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

About the Journal

  Current Issue
    , Volume 49 Issue 5 Previous Issue    Next Issue
    For Selected: View Abstracts
    LARGE EDDY SIMULATION OF UNSTEADY ARGON/STEEL TWO PHASE TURBULENT FLOW IN A CONTINUOUS CASTING MOLD
    LIU Zhongqiu1, LI Baokuan1, JIANG Maofa1, ZHANG Li2, XU Guodong2
    Acta Metall Sin, 2013, 49 (5): 513-522.  DOI: 10.3724/SP.J.1037.2012.00760
    Abstract   PDF (7358KB) ( 729 )

    A large eddy simulation of unsteady argon/steel two phase flow in a continuous casting mold is presented in this work. The inhomogeneous Euler-Euler approach is used to describe the equations of motion of the two phase flow. Large eddy simulation model is used to solve the turbulent viscosity force. The drag force, lift force and virtual mass force are incorporated in the model. And this model has been validated with a water model experiment and the measurements of the ultrasonic flaw detection. The predicted unsteady two phase turbulent flow characteristics were validated, indirectly, by the measurements of ultrasonic flaw detection. The predicted asymmetric flow was validated by the water model experiment. The simulation results show that the distribution of the bubble diameter is seen to depend on the argon gas and molten steel flow rates. The mean bubble diameter decreases with increasing the molten steel flow rate, but increases with increasing the argon gas flow rate. The asymmetric flow inside the mold was not stationary; the time intervals for changeover appeared to be vary random. It can be concluded that the shape of the bending part of the secondary cooling zone of mold is important to the asymmetric flow. The asymmetric flow occurrs near the bending section.

    References | Related Articles | Metrics
    NUMERICAL SIMULATION OF RECRYSTALLIZATION PROCESS OF NICKEL BASE SINGLE CRYSTAL SUPERALLOY
    WU Chunlong, XU Qingyan, XIONG Jichun, LI Zhonglin, LI Jiarong,LIU Baicheng
    Acta Metall Sin, 2013, 49 (5): 523-529.  DOI: 10.3724/SP.J.1037.2013.00036
    Abstract   PDF (1856KB) ( 998 )

    The formation of recrystallization can cause great damage on the performance of nickel base single crystal superalloy turbine blade. The stress, strain and stored energy distribution of single crystal superalloy were modeled and simulated under the condition of inhomogeneous deformation. The critical stored energy was established which takes the anisotropic mechanical properties of single crystal superalloy and the recrystallization grain boundary migration process into account. Based on the calculation of the critical stored energy, the recrystallization microstructure in the local deformation zone after heat treatment was simulated by using cellular automaton (CA) method. DD6 single crystal test bars were treated by the cold deformation process and then annealed at the temperature of 1280 ℃ for 30, 60 and 240 min. The experimental results showed that the recrystallization occurred on the surface near the deformation area. The recrystallization area and the grain size increased gradually with time. The simulated microstructure results  agree well with the experimental ones (EBSD) and indicate that the recrystallization process of single crystal superalloy can be well simulated by the proposed model.

    References | Related Articles | Metrics
    MOLECULAR DYNAMICS STUDY ON TEMPERATURE EFFECT AND RADIATION-DEDUCED DEFECT FORMATION IN hcp-Ti
    YAO Man, GAO Xiao, ZENG Weipeng, WANG Xudong, XU Haixuan, Simon R. Phillpot
    Acta Metall Sin, 2013, 49 (5): 530-536.  DOI: 10.3724/SP.J.1037.2013.00007
    Abstract   PDF (1858KB) ( 888 )

