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

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    THE UNDERSTANDING AND CALCULATION OF MISORIENTATION BETWEEN VARIANTS BASED ON THE PHASE TRANSFORMATION
    WU Jing ZHANG Wenzheng
    Acta Metall Sin, 2009, 45 (8): 897-905. 
    Abstract   PDF (951KB) ( 1272 )

    Misorientation between crystalline grains generated from a solid state phase transformation can be understood according to the orientation relationship (OR) between the parent and product phases, when such an OR is reproducible and unique. Due to the symmetry of the parent phase, the product phase can be related to the parent phase by various crystallographically equivalent variants of the same OR. When a pair of product particles with different variants meet at a grain boundary, the misorientation between the adjacent grains can be determined based on the OR and the symmetry of the parent phase. Misorientations for ideal rational ORs of phase transformations in some systems have been tabulated in literatures, and also, irrational ORs have been reported, with an increasing tendency because of the improvement of measurement technologies. This paper describes in details how to calculate the misorientation of different variants with a general OR. It starts from constructing the transformation matrix for the phase transformation with a rational or irrational OR, from either measurements or calculations. By applying the symmetry operators in the parent phase with the matrix manipulation, the misorientations between different variants have been derived. Since the misorentations are due to the symmetry of the parent phase, the determined values of misorientation nles between different vriants are firstly independent of the OR, but the misorientations axes are dependent on the OR. Nevertheless, the final results of the minimum rotation angles usually vary with the OR when the symmetry of the product phase is taken into consideration to derive misorientation angle/axis. The present approach elaborates the quantitative relationship between the OR of a phase transformation and the misorientation between product particles of different variants. It contributes to a better understanding of the cause of the misorientation, and provides simple formulas to determine the misorientations for a general OR. For simplicity, an example of applications of the present approach is given to an fcc→bcc phase transformation system with the N–W OR. The calculated results are consistent with thosgot from othr approaches. In additin, the nuber of indepndent variants with different ORs, such as the N–W, K–S, Bain ORs in the fcc/bcc system is analyzed, by following Cahn and Kaloji approacof uperimposing point groups of te parent and product phases. The relationship betweethe transformation matrix, the orintation relationship matrix and the correspondinmatrix in cuic systems is also discussed.

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    STRAIN--INDUCED MARTENSITIC TRANSFORMATION IN 304L AUSTENITIC STAINLESS STEEL UNDER ECAP DEFORMATION
    YANG Gang HUANG Chongxiang WU Shiding ZHANG Zhefeng
    Acta Metall Sin, 2009, 45 (8): 906-911. 
    Abstract   PDF (2958KB) ( 1760 )

    The strain–induced martensitic transformation (SIMT) is considered to be an effective route to enhance the mechanical properties of metastable austenitic steels. Recently, it was found that the SMIT was favourable for the formation of nanocrystalline microstructures in some austenitic steels and titanium alloys, by using the technique of severe plastic deformation (SPD) for grain refinement. It is well known that austenitic stainless steel is sensitive to martensite transformation under plastic deformation at low temperature. However, the mechanisms of SIMT in austenitic stainless steel (AISI 304 series) under SPD, particularly the transformation mechanisms in small grains with sizes of submicronmeter and nanometer, are still lack of investigation. Equal channel angular pressing (ECAP) is one of the popular methods of SPD, which can produce bulk nanostructured metallic materials without any reduction in the cross–sectional area of specimen. It has been clarified that the shear deformation imposed by ECAP was the most effective route to trigger SIMT in austenitic stainless steel in comparison with uniaxial tension and compression. In this paper, the SIMT in 304L austenitic stainless steel was invesigated under ECAP deformation at room temperature, in order to reveal the mechanisms of nucleation, grwth and crystallography of strain–induced martensite. The microstructures of strain–induced martensite during ECAP deformation were carefully examined by X–ray diffraction and transmission eectron microscope (TEM). It was found that in the case of coarse austenitic grains, the strain–induced marteniste nucleated at the intersection of deformation bands (including the bundles of stacking faults, deformation twins and platelets of epsilon phase) and kept the K–S (Kurdjumov–Sachs) but not the Nishiyama–Wassermann orientation relationships with austenitic grains. While in the case of small austenitic grains with sizes of several hundred nanometers, the strain–induced martensite preferred to nucleate at grain boundaries and grew up via swallowing the matrix of austenite. The martensitic grains followed the K–S crystallographic relationships with austenite too. Furthermore, the new nanocrystallne martensitic grains were easily rotated against each other by shear deformation, which prevented the coalescence of martensitic grains and was beneficial for the formation of nanocrystalline stuctures. Accoring to the K–S orientation relationship, the {110} planes of martensite are converted from the {111} lanes of austenite, keeping the <110> direction of martensite parallel to the <111> direction of austenite as well. The difference and mechanism of SIMT occurring in coarse austenitic grains and submicron austenitic grains were discussed in detail.

