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CN: 21-1361/TG
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Research Progress of Electromagnetic Properties of MgB2 Induced by Carbon-Containing Materials Addition and Process Techniques
Jiancheng Li, Haobo Liu, Ying Li, Chuanbing Cai, Shixue Dou, Wenxian Li
Acta Metallurgica Sinica (English Letters)    2020, 33 (4): 471-489.   doi:10.1007/s40195-020-01016-0
Abstract48)   HTML1)    PDF (9837KB)(79)      

For the high transition temperature (Tc) and low cost taking both raw materials and fabrication process into account, MgB2 has been a competitive candidate to replace the conventional NiTi superconductor for high-temperature application in fault current limiters, transformers, motors, magnetic resonance imaging, adiabatic demagnetization refrigerators, generators, etc. The carbon-containing materials addition induced high critical current density (Jc) is reviewed based on their influences on the upper critical field (Hc2), flux pinning force, and connectivity. The doping effects were compared in the overview focusing on SiC, organic dopants, and graphene-related dopants. SiC doping is featured for the high-field critical current density, which is caused by the increased Hc2 attributed to the substitution of carbon on boron site and the strong flux pinning force offered by the nanosized secondary phases in the MgB2 matrix. Organic dopants have the advantage over SiC dopant for their relatively homogeneous distribution in the MgB2 matrix based on wet mixing of the organics and the raw boron powders. Low doping level of two-dimensional materials can improve the superconducting properties in all measured fields because of the combined advantages of carbon substitution effect and grain connectivity. MgB2 fabricated with carbon-encapsulated boron also introduces strong flux pinning centers in MgB2, which show weak destruction of the connectivity of the MgB2 grains as reflected by the low-magnetic-supercurrent behavior. High-pressure treatment and diffusion method can fabricate high-density MgB2 superconductors with better connectivity and increase the Jc compared with the in situ and ex situ methods.

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Effect of Alloyed Mo on Mechanical Properties, Biocorrosion and Cytocompatibility of As-Cast Mg-Zn-Y-Mn Alloys
Longlong Zhang, Yatong Zhang, Jinshan Zhang, Rui Zhao, Jiaxin Zhang, Chunxiang Xu
Acta Metallurgica Sinica (English Letters)    2020, 33 (4): 500-513.   doi:10.1007/s40195-019-00995-z
Abstract33)   HTML1)    PDF (5509KB)(160)      

The influences of Mo contents on mechanical properties, biocorrosion and cytocompatibility of as-cast Mg-6Zn-8.16Y-2.02Mn-xMo (x = 0.0, 0.1, 0.3, 0.5, 0.7 wt%) alloys were firstly investigated. Appropriate amount of Mo was conducive to grain refinement and the formation of long-period stacking ordered structure with continuous distribution, which was advantageous to mechanical properties and corrosion resistance. Mg-6Zn-8.16Y-2.02Mn-0.3Mo exhibited the ultimate tensile strength of 265.0 MPa, elongation of 13.5% and the lowest weight loss rate in Hank’s solution. Moreover, the cell toxicity cultured in 25% extract was evaluated and the alloy with 0.3 wt% Mo exhibited the best cytocompatibility. Thus, the alloy was expected to become a novel biodegradable implant material.

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Enhanced Electromagnetic Interference Shielding in a Duplex-Phase Mg-9Li-3Al-1Zn Alloy Processed by Accumulative Roll Bonding
Jiahao Wang, Lin Xu, Ruizhi Wu, Jing Feng, Jinghuai Zhang, Legan Hou, Milin Zhang
Acta Metallurgica Sinica (English Letters)    2020, 33 (4): 490-499.   doi:10.1007/s40195-020-01017-z
Abstract26)   HTML5)    PDF (2441KB)(42)      

High electromagnetic shielding performance was achieved in the Mg-9Li-3Al-1Zn alloy processed by accumulative roll bonding (ARB). The microstructure, electromagnetic interference shielding effectiveness (SE) in the frequency of 30-1500 MHz and mechanical properties of the alloy were investigated. A model based on the shielding of the electromagnetic plane wave was used to theoretically discuss the EMI shielding mechanisms of ARB-processed alloy. Results indicate that the SE of the material increases gradually with the increase in the ARB pass. The enhanced SE can be attributed to the obvious microstructure orientation caused by ARB, and the alternative arrangement of alpha(Mg) phase and beta(Li) phase. In addition, with the increase in ARB pass, the number of interfaces between layers increases and the grain orientation of each layer tends to alignment along c-axis, which is beneficial to the reflection loss and multiple reflection loss of the incident electromagnetic wave.

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Hot Deformation Behavior and Processing Maps of 0.3C-15Cr-1Mo-0.5N High Nitrogen Martensitic Stainless Steel
Xin Cai, Xiao-Qiang Hu, Lei-Gang Zheng, Dian-Zhong Li
Acta Metallurgica Sinica (English Letters)    2020, 33 (5): 693-704.   doi:10.1007/s40195-019-00991-3
Abstract22)   HTML2)    PDF (6145KB)(4)      

Hot deformation behavior of 0.3C-15Cr-1Mo-0.5N high nitrogen martensitic stainless steel (HNMSS) was investigated in the temperature range of 1173-1473 K and at strain rates of 0.001-10 s-1 using a Gleeble 3500 thermal-mechanical simulator. The true stress-strain curves of the studied HNMSS were measured and corrected to eliminate the effect of friction on the flow stress. The relationship between the flow stress and Zener-Hollomon parameter for the studied HNMSS was analyzed in the Arrhenius hyperbolic sine constitutive model by the law of $Z = 3.76 \times 10^{15} \sinh \left( {0.004979\sigma_{\text{p}} } \right)^{7.5022}$. The processing maps at different strains of the studied HNMSS were plotted, and its flow instability regions in hot working were also confirmed in combination with the microstructure examination. Moreover, the optimal hot deformation parameters of the studied HNMSS could be suggested at T = 1303-1423 K and $\dot{\varepsilon }$ = 5-10 s-1 or T = 1273-1473 K and $\dot{\varepsilon }$ = 0.005-0.04 s-1.

