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

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    Mechanism of TiN Fracture During the Tensile Process of NM500 Wear-Resistant Steel
    WU Xiang,ZUO Xiurong,ZHAO Weiwei,WANG Zhongyang
    Acta Metall Sin, 2020, 56 (2): 129-136.  DOI: 10.11900/0412.1961.2019.00209
    Abstract   HTML   PDF (16758KB) ( 104 )

    Low-alloy high-strength martensitic wear-resistant steel has been widely used in the field of construction machinery due to its low cost and excellent mechanical properties. Microalloying elements, especially Ti, B and other elements, have been widely used to improve the performance of low carbon steel. However, addition of Ti will cause micron-sized Ti precipitates in the continuous casting process, causing cleavage fracture. Therefore, it is necessary to study the micron-sized TiN to reduce its influence on the toughness of the material. SEM, EDS, TEM and EBSD methods were combined with thermodynamic theory to study the precipitation rule of micron-sized TiN in NM500 wear-resistant steel, the fracture mechanism and the influence of matrix on the fracture mechanism. The results show that the tensile fracture mechanism of NM500 steel is mixed mode. There are two fracture morphology of micron-sized TiN on fracture surface: TiN is on the fracture surface, being on the tear ridge; TiN is at the bottom of a deep dimple. The Ti element in the steel precipitates at high temperature and forms a large number of micron-sized TiN. There are three kinds of fracture mechanisms in TiN when subjected to tensile stress: A single crack appears in TiN initiates and spreads to the matrix; A single crack appears in TiN initiates but stops at the matrix; A plurality of cracks are generated in the TiN, and the crack stops at the base, with the TiN shape being preserved intact. There are high strain zones and micron-sized TiN in NM500 steel, and the prior austenite grains are coarse. When the TiN cracks, the matrix has a poor ability to arrest the cracks, then the crack can extend on the substrate easily. When a plurality of TiN clusters are formed, the cracks are connected into one piece to be a weak band, leading to a poor plasticity to the steel.

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    Corrosion Behavior of X80 Steel Welded Joint in Simulated Natural Gas Condensate Solutions
    CHEN Fang,LI Yadong,YANG Jian,TANG Xiao,LI Yan
    Acta Metall Sin, 2020, 56 (2): 137-147.  DOI: 10.11900/0412.1961.2019.00237
    Abstract   HTML   PDF (10336KB) ( 156 )

    Carbon steels are widely used as transportation pipelines in oil and gas fields and welding is one of the main ways of connecting pipeline steel. The welded joints are easily corroded due to the difference in composition, structure and properties of the various components. The effect of content of HAc, Cl-, ethylene glycol (MEG) and temperature (T) on the corrosion behavior of welded joint of X80 steel in a simulated natural gas condensate saturated with CO2 was studied by the orthogonal experimental design, weight loss experiment and electrochemical experiment. It is demonstrated that the corrosion rate of the welded joint is significantly higher than that of the base metal because of the formation of macroscopic corrosion galvanic cells, and the corrosion of the weld metal as an anode region is accelerated to become a weak link of the welded joint. The significance of the four factors on the corrosion process is: c(HAc)>T $\gg$c(Cl-)>c(MEG), c(HAc) and T are the main factors affecting the corrosion behavior, and c(Cl-) and c(MEG) are secondary factors. Base metal and weld metal corrosion tendencies increase with increasing temperature. Because ferrous acetate is less protective as a corrosion product, as the c(HAc) increases, the impedance of the base metal and weld metal decreases, and the corrosion rate increases. Based on the results of orthogonal experiments, a multivariate linear regression equation was established.

