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

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    STUDY ON TENSILE BEHAVIOR OF SiCf/TC17 COMPOSITES
    Xu ZHANG, Yumin WANG, Qing YANG, Jiafeng LEI, Rui YANG
    Acta Metall Sin, 2015, 51 (9): 1025-1037.  DOI: 10.11900/0412.1961.2015.00127
    Abstract   HTML   PDF (16436KB) ( 774 )

    Tensile properties and fracture mechanisms of SiCf/TC17 composites at room temperature and 773 K were studied. The results show that fiber elastic deformation and matrix yielding contributed to the shapes of the stress-strain curves of SiCf/TC17 composites, which were the bilinear appearance at 298 K and the slight curvature at 773 K. Major fracture mechanism of SiCf/TC17 composites at room temperature were as follows: multiple fractures of the interfacial reaction layer, single fiber fracture, matrix brittle fracture etc.. Typical fracture mechanism of SiCf/TC17 composites at elevated temperature were as follows: multiple fiber fracture, matrix plastic fracture, interface debonding etc.. Fiber cumulating damage theory was proved to be suitable for estimation of the fracture strength of this composite. The calculations of local loading sharing model while taking three or more fibers failure into account and global loading sharing model were close to the experimental values of room temperature and elevated temperature respectively. In addition, according to fracture mechanisms and strength prediction, tensile fracture process of SiCf/TC17 composites at room and elevated temperature were explained in detail. At room temperature, multiple fractures of the interfacial reaction layer started at first, and then the weak fiber fractured gradually and randomly. After critical fiber cluster has been formed by nearby broken fibers, the crack extended into the matrix from these fibers. With the increase of load, the fibers and the matrix at the tip of crack gradually destroyed. At the same time, the cracks from other critical fiber clusters were also expanding and connecting to each other. When the crack area has reached the critical level, the remaining fiber and matrix quickly fractured. However, at elevated temperature the matrix yielded firstly, and then multiple fracture randomly of the interfacial reaction layer and the weak fiber occurred sequentially. The crack from broken fiber deflected at interface between fiber and matrix, caused interface debonding. With the increasing of broken fiber number, the micro-cavities of matrix emerged gradually in the stress concentration area. When the total crack area accumulated by the broken fibers and micro-cavities of matrix has reached the critical level, the remaining fiber and matrix quickly fractured.

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    EVALUATION OF THE UNIFORM DISTRIBUTION OF DENDRITIC MICROSTRUCTURE IN DIRECTIONALLY SOLIDIFIED SINGLE-CRYSTAL DD6 SUPERALLOY
    Yumin WANG,Shuangming LI,Hong ZHONG,Hengzhi FU
    Acta Metall Sin, 2015, 51 (9): 1038-1048.  DOI: 10.11900/0412.1961.2015.00035
    Abstract   HTML   PDF (9068KB) ( 954 )

    Homogeneous distribution of primary dendritic arm spacing (PDAS) is required to achieve uniform mechanical properties in final product of single-crystal superalloys. In this work, the dendrite characterization and orientation of Ni-based single-crystal DD6 superalloy have been deeply investigated using different methods, which include minimum spanning tree (MST), Voronoi polygon-based approach, fast Fourier transform (FFT), as well as EBSD and RO-XRD. The investigation results indicate that the mean PDAS of DD6 superalloy is about 325.7 mm and its variation ratio is 7.38%. The measured Voronoi polygon parameters suggest that the number of nearest-neighbor dendrite ranges from 5.87 to 5.93, approximating six nearest neighbors in the spatial distribution of dendrite microstructures. However, the change in ratio of six nearest number proportion has exceeded 30% for the twenty specimens. The MST method shows that the change in branch length measured from the twenty specimens achieves 26.95%. Also, the analysis results of FFT imply that the dendrite microstructures of DD6 superalloy evolve apparently. These results give the proof that the dendrite microstructures of DD6 superalloy vary with the solidified distance. Additionally, the deviation angles between preferential orientations of DD6 with the axial direction of specimen were measured by EBSD and RO-XRD, respectively. The deviation angle values of DD6 superalloy in this experiment are both within 10°. The reason for the deviation angle measured by RO-XRD being smaller is well explained due to the fact of selecting the diffraction intensity maximum angles. Furthermore, the EBSD results indicate that the orientations of DD6 superalloy prepared by grain selector can be well controlled along the Z-axial direction, but do not work in other two X and Y directions.