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    MODELING OF Ag3Sn COARSENING AND ITS EFFECT ON CREEP IN Sn-Ag-Cu SOLDER
    WANG Xiaojing ZHU Qingsheng WANG Zhongguang SHANG Jianku
    Acta Metall Sin, 2009, 45 (8): 912-918. 
    Abstract   PDF (2231KB) ( 1424 )

    Coarsening of the microstructure in Sn–Ag–Cu (SAC) solder joints under current stressing was bserved experimentally and modeled by a disocation–creep model which incorporates te carsening f second pase particles in lead–free solder aoys. Both the effects of electric current and strain–enhanced coarsening were conidered in thimodel. The straining effect took into account of both the inelastic–strain hstory and hydrostatic constraint. The model describes well the evolution of thutectic microstructure and the predictions of the model agree reasonably well with experimentally observed trends.

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    EFFECT OF AGING TIME ON ELECTROCHEMICAL CORROSION BEHAVIOR OF 2101 DUPLEX STAINLESS STEEL
    HAN Dong JIANG Yiming DENG Bo ZHANG Lihua ZHANG Wei LI Jin
    Acta Metall Sin, 2009, 45 (8): 919-923. 
    Abstract   PDF (1146KB) ( 1286 )

    Duplex stainless steels (DSS) are extensively applied in many fields such as petroleum industry, petrochemistry, desalination service and paper–making industry. They are characterized by a two–phase structure, offering an attractive combination of corrosion resistance and mechanical properties. Their best general properties are obtained by keeping approximately equal volume fractions of austenite and ferrite but averting third harmful phases, such as σγ2, χ, carbide and nitride, to be precipitated. These harmful phases can be formed from ferrite when DSS are welded or worked at improper high temperatures. Recently, a kind of economical DSS 2101 of typical composition Fe– 21.4Cr–1.2Ni–5.7Mn–0.23N–0.31Mo was developed and shows a wide application potential due to its higher yield strength, better pitting corrosion resistance and lower cost than traditional 304 austenite steel. However, limited research has been dedicated to studying the microstructure and mechanical properties of DSS 2101. Especilly research n the nature of precipitations and their effects on mechanicl properties in DSS 101 ws stilacking in comparison to many researches on the recipitations in DSS 2205 and DSS 2507. In the present work, it aims at investigating how the precipitations can affect the corrosion resistance of DSS 2101. Pitting corrosion resistance of samples aged at 700 ℃ in 1 mol/L NaCl solution was evaluated by potentiodynamic polarization curve test. The electrochemical potentiokinetic reactivation (EPR) test was also used to evaluate the degree of sensitization to intergranular corrosion. Scanning electron microscopy (SEM) and optical microscopy (OM) were used to observe the surface morphology after electrochemical tests. The results demonstrate that both the resistance to pitting (Eb) and intergranular corrosion (Ra) decreases with aging time increasing. Pitting nucleates preferentially in the ferrite phase for the solution–annealed sample, while the initiation of pitting corrosion takes place around Cr2N, in the newly formed secondary austenite for the aged specimen. EPR tetconducted at room temperature are able to detect the sensitization to Ra due to Cr2N precipitation. With increasing aging time, more Cr2N particles are precipitated at phase interfaces and grain boundaries, causing a continuous decrease of resistance to intergranular corrosion.

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    EFFECT OF Ce ON THE DYNAMIC RECRYSTALLIZATION AND TEXTURE OF AZ31 MAGNESIUM ALLOY DURING HOT ROLLING
    LI Zaijiu JIN Qinglin JIANG Yehua ZHOU Rong
    Acta Metall Sin, 2009, 45 (8): 924-929. 
    Abstract   PDF (3614KB) ( 1375 )

    The effects of Ce on the dynamic recrystallization (DRX) and texture of AZ31 magnesium alloy during hot rolling were investigated. It was found that the twin DRX is suppressed in AZ31–1.0Ce alloy, the DRX is accelerated and the basal texture is weakened significantly by adding 1%Ce (mass fraction) into AZ31 alloy. The EBSD resuls show that besides (0001) basatexture, there is an apparent peak intensity beteen (0001) and 10¯10) in AZ31–1.0Ce alloy, which means the actvaion of non–basal slip system. The addition of Ce does not result in a decrease of c/a ratio. On the contrary, the c/a ratio is sightly increased by adding of 1%Ce. It indicates tht te activation of non–bsal slip sysem does not stem from the change of crystal structure. The activation of non–basal slip and the corresponding basal texture weakening are attributed to the change in atomic binding states anstacking fault energy.

