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

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    Original Article
    EFFECT OF ALUMINUM ON THE SOLIDIFICATION MICROSTRUCTURE OF M2 HIGH SPEED STEEL
    ZHOU Xuefeng , FANG Feng , TU Yiyou , JIANG Jianqing , XU Huixia , ZHU Wanglong
    Acta Metall Sin, 2014, 50 (7): 769-776.  DOI: 10.3724/SP.J.1037.2013.00621
    Abstract   HTML   PDF (8944KB) ( 2375 )
    The effect of aluminum on the solidification microstructure of M2 high speed steel, particularly the morphology and microstructure of eutectic carbides, has been investigated by OM, TEM, SEM, EBSD and XRD. The results show that the as-cast microstructure consists of dislocation martensite and M2C eutectic ledeburite. Excessive amount of aluminum, 1.2%, favors the formation of ferrite and needle-like carbides. After the addition of aluminum, eutectic carbides are distributed more homogeneously. Additionally, the morphology of M2C eutectic carbides transforms from the fibrous to the plate-like, and their microstructure also changes significantly. The plate-like M2C has crystal defects, such as micro-twins and stacking faults, and different growing orientation between adjacent plates whereas the fibrous carbides have few defects and single crystal orientation. Compared to fibrous carbides, the plate-like carbides are much difficult to get spheroidized at high temperature, which is unfavorable for carbide refinement. The ferrite, formed by adding excessive amount of aluminum, cannot be eliminated by ordinary heat treatments, decreasing remarkably the hardness of high speed steel after quenching.
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    INFLUENCE OF MICROSTRUCTURE ON IMPACT TOUGHNESS OF G18CrMo2-6 STEEL DURING TEMPERING
    LI Zhenjiang , XIAO Namin , LI Dianzhong , ZHANG Junyong , LUO Yongjian , ZHANG Ruixue
    Acta Metall Sin, 2014, 50 (7): 777-786.  DOI: 10.3724/SP.J.1037.2013.00747
    Abstract   HTML   PDF (9571KB) ( 1067 )
    Low alloy CrMo steels are widely used for high temperature applications in the power and petrochemical industries as structural materials. The most crucial mechanical properties for these steels are sufficient strength to withstand internal pressure and high impact toughness to assure safety from momentary shock owing to unexpected accidents. Particularly, impact toughness deteriorates because of continuous high temperature during the operation of a high pressure vessel and embrittlement can occur. Thus, the use of steels with high impact toughness is extremely important to guarantee sufficiently the safe operation of the nuclear reactor. Usually low alloy CrMo steels enter service after the normalized and tempered treatment or annealed treatment with a mixed ferrite-bainite or full bainite microstructure. The G18CrMo2-6 steel is one of the most popular materials with the mixed microstructures of ferrite and bainite for the pressure vessel in nuclear industry due to its good impact toughness, high strength and good creep resistance. In this work, the influence of microstructures, including the parent phases and precipitates, on the impact toughness is investigated in detail. The experimental results show that the constituent of the parent phases, namely the ferrite, pearlite or bainite, is not the reason resulting in the ultra-low impact energy. The microstructure characterization implies that the morphology and the distribution of precipitates play the key role in controlling the impact toughness of the G18CrMo2-6 steel. The lower tempering temperatures result in the blocky martensite/austenite (M/A) island and lathy M3C carbides with the large particle size. The finely granular M3C carbides with the uniform distribution on the bainite matrix can be found at the higher tempering temperatures. As the tempering temperature increased, the Charpy absorbed energy at room temperature increased. After the tempering below 600 ℃, Charpy absorbed energy has the ultra-low value of 17 and 29 J. Generally speaking, the weak softening of matrix during the lower tempering temperature increases the accumulative residual stress at particle-ferrite interface. The other important factor should be attributed that the blocky M/A island and lathy M3C carbides result in the lower critical fracture stress of a particle-ferrite interface.