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    EFFECT OF Zr ADDITION ON MICROSTRUCTURE AND OXIDATION RESISTANCE OF Nb-Ti-Si BASE ULTRAHIGH-TEMPERATURE ALLOYS
    Yuxiang ZENG,Xiping GUO,Yanqiang QIAO,Zhongyi NIE
    Acta Metall Sin, 2015, 51 (9): 1049-1058.  DOI: 10.11900/0412.1961.2015.00092
    Abstract   HTML   PDF (10943KB) ( 605 )

    Nb-Ti-Si base in situ composites which consist of Nb solid solution (Nbss) and silicides (a-Nb5Si3, b-Nb5Si3, g-Nb5Si3 and/or Nb3Si) phases, have shown great potential as alternative materials to Ni-based superalloys due to their high melting points (beyond 1700 ℃), good formability, low density (6.6~7.2 g/cm3) and high strength. However, a major hindrance to the applications of these alloys at elevated temperatures is their poor oxidation resistance. Alloying is an effective method to improve the integrated properties of the alloys, especially for the oxidation resistance. Up to now, many beneficial elements such as Ti, Al, Cr and Sn have been employed to ameliorate their oxidation resistance. Nevertheless, there is no systematic and comprehensive investigation on the effect of Zr contents on the microstructure and oxidation behavior of the alloys based on Nb-Ti-Si system. The aim of this work is to clarify the effects of Zr contents on phase selection, microstructure and high temperature oxidation resistance of Nb-Ti-Si based alloys in detail. The constituent phases, microstructure and composition of the alloys under as-cast state and after oxidation were investigated by OM, XRD, SEM and EDS. Thus, six Nb-Ti-Si base ultrahigh-temperature alloys with compositions of Nb-22Ti-15Si-5Cr-3Hf-3Al-xZr (x=0, 0.5, 1, 2, 4, 8, atomic fraction, %) were prepared by vacuum non-consumable arc-melting. The results show that the alloys with different Zr contents are mainly composed of Nbss and g-(Nb, X)5Si3 (X represents Ti, Hf, Cr and Zr). However, the addition of Zr has an obvious affect on the microstructure of Nb-Ti-Si base alloys. Both the sizes and amounts of primary g-(Nb, X)5Si3 increase with increase in Zr contents. Alloys with different Zr contents were oxidized at 1250 ℃ for 1~50 h, respectively. It is found that both adhesion and compactness of the scales are improved effectively by increase in Zr contents. The scales of alloys with higher Zr contents (x=4 and 8) after oxidation for 50 h show an obvious layered structure: the outmost layer is only composed of TiO2, the middle layer mainly consists of ZrO2, TiNb2O7 and TiO2, and the inner layer is mainly comprised of Si-rich oxides. The mass gain per unit area and the thickness of the scale after oxidation decrease with increase in Zr contents in the alloys, indicating that the addition of Zr can improve the oxidation resistance of the alloys significantly.

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    CORROSION BEHAVIOR OF GH3535 SUPERALLOY IN FLiNaK MOLTEN SALT
    Tao LIU,Jiasheng DONG,Guang XIE,Yisheng WANG,Hui LI,Zhijun LI,Xingtai ZHOU,Langhong LOU
    Acta Metall Sin, 2015, 51 (9): 1059-1066.  DOI: 10.11900/0412.1961.2015.00132
    Abstract   HTML   PDF (7887KB) ( 1142 )

