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

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Deformation Behavior and Toughening of High-Entropy Alloys
A new alloy design concept, high-entropy alloys (HEAs), has attracted increasing attentions and becomes a new research highlight recently. Different from traditional alloy design strategy which usually blends with one or two elements as the principal constituent and other minor elements for the furt. . .
Acta Metall, 2018 Vol. 54 (11): 1553-1566    DOI: 10.11900/0412.1961.2018.00372
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Research Progress of Laser Additive Manufacturing of Maraging Steels PDF (3259KB)
Composition Rule of High Hardness and Electrical Conductivity Cu-Ni-Si Alloys PDF (1334KB)
Research and Development of Equiaxed Grain Solidification and Forming Technology for Nickel-based Cast Superalloys PDF (1651KB)
Effect of Secondary Orientation on Oxidation Anisotropy around the Holes of Single Crystal Superalloy during Thermal Fatigue Tests PDF (2070KB)
Current Issue More>>
      11 June 2019, Volume 55 Issue 6 Previous Issue   
    Current Research and Future Prospect on the Preparation and Architecture Design of Nanomaterials Reinforced Light Metal Matrix Composites
    Huiyuan WANG,Chao LI,Zhigang LI,Jin XU,Hongjiang HAN,Zhiping GUAN,Jiawang SONG,Cheng WANG,Pinkui MA
    Acta Metall. 2019, 55 (6): 683-691.   DOI: 10.11900/0412.1961.2018.00517
    Abstract   HTML   PDF (5830KB)

    In recent years, nanomaterials reinforced light metal matrix composites (LMMCs) have been researched widely, due to the enhancement in strength and ductility at room temperature, good wear resistance, excellent high temperature performance and structural-functional integration. However, there remain many challenges in developing high-performance nanomaterials reinforced LMMCs to date. The challenges mainly concentrate in the attainment of homogeneous dispersion or a controlled inhomogeneous microstructure of nanomaterials reinforcements, and the formation of the strong interfacial bonding. In the present review, therefore, current developments in fabrication, multi-scale hybrid reinforcement, novel architecture design and new processing method have been addressed. Moreover, further research interests related to the designs of nanomaterials reinforced LMMCs exhibiting high strength and plasticity, optimal architecture design and structural-functional integration have been proposed.

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    Microstructure and Tensile Property of TC4 Alloy Produced via Electron Beam Rapid Manufacturing
    Zheng LIU,Jianrong LIU,Zibo ZHAO,Lei WANG,Qingjiang WANG,Rui YANG
    Acta Metall. 2019, 55 (6): 692-700.   DOI: 10.11900/0412.1961.2019.00007
    Abstract   HTML   PDF (10313KB)

    Electron beam rapid manufacturing (EBRM) is one of the 3D printing technologies. The main attractions of EBRM technology are its high efficiency and economy in fabricating large, complex near net shape components dielessly and only needing limited machining. In general, the microstructure and texture of titanium alloy can play a significant role in determining its mechanical behaviors. In the present work, the microstructure, texture and tensile property of TC4 alloy produced by electron beam rapid manufacturing (EBRM) are investigated. Results show that the microstructure is comprised of columnar prior β grains that orient parallel to the building direction. The width of the columnar β grains increased rapidly at the initial several build layers, and the subsequent increase rate of the width of the columnar β grains tends to slow down. Fine α lamellae with gradient size are observed inside the columnar prior β grains, which occur because the alloy experiences different complex thermal histories during the EBRM-produced process. The size of α lamellae tends to decrease with the increase of build layers. The XRD result shows that the TC4 alloy has a typical α phase texture, (the c-axes are either concentrated at about 45° or are perpendicular to the building direction). At the same time, the <$10\bar{1}0$> poles are relative to random distribution. For the tensile samples along the electron beam scanning direction, the yield strengths do not show significant change with the increase of build layers, but the tensile strengths increase. The ductility of the alloy also has an upward trend, despite of a slightly decreasing ductility in the top sample. The tensile samples at the bottom of the alloy (10 mm and 20 mm away from the substrate) have similar work hardening exponents, which are lower than the top sample. The top sample shows the highest work hardening exponent. This difference in the tensile properties can be highly attributed to the gradient microstructure. The alloy also presents obvious anisotropy in tensile strength. The tensile sample along the 45° direction has a higher strength than the sample along the X direction, while the tensile sample along the Z direction shows the lowest strength. This anisotropic strength is strongly associated with the α phase texture. When the loading direction is 45° to the building direction, most of the c-axes of α phase are about parallel to the loading direction, showing a "hard" orientation, leading to a higher strength than other oriented samples. Conversely, when the loading direction is along the building direction, most of the α phase present a "soft" orientation, resulting in lower strength compared to the tensile samples along the 45° or the X direction.