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Integrated Modeling of Process-Microstructure-Property Relations in Friction Stir Additive Manufacturing
Zhao Zhang, Zhi-Jun Tan, Jian-Yu Li, Yu-Fei Zu, Jian-Jun Sha
Acta Metallurgica Sinica (English Letters)    2020, 33 (1): 75-87.   doi:10.1007/s40195-019-00945-9
Abstract21)   HTML0)    PDF (2290KB)(43)      

Friction stir additive manufacturing is a newly developed solid-state additive manufacturing technology. The material in the stirring zone can be re-stirred and reheated, and mechanical properties can be changed along the building direction. An integrated model is developed to investigate the internal relations of process, microstructure and mechanical properties. Moving heat source model is used to simulate the friction stir additive manufacturing process to obtain the temperature histories, which are used in the following microstructural simulations. Monte Carlo method is used for simulation of recrystallization and grain growth. Precipitate evolution model is used for calculation of precipitate size distributions. Mechanical property is then predicted. Experiments are used for validation of the predicted grains and hardness. Results indicate that the average grain sizes on different layers depend on the temperature in stirring and re-stirring processes. With the increase in building height, average grain size is decreased and hardness is increased. The increase in layer thickness can lead to temperature decrease in reheating and re-stirring processes and then lead to the decrease in average grain size and increase of hardness in stir zone.

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Experimental Investigation on the LCF Behavior Affected by Manufacturing Defects and Creep Damage of One Selective Laser Melting Nickel-Based Superalloy at 815 °C
Xiao-An Hu, Gao-Le Zhao, Yun Jiang, Xian-Feng Ma, Fen-Cheng Liu, Jia Huang, Cheng-Li Dong
Acta Metallurgica Sinica (English Letters)    2020, 33 (4): 514-527.   doi:10.1007/s40195-019-00986-0
Abstract18)   HTML2)    PDF (4799KB)(29)      

Uniaxial tensile tests and stress-controlled low-cycle fatigue (LCF) and creep-fatigue interaction (CFI) tests of Inconel 625 alloy manufactured by selective laser melting (SLM) were performed at 815 °C in air environments. The microstructure was characterized by optical microscopy and scanning electron microscopy after testing. The results confirmed that significant embrittlement and large scatter in LCF life are resulted from manufacturing defects. The CFI life is decreased sharply to approximately dozens of cycles with the accumulated creep strain; however, the selected dwell time (i.e., 60 s and 300 s) exhibits low sensitivity to the fracture time and elongation to failure. The embrittlement of SLM Inconel 625 was proposed to be due to the low grain uniformity and precipitation of carbides at the grain boundaries. Due to the quality of the SLM process, the accelerated initiation and propagation of fatigue crack are caused by the present unmelted powder particles, which result in the large dispersion of LCF life. Meanwhile, due to the accumulation of creep damage, cracks in the CFI test are initiated along the grain boundaries and then linked together, contributing to a significant decline in fatigue life.

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Effect of Sleeve Plunge Depth on Interface/Mechanical Characteristics in Refill Friction Stir Spot Welded Joint
Guang-Da Sun, Li Zhou, Ren-Xiao Zhang, Ling-Yun Luo, Hao Xu, Hong-Yun Zhao, Ning Guo, Di Zhang
Acta Metallurgica Sinica (English Letters)    2020, 33 (4): 551-560.   doi:10.1007/s40195-019-00968-2
Abstract17)   HTML1)    PDF (3433KB)(35)      

Refill friction stir spot welding was employed to produce 6061-T6 aluminum alloy joints with different sleeve plunge depths. The interface characteristics of joint-line remnant and hook are investigated by optical and scanning electron microscopy. The joint-line remnant consists of primary bonding region and secondary bonding region, and two types of hook can be identified as downward hook and upward hook. Tensile shear results demonstrate that joint-line remnant and hook make interaction effects on tensile shear properties. The optimal joint is achieved when sleeve plunge depth was 2.0 mm with the corresponding failure load of 8673.4 N. Three different types of fracture mode are exhibited in joints produced at different sleeve plunge depths, which are closely related with the morphology of interface characteristics.