    As one of the most promising next generation reactors, the molten salt breeder reactor (MSBR) with excellent inherence security has attracted more and more attentions in recent years due to energy shortage and the security problem of traditional nuclear reactor. The most significant service characteristic of the structural material used in MSBR is the existence of FLiNaK molten salt compared with other nuclear reactors. FLiNaK molten salt is very corrosive to the structural material in the reactor, and affects the safety operation of nuclear power plants. A polycrystalline Ni-Mo-Cr-Fe superalloy was developed and used as an important structural material in MSBR at Oak Ridge National Laboratory (ORNL), but the corrosion mechanism of the alloy in FLiNaK molten salt has not been determined since the study terminated in 1970' s as some politic reasons. Alloy served in harsh environments, often using protective coating to improve the corrosion properties. While few works about the coating corrosion resistance in FLiNaK molten salt were reported at present. Al2O3 and Cr2O3 coatings usually have excellent corrosion resistance in molten salt, such as sulphate, nitrate and halide molten salt. But, whether the oxide film has corrosion resistance in FLiNaK molten salt has not been determined. In this work, the corrosion mechanism of alloy in FLiNaK molten salt was studied by using immersion corrosion experiment through the method of SEM, EDS and XRD. The influence of Al2O3 coating on corrosion resistance in FLiNaK molten salt was also investigated. The results show that the Al2O3 coating does not affect the exsolution corrosion characteristics of Cr and Mo elements in FLiNaK molten salt at 700 ℃ for 400 h. The different is that naked alloy exhibits intergranular corrosion characteristic, and the alloy with Al2O3 coating exhibits spot corrosion characteristic. The Al2O3 coating cannot improve the corrosion resistance of the alloy in FLiNaK molten salt. The Al2O3 film dissolved in molten salt and resulted in the exposure of the alloy surface. The corrosion rate was increased since the formation of corrosion cell between oxide film and the exposed alloy surface.

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    EFFECTS OF SO42- ON THE CORROSION BEHAVIOR OF NiCu LOW ALLOY STEEL IN DEAERATED BICARBONATE SOLUTIONS
    Yunfei LU,Junhua DONG,Wei KE
    Acta Metall Sin, 2015, 51 (9): 1067-1076.  DOI: 10.11900/0412.1961.2015.00133
    Abstract   HTML   PDF (3021KB) ( 781 )

    High level radioactive waste (HLW) is an extremely dangerous by-product of the global nuclear industry. Due to its intensely radioactive nature and ultra long half-life, HLW has to be safely managed and disposed for thousands of years, isolated from the biosphere. Deep geological repository (DGR) is considered to be the most feasible option worldwide because of its operability, stability, durability, environmental protection and so on. Basically, DGR relies on a multibarrier system and it consists of metallic canisters, backfill materials and a stable geologic formation. Since radionuclides could be moved into the biosphere by action of groundwater, both the geologic formation and backfill materials have to be of very low hydraulic permeability and metal canisters have to be corrosion resistant and prevent contact between the groundwater and the radioactive waste for as long as possible. Low carbon steel has been selected and studied as a candidate canister material in many countries because its long industrial experience, high-strength, low cost and it is less prone to localized corrosion than materials that passivity, but its larger corrosion rate may also set an insuperable barrier for the practical application. Recently, our studies revealed that NiCu low alloy steel is a more promising candidate for the canister material compared with the popular one, low carbon steel, since the former performs a more acceptable corrosion rate without increasing much cost and has better resistance to localized corrosion in environments with high concentration of Cl-. In this work, effects of SO42-, another ubiquitous species in deep groundwater, on the corrosion behavior of NiCu low alloy steel during immersion in simulated deep groundwater environments were investigated by in situ electrochemical measurements and surface analysis techniques. Results show that the addition of SO42- can promote the substrate dissolution during the initial stage of immersion. In the later stage, SO42- weakens the protectiveness of formed films and consequently, active dissolution prevails on the electrode surface rather than the prepassivation. Concentrated SO42- and HCO3- can both promote the formation of Fe6(OH)12CO3. The main components of corrosion products are a-FeOOH, Fe3O4 and Fe6(OH)12CO3, and uniform corrosion is observed.

