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

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    INVESTIGATION OF THE MICROSTRUCTURE AND IMPACT PROPERTIES OF THE HIGH NITROGEN STAINLESS STEEL WELD
    LI Dongjie, LU Shanping, LI Dianzhong, LI Yiyi
    Acta Metall Sin, 2013, 49 (2): 129-136.  DOI: 10.3724/SP.J.1037.2012.00514
    Abstract   PDF (3997KB) ( 922 )

    The high nitrogen stainless steel (HNS) is widespread attention in recent years because of the high strength, good toughness and corrosion resistance. Weld ability and welding efficiency are received extensive attention in the HNS welding. Using the double shielded TIG welding method, the active element, oxygen, was transferred into weld pool so as to change the convection of the liquid metal from outward to inward. This proposed welding process could effectively reduce the width of the pool surface, which is beneficial to reducing the overflow of nitrogen content in the weld. The influence of heat input (different welding current) on the weld pool shape and the impact properties of welds were investigated. The weld penetration increased with the increasing heat input and the 10 mm thick work piece was welded thoroughly under the 250 A welding current. The XRD test and calculation results showed that the δ ferrite content in the welds was increased with the increasing welding current, while the impact properties of welds change little. The poor impact properties of the weld are another focus of this study. The fracture surfaces of the Charpy V-notch specimens were characterized using the scanning electron microscope (SEM).

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    EBSD STUDIES OF 30MnB5 HOT STAMPING STEEL TEMPERED AT DIFFERENT TEMPERATURE
    CHENG Junye, ZHAO Aimin, CHEN Yinli, DONG Rui, HUANG Yao
    Acta Metall Sin, 2013, 49 (2): 137-145.  DOI: 10.3724/SP.J.1037.2012.00451
    Abstract   PDF (6662KB) ( 2028 )

    Currently, the senior-level hot stamping steels, such as 30MnB5, could not be used widely in automotive body like 22MnB5 hot stamping steel due to its low plasticity and high hydrogen induced cracking sensitivity after hot stamping. However, its good mechanical properties after tempering processes made it suitable for automotive structural parts. In this paper, the 30MnB5 hot stamping steel was quenched and tempered at 200-600℃ for 2 min. The orientation relationship (OR) with parent phase and misorientation evolution of martensite variants were characterized by EBSD. The OR between martensite variants and parent phase was determined by using {100} pole figure of martensite variants inside a single prior austenite grain and the method of pole figure contouring. The results showed that, the OR between martensite variants and parent phase in 30MnB5 hot stamping steel after quenching was closer to N-W OR than K-S OR, and the actual pole points were distributed around the theoretical pole points. The OR between martensite and parent phase variants did not change in the samples tempered at different temperatures for 2 min, but the number of martensite variants inside a single prior austenite grain tended to decrease. The misorientation of martensite variants after quenched and tempered at different temperatures were both mainly distributed in the angle range of less than 5° and more than 50°. As tempering temperature increased, the low angle grain boundaries below 5° were decreased slightly while the high angle grain boundaries above 50° reflected an upward trend, but the former still accounted for a large percentage. These low angle grain boundaries below 5°, mainly existing inside martensite variants and  derived from the misorientation between martensite laths, were the main reason that led to discrete distribution of angles between different martensite variants around theoretical values.

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    STUDY ON GROWTH MECHANISM OF SALT BATH VANADIZING COATING BY TD PROCESS ON SDC99 STEEL
    YANG Haopeng, WU Xiaochun, QIN Fang, YANG Longjiao
    Acta Metall Sin, 2013, 49 (2): 146-152.  DOI: 10.3724/SP.J.1037.2012.00512
    Abstract   PDF (3067KB) ( 650 )