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    STRUCTURE AND MIGRATION CHARACTERISTIC OF HETEROINTERFACES DURING THE PHASE TRANSFORMATION FROM DO22 TO L12 PHASE IN Ni75AlxV25−x ALLOYS
    ZHANG Mingyi CHEN Zheng WANG Yongxin LU Yanli ZHANG Jing FAN Xiaoli
    Acta Metall Sin, 2009, 45 (8): 930-936. 
    Abstract   PDF (2594KB) ( 811 )

    The behavior of heterointerfaces governs the evolution of microstructures during processing and heat treating, and ultimately controls the resultant morphology of transformation products and therefore the resultant physical and mechanical properties of an engineering material. Atomistic numerical simulation has been proven to be one of the most powerful methods for exploring solid interface behaviors today, especially in the case where they are hardly investigated by experimental techniques, such as atomistic interface migration mechanisms during phase transformations. The micrscpic phase–field model can be used tstudy the microstructure evolution during phase transformations and to trck the migration of an interface at atomic scale at the same time. In this paper, this model was used to investigate the interface migration during the phase transformation from DO22 (Ni3V) to L12 (Ni3Al) in Ni75AlxV25−x(x=4.2, 5.0) alloys. The DO22 phase is precipitated from the disordered fcc phase and then transformed to the L12 phase during aging process. Using the simulated microstructures and the occupation probabilities of alloy elements at interfaces, the structure and migration characteristics of ordered domain interfaces formed between DO22 and L12 phases are investigated and the mechanism of the phase transformation from DO22 to L12 is proposed. The results show that there are five kinds of heterointerface structures to be formed between DO22 and L12 phases, four of them are able to migrate during the phase transformation from DO22 to L12 except for (002)D//(001)L. The structures of (100)D//(200)L and (100)D// (200)L·1/2[001] themselves are kept unchanged with their migrating, only (002)D//(002)L and (002)D//(002)L·1/2[100] alternate with each other, and therefore, these two kinds of interfaces appear alternatively. Ni atoms present a site selective behavior during interface migration, they would jump to their nearest neighbor sites and substitute for V atoms with V atoms migrating to the inside of DO22 phases. Al atoms would migrate to interfaces and substitute for Ni or V atoms there. Such a jump and substitution mode of aoms may be an optimization way in thermodynamics and kinetics for interface migration during phase transformaion. All possible atom jump modes inducing the interface migration would obey that the number of jumping atoms during migration must be the least and the jumping distance of atoms the shortest.

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    EFFECT OF PRE-DEFORMATION ON STACKING FAULT PROBABILITY AND DAMPING CAPACITY OF Fe-Mn ALLOY
    HUANG Shuke LIU Jianhui LI Chang'an ZHOU Danchen LI Ning WEN Yuhua
    Acta Metall Sin, 2009, 45 (8): 937-942. 
    Abstract   PDF (1797KB) ( 851 )

    Fe–Mn alloys have been widely applied to industrial facilities because of their good mechanical properties, high damping capacity at great strain amplitude and low cost. However, the damping mechanism of Fe–Mn alloys is not fully clear now. In this paper, the high damping mechanism of a Fe–Mn alloy and its effect of pre–deformation on damping capacity were studied by using G–L dislocation model and measuring stacking fault probability. The damping capacity was measured using reversal torsion pendulum. The stacking fault probability and volume fraction of  ε–martensite were determined by XRD and the microstructure was observed by SEM and TEM. The results show that the high damping mechanism can be described in terms of the breakaway movement of Shockley partial dislocations on stacking faults. Pre–deformation has little effect on the volume fraction of  ε–martensite, but it increases the stacking fault probability in γ–austenite and ε–martensite, so the damping capacity of Fe–Mn alloy is improved. With pre–deformation increasing (greater than 4%), however, the stacking faults and ε–martensite segment each other, so the breakaway movement of Shockley partial dislocations is difficult and the damping capacity of Fe–Mn alloy decreases gradually.

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    INFLUENCE OF HEAT TREATMENT ON MICROSTRUCTURES AND ADHESIVE STRENGTH OF HIGH TEMPERATURE SOLID SELF–LUBRICANT COATING
    XU Na ZHANG Jia HOU Wanliang QUAN Mingxiu LI Rongde CHANG Xinchun
    Acta Metall Sin, 2009, 45 (8): 943-948. 
    Abstract   PDF (2661KB) ( 1114 )