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Stresses State and Mechanical Behaviors of the Green Body During Die Compaction and Ejection Process
S. M. Wang, Y. Wang, Y. X. Wang, F. P. Liu, J. Cao
Acta Metallurgica Sinica (English Letters)    2020, 33 (4): 605-614.   doi:10.1007/s40195-020-01018-y
Abstract17)   HTML2)    PDF (3398KB)(33)      

Finite element simulations for metal powder compaction of a clutch plate were performed to examine the stresses during compaction, unloading, and ejection. To describe the mechanical behavior of compacted green body, a modified density-dependent Drucker-Prager Cap model was utilized to predict the stress and density distribution of the compacted clutch plate during loading and ejection stages. The results indicate that maximum tensile principal stress was a main driving force for the tensile crack initiation during ejection stage, and shear stress may be another driving force in both compaction and ejection stages for shear crack initiation. There were peak value of the stresses during ejection stage, and the stresses are in compressive state only during compaction stage. Therefore, the tensile crack initiation is not possible during compaction except shear crack. Hoop stress in the clutch plate is of less contribution to the crack initiation during compaction, unloading and ejection. Study of criteria of the crack initiation and fracture is necessary in order to obtain uniform density and crack-free components in the manufacturing of metal powder compaction.

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Relationship Between Microstructures and Microhardness in High-Speed Friction Stir Welding of AA6005A-T6 Aluminum Hollow Extrusions
Xiang-Qian Liu, Hui-Jie Liu, Yan Yu
Acta Metallurgica Sinica (English Letters)    2020, 33 (1): 115-126.   doi:10.1007/s40195-019-00970-8
Abstract17)   HTML1)    PDF (3656KB)(52)      

AA6005A-T6 aluminum hollow extrusions were friction stir welded at a fixed high welding speed of 2000 mm/min and various rotation speeds. The results showed that the heat-affected zone (HAZ) retained the similar grain structure as the base material except some grain coarsening, and the density of dislocations and β′ precipitates were almost unchanged, indicating that the high welding speed inhibited the coarsening and dissolution of β″ precipitates via fast cooling rate. The thermo-mechanically affected zone (TMAZ) was characterized by elongated and rotated grains, in which a low density of β′ precipitates and the highest density of dislocations were observed. The highest heat input and severest plastic deformation occurring in the nugget zone (NZ) resulted in the occurrence of dynamic recrystallization and a high density of dislocations. Hence, all the β″ precipitates and most of the β′ precipitates dissolved into the matrix, and a few β′ precipitates were transformed into β precipitates. The microhardness was controlled by the precipitation and solution strengthening in the HAZ, by the dislocation and precipitation strengthening in the TMAZ, and by the fine-grain and dislocation strengthening in the NZ. With the increase in rotation speed, the peak and the lowest microhardness value increased monotonously.

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Biodegradation Behavior of Coated As-Extruded Mg–Sr Alloy in Simulated Body Fluid
Ming-Chun Zhao, Ying-Chao Zhao, Deng-Feng Yin, Shuo Wang, Yong-Ming Shangguan, Chao Liu, Li-Li Tan, Ci-Jun Shuai, Ke Yang, Andrej Atrens
Acta Metallurgica Sinica (English Letters)    2019, 32 (10): 1195-1206.   doi:10.1007/s40195-019-00892-5
Abstract16)   HTML8)    PDF (2428KB)(119)      

As-extruded Mg-Sr alloy, a kind of promising biodegradable biomedical material, was coated using micro-arc oxidation and also using a phosphate conversion coating. The corrosion behaviors were investigated using Hanks’ solution. The corrosion of the as-extruded Mg-Sr alloy became more serious with increasing immersion time; that is, the corrosion pits became more numerous, larger and deeper. The micro-arc oxidation coating and the phosphate conversion coating were effective in improving the corrosion resistance of the as-extruded Mg-Sr alloy. The micro-arc oxidation coating was much more effective. Moreover, the as-extruded Mg-Sr alloy and the coated as-extruded Mg-Sr alloy exhibited lower corrosion rates than the as-cast Mg-Sr alloy and the corresponding coated as-cast Mg-Sr alloy, indicating that the corrosion properties of the coated samples are dependent on their substrates. The finer microstructure of the substrate of the as-extruded condition corroded much slower. The corrosion resistance of the coated Mg-Sr alloy depended on the coating itself and on the microstructure of the substrate.

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Effect of Submillimeter Variation in Plunge Depth on Microstructure and Mechanical Properties of FSLW 2A12 Aluminum Alloy Joints
Dan Chen, Jinglong Li, Huaxia Zhao, Zhejun Tan, Jiangtao Xiong
Acta Metallurgica Sinica (English Letters)    2020, 33 (1): 165-171.   doi:10.1007/s40195-019-00981-5
Abstract16)   HTML1)    PDF (2742KB)(42)      

Friction stir lap welding was conducted on 2 mm?+?2 mm sheets of aluminum alloy 2A12-T4. The plunge depth (PLD) was designed as 2.45-2.58 mm, which was varied in submillimeters as 2.45, 2.50, 2.53, 2.55, and 2.58 mm, and the axial force was recorded in the welding process. The results show that the PLD fluctuation in submillimeters causes significant variation in the axial force and affects the voids (i.e., Void I and Void II), hook, and effective sheet thickness (EST), among which Void I is the main factor that affects the EST. The fracture load-PLD function in the tensile shear test of the joints follows the rule of the EST-PLD function. An optimized PLD is approximately 2.55 mm, at which the EST reaches 1.71 mm, corresponding to a peak fracture load of 11.03 kN. Thus, a PLD of 2.55 mm is suggested with a tolerance of 0.02 mm, corresponding to a fracture load of 9.6-11.0 kN, i.e., within a fluctuation of 12%.