    The SDC99 steel is a new kinds of cold work die steel with high strength, high wear resistant and toughness. However, in order to extend its service life, the surface modification technology is required. Many surface treatments can be used to improve the wear resistant of cold work die steel. Among these methods, transition metal nitrides and carbides (such as VC) fabricated by TD (thermal diffusion carbide coating process) have been commonly used in tribological applications to enhance the life of mechanical components because they possess high hardness, excellent wear resistance, low coefficient of friction and good corrosion resistance. During the TD process, materials are immersed in to a molten borax bath that contains the relevant carbide/nitride forming element such as V, Nb, Ta, Cr, Mo and W, and the coating is formed by the combination of metal elements as mention above with the carbon/nitrogen diffused from the substrate. In order to investigate the growth mechanism of VC coating on SDC99 steel, salt bath vanadizing by TD process were performed by molten borax bath containing Na2B4O7(75.6%),BaCl2(8.4%), V2O5(10%) and Al powders(6%). The experiments were carried at 850, 900, 950,1000 and 1050 ℃ for various time. The microstructure and morphology of VC coating layer were observed using SEM and metallographic measurements, and the elemental analysis on the surface were carried out with EDS. The crystalline structures of samples were characterized with XRD.The hardness gradient of nanoindentation test was performed on Triboindenter in-situ nanomechanical test system. The results indicated that at the same immersion time, the higher the temperature of salt bath was, the thicker of prepared coating was. The VC coating layer possesses much greater hardness (about 22 GPa) than that of the substrate (about 7.0 GPa). The hardness between VC coating layer and substrate decreases slowly, indicating that the microstructure of substrate adjacent to the interface can provide effective supporting for VC coating layer. The growth mechanism was the nucleation and growth of initial grains, and the formation of sub--micron sized grains on them. Moreover, the crystalline grain of coating grew into equiaxed grains, due to the influence of carbon activity in substrate, and the grain size decreased with the decrement of its distance to the substrate. In addition, at the initial growth stage of crystalline grain of coating, the growth preferred orientation of crystalline grains on SDC99 steel transformed from crystal plane (200) to (111). However, with the increment of time, the growth preferred orientation of crystalline grains was unsignificance and they grew into equiaxed grains.

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    TEM CHARACTERIZATION OF DISPERSOIDS AND MICROSTRUCTURE OF ODS-316 AUSTENITIC STEEL
    WANG Man, ZHOU Zhangjian, YAN Zhigang, YU Pengfei, SUN Hongying
    Acta Metall Sin, 2013, 49 (2): 153-158.  DOI: 10.3724/SP.J.1037.2012.00466
    Abstract   PDF (3375KB) ( 814 )

    Super-critical water-cooled reactor (SCWR) has been selected to be one of the most potential advanced nuclear reactors due to its simplification and high efficiency. Oxide dispersion strengthened (ODS) austenitic steels are promising candidate materials for SCWR, which combine the advantages of both austenitic stainless steels and ODS steels. In this work, ODS-316 austenitic steel (316L+0.35%Y2O3+0.3%Ti, mass fraction) was fabricated by the process of mechanical alloying (MA) and hot isostatic pressing (HIP). The microstructural evolution after milling and hipping were studied by XRD. Morphologies and electron diffraction patterns of dispersed particles were characterized by TEM. Tensile properties of ODS austenitic steel were measured at room temperature. It was found that there were two phases of α (FeCr) and γ (Fe, Ni) in the powders after mechanical alloying, which was resulted from strain induced transformation. ODS-316 austenitic steel was single austenitic phase, since the strain disappeared during the consolidation of HIP. According to the observation and characterization of TEM, three kinds of dispersoids were found in samples: cubic Y2Ti2O7 and TiN and orthorhombic Y2TiO5. However, most of the dispersoids were round particles that enriched in Y, Ti and O. The tensile strength of ODS-316 austenitic steel was improved significantly, and its UTS reached 726 MPa. 