    The radiation-induced defects formed in nuclear structure materials in the initial stage of displacement cascade and the subsequent changes in their microstructures and mechanical properties are the primary causes of their failure. Unfortunately the formation and aggregation of point defects are hard to be found only by probed experiments. In this work, the displacement cascade occurring in hcp-Ti system has been investigated by using molecular dynamics. After building the atomic model, by setting different PKA (primary knock-on atom) directions and PKA energies respectively, the evolution of simulated radiation-induced point defects in displacement cascade was studied and its two major characteristics, displacement and thermal spikes, have been given. The transient temperature distribution, local “melting region” variation in the damage zone caused by radiation as well as the peak and surviving numbers of Frenkel pairs were quantitatively estimated. For hcp-Ti, in the PKA energy region of 0.5-9.0 keV, the effect of PKA velocity direction on survival defect number is very small. Both the defect number at stable state and its maximum value have approximately linear relationship with the PKA energy.In the radiation evolution stage, the defects about 95 percent have recovered, the defect and temperature distribution caused by displacement cascade are asymmetric. The simulation method used in this paper would provide an effective way for the study on the evolution and micromechanism of radiation-induced point defects in displacement cascade.

    References | Related Articles | Metrics
    SOLIDIFICATION OF MONOTECTIC ALLOY UNDER LASER SURFACE TREATMENT CONDITIONS
    CHEN Shu, ZHAO Jiuzhou
    Acta Metall Sin, 2013, 49 (5): 537-543.  DOI: 10.3724/SP.J.1037.2013.00014
    Abstract   PDF (2052KB) ( 633 )

    Monotectic alloys have great potentials to be used in industry due to their special physical and mechanical properties. But these alloys have an essential drawback that just the miscibility gap in the liquid state poses problem during solidification. When a homogeneous, single-phase liquid is cooled into the miscibility gap, the components are no longer miscible and two liquid phases develop. Generally, the liquid-liquid decomposition causes the formation of the microstructure with serious phase segregation. Recent researches demonstrate that the only effective method to prevent the formation of the phase segregated microstructure in the immiscible alloys is using the rapid or sub-rapid solidification processing techniques. Laser surface treatment is a well known rapid solidification technique. It is widely applied in the industry to improve the surface properties of different type of alloys. But up to date, the solidification behaviors of the monotectic alloys under the laser surface treatment conditions have not been investigated. In this work, laser surface treatment experiments with Al-Pb alloys were carried out. A numerical model was developed to describe the microstructure evolution in the surface layer of Al-Pb alloys under the conditions of laser surface treatment. The model was applied to calculate the microstructure formation in the surface layer. The numerical results have a good agreement with the experimental ones. Both of the numerical and experimental results indicate that the microstructure of the laser treated surface layer is determined by the re-melting, composition homogenization and solidification of the alloy. Laser surface treatment can lead to the formation of an Al-Pb surface layer with well dispersed microstructure.

    References | Related Articles | Metrics
    LIQUID METAL FLOW DRIVEN BY A MODULATED HELICAL MAGNETIC FIELD
    WANG Songwei, WANG Xiaodong, NI Mingjiu, ZHANG Xinde, WANG Zenghui,NA Xianzhao
    Acta Metall Sin, 2013, 49 (5): 544-552.  DOI: 10.3724/SP.J.1037.2012.00690
    Abstract   PDF (5831KB) ( 653 )

    Magnetic fields and electromagnetic forces have long been used to control the flow of a solidifying melt. In this study, low-frequency modulated electromagnetic stirring was provided by a helical permanent magnet stirrer, which can be considered as superposition of traveling and rotating magnetic fields. Flow behaviors of liquid metal driven by this helical magnetic field was investigated. Moreover, the rotating direction of the helical magnetic field was periodically reversed to form a modulated helical magnetic field. The helical magnetic field was constructed on some units of the permanent magnets magnetized in their radius directions. The liquid metal was driven by the Lorentz force via this rotating magnetic stirrer. The azimuthal and axial velocity distribution of liquid GaInSn alloy was quantitatively measured using an ultrasonic Doppler velocimetry (UDV), which revealed the time-dependent flow structure and flow pattern varying with the modulation parameters: the rotating speed of the magnetic stirrer and the modulation frequency. The main results of the velocity measurement of the liquid metal were as followed: the  azimuthal velocity of the screw flow was periodically reversed with the same modulated frequency, and the flow intensity gradually saturated when the modulation period Tm40 s; The axial flow resulted from the competition of large vertical vortex driven by the traveling component of the helical magnetic field and the secondary flow driven by the rotating component of the helical magnetic field. There was a critical modulated period Tm* with respect to the reversed characteristic and flow intensity. When Tm<Tm*, the axial velocity exhibited typically reversed flow, when Tm > Tm*, the secondary flow appeared and gradually dominated in the axial flow pattern. The prospective goals of the present study is to develop proper magnetic fields, which can generate a three-dimensional modulated metal flow in front of solidifying front to adapt the varying casting slabs. The role of electromagnetically driven flow is to transport the solute rejected by the solidifying interface at significant distances in the melt, and to periodically reverse its circulation so that macrosegregation is minimized.