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    INVESTIGATION ON PITTING CORROSION BEHAVIOR OF ULTRAFINE-GRAINED 304L STAINLESS STEEL IN Cl- CONTAINING SOLUTION
    Nan PIAO,Ji CHEN,Chengjiang YIN,Cheng SUN,Xinghang ZHANG,Zhanwen WU
    Acta Metall Sin, 2015, 51 (9): 1077-1084.  DOI: 10.11900/0412.1961.2015.00062
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    The electrochemical behavior and pitting corrosion in a Cl- containing solution (0.05 mol/L H2SO4+0.05 mol/L NaCl) of the ultrafine-grained 304L stainless steel (304L SS) with average grain size of (130±30) nm prepared by equal channel angular pressing (ECAP) technique were examined using potentiodynamic polarization curves, cycle polarization curves, electrochemical impedance spectroscopy (EIS), Mott-Schottky (M-S) curve measurements together with SEM observation of surface morphology. As compared to the coarse-grained counterpart, the ultrafine-grained sample exhibited a higher corrosion current density icorr of 81.74 Acm2 and a lower corrosion potential Ecorr (vs SCE) of -466 mV, and having a higher passivation current density ip of 32.38 mAcm2 and a narrower passive region (-315~450 mV) together with a breakdown potential Eb decrease of 100 mV and a protection potential Ebp decrease of 190 mV. On one hand, the grain refinement induced by severe plastic deformation deteriorates the compactness of the passive films and is helpful for the Cl- absorption, resulting in a 1.6 times increase of the carrier density and one order of magnitude increase of the diffusion coefficient in the passive films. On the other hand, the significant increase of grain boundaries provides more possibility for Cl- diffusion along grain boundaries, and thus promotes the pitting nucleation and growth.

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    EFFECT OF SOLUTION TEMPERATURE ON MICRO- STRUCTURE AND PITTING CORROSION RESISTANCE OF S32760 DUPLEX STAINLESS STEEL
    Yulai CHEN,Zhaoyin LUO,Jingyuan LI
    Acta Metall Sin, 2015, 51 (9): 1085-1091.  DOI: 10.11900/0412.1961.2015.00044
    Abstract   HTML   PDF (6431KB) ( 829 )

    In order to obtain the optimal corrosion resistance, the characteristics of microstructure and alloy elements distribution of S32760 duplex stainless steel were studied after solid solution treatment at various temperatures from 1000 ℃ to 1300 ℃ by means of OM, EPMA, SEM, EDS and TEM. In addition, the pitting corrosion resistance was measured by the electrochemical workstation. The results show that the N atoms diffused into d phase from g phase during solution treatment when the temperature was higher than 1080 ℃. N atoms migrated back into g phase when the subsequent cooling was slow enough. However, Cr2N phase in situ precipitated during quenching because there was not enough time for the N atoms to diffuse back into g phase. Cr2N particles increased with the solution temperature increasing. Furthermore, s phase precipitated when the tested sheet was heat treated at or below 1040 ℃ due to the high content of N. Thus it is obvious that the solution temperature range of the S32750 duplex stainless steel is quite narrow, which is between 1040 ℃ and 1080 ℃, and it is confirmed that the optimal temperature is 1060 ℃. After treated at 1060 ℃ for 60 min, the Brinell hardness of S32760 steel is 249 HBW, pitting potential is up to 1068 mV and the passive current density is as low as 1.48×10-4 A/cm2.

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    EFFECT OF MARTENSITE DISTRIBUTION ON MICROSCOPIC DEFORMATION BEHAVIOR AND MECHANICAL PROPERTIES OF DUAL PHASE STEELS
    Jie DENG,Jiawei MA,Yiyang XU,Yao SHEN
    Acta Metall Sin, 2015, 51 (9): 1092-1100.  DOI: 10.11900/0412.1961.2015.00083
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    Investigation of the relationship between microstructure and microscopic deformation behavior of dual phase steel is very important for high property dual phase steel development. In this work, step quenching (SQ) and intercritical annealing (IA) heat treatments were optimized to produce dual phase steels of similar martensite volume fraction, but with respectively isolated and continuous martensite distribution. The tensile and dynamic fracture properties of dual phase steels were investigated. Strain distribution of steels was measured by digital image correlation (DIC) method. Combined with observations of microcracks/microvoids, different deformation and fracture mechanisms were revealed. Compared to IA steel, SQ steel has lower strength, but longer elongation and higher fracture toughness, and the latter were attributed to larger deformation in ferrites that results in more stress relaxation of martensite during deformation. While in IA steel, the deformation in ferrites is blocked by adjacent martensites, so that a relatively small strain of ferrite cannot effectively relax the stress in martensites, which resulted in higher plastic deformation in martensite than in SQ steel; therefore, cracks preferentially initiate in martensite, and IA steel exhibits higher strength and lower plasticity.

