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

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    , Volume 53 Issue 8 Previous Issue    Next Issue
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    Orginal Article
    Quantitative Characterization on the Precipitation of AA 7055 Aluminum Alloy by SAXS
    Junzhou CHEN, Liangxing LV, Liang ZHEN, Shenglong DAI
    Acta Metall Sin, 2017, 53 (8): 897-906.  DOI: 10.11900/0412.1961.2016.00559
    Abstract   HTML   PDF (1518KB) ( 1038 )

    AA 7055 aluminum alloy is a newly advanced Al-Zn-Mg-Cu alloy. It has been wide applied in aviation and aerospace field due to its attractive combined properties, such as high strength, high fracture toughness, good resistance to the growth of fatigue cracks and good stress corrosion resistance, and so on. It is generally believed that the optimum ageing precipitates are responsible for these good properties. However, the detailed information, such as size and its distribution, volume fraction, and morphology of precipitate in this alloy is still not clear. Although TEM is used to determine these information, the results are mostly qualitative. Small angle X-ray scattering (SAXS) provides a direct technique to determine the size, morphology and volume fraction of nano-scale particles and the sampling size is much larger than that in TEM. In this work, the evolution of the precipitates during ageing at 120 and 160 ℃ in AA 7055 aluminum alloy were investigated systematically and quantitatively by SAXS technique. The results show that, when ageing at 120 ℃, the average radius of the precipitates increases with increasing the ageing time. After ageing for 5 h and later, the average radius of the precipitates is 3.3 nm, and its distribution almost keeps stably. The volume fraction of the precipitates is also increased with increasing the ageing time. When ageing from 5 h to 60 h, the volume fraction increases from 2.4% to 5.2%. When ageing at 160 ℃, however, the average radius of the precipitates increases from 3.1 nm to 11.7 nm with increasing the ageing time from 0.5 h to 72 h. The volume fraction of the precipitates increases from 1.4% to 5.4% with increasing the ageing time from 0.5 h to 16 h. After ageing for 16 h and later, the volume fraction of the precipitates keeps stably. Both ageing at 120 and 160 ℃, the morphology of the precipitates is similar to a flat ellipsoid with an axis ratio between 0.2 and 0.3. Based on these quantitative results of the precipitates, the strength models during ageing will be built possibility.

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    Optimization and Controlling on the Microstructure, Texture and Properties of an Advanced Al-Mg-Si-Cu-Zn Alloy Sheet
    Yi CHEN, Mingxing GUO, Long YI, Bo YUAN, Gaojie LI, Linzhong ZHUANG, Jishan ZHANG
    Acta Metall Sin, 2017, 53 (8): 907-917.  DOI: 10.11900/0412.1961.2016.00480
    Abstract   HTML   PDF (2082KB) ( 1220 )

    To reduce the weight of car body, Al-Mg-Si-Cu series alloys have been widely used to produce outer body panels of automobiles due to their favorable high-strength-to-weight ratio, corrosion resistance and good formability. Moreover, the strength of Al-Mg-Si-Cu series alloys can be enhanced by artificial ageing treatments. However, their formability and final strengths still need to be further improved compared to steels, which are the major obstacles to wide-scale application of aluminum in the automotive fields. In this work, both the effect of different thermomechanical processes on formability, microstructure and texture of Al-Mg-Si-Cu-Zn alloy, and the influence of ageing treatment on its precipitation behavior were studied through mechanical property tests, OM, SEM, TEM and EBSD measurements. The results reveal that both the strengths and strain-hardening component n value of the T4P treated alloys are not affected by the two thermomechanical processes, but the r?, Δr and elongations in the different directions are significantly affected. The microstructure and texture evolution of the alloy in the two thermomechanical processes are different from each other. Both the microstructure of a little coarser and bi-modal grain size distribution, and the texture characteristics of much more components but with quite lower intensities can be seen in the solution treated alloy sheet which possesses a better formability after the T4P treatment. The hardness increment of 65 HV can be achieved in the quenched alloy after artificial ageing treatment of 185 ℃, 20 min. And then the peak-ageing state can be obtained after ageing 5 h, the hardness, yield strength, ultimate tensile strength and elongation, are as follows, 132 HV, 318 MPa, 364 MPa and 13%, respectively, and ductile fracture is the main fracture feature as observed by SEM examination of fracture surface. Mg-Si precipitates, such as β", β' and Q' phases, are still the main precipitates formed after artificial aging at 185 ℃, and β" phases mainly grow along its b axis and finally transform into β' and Q' phases, which is the main reason for the observed better ageing stability during long time artificial ageing treatment.

