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

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Grain Refinement of As-Cast Mg-RE Alloys: Research Progress and Future Prospect
Magnesium rare-earth (Mg-RE) alloy castings with a large size and complex structure exhibit versatile prospects in critical aircraft, aerospace, and defense fields owing to their ultralow density, excellent specific strength, and high-temperature resistance. The grain refinement of cast Mg-RE alloys. . .
Acta Metall Sin, 2022 Vol. 58 (4): 385-399    DOI: 10.11900/0412.1961.2021.00519
 
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Initial Corrosion Behavior and Local Corrosion Origin of 9% Cr Alloy Steel in Cl Containing Environment

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2022-05-25
Hot isostatic densification of Inconel 718 powder alloy, and microstructure and mechanical properties PDF (1539KB)
2022-05-24
Effect of Y on the Solidification Microstructure, Warm Compression Behavior and Softening Mechanism of Non-Oriented 6.5%Si Electrical Steel electrical steel PDF (2760KB)
2022-05-24
Present Status For Rolling TiAl Alloy Sheet PDF (3702KB)
2022-05-20
High Temperature Steam Oxidation Behavior of Fe22Cr5Al3Mo-xY Alloy Under Simulated LOCA Condition PDF (2756KB)
2022-05-20
Current Issue More>>
      11 May 2022, Volume 58 Issue 5 Previous Issue   
    Overview
    Controlling the Residual Stress in Metallic Solids by Pulsed Electric Current
    ZHANG Xinfang, XIANG Siqi, YI Kun, GUO Jingdong
    Acta Metall Sin. 2022, 58 (5): 581-598.   DOI: 10.11900/0412.1961.2021.00367
    Abstract   HTML   PDF (2606KB)

    The generation of residual stress is unavoidable during the preparation and processing of metallic materials. This residual stress reduces the stability of material preparation and processing, particularly the surface tensile stress, which will reduce the fatigue and corrosion resistances of the material. Therefore, the effective regulation of the residual stress is generally required. However, traditional residual stress control methods, such as heat treatment, exhibits low efficiency because they are limited by material size and type. It is crucial to develop a new method for regulating residual stress that is green, low energy consumption, stable, effective, and applicable to various metallic materials. Pulsed electric current processing is a new material processing technology. It has been widely used in the elimination and control of residual stress in materials in the recent years. Herein, the generation, disadvantages, and traditional control methods of residual stress are briefly reviewed; the characteristics of residual stress under various pulsed electric current treatment modes are reviewed in detail; and the mechanism of residual stress under pulsed electric current is briefly discussed. Based on the obtained results, under the action of high energy pulsed electric current, the residual stress inside and on the surface of the metallic materials can be effectively eliminated in a very short period (approximately 1 s) and the maximum elimination rate can reach 100%. The higher the current density, the higher is the rate of residual stress elimination. Furthermore, the greater the initial residual stress in the material, it is simpler to eliminate residual stress. The experimental results of low energy continuous pulsed electric current treatment show that there are numerous types of response modes, such as increasing, decreasing, and unchanged residual stress, which are associated with the type of material and the pulse parameters. To control the residual stress in the material, the treatment method for coupling pulsed electric current and external stress is effective. Coupling low energy continuous pulsed electric current during material processing can effectively introduce residual compressive stress on the surface of the material and improve the fatigue and corrosion resistances of the material. The electrodynamic treatment technology, which produces hammering when the material is subjected to pulsed electric current, can transform the tensile stress on the material surface into compressive stress to improve the performance of the material. This effectively breaks through the high energy requirement of eliminating residual stress and allowing the workpiece directly in the setting regional area. Residual stress in pulsed electric current processing is removed via the following mechanism: Joule heating and pulsed electric current effects caused by pulsed electric current promote dislocation movement and reduce the flow stress of the material; therefore, the material can undergo plastic deformation at a low stress level, for which the pulsed electric current effect is crucial. The combined action of stress changes caused by pulsed electric current (thermal stress, pinch effect, magnetostrictive effect, and instantaneous thermal expansion stress), external stress (deformation and impact), and residual stress constitute the driving force to promote plastic deformation.