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    Anomalous Thermal Expansion Behavior of Cold-RolledTi-35Nb-2Zr-0.3O Alloy
    Chunbo LAN,Jianeng LIANG,Yuanxia LAO,Dengfeng TAN,Chunyan HUANG,Xianzhong MO,Jinying PANG
    Acta Metall. 2019, 55 (6): 701-708.   DOI: 10.11900/0412.1961.2018.00347
    Abstract   HTML   PDF (10109KB)

    Thermal expansion behavior is one of the intrinsic properties of most materials, which is very difficult to control their thermal expansion behavior. Metallic material with ultra-low coefficient of thermal expansion named Invar effect was first found in Fe-Ni alloys. Recently, a multifunctional titanium alloy termed Gum metal (the typical composition is Ti-36Nb-2Ta-3Zr-0.3O, mass fraction, %; three electronic parameters: electron per atom ratio e/a≈4.24, bond order Bo≈2.87 and d electron orbital energy level Md≈2.45 eV) has been developed, and the alloy exhibits Invar effect after severe cold working. It is well known that the Invar effect of Fe-Ni alloys is related to the magnetic transition. However, titanium and its alloys are paramagnetic, and thus this mechanism cannot be used to explain Invar effect of Gum metal. In addition, the Invar effect of Gum metal is related to a dislocation-free plastic deformation mechanism. So far, there is still some controversy about this mechanism. In this study, a new β-type Ti-Nb base alloy Ti-35Nb-2Zr-0.3O (mass fraction, %) was developed whose three electronic parameters are different from those of the above mentioned Gum metal. The alloy was melted under high-purity argon atmosphere in an electric arc furnace, and the effects of cold rolling on microstructures and thermal expansion behaviors were characterized by OM, XRD, SEM, TEM and thermal mechanical analyzer (TMA). Results showed that the stress-induced martensitic α" (SIM α") phase transformation occurs after cold rolling, and the dominant <110> texture forms after severe plastic deformation. The equiaxed grains of Ti-35Nb-2Zr-0.3O alloy exhibit ordinary positive thermal expansion behavior and the thermal expansion rate increases with the increase of temperature. After cold deformation, negative thermal expansion occurs along rolling direction, and normal thermal expansion higher than solution treated sample occurs along transverse direction. The abnormal thermal expansion extent of the alloy increases with the increase of deformation reduction. The 30% cold deformed alloy along rolling direction possesses Invar effect between room temperature to 250 ℃, which is possibly related to SIM α" phase transformation, lattice distortion and <110> texture formation. The anomalous thermal expansion of the cold deformed samples in a temperature range from 25 ℃ to 110 ℃ is attributed to the lattice transition of SIM α" to β phase, while above 110 ℃ is attributed to the precipitation of ω and α phases.