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    EFFECT OF STRAIN RATE ON DYNAMIC DEFORMATION BEHAVIOR OF DP780 STEEL
    DONG Danyang, LIU Yang, WANG Lei, SU Liangjin
    Acta Metall Sin, 2013, 49 (2): 159-166.  DOI: 10.3724/SP.J.1037.2012.00515
    Abstract   PDF (5553KB) ( 1800 )

    Tensile properties and deformation behavior of DP780 steel were studied using servo-hydraulic high-speed material testing machine, SEM and TEM. The effects of strain rate and the mechanism were investigated. The results showed that the strength and ductility of DP780 steel remained almost unchanged as the strain rate increased at strain rates lower than 100 s-1. When the strain rate was over 101 s-1, the strength and the strain-hardening coefficient increased remarkably. Ductility of DP780 steel increased significantly at the strain rates ranging from 3~101 to 5~102 s-1. The deformation resistance increased with increasing the strain rate due to the stronger short range resistance induced by the acceleration of dislocation motion in the ferrite matrix. Increasing strain rate up to 3~101 s-1 resulted in a considerable increase of the amount of mobile dislocation, which was the main reason for the increasing uniform elongation and fracture elongation of DP780 steel at the strain rate ranging from 3~101 to 5~102 s-1. Interface of ferrite-martensite in DP780 steel was the main location for pile-up of dislocation, crack initiation and propagation. The ability of inhomogeneous plastic deformation of DP780 steel increased due to the decreasing plastic strain energy difference between the ferrite matrix and ferrite-martensite interface and the consequent delaying initiation and propagation of microvoids at ferrite-martensite interface induced by the increasing work hardening degree of ferrite matrix with the increasing strain rate.

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    SPLIT FRACTURE PHENOMENON AND MECHANISM IN TENSILE TESTS OF HIGH STRENGTH LOW CARBON BAINITIC STEEL
    LI Xiucheng, XIE Zhenjia, WANG Xuelin, WANG Xuemin, SHANG Chengjia
    Acta Metall Sin, 2013, 49 (2): 167-174.  DOI: 10.3724/SP.J.1037.2012.00545
    Abstract   PDF (3166KB) ( 771 )

    Split fracture phenomenon in tensile tests of a high strength low carbon bainitic steel plate was investigated. Regular tensile tests in the longitudinal direction and short bar tensile tests in three principal axis directions were performed. The results showed that split fracture always occurred when the tensile direction was longitudinal or transverse. The splitting surface was almost parallel to the direction of thickness. According to SEM observation of splitting crack, it was found that the ‘split’ had features of cleavage fracture. Meanwhile, the original steel plate was experimentally proved to have similar strength and ductility in the longitudinal direction, the transverse direction and the direction of thickness. Based on a finite element analysis simulation, the lateral tensile stress within the necked zone was calculated to be much lower than the principle tensile stress. Thus, a mechanism transition from ductile to brittle during the tension process was proposed. Moreover, it was revealed that the split fracture is not definitely the result of lower performance along the direction of thickness, but it is caused by the comprehensive influence of texture evolution, the redistribution of grain boundaries and the state of three dimensional stresses with large tensile plastic deformation, which can be considered as the characteristic of mechanical property of bainitic steel.

     
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    THE MICROSTRUCTURE AND TENSILE FRACTURE BEHAVIOR OF LONG TERM THERMAL AGED Z3CN20-09M STAINLESS STEEL
    ZHENG Kai, WANG Yanli, LI Shilei, LU Xuming, WANG Xitao, XUE Fei
    Acta Metall Sin, 2013, 49 (2): 175-180.  DOI: 10.3724/SP.J.1037.2012.00560
    Abstract   PDF (2430KB) ( 1153 )