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    Research paper
    Corrosion and Cavitation Erosion Behavior of GLNN/Cu Composite in Simulated Seawater
    PAN Chengcheng, ZHANG Xiang, YANG Fan, XIA Dahai, HE Chunnian, HU Wenbin
    Acta Metall Sin. 2022, 58 (5): 599-609.   DOI: 10.11900/0412.1961.2021.00333
    Abstract   HTML   PDF (3618KB)

    Herein, three-dimensional graphene-like nanosheet network (3D-GLNN)/copper (Cu) materials were synthesized using hop-pressing (HP) and hot-rolling (HR) methods and their corrosion resistance and mechanism were investigated using polarization curves, electrochemical impedance spectroscopy (EIS), and weight loss data after a cavitation corrosion test. Microstructural characterization results revealed that the 3D-GLNN structure was intact in the bulk composites, thereby restricting the effective grain growth of the Cu matrix. Compared with pure Cu, the Vickers hardness of 3D-GLNN/Cu fabricated using the HP and HR methods improved by 8% and 46%, respectively. Polarization curve results indicated that the anodic dissolution current of 3D-GLNN/Cu was considerably lower than that of pure Cu, indicating that 3D-GLNN/Cu exhibited better corrosion resistance. EIS measurements under a corrosion potential revealed that the electrode process kinetics was complex, with both charge and mass transfer controlling it. By extending the immersion time from 1 h to 9 d, the corrosion potential first became positive and then became negative. The capacitance arc at a high-frequency EIS range first increased and then decreased, attributed to the formation and detachment of a CuCl salt film. Diffusion impedance was observed in the low-frequency EIS range, with a phase angle of 18°-23°, indicating that the mass transfer process was not attributed to a single species but controlled by anodic and cathodic reactants. The constant phase angle element (CPE) behavior of the electrochemical system was further evaluated using the ohm-corrected phase angle and impedance modulus. The high-frequency phase angle was greater than -90 °, while the slope of impedance modulus was approximately -0.9; thus, the CPE was used to model the EIS data. The CPE behavior was attributed to the surface distribution of the charge transfer resistance and interface capacitance, implying a time-constant dispersion on the surface. Weight loss data after the cavitation corrosion test indicated that pure Cu showed better cavitation resistance than 3D-GLNN/Cu fabricated using the HR and HP methods. This is because of the difference in the elastic modulus between the graphene and Cu matrix that caused deformation dissonance during cavitation erosion.

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    Effect of Ti on the Corrosion Behavior of Fe22Cr5Al3Mo Alloy in 500oC Superheated Steam
    SUN Rongrong, YAO Meiyi, LIN Xiaodong, ZHANG Wenhuai, QIU Yunlong, HU Lijuan, XIE Yaoping, YANG Jian, DONG Jianxin, CHENG Guoguang
    Acta Metall Sin. 2022, 58 (5): 610-622.   DOI: 10.11900/0412.1961.2021.00200
    Abstract   HTML   PDF (5670KB)