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    Influence of Deformation Parameters on Dynamic Recrystallization of 2195 Al-Li Alloy
    Xu LI,Qingbo YANG,Xiangze FAN,Yonglin GUO,Lin LIN,Zhiqing ZHANG
    Acta Metall. 2019, 55 (6): 709-719.   DOI: 10.11900/0412.1961.2018.00430
    Abstract   HTML   PDF (34712KB)

    Al-Li alloys have attracted extensive attentions as promising structural materials in aerospace industries due to their excellent mechanical properties, and are usually formed through a variety of hot workings such as rolling and forging. Dynamic recrystallization (DRX) is considered as one of the key microstructure evolutions of Al alloys during hot working, and many works have been done concerning with DRX. However, the influence of deformation parameters on different types of DRX of 2195 Al-Li alloy is still unclear. In this work, hot plane strain compression tests were conducted at the strain rate range from 0.01 s-1 to 1 s-1 and the temperature range from 350 ℃ to 500 ℃ to investigate the critical condition of dynamic recrystallization of 2195 Al-Li alloy under different hot deformation conditions, DRX mechanisms were discussed, and the influence of deformation parameters on different types of DRX was revealed using EBSD and TEM. The results showed that the critical strain decreased with the decrease of Zener-Hollomon parameter (Z), DRX was more sufficient in lower Z value, and discontinuous dynamic recrystallization (DDRX) was primary type while only a little continuous dynamic recrystallization (CDRX) was found. Both CDRX and DDRX were promoted in lower Z value, geometric dynamic recrystallization (GDRX) only occurred in high Z value and increased with further increase of Z value, and the appearance of GDRX was accompanied by the increase of the number of DRX grains so that the DRX fraction slightly increased.

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    Effect of Boron Content on Microstructure Evolution During Solidification and Mechanical Properties of K417G Alloy
    Qiaomu LIU,Shunzhou HUANG,Fang LIU,Yan YANG,Hongqiang NAN,Dong ZHANG,Wenru SUN
    Acta Metall. 2019, 55 (6): 720-728.   DOI: 10.11900/0412.1961.2018.00491
    Abstract   HTML   PDF (54241KB)

    Boron is a key element in superalloys and many other metallic materials for strengthening the grain boundaries. However, it also has harmful effect on aggravating the solidification segregation of the alloys. Although the mechanism for the influences of B on the alloys has been studied extensively, it is still required to study in some alloys currently because the compositive effects of boron in different alloys are sometimes distinct. K417G, a cast superalloy with good comprehensive properties, has been applied in aero engines of China. In the present work, the effects of boron content on the microstructure evolution during the solidification and the mechanical properties of the as cast K417G alloy have been investigated, providing some fundamental information for the control of boron addition in the alloy. It has been found that boron aggravated the elemental segregation and promoted the eutectic (γ+γ') precipitation at the final stage of the solidification of K417G alloy. In addition, boron decreased the precipitation temperature, and hence reduced the nucleation rate of the γ matrix. When the boron content was below 0.036%, the grain size was increased with the increment of B content, which is caused by the decreased nucleation of the γ phase. When the B addition was increased up to 0.060%, the grain was refined at some local places, because the growth of the dendrites was inhibited and the γ phase could nucleate at the inner part of the subcooled liquids. The mechanical properties of K417G alloy were significantly influenced by the precipitation of the boride at the grain boundaries. The borides were precipitated as fine particles at the grain boundaries when the B addition was below 0.036%, and the tensile properties at 900 ℃ and the stress rupture properties at 900 ℃ and 315 MPa were markedly improved with the increasing B content in this addition range. When the B content was increased to 0.060%, the boride was precipitated as eutectic form in front of the eutectic (γ+γ'). The tensile and stress rupture properties were decreased due to the weak cohesion between the eutectic (γ+γ') and the eutectic form borides.

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    Ring Rolling Forming and Properties of Ti2AlNb Special Shaped Ring Prepared by Powder Metallurgy
    Zhengguan LU,Jie WU,Lei XU,Xiaoxiao CUI,Rui YANG
    Acta Metall. 2019, 55 (6): 729-740.   DOI: 10.11900/0412.1961.2019.00015
    Abstract   HTML   PDF (23413KB)