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    Study on the Selective Laser Melting of AlSi10Mg
    Wenqi ZHANG, Haihong ZHU, Zhiheng HU, Xiaoyan ZENG
    Acta Metall Sin, 2017, 53 (8): 918-926.  DOI: 10.11900/0412.1961.2016.00472
    Abstract   HTML   PDF (1488KB) ( 1958 )

    The growing interest for a wide range of usable Al alloy parts with complex shape in industrial field makes selective laser melting (SLM) stand out as a new technology for rapid prototyping manufacturing. The objective of this work is to investigate AlSi10Mg cast aluminum alloy manufacturing by SLM. The investigation involved the influence of process parameters on the relative density and the influence of heat treatment on the microstructure and mechanical properties. High density and performance were achieved. The results show that the tensile strength of the SLMed AlSi10Mg is much higher than that of press wrought AlSi10Mg, but the elongation is as almost same as that of the press wrought AlSi10Mg. The heat treatment has a significant effect on the mechanical properties and microstructure of SLMed AlSi10Mg parts. The mechanical properties changes with the annealing temperature. Compared with the mechanical properties without annealing process, the tensile strength decreases from 507~518 MPa to 378~406 MPa and the elongation increases from 3.0%~3.5% to 6.5%~9.0% when the annealing temperature is 300 ℃ and the soap time is 2 h because of the changes in the morphology and distribution of the Si.

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    Microstructure, Ordered Structure and Warm TensileDuctility of Fe-6.5%Si Alloy with Various Ce Content
    Xuan YU, Zhihao ZHANG, Jianxin XIE
    Acta Metall Sin, 2017, 53 (8): 927-936.  DOI: 10.11900/0412.1961.2017.00055
    Abstract   HTML   PDF (2013KB) ( 867 )

    Fe-6.5%Si (mass fraction) alloy is an important soft magnetic material due to its excellent magnetic properties. However, the existence of ordered structure in a great amount is the fundamental cause of poor ductility of the alloy, which restricts the application of the alloy seriously. To modify the microstructure and crystal structure of Fe-6.5%Si alloy by rare earth micro-alloying is one of the significant methods to reduce brittleness and improve plastic deformation ability of the alloy. Whereas, there still lack of elaborate studies on order degree reduction mechanism, ductility improvement evaluation and its connections to a varying microstructure, rare earth distribution, etc., caused by rare earth doping, which restricts a deep understanding on rare earth micro-alloying mechanism and its application in this alloy. In this work, influences of Ce content (mass fraction) on microstructure, ordered structures and warm tensile property of the as-cast alloy were investigated, and the ductility improvement mechanism of the alloy caused by Ce micro-alloying was analyzed. The results indicate that, there is no evident variation of as-cast microstructure when Ce content is below 150×10-6, while the obvious microstructure refinement is observed when Ce content exceeds 210×10-6. Ce addition reduces the alloy's order degree significantly and thus improves its warm tensile ductility obviously. Compared with Ce undoped specimens, average tensile elongation to failure at 400 ℃ increases from 7.4% to 10.1%, 19.3% and 23.0% by 62×10-6, 150×10-6 and 210×10-6 Ce doping, respectively. Inter-granular brittle fracture characteristic occurs in fractured tensile specimens due to the obvious Ce enrichment at grain boundary when Ce content increases to 260×10-6 and 790×10-6, hence the average tensile elongation to failure at 400 ℃ reduces to 15.5% and 14.2%. A reasonable Ce content is within the range of (150~210)×10-6 to improve effectively the ductility of Fe-6.5%Si alloy.