    Zirconium alloys can react with water to produce hydrogen under a loss of coolant accident, which can lead to a hydrogen explosion. Therefore, the idea of developing accident tolerant fuel (ATF) is proposed, which involves nuclear fuel and cladding. FeCrAl alloy is a promising candidate material for ATF cladding. Studying the effects of alloying elements on the corrosion behavior and mechanism of FeCrAl alloy can provide a theoretical basis and guidance for optimizing its composition. Therefore, in this study, the effect of Ti on the corrosion behavior of Fe22Cr5Al3Mo alloy in 500oC superheated steam was investigated. Three types of Fe22Cr5Al3Mo-xTi (x = 0, 0.5, 1.0, mass fraction, %) alloys, designated as 0Ti, 0.5Ti, and 1.0Ti alloys, respectively, were fabricated and corroded in 500oC and 10.3 MPa superheated steam using a static autoclave. The microstructure, crystal structure and composition of the samples before and after corrosion were observed using XRD, OM, FIB/SEM, EDS, and TEM. The results show that the oxide films formed on the Fe22Cr5Al3Mo-xTi alloys in 500oC and 10.3 MPa superheated steam present a trilayer structure consisting of an outer oxide layer of Fe2O3, a middle layer of hcp-Cr2O3, and an inner layer of Al2O3. There is α-(Fe, Cr) in the Al2O3 layer near the oxide/metal interface. The ratio, R, of Cr oxide film thickness to total oxide film thickness for 0Ti, 0.5Ti, and 1.0Ti alloys follows the order R0.5Ti > R1.0Ti > R0Ti, which may explain the better corrosion resistance of 0.5Ti alloy than 1.0Ti and 0Ti alloys. The addition of Ti can reduce the total thickness of the oxide films and improve the corrosion resistance of the alloys by increasing the thickness of the protective hcp-Cr2O3 film and inhibiting the precipitation of Cr23C6.

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    Effects of Heat Treatment on Microstructure and Mechanical Properties of a Bimodal Grain Ultra-Low Carbon 9Cr-ODS Steel
    ZHANG Jiarong, LI Yanfen, WANG Guangquan, BAO Feiyang, RUI Xiang, SHI Quanqiang, YAN Wei, SHAN Yiyin, YANG Ke
    Acta Metall Sin. 2022, 58 (5): 623-636.   DOI: 10.11900/0412.1961.2020.00507
    Abstract   HTML   PDF (5923KB)

    Oxide dispersion strengthened (ODS) steel is a promising structural material for advanced nuclear power systems. In this study, an ultra-low carbon 9Cr-ODS steel with a bimodal grain structure was prepared using powder metallurgy, and a superior matching of strength and plasticity was expected by adjusting the soft-hard matching of the coarse-grained and fine-grained regions. The effects of heat treatment on microstructure and mechanical properties of the ultra-low carbon 9Cr-ODS steel were evaluated through OM, SEM, TEM, microhardness, and tensile tests. The results demonstrated that the ultra-low carbon 9Cr-ODS steel exhibited a tempered martensite structure after normalizing at 1050-1200oC, and then tempering at 700 and 750oC. Moreover, it presented the microstructure characteristics of coarse-grained and fine-grained regions, in which the average grain size of fine-grained regions was 1.6 μm and that of coarse-grained regions was 4.3 μm. The dislocation density in the ultra-low carbon 9Cr-ODS steel was very high and the number density of nano-scale oxide particles was up to about 1022 m-3. The microhardness in fine-grained regions was higher than that in coarse-grained regions. As the normalizing temperature increased, the microhardness of the ultra-low carbon 9Cr-ODS steel first increased and then decreased. The microhardness reached the highest after normalizing at 1100oC. When the normalizing temperature increased to 1200oC, the microhardness decreased due to the growth of austenitic grains. Regarding the tempering temperature, the microhardness first decreased and then increased as the tempering temperature increased from 700oC to 800oC. Furthermore, the decrease in microhardness when tempering at 700 and 750oC was because the microstructure was recovered and softened. The higher the tempering temperature, the lower the microhardness. However, when tempering at 800oC, the microhardness increased significantly, mainly due to the partial austenite transformation of martensite. The tensile test results at 25oC showed that the strength of the ultra-low carbon 9Cr-ODS steel first decreased and then increased by increasing the tempering temperature, which was consistent with the microhardness change while the opposite was observed for elongation. The tensile test results at 700oC showed that the strength of the ultra-low carbon 9Cr-ODS steel slightly decreased by increasing the tempering temperature. Moreover, the fracture morphology was dominated by fine dimples and secondary tearing, indicating that the ultra-low carbon 9Cr-ODS steel underwent ductile fracture. Combined with the mechanical property and fracture analysis results, the ultra-low carbon 9Cr-ODS steel exhibited superior matching of strength and plasticity after normalizing at 1150oC for 1 h and tempering at 750oC for 1 h.