    Cast austenite stainless steels (CASS) were widely used in the primary circuit piping of pressurized water nuclear reactors (PWRs), for their excellent mechanical behavior, corrosion resistance and good weldability.CASS, however, were known to have a tendency of thermal aging embrittlement after long term service at mid-temperature (about 280-320℃). The microstructures, micro-mechanical properties and tensile fracture behaviors of Z3CN20-09M stainless steels, thermal aged at 400 ℃ for 2~104 h, were studied in this work. TEM was utilized to observe the microstructure. A nano-indenter was used to investigate the micro-mechanical properties. The tensile tests were completed by an in situ fatigue tester. The tensile surface morphology was observed in SEM and the fracture of thermal aged CASS was examined by an electron probe micro-analyzer (EPMA). The results indicated that the spinodal decomposition and G-phase precipitation distributed in ferrite of the aged CASS. The nano-hardness of ferrite increased and the plastic deformation ability of ferrite phase declined. After long-term aging, the yield strength and ultimate strength increased but the elongation declined. Cleavage features were observed in the ferrite phase of thermal aged CASS. It was also found that micro-cracks initiated at phase boundaries and extended toward the ferrite phase. Ductile dimples and tearing at some areas were shown in the austenite phase in the thermal aged CASS. The spinodal decomposition was considered to be the primary mechanism for the change of the tensile fracture behaviors of Z3CN20-09M stainless steel before and after thermal aging.

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    EFFECT OF WELDING PROCESSING PARAMETERS ON POROSITY FORMATION OF MILD STEEL TREATED BY CO2 LASER DEEP PENETRATION WELDING
    CHEN Gao, GAO Ziying
    Acta Metall Sin, 2013, 49 (2): 181-186.  DOI: 10.3724/SP.J.1037.2012.00574
    Abstract   PDF (1139KB) ( 724 )

    The porosity in the welded seam can be generated easily during the CO2 laser nonpenetration deep welding of low carbon steel, which affects the quality of welding. This research uses the mild steel as the object for the high quality requirements of welding. The advanced high-power CO2 laser generator was used for the welding experiment. The method of cutting cross-section of weld seam was used to analyze the porosity number and observe the morphology and location of porosity in the weld. The effects of such process parameters as shielding gas flow, laser beam inclination, laser power and welding speed on porosity generating were discussed. The research results show that the generating of porosity is due to the unstable collapse of the keyhole in the process of CO2 laser nonpenetration welding of low carbon steel. The porosity would be formed when the speed of bubble escaping form the weld pool is lower than the speed of melting metal solidifying. The results also show that with the increase of shielding gas flow, the porosity number presents a curve of increase firstly and then decrease. The lowest porosity number can be obtained at a 35 L/min of gas flow. With the increase of laser beam inclination angle, the porosity number shows a trend of decrease after increase. Under the condition of deeper penetration welding, the relatively lower porosity number can be obtained at the inclination angle of 22.5°.When the laser power is 4 kW, the porosity number is lowest. At the condition of lower welding speed, the bubble can escape easily for the longer existence period of melting weld pool. Thus lower porosity number and porosity number can be achieved. The porosity can be inhibited effectively at a welding speed of 0.75 m/min.

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    STUDY ON THE DISLOCATION STRUCTURE OF INTERPHASE INTERFACE AND MARTENSITE TRANSFORMATION CRYSTALLOGRAPHY IN Ni2 MnGa ALLOY
    WEI Zhaozhao, MA Xiao, ZHANG Xinping
    Acta Metall Sin, 2013, 49 (2): 187-198.  DOI: 10.3724/SP.J.1037.2012.00465
    Abstract   PDF (4476KB) ( 946 )