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    HYDROGEN PERMEATION PARAMETERS OF X80 STEEL AND WELDING HAZ UNDER HIGH PRESSURE COAL GAS ENVIRONMENT
    Timing ZHANG,Yong WANG,Weimin ZHAO,Xiuyan TANG,Tianhai DU,Min YANG
    Acta Metall Sin, 2015, 51 (9): 1101-1110.  DOI: 10.11900/0412.1961.2015.00039
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    Hydrogen gas is usually included in coal gas environment, so hydrogen induced permeation would happen to pipeline, especially in welding heat affected zone (HAZ). Hydrogen permeation process in pipeline is the preconditions for the following hydrogen embrittlement failure. With the development of coal gas industry, the basic research to the hydrogen permeation behavior in pipeline under coal gas circumstance is still unfortunately lack and urgently needed to supplement. In this work, X80 pipeline steel was used, and the HAZ samples, including intercritical heat affected zone (ICHAZ), fine grained heat affected zone (FGHAZ) and coarse grained heat affected zone (CGHAZ), were experimentally simulated using a Gleeble 3500 simulator. Next, hydrogen permeation tests were conducted on X80 pipeline steel and HAZs in coal gas environment. Calculated results indicated that the hydrogen diffusion coefficient increased with the rise of peak temperature in HAZs, but it was opposite to other parameters, such as sub-surface hydrogen concentration, hydrogen solubility and hydrogen trap density. The mechanism of the difference in HAZ hydrogen permeation parameters was analyzed combined with OM, EBSD and TEM analysis. It turned out that the content of large-angle grain boundaries, the grain boundary straightness and dislocation density were the main factors, where the large-angle grain boundaries and dislocations could dramatically arrest hydrogen atoms while the straight grain boundaries may act as hydrogen diffusion path. For FGHAZ, the straight grain boundary and low dislocation density compared with matrix played the predominant role in hydrogen diffusion process, and thus the hydrogen diffusion coefficient increased compared with steel substrate. For ICHAZ and CGHAZ, the decrease of large-angle grain boundaries and dislocation density acted as the main factor, especially for CGHAZ, the microstructures was mainly composed of tabular bainite ferrite (BF) with large grain size and straight grain boundaries because of the highest peak temperature, and the content of large-angle grain boundaries decreased obviously. In comparation with other regions, CGHAZ had the highest hydrogen diffusion coefficient and the lowest hydrogen trap density and hydrogen solubility.

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    EFFECT OF ELECTRON BEAM POWER ON TC4 ALLOY RIGID RESTRAINT THERMAL SELF-COMPRESSING BONDING, MICRO- STRUCTURE AND MECHANICAL PROPERTIES OF JOINTS
    Yunhua DENG,Qiao GUAN,Jun TAO,Bing WU,Xichang WANG
    Acta Metall Sin, 2015, 51 (9): 1111-1120.  DOI: 10.11900/0412.1961.2015.00105
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    Rigid restraint thermal self-compressing bonding is a new solid-state bonding process. During the process, localized non-melted heating method is employed to heat the butted interface of the rigid restrained plates to be bonded. Under the localized heating, materials close to the butted interface are expanded. However, due to the existence of surrounding cool metals and rigid restraints, the expansion of the high temperature materials is restrained and thus, a compressive pressure is developed which compresses the high temperature metals near the bond interface and facilitates the atom diffusion between butt-weld specimens to produce a permanent solid-state joint. Utilizing the localized stress-strain field to accomplish atomic bonding, this process can avoid the use of external forces on which diffusion bonding and other solid-state bonding methods rely. Previous study has proven the feasibility of this process to join titanium alloys. In present work, the effect of beam power on bond interface, microstructure and mechanical properties of the TC4 joints bonded at different beam powers were analyzed through the OM observation, EBSD analysis, mechanical property test and fracture morphology analysis. Meanwhile, in order to reveal the mechanism about the effect of beam power on bond interface, the experiment study on microstructure and mechanical property and finite element analysis on present bonding were conducted to investigate the effect of beam power on the thermal stress-strain process during bonding. The results show that with the increase of beam power, the heating temperature, dwell time over high temperature, volume of materials with high temperature and the compressive plastic strain increase which promote the atom diffusion and thus bond quality of the interface is improved. At low beam power, the microstructure of the joints is homogeneous, while coarse grain with acicular a phase forms in the joint when the beam power is high. Mechanical properties of the joint are dependent on bond rate and microstructure. When the beam power is lower or higher, the compressive mechanical properties of the joints are poor because of the poor bonding quality of the interface or the coarse microstructure developed in the joint. Good comprehensive mechanical properties are obtained at the beam power of 330 W.