    Ti2AlNb alloy was considered as the candidate material to replace superalloys such as GH4169 in gas turbine engine applications due to higher strength-weight ratio at elevated temperatures. Powder metallurgy (PM) offers the potential for solving many of the problems associated with the large ingots, such as center-line porosity and chemical inhomogeneity. In order to study the feasibility of preparing Ti2AlNb special shaped ring with large size, PM + ring rolling combined process is considered as a potential method and discussed in this work. PM Ti2AlNb alloy and special shaped ring (D>800 mm) with a nominal composition of Ti-22Al-24.5Nb-0.5Mo (atomic fraction, %) were prepared from pre-alloyed powder using hot isostatic pressing (HIP). Hot compression tests of PM Ti2AlNb alloy and wrought alloy with the same chemical composition were conducted on Gleeble-3800 testing machine under 930~1050 ℃ and 0.001~1 s-1 conditions. Ring rolling was conducted on PM Ti2AlNb special shaped ring by horizontal rolling machine, and the microstructure evolution and properties performance of PM ring after rolling forming process were studied. The results show that the processing window for PM Ti2AlNb alloy is broader than that for wrought alloys, and wrought Ti2AlNb alloy is easier to crack at low temperature or relative high strain rate. PM Ti2AlNb alloy has more homogeneous chemical composition and uniform α2 phase distribution. Stress instability phenomenon of PM Ti2AlNb alloy is more obvious than that of wrought alloy which is related to phase transition of Ti2AlNb alloy. Optimized deformation temperature for PM Ti2AlNb special shaped ring was set as 1030~1045 ℃ with reference to the hot compression results. Ti2AlNb special shaped ring after two rolling steps has no any kinds of defects presented by X-ray testing, ultrasonic testing and fluorescence detection. O laths inside PM Ti2AlNb alloy become shorter and narrow, and α2 phase tends to be a coarser and spherical structure due to the hot deformation. After a typical heat treatment (980 ℃, 2 h, AC+830 ℃, 24 h, AC), nearly B2+O microstructure is obtained in Ti2AlNb special shaped ring. Compared with the undeformed alloy, tensile ductility at room temperature and 650 ℃ of Ti2AlNb ring after hot deformation improves due to the refined O phase structure.

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    Influence of Alloying Elements Partitioning Behaviors on the Microstructure and Mechanical Propertiesin α+β Titanium Alloy
    Sensen HUANG,Yingjie MA,Shilin ZHANG,Min QI,Jiafeng LEI,Yaping ZONG,Rui YANG
    Acta Metall. 2019, 55 (6): 741-750.   DOI: 10.11900/0412.1961.2018.00460
    Abstract   HTML   PDF (22782KB)

    During the thermal treatments of α+β titanium alloys in (α+β) phase field, alloying element partitioning effect takes place accompanying with the α?β transformation, which results in the segregation of α stabilizing elements (Al, O) and β stabilizing elements (V, Mo, etc.) into the corresponding phases respectively. The element partitioning effect will further affect the microstructure characteristics (phase constitution, microstructure size), plastic deformation modes and the final mechanical properties of the alloy. In this work, the influences of solution temperature and cooling rate on the element partitioning behavior during solution process of Ti-6Al-4V alloy in (α+β) phase field were investigated. The element concentrations in primary α phase (αp) and β transformed region (βt) were characterized by EPMA technique. The microstructural variation of βt with respect to solution temperature was analyzed. It was found that βt showed an obvious increase of Al content and decrease of V content with the increasing of solution temperature, while the αp exhibited less noticeable change, which led to the reduction of concentration difference between the two phases. Under the same solution temperature, the microstructures and element distributions at different cooling rates (water quenching, air cooling, furnace cooling) were exhibited. The slow cooling processing especially furnace cooling would induce higher volume fraction of αp phase and more pronounced element partitioning. The microstructural characteristics of βt cooled from different solution temperatures were further analyzed. During the water or air cooling process, the transformations of β→matensite/αs happened, and the sizes of martensite or αs were postulated to be dependent on the element concentration of β phase. The properties of local microstructure (αp, βt) were further measured by nanoindentation. It indicates that the intrinsically anisotropic character of the hexagonal crystal structure (hcp) of the αp phase has decisive consequences for the properties, while the elastic modulus and hardness of βt calculated by nanoindentation are mainly dominated by the width of αs lamellas. On the basis of the above results, the relationship between solution temperature, element concentration of local microstructure, microstructure size and mechanical properties of local microstructure was finally discussed.