    Ferromagnetic shape memory alloys (FSMAs), such as Ni2MnGa alloy, promise to be the keymaterials in manufacturing new type of actuator and sensor because of their high responsive speed and large strainoutput. In order to find the best functions of FSMAs, the understanding of their martensite transformationcrystallography is of enormous necessity and importance. In the present study, the topological model of martensite transformation was applied to the analysis of the interphase interfacial defect structure and crystallography ofmartensite transformation, such as the habit plane index and orientation relationship, in a Ni2MnGa alloy. The results obtained in this work were compared with the prediction from the phenomenological theory of martensite crystallography (PTMC). When the transformation dislocation b+1/+1D1 with smaller Burgers vectors and the lattice invariant deformation (LID) dislocation bL=0.186[111]M are activated to accommodate the coherency strain with a twist angle ω=3.2°, the habit plane is determined to be {0.691-0.117 0.713}p represented in the parent crystal frame, having an inclination angle Ψp=42.000°with the terrace plane (111)p. Furthermore, the orientation relationship between the parent and martensite phases is found to be, namely, {0.69-0.117 0.713}p 0.317° away from (101)M and [110]p about 3.200° away from [111]M, which are close to the calculated values based on the phenomenological theory of martensite crystallography. It is evident that the transformation dislocation with smaller Burgers vectors can be activated more easily during a martensite transformation. Additionally, for a range of twist ω, the alternative combination of disconnection and LID might be introduced in the topological model to obtain multiple predictions of martensite transformation crystallography, which could provide an explanation for the diversity and complexity of martensite transformation crystallography resulting from the different measured positions or external fields. Moreover, the parameters of the interfacial defect network, i.e., the line direction and the spacing of the interfacial defects, were also determined according to the topological model of martensite transformation, while the PTMC provides limited information of the microstructure of the martensite interface. It has been shown that the topological model of martensite transformation exhibits greater flexibility and advantage compared to the PTMC.

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    PHASE TRANSFORMATION TEMPERATURE CONTROL OF WELD METAL OF LASER WELDED TiNi SHAPE MEMORY ALLOY JOINT
    YANG Chenggong, SHAN Jiguo, REN Jialie
    Acta Metall Sin, 2013, 49 (2): 199-206.  DOI: 10.3724/SP.J.1037.2012.00482
    Abstract   PDF (4255KB) ( 645 )

    The phase transformation temperature (PTT) of laser welded TiNi alloy joint is mainly determined by the PTT of its weld metal. A two-step approach is developed to control the PTT of weld metal, including Ni-plating and aging treatment. The Ni-plating increases the Ni content of weld metal, and the aging treatment improves the formation of precipitation phase. The effect of plating thickness and aging time on the PTT of weld metal is studied. The chemical composition, microstructure, crystal structure and PTT are analyzed by DSC, EDS, OM, SEM and XRD, respectively. The PTT controlling mechanism of the developed two-step approach is understood by investigating the effect of precipitation phase, grain preferred orientation, and lattice distortion on the elastic energy resilience of martensite and the resistance of phase transition. The increasing of Ni content in weld metal by Ni-plating can just rise up the elastic energy resilience of martensite, which causes the drop of PTT. The aging treatment not only increases the resistance of phase transition, but also reduces the elastic energy resilience of martensite. The compound effect increases the PTT of weld metal. By adjusting the plating thickness and the aging time of the two-step approach, both the elastic energy resilience of martensite and the resistance of phase transition can be controlled within a reasonable range. These measures keep the austenite start temperature (As) and finish temperature (Af) of weld metal as the similar as that of base metal, thus the PTT of TiNi alloy joint is well controlled.

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    INVESTIGATION ON PITTING CORROSION BEHAVIOR OF COPPER IN THE MIXED SOLUTION OF HCO3-, SO42- AND Cl-
    WANG Changgang, DONG Junhua, KE Wei, LI Xiaofang
    Acta Metall Sin, 2013, 49 (2): 207-213.  DOI: 10.3724/SP.J.1037.2012.00357
    Abstract   PDF (1470KB) ( 478 )

    In this work, the pitting corrosion behavior of Cu in the mixed solution of HCO3-, Cl- and SO42- simulating groundwater was investigated by means of cyclic polarization test and SEM microscopy. The results showed that both SO42- and Cl- can synergistically promote the anodic dissolution of Cu electrode. Cl- can decrease the corrosion potential of Cu to enhance its electrochemical activity. The plot of pitting sensitivity of Cu showed that the pitting critical concentration of Cl- was 0.02 mol/L. When the concentration of Cl- was low, SO42- did not affect the pitting susceptibility; when the concentration of Cl- was in the middle, SO42- strongly inhibited the pitting corrosion of Cu; when the concentration of Cl- was high, the pitting susceptibility will first increase and then reduce with increasing the concentration of SO42-. Regardless of what concentration of SO42-, Cl- can promote the pitting corrosion of Cu. In the current solution system, it is found that the pitting corrosion of Cu was sorely sensitive to SO42- and Cl-.