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    Surface Characteristics and Stochastic Model of Corroded Structural Steel Under General Atmospheric Environment
    WANG Youde,XU Shanhua,LI Han,ZHANG Haijiang
    Acta Metall Sin, 2020, 56 (2): 148-160.  DOI: 10.11900/0412.1961.2019.00156
    Abstract   HTML   PDF (18377KB) ( 83 )

    Steel structures exposed to corrosive atmospheres for a long time are highly susceptible to corrosion damage. The safety assessments of existing corroded steel structures rely heavily on the quantification of corrosion itself. In order to study the corrosion characteristics of structural steel in general atmospheric environment, 6 batches of artificial accelerated corrosion experiments and 8 a of natural exposure experiments were carried out. The surface characteristic parameters and evolution rules of corroded structural steel were studied by the surface morphology tests and self-programmed morphology analysis program. The distribution characteristics of corrosion depth, pit depth and aspect ratio were clarified, and the changing laws of statistical parameters (such as mean value and standard deviation) and pitting shapes were revealed. The results indicated that the corrosion depth of structural steel in general atmospheric environment obeyed the normal distribution, and the pit depth and aspect ratio obeyed the lognormal distribution. With the increase of corrosion degree, the mean value and standard deviation of corrosion depth, the peak value of power spectrum density of corrosion depth, and the logarithmic mean value of pit depth gradually increased, and the logarithmic mean value of pit aspect ratio decreased. Meanwhile, the shape of pits was gradually changed from a cylinder or hemisphere to a cone. Finally, based on the statistical analysis results of corrosion depth parameters and pit parameters, and taking the variation laws and internal relationships of characterization parameters into consideration, the stochastic field model of corrosion depth and the random distribution model of corrosion pits were established, which achieved the accurate simulation and reconstruction of surface characteristics of corroded steel under general atmospheric environment.

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    Influence of W Content on the Microstructure of Nickel Base Superalloy with High W Content
    HUA Hanyu,XIE Jun,SHU Delong,HOU Guichen,Naicheng SHENG,YU Jinjiang,CUI Chuanyong,SUN Xiaofeng,ZHOU Yizhou
    Acta Metall Sin, 2020, 56 (2): 161-170.  DOI: 10.11900/0412.1961.2019.00193
    Abstract   HTML   PDF (23570KB) ( 151 )

    Nickel base superalloys are widely used in the preparation of hot end parts for aircraft engines because of their good comprehensive mechanical properties, oxidation resistance and structural stability. It's strengthened mainly by solid solution strengthening, γ' phase strengthening and carbide strengthening. High alloying is one of the main methods to improve the solid solution strengthening level of nickel base superalloys, where the element W is an efficient alloying element with low price. The control of the W content is extremely important for high W content nickel base superalloys. However, there are few reports on the influence of W content on the microstructure of high W alloy. According to this background, by means of OM, SEM observation and XRD analysis, the influence of W content on the solidified microstructure in nickel base superalloy have been investigated in this work. Results show that when the W content is about 14% (mass fraction, the same below), there is no α-W phase being precipitated in the alloy. While as the content of W is higher than 16%, α-W could be precipitated during the solidification. On another hand, the grain size of the alloy decreases from 1.04 mm to 0.17 mm and the volume fraction of eutectic increases from 6% to 10% with the increase of the W content. While the content of W has no obvious effect on the sizes and morphologies of γ' phase in the dendrite and inter-dendrite areas. During solidification, the α-W phase will be first precipitated due to its higher precipitation temperature, and the shrinkage of the residual liquid phase may cause the shift and growth of the α-W to the core of the liquid phase. The α-W could be as the core of the heterogeneous nucleation to reduce the critical nucleation energy, which is the main reason that the grain size of the 18%W alloy is smaller. During the growth of the dendrites with various orientations, the concentration of Al and Ti in the residual liquid phase may have a higher concentration gradient to cause the occurrence of eutectic transformation, which is the main reason that there is a higher volume fraction of eutectic in 18%W alloy.

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    Effect of Orientation Deviation on Microstructure and Mechanical Properties of Nickel-Based Single Crystal Superalloy Brazing Joints
    ZHAO Xu,SUN Yuan,HOU Xingyu,ZHANG Hongyu,ZHOU Yizhou,DING Yutian
    Acta Metall Sin, 2020, 56 (2): 171-181.  DOI: 10.11900/0412.1961.2019.00258
    Abstract   HTML   PDF (38062KB) ( 156 )

