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

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    EFFECT OF INDUCTION TEMPERING ON CARBIDE PRECIPITATION BEHAVIOR AND TOUGHNESS OF A 1000 MPa GRADE HIGH STRENGTH LOW ALLOY STEEL
    FANG Yupei, XIE Zhenjia, SHANG Chengjia
    Acta Metall Sin, 2014, 50 (12): 1413-1420.  DOI: 10.11900/0412.1961.2014.00306
    Abstract   HTML   PDF (11158KB) ( 4741 )

    By comparing induction tempering with conventional tempering, the effect of induction reheating tempering on carbide precipitation behavior and toughness of a 1000 MPa grade high strength low alloy steel was investigated. Microstructures of the steel in different heat treatment stages were characterized using SEM and TEM (with EDS), mechanical properties inclusive of Vickers hardness and toughness were tested. The results showed that microstructure of quenched samples consisted of lath martensite and lower bainite, needle like carbides were observed in lower bainitic lath. With tempering temperature increasing from 400 ℃ to 550 ℃, the shape of carbides located within the bainitic lath gradually changed from needle like to short rod like type. Carbides were fine and well distributed using induction tempering. When the tempering temperature was 550 ℃, the long axis length of short rod like carbides located within the bainitic lath by conventional reheating tempering was 200 nm, whereas the long axis length of short rod like carbides located within the bainitic lath by induction reheating tempering was about 60 nm. When tempering by conventional reheating, carbides mainly precipitated along martensite lath boundaries, while carbides were more dispersed in the matrix lath by induction reheating, the size of these dispersed carbides was less than 100 nm when tempering temperature was 550 ℃. As a result, a superior of mechanical properties with 344 HV and Charpy impact energy of 133 J at -20 ℃ was obtained with induction reheating tempering at 550 ℃.

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    MICROSTRUCTURES AND PROPERTIES OF SPRAY FORMED Nb-CONTAINING M3 HIGH SPEED STEEL
    WANG Hebin, HOU Longgang, ZHANG Jinxiang, LU Lin, YU Yipeng, CUI Hua, ZHANG Jishan
    Acta Metall Sin, 2014, 50 (12): 1421-1428.  DOI: 10.11900/0412.1961.2014.00216
    Abstract   HTML   PDF (6894KB) ( 724 )

    The billets of M3 high speed steel (HSS) with or without niobium addition were prepared via spray forming and compared with traditional cast steels with same composition, followed by hot forged and heat treated. The corresponding microstructure evolutions of steels induced by niobium have been investigated using SEM with EDS, XRD, OM, TEM and HRTEM. The results show that finer and uniformly-distributed grains without macrosegregation appear in the as-deposited HSS that are different to the as-cast HSS, 1% (mass fraction) niobium addition can promote the formation of primary MC-type carbides before onset of eutectic reaction, which can make the MC particles refined and evenly distributed. Niobium mainly contribute to the primary MC-type carbides by consuming carbon, the eutectic reaction is suppressed and the quantity of M2C eutectic carbides decrease, leading to more W and Mo atoms dissolve into matrix. Compared to spray formed M3 HSS, the niobium alloying M3 HSS possesses higher stability during austenitization, induced by the high stabilization of Nb-containing MC carbides, which can pin the grain boundaries and keep the grain size of primary austenite below that of spray formed M3 HSS. The quenched hardness of niobium-containing steel is remarkably higher, while the over tempering hardness of it is a little below than that of M3 HSS, it is related to the difference of dissolution rate of carbides during austenitization and the precipitation behavior of the secondary carbides after tempering. The amount of Nb-containing MC carbides are hard to dissolve into matrix, additionally, lower content of M2C carbides are in the as-deposited steel, leading to the larger numbers of nano-scaled M2C secondary carbides precipitate after tempering. Spray formed niobium-containing steel has a more advanced hardness and bending strength compared with ASP23, but possesses a lower impact toughness due to that the stress concentration can easily caused by mass of harder MC carbides distributed in matrix.