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    Strengthening Mechanism of 600 MPa Grade Nb-Ti Microalloyed High Formability Crossbeam Steel
    Yajun HUI, Hui PAN, Kun LIU, Wenyuan LI, Yang YU, Bin CHEN, Yang CUI
    Acta Metall Sin, 2017, 53 (8): 937-946.  DOI: 10.11900/0412.1961.2017.00038
    Abstract   HTML   PDF (1476KB) ( 682 )

    Automobile beam steel with high strength is the development trend of the automotive industry, which will help to conserve resources and protect the environment. The crossbeam as an important part of the frame, its strength also affects the overall strength of the frame, which will affect the safety performance of vehicles. At present, the crossbeam steel of heavy-duty vehicles is mainly Q235B and Q345C. About 3% of the crossbeams occur cracking problems when heavy-duty vehicles drive about 10000 km or about half a year or so. The strength increasing will cause the cracking in the forming process. Therefore, it is of great significance to develop a high strength crossbeam steel with high fatigue and high formability. In this work, the microstructure, properties and strengthening mechanism of 600 MPa grade Nb-Ti microalloyed high formability crossbeam steel were investigated by OM, SEM and TEM. The results show that the finish rolling temperature has a considerable influence on the microstructure and mechanical strength, with the decreasing of finish rolling temperature, the ferrite grain size and the size of precipitates decreases gradually; dislocation density, the number of precipitates and the ratio of Nb/Ti increase gradually; both the yield strength and tensile strength increase monotonously, while the elongation has an optimum temperature. The optimal mechanical properties were obtained when the finish rolling temperature is 840 ℃, and the yield strength, tensile strength, elongation and impact energy at -60 ℃ reached 541 MPa, 615 MPa, 31.0% and 117 J, respectively. The NrT and PTT curves of (Nb, Ti)C precipitated in the austenite showed that the nucleation rate of uniform nucleation and dislocation linear nucleation increases and the nucleation time is shortened with the decrease of temperature in the experimental temperature range, which is consistent with the observation result that the size of precipitates decreases with the reduction of the finish rolling temperature, while the number of precipitates increases. The grain refinement strengthening and dislocation strengthening are the main strengthening modes of the steel, the grain refinement strengthening accounts for 46%~48% of the yield strength, the dislocation strengthening accounts for 18%~25%, while the precipitation strengthening only contributes 2% of the yield strength.

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    Effects of Ultra-Fast Cooling After Hot Rolling and Lamellarizing on Microstructure and Cryogenic Toughness of 5%Ni Steel
    Meng WANG, Zhenyu LIU, Chenggang LI
    Acta Metall Sin, 2017, 53 (8): 947-956.  DOI: 10.11900/0412.1961.2016.00474
    Abstract   HTML   PDF (1977KB) ( 675 )

    In recent years, the demands for liquefied ethylene gas (LEG) are rapidly increased in China. 5%Ni steel is being widely used to build LEG tanks, due to the excellent toughness, high strength and ductility of the material. Along with the continuous increase in the size of LEG tanks, higher cryogenic toughness has been required for new generation 5%Ni steel. In this work, controlled rolling (CR) has been developed in the aim of microstructure refinement for Ni-containing steels, and ultra-fast cooling (UFC) after hot rolling has been successfully applied to replace on-line direct quenching, which formed the integrated CR-UFC for 5%Ni steel. A new processing technologies, named UFC-LT treatment which consisted of CR-UFC, lamellarizing and tempering has been developed for 5%Ni steel in this work. The microstructure and mechanical properties of 5%Ni steel treated by UFC-LT were investigated, as well as quenching and tempering (QT), quenching, lamellarizing and tempering (QLT) treatments. The results show that the microstructure of 5%Ni steel treated by UFC-LT treatment consisted of tempered martensite, intercritical ferrite and about 5.83% reversed austenite. The reversed austenite has two types of morphologies: one type is acicular reversed austenite which forms along the lath boundaries; another type is block reversed austenite which mainly forms at prior austenite grain boundaries. An optimum combination of strength and toughness were obtained by UFC-LT treatment (ultimate tensile strength is 608 MPa, yield strength is 491 MPa, elongation is 34%, Charpy impact energy at -196 ℃ is 185 J). The ductile-brittle transition temperature of 5%Ni steel treated by QT and UFC-LT heat treatments were -152 ℃ and lower than -196 ℃, respectively. The superior cryogenic toughness compared to QT treatment contributed to the dissolution of cementite, high percentage of large angle grain boundaries and the formation of 5.83% reversed austenite.