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    Liquid-Liquid Phase Separation of Fe-Cu-Pb Alloy and Its Application in Metal Separation and Recycling of Waste Printed Circuit Boards
    Bin CHEN,Jie HE,Xiaojun SUN,Jiuzhou ZHAO,Hongxiang JIANG,Lili ZHANG,Hongri HAO
    Acta Metall. 2019, 55 (6): 751-761.   DOI: 10.11900/0412.1961.2018.00486
    Abstract   HTML   PDF (13341KB)

    The pyrolysis processing was carried out on the waste printed circuit boards (WPCBs) of mobile phones to dissociate metals from non-metals and obtain mixed metals with Fe, Cu and Pb as main components. Based on the main compositions of Fe, Cu and Pb, the liquid-liquid phase separation behavior of (Fe0.4Cu0.6)100-xPbx ternary alloy has been studied experimentally. The results show that the liquid-liquid phase separation of L→L(Fe)+L(Cu, Pb) may occur during the ternary Fe-Cu-Pb alloy melt cooling in the miscibility gap. After the liquid L(Fe) solidified, the secondary liquid-liquid phase separation L(Cu, Pb)→L(Cu)+L(Pb) takes place in the residual L(Cu, Pb) liquid phase, finally resulting in a three-zone separation structure. On the basis of the behavior of the liquid-liquid phase separation, a self-organized hierarchical separation system has been designed to separate and recycle these mixed metals from WPCBs. The enrichment behavior of the minor components like Cr, Au and Cd in the separation system was explored. The effect of super-gravity level on the metal separation and recycling rates has been discussed. As a result, a new harmless route has been established to recycle metal resources in WPCBs.

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    Characteristics and Evolution of the Spot Segregations and Banded Defects in High Strength Corrosion Resistant Tube Steel
    Bo LI,Zhonghua ZHANG,Huasong LIU,Ming LUO,Peng LAN,Haiyan TANG,Jiaquan ZHANG
    Acta Metall. 2019, 55 (6): 762-772.   DOI: 10.11900/0412.1961.2018.00557
    Abstract   HTML   PDF (35222KB)

    C110 casing tube is one of the high strength corrosion resistant steel products for deep well oil exploration. Due to the co-existence of acidic media such as H2S and the high pressure, there are frequently sulfide stress corrosion cracking (SSC) failures produced in the tubes, which are supposed to be closely connected with their banded segregation defects. The relationship between the as-cast spot segregation and the following as-rolled banded defects, together with the impacts of quenching and tempering (QT) treatment have been revealed. The banded defects in high strength corrosion resistant oil tube have been studied experimentally from its very beginning of as-cast state. With aids of OM, SEM, EDS and EPMA observation and analysis, the various spot like segregations in round casting were revealed along with their following banded structure in both as-rolled and QT tubes. The mechanism and appearance of the segregation induced banded defects were investigated comparatively of the both tubes. It is pointed out that there are normally two kinds of spot like segregations in steel castings, speckle type and porosity type, respectively. There are not only severe positive segregations of solutes, such as C, Cr, Mo and Mn etc., in the macro-etched spot like areas, a finer dendritic sub-structure has also been observed in the speckle type spot segregation zones. It has been found that the width of the banded defects in the as-rolled tubes is closely related to the types of segregations, and the severe banded defects, which are difficult to remove by heat treatment, are recognized to originate directly from the spot like segregations. Solute segregations are found in the microstructure of banded defects of the both as-rolled and QT tubes but with different existences. A kind of pearlite plus bainite banded structure is present in the former tube, while the banded defect of latter is composed of concentrated granular carbides, which explains the difference of their hardness behavior.