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    EFFECTS OF TEMPERING ON THE MICROSTRUC-TURE AND MECHANICAL PROPERTY OF ELECTRON BEAM WELDING JOINT OF 9Cr2WVTa STEEL
    GAO Heng, SONG Yuanyuan, ZHAO Mingjiu, HU Xiaofeng, RONG Lijian
    Acta Metall Sin, 2014, 50 (12): 1429-1436.  DOI: 10.11900/0412.1961.2014.00333
    Abstract   HTML   PDF (8132KB) ( 729 )

    9Cr2WVTa steel is one kind of reduced activation ferritic/martensitic (RAFM) steels which exhibit lower thermal expansion coefficient, higher thermal conductivity and less irradiation swelling compared with austenitic steel. It has been considered as the candidate structural material for the accelerator driven subcritical system (ADS). Due to the narrow heat affected zone and large depth to width ratio, electron beam (EB) welding is expected as a potential technique to connect components of ADS. However, few previous studies have focused on the weldability of 9Cr2WVTa steel by EB welding. In this work, EB welding was applied to join the 9Cr2WVTa steel and tempering was used to modify the microstructure and mechanical properties of the weld joint. Microstructure analysis shows that the weld metal consists of coarse lath martensite and d-ferrite. After high temperature tempering, large amounts of M23C6 type carbides precipitate in the matrix. Tempering lowers the hardness of the weld metal. Tensile tests show that the strength of the weld joint decreases, but the total elongation increases after tempering. It was found that the accurate impact energy of the weld metal was not able to be measured with standard Charpy impact specimen due to the crack deviation from the weld metal to the base metal. In order to estimate the impact property of the weld metal accurately, Charpy V-notch specimens with side-grooves were used. The result shows that tempering induces a significant improvement of the impact energy to the weld metal compared with the as-welded condition. Furthermore, the weld metal demonstrates better impact property than the base metal, which results from the existence of low fraction of d-ferrite in the weld metal.

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    INFLUENCE OF HEATING RATE ON THE DECARBU- RIZED ANNEALING MICROSTRUCTURE AND TEXTURE IN LOW-CARBON NON-ORIENTED ELECTRICAL STEEL
    XIA Dongsheng, YANG Ping, XIE Li, MAO Weimin
    Acta Metall Sin, 2014, 50 (12): 1437-1445.  DOI: 10.11900/0412.1961.2014.00311
    Abstract   HTML   PDF (8100KB) ( 866 )

    The present work investigates the effect of heating rate on the evolution of decarburized microstructures and textures in low-carbon electrical steels within the inter-critical temperature region. The results show that heating rate has a significant effect on both the final microstructures and textures during the process of decarburization annealing. The ''nucleation'' sites of columnar grains are determined by the heating rate. Slow heating rate would have the ''nuclei'' formed within a certain range of the surface layer, and finally leading to a fine-grained layer near the sample surface. By comparison, a complete columnar microstructure is acquired under the rapid heating condition. Strong g-fiber and relatively weak a-fiber components were obtained at the slow heating rate. In contrast, g-fiber texture is greatly weakened and a-fiber component slightly strengthened under the rapid heating condition, and a relatively strong {001}<120> texture is formed at the same time. The experimental results prove that the final decarburized textures are mainly dependent upon the texture component of recrystallized grains in the ''nucleation'' sites.

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    ROLLING CONTACT FATIGUE BEHAVIOR OF AN ULTRAHIGH CARBON STEEL
    LIU Hongji, SUN Junjie, JIANG Tao, GUO Shengwu, LIU Yongning, LIN Xin
    Acta Metall Sin, 2014, 50 (12): 1446-1452.  DOI: 10.11900/0412.1961.2014.00260
    Abstract   HTML   PDF (6778KB) ( 716 )