    Foil air bearings are self–acting hydrodynamic bearings relying on solid lubricants to reduce friction and wear during sliding at start–up and shut–down when speeds are too low to allow the formation of a hydrodynamic air film. The service temperature of traditional solid lubricants such as graphite, PTFE and MoS2 is limited to only about 300 ℃, The NiCr–Cr2O3–Ag–BaF2/CaF2 solid lubricants can make foil air bearings run from room temperature to 650 ℃ In this paper, the NiCr–Cr2O3–Ag–BaF2/CaF2 high temperature solid self–lubricant coatings were fabricated on GH4169 matrix using plasma spray technology. Microstructures and adhesive strengths of the coatings as sprayed and treated at 500, 650 and 800 ℃ were analyzed by SEM, XRD and tensile test. The results show that the as–sprayed coating consists of Ni(Cr) phase, Cr2O3 phase, Ag phase and (Ba,Ca)F phase and has coarse lamellar–likstructures, the adhesive strength reaches 30.4 MPa. After heat treatments at 500 and 650 ℃ (Ba, Ca)F phase transforms into BaF2/CaF2 phase, meanwhile the BaF2/CaF2 becames compacted. The formatin and growth of lots of black Cr2Oand iO precipitations make Ni(Cr) phase refine. After heat treatment at 650 ℃ for 12 or 24 h, the adhesive strength increases to 46.5 MPa. But after heat treatment at 800 ℃ BaF2 was oxidized, which makes the microstructures loose and the porosities are formed in a large area. The adhesive strength is decreased evidently.

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    MICROSEGREGATION OF SOLUTE ELEMENTS IN SOLIDIFYING MUSHY ZONE OF STEEL AND ITS EFFECT ON LONGITUDINAL SURFACE CRACKS OF CONTINUOUS CASTING STRAND
    CAI Zhaozhen ZHU Miaoyong
    Acta Metall Sin, 2009, 45 (8): 949-955. 
    Abstract   PDF (1522KB) ( 1658 )

    The solidification of molten steel in continuous casting mold is a complicated nonequilibrium process with high cooling rate of 10—100 ℃/s. At such a cooling rate, the segregation of the solute elements such as C, Si, Mn, P and S in brittle temperature range (ΔθB) will vary with their initial contents and influence on the thermal strain significantly which could greatly increase the incidence of surface defects of strand. In this paper, a microsegregation model of solute elements in mushy zone with δ/γ transformation during solidification was established based on the regular hexagon transverse cross section of dendrite shape proposed by Ueshima by finite difference method under the non-equilibrium solidification condition at 10 ℃/s of cooling rate and the brittle temperaturerange ΔθB was determined. The distribution characteristics of solute elements and the effect of their segregations on ΔθB  and thermal strin were nvestigatedTe results show that both P and S are the most serious segregation elements in final stage of solidification and affect on ΔθB  sinificantly together with carbon content in molten steel. The mechanism that increasing contents of P and S may incrasthe probability of longitudinal surface crack for continuous casting strand was presented by calculatng the change law of thermal strain with carbn content under different of P and S contents.

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    THERMO–CALC LINKED COMPUTATIONS OF SOLIDIFICATION PATHS OF TERNARY ALLOYS USING AN EXTENDED UNIFIED MICROSEGREGATION MODEL
    ZHAO Guangwei XU Daming SONG Menghua FU Hengzhi DU Yong HE Yuehui
    Acta Metall Sin, 2009, 45 (8): 956-963. 
    Abstract   PDF (1734KB) ( 1382 )

    A model for predicting the microsegregation of ternary alloys was developed via extending a previously proposed unified microsegregation model for binary alloys. The present multicomponent/multiphase model retains the advanced features of the previous binary microsegregation model, in which the unified microscale parameter Φ takes a general function form to account for more possible influential factors, including the partition coefficient, solid fraction, solid diffusion coefficient, dendrite geometrical morphologies and solidification rate, etc.

    The algorithms fr cacuating the solidification paths of ternary isomorphous and eutectic alloys were proposed, which closely couples with a commercial software package/database of Thermo–Calcvia its TQ6–interface in order to directly access to thermodynamic data needed in the multicomponent/multiphase solidification path computations. In the solidification of primary phase and three phases eutectic, solid fraction fs was selected to be a control variable when solving the equations of the microsegregation models, while in the solidification of two phases eutectic, temperature was selected to be a control variable as the relationship between the concentrations of solutes B and C was unknown. In each calculation iterative step, the names and number of the equilibrium phases were obtained from Thermo–Calc, and then saved and compared with the calculation results of the previous step. As the names and number of the equilibrium phases in each solidification stage of ternary eutectic alloys are different, the three solidification types were determined by comparing with the results in the computation process.

    The availability and reliability of the proposed multicomponent/multiphase model and algorithms were demonstrated by sample computations for the solidification paths of Fe–40V–40Cr, Al–4.5Cu–1Si, Al–10Cu–2.5g and Al–1.49Si–0.64Mg (mass fraction, %) alloys under different solidification/cooling rates, and by comparisons with the experimental results of quantitative measurementof corresponding solidification microstructures.