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    EFFECT OF B MICRO-ALLOYING ON MICRO-POROSITIES IN AS-CAST HK40 ALLOYS
    Xianfei DING,Dongfang LIU,Yunrong ZHENG,Qiang FENG
    Acta Metall Sin, 2015, 51 (9): 1121-1128.  DOI: 10.11900/0412.1961.2015.00126
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    Casting microporosity defect is one of the important issues for as-cast HK40 alloys preparation, which is of great importance to application performance of the alloy castings. A comprehensive understanding of the mechanism on formation of the casting microporosity defect is still unclear for the alloys. In this work, the casting microporosity defect and influences of boron micro-alloying on the as-cast microstructures and microporosities in HK40 alloys castings were investigated by means of SEM, OM and XRD, etc.. The microstructures in the HK40 alloys with and without boron micro-alloying after quenching at high temperatures were also examined to check the solidification characteristic change attribute to boron addition. The results show that there are two types of casting microporosities in the castings. Type A is mainly caused by the rapid growth of dendrites and thus dendritic bridge connecting which lead to feeding shortages between the bridge dendrites. Type B is, however, resulted by the growth of M7C3 carbides in coarsened dendritic morphology which induce to the feeding channel blockage in adjacent interdendritic regions. Boron micro-alloying decreases the tendency of columnar grain formation and refines the dendrites in HK40 alloys which therefore suppresses the casting microporosity defect of type A. Additionally, boron micro-alloying not only increases the volume fraction of eutectic phases, but also changes the M7C3 carbides in dendritic morphology into the M23C6 carbides in lamellar morphology, which prevents the feeding channal blockage in adjacent interdendritic regions, thus reduces the casting microporosity defect of type B.

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    INTERFACIAL REACTION AND STRENGTHENING MECHANISM OF CERAMIC MATRIX COMPOSITE JOINTS USING LIQUID PHASE DIFFUSION BONDING WITH AUXILIARY PULSE CURRENT
    Mingfang WU,Fei LIU,Fengjiang WANG,Yanxin QIAO
    Acta Metall Sin, 2015, 51 (9): 1129-1135.  DOI: 10.11900/0412.1961.2015.00100
    Abstract   HTML   PDF (4079KB) ( 909 )

    Ceramic matrix composites (CMCs) is attracted in airspace and nuclear engineering due to their high temperature, corrosion and wearing resistance, but the usage is limited by the joining between CMC and other metals due to obvious incompatibility on physical and chemical aspects on them. The liquid phase-diffusion bonding (LPDB) on Ti(C, N)-Al2O3 CMC/Cu joint was studied using the Cu-Zr foil/Cu foil/Cu-Zr foil sanwich as an interlayer in this work. Auxiliary pulse current was also added to control the elemental diffusion and interfacial reaction during LPDB. The element diffusion and reacted products at the interface were analyzed with SEM, EPMA and EDS, and the joint strength was tested with four points bending method. The results show that with an auxiliary pulse current during LPDB, a higher joint strength is reached with a lower bonding temperature and a shorter holding time. The diffusion behavior of element Zr and Cu in CMC and the interfacial area is obviously changed, and the activity of Zr element and its chemical reaction with Al2O3 are depressed by the auxiliary pulse current during LPDB. The diffusion of ceramic partilces into the interface and the thickness of corresponding diffusion transition zone (DTZ) and Zr-Cu interfacial reaction zone (IRZ) at the interface are also depressed by the auxiliary pulse current, which strengthens the interface, and is always kept an higher joint strength.