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    MICROSTRUCTURES AND MECHANICAL PROPERTIES OF SPRAY FORMED H13 TOOL STEEL
    ZHANG Jinxiang , HUANG Jinfeng , WANG Hebin , LU Lin , CUI Hua , ZHANG Jishan
    Acta Metall Sin, 2014, 50 (7): 787-794.  DOI: 10.3724/SP.J.1037.2013.00820
    Abstract   HTML   PDF (14552KB) ( 525 )
    H13 steel is well-established for hot-work applications in different parts of the world, combining good hot strength, plasticity, toughness, oxidation resistance and thermal fatigue resistance due to its reasonable design of alloying elements. However, the alloying elements segregation and coarse primary carbides are serious and unavoidable in traditional cast H13 steel, which develops into band structure after the subsequent hot-working process, causing a poor isotropy, and finally reduces the service life of tool. Spray forming, as one of the rapid solidification technologies, fills the gap between casting and powder metallurgy, combining advantages of rapid solidification, and gaining a fine grained microstructure without macro-segregations. H13 steel was prepared by traditional casting and spray forming respectively, subsequently forged and conventional heat treated. The microstructures of as-cast and spray-formed H13 steels at various stages of processing were studied by OM, SEM and XRD. Mechanical properties were tested at ambient and elevated temperatures for both steels under the same heat treatment processes. The microstructure of as-cast H13 steel is characterized by coarse dendritic structure and primary carbides, while spray forming resulted in a much refined equiaxed grain structure without segregation of carbides. Meanwhile, the as-deposited H13 steel contains higher volume of austenite than that of the as-cast H13 steel, induced by the high cooling rate during the atomization process in spray forming. The density of the spray-formed H13 billet before and after hot-forging process are 98.2% and 99.7% respectively, indicating pores are completely eliminated. Compared to the as-cast H13, the spray-formed H13 has better temper resistance, higher room temperature tensile strength and hot strength. Moreover, the spray-formed H13 increases the room temperature impact toughness two times than that of the as-cast H13. The band structure which is obvious in the cast H13 steel can hardly be seen in the spray-formed H13 steel, thereby better isotropy can be expected in the spray-formed H13 steel. The improvement of mechanical properties of the spray-formed H13 can be attributed, to a large extent, to the refined structures without coarse primary carbides and macro-segregation, leading more alloying elements dissolved into the matrix, distributed more uniformly. Consequently, the secondary precipitates during tempering are more uniform and the grains are finer.
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    RECALESCENCE EFFECT SIMULATION AND MICROSTRUCTURE EVOLUTION OFUNDERCOOLED Fe82B17Si1 ALLOY
    CHEN Zheng , YANG Yanan , CHEN Qiang , XU Junfeng , TANG Yueyue , LIU Feng
    Acta Metall Sin, 2014, 50 (7): 795-801.  DOI: 10.3724/SP.J.1037.2013.00813
    Abstract   HTML   PDF (4897KB) ( 1007 )
    Eutectic alloys (e.g. Fe-B-Si) as important casting alloys are the most commonly used material in industrial production. Fe-B-Si eutectic alloy is a kind of soft magnetic materials, has the very important application prospect in the market. In the past few decades, research on eutectic solidification alloys has achieved important results. The microstructure evolution, nucleation and growth theory of Fe-B-Si eutectic alloy under non-equilibrium condition have been perfected. But the recalescence effect on microstructure evolution has not been accurately described theoretically. The microstructure evolution of undercooled Fe82B17Si1 alloy in the obtained undercooling range ΔT=6~280 K were investigated by employing the glass fluxing technique in combination with cyclical superheating. The cooling curves of solidification process were fitted in combination of break equation and JMAK model, and the calculated results were consistent with the evolution of pattern of organization and microstructure of Fe82B17Si1 eutectic alloy. When 6 K≤ΔT<75 K, Fe82B17Si1 alloy was made of complex rules eutectic and eutectic mixture of quasi regular eutectic. When 75 K≤ΔT<180 K, the solidification microstructure was composed of a mixture of eutectic and deeply undercooled irregular eutectic. When 180 K≤ΔT<250 K, the solidification microstructure consists of primary a-Fe phase as well as the irregular eutectic among dendrite. When ΔT >250 K, the solidification structure was completely non-eutectic organization.