    References | Related Articles | Metrics
    CONTROL OF STRING SHAPED NON-METALLIC INCLUSIONS OF CaO-Al2O3 SYSTEM IN X80 PIPELINE STEEL PLATES
    WANG Xinhua, LI Xiugang, LI Qiang, HUANG Fuxiang,LI Haibo, YANG Jian
    Acta Metall Sin, 2013, 49 (5): 553-561.  DOI: 10.3724/SP.J.1037.2012.00505
    Abstract   PDF (5133KB) ( 1414 )

    The string shaped B type non-metallic inclusions in API (American Petroleum Institute) X80 pipeline steel plates, produced by the BOF-LF-RH-Ca treatment steelmaking route, were mainly of CaO-Al2O3 system with lower melting temperatures. The formation reasons are as follows: (1) there existed many small sized liquid inclusions of CaO-Al2O3 system in liquid steel after the secondary refining and Ca treatment. These small inclusions could aggregate to larger ones of 10- 20μm in continuous casting and be deformed into string shaped inclusions in steel plates during rolling. (2) for the larger sized and low melting temperature CaO-Al2O3 inclusions, in Ca treatment, their surface layers could be modified into high melting temperature CaO, CaS or CaO-CaS system, but the centers remained to be CaO-Al2O3 system. During rolling, these inclusions could also be elongated to string typed ones because of their soft CaO-Al2O3 centers. A new strategy for control of B type  inclusions in X80 pipeline steel plates was adopted. The key of the control was shifted from removing low melting temperature inclusions of CaO-Al2O3 system after Ca treatment to remove as much as possible inclusions especially large sized inclusions before the Ca treatment. With the new strategy, the amount of inclusions after RH refining was remarkably decreased and the efficiency of Ca treatment significantly improved. The non-metallic inclusions found in steel plates were all of high melting temperature CaO-CaS system and theseverity of B-type inclusions has been lowered from ≤2.0 to 0.

    References | Related Articles | Metrics
    STUDY ON MICROSTRUCTURE AND PRECIPITATES AT DIFFERENT NORMALIZING IN Fe-3.15%Si LOW TEMPERATURE ORIENTED SILICON STEEL
    LI Hui, FENG Yunli, QI Xuejing, CANG Daqiang, LIANG Jinglong
    Acta Metall Sin, 2013, 49 (5): 562-568.  DOI: 10.3724/SP.J.1037.2012.00644
    Abstract   PDF (3841KB) ( 803 )

    The decreasing of slab heating temperature for grain-oriented silicon steel will reduce the amount of precipitates in hot rolled plate, and be disadvantage to the formation of ultimate Goss texture. The aim of normalizing is to control and adjust the amount, size and distribution of precipitates. Microstructures, precipitates and magnetic characteristics of finished products with different normalizing technologies for Fe-3.15%Si low temperature hot rolled grain-oriented silicon steel are researched, and the textures of cold rolled plates which are  original hot rolled plate and normalized plate are analyzed by means of OM, TEM, EDS and XRD, respectively. The results show that, normalizing technology with a temperature of 1120 ℃, holding 3 min, and a two-stage cooling is a most advantaged to obtain oriented silicon steel with sharper Goss texture and higher magnetic, owing to the uniform surface microstructures and the obvious inhomogeneity of microstructures along the thickness; the normalizing technology with two-stage cooling is the optimum process, which due to more finer precipitates are dispersively distributed in the matrix, and it is beneficial for finished products to get a higher magnetic; in these two processes, they obtain the same textures  which are mainly consist of {111}<110> and {111}<112>, however,comparing with the cold rolled textures without normalizing, the texturesdensity of γ orientation line on cold rolled plate which treated by normalizing are significantly increased.Therefore, it is confirmed that normalizing is helpful for grain--oriented silicon steel to get sharper Goss texture.