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    Effect of Ce on the Microstructure, High-Temperature Tensile Properties, and Fracture Mode of Strip Casting Non-Oriented 6.5%Si Electrical Steel
    LI Min, LI Haoze, WANG Jijie, MA Yingche, LIU Kui
    Acta Metall Sin. 2022, 58 (5): 637-648.   DOI: 10.11900/0412.1961.2021.00115
    Abstract   HTML   PDF (5672KB)

    Non-oriented 6.5%Si electrical steel exhibits excellent high-frequency magnetic properties, such as low iron loss and near-zero magnetostriction. Moreover, the high Si content causes poor deformability due to the solution strengthening effect of Si and the resulting ordering transformations, delaying the commercial application. Strip casting is a near-net forming technology that directly produces thin strips from the melt and reduces the required rolling deformation for the fabrication of thin sheets. This technology could be a viable option for industrializing the production of non-oriented 6.5%Si electrical steel. Despite this, even in the strip casting process, this brittle material is prone to edge cracks. Thus, improving the intrinsic plasticity of strip casting non-oriented 6.5%Si electrical steel is necessary through chemical modification to eliminate the deforming defects. The effect of Ce on the ordered phase of the solidification microstructure, high-temperature tensile properties, and fracture mode of strip casting non-oriented 6.5%Si electrical steel was investigated in this work. The results showed that the addition of Ce introduced high-melting Ce2O2.5S and Ce4O4S3 during strip casting, which promoted heterogeneous nucleation and refined the solidification microstructure of the as-cast strip. The presence of Ce did not affect the ordering condition of the as-cast strip. In decreasing order, the tensile temperatures corresponding to the ordered degree of the as-cast strip were 650, 400, and 800oC. With the tensile temperature increasing, the yield and tensile strengths of the as-cast strip decreased, whereas the elongation gradually increased. When the tensile temperature exceeded 500oC, the purification and microstructure refining effects of Ce improved grain-boundary cohesion and prevented intergranular cracking. Moreover, the occurrences of dynamic recovery and recrystallization eventually made the as-cast strip doped with Ce fractured by dimples, leading to a considerably enhanced tensile ductility. According to the findings, the rare-earth treatment should be considered as an effective method for increasing the ductility of strip casting non-oriented 6.5%Si electrical steel.

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    Mechanism of Dynamic Strain-Induced Ferrite Transformation in a 3Mn-0.2C Medium Mn Steel
    SUN Yi, ZHENG Qinyuan, HU Baojia, WANG Ping, ZHENG Chengwu, LI Dianzhong
    Acta Metall Sin. 2022, 58 (5): 649-659.   DOI: 10.11900/0412.1961.2021.00192
    Abstract   HTML   PDF (5862KB)

    Medium Mn steels (MMSs) have Mn contents of 3%-12% (mass fraction), and have been energetically investigated as the most promising candidates of the third-generation advanced high-strength steel. Their phase transformations and microstructures during various heat treatments and thermomechanical processes have received wide attention with the purpose to achieve an optimal balance of cost-efficient alloying compositions and mechanical properties. The aim of this work is to investigate the microstructural behavior of deformation-induced ferrite transformation (DIFT), starting from austenite, which occurs in MMS. Then, improved understandings of the formation of ultrafine ferrite via the DIFT and conservation of this microstructure during the post-deformation period can be obtained. For this purpose, a 3Mn-0.2C MMS with lower contents of alloying elements was selected. Microstructures and alloying element distributions of the thermomechanically processed samples were analyzed via EBSD and EPMA. The results showed that the DIFT occurred in the thermomechanically processed 3Mn-0.2C MMS in the α + γ region. Characteristic multiphase microstructures consisting isolated martensite and fine-grained equiaxed ferrite concomitant with fine islands of retained austenite dispersed between ferrite grains can be obtained. During the DIFT, the enhanced nucleation of ferrite at α/γ interfaces can not only increase the ferrite nucleation density but also facilitate extensive impingement among the neighboring grains. Formation of ultrafine ferrite via the DIFT in MMS can be interpreted in terms of unsaturated nucleation and limited growth. In addition, partitioning of Mn between the ultrafine ferrite and austenite is accelerated during the DIFT such that a large number of Mn-enriched fine islands of austenite are left untransformed at the α/α grain boundaries or at triple junctions. These islands of austenite are considered to play critical roles not only for obtaining retained austenite at room temperature but also for conserving the ultrafine microstructure of the DIFT during the post-deformation processing.