    Ni-based single crystal superalloy has excellent high temperature properties, which is the main materials for aero-engine turbine blade. In order to improve the yield strength of single crystal blades, the reliable bonding technology has become an increasingly indispensable key technology in the process of producing single crystal blades. However, there is inevitably an orientation deviation in the bonded single crystal component, owing to its shape complexity and randomness during assembly in the practice of bonding single crystal components. The CMSX-4 single crystal superalloy with the orientation combination of 0°+0°, 0°+45° and 0°+90° were brazed by Ni-based filler alloy at 1210 ℃ for 30 min and carried out ageing heat treatment. The effect of base material orientation combination on the microstructure was analyzed by SEM, EBSD and EPMA. The mechanical properties of joints after bonding and ageing treatment were tested. The result indicates that the microstructures and phase compositions of three orientation combination joints were similar in the filler alloy zone, consisting of γ-Ni, γ′, γ+γ′ eutectic, M3B2 type boride, CrB, nickel-silicon compound and γ-Ni+Ni3B+CrB ternary eutectic phase. The melting point depressant B in the filler alloy is not diffused significantly to the base material, and no brittle compound phase is precipitated in the diffusion affected zone of the joint. After ageing treatment, elements diffusion is uniform and brittle precipitates are reduced, and the continuous grain boundary can be observed at the center of the joint when the base material on both sides of the joint has orientation deviation. The testing results of mechanical properties show that the base material orientation deviation has no distinctly effect on the room and high temperature tensile properties of the joint. However, the tensile strengths of the joint at room and high temperature both reduce with orientation deviation after ageing treatment, but the degree of orientation deviation has no obvious influence on the tensile strength of the joint. The fracture of the three joints occurs in the filler alloy zone.

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    Effect of Heat Treatment Parameters on Microstructure and Hot Workability of As-Cast Fine Grain Ingot of GH4720Li Alloy
    WANG Tao,WAN Zhipeng,LI Zhao,LI Peihuan,LI Xinxu,WEI Kang,ZHANG Yong
    Acta Metall Sin, 2020, 56 (2): 182-192.  DOI: 10.11900/0412.1961.2019.00205
    Abstract   HTML   PDF (23934KB) ( 205 )

    GH4720Li was Ni-Cr-Co base precipitation strengthened superalloy and widely used for high performance applications such as disks and blades of either aircraft engines or land-based gas turbines attributing to its excellent properties including resistance to creep and fatigue, together with corrosion, fracture and microstructural stability for the intended applications. Compared with the double-melting process (vacuum induction melting (VIM)+electroslag remelting (ESR) or VIM+vacuum arc remelting (VAR), a triple-melting process (VIM+ESR+VAR) can eliminate the segregation coefficient of the alloying elements and reduce the content of impurity elements, while the ingot fabricated by the triple-melting process also exhibited lots of shortcomings, such as the coarse grains, dendritic structure, microstructure defects and high forging temperature. The as-cast fine grain ingot prepared by grain refining casting process can eliminate the microscopic shrinkage, reduce the differences among three crystalline regions and improve the hot workability as a result. However, it was hardly to avoid the microstructure defects by simply improving the casting process attributing to its large number of alloying elements. Therefore, the homogenization treatment was always performed on the superalloy ingot. In this work, the optimized homogenization parameter was identified according to the investigation on the microstructure evolution under various homogenization treatment conditions and hot workability of as-cast fine grain ingot after homogenization treatment. The relationships of one-stage as well as two-stage homogenization treatment parameters and segregation coefficient as well as volume fraction of eutectic phase were investigated indepth. The hot workability of the homogenized samples under various conditions was also analyzed with the help of hot compression tests. Experimental results revealed that the increased homogenization treatment temperature and extended holding time were able to decrease the volume fraction of eutectic phase and segregation coefficient of the alloying element significantly. Hot compression tests by the Gleeble 3800 dynamic thermal-mechanical testing machine indicated that the samples suffered two-stage homogenization treatment followed by the slowly cooling rate (1140 ℃, 12 h+1170 ℃, 10 h, 0.2 ℃/min furnace cooling to 1010 ℃, and then air cooling) exhibited better hot workability (the maximum reduction rate of 50% deformed at 1120 ℃, 1 s-1). Discontinuous dynamic recrystallization was identified as the mainly nucleation mechanism of the alloy, and the recrystallized grains preferred to nucleate at the boundaries of the original grains according to the microstructure observation of hot compressed samples. In additions, the M(C, N) type precipitates were able to promote the occurrence of dynamic recrystallization behavior. Homogenization treatment experiments and microstructure observation suggested that the optimized treatment parameters of the as-cast fine grain ingot was 1140 ℃, 12 h+1170 ℃, 10 h, 0.2 ℃/min furnace cooling to 1010 ℃, and then by air cooling.