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    SYNTHESIS AND ELECTRICAL PROPERTIES OF NiCrAl ELECTRO-THERMAL ALLOY FOAMS
    ZHANG Yuelai, DUAN Deli, ZHAO Yuhang, HOU Sihan, LI Shu
    Acta Metall Sin, 2013, 49 (2): 214-220.  DOI: 10.3724/SP.J.1037.2012.00446
    Abstract   PDF (2748KB) ( 859 )

    NiCrAl alloy is a kind of electro--thermal material with high temperature performance and metal foams have high specific surface area and high porosity. Therefore NiCrAl alloy foams can be used in many applications such as fluids heating, electro-thermal catalyst bed and so on. In this work the NiCrAl foams were made from NiCr foams by using pack-aluminizing and vacuum homogenizing heat treatment. Effects of the vacuum homogenizing heat treatment temperature on evaporation of metal elements were investigated. The composition and structure of the NiCrAl foams were analysed by SEM and XRD and their relationship with the apparent resistivity of NiCrAl foams was discussed. The results indicate that NiCrAl foams with uniform composition and structure can be prepared by the process of pack-aluminizing on NiCr foams. The composition of NiCrAl foams can be controlled by adjusting temperature of pack-aluminizing, and the uniform structure of NiCrAl foams is obtained after appropriate vacuum homogenizing heat treatment. After pack-aluminizing, the foams possessed multiphase structure consisting of Ni solid solution, Cr, Ni2Al3, NiAl, Ni3Al and their resistivity decreased with the Al content increasing. NiCrAl foams with low content of Al after vacuum homogenizing heat treatment had single Ni solid solution phase. The resistivity of NiCrAl foams is higher than NiCr foams's. NiCrAl foams exhibited the maximum apparent resistivity when the content of Al was about 5% (mass fraction). With the content of Al above 5% the resistivity of NiCrAl foams decreased due to the formation of Ni3Al phase. If the content of Cr and Al increased further Cr precipitation occurred in the alloy foams. Cr precipitation greatly lowered the resistivity of NiCrAl foams. NiCr30Al9 foam is not suitable as electro-thermal alloy.

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    CHARACTERISTIC BEHAVIORS OF PARTICLE PHASES IN NiCrBSi-TiC COMPOSITE COATING BY LASER CLADDING ASSISTED BY MECHANICAL VIBRATION
    WANG Chuanqi, LIU Hongxi, ZHOU Rong, JIANG Yehua, ZHANG Xiaowei
    Acta Metall Sin, 2013, 49 (2): 221-228.  DOI: 10.3724/SP.J.1037.2012.00318
    Abstract   PDF (4460KB) ( 1159 )

    The good high temperature wear resistance and corrosion behavior of particle reinforced Ni-based alloy composite coating have attracted extensive attention in material science and engineering. It is necessary to analyze the morphologic characteristics and distribution of particles in composite coating. TiC particle reinforced NiCrBSi composite coating on medium carbon steel surface was fabricated by mechanical vibration assisted laser cladding technique. According to the distribution characteristics of hard phase particles in laser cladding molten pool, the growth morphology of TiC particle and endogenous M23C6 carbide, formation mechanism and its distribution characteristics in γ-Ni solid solution were analyzed by XRD, SEM and EDS. The results showed that most of TiC particles dissolved into the melted Ni-based alloy, but some supersaturated Ti and C atoms were precipitated in the form of TiC particles eutectic during cooling process. The TiC particles lateral growth with heterogeneous nucleation way depended on M23C6 type carbide substrate. At the same time, some composite carbide core-shell structure with (Ti, Cr, Ni, Fe, Si)C encapsulated TiC were generated in the coating. Under the effect of vibrant force, the bulky branch crystal eutectic structure disappeared in the bottom of laser cladding coating, the TiC particle floatation trend slowed down with the fluid stratosphere which caused by vibratory force and high-pressure gas, and some double and petal shape TiC particle clusters were also formed. The precipitated TiC particle increases with the Cr content in the inter-dendrite reticular (Fe, Ni) solid solution, and the average particle size was increased by more than 25%. The XRD results indicated that the diffraction peak intensity and lattice integrity of the main hard phases were enhanced, the half peak width was broadened and the crystalline grain size become smaller. The mechanical vibration promoted the dispersion of particles in the matrix dendrites and inter-dendrite.

