is enclosed in square brackets, the glue atom is out square brackets) is then determined as an ideal cluster formula for Fe-B binary amorphous. To further enhance the glass-forming ability (GFA) of the alloy, the center B and shell Fe atoms in [B-B2Fe8]Fe are replaced with Si and Ta, respectively, due to their large negative enthalpy of mixing between Si-Fe and (B, Si)-Ta atomic pairs, and Fe-B-Si-Ta quaternary composition series, namely [Si-B2Fe8-xTax]Fe, are thus derived. The experimental results reveal that the bulk amorphous alloys with a diameter of 1.0 mm can be achieved for [Si-B2Fe8-xTax]Fe (x=0.4~0.7) compositions. Among them, [Si-B2Fe7.4Ta0.6]Fe (i.e. Fe70B16.67Si8.33Ta5, atomic fraction, %) is the best glass former, its glass transition temperature Tg, supercooled liquid region ΔTx and the reduced glass transition temperatures Trg are 856 K, 33 K and 0.584, respectively. The Vickers hardness, saturation magnetization and coercivity of the [Si-B2Fe7.6Ta0.4]Fe (i.e. Fe71.67B16.67Si8.33Ta3.33) amorphous alloy are measured to be 1117 HV, 1.37 T, and 3.0 A/m, respectively.

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