    References | Related Articles | Metrics
    STUDY ON Cu PRECIPITATE OF THE LOW C HIGH STRENGTH STEEL CONTAINING Cu AND Ni DURING ISOCHRONAL TEMPERING
    YU Ximo, ZHAO Shijin
    Acta Metall Sin, 2013, 49 (5): 569-575.  DOI: 10.3724/SP.J.1037.2012.00666
    Abstract   PDF (2789KB) ( 799 )

    Cu precipitation strengthening plays an important role in the fabrication of high-strength low-alloy (HSLA) steels. The nature of Cu precipitation and the actual distributional morphology of Cu precipitates have a significant effect or directly determine the strength and toughness of HSLA steels. HSLA steel is weldable without preheat by reducing C to a low concentration. To compensate for the decrease of strength caused by reducing C, Cu was added to HSLA steel for precipitation strengthening by nanoscale Cu precipitates. The size, number density and composition of Cu nanophases could be well characterized by the atom probe tomography (APT), and the Cu nanophases obtained by APT analysis are usually termed Cu clusters. In the study, the specimens were austenitized for 30 min at 900 ℃followed by water quenching, and tempered isochronally for 60 min at different temperatures. The hardness was conducted, the microstructure and Cu precipitate were analyzed by HRTEM and APT. During tempering, Cu precipitation happened, Cu precipitate Moire fringe formed and the Cu precipitate transformed to fcc structure; the lath boundary gradually bulged out and migrated, a repeat of bulging and migration of local parts of lath boundary resulted in migration of the whole boundary, and lath martensite transformed to equiaxed ferrite finally. At 500℃, the strengthening peaked by Cu precipitates. During 400-500℃, the number of Cu clusters changed greatly when the Cu isoconcentration set at different values, this indicated that the Cu precipitates  were on the stage of nucleation; while the number of Cu clusters changed little during 500-650 ℃, this indicated that the Cu precipitates were on the stage of coarsening. The Cu, C, Mo and P segregated at the grain boundary. The boundary could provide Cu solutes and nucleation sites for Cu precipitation, leading to the segregation of Cu clusters at the grain boundary. The Ni, Mn and Al segregated at the heterophase interface between Cu precipitate and ferrite matrix forming a core-shell structure.

    References | Related Articles | Metrics
    FRACTURE TOUGHNESS OF WELDED JOINTS OF X100 HIGH-STRENGTH PIPELINE STEEL
    BI Zongyue, YANG Jun, NIU Jing, ZHANG Jianxun
    Acta Metall Sin, 2013, 49 (5): 576-582.  DOI: 10.3724/SP.J.1037.2012.00703
    Abstract   PDF (3553KB) ( 1256 )

    Fracture toughness of base metal, weld seam and heat-affected-zone (HAZ) in X100 high-strength pipeline steel welded joints was investigated by three-point crack tip opening displacement (CTOD) test. Microstructure and inclusion near fracture zones were observed by means of SEM and TEM. The results indicated that fracture toughness of X100 high-strength pipeline steel welded joints was greatly influenced by test temperature. At the same temperature, the numerical values of apparent crack initiation δ0.05, conditional crack initiation δ0.2 and δ0.2BL of base metal are higher than that of weld seam and HAZ, and low temperature fracture toughness of base metal is better than those of weld seam and HAZ. With temperature decreasing, the fracture toughness of base metal, weld seam and HAZ decreased. The microstructure of near fracture zones of base metal specimen was composed of granular bainite (GB), a small quasi polygonal ferrite (QF) and lath bainite ferrite (BF), and the fine and equally dispersed M-A structure distributed on the grain boundary. The microstructure of near fracture zones of weld seam specimen was composed of acicular ferrite (AF), and the form of M-A constituents shows diversity, sharp-angled clearly. The microstructure  of near fracture zones of coarse-grain HAZ was composed of GB and parallel LBF, and the square, wedged and bar M-A constituents distributed on the interior of grain, grain boundary and lath boundary. The poor fracture toughness of weld seam and HAZ specimen results from large size and cusp type M-A structure. While the higher distribution of inclusion in weld seam makes fracture toughness worse.