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    EFFECT OF SO2 ON CORROSION EVOLUTION OF Q235B STEEL IN SIMULATED COASTAL-INDUSTRIAL ATMOSPHERE
    CHEN Wenjuan, HAO Long, DONG Junhua, KE Wei, WEN Huailiang
    Acta Metall Sin, 2014, 50 (7): 802-810.  DOI: 10.3724/SP.J.1037.2013.00738
    Abstract   HTML   PDF (1251KB) ( 965 )
    The atmosphere in many cities along the coastal lines such as Qingdao in China has been polluted with SO2 due to the development of industry, and has been changed to coastal-industrial atmosphere. The corrosion behavior and mechanism of steel in coastal-industrial atmosphere with the co-existence of SO2 and Cl- is different from that in the coastal atmosphere containing only Cl- or the industrial atmosphere containing only SO2. It is necessary to study the corrosion mechanism of steel in the coastal-industrial atmosphere. However, almost all the atmospheric corrosion data of steels was obtained by the field exposure test, which could not reflect the dependence of SO2 or Cl- on the atmospheric corrosion evolution of steels due to the difficulties in controlling the field conditions. So the effect of SO2 on the corrosion evolution of Q235B steel in the simulated coastal-industrial atmospheres was investigated by the dry/wet cyclic corrosion test (CCT), as well as XRD, electrochemical impedance spectroscopy (EIS) and polarization curve measurements. The results indicate that in the initial stage, the SO2 inhibits corrosion of Q235B steel. During the later stage, the corrosion rate of the steel increases with increasing the SO2 concentration to a certain level but further increasing the SO2 concentration contributes to a decreased corrosion rate. Besides, higher SO2 concentration can promote the formation of a-FeOOH while inhibit the formation of g-FeOOH and b-FeOOH. Evolution of Q235B steel corrosion rate in the atmosphere with SO2 concentration is due to the change of corrosion products.
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    CORROSION MECHANISM OF OCTG CARBON STEEL IN SUPERCRITICAL CO2/OIL/WATER SYSTEM
    SUN Chong , SUN Jianbo , WANG Yong , WANG Shijie , LIU Jianxin
    Acta Metall Sin, 2014, 50 (7): 811-820.  DOI: 10.3724/SP.J.1037.2013.00812
    Abstract   HTML   PDF (9636KB) ( 647 )
    The supercritical CO2 corrosion problems of oil country tubular goods (OCTG) become increasingly prominent along with the application of CO2 flooding enhanced oil recovery (EOR) technique and the exploitation of deep oil wells under high temperature and high pressure. Actually, OCTG steel often suffers from multiphase fluid corrosion of crude oil, water and supercritical CO2 at different stages of oil and gas production. However, studies about CO2 corrosion of carbon steel used for oil and gas production generally are carried out considering only the aqueous phase without proper consideration of the oil phase that may be present. The crude oil in crude oil/water production environments is a key factor affected the corrosion behavior of carbon steel. Corrosion rate and corrosion type of J55 steel were investigated under the conditions of different oil/water ratios saturated with supercritical CO2. SEM, EDS and XRD were employed to analyze the morphology and characteristic of corrosion scale on the steel. The corrosion models were developed to understand the corrosion mechanism with consideration of a variation of oil/water ratio in reality. The results show that uniform corrosion occurs along with the lower corrosion rate due to the protection of crude oil when the water cut of crude oil is within the range, i.e., 0~30% and the water-in-oil fluid is formed. But the local corrosion rate of the steel increases rapidly due to the inhomogeneous adsorption of crude oil with the fluid changing from water-in-oil to oil-in-water emulsion when the water cut is between 30% and 75%. The corrosion products deposited on the steel surface change the wettability of oil and water phase, therefore, water phase can preferentially wet the localized deposited scale, leading to the development of pitting corrosion under the scale. However, when the water cut is higher than 75% and the oil-in-water fluid is formed, water phase infiltrates the metal surface that blocks the corrosion inhibition of crude oil for the steel, hence, the corrosion rate increases dramatically. The localized failure of corrosion scale due to scouring action of the fluid and the dissolution of aggressive medium leads to mesa corrosion on the steel. When the water cut is 100%, serious uniform corrosion occurs as a result of the strong corrosiveness of supercritical CO2 dissolved in the water phase. Furthermore, crude oil can weaken the dissolution of corrosion scale in the supercritical corrosive medium, which modifies the grain size, morphology and chemical composition of corrosion scale and improves the protection performance of the scale.