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    Microstructure, Texture and Mechanical Property ofTA32 Titanium Alloy Thick Plate
    CHENG Chao,CHEN Zhiyong,QIN Xushan,LIU Jianrong,WANG Qingjiang
    Acta Metall Sin, 2020, 56 (2): 193-202.  DOI: 10.11900/0412.1961.2019.00226
    Abstract   HTML   PDF (28271KB) ( 218 )

    TA32 alloy is a new near α titanium alloy designed by optimizing the alloy elements ratio based on a series of elements Ti-Al-Sn-Zr-Mo-Si-Nb-Ta which has less β-stabilizing elements. This alloy has an excellent match of heat resistance and heat stability at 550 ℃, and good short-term mechanical properties at 600~650 ℃. TA32 titanium alloy thick plate can be applied to the key components in high temperature service of the hypersonic vehicle. Due to the low deformation degree of thick plate during rolling process, the heterogeneity of microstructure, texture and mechanical properties of the thick plate increases. In order to provide theoretical basis and experimental basis for the subsequent optimization of mechanical properties of TA32 titanium alloy thick plate, the microstructure, texture and mechanical properties of this alloy with a thickness of 60 mm are investigated in this work. Results show that the microstructure of the as-received material is mainly composed of lamellar α grains with few retained thin β layers, and the microstructure difference is not obvious from the surface to the center along the thickness direction of the plate no matter of the RD (rolling direction)-ND (normal direction) plane or the TD (transverse direction)-ND plane. Moreover, the rolling streamline can be obviously observed on the two planes. The morphology of α grains of the alloys presents either straight or wavy depending on their orientations with respect to the principal rolling directions. XRD results show that the as-received material has a typical T-type texture with c-axis of α phase approximately parallel to TD. At the same time, the <$10\bar{1}0$> poles are parallel to RD while <$10\bar{1}1$> poles present random distribution. As the c-axis gradually deviates from the TD of the surface to the center along the thickness direction of the plate, the Schmidt factors gradually increase, which is one of the main reasons for the gradual decrease of tensile strength; and the decrease of fraction of intragranular substructure from the surface to the center along the thickness direction is another important factor. The tensile properties have no obvious difference along the TD and RD at the same thickness position of the as-received material, but slightly worse along the ND. In addition, the influences of microstructure and texture on tensile properties are further clarified by adding two sets of heat treatment experiments (920 ℃, 30 min, AC+600 ℃, 5 h, AC; 950 ℃, 30 min, AC+600 ℃, 5 h, AC). The results show that the texture is the main factor affecting the tensile strength of TA32 titanium alloy plate at different positions under the condition of no obvious difference in microstructure. After double annealing, microstructure difference is the main factor affecting tensile strength.

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    Effect of Cooling Rate on Boride and Room Temperature Tensile Properties of β-Solidifying γ-TiAl Alloys
    WANG Xi,LIU Renci,CAO Ruxin,JIA Qing,CUI Yuyou,YANG Rui
    Acta Metall Sin, 2020, 56 (2): 203-211.  DOI: 10.11900/0412.1961.2019.00100
    Abstract   HTML   PDF (18495KB) ( 177 )