    References | Related Articles | Metrics
    EFFECTS OF AUSTENITIZATION AND COOLING RATES  ON THE MICROSTRUCTURE IN A HYPEREUTECTOID STEEL
    LI Junjie, Godfrey Andrew, LIU Wei
    Acta Metall Sin, 2013, 49 (5): 583-592.  DOI: 10.3724/SP.J.1037.2012.00699
    Abstract   PDF (4049KB) ( 939 )

    Cold-drawn pearlitic steel wires have the highest strength of all steel products. It is a promising way to enhance the mechanical properties by increasing the carbon content. However, the proeutectoid cementite forms easily due to the hypereutectoid composition and deteriorates the mechanical and processing properties of steel wires. It is important for hypereutectoid steel wire drawing to achieve a fine and fully pearlitic microstructure without proeutectoid cementite. The austenitization and following continuous cooling process were simulated in the dilatometer for a hypereutectoid steel. The microstructure was observed with OM and SEM. The transformation temperature, prior--austenite grain size, pearlitic interlamellar spacing and proeutectoid cementite thickness were determined by dilatometric curves or OM/SEM images. The austenite grain size increases rapidly with a higher temperature and almost keeps invariant with a longer austenitization time. Faster cooling rate, higher austenitization temperature or longer austenitization time decrease the starting and finishing temperature of phase transformation, widen the temperature range, refine the pearlitic interlamellar spacing and suppress the proeutectoid cementite precipitation (reduce the thickness or make it discontinuous). However, it is easy to form martensite which is bad for the homogeneity of pearlitic microstructure by increasing the cooling rate or austenitization temperature simply. A fine pearlite in a pseudoeutectoid microstructure is achieved by extending the austenitization time to 60 min and controlling the austenitization temperature and cooling rate. Discontinuous proeutectoid cementite is observed in the samples with the higher austenitization temperature. Higher austenitization temperature and longer time are helpful to weaken the carbon concentration gradient. The homogeneous carbon distribution restrains carbon diffusion for cementite nucleation during the proeutectoid cementite precipitation and pearlite transformation, which decreases the transformation temperature. Compare to pearlite transformation, the proeutectoid cementite precipitation is affected more strongly by the carbon diffusion due to a longer diffusion distance. Therefore, the precipitation amount of proeutectoid cementite is reduced if the carbon diffusion is restrained. The amount of grain corner and grain edge reduce more dramatically than that of grain boundary if austenite grain size increases. The proeutectoid cementite only nucleates in grain corner and grain edge. The pearlite nucleation can form in grain boundary. Therefore, the larger austenite grains result in the sharp reduction in the sites for nucleation of proeutectoid cementite. Then, the amount of proeutectoid cementite are reduced and the morphology becomes discontinuous.

    References | Related Articles | Metrics
    STABILIZATION AND CORROSION RESISTANCE UNDER HIGH-TEMPERATURE OF NANOSTRUCTURED CeO2/ZrO2-Y2O3 THERMAL BARRIER COATING
    GONG Wenbiao, LI Renwei, LI Yupeng,SUN Daqian, WANG Wenquan
    Acta Metall Sin, 2013, 49 (5): 593-598.  DOI: 10.3724/SP.J.1037.2012.00702
    Abstract   PDF (1580KB) ( 665 )