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    Study on the Method of Improving the Toughness of CGHAZ for High Heat Input Welding Steels
    Zongyuan ZOU, Xiaokui XU, Yinxiao LI, Chao WANG
    Acta Metall Sin, 2017, 53 (8): 957-967.  DOI: 10.11900/0412.1961.2016.00551
    Abstract   HTML   PDF (2131KB) ( 868 )

    Compared with the low heat input welding steel structures, the high strength low alloy (HSLA) steel structures after high heat input welding keep high temperature with longer time, and the cooling speed is slower, then the austenite crystal grains of coarse-grained heat affected zones (CGHAZ) grow up sharply, and coarse upper bainite (UB) and ferrite side plate (FSP) are generated easily in original austenite crystal, thus toughness of CGHAZ deteriorates seriously. At present, the approach of improving toughness of CGHAZ is to produce massive interleaved acicular ferrite (AF) in the original austenite crystal. However, with the improvement of welding capability for thick plate, welding heat input will be greater, and the hold time of high temperature will be more prolonged. In this case, AF coarsens much seriously, thus the improvement of CGHAZ toughness is limited severely. In this work, a new method for improving the toughness of CGHAZ in high heat input welding steels by studying the distribution map of HAZ impact value was proposed. This new method changes the grain boundary ferrite (GBF) and AF of the CGHAZ to polygonal ferrite (PF) of the fine-grained heat affected zones (FGHAZ) at same peak temperature, which improves the toughness of CGHAZ significantly. Comparing the microstructures and toughness of CGHAZ in Ti-V-N and Al-Ti-V-N micro alloy welding steels, the transformation condition and nucleation mechanism of PF in the CGHAZ of Al-Ti-V-N steel were analyzed. It is found that micron oxide inclusions is a key factor to inducing the nucleation of massive PF in CGHAZ, and nanoscale carbonitride is a key factor to draging and pinning the grain boundaries of austenite and ferrite. Therefore, the effective combination of above two factors guarantees the generation of a large number of PF, which improves the impact toughness greatly at low temperature.

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    Effect of Temperature and Strain Rate on Deformation Behavior of Invar 36 Alloy
    Xifeng LI, Nannan CHEN, Jiaojiao LI, Xueting HE, Hongbing LIU, Xingwei ZHENG, Jun CHEN
    Acta Metall Sin, 2017, 53 (8): 968-974.  DOI: 10.11900/0412.1961.2017.00043
    Abstract   HTML   PDF (1553KB) ( 1232 )

    Since the thermal expansion coefficient of Invar 36 alloy is so low that it matches the composite materials well. It is very suitable as the material of composite material forming mould. Invar 36 alloy mould surface is usually produced by hot pressing technology. The hot pressing temperature and strain rate severely affect the quality of mould surface. In this work, the mechanical properties of Invar 36 alloy were studied in the temperature range from room temperature to 900 ℃ under different initial strain rates (8×10-5, 8×10-4 and 8×10-3 s-1) by using uniaxial tensile tests. The effect of temperature on the springback trend of thick Invar 36 alloy sheet by three-point bending tests at room temperature, 600 ℃ and 800 ℃ was investigated. The results indicate that the yield strength and ultimate tensile strength of Invar 36 alloy significantly decrease with increasing temperature. Meanwhile, the elongation firstly increases and then decreases with the increase of temperature. It reaches a peak value of 69.2% at 600 ℃ and increases by 55% than that at ambient temperature, which mainly results from the plasticity improvement by dynamic recrystallization. Invar 36 alloy at lower temperature (room temperature and 500 ℃) shows insensitive to the strain rate. Nevertheless, the strength and plasticity at 800 ℃ substantially decrease with decreasing strain rate. When the strain rate decreases from 8×10-3 s-1 to 8×10-5 s-1, the yield strength, ultimate tensile strength and elongation reduce by 38%, 47% and 50%, respectively. The three-point bending springback value decreases by 87.0% when the tested temperature increases from room temperature to 800 ℃.