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Microstructural Evolution and Mechanical Behavior of Friction-Stir-Welded DP1180 Advanced Ultrahigh Strength Steel
Z. W. Wang, G. M. Xie, D. Wang, H. Zhang, D. R. Ni, P. Xue, B. L. Xiao, Z. Y. Ma
Acta Metallurgica Sinica (English Letters)    2020, 33 (1): 58-66.   doi:10.1007/s40195-019-00949-5
Abstract15)   HTML3)    PDF (3516KB)(69)      

Friction stir lap welding of a DP1180 advanced ultrahigh strength steel was successfully carried out by using three welding tools with different pin lengths. The effects of the welding heat input and material flow on the microstructure evolution of the joints were analyzed in detail. The relationship between pin length and mechanical properties of lap joints was studied. The results showed that the peak temperatures of all joints exceeded Ac3, and martensite phases with similar morphologies were formed in the stir zones. These martensite retained good toughness due to the self-tempering effect. The formation of ferrite and tempered martensite was the main reason for the hardness reduction in heat-affected zone. The mechanical properties of the lap joints were determined by loading mode, features of lap interface and the joint defects. When the stir pin was inserted into the lower sheet with a depth of 0.4 mm, the lap joint exhibited the maximum tensile strength of 12.4 kN.

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Single Crystal Castability and Undercoolability of PWA1483 Superalloy
De-Xin Ma, Fu Wang, Jian-Zheng Guo, Wen-Liang Xu
Acta Metallurgica Sinica (English Letters)    2019, 32 (11): 1415-1420.   doi:10.1007/s40195-019-00891-6
Abstract15)   HTML0)    PDF (1429KB)(39)      

Both of the single crystal (SX) castability and undercoolability of PWA1383 superalloy were investigated during the directional solidification and isothermal cooling. In all the six SX parts of a casting cluster, no stray grains were found, revealing a defect-free SX structure. This excellent SX castability of the superalloy was attributed to its good undercoolability. The melting point (TL) and the critical nucleation temperature (TN) of the alloy were measured to be 1327 °C and 1306 °C, respectively. The statistic average of the critical nucleation undercooling ΔTN?=?TL?-?TN of the alloy was determined to be about 21 K, exhibiting a relatively great capacity to be deeply cooled to a temperature below the melting point without the onset of solidification.

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Influence of Static Low Electromagnetic Field on Copper Corrosion in the Presence of Multispecies Aerobic Bacteria
Xiao-Yang Wei, Masoumeh Moradi, Li-Jing Yang, Zhen-Lun Song, Bi-Zhang Zheng, Zhan-Peng Lu
Acta Metallurgica Sinica (English Letters)    2019, 32 (10): 1287-1297.   doi:10.1007/s40195-019-00915-1
Abstract14)   HTML3)    PDF (5238KB)(97)      

The effects of low electromagnetic field (EMF) (B = 2 mT) on the corrosion of pure copper in the absence and presence of multispecies marine aerobic bacteria were investigated in this work. The results showed that EMF has an inhibitory effect on copper metals and decreases the corrosion rate of copper metals in sterile artificial seawater. However, microbiologically influenced corrosion of Cu was increased in the presence of electromagnetic field due to its effect on the biofilm morphology and structure. EMF reduced the growth rate of bacteria and decreased bacterial attachment, thereby forming a heterogeneous and non-stable biofilm on the Cu surface in the presence of EMF. Moreover, the biofilm was dispersed throughout the surface after 7 days, whereas the scattered bacteria were observed on the surface after 10 days. Confocal laser scanning microscopy images showed large and deep pits on the surface in the presence of EMF and confirmed the acceleration of Cu corrosion in the presence of EMF and multispecies bacteria. Furthermore, XPS and FTIR results demonstrated that the corrosion products and metabolic by-products were significantly changed in the presence of EMF.

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Research Progress of Bobbin Tool Friction Stir Welding of Aluminum Alloys: A Review
Guo-Qing Wang, Yan-Hua Zhao, Ying-Ying Tang
Acta Metallurgica Sinica (English Letters)    2020, 33 (1): 13-29.   doi:10.1007/s40195-019-00946-8
Abstract14)   HTML0)    PDF (4521KB)(78)      

Bobbin tool friction stir welding (BT-FSW) is a variant of conventional friction stir welding (FSW). It can be used to weld complex curvature structures and closed sections by adding an extra shoulder instead of a rigid backing anvil, which expands the potential application of FSW in aerospace, railway, automotive and marine industries. BT-FSW has some significant advantages over conventional FSW such as no root flaws, full weld penetration, low stiffness requirements for machines and fixtures, balanced heat input, lower distortion and thus has broad prospects for development. At present, there have been numerous research reports on BT-FSW, but its widespread use is still restricted due to various factors such as tool life, process stability, control complexity and implementation cost. In this paper, the domestic and foreign research progress of BT-FSW is reviewed from four aspects of bobbin tool design and classification, temperature field and flow field during welding, microstructure and mechanical properties of welded joints as well as industrial application, and then the possible research hotspots of BT-FSW in the future are pointed out. This paper mainly aims to help researchers have a comprehensive and in-depth understanding of BT-FSW.