    β-solidifying γ-TiAl alloys have attracted much attention for their higher specific strength and better mechanical properties at elevated temperature. They usually need some boron addition to refine the lamellar grain size, which is believed to improve their poor room temperature ductility. However, the boron addition may cause some side effects on mechanical properties for the formation of borides with unfavorable morphology and crystal structure, which is severely influenced by the alloy composition and cooling rate during casting. The components of γ-TiAl applied usually have complex structure, such as different thicknesses, which leads to different cooling rates and therefore different microstructures and mechanical properties. To evaluate the influence of cooling rate on the microstructure and mechanical properties of γ-TiAl investment casting, plate with step thicknesses was designed to achieve different cooling rates. Step plates of β-solidifying boron-containing TiAl alloy were fabricated by centrifugal casting in Y2O3 facing coating ceramic moulds. It was found that boride mainly distributed on grain boundary, and its aspect ratio increased with increasing cooling rate, with its morphology varying from short, flat plate to long, curvy ribbon. The short plate and curvy ribbon borides were TiB with B27 and Bf structure, respectively. Both types of boride exhibit anisotropic growth characteristics (especially for Bf structure), with the slowest growth rate along [100] and [010] for B27 structure and Bf structure, respectively. This is attributed to the difficulty of atomic rearrangement along corresponding directions during solidification. The cooling rate increase caused the increase of yield strength but the decrease of room temperature ductility, the former results from the decreasing of grain size and lamellar spacing, while the latter results from the easy cracking nucleation and propagation of the long curvy boride, leaving smooth curvy surfaces on the fracture surface. Samples containing short flat plate boride showed better ductility, and no smooth curvy surface was observed.

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    Study on the Solidification of Ag-Ni Monotectic Alloy
    DENG Congkun,JIANG Hongxiang,ZHAO Jiuzhou,HE Jie,ZHAO Lei
    Acta Metall Sin, 2020, 56 (2): 212-220.  DOI: 10.11900/0412.1961.2019.00192
    Abstract   HTML   PDF (8344KB) ( 67 )

    The Ag-Ni alloy has high electrical conductivity, good thermal conductivity, high specific heat capacity, and excellent electrical wear resistance if the Ni-rich phase is dispersedly distributed in the Ag-based matrix. It has been widely used in the medium load contactors, magnetic starters, relays, etc. However, Ag-Ni alloy is a typical monotectic system. Generally, the liquid-liquid phase transformation leads to the formation of a solidification microstructure with serious phase segregation. So far, there have been few studies on the solidification process of Ag-Ni alloys and powder-metallurgical techniques are commonly used to prepare Ag-Ni alloys in industry. In this work, casting experiments and microhardness test were carried out with the Ag-Ni monotectic alloy. The samples with composite microstructure, in which the Ni-rich particles dispersed homogeneously in Ag matrix, were obtained. The microhardness of Ag-Ni alloy increases with the increase of nickel content and the cooling rate of the sample during solidification. When the cooling rate during the liquid-liquid phase transition of the Ag-4.0%Ni alloy reaches 1800 K/s, the microhardness of the Ag-4.0%Ni alloy is close to that of the Ag-10.0%Ni sheet electrical contacts produced by powder metallurgy. A model describing the microstructure evolution during cooling Ag-Ni monotectic alloy melt has been proposed. The process of microstructure formation has been simulated and discussed in details. The results indicate that the cooling rate during the nucleation of the Ni-rich droplets/particles has a dominant influence on the solidification microstructure. The average radius of the Ni-rich particles increases with the increase of nickel content, while it decreases with the increase of the cooling rate during solidification. The average radius of the Ni-rich particles shows an inverse square root dependence on the cooling rate during the nucleation of the Ni-rich droplets/particles. The Ostwald coarsening of the Ni-rich droplets/particles is very weak during cooling Ag-Ni monotectic alloy melt. Rapid/sub-rapid solidification has a good application prospect in the preparation of the high-performance Ag-Ni contact materials.

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    The Initial Corrosion Behavior of Zr-0.75Sn-0.35Fe-0.15Cr Alloy in Deionized Water at 250 ℃
    YAO Meiyi,ZHANG Xingwang,HOU Keke,ZHANG Jinlong,HU Pengfei,PENG Jianchao,ZHOU Bangxin
    Acta Metall Sin, 2020, 56 (2): 221-230.  DOI: 10.11900/0412.1961.2019.00191
    Abstract   HTML   PDF (18461KB) ( 45 )