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    INTER-DIFFUSION BEHAVIOR OF RECOATED CoCrAlY COATING/DZ125 DIRECTIONALLY SOLIDIFIED SUPERALLOY
    QIN Lei, PENG Hui, GUO Hongbo, GONG Shengkai, XU Huibin
    Acta Metall Sin, 2013, 49 (2): 229-235.  DOI: 10.3724/SP.J.1037.2012.00532
    Abstract   PDF (3922KB) ( 605 )

    The thermal barrier coatings (TBCs) are widely used in gas turbines which could lower the surface operating temperature and improve the service life of the coated part component. When repair or refurbishment of these coated components, it is necessary to remove the coating and replace it with a new coatings. The CoCrAlY coatings were deposited by EB-PVD on the directionally solidified nickel-based superalloy DZ125. The coatings were removed via physical methods in varying thicknesses and recoated subsequently. The results showed that the depth of the removal had a significant impact on the inter-diffusion behavior as well as the oxidation resistance. The thickness of Al-depleted zone formed in the recoated coatings after oxidation are thinner than the first coated coatings. Moreover, the more the coating was removed, the thinner the Al-depleted zone was formed. The recoated coatings deposited on the substrate removed the entirely inter-diffusion zone (IDZ) showed the best oxidation resistance, and the IDZ formed after oxidation was similar to the first coated coatings.

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    MICROSTRUCTURES AND MECHANICAL PROPERTIES OF Mg-(2, 3, 4)Y-1Zn ALLOYS WITH LONG PERIOD STACKING ORDERED STRUCTURE
    LIU Huan, XUE Feng, BAI Jing, ZHOU Jian, SUN Yangshan
    Acta Metall Sin, 2013, 49 (2): 236-242.  DOI: 10.3724/SP.J.1037.2012.00548
    Abstract   PDF (3188KB) ( 741 )

     

    Recently, the Mg-Y-Zn alloy systems have received great attention due to their unique microstructures and excellent mechanical properties. Three kinds of ternary equilibrium Mg-Y-Zn phases have been reported in the systems: the W phase (Mg3Y2Zn3), the I phase (Mg3YZn6) and the X phase (Mg12YZn, long period stacking ordered (LPSO)structure). To further study the evolutions of LPSO structures in Mg-Y-Zn alloys, three Mg-(2, 3, 4)Y-1Zn (atomic fraction, %) ternary alloys were prepared by casting and extrusion. Based on the OM, SEM and TEM observations, the microstructures of the as-cast WZ21 and WZ31 alloys are mainly composed ofα-Mg and Mg12YZn duplex microstructures, while that of the as-cast WZ41 alloy consists of α-Mg,Mg12YZn and Mg24Y5 phases. The Mg12YZn phase which forms a network is a kind of 18R-LPSO structures and the Mg24Y5 phase is inclined to be located within 18R phases. During homogenization treatment, part of 18R phase dissolves and 14H lamellas are precipitated in the matrix of the WZ21 and WZ31 alloys. After extrusion, the 18R phases are aligned along the extrusion direction, whereas the 14H lamellas in the matrix are still parallel to each other. During solution treatment (T4), the 18R structures continue to dissolve and 14H lamellas further develop. With increase of the Y/Zn atomic ratio, the volume fraction of 14H-LPSO phase increases after T4 treatment. The mechanical properties for the extruded alloys are better than alloys in as-cast, as-annealed and T4-treated stages. With increasing Y content, the strength of the alloys increases, but the ductility decreases. Tensile strength of the extruded WZ41 alloy reaches 350 MPa at room temperature.
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    EFFECTS OF T6 AND T78 TEMPERS ON THE MICROSTRUCTURES AND PROPERTIES OF Al-Mg-Si-Cu ALLOYS
    LI Xiangliang, CHEN Jianghua, LIU Chunhui, FENG Jiani, WANG Shihao
    Acta Metall Sin, 2013, 49 (2): 243-250.  DOI: 10.3724/SP.J.1037.2012.00509
    Abstract   PDF (3585KB) ( 1303 )