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A Novel Series of Refractory High-Entropy Alloys Ti2ZrHf0.5VNbx with High Specific Yield Strength and Good Ductility
Dong-Xu Qiao, Hui Jiang, Wen-Na Jiao, Yi-Ping Lu, Zhi-Qiang Cao, Ting-Ju Li
Acta Metallurgica Sinica (English Letters)    2019, 32 (8): 925-931.   doi:10.1007/s40195-019-00921-3
Abstract14)   HTML9)    PDF (2945KB)(607)      

A series of Ti2ZrHf0.5VNbx (x=0, 0.25, 0.5, 0.75 and 1.0) refractory high-entropy alloys were prepared to investigate the alloying effect of Nb on the microstructures and mechanical properties. All the alloys displayed a simple BCC structure. The microstructures of the alloys changed from the initial single-phase columnar structure (x=0) to dendrite microstructure (x>0). At room temperature, all the alloys exhibited high ductility (with the compressive strains of more than 50%). With the increase in Nb content, the yield strength slightly decreased from 1160 to 980 MPa and the hardness dropped from 338 to 310 HV. Moreover, the alloys exhibited low density from 6.47 to 6.84 g/cm3 and high specific yield strength (SYS) from 143 to 179 kPa m3/kg. The comprehensive performance of ductility and SYS was superior to most of the reported high-entropy alloys. The yield strength of the alloys increased from 405 to 859 MPa and from 85 to 195 MPa with the addition of Nb element at 873 K and 1073 K, respectively.

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Effect of laser power and deposition environment on the microstructure and properties of direct laser metal-deposited 12CrNi2 steel
Mohamad Ebrahimnia, Yujiang Xie, Changtai Chi
Acta Metallurgica Sinica (English Letters)    2020, 33 (4): 528-538.   doi:10.1007/s40195-019-00974-4
Abstract14)   HTML1)    PDF (5681KB)(32)      

Direct laser metal deposition was used for preparing blocks of steel 12CrNi2 using four different laser powers under two different deposition environments including atmospheric environment and Ar-protected chamber. The results showed that microstructures and mechanical properties were significantly affected by different laser powers. Increasing laser power and deposition in Ar chamber will lead to a decrease in the quantity and size of the voids, which brings more elongation to the samples. Bainitic microstructure was replaced by Widmanstatten ferrite and pearlite, and the amount of proeutectoid ferrite increased with increasing laser power. Moreover, microstructures of previous layers were completely altered in high laser power. Excessive heat accumulation by using high heat input can produce equiaxed ferritic grains with the pearlites in previously deposited layers. Hardness of deposited samples increased from the bottom layer toward the top layer. By using a diode laser with a spot diameter size of 2 mm, the 900-W laser power is suitable for producing crack- and void-free samples. However, post-deposition heat treatment is necessary for obtaining homogeneous desired microstructure and grain size in the manufactured samples.

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Site Occupation of Nb in γ-TiAl: Beyond the Point Defect Gas Approximation
Wei Diao, Li-Hua Ye, Zong-Wei Ji, Rui Yang, Qing-Miao Hu
Acta Metallurgica Sinica (English Letters)    2019, 32 (12): 1511-1520.   doi:10.1007/s40195-019-00925-z
Abstract14)   HTML0)    PDF (2063KB)(68)      

Microalloying is an effective approach to improve the mechanical properties of γ-TiAl intermetallic compound. Knowledge about the site occupancy of the ternary alloying element in the crystal lattice of γ-TiAl is highly demanded in order to understand the physics underlying the alloying effect. Previous first-principle methods-based thermodynamic models for the determination of the site occupancy were based on the point defect gas approximation with the interaction between the point defects neglected. In the present work, we include the point defect interaction energy in the thermodynamic model, which allows us to predict the site occupancy of the ternary alloying element in γ-TiAl beyond the point defect gas approximation. The model is applied to the γ-TiAl-Nb alloy. We show that, at low temperature, the site occupancy of Nb atoms depends on the composition of the alloy: Nb atoms occupy the Al sublattice for the Ti-rich alloy but occupy Ti sublattice for the Al-rich alloy. The fraction of Nb atoms occupying Al sublattice in the Ti-rich alloy decreases drastically, whereas the fraction of Nb atoms on the Ti sublattice in the Al-rich alloy decreases slightly with increasing temperature. At high temperature, Nb atoms occupy dominantly the Ti sublattice for both the Ti-rich and Al-rich alloys. The interaction between the point defects makes the Ti sublattice more favorable for the Nb atoms to occupy.

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Effect of Cooling Rate on Microstructure and Mechanical Properties of Sand-Casted Al-5.0Mg-0.6Mn-0.25Ce Alloy
Hua-Ping Tang, Qu-Dong Wang, Chuan Lei, Kui Wang, Bing Ye, Hai-Yan Jiang, Wen-Jiang Ding
Acta Metallurgica Sinica (English Letters)    2019, 32 (12): 1549-1564.   doi:10.1007/s40195-019-00922-2
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This study examines the relationship among cooling rate, microstructure and mechanical properties of a sand-casted Al-5.0Mg-0.6Mn-0.25Ce (wt%) alloy subjected to T4 heat treatment (430 °C × 12 h + natural aging for 5 days), and the tested alloys with wall thickness varying from 5 to 50 mm were prepared. The results show that as the cooling rate increases from 0.22 to 7.65 K/s, the average secondary dendritic arm spacing (SDAS, λ2) decreases from 94.8 to 27.3 μm. The relation between SDAS and cooling rate can be expressed by an equation: $\lambda_{2} = 53.0R_{\text{c}}^{ - 0.345}$. Additionally, an increase in cooling rate was shown not only to reduce the amount of the secondary phases, but also to promote the transition from Al10Mn2Ce to α-Al24(Mn,Fe)6Si2 phase. Tensile tests show that as the cooling rate increases from 0.22 to 7.65 K/s, the ultimate tensile strength (UTS) increases from 146.3 to 241.0 MPa and the elongation (EL) increases sharply from 4.4 to 12.2% for the as-cast alloys. Relations of UTS and EL with SDAS were determined, and both the UTS and EL increase linearly with (1/λ2)0.5 and that these changes can be explained by strengthening mechanisms. Most eutectic Al3Mg2 phases were dissolved during T4 treatment, which in turn further improve the YS, UTS and EL. However, the increment percent of YS, UTS and EL is affected by the cooling rate.