    Zirconium alloys are important structural materials in pressurized water reactors. During actual operation, the corrosion resistance of water side is the most important factor affecting its service life. The oxide film of zirconium alloys formed during the corrosion process will reduce the heat transfer performance, mechanical properties and service life of the cladding material, thus becoming a factor restricting the development of nuclear power. The initial phase composition and the defect state in the crystal affect the microstructural evolution of the oxide film during the corrosion process, which in turn determines the late growth of the oxide film. In order to study the phase composition and crystal structure evolution of zirconium alloys from the initial oxidation to the formation of ZrO2, the initial corrosion behavior of Zr-0.75Sn-0.35Fe-0.15Cr alloy was studied by using TEM thin foil specimens with coarse grains. The oxygen content varied due to the change of sample thickness at different distances along the perforation of TEM thin foil specimens with coarse grains, which could be investigated the crystal structure evolution of oxide film with the variation of oxygen content. Corrosion tests of these TEM specimens were conducted in an autoclave at 250 ℃ and 3 MPa in deionized water for short time exposure. The results showed a variation of the crystal structure along with the increase of oxygen contents at the initial oxidation stage. When the Zr/O atomic ratio reached 5~7, a commensurable long period super-lattice structure was formed. The lattice constants of the super-lattice (a, c) and α-Zr matrix (a0, c0) satisfied the relationship of a=9a0 and c=2c0, which was called 9a0-2H structure. When the Zr/O atomic ratio reached 3 and 1, sub-oxides Zr3O with hcp and ZrO with fcc ordered structures were formed, respectively. When the Zr/O atomic ratio was 0.85, monoclinic ZrO2 was detected.

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    Preparation of Steel/Aluminum Laminated Composites by Differential Temperature Rolling with Induction Heating
    XIAO Hong,XU Pengpeng,QI Zichen,WU Zonghe,ZHAO Yunpeng
    Acta Metall Sin, 2020, 56 (2): 231-239.  DOI: 10.11900/0412.1961.2019.00150
    Abstract   HTML   PDF (10940KB) ( 212 )

    Both cold-rolled and hot-rolled steel/aluminum laminated composites exhibited obvious strain-hardening of steel layer because the rolling temperature, limited by the melting point of aluminum (about 660 ℃), was lower than dynamic recrystallization temperature of steel (about 710 ℃). This led to poor deformation ability of composite plates and subsequent processing cracks. And the initial bonding of cold-rolled steel/aluminum composite plates usually required more than 50% highly first pass reduction, which resulted in high requirement for rolling mill capacity, especially for medium or thick size composite plates. To solve above two problems simultaneously, in this study, the steel/aluminum composite plates were prepared by differential temperature rolling (DTR) with induction heating in an argon atmosphere. The bonding properties and microstructure of the steel/aluminum laminated composites were studied, and the effect of DTR process on the bonding properties was analyzed compared with the cold rolling process. The results show that dynamic recovery and recrystallization occurred with equiaxed grains appearing in the structure of the rolled carbon steel due to the higher heating temperature of the steel layer, and an equiaxed fine grain zone with an average grain size of approximately 5 μm was formed near the interface of the steel side, which greatly reduced the hardening phenomenon of the laminated composites compared with the cold rolled clad plate. The micro-interface of DTR steel/aluminum clad plate was tightly bonded without holes and gaps. The diffusion width of Al and Fe elements across the interface reached 2.4 μm, indicating the clad plate achieved a good metallurgical bonding state, and the fine grained zone near the interface improved the properties of the sheet. The combined effect made the shear strength of the DTR clad plates much higher than that of the cold-rolled plate. At 45% reduction, the shear strength of DTR composite plate reached 85 MPa, which was 7 times of cold-rolled composite plate with the same reduction (12 MPa). The fracture of cold-rolled composite plate occurred at the steel/aluminum interface, showing brittle fracture, while the fracture of DTR clad plates occurred in the aluminum alloy matrix with a large number of dimples in the shear section, showing the characteristics of plastic fracture.