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    INTEGRATED SIMULATION OF THE FORGING PROCESS FOR GH4738 ALLOY TURBINE DISK AND ITS APPLICATION
    LI Linhan, DONG Jianxin, ZHANG Maicang, YAO Zhihao
    Acta Metall Sin, 2014, 50 (7): 821-831.  DOI: 10.3724/SP.J.1037.2013.00675
    Abstract   PDF (8158KB) ( 909 )
    In order to control the grain size of forged turbine disk of wrought superalloy like GH4738 more effectively, constitutive equations and grain structure evolution models of GH4738 alloy are used in Deform 3DTM for achieving integrated simulation of whole forging process of GH4738 alloy turbine disk (from preheating billet for upsetting to die forging). By using of integrated simulation, the variation of temperature, average grain size, etc., during the whole forging process has been explored, making it possible to control these parameters quantitatively. Comparing with traditional simple stage simulation, results of integrated simulation are more consistent with corresponding experimental results of forged turbine disk (300 mm in diameter). Therefore, the reliability of the integrated simulation is verified. Finally, with the application of integrated simulation, GH4738 alloy turbine disk with a diameter of 1450 mm has been successfully forged by 8×104 t forging press. This work provides a more practical simulation method for helping the process design of forging large turbine disk.
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    FORMATION AND EVOLUTION OF PRECIPITATE IN TiAl ALLOY WITH ADDITION OF INTERSTITIAL CARBON ATOM
    ZHOU Huan, ZHANG Tiebang, WU Zeen, HU Rui, KOU Hongchao, LI Jinshan
    Acta Metall Sin, 2014, 50 (7): 832-838.  DOI: 10.3724/SP.J.1037.2013.00746
    Abstract   HTML   PDF (5840KB) ( 917 )
    As promising light-weight high-temperature materials, g-TiAl base alloys are considered as prospective candidates for automobile and aerospace application due to their high specific yield strength. Adding Nb to TiAl alloys increases the liquidus temperature and results in improvents of creep resistance, high temperature strength and oxidation resistance. High Nb-containing TiAl alloys have attracted much attention during past decades. With the addition of carbon in Ti-46Al-8Nb-xC alloys (x=0, 0.7, 1.4, 2.5, atomic fraction, %), the formation of precipitates, the orientation relationship between precipitates and the TiAl matrix and the evolution of the precipitates during heat treatments have been investigated in this work by XRD, SEM and TEM. The results show that lath-shaped precipitates of Ti2AlC can be formed during the preparation of ingots with the addition of 1.4% and 2.5% of C. With good thermal stability, the size, amount and distribution of Ti2AlC precipitates remain almost stable during the aging process. Needle-shaped precipitates of Ti3AlC are formed in the aged alloys with 0.7%, 1.4% and 2.5% of C. And the precipitates are preferentially formed in g grains. The orientation relationship between Ti3AlC precipitates and g phase is found to be and . Meanwhile, the precipitation behavior and morphology of Ti3AlC are also discussed. Ti3AlC precipitates grow slightly after prolonged aging, while the amount of the precipitates remains small. With a higher aging temperature, the size of Ti3AlC precipitates increases significantly and an increasing amount of the precipitates is observed.