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Hot Deformation Behavior and Processing Map of a Cu-Bearing 2205 Duplex Stainless Steel
Tong Xi, Lu Yin, Chun-Guang Yang, Ke Yang
Acta Metallurgica Sinica (English Letters)    2019, 32 (12): 1537-1548.   doi:10.1007/s40195-019-00910-6
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The hot deformation behavior and processing map of Cu-bearing 2205 duplex stainless steel (2205-Cu DSS) were investigated at temperatures of 950-1150 °C and strain rates of 0.01-10 s-1. The effects of Cu addition and different deformation parameters on deformation behavior were, respectively, characterized by analyzing flow curves, constitutive equations and microstructures. The results indicated that the shapes of flow curves strongly depended on the volume fraction of two phases. When deformed at low strain rate, DRV in ferrite was prompted with increase in the temperature and was further developed to continuous DRX. At high strain rate, flow localization preferentially occurred in ferrite at low deformation temperature due to the strain partitioning and relatively less fraction of ferrite. The activation energy for 2205-Cu DSS was 452 kJ/mol and was found to connect with the variation of strain, strain rate and deformation temperature. The optimum hot deformation parameters for 2205-Cu DSS were obtained in the temperature range of 1100-1150 °C and strain rate range of 0.1-1 s-1 with a peak power dissipation efficiency of 41%. Flow localization was the main way to lead to flow instability. Meanwhile, the Cu-rich precipitates were generated within a few ferrite grains when deformed at temperature lower than 1000 °C. The interaction between dislocations and Cu-rich precipitates at high strain rate, as well as the limited DRV in ferrite and DRX in austenite, contributed to the complex microstructure and flow behavior.

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Detailed Structures and Formation Mechanisms of Well-Known Al10RE2Mn7 Phase in Die-Cast Mg-4Al-4RE-0.3Mn Alloy
Qiang Yang, Shu-Hui Lv, Fan-Zhi Meng, Kai Guan, Bai-Shun Li, Xu-Hu Zhang, Jing-Qi Zhang, Xiao-Juan Liu, Jian Meng
Acta Metallurgica Sinica (English Letters)    2019, 32 (2): 178-186.   doi:10.1007/s40195-018-0819-0
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The detailed structures and the corresponding formation mechanisms of the well-known Al10RE2Mn7 phase in the conventional die-cast Mg-4Al-4RE-0.3Mn alloy were thoroughly investigated using transmission electron microscopy (TEM). The results indicate that the Al10RE2Mn7 phase ordinarily contains both normal $(11\overline{2} 1)$ twins and orientation twins. Both detailed TEM observations and density functional theory calculations indicate that the Al10RE2Mn7 phase is transferred from the Al8REMn4 phase following an orientation relationship as $[010]_{{{\text{Al}}_{ 8} {\text{REMn}}_{ 4} }} / /[\overline{1} 101]_{{{\text{Al}}_{ 1 0} {\text{RE}}_{ 2} {\text{Mn}}_{ 7} }}$ and $(101)_{{{\text{Al}}_{ 8} {\text{REMn}}_{ 4} }} / /(11\overline{2} 0)_{{{\text{Al}}_{ 1 0} {\text{RE}}_{ 2} {\text{Mn}}_{ 7} }}$. Moreover, forming orientation twins in the Al10RE2Mn7 phase is attributed to the blurry regions at incoherent twin boundaries in the Al8REMn4 phase. Finally, these formed orientation twins result in the $(11\overline{2} 1)$ twins in the Al10RE2Mn7 phase.

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Microstructural Evolution, Mechanical Properties and Thermal Stability of Gradient Structured Pure Nickel
Xiao Li, Bo Guan, Yun-Fei Jia, Yun-Chang Xin, Cheng-Cheng Zhang, Xian-Cheng Zhang, Shan-Tung Tu
Acta Metallurgica Sinica (English Letters)    2019, 32 (8): 951-960.   doi:10.1007/s40195-018-00870-3
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The microstructural evolution of pure nickel treated by deep rolling (DR) technique with different indent depths was investigated by means of optical microscopy and transmission electron microscopy. The surface roughness, hardness and residual stress distribution along the depth from surface were measured. Moreover, the DR-treated sample was annealed at temperatures from 300 to 700 °C for 2 h. The results reveal that dislocation movements are the fundamental mechanisms of gradient grain refinement during the DR process. With increasing indent depth of the DR, the gradient microhardness on the cross section of sample significantly increases, the maximum compressive residual stress decreases, and the affecting region of residual stress increases. The results of thermal stability depict that the microstructure can be stable as temperature up to 300 °C, and the abnormal grain growth and annealing twins are observed at 600 °C.