    With the development of modern industrial equipment and the requirement in energy conservation and emission reduction, the traditional high carbon and high chromium steel which is widely used as bearing material cannot meet these demands. Therefore, it is of paramount importance to exploit novel materials used as bearings with long life. In recent years, some new techniques have been used to improve the bearing life, such as physical vapor deposition (PVD), chemical vapor deposition (CVD) and plasma immersion implantation and so on. All these techniques are attempting to increase the surface hardness of the bearing. Though the bearing life has been extended to some extent, the application range of these techniques is limited by the price factor and the dimensions of components. Ultrahigh-carbon steels (UHCSs) have been studied for many years, and they possess outstanding mechanical properties and wear resistance. Therefore, it is interesting to explore the probability whether UHCSs can be used in the bearing application. It is well known that if bearings are well assembled, lubricated and loaded, rolling contact fatigue (RCF) is the main failure form. Accordingly, the evaluation of the resistance to RCF is of paramount importance for bearing materials. The RCF properties of UHCSs have never been studied in the past decades. Therefore, in this work, the RCF behavior of a UHCS with 1.29%C (mass fraction) was investigated in well lubricated conditions, using a flat washer-type RCF tester. In order to shorten the testing time, the maximum Hertzian stress was set as 4400 MPa. For comparison, the RCF lives of conventional GCr15 and SKF3 bearing steels were also tested under the same conditions. The results showed that the rated life L10 of the UHCS was 2.14 and 1.81 times longer than those of the GCr15 and SKF3 steels, respectively. Since more spherical residual carbide particles could be used to retard the grain growth during austenitization for the UHCS, the prior austenite grain size of the UHCS was only 6.91 μm. However, the prior austenite grain sizes of the GCr15 and SKF3 steels were 13.52 and 11.41 μm, respectively. Therefore, the average size of martensite plate of the UHCS was approximately half of those of the GCr15 and SKF3 steels. Finer grains were expected to retard the crack initiation and propagation, and then the RCF life would be prolonged. On the other hand, the carbon content and the volume fractions of precipitates in the martensite plates of the quenched and tempered UHCS were both higher than those of the GCr15 and SKF3 steels. These factors made the UHCS harder than GCr15 and SKF3 steels, which was beneficial for the improvement of RCF life.

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    EFFECT OF HEAT TREATMENT ON ANTIBACTERIAL PERFORMANCE OF 3Cr13MoCu MARTENSITIC STAINLESS STEEL
    WANG Shuai, YANG Chunguang, XU Dake, SHEN Minggang, NAN Li, YANG Ke
    Acta Metall Sin, 2014, 50 (12): 1453-1460.  DOI: 10.11900/0412.1961.2014.00184
    Abstract   HTML   PDF (3835KB) ( 318 )

    The effect of aging on antibacterial performance of 3Cr13MoCu martensitic stainless steel was studied by antibacterial test, Vickers hardness measurement, TEM observation, confocal laser scanning microscope (CLSM) and SEM observations. The results showed that increase of the aging temperature did favor to rapid precipitation of Cu-rich phases in the steel matrix, and correspondingly, the antibacterial rate was increased, but the hardness declined. Aging at 500 ℃ for 10~14 h could enhance both antibacterial rate and hardness due to the increase of precipitation of Cu-rich phases. Therefore the optimal heat treatment for 3Cr13MoCu martensitic stainless steel was proposed to solution at 1080 ℃ for 30 min, water cooling to room temperature, then aging at 500 ℃ for 10~14 h, air cooling to room temperature. CLSM and SEM observations indicated that 3Cr13MoCu martensitic stainless steel with optimum heat treatment could effectively killed the free bacteria and inhibited the formation of bacterial bio-films on its surface.