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    Detwinning Behaviors and Dynamic Mechanical Properties of Precompressed AZ31 Magnesium Alloy Subjected to High Strain Rates Impact
    CHEN Yang, MAO Pingli, LIU Zheng, WANG Zhi, CAO Gengsheng
    Acta Metall Sin. 2022, 58 (5): 660-672.   DOI: 10.11900/0412.1961.2021.00117
    Abstract   HTML   PDF (4382KB)

    To investigate the detwinning behaviors and dynamic mechanical properties of a precompressed rolled AZ31 magnesium alloy sheet impacted under high strain rates, the as-received sheet was precompressed along the rolling direction (RD) to the true strain of 4% for inducing { 101¯2} tensile twins. The as-received and precompressed rolled AZ31 magnesium alloy sheets were impacted along the normal direction (ND) using a split Hopkinson pressure bar experiment apparatus at strain rates of 700, 1000, 1300, and 1600 s-1. Microstructural characteristics of the as-received, precompressed, and impacted specimens were analyzed and compared by an electron backscatter diffraction technology. The results show that in the precompressed specimen, the density of the basal texture was weakened and a new twin texture with the c-axis paralled to RD was formed. The average grain size of the precompressed specimen decreased visibly as a result of the parent grains being subdivided by tensile twin boundaries. The dominant deformation mechanism of the precompressed rolled AZ31 magnesium alloy impacted along ND is detwinning. With increasing the strain rate, the initial basal texture recovered, the average grain size increased, and the average twin thickness decreased. Compared with the precompressed specimen, the as-received specimen impacted along ND exhibited higher strength and lower formability. The precompressed specimen demonstrated greater strain rate sensitivity during plastic deformation.

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    Creep Behavior of FGH96 Superalloy at Different Service Conditions
    PENG Zichao, LIU Peiyuan, WANG Xuqing, LUO Xuejun, LIU Jian, ZOU Jinwen
    Acta Metall Sin. 2022, 58 (5): 673-682.   DOI: 10.11900/0412.1961.2021.00207
    Abstract   HTML   PDF (4862KB)

    The FGH96 superalloy is extensively used as a gas turbine disk under high temperature due to its excellent tensile and creep properties. With recent developments in the aviation industry, the velocity of the aircraft has increased significantly, thereby increasing the temperature and stress on the turbine disk materials during their service. Therefore, creep deformation is crucial in the turbine disk superalloy. In this study, the creep characteristics of FGH96 superalloy were systematically studied at 650-750oC and 690-810 MPa and the creep mechanism of the alloy under different service conditions was investigated via SEM, EBSD, and TEM. For the creep temperature of 704oC, the creep properties of the alloy decreased with the increase in stress level. When the applied loading stress was 690 MPa, the creep properties of FGH96 alloy decreased significantly with the increase in temperature, and its steady-state creep strain rate was more sensitive to the service temperature. Further, every 30oC increase in the service temperature increased the creep rate by an order of magnitude. For the temperature in the range 650-750oC and the applied loading stress in the range 690-810 MPa, the creep deformation of the alloy was dominated by dislocation slip and resulted in various micro-twins on the continuous (111¯) planes. Moreover, the creep fracture of FGH96 alloy presented typical intergranular fracture under different service conditions in this study.