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    MICROSTRUCTURE AND MECHANICAL PROPERTIESOF A Ni-BASED SUPERALLOY WITH REFINED GRAINS
    YANG Jinxia, SUN Yuan, JIN Tao, SUN Xiaofeng, HU Zhuangqi
    Acta Metall Sin, 2014, 50 (7): 839-844.  DOI: 10.3724/SP.J.1037.2013.00745
    Abstract   HTML   PDF (7395KB) ( 1396 )
    A new Ni-based superalloy with the refined grains is to be used in industrial and aircraft turbines because of its high strength and excellent fatigue resistance at lower and medium temperatures (500~800 ℃). The grains with six different sizes have been made by decreasing the pouring temperature from 1460 to 1480 ℃ then 1500 ℃ and adding refiner to alloy and planting seed on the surface of mold. The size of equiaxed crystal grain is reduced to 0.5 mm in the center part of specimen with the columnar crystals in the outside of specimen made by the refining process which is finer than those of traditional process. It has been found that g' phase and carbide are finer in refined grains than those in the coarse grains made by decreasing the pouring temperature. The room-temperature tensile properties and high cycle fatigue properties of tested alloy are improved with decreasing grain size. The stress-rupture properties are increased under the conditions of 760 ℃ and 662 MPa while are decreased with decreasing the grain size. The grain structure and size are refined by the refining process that dominated the excellent mechanical properties of tested alloy at lower and medium temperatures. However, it is not good for the mechanical properties at high temperatures.
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    EFFECTS OF Ni ON MICROSTRUCTURAL EVOLUTION AND g ′ DISSOLUTION OF NOVEL Co-Al-W BASE ALLOYS
    XUE Fei , MI Tao , WANG Meiling , DING Xianfei , LI Xianghui , FENG Qiang
    Acta Metall Sin, 2014, 50 (7): 845-853.  DOI: 10.3724/SP.J.1037.2013.00786
    Abstract   HTML   PDF (9834KB) ( 715 )
    The influences of Ni on the phase transformation temperatures, g /g ′ two-phase microstructural evolution, g ′ dissolution behavior and microhardness have been investigated in four Co-Al-W base alloys containing various Ni contents (15%~45%, atomic fraction). The results show that the g ′ solvus temperatures continuously increase and the solidus temperatures are nearly unchanged with increasing the Ni content. The g /g ′ two-phase microstructure is generated in four experimental alloys after the heat treatment at 900 ℃ for 50 h, whereas the g ′ morphology changes from cuboidal to nearly spherical and the g ′ volume fraction reduces as the Ni content increases. When prolonged heat treatment at 900 ℃ for 300 h is employed, no significant change in the g ′ morphology is observed in four experimental alloys but the g ′ volume fraction decreases to different degree as a function of Ni concentration. High temperature treatments at 970~1060 ℃ are conducted after experimental alloys are heat treated at 900 ℃ for 300 h. In the high temperature range, the dissolution of the g ′ phase is more pronounced as the temperature elevates, whilst the g ′ morphology becomes spherical and cuboidal in alloys containing the low and high levels of Ni, respectively. The microhardness results of the experimental alloys after heat treatment at 900 ℃ for 50 h and 300 h indicate that the microhardness is lowered in alloys with higher Ni content, but it increases as the heat treatment time is prolonged.
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    EFFECT OF C ON THE RUPTURE PROPERTIES OF SINGLE CRYSTAL SUPERALLOYS
    YU Zhuhuan , LIU Lin
    Acta Metall Sin, 2014, 50 (7): 854-862.  DOI: 10.3724/SP.J.1037.2013.00790
    Abstract   HTML   PDF (13024KB) ( 394 )
    The effects of carbon addition on the solidification microstructure and rupture life were investigated in five different carbon level single crystal superalloys. With the increasing of carbon level, the volume fraction of eutectic decreased markedly and the volume fraction of carbide increased. The carbides mainly distributed in interdendrite zone, when the carbon level was high, there were little carbides in the dendrite core. After heat treatment, coarse g /g ′ eutectics in interdendrte zone mainly were dissolved, a little g /g ′ eutectic was not dissolved. Morphologies of carbide became much simpler, the size of carbide decreased, the volume fraction of carbide decreased, and the distribution of carbide became much more dispersion, and the type of carbide became much more variety. Grain and chainlike M23C6 appeared after heat treatment. With the increasing of carbon level, the rupture life of single crystal superalloy increased at first and then decreased, and the rupture life came up to the maximum when the carbon level was 0.045%. SEM observation indicates that the cracks of alloys mainly originate from shrinkage, carbides and eutectics. The change of rupture life was mainly because the un-dissolved eutectic and carbides of alloy which act as the source of cracks. The variation trend of carbide and eutectic was contrary with the increasing of carbon level; therefore, the carbon content should be controlled in the perfect level.