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    HIGH TEMPERATURE OXIDATION BEHAVIOR OF TWO NiCoCrAlSiY COATINGS DEPOSITED BY ARC ION PALTING
    XU Chaozheng JIANG Sumeng MA Jun GONG Jun SUN Chao
    Acta Metall Sin, 2009, 45 (8): 964-970. 
    Abstract   PDF (2947KB) ( 1051 )

    Two Ni-Co-Cr-Al-Si-Y (M14 and M19) coatings with different Al and Cr contents were deposited onto DD32 Ni--base superalloy substrate using arc ion plating (AIP) method. The microstructures, isothermal oxidation behaviors at 1000 and 1100 ℃ and cyclic oxidation behaviors between room temperature and 1000 ℃ of the both coatings were investigated. The results show that these two coatings are composed of γ'/γβ-NiAl and some α-Cr precipitate phases. The only difference is that there are more β-NiAl phase precipitates in M19 coating with high Al content. M19 coating, in contrast to M14 coating, can not only repair destroyed alumina scales effectively during isothermal oxidation, but also resist further cracking and spalling of alumina scales during cyclic oxidation. High Al content in M19 coating can be kept under long time oxidation, making M19 coating long-term service.

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    EFFECTS OF NANO–CeO2p ON OXIDATION BEHAVIORS OF NiCoCrAlY LASER CLADDING COATINGS ON Ni–BASED SUPERALLOYS
    WANG Hongyu ZUO Dunwen WANG Mingdi SHAO Jianbing
    Acta Metall Sin, 2009, 45 (8): 971-977. 
    Abstract   PDF (3103KB) ( 982 )

    Nano-CeO2p, which is most commonly used as a kind of rare earth modified material of alloys, possesses a prominent advantage in improving oxidation resistance. To study how nano-CeO2p will affect the oxidation resistance of NiCoCrAlY bondcoat in double--layer thermal barrier coatings, in this work, four kinds of the NiCoCrAlY coatings with different contents of nano-CeO2p were prepared on the surface of a Ni--based superalloy using laser cladding technology, their isothermal oxidation behaviors in atmosphere under 1050℃ were investigated, and the influence mechanism of nano-CeO2p on the oxidation resistance of coatings was preliminary discussed. The results show that all these coatings with nano-CeO2p have better oxidation resistance than those without nano-CeO2p, among which the coating with 2% (mass fraction) nano-CeO2p has the best oxidation
    resistance. After 100 hours of oxidation, its oxidation mass gain is only 1.56 mg/cm2, reducing by more than half compared with the coatings without nano-CeO2p; meanwhile, it takes extremely short time to enter the steady oxidation stage, only about 1/20 of the coating without nano-CeO2p. Besides, the oxide layer of the coating is still very dense even after a long time oxidation and only a slight internal oxidation takes place within the coating, however, in that case, the oxide layer of the coatings without nano-CeO2p is out of operation.

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    TEM OBSERVATION OF THE Al3Er PHASE DURING HOMOGENIZING OF THE 5083 ALLOY WITH Er ADDITION
    LIN Shuangping HUANG Hui WEN Shengping NIE Zuoren
    Acta Metall Sin, 2009, 45 (8): 978-982. 
    Abstract   PDF (4180KB) ( 1580 )

    The research on scandium additions in aluminium alloys has gained some interests over the last decades, due to the precipitation of Al3Sc phase particles, which have the beneficial effect on mechanical properties, weldability and corrosion resistance. But Sc is a very expensive strategic
    metal. Therefore, a breakthrough of the selection of RE is expected. However, only a limited number of possibilities exist in Al–based alloys for the formation of coherent strengthening particles that are thermodynamically stable and have the ordered, L12 structure. It involves the addition of some suitable alloying elements such as La, Ce, Y, Sc and especially to Sc, while the effects of other single elements are scarcely referred. Recent studies have considered Er as a promising alloying element in Al–alloy, and it has been shown that a small Er addition can improve the material properties in several Al–alloy systems. Most of the beneficial effects are attributed to the formation of the Al3Er phase in these alloys.
    However, few research have been done on the effect of homogenizing treatment on the precipitation of Er modified 5083 alloy. Although the Al3Sc and Al3(Sc,Zr) particles are wildly studied, but few studies about coherency and coarsening behaviors of Al3Er have been reported. The present investigation provides some information about orientation relationships of the precipitates with matrix and the coherency behavior of Al3Er phase.
    Al3Er phase precipitates durinhomogenizing of the Er modified 5083 alloy produced by direct chill casting were analyzed by transmission electron microscopy (TEM).The results show that many particles with different shapes and sizes precipitate during homogenizing. Spheric precipitates with the size between 10—50 nm is Al3Er phase which are good coherent/semi–coherent with matrix; the orientation relationships between matrix and the spheric precipitates are derived:[111]m//[111]sp, (110)m//(110)sp, (101)m//(101)sp; [011]m//[011]s, (100)m//(200)sp, where the indexs m and sp indicate matrix and spheric precipitate, respectivety.

    The radius for coherent/semi-conherent transition of Al3Er particle is determinined as 12---20 nm by TEM analysis; The calculated value of critical transition radius (rt=9.44 nm) of the Al3Er precipitates is correspondent with the experiments.