    Thermal barrier coating (TBC) systems are being used in thermal insulation components in the hot sections of gas turbines in order to increase operation temperature with better efficiency. The typical material of TBC is yttria stabilized zirconia (YSZ) because of its high thermal expansion coefficient, which closely matches that of the substrate, and low thermal conductivity. However, YSZ based TBC systems cannot be successfully applied due to hot corrosion problems caused by molten salts, such as Na, S and V, contained in low quality fuels. In recent years, nanostructured TBC attracted intense attentions due to their enhanced thermal physical properties, but the thermal stability and resistance to molten salt performance are rarely studied.As a new candidate TBC material, ceria and yttria stabilized zirconia currently looks to be promising. The purpose of this work was to obtain the better understanding of microstructure and molten salt corrosion capability of plasma-sprayed nanostructured CSZ coating and to provide some foundation for improving the properties of TBC. In this work, nano-sized ZrO2-8%Y2O3 (YSZ, mass fraction) and nano-YSZ doped with 25% of nanometer CeO2 (CeO2/ZrO2-8%Y2O3, CSZ) were deposited on GH30 superalloy surface through air plasma spray process (APS) to form a thermal barrier coating. The morphology and microstructure of the CSZ coating were characterized using FESEM and XRD. The grain size of CSZ coating under the following two conditions were examined, firstly, the CSZ coating was heated to 1100 ℃ and held for various durations, thenthe CSZ coating was heated for a fixed 10 h but up to various temperatures. Its corrosion resistance under high temperature molten Na2SO4 salt was also tested. The results showed that average grain size of CSZ coating grown from 45 to 63 nm under prolonged exposure to high temperature and CSZ has showed better corrosion resistance than YSZ coating with no m-ZrO2 phase precipitated at under prolonged high temperature at 900 ℃ in Na2SO4 salt corrosion.

    References | Related Articles | Metrics
    FORMATION OF NANOCRYSTALLINES IN THE SURFACE LAYER OF COMMERCIAL PURE TITANIUM THIN SHEET DURING ASYMMETRIC ROLLING
    LIU Gang, LIU Jinyang,WANG Xiaolan,WANG Fuhui,ZHAO Xiang,ZUO Liang
    Acta Metall Sin, 2013, 49 (5): 599-604.  DOI: 10.3724/SP.J.1037.2012.00733
    Abstract   PDF (2081KB) ( 537 )

    Commercial pure titanium sheet was rolled by means of asymmetric rolling in order to obtain nanocrystallines in the top-surface layer, the microstructural evolution was examined by using different experimental techniques. Experimental results show that, after the asymmetric rolling, equiaxed nanocrystallines of about 30-60 nm in size form in the top-surface layer of sheet. The research work indicates that the surface nanocrystallization can be realized for large-dimensional metal sheets during the rolling process by using the asymmetric rolling. The asymmetric rolling induced surface nanocrystallization mechanism was summarized as follows: upon the application of repeated loads, dislocation cells/sub-micro-grains form through slips, annihilations and recombinations of high density of dislocations; with the increment of rolling pass and reduction,high density of dislocations in the refined cells/grains developing in above route lead to the reduction of grain size and the increment of misorientations between the refined grains, and finally equiaxed nanocrystallines with medium to large angle grain boundaries form. A gradient-structured surface layer of about 30μm thick was observed to form in the initial stage,its thickness increases with increasing rolling reduction, and the hardness variation along the depth was found to relate to the grain size.

    References | Related Articles | Metrics
    MICROSTRUCTURE OF HEAT-AFFECTED ZONE OF LASER FORMING REPAIRED 2Cr13 STAINLESS STEEL
    XU Qingdong, LIN Xin, SONG Menghua, YANG Haiou, HUANG Weidong
    Acta Metall Sin, 2013, 49 (5): 605-613.  DOI: 10.3724/SP.J.1037.2012.00708
    Abstract   PDF (5423KB) ( 1271 )