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    Investigation on AC-Induced Corrosion Behavior and Product Film of X70 Steel in Aqueous Environment with Various Ions
    Hui ZHANG, Yanxia DU, Wei LI, Minxu LU
    Acta Metall Sin, 2017, 53 (8): 975-982.  DOI: 10.11900/0412.1961.2016.00566
    Abstract   HTML   PDF (1551KB) ( 834 )

    During the past decades, more buried oil or gas pipeline failures have been attributed to alternating current (AC) interference, and finally, those corrosion failures were investigated and the AC current density was identified as the critical influence parameter. There is general agreement on AC current density as a chief factor in determining metal wastage condition or assessing the AC corrosion risk for any type of soil in the presence of AC corrosion. Different degrees of AC corrosion may occurr if long distance pipelines pass through different kinds of soil environments, however, the effect of soil ions on pipeline steel without cathodic protection is still not well understood. Therefore, it is imperative to study the AC corrosion behaviors of pipeline steel in different soil environments. In the AC corrosion simulation experiment, by investigating the AC corrosion behaviors of X70 steel in 4 kinds of environmental media, the corrosion rates (Kd) were obtained, whose order was Kd (NaCl)>Kd Na2SO4)>Kd (CaCl2)>Kd (MgCl2). SEM, EDS and XRD were used to characterize and analyse the microscope morphologies, elements and phase compositions of the corrosion product films surfaces/cross-sections. Under the same AC current density, the reasons of differences of X70 steel's corrosion rate in different media were discussed. The presence of corrosive ions SO42- and Cl- would accelerate the corrosion rate, Na+ would not affect the formation of corrosion product film in NaCl and Na2SO4 solution media, however, the presence of Ca2+ and Mg2+ were helpful to form protective corrosion product films in MgCl2 and CaCl2 solution media.

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    Effects of Nitrogen Addition on Microstructure and Grain Boundary Microchemistry of Inconel Alloy 690
    Bo CHEN, Xianchao HAO, Yingche MA, Xiangdong CHA, Kui LIU
    Acta Metall Sin, 2017, 53 (8): 983-990.  DOI: 10.11900/0412.1961.2016.00545
    Abstract   HTML   PDF (938KB) ( 685 )

    Inconel alloy 690 is an austenitic nickel-based corrosion resistant alloy with about 30%Cr, which is considered as the most ideal steam generator tubing materials in nuclear power plants because of its superior resistance to intergranular attack (IGA). However, the existence of impurities and the addition of minor alloying elements cause significant difference of carbide morphology, microstructure and chromium depletion of Inconel alloy 690. In this work, the microstructure and grain boundary chemistry of Inconel alloy 690 with four different nitrogen contents have been investigated by SEM and TEM. Stacking fault probability (SFP) and IGA with respect to the microstructure was tested and analyzed. The results indicated that thermal treatment at 715 ℃ following solution annealing (SA) at 1080 ℃ caused a wide range of intergranular carbide morphology with the associated chromium depletion in the vicinity of grain boundaries. With the increasing of nitrogen content, the characters of the carbides ranged from thin continuous bands along boundaries to coarse discrete particles. Stacking fault probability was increased with the increasing of nitrogen content, and the value reached the peak at 100×10-6 of nitrogen content, then it dropped. The corrosion tests showed that moderate nitrogen content alloy performed favorable intergranular attack correlated with the presence of semi-continuous grain boundary carbide and chromium depletion was mitigated. The consequent nitrides were appeared in high nitrogen alloy. So, about 100×10-6 contents of nitrogen in alloy 690 is suitable by synthesis considering of carbides, nitrides and chromium depletion.