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    PREPARATION OF ALUMINIDE COATINGS ON HR-2 STAINLESS STEEL
    ZHANG Guikai LI Ju CHEN Chang'an DOU Sanping LING Gouping
    Acta Metall Sin, 2009, 45 (8): 983-987. 
    Abstract   PDF (2227KB) ( 947 )

    The aluminized coating has been widely applied on structural materials in fusion reactors as a tritium permeation barrier (TPB). In present study, we proposed a new two-step method for preparing an aluminized coating on HR-2 stainless steel: room temperature molten salt electroplating followed by heat treating at 500 and 700 ℃ in air. In the electroplating process, aluminum deposition from AlCl3/EMIC to the surface of HR-2 stainless steel was performed with a deposition rate of\linebreak 15 μm/h at 25 ℃ under the protection of argon gas. This method permits coating of large complex shapes that have few blind areas. The coating microstructure has been characterized by optical microscope (OM), scanning electronic microscope (SEM), energy dispersive spectroscope (EDS) and X-ray diffraction (XRD). These results show that after heat treated at 500 ℃ for 10 h, the coating thickness is about 10-15 μm consisting of two distinct layers with a clear interface between the coating and substrate. however, after heat treated at 700 ℃ for 2 h, the coating thickness is changed to about 10-13 μm and it consists of an external (Fe, Cr, Mn and Ni)2Al5 layer, inner (Fe, Cr, Mn and Ni)Al layer and transitional (Fe, Cr, Mn and Ni)3Al layer. No clear interface between the coating and substrate and defects and cracks in the coating were observed.

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    EFFECTS OF Nd CONTENT ON MAGNETIC PROPERTIES OF NANOCRYSTALLINE NdxFe94-xB6 ALLOYS
    BAO Xiaoqian GAO Xuexu ZHU Jie ZHANG Maocai ZHOU Shouzeng
    Acta Metall Sin, 2009, 45 (8): 988-993. 
    Abstract   PDF (1317KB) ( 1056 )

    Nanocomposite magnets have attracted considerable attention as a probable new generation of permanent magnets with a potential theoretical maximum energy product of 1 MJ/m3 due to remanence enhancement resulting from intergrain exchange interactions between magnetically
    soft and hard grains. However, the presence of soft phase increases remanence but coercive field is significantly decreased. Alloys with Nd content higher than 11%(atom fraction), i.e., close to the stoichiometry composition of Nd2Fe14B, are being studied to further improve coercivity. Nanocrystalline NdxFe94−xB6(x=11.0—16.8) alloys were prepared by melt–spinning at a rate of 22 m/s cooperating with subsequent annealing. The effects of Nd content on microstructure, magnetic properties, exchange coupling interactions and coercivity mechanism have been studied by X–ray diffraction (XRD), differential scanning calorimeter (DSC), high resolution scanning electron microscope (HRSEM) and vibrating sample magnetometer (VSM). The intrinsic coercivity Hci increases from 601.3 kA/m for x=11.0 to 1277.3 kA/m for x=16.8 monotonously, and on the contrary, the remanent polarization Jr reduces from 1.047 T for x=11.0 to 0.721 T for x=16.8. The maximum energy product (BH)max first increases
    and then reduces with increasing Nd content. The optimum magnetic properties with Jr=0.992 T, Hci=727.9 kA/m and (BH)max=137.2 kJ/m3 are achieved by annealing melt–spun Nd11.8Fe82.2B6 ibbons. Although the exchange coupling interactions are degraded by increasing Nd content in the ribbons, it is still relatively strong for the alloy with x=16.8. The coercivities of alloys are determined mainly by pinning field. Nd content almost does not influence microstructure under optimal annealing conditions. The microstructral model of nanocrystalline Nd–Fe–B with different Nd content is presented and used to analyze the effect of Nd content on magnetic properties and exchange coupling effect well.

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    NUMERICAL ANALYSIS ON THE FUNCTIONS OF STIR TOOL'S MECHANICAL LOADS DURING FRICTION STIR WELDING
    YAN Dongyang SHI Qingyu WU Aiping Juergen Silvanus
    Acta Metall Sin, 2009, 45 (8): 994-999. 
    Abstract   PDF (772KB) ( 1017 )