    2Cr13 martensitic stainless steel is widely used in the manufacturing of heavy load components,which are easy to be damaged due to their severe service environment. If these damaged components can be repaired rapidly, considerable savings in materials, processing and time costs can be achieved. Four kinds of laser forming repairing for 2Cr13 stainless steel sample, single-track single-layer, multi-track single-layer, single-track multi-layer and multi-track multi-layer, was conducted to investigate their microstructure characteristic and evolution of heat-affected zone (HAZ). The formation mechanism of microstructure was analyzed based on the temperature field simulation. It is found that microstructure varies continuously from substrate zone (SZ) to the bottom of laser repaired zone (RZ), in which the main phases varied asα ferrite→α ferrite+ martensite →martensite+α ferrite→martensite, and the appearance of the martensite led to a rapid increase in hardness. Meanwhile, the primary M23C6 dissolved gradually and disappeared eventually. It is interesting to note that the dissolving of intragranular carbides occurred prior to the intergranular carbides. With the carbides dissolving, δ ferrite particles appeared, coarsened and connected into skeleton patterns eventually when closing to the bottom of RZ. As the deposited layers increased, the hardness peak decreased, and the grains were refined in the partial region of the middle of HAZ. The carbides precipitated again in the grain boundary at the top of HAZ, meanwhile, δ skeleton is gradually interrupted by the grain boundary.

    References | Related Articles | Metrics
    SURFACE LAYER NANO-MECHANICAL RESPONSES TO INTERACTION BETWEEN CAVITATION AND ELECTROCHEMICAL CORROSION
    ZHANG Ru, SHEN Hanjie, ZHANG Yaqin, LI Dongliang, LI Yanjia, YONG Xingyue
    Acta Metall Sin, 2013, 49 (5): 614-620.  DOI: 10.3724/SP.J.1037.2012.00716
    Abstract   PDF (1089KB) ( 681 )

    Cavitation corrosion resistances of metals have a very much closed relationship with the mechanical properties of their surface layer. It is very important to investigate effects of surface layer mechanical properties on cavitation corrosion to understand synergistic mechanism. Nano-indentation technology is a sensitively power tool for measuring surface layer mechanical properties in nanometer scale. In this work, it was used to study the effects of anodic polarization on the surface layer nano-mechanical properties (nano-hardness, Hnano and nano-elastic modulus, Enano) of austenitic stainless steel under cavitation. The synergistic mechanism of cavitation corrosion caused by anodic polarization was also investigated by weight loss in conjunction with SEM. The surface layer comprehensive nano-mechanical parameter was defined as (H/E)nano. It was found that the profiles of Hnano, Enano and (H/E)nano with displacement into surface (L) are very different at different anodic polarized potentials. Hnano and (H/E)nano decrease and Enano increases with the increment of logarithm of anodic current density. When the samples under cavitation were polarized at passive region, cavitation corrosion of austenitic stainless steel is mainly controlled by erosion. However, it is mainly dominated by  corrosion-induced erosion and erosion--induced corrosion, respectively, if anodic polarized potentials were controlled at the beginning of passive to super-passive and super-passive regions. The surface layer nano-hardness is a key factor dominating cavitation corrosion resistances of metals. During the interaction of cavitation with electrochemical corrosion, mass loss of corrosion-induced erosion, which is called non-Faraday weight loss, increases as a result of electrochemical corrosion. The corresponding corroded morphologies with microgrooves, micro-holes and micro-pits were observed.

    References | Related Articles | Metrics
    EFFECTS OF Al AND Zn ADDITION ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF CAST Mg-5Sn ALLOY
    DONG Xuguang, FU Junwei, YANG Yuansheng
    Acta Metall Sin, 2013, 49 (5): 621-628.  DOI: 10.3724/SP.J.1037.2012.00720
    Abstract   PDF (3870KB) ( 806 )

    A cast Mg-5Sn-4Zn-2Al alloy was developed by adding 2%Al and 4%Zn (mass fraction) into a Mg-5Sn base alloy. The results showed that the combined addition of Al and Zn had more obvious effect on dendrite refinement than the single Al addition. The single Al addition remarkably increased the elongation from 6.6% to 22.4%. With further addition of Zn, Mg32(Al, Zn)49 phase was introduced into the solidified microstructure, and the yield strength and ultimate tensile strength were increased to 96 and 232 MPa while elongation was decreased to 14.8%. After aging treatment at 175 ℃ for 24 h, there appeared peak hardness of 83.5 HV by forming the rod-shaped MgZn2 along c-axis and cubic Mg2Sn precipitates into α-Mg matrix for Mg-5Sn-4Zn-2Al alloy. With the aging strengthening, the yield strength and ultimate tensile strength were further increased to 144 and 264 MPa, respectively. When the testing temperature was elevated to 150 ℃, the yield strength of peak-aged Mg-5Sn-4Zn-2Al alloy still attained to 138 MPa, which indicated that both the MgZn2 and Mg2Sn precipitates possess good thermal stability for elevated temperature property.