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Hot Deformation Behavior and Processing Maps of a Medium Manganese TRIP Steel
Ning Yan, Hong-Shuang Di, Hui-Qiang Huang, R D. K. Misra., Yong-Gang Deng
Acta Metallurgica Sinica (English Letters)    2019, 32 (8): 1021-1031.   doi:10.1007/s40195-018-0854-x
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The hot deformation behavior of a medium-Mn steel was studied in terms of hot compression flow curves in the temperature range of 850-1050 °C and strain rates of 0.05-10 s-1. The thermo-mechanical analysis was carried out and suggested that the microstructure during deformation was completely austenite which had high tendency for dynamic recrystallization (DRX). The flow behavior was characterized by significant flow softening at deformation temperatures of 950-1050 °C and lower strain rates of 0.05-5 s-1, which was attributed to heating during deformation, DRX and flow instability. A step-by-step calculating procedure for constitutive equations is proposed. The verification of the modified equations indicated that the developed constitutive models could accurately describe the flow softening behavior of studied steel. Additionally, according to the processing maps and microstructure analysis, it suggested that hot working of medium-Mn steel should be carried out at 1050 °C, and the strain rate of 0.05-10 s-1 resulted in significantly recrystallized microstructures in the in steel. The flow localization is mainly flow instability mechanism for experimental steel.

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Corrosion Resistance of AZ91 Mg Alloy Modified by High-Current Pulsed Electron Beam
Peng-Peng Wu, Kun-Kun Deng, Kai-Bo Nie, Zhong-Zhong Zhang
Acta Metallurgica Sinica (English Letters)    2019, 32 (2): 218-226.   doi:10.1007/s40195-018-0798-1
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The high-current pulsed electron beam (HCPEB) treatment with current density 6 J/cm2 was applied on AZ91 Mg alloy to improve its corrosion resistance. Results showed that the net-like Mg17Al12 disappeared on the surface of AZ91 Mg alloy after irradiation by HCPEB, which was instead of supersaturated Al element on the surface. Nevertheless, the application of HCPEB also led to the formation of crater-like and groove-like structures as well as micro-cracks on the surface of AZ91 Mg alloy. After HCPEB treatment by 3, 5 and 10 pulses, the AZ91 Mg alloy exhibited better corrosion resistance. However, the increasing amount of micro-cracks reduced the anti-corrosive properties of AZ91 Mg alloy as the pulse increased to 20 and 30.

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Microstructure and Its Influence on the Mechanical Properties of Ni-28W-6Cr-Based Alloy-Welded Joints by GTAW
Shuang-Jian Chen, Xiang-Xi Ye, D.K. L.Tsang, Li Jiang, Chao-Wen Li, Kun Yu, Zhi-Jun Li
Acta Metallurgica Sinica (English Letters)    2019, 32 (8): 1032-1040.   doi:10.1007/s40195-019-00881-8
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Microstructure and mechanical properties of Ni-28W-6Cr alloy-welded joints produced by gas tungsten arc welding were investigated in this work. Results showed that original fine-grain base metal near fusion line totally transformed into coarse heat affected zone after welding. Carbides with different shapes were found in the weld metal and base metal, which all were determined as M6C carbides. In comparison with carbides in base metal, M6C carbides in weld metal are rich in Si and Cr but deficient in W. Moreover, M6C carbides are extremely scarce and hard to be detected in weld metal. Mechanical tests show that the hardness value of weld metal is only about 60% of base metal; the yield strength and tensile strength of welded joint are much lower than those of base metal due to the absence of carbides in weld metal.

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Microstructure and Mechanical Properties of RAFM-316L Dissimilar Joints by Friction Stir Welding with Different Butt Joining Modes
Bin He, Lei Cui, Dong-Po Wang, Hui-Jun Li, Chen-Xi Liu
Acta Metallurgica Sinica (English Letters)    2020, 33 (1): 135-146.   doi:10.1007/s40195-019-00951-x
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Dissimilar welded joints of reduced activation ferritic/martensitic (RAFM) steel and 316L austenitic stainless steel were prepared by friction stir welding with different butt joining modes and welding parameters. The weld quality of the joint was improved by placing the 316L steel on the advancing side and the RAFM steel on the retreating side, and using a relatively high rotational speed of 400 rpm. The microstructure of the stir zone on the 316L steel side consisted of single-phase austenite, and the microstructure of the stir zone on the RAFM steel side mainly consisted of lath martensite and equiaxed ferrite. A mechanical mixture of the two steels and diffusion of Cr and Ni could be detected near the bonding interface. Diffusion of Ni from the 316L steel to the RAFM steel resulted in the formation of a dual-phase structure consisting of austenite and ferrite. The as-welded joints showed good strength and ductility at room temperature and 550 °C, which were nearly equal to those of the 316L base material. The heat-affected zone on the RAFM side had the lowest impact toughness throughout the weld with a value of 13.2 J at - 40 °C, ~ 52% that of the RAFM base material.