    Besides lower welding temperature and solid phase welding process, another significant difference between friction stir welding and conventional fusion welding (FSW) is that the forming of weld seam in FSW is effected by both thermal load and mechanical loads of stir tool. However, the definite functions of mechanical loads of stir tool in FSW process are not clearly described. With FSW experiments, only the material flow around stir tool can be observed, from which the results were obtained that the mechanical loads of stir tool gave birth to the plastic flow, a key factor of weld formation, of intenerated metal around the tool. In the simulation of FSW process, mechanical loads are always neglected in most analysis models, which can’t describe the unique characteristic of FSW. Although the mechanical loads have been considered in FSW simulation by some researchers, the loads are greatly simplified and the analysis is only localized in the asymmetric distribution of residual stress. In this paper, numerical simulation was applied to investigate the functions of mechanical loads during FSW process. Based on experiments of FSW aluminium alloy sheets, three simulation models were establised with different conditions of considering thermal load only, considering both thermal load and down force, and considering thermal load, down force and working torque synthetically. The results show that the down force significantly reduces residual stress and the torque leads to the unsymmetrical distribution of residual stress. On the other hand, mechanical loads absolutely change the residual distortion pattern from saddle state into anti–saddle state. Mechanical loads strengthen the mechanical restriction on sheet, forge and extrude the materials around tool, so the residual strain in weld zone is reduced, which leads to the eduction of residual stress. Furthermore, the mechanical loads change the correlaton of strain on top surface and bottom surface of sheet, which resultes in the change of distortion pattern.

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    EFFECT OF ARC POWER ON THERMAL FIELD CHARACTERISTICS OF LASER+GMAW-P HYBRID WELDING
    WU Chuansong XU Guoxiang QIN Guoliang WANG Xuyou LIN Shangyang
    Acta Metall Sin, 2009, 45 (8): 1000-1005. 
    Abstract   PDF (571KB) ( 1427 )

    Laser + GMAW–P (pulsed gas metal arc welding) hybrid welding is a combination of laser welding with GMAW–P which can not only bring the advantages of heatsources used in these two welding processes into full play, but also compensate for each other’s drawback. Therefore, laser + GMAW–P hybrid welding is a promising joining technology for industrial applications. With its applications becoming more widespread, there is a growing need to understand more deeply of this new welding process, such as the relationship between process parameters and weld quality. As the key factors determining weld quality, the thermal field characteristics of HAZ (heat affected zone) in welding, such as HAZ width and thermal cycle parameters, have significant effects on the microstructure and properties of welded joint. In this paper, an adaptive combined heat source model developed for laser + GMAW–P hybrid welding was employed to conduct the numerical analysis of thermal field in hybrid welding. The influence of different arc power levels on the HAZ width and thermal cycle parameters in hybrid welding is quantitatively analyzed, and the relevant data obtained respectively during single laser welding, single GMAW–P and laser + GMAW–P hybrid welding are compared to each othe. It has bn found that the HAZ width is less in hybrid welding than in GMAW–P even if the poweinput of lasr + GMAW–P hybrid welding is 2 kW largr than that of GMAW–P. When the arc power is lower, there is a small difference of HAZ width between GMAW–P and laser + GMAW– P, while the difference is narrowed down when the arc power becomes higher. Laser + GMAW–P hybrid welding causes a larger cooling time and lower peak temperature which are benificial to the improvement of microstruture and properties of HAZ. The results are very useful for demonstrating the process features and understanding more deeply the advantages of hybrid welding.

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    EXPERIMENTAL STUDY ON THE REASON OF PORE FORMATION IN LASER WELDING OF DIE–CAST MAGNESIUM ALLOY
    SHAN Jiguo ZHANG Jing ZHENG Shiqing CHEN Wuzhu REN Jialie
    Acta Metall Sin, 2009, 45 (8): 1006-1012. 
    Abstract   PDF (1582KB) ( 977 )

    Because of their low density, high strength-to-weight ratio and high damping capacity, magnesium alloys have gained increasing applications in many fields, especially for die-cast magnesium alloys. The application of die-cast magnesium alloys in structures and repair welding of casting defects necessitates the development of welding techniques. As a high energy density thermal source, laser welding can reduce many welding defects compared with conventional welding methods, but pore formation during welding of die-cast magnesium alloys is still a severe problem that has not been solved. This paper seeks to identify the mechanism of pore formation during laser welding of die-cast magensium alloys and search for a metallurgic solution of porosity. The great tendency to pore formation in die-cast magnesium alloys is attributed to high gas content in the base metal. The characteristic of gas source in die-cast magnesium alloys and the process of pore formation during laser welding are studied by some experiments including analysis of characteristic parameters of pores (porosity, average diameter and number densities), change of densities before and after heating(350℃, holding time 1 h) and pore formation tendency under different vacuo heating conditions (heating the base metal to 350℃ under vacuum environment of different holding time). The results show that heating increases porosity, average diameter and number densities, which lead to the reduction of bulk density. Vacuo heating before welding can reduce porosity to a great extent, and with holding time prolonging, porosity reduces. In die casting, there are molecular hydrogen and atomic hydrogen in die-cast magnesium alloys. The hydrogen gas (molecular hydrogen) in high pressure pores in the base metal expands sharply in the molten pool, and the oversaturated atomic hydrogen can also form new pores or contribute to the growth of exsisting pores. That is to  say that both molecular hydrogen and atomic hydrogen contribute to the pore formation of die-cast magnesium alloys during laser welding. During vacuo heating before welding, atomic hydrogen can escape in part through diffusion, thus reducing the tendency to form pores. With holding time of vacuo heating prolonging, the amount of atomic hydrogen escaping from the base metal increases, thus porosity reduces. Adding elements, which can combine hydrogen to form hydride, may be a possible solution to pore formation in die-cast magnesium alloys. Preliminary experiments show that adding Zr to welds during welding can reduce the tendency to form pores obviously, the reason for this needs further research.