    Although AlMgSi (6000-series) are generally considered to have better corrosion resistance than other aluminum alloys, it may introduce susceptibility to intergranular corrosion (IGC) by some factors, for instance, improper thermo-mechanical treatment and high Cu content. So the T78 treatment has been designed to desensitize it to IGC. In this paper, effects of T6 and T78 tempers on the microstructures and properties of an Al-0.75Mg-0.75Si-0.8Cu (mass fraction, %) were investigated by hardness test, electric conductivity test,accelerated corrosion test, SEM, TEM and energy dispersive X-ray elemental mapping. The hardness and conductivity of the T6 peak-aged sample, which was obtained by artificial aging for 8 h at 180 ℃, were 128.3 HV and 27.3~106 S/m, respectively. T78 tempers involved a first step aging (180 ℃, 5 h) followed by a second step aging at 195, 205 and 215 ℃, respectively.With the prolonging of second step aging, the hardness firstly decreased then increased, while the conductivity increased gradually. The optimum T78 process was(180 ℃, 5 h)+(195 ℃, 2 h), at which condition the hardness was 129.2 HV and the electric conductivity was 27.6~106 S/m. TEM observation results show that it was mostly needle-likeβ” phase in the Al matrix for the peak-aged sample. After T78 treatment, a large amount of lath-like precipitates formed in the matrix, while the precipitate free zones (PFZ) broadened slightly. The segregation of Cu was found at the interface between lath-like precipitates and the matrix, so more Cu precipitated out from the Al matrix and thus reduced the electrochemical potential difference between the PFZ and the matrix. The above finding may explain why T78 temper can desensitize intergranular corrosion without sacrificing strength.

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    MICROSTRUCTURE AND PROPERTIES OF METAMORPHIC LAYER FORMED ON Mg-RE ALLOY IN MICRO-EDM PROCESS
    SUN Shufa, DI Shichun, LU Pengxiang, WEI Dongbo,YU Jie, GUO Yupeng
    Acta Metall Sin, 2013, 49 (2): 251-256.  DOI: 10.3724/SP.J.1037.2012.00557
    Abstract   PDF (3393KB) ( 804 )

    In this work, the Mg-RE alloy metamorphic layer was formed in the micro-EDM process in different process parameters. The morphology and composition of the metamorphic layer were detected by SEM and XPS, the hardness of the metamorphic layer was measured by a nanoindentor, the corrosion resistance property was investigated by using a potentiodynamic polarization test. The results show that there are microcracks on the surface of metamorphic layer, the microcracks can be reduced by decreasing processing voltage and current, and it can be further eliminated by increasing pulse wide and pulse interval. The oxidation caused on the surface of metamorphic layers in the micro-EDM process in three different dielectrics among which there was significantly electrochemical corrosion on the surface in deionized water. The hardness of metamorphic layer is enhanced compared to the substrate of Mg-RE alloy, and its maximum value is about 1.664 GPa. The potentiodynamic polarization test shows that the corrosion resistance of the Mg-RE alloy was improved significantly due to the existence of the metamorphic layer.

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