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    Mechanical Properties and Strengthening Mechanism of Graphene Nanoplatelets Reinforced Magnesium Matrix Composites
    ZHOU Xia,LIU Xiaoxia
    Acta Metall Sin, 2020, 56 (2): 240-248.  DOI: 10.11900/0412.1961.2019.00158
    Abstract   HTML   PDF (31802KB) ( 101 )

    To improve the mechanical properties of Mg alloys and broaden their application fields, high performance Mg matrix nanocomposites have received more and more attention nowadays. Therefore, the research on the basic mechanical properties and strengthening mechanism of new Mg matrix composites at nanoscale has important theoretical and practical significance. The mechanical properties of pristine single-layer graphene nanoplatelets (GNPs) and single-side and double-side nickel-coated GNP (Ni-GNP, Ni-GNP-Ni) reinforced Mg composites (GNP/Mg, Ni-GNP/Mg, Ni-GNP-Ni/Mg) are studied under uniaxial tension by molecular dynamics (MD) simulations. Meanwhile, their tensile properties are also compared with those of double-side nickel-coated GNP with vacancy defects (Ni-defected GNP-Ni) and double-side nickel-coated multilayer GNPs (Ni-nGNPs-Ni) reinforced Mg-based composites. The simulated results show that the mechanical properties of Mg matrix composites are improved significantly by the addition of GNPs. Compared with single crystal Mg, the tensile strength and elastic modulus of GNP/Mg nanocomposites at 300 K and 1×109 s-1 are increased by 32.60% and 37.91%, respectively; while the tensile strength and elastic modulus of Ni-GNP-Ni/Mg composites are increased by 46.79% and 54.53%, separately. In addition, there is a larger increase in the elastic modulus and tensile strength but a smaller increase in the fracture strain for Ni-defected GNP-Ni/Mg composites, while there is a larger increase in the tensile strength and fracture strain but a smaller increase in the elastic modulus for Ni-GNP/Mg composites as compared with those of GNP/Mg composites. The elastic modulus, tensile strength and fracture strain of Ni-GNP-Ni/Mg composites decreases with increase in temperature, showing a temperature softening effect, but the variation in the elastic modulus of the composites is insensitive to temperature. With increasing of the layers or volume fractions of GNPs in Ni-nGNPs-Ni, the elastic modulus, tensile strength and fracture strain of the composites are all increased significantly, and the composites show excellent comprehensive mechanical properties. It is concluded that the main strengthening mechanisms for Ni-GNP-Ni/Mg nanocomposites are strong interface bonding, effective load transfer from the Mg matrix to the Ni-GNP-Ni and dislocation strengthening by analysis of the evolution of atomic structure.

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    Molecular Dynamics Simulation of DisplacementCascades in Nb
    MA Xiaoqiang,YANG Kunjie,XU Yuqiong,DU Xiaochao,ZHOU Jianjun,XIAO Renzheng
    Acta Metall Sin, 2020, 56 (2): 249-256.  DOI: 10.11900/0412.1961.2019.00203
    Abstract   HTML   PDF (6060KB) ( 84 )

    Refractory metal Nb and its alloys are considered as promising materials in fusion reactor, where they are required to withstand a high neutron irradiation, because their excellent high temperature properties such as high temperature strength, good thermal conductivity and compatibility with most liquid metal coolants. The defects are created in atomic displacement cascade from the primary state of damage and subsequent evolution gives rise to important change in their microstructures and engineering properties. However, the evolution and aggregation of induced radiation defects in atomic level cannot be observed by experiment so far. In this work, molecular dynamics (MD) method is used to explore the microstructural formation and evolution of defects from the atomic displacement cascades in bcc-Nb. In the simulation, the energy range of primary knock-on atom (PKA) is chosen 5~50 keV and the simulation temperature 300 K. It is observed that the most of defects in bcc Nb are point defects at different PKA energies. The vacancy cluster rate varies from 17% to 35% and self-interstitial cluster rate varies from 23% to 40%. As the PKA energy increasing, vacancies usually tend to form larger clusters. The self-interstitial atoms form a dumbbell structure along the direction <110>. The 1/2<111> intermittent dislocation loop and <100> vacancy dislocation loop are produced when the PKA energy greater than 30 keV. The quantitative relationship between energy of PKA (EPKA) and number of survivals Frenkel pairs (NFP) is fitted by a power function with different parameters at low-energies (5~30 keV) and the high-energies (30~50 keV).

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