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    Fatigue and Cycle Plastic Behavior of 316L Austenitic Stainless Steel Under Asymmetric Load
    Jian PENG,Yi GAO,Qiao DAI,Ying WANG,Kaishang LI
    Acta Metall. 2019, 55 (6): 773-782.   DOI: 10.11900/0412.1961.2018.00377
    Abstract   HTML   PDF (11728KB)

    Due to excellent mechanical property and corrosion resistance of 316L austenitic stainless steel, it is widely used in chemical industry, but its fatigue behavior under asymmetric cycle load is not well understood. In this work, the fatigue and cyclic plastic deformation behavior of 316L austenitic stainless steel under asymmetric tensile-tensile cycle loading are studied, focusing on the variations of fatigue life, cycle plastic deformation and fracture mechanism with applied cycle load. The high and low stress regions can be clearly divided based on the differences of fatigue life, cyclic strain amplitude, mean strain, mean strain rate and failure strain. In the high stress region, mean strain, mean strain rate and failure strain are large, resulting in the significant cyclic plastic deformation, and the fatigue life is short. In the low stress region, the cyclic plastic deformation accumulation is limited, and the fatigue life is significantly increased. Through microstructural observations near fracture area and fracture surface analyses, the differences between large stress region and low stress region can be found. In the high stress region, a large number of voids are generated near the fracture surface, and the fracture surface is mainly featured by dimples. In contrast, in the low stress region, the fatigue crack is found near the fracture surface, and its propagation direction is perpendicular to the loading direction. The fatigue crack initiation site, the fatigue crack propagation zone, transition zone and rapid fracture zone are found on the fracture surface. Results of fracture mechanism analyses suggest that, the high stress region of 316L austenitic stainless steel is the cyclic plastic deformation dominant region, and the failure mechanism is the ductile failure caused by the accumulation of cyclic plastic deformation; while the low stress region is the fatigue dominant zone, and the failure mechanism is the fatigue crack propagation failure.

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    The Strengthening Mechanism of Cu Bearing High Strength Steel As-Quenched and Tempered and Cu Precipitation Behavior in Steel
    Zhengyan ZHANG,Feng CHAI,Xiaobing LUO,Gang CHEN,Caifu YANG,Hang SU
    Acta Metall. 2019, 55 (6): 783-791.   DOI: 10.11900/0412.1961.2018.00485
    Abstract   HTML   PDF (18289KB)

    High strength low alloy (HSLA) steels are widely used in the construction of ship structures, oil pipelines, offshore platforms and so on because of their good strength, toughness and weldability. HSLA steel is generally designed with low carbon and Cu alloying. Tempered lath bainite or martensite and nano-precipitate phase of Cu can be obtained by quenching and ageing process after rolling to ensure the excellent matching of strength, low temperature toughness and weldability of HSLA steel. At present, increasing attention has been focused on the precipitation behavior and strengthening mechanism of Cu particles in HSLA steel which was aged at the peak hardness of ageing curve. However, in practical engineering applications, overageing heat treatment is generally used to make HSLA steel achieve a good match of strength and toughness. In this work, the microstructure and nano-sized Cu precipitates of an industrial production HSLA steel plate with thickness of 35 mm were characterized by SEM, EBSD, HRTEM and APT. Meanwhile, the strengthening mechanism of the tested steel was investigated. The results show that Cu precipitates in the tested steel processed by overageing are mainly in the range of 6~50 nm, Cu particles exhibiting short rod or spherical shape within 30 nm are 9R structure, and other particles size larger than 30 nm exhibiting long rod or spherical shape are fcc structure. The segregation of trace Mn and Ni in rod particles on the interface between Cu particles and matrix is more obvious. After ageing at a higher temperature range, the yield strength of the tested steel decreases linearly with the increase of tempering temperature. The main strengthening mechanism of the HSLA steel is fine grain strengthening, followed by dislocation strengthening and precipitation strengthening. The calculated results show that every 1%Cu added in the tested steel can produce about 90 MPa precipitation strengthening increment under the condition of overageing heat treatment. The strength difference between the surface and the center of the tested steel plate is about 40 MPa, which is mainly due to the difference of grain size and dislocation density of steel.