    References | Related Articles | Metrics
    FABRICATION OF SUPERHYDROPHOBIC Ti SURFACE BY THERMAL OXIDATION AND ITS ANTICORROSION PROPERTY
    KANG Zhixin, GUO Mingjie
    Acta Metall Sin, 2013, 49 (5): 629-634.  DOI: 10.3724/SP.J.1037.2013.00027
    Abstract   PDF (1424KB) ( 571 )

    Ti and its alloys, due to their good stability and high strength-to-density ratio, have been widely used in many industry fields, such as aviation, navigation, biomedical devices, etc. It is quite necessary to improve their performance against corrosion of water pollution or other corrosive mediums in these fields. The process of thermal oxidation is an effective way to enhance their corrosion resistance while high-temperature oxidation is usually thought to have detrimental effects. However, the porous structure caused by high-temperature oxidation is found to be beneficial for preparation of superhydrophobic surface, which has gotten extensive application in improvement of metals' antcorrosion ability. In this study, a rough surface with hierarchical micro- and nano- structures was formed on Ti by a heat treatment process in atmospheric environment at 1000 ℃ for 1 h. The following air-cooling process separated the flaky yellow oxide layer formed on Ti plate during the oxidation from the substrate and a grey porous substrate (TO) was obtained. Furthermore, TO was modified with n-octadecyltrichlorosilane (OTS), leading to the formation of superhydrophobic Ti surface (TO-OTS). The TO-OTS film exhibited a static contact angle of 166.0° and a rolling angle of 2.0° for 5 μL water droplets. The as-prepared film was characterized by XRD, FE-SEM, XPS and contact angle measurements. The results indicated that dual-scale roughness leaved by thermal oxidation endowed TO-OTS with excellent non-sticking superhydrophobicity and durability, even for some corrosive liquids including salt solution and acidic and alkali solutions at different pH values. By means of immersion test, TO-OTS displayed great non corrodibility against HF solution, with a protective mirror-like air film formed above it. Moreover, based on potentiodynamic polarization measurement in 3.5%NaCl solution, the corrosion resistance of TO-OTS was proved to have a significant enhancement with a protection efficiency of 99.1%. This is a facile method for preparation of large-scale or complex shaped superhydrophobic surfaces without requirement of expensive instrument, which may provide an effective protection for Ti under harsh environment.

    References | Related Articles | Metrics
    MICROSTRUCTURE DEPENDENCE OF DIRECTCURRENT ELECTRODEPOSITED BULK Cu WITH PREFERENTIALLY ORIENTED NANOTWINS ON THE CURRENT DENSITIES
    JIN Shuai, PAN Qingsong, LU Lei
    Acta Metall Sin, 2013, 49 (5): 635-640. 
    Abstract   PDF (2175KB) ( 594 )

    Bulk Cu with preferentially oriented nanoscale twins was synthesized by means of direct-current electrodepositon. The nanotwinned Cu sample is composed of columnar grains with high density nanoscale coherent twin boundaries, most of which are parallel to the growth plane. It was found that the current densities play an important role on the grain size of the nanotwinned Cu samples, however, a non-obvious effect was found on the twin lamellar thickness. With increasing the current densities from 10 mA/cm2 to 30 mA/cm2, the average grain size of the bulk nanotwinned Cu samples was decreased from about 10.1 μm to 4.2 μm. While the twin lamellar thickness was unchanged obviously and in the range of 30-50 nm. This is because that with the increase of current density, the overpotential of cathode increases and the grain refinement occurs.

    References | Related Articles | Metrics