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    Propagation Behaviors of Small Cracks in Powder Metallurgy Nickel-Based Superalloy FGH4096
    YANG Qinzheng, YANG Xiaoguang, HUANG Weiqing, SHI Duoqi
    Acta Metall Sin. 2022, 58 (5): 683-694.   DOI: 10.11900/0412.1961.2021.00221
    Abstract   HTML   PDF (4352KB)

    Inevitable nonmetallic inclusions (NMIs) exist in powder metallurgy (PM) superalloys. These inclusions serve as preferred sites for crack initiation either by fracture of inclusions or inclusion/matrix decohesion. After initiating from NMIs, fatigue cracks will experience the small crack propagation phase. Small fatigue cracks could grow under the fatigue crack growth threshold and propagate at a vibrated rate. To investigate the propagation behavior and reveal the underlying mechanisms, small crack propagation experiments under fatigue loads of different maximum stresses were conducted on PM superalloy FGH4096 using the small fatigue crack-propagation experiment system. The characterization of microstructure was conducted and orientations of grains were calibrated using SEM integrated with EBSD. Focusing on the three-dimensional nature and the physical basis, the propagation and stagnation behavior of small cracks were revealed. Experimental results showed that the small cracks propagated along octahedral slip planes, from initiation to a length even longer than 1.0 mm. During the propagation in the grain-containing twin, small cracks grew along the direction parallel to the twin boundary. However, several twin boundaries impeded crack growth. Small cracks were stagnated at grain and twin boundaries of which M factors were lower than adjacent ones. Three behaviors were observed after the stagnation of small cracks due to the different properties of grain/twin boundaries and the applied load; first, stagnated small cracks could continue to propagate by consuming more cycles; second, small cracks could propagate by alternating to another slip plane in current or other grains on the crack path; third, secondary cracks would initiate within 1-2 grains from the tip of the fully stagnated cracks and connected to the main crack. This behavior was observed only in the specimen in which the maximum stress is close to the lower limit of the yield strength.

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    Effect of Deformation Rate on the Elastic-Plastic Deformation Behavior of GH3625 Alloy
    GAO Yubi, DING Yutian, LI Haifeng, DONG Hongbiao, ZHANG Ruiyao, LI Jun, LUO Quanshun
    Acta Metall Sin. 2022, 58 (5): 695-708.   DOI: 10.11900/0412.1961.2020.00508
    Abstract   HTML   PDF (4069KB)

    GH3625 alloy is a typical polycrystalline material. The mechanical properties of a crystal within the alloy depend on the single crystal properties, lattice orientation, and orientations of neighboring crystals. However, accurate determination of single crystal properties is critical in developing a quantitative understanding of the micromechanical behavior of GH3625. In this study, the effect of deformation rate on the elastoplastic deformation behavior of GH3625 was investigated using in situ neutron diffraction room-temperature compression experiments, EBSD, and TEM. The results showed that the microscopic stress-strain curve included elastic deformation (applied stress σ ≤ 300 MPa), elastoplastic transition (300 MPa < σ ≤ 350 MPa), and plastic deformation (σ > 350 MPa) stages, which agreed with the mesoscopic lattice strain behavior. Meanwhile, the deformation rate was closely related to the crystal elastic and plastic anisotropy. The results of the lattice strain, peak width, and peak intensity reflected by the specific hkl showed that the deformation rate had little effect on the elastic anisotropy of the crystal, but had a significant effect on the plastic anisotropy of the crystal. With the increase in the deformation rate, the high angle grain boundaries gradually changed to the low angle grain boundaries, and the proportion of twin boundaries gradually reduced. Also, the grains transformed from uniform deformation to nonuniform deformation. Moreover, with the increase in deformation rate, the total dislocation density (ρ) of the alloy first decreased and then increased, whereas the geometrically necessary dislocation density (ρGND) monotonically increased, and the statistically stored dislocation (SSD) density (ρSSD) monotonically decreased. Meanwhile, the abnormal work hardening behavior of the sample at a deformation rate of 0.2 mm/min was mainly related to the SSD generated by uniform deformation. Additionally, the contribution of dislocation strengthening and TEM observation confirmed that the dominant deformation of GH3625 was dislocation slip, and its work hardening mechanism was dislocation strengthening.

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