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    Effect of Annealing Temperature on Microstructure, Properties and Hydrogen Embrittlement of TWIP Steel
    Futao DONG,Fei XUE,Yaqiang TIAN,Liansheng CHEN,Linxiu DU,Xianghua LIU
    Acta Metall. 2019, 55 (6): 792-800.   DOI: 10.11900/0412.1961.2018.00566
    Abstract   HTML   PDF (12174KB)

    TWIP steel as the representative of advanced high strength steel (AHSS) has a bright future in market and application owing to its excellent strength and ductility. Hydrogen embrittlement (HE) as a difficult problem of TWIP steel, researches on solving it mainly focus on alloying, few effort has been made on the mechanism of improving HE resistance by process adjustment and microstructure optimization. In this work, electrochemically combined with slow strain rate tensile test (SSRT) and OM, SEM have been used to study the effect of annealing temperature on mechanical property and deformation behavior of a Fe-18Mn-0.6C (mass fraction, %) twinning-induced plasticity (TWIP) steel, and also the influence of various microstructures on HE were discussed. The results showed that the grain size of TWIP steel increased with the increasing annealing temperature. In 700 ℃ annealed sheet, grain boundary (Fe, Mn)3C cementite was obvious. TWIP steel with uniform medium-sized grains by 900 ℃ annealing had the highest strength-ductility balance. After SSRT under ongoing hydrogen charging, strength and plasticity reduced significantly. The strength-ductility balance loss rate (R) showed a tendency of increasing with the increasing annealing temperature. Deformation twins were more likely to be produced in large-sized grains by high temperature annealing. The junctions of twin/twin and twin/grain boundary were the main sources of hydrogen induced cracks. Although relatively low temperature annealing resulted in fine grains and the change of stacking fault energy (SFE) along grain boundary, deformation twins were not easily formed. But it was very vulnerable to generate vacancies where the stress concentrated between the local coarse carbides and deformation twins, then evolved into crack sources. As a result, the plasticity of TWIP steel itself was not high when annealed at a relatively low temperature. It had no apparent effect on improving HE resistance for TWIP steel annealed below 800 ℃.

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    Reconstruction and Characterization of Galvanic Corrosion Behavior of X80 Pipeline Steel Welded Joints
    Yadong LI,Qiang LI,Xiao TANG,Yan LI
    Acta Metall. 2019, 55 (6): 801-810.   DOI: 10.11900/0412.1961.2018.00562
    Abstract   HTML   PDF (10490KB)

    Welding is widely used for pipeline connection. Composition, microstructures and properties of the welded joints are highly heterogeneous and the resultant corrosion such as galvanic corrosion between different parts is widely present and influence the long-time service and safety. In this sense, the fundamental research in the electrochemical behavior of such joint parts is required. Electrochemical corrosion behavior of simulated X80 steel welded joint, accurately modeled by wire beam electrode (WBE) technique, was investigated by classical electrochemical techniques and microelectrode array (MEA) technique. A new index, namely the galvanic corrosion intensity factor, was proposed and verified to succeed in characterizing the degree of galvanic corrosion. Results showed that microstructure of granular bainite mixed with ferrite showed the highest positive open circuit potential and lowest polarization resistance. Furthermore, the corrosion tendency of the isolated electrodes that constituted the X80 steel welded joint was found to increase in the following order: fine grain heat affected zone (FGHAZ) < intercritical heat affected zone (ICHAZ) < base metal (BM) < coarse grain heat affected zone (CGHAZ) < weld metal (WM). Due to the difference in potential and the polarization characteristics, the WM displayed the highest polarization resistance but the most positive current density. The CGHAZ possessed a lower polarization resistance and a higher positive current density. In comparison, the FGHAZ and ICHAZ performed a lower polarization resistance but higher negative current densities. The WM and CGHAZ acted as the main anode, while the FGHAZ and ICHAZ acted as the main cathode and the galvanic current polarity of some BM electrodes changed with time during the immersion test. The intensity of galvanic corrosion of simulated X80 steel welded joint plateaued with immersion time. The results revealed that WM and CGHAZ were the weak links in the simulated X80 pipeline steel welded joints during its long-term service.

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