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    STUDY ON WEAR RESISTANCE OF LASER CALDDING Fe–BASED COMPOSITE COATINGS REINFORCED BY IN–SITU MULTIPLE CARBIDE PARTICLES
    WU Chaofeng MA Mingxing LIU Wenjin ZHONG Minlin ZHANG Weiming ZHANG Hongjun
    Acta Metall Sin, 2009, 45 (8): 1013-1018. 
    Abstract   PDF (2273KB) ( 1371 )

    Particle reinforced metal matrix composites (PR–MMCs) have attracted extensive investigation in material science and engineering. They combine both strength and toughness and show excellent properties such as good wear resistance, corrosion resistance and high temperature properties. Laser depositing of metal matrix composite coatings containing in–situ carbide particles is a research focus in laser surface processing. The essential advantage of the in–situ synthesis technology is that the reinforcements are much more compatible with the matrix and the interface between the reinforcements and the matrix is much cleaner. It has a special advantage in high distribution density and dimension uniformity to in–situ synthesize particles by laser melting precursor powders containing several strong carbide-forming elements (SCFEs) rather than one. Each SCFE has significant effects on the precipitation and distribution of the carbide particles. To investigate the microstructure and properties of the cladded ayers, in this paper, the coatings were produced by laser cladding powders containing 2%Ti, 1%Ti+1%Zr and 1%Ti+1%Zr+10%WC (mass fraction) respectively on the surface of a medium carbon steel. The microstructure of the coatings and the carbide particles were studied by X–ray diffractometer (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that the microstructure of the coatings is typically hypo–eutectic. The in–situ particles have a TiC structure with high content of Zr or W when 1%Ti+1%Zr and 1%Ti+1%Zr+10%WC are added into the cladded powders. Therefore, they are multiple carbides. The interface between the article and the ae material is strong. The optimized istribution of the particles with high distribution density (2×104 mm−2) and high dimension uniformity (about 1μm) are obtained when 1%Ti+1%Zr+10%WC are added. The wear resistance of the coating was tested throgh ring–on–block wear eperiment. It is indicated that laser cladding Fe–based composite coating reinforced by in–situ multiple carbide particles presents excellent wear resistance.

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    EFFECTS OF PULSED ELECTROMAGNETIC FIELD ON CoC2O4·2H2O POWDER SIZE
    DU Huiling WANG Jianzhong QI Jingang HE Lijia CANG Daqiang
    Acta Metall Sin, 2009, 45 (8): 1019-1024. 
    Abstract   PDF (1079KB) ( 1312 )

    Metal cobalt powder is a promising material added in steels for cutting tools, alkaline rechargeable batteries or heterogeneous catalysis and others for fabrication of abrasion strengthened composites. The final mechanical performance of composites is affected by the size of cobalt powder, which is closely dependent on the use of cobalt oxalate precursor. Developments of size controlled preparation methodologies are of great interest in materials chemistry. Previous works have revealed that the size of powders can be controlled through addition of some organic surfactants in preparation or by homogeneous precipitation methods. However, the former has a difficulty of separation and the latter has a shortcoming of low yield. In this study, the cobalt oxalate particles were prepared by precipitation using CoCl2 and (NH4)2C2O4 solutions. Meanwhile a pulsed electromagnetic field (PEMF) was applied to the reaction system to improve the size of these particles. The effects of PEMF on the size of cobalt oxalate particles were investigated, and the mechanism model of interaction between PEMF and reaction system was established. The phase structures of products were characterized by X–ray powder diffraction (XRD). Packing density instrument and Laser Particle Size Analyzer were used to measure the sizes of obtained cobalt oxlte powders. Thermal decomposition was performed using TG–DSC thermogravimetric analyzer. The results indicate cobalt oxaldate ( β–CoC2O4·2H2O) particles are formed of the packindensity of 0.393 g/cm3 and the average diameter of 3.5403 μm at PEMF voltage of 800 V. Compared with the paticles prepared without PEMF treatment, the packing density and average diameter decrease by 26.95% and 60.13%, respectively. The results of cobalt oxalate thermolysis show that products prepared from cobalt oxalate with and without PEMF both are β–Co particles. The average diameter of cobalt powders prepared from cobalt oxalate precursor with PEMF is decreased by 71.18% compared with that without PEMF.

    This paper offers a simple and rapid separation technique for preparing lower cost cobalt powders, which would be susceptible for industrial application.

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