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    Deformation Characteristics and Mechanical Properties of Single Crystal Copper During Equal Channel Angular Pressing by Route A
    Tingbiao GUO, Qi LI, Chen WANG, Feng ZHANG, Zhi JIA
    Acta Metall Sin, 2017, 53 (8): 991-1000.  DOI: 10.11900/0412.1961.2016.00582
    Abstract   HTML   PDF (1572KB) ( 880 )

    The single crystal copper has got more and more attention in the important areas of the national economy due to its good conductivity and thermal conductivity and elongation. Whereas the lower strength limits its application and strengthening methods of single crystal copper are of great concern. The traditional strengthening methods, such as solid solution strengthening, fine grain strengthening and deformation strengthening, can seriously damage the conductivity of single crystal copper. As an effective method for severe plastic deformation (SPD), equal channel angular pressing (ECAP) can effectively improve the material strength and keep its excellent performance by controlling the deformation and strain. Deformation texture of the single crystal copper (99.999%) during ECAP by A route was investigated by XRD, EBSD and TEM, the mechanical properties and conductivity were tested, and the mechanism of texture evolution and influence factors of mechanical and electrical properties during deformation process were analyzed. The results show that equiaxed deformation structure with small sizes appeared in single crystal copper after two passes extruded. After four passes of deformation, deformation band structure with same (110) orientation was formed. And the grain orientation of the highly refined grains gradually tended to the (111) surface, the {111}<110>, {111}<112> textures and the little {001}<100> recrystallization texture formed. The scattering of electrons by grain boundaries (GBs) can effectively get reduced and conductivity increases slightly, at the same time, the work hardening rate of the material is significantly improved when {hkl}<110> texture with stable orientation forms under medium and low strains. A large number of low-angle grain boundaries (LAGBs) are formed in the initial deformation stage of single crystal copper. With the increase of strain, the LAGBs gradually change to the high-angle grain boundaries (HAGBs). Dislocation accumulation and GB density increase that dislocation movement is obstructed during deformation process. The tensile strength increases from 168 MPa to 400 MPa, the elongation decreases from 63% to 27.3% after three passes deformation. With the extrusion process, the tensile strength increases slowly, whereas the elongation increases slightly. When the extrusion pass is less than eight times, hardness increases continuously, and recrystallization occurs after eighth passes extrusion that hardness tends to be unstable.

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    Effect of Annealing Temperature on Tensile Fracture Behavior of ARB-Cu at Room Temperature
    Min LI, Jing LIU, Qingwei JIANG
    Acta Metall Sin, 2017, 53 (8): 1001-1010.  DOI: 10.11900/0412.1961.2016.00475
    Abstract   HTML   PDF (1504KB) ( 822 )

    Annealing treatment is an effective method for improving structural stability of ultrafine-grained (UFG) or nanostructured (NS) materials produced by severe plastic deformation (SPD). This work focuses on the effect of annealing temperature on the tensile fracture behavior of UFG Cu produced by accumulative roll bonding (ARB). Annealing treatment was performed for 10 min at temperatures of 100, 150, 200 and 250 ℃. The microstructure of annealed and ARBed UFG Cu was observed by TEM. The uniaxial static tensile test was performed by utilizing fatigue testing machine (IBTC-5000) with an initial strain rate of 10-2 s-1. Fracture morphology was observed by SEM. The results suggested that yield strength and tensile strength decreased after annealing treatment compared with initial sample. However, yield strength and tensile strength of ARB-Cu increased with increasing annealing temperature below recrystallization temperature. When annealing temperature is higher than recrystallization temperature, the strength decreased rapidly. With increasing the annealing temperature, the grain size of ARB-Cu increases and gradually tends to bimodal distribution, and the fracture morphology shows a trend of increasing plasticity gradually. The annealing treatment is helpful to bonding efficiency E. The relationship between the theoretical bonding efficiency E and the ARB passes n can be expressed in E=(1-0.5n)×100%.