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    PRE-AGING OF RETROGRESSION AND RE-AGINGOF SPRAY FORMED 7075 ALLOY
    SU Ruiming, QU Yingdong, LI Rongde
    Acta Metall Sin, 2014, 50 (7): 863-870.  DOI: 10.3724/SP.J.1037.2013.00756
    Abstract   HTML   PDF (4248KB) ( 856 )
    Retrogression and re-aging (RRA) treatments are divided into pre-aging, retrogression and re-aging. Although peak aging was used as the pre-aging of RRA treatment in the past, some different opinions were reported in recent years. The effects of pre-aging of RRA treatment on microstructure, mechanical properties and conductivity of spray formed 7075 aluminum alloy were investigated by TEM, tensile and conductivity test. The results show that the mechanical properties and conductivity of spray formed 7075 alloy could be improved by the under aging at 120 ℃ for 16 h as the pre-aging of RRA treatment. The properties of the 7xxx series aluminum alloys depend on matrix precipitates (MPt), grain boundary precipitates (GBPs) and precipitate free zones (PFZs). The tiny MPt can increase the tensile strength. The connected GBPs and narrow PFZs will lower the conductivity and the elongation of the alloy. The under aging is more beneficial for the re-dissolution of the MPts at retrogression treatment at 200 ℃ for 10 min, and is more conducive to interrupt distributions of the GBPs than the early aging after RRA treatment. With the under aging as the pre-aging treatment, the growth of the MPts was actively suppressed and the GBPs at grain boundaries are continuous. During retrogression treatment, the MPts were re-dissolved absolutely and the GBPs were transformed from a chain to dissociation. After re-aging treatment, lots of tiny dispersive MPts precipitated out again in matrix, the GBPs were totally separated and the PFZ were widened. After pre-aging at 120 ℃ for 16 h and RRA treatment, the tensile strength and yield strength of the alloy are 782 and 726 MPa, respectively, which are higher than that after peak aging treatment or conventional RRA treatment, the conductivity of the 7075 alloy is excellent with 22.7 MS/m.
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    ANALYSIS ON THE CONSTITUTIVE RELATIONSHIP OF TC18 TITANIUM ALLOY BASED ON THE SOFTENING MECHANISM
    LIANG Houquan, GUO Hongzhen, NING Yongquan, YAO Zekun, ZHAO Zhanglong
    Acta Metall Sin, 2014, 50 (7): 871-878.  DOI: 10.3724/SP.J.1037.2013.00801
    Abstract   HTML   PDF (1580KB) ( 848 )
    The true stress-true strain curves have been attained through the isothermal compression experiment of TC18 titanium alloy. The influence of deformation parameters on the shape of stress-strain curves and peak stress has been analyzed through the working-hardening and dynamic softening effects. The true stress-true strain curves show the similar characteristics under different deformation conditions. However, the peak stress is sensitive to the changes of deformation parameters. The type of dynamic softening mechanisms in hot deformation under certain conditions can be obtained through Poliak-Jonas criterion. The dynamic recrystallization process tends to take place during the deformation with lower strain rates. And the choice of the dynamic softening mechanisms is not sensitive to deformation temperatures. The suitable constitutive models under different softening mechanisms have been constructed based on the traditional Arrhenius equations. With the identification of the dynamic softening mechanisms in hot deformation of TC18 alloy with different conditions, the response of true stress, at strain of 0.7, to the deformation temperatures and strain rates can be described through the proposed models. And the sensitivity coefficient of strain rates and deformation activation energy, of the process with the dynamic recrystallization as the major softening mechanism, are much larger than the ones of process with dynamic recovery.