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Corrosion Resistance of Friction Stir Welded Al-Cu-Li Alloy AA2099-T8
Yan-Long Ma, Hui-Bin Xu, Zhao-Yuan Liang, Lei Liu
Acta Metallurgica Sinica (English Letters)    2020, 33 (1): 127-134.   doi:10.1007/s40195-019-00944-w
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In order to study the effect of friction stir welding (FSW) on corrosion resistance of Al-Cu-Li alloy AA2099-T8, the microstructure and microhardness of FSW joints were characterized, and then, the corrosion behavior of the FSW joints was investigated by the immersion and potentiodynamic polarization tests in a 3.5% NaCl solution at room temperature. It is indicated that the alloy was softened by FSW, with the lowest hardness appearing at the boundary between the nugget zone and the thermo-mechanically affected zone. When exposed to the NaCl solution, the FSW joint was characterized by shallow pits and was free of severe localized corrosion, probably due to dissolution of T1 (A2CuLi) phase in the FSW joint. It is suggested that further work should be carried out to evaluate the galvanic coupling effect between the FSW joint and the base metal, as well as the stress corrosion cracking resistance of the FSW joint.

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Element Segregation and Solidification Behavior of a Nb, Ti, Al Co-Strengthened Superalloy ЭК151
Yuan-GuoTan, Fang Liu, An-Wen Zhang, Da-Wei Han, Xiao-Yu Yao, Wei-Wei Zhang, Wen-Ru Sun
Acta Metallurgica Sinica (English Letters)    2019, 32 (10): 1298-1308.   doi:10.1007/s40195-019-00894-3
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The as-cast microstructure, element segregation and solidification behavior of a multi-alloyed superalloy ЭК151 have been investigated. The results show that the severe element segregation leads to the complicated precipitations at the inter-dendritic region, including η-Ni3(Ti, Nb), eutectic (γ + γ′) and Laves, which shows the characteristics of both Ti, Al-strengthened and Nb-strengthened alloys. Differential thermal analysis, heating and quenching tests reveal the solidification sequence as follows: Liquids → γ matrix → (Nb, Ti)C → η-Ni3(Ti, Nb) → eutectic (γ + γ′) → Laves. The melting points are between 1250 and 1260 °C for (Nb, Ti)C, between 1200 and 1210 °C for η phase, between 1180 and 1190 °C for eutectic (γ + γ′) and Laves. γ′ initially precipitates from matrix at 1150 °C and achieves the maximum precipitation at 1130 °C. According to the microstructure evolution captured during solidification and composition analysis by an energy dispersive spectrometer and electron probe microanalyzer, (Nb, Ti)/Al ratio is put forward to explain the formation of η-Ni3(Ti, Nb) and eutectic (γ + γ′). The solidification of γ matrix increased the Nb, Ti concentration in the residual liquids, so the high (Nb, Ti)/Al ratio would result in the formation of η-Ni3(Ti, Nb); the precipitation of the phase consumed Nb and Ti and decreased the (Nb, Ti)/Al ratio in the liquid, which led to the precipitation of eutectic (γ + γ′). Laves formed by the sides of η-Ni3(Ti, Nb) and in front of the eutectic (γ + γ′) after Al, Ti were further depleted by the two phases and Cr, Co, Mo were rejected to liquids.

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Influence of Pin Offset on Microstructure and Mechanical Properties of Friction Stir Welded Mg/Ti Dissimilar Alloys
Qi Song, Zhong-Wei Ma, Shu-De Ji, Qing-Hua Li, Liu-Fang Wang, Rui Li
Acta Metallurgica Sinica (English Letters)    2019, 32 (10): 1261-1268.   doi:10.1007/s40195-019-00899-y
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The influences of pin offset on the formation, microstructure and mechanical properties of friction stir welded joint of Ti6Al4V and AZ31B Mg dissimilar alloys were investigated. The results show that sound joints are obtained at different offsets. With the offset decreasing from 2.5 to 2.1 mm, the number of Ti alloy fragments is increased, and the stir zone (SZ) is enlarged and the grains in SZ become coarser. A hook-like structure is formed at the Mg/Ti interface and its length is increased with the decrease in pin offset. The Al element has an enrichment trend at the Ti alloy side near the Mg/Ti interface when the offset is decreased, which is beneficial to the bonding of the interface. An Al-rich layer with a thickness of 3-5 μm forms at the offset of 2.1 mm. All the joints fracture at the interface and present a mixed ductile-and-brittle fracture mode. The joint tensile strength is increased with the offset decreasing from 2.5 to 2.1 mm, and the maximum tensile strength of 175 MPa is acquired at the offset of 2.1 mm.

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Velocity and Temperature of In-Flight Particles and Its Significance in Determining the Microstructure and Mechanical Properties of TBCs
Lei Zhang, Tao He, Yu Bai, Fang-Li Yu, Wei Fan, Yu-Shan Ma, Zhan-Dong Chang, Hai-Bo Liu, Ben-Qiang Li
Acta Metallurgica Sinica (English Letters)    2019, 32 (10): 1269-1280.   doi:10.1007/s40195-019-00886-3
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The correlation between particle in-flight parameter, defect content and mechanical property of yttria-stabilized zirconia coating was systematically studied in the present work. The melting state of in-flight particle during spraying was simulated using computational fluid dynamics. The results suggested that, with the increase of velocity and temperature of in-flight particles in the plasma jet, the particles changed from partially melted state to fully melted one. As a result, the total defect content of as-sprayed coating gradually decreased, while elastic modulus and microhardness increased correspondingly. However, the fracture toughness of as-sprayed coating reached a maximum value when the total defect content reached approximately 9.1%.

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