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    Effect of Interfacial Energy on Distribution of Nanoparticle in the Melt During the Preparation of Fe-Based ODS Alloys by Thermite Reaction
    Jianxue LIU, Wenjun XI, Neng LI, Shujie LI
    Acta Metall Sin, 2017, 53 (8): 1011-1017.  DOI: 10.11900/0412.1961.2016.00438
    Abstract   HTML   PDF (1109KB) ( 755 )

    Fe-based oxide dispersion strengthened (ODS) alloys are conventionally manufactured through mechanical alloying. Such route even involves an expensive milling step but the oxide surface still could not avoid being contaminated. This work developed a new method by combination of thermite reaction and rapid solidification (RS) to prepare ODS alloys. Attributing to the optimization of thermite mixture composition, nanoparticle α-Al2O3 was synthetized in situ and the molten alloy was modulated by spinodal decomposition (SD) into Fe, Cr-rich and Ni, Al-rich regions. During the cooling of the melt, the low interfacial energy between α-Al2O3 and Ni, Al-rich region was also considered in the process for nanoparticles α-Al2O3 to assemble into NiAl, thus they could uniformly distribute in matrix. This work focuses on the thermodynamic analysis of SD in the melt alloy and the speed of the nanoparticles α-Al2O3 under the influence of interfacial energy and Brownian motion. Experiment results shows that the spherical NiAl segregated by SD has a mean diameter of about 50 nm, whose volume fraction reaches up to 50%; and nanoparticle α-Al2O3, formed during thermite reaction, has a diameter of 5 nm combined into NiAl under the influence of interfacial energy. Computation results indicate that, driven by interfacial energy and Brownian motion, nanoparticle α-Al2O3 could move fast enough into Ni, Al-rich region before solidification accomplishes during RS. Test results imply that the tensile strength of Fe-based ODS alloy is 602 MPa with ultimate elongation of 21% and its mass gain under 1000 ℃ in air for 100 h is 0.4 mg/cm2.

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    Thermophysical Properties and Atomic Distribution of Undercooled Liquid Cu
    Jianglei ZHU, Qing WANG, Haipeng WANG
    Acta Metall Sin, 2017, 53 (8): 1018-1024.  DOI: 10.11900/0412.1961.2017.00053
    Abstract   HTML   PDF (863KB) ( 942 )

    Cu is commonly used in the field of electricity and electronics because of its high ductility, and electrical and thermal conductivity. The thermophysical properties and the atomic structure of liquid Cu, especially for undercooled state, are of practical significance in both application and fundamental researches. The major approaches to obtain thermophysical properties of undercooled metals are containerless techniques based on electrostatic levitation, electromagnetic levitation and ultrasonic levitation et al. However, the strong volatility of liquid Cu results in great difficulties to measure the thermophysical properties. Accordingly, computational prediction is becoming an expected method to obtain the thermophysical data of liquid Cu. The molecular dynamics (MD) simulation, in combination with a resonable potential model, has been extensively employed in studying the physical properties of several metals as a powerful approach. In this work, the atomic distribution and thermophysical properties including melting temperature, density, specific heat and self-diffusion coefficient of liquid Cu were studied by molecular dynamics simulation. Mishin's and Zhou's embedded-atom method potentials, and the modified embedded-atom method potential proposed by Baskes were used over the temperature range of 800~2400 K, reaching the maximum undercooling of 556 K. The simulated results are in good agreement with the reported experimental results. The crystal-liquid-crystal sandwich structure has been used to calculate the melting point. The melting point calculated by Baskes' potential model is 1341 K, just a difference of 1.11% from the experimental value. The density at the melting point calculated by Mishin's potential is 7.86 g/cm3, with a difference less than 2% compared with the reported data. It is found that the enthalpy of liquid Cu increases linearly with the increase of temperature. The specific heat is obtained to be 31.89 J/(molK) by Mishin's potential, which is constant in the corresponding temperature range. The self-diffusion coefficient is exponentially dependent on the temperature. The maximum error between the reported value and the present value of the self-diffusion coefficient calculated by Mishin's potential is only 4.93%. The pair distribution function was applied to investigate the atomic structure of liquid Cu, which suggests that the simulated system is still ordered in short range and disordered in long range for both normal liquid and undercooled state. It is found that the atomic ordered degree is weakened with the increase of temperature, and it is kept within 3~4 atom neighbor distance.

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