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    DISCHARGE CHARACTERISTICS OF Ti AND FILM PREPARATION USING HYBRID HIGH POWER IMPULSE MAGNETRON SPUTTERING
    LI Xiaochan , KE Peiling , LIU Xincai , WANG Aiying
    Acta Metall Sin, 2014, 50 (7): 879-885.  DOI: 10.3724/SP.J.1037.2013.00744
    Abstract   HTML   PDF (1801KB) ( 892 )
    Hybrid high power impulse magnetron sputtering (HIPIMS) is a new-generation HIPIMS technique with a pulse and dirrect current power supply parallelled connection operation. In this work, the influence of dirrect current from 0 to 4.0 A supplied by the dirrect current power is investigated on hybrid HIPIMS Ti discharge characteristics, plasma parameters (plasma potential, electron temperature and electron density) and Ti film properties in an Ar atmosphere. The results show that target voltage and current are characterized by a peak with variation of time in different dirrect currents. Although the target voltage is barely affected, the target current decreases with increasing the dirrect current during the pulse turn-on stage. The plasma parameters determined by a Langmuir probe have been significantly influenced by the dirrect current. Moreover, the deposition rate and average roughness increase while the hardness and elastic modulus have a slight decrease with the variation of dirrect current from 1.0 to 3.0 A. The samples are selected for comparison with that prepared by conventional direct current magnetron sputtering (DCMS) at the same average target power 650 and 1500 W. The results demonstrate that Ti films using hybrid HIPIMS have a close deposition rate and a superior quality and performance to those prepared using DCMS especially at the low target power 650 W when the direct current is 1.0 A.
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    PHASE FIELD CRYSTAL SIMULATION OF STRAIN EFFECTS ON DISLOCATION MOVEMENT OF PREMELTING GRAIN BOUNDRIES AT HIGH TEMPERATURE
    GAO Yingjun , ZHOU Wenquan , DENG Qianqian , LUO Zhirong , LIN Kui , HUANG Chuanggao
    Acta Metall Sin, 2014, 50 (7): 886-896.  DOI: 10.3724/SP.J.1037.2013.00816
    Abstract   HTML   PDF (16527KB) ( 479 )
    The properties of modern materials, especially superplastic, nanocrystalline or composite materials, depend critically on the properties of internal interfaces such as grain boundaries (GBs) and interphase boundaries (IBs). All processes which can change the properties of GBs and IBs affect drastically the behaviour of polycrystalline metals and ceramics. In this work, the annihilation processes of low-angle symmetric tilt GBs and dislocations during plastic deformation in the representative system of these materials near but below the melting point and the temperature at liquid-solid coexistence line were simulated using the phase-field crystal model, respectively. The results show that local premelting occurs around lattice dislocations near the melting point but the dislocation structure in the premelting region does not change, while the region become significantly larger when the system reaches the melting temperature. After premelting, deformation to the system causes dislocations in the premelting GB to begin to glide then annihilate with opposite Burgers vectors via the movement, finally the GB and the premelting region disappear. The annihilation mechanisms of dislocations are similar to those for premelting conditions. The more the temperature is closer to the melting point, the more obvious the atomic lattice around the premelting region is softened leading to the atomic binding strength around the dislocations being lowered. Only at this moment, the lattice atoms enable to reduce the resistance of the dislocation motion and accelerate its velocity during deformation. At the temperature reaching to the liquid-solid coexisting region in the simulation, the original premelting regions are induced to develop into bigger ones by the external strain acting. During this process, it can be seen some interactions including the multiplication dislocation pairs, the rotation of dislocation pairs and their annihilation. Furthermore, the shape of the premelting region changes with the variation of the interaction of dislocations inside the region, it is observed that the premelting regions approach each other and consolidate together, then decompose and segregate from each other. Although the shape of the premelting region changes with the applied strain, these regions do not disappear at the end of the simulation, totally different those in lower premelting temperature.
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