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    SIMULATION OF GROWTH KINETICS OF PRO-EUTECTOID FERRITE USING MIXED CONTROL MODEL WITH CONSIDERATION OF DISLOCATION INTERACTION
    Huidong WU,Chi ZHANG,Wenbo LIU,Zhigang YANG
    Acta Metall Sin, 2015, 51 (9): 1136-1144.  DOI: 10.11900/0412.1961.2015.00091
    Abstract   HTML   PDF (1751KB) ( 733 )

    During austenite to ferrite transformation, the lattice structure transforms from fcc to bcc, resulting in a clearly distinguishable austenite and ferrite interface. The short range diffusion of the Fe and C atoms across the interface causes its movement, referred to as interface migration. On the other hand, the C rejected by the ferrite during the austenite to ferrite transformation in Fe-C alloys accumulates ahead of the moving interface. This pile-up of C atom is dependent on the long range diffusion of C in austenite and also influences the ferrite growth kinetics. Experimental observations indicate that dislocations are always migrating with ledges during ledgewise growth. The local stress field of dislocations is considered to alter the solute concentration at the riser of ledges and causes a complex diffusion field interaction among ledges as they migrate. Some established works by other researchers have already taken the effect into consideration when studying phase transformation kinetics. However, these works were limited in diffusion control cases and could hardly explain some experimental results. In this work, a ledgewise growth model considering migration of austenite/ferrite interface, C diffusion in austenite and especially elastic interactions between dislocations moving with ferrite ledges was established, and all the simulated results were qualitatively similar to the reported experimental results. Calculated results showed that the C concentration at the riser of ledges was changed by the elastic stress of these dislocations, which would further change the growth behavior of ledges. In the growth behavior simulations of two ledges, the horizontal distance of the two ledges was found to be a key role to determine the growth kinetics. When the horizontal distance of two ledges was larger than the critical distance, an attractive phenomenon of the two ledges was found to decelerate the leading step; while a repulsive phenomenon of the two ledges which would accelerate the leading ledge if the horizontal distance was smaller than this value. Compared with the simulation results without considering elastic interactions between dislocations, however, in the growth behavior simulations of multi-ledge with elastic dislocation interactions, the coalescence behavior of ledges and growth rate of the leading step were both changed.

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    EFFECT OF THERMAL BARRIER COATINGS ABOVE MOULD MENISCUS ON MOULD HEAT TRANSFER AND OSCILLATION MARK MORPHOLOGY OF STRANDS
    Xiaoguang HOU,Engang WANG,Xiujie XU,Anyuan DENG,Wanlin WANG
    Acta Metall Sin, 2015, 51 (9): 1145-1152.  DOI: 10.11900/0412.1961.2015.00041
    Abstract   HTML   PDF (3511KB) ( 808 )

    Oscillation marks are closely related to the surface quality of bloom. Excellent surface quality of bloom is essential assurance of the technology of continuous casting and rolling. The improvement of heat transfer process through mold is beneficial to alleviate oscillation marks. A new method of inhibiting formation of oscillation marks in continuous casting, namely spraying or embedding thermal barrier coatings above mold meniscus (TBCMM) was proposed, by which the temperature and heat flux fluctuation of meniscus was reduced, and then the surface quality of strands was improved. Using an one-dimensional heat transfer simulating apparatus, the effect of location of thermal barrier coatings on heat transfer near meniscus was investigated, and the possible mechanism of TBCMM on inhibiting formation of oscillation marks was also discussed. With a dip simulator for continuous casting, lower melt point Sn-12.5%Pb alloy was casted with thermal barrier coating (TBC) and without TBC molds respectively, and the temperature fluctuation was also measured. The heat flux near meniscus in mold and the oscillation marks morphology on strands confirm the effectiveness of the proposed TBCMM. Finally, in a pilot continuous caster, casting experiments of steel with TBCMM was conducted, and the oscillation marks on billet surface were alleviated or removed.

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