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    CORROSION BEHAVIOR OF B10 ALLOY EXPOSED TO SEAWATER CONTAINING VIBRIO AZUREUS, SULFATE-REDUCING BACTERIA, AND THEIR MIXTURE
    WEI Renchao, XU Fengling, LIN Cunguo, TANG Xiao, LI Yan
    Acta Metall Sin, 2014, 50 (12): 1461-1470.  DOI: 10.11900/0412.1961.2014.00204
    Abstract   HTML   PDF (4890KB) ( 464 )

    With increasing attention paid to the security issues of onshore engineering structure, corrosion researches of copper alloy were focused on the influence of single bacteria, especially the anaerobic sulfate-reducing bacteria (SRB). However, a part of documents indicated that comprehensive influence of natural bacteria on the copper alloy does exist, and whether the influence of single bacteria could represent the real impact of natural complex bacteria is remaining unclear. Under this consideration, electrochemical measurements, incorporated with surface morphology and composition analysis, were employed to investigate the corrosion behavior of B10 alloy in seawater which was inoculated into Vibrio azureus, SRB and their mixed strains, respectively, in this work. The results showed that these marine micro-organisms could affect the corrosion process of B10 alloy in relatively different ways. Compared with the sterile condition, Vibrio azureus could inhabit the corrosion of B10 alloy to some extent by blocking cathodic oxygen reducing process, while SRB could significantly promote its corrosion by accelerating anodic dissolution of B10 alloy via hydrogen depolarization and forming loose and bulky corrosion products without complete protection. In the mixed microbial medium, SRB multiply rapidly in the local anaerobic environment created by the biological membrane of Vibrio azureus, their interacting changed the corrosive micro-environment on the surface of B10 alloy. The smaller and complicated corrosion products formed in the seawater containing mixed strains obviously performed better than that produced in the medium containing SRB only, giving rise to a significant increase in anodic polarization; at the same time, similar cathodic process was still occurred in the mixed culture. As a result, the corrosion current density of B10 alloy fell in between those detected in two single microbial media. For the practice engineering applications, therefore, the conclusions drawn from single microbe medium should be cautiously and carefully adopted as the criterion to evaluate corrosion behavior of B10 alloy in actual microbial environment.

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    EFFECT OF ELECTROMAGNETIC MELT TREATMENT NEAR LIQUIDUS ON THE FORMATION OF NON-DENDRITE MICROSTRUCTURE OF SUPERALLOY
    GAO Zhongtang, HU Rui, WANG Jun, YANG Jieren, LI Jinshan
    Acta Metall Sin, 2014, 50 (12): 1471-1477.  DOI: 10.11900/0412.1961.2014.00153
    Abstract   HTML   PDF (4190KB) ( 812 )

    The effects of electromagnetic field and melt treatment near liquidus on the grain refinement of the Ni-20Cr-18W (mass fraction, %) superalloy have been studied. The average grain size of 60 kg ingot can be refined to 127 μm and the grains are both ?ne and globular. Based on the effect of electromagnetic field on atomic cluster, interface stability and transformation from dendritic grain to globular grain, the mechanism of grain refinement has been studied from the aspect of nucleation thermodynamics by OM, SEM, EBSD. The results show that electromagnetic melt treatment near liquidus results in signi?cant re?nement. The grain refinement can be attributed to the mechanism that atomic clusters and globular structures can become the nucleus when the initial undercooling reaches certain level. Electromagnetic field not only improves homogeneity of the macroscopic temperature field, but also plays a positive role in interface stability and dendritic to globular transformation, which increases the nucleation rate.

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    EFFECT OF THERMAL EXPOSURE AT 750 ℃ ON ROOM TEMPERATURE TENSILE DUCTILITY OF CAST TiAl ALLOY WITH DIRECTIONAL LAMELLAR MICROSTRUCTURE
    ZHU Chunlei, LI Sheng, LI Haizhao, ZHANG Ji
    Acta Metall Sin, 2014, 50 (12): 1478-1484.  DOI: 10.11900/0412.1961.2014.00248
    Abstract   HTML   PDF (3565KB) ( 514 )

    The effect of thermal exposure on room temperature tensile ductility of cast TiAl alloy with directional lamellar microstructure was evaluated at 750 ℃ for 48~300 h in atmosphere. By preloading, unloading, dye-penetrating followed by reloading until fracture for exposed samples, initiation and propagation behavior of the microcrack triggered by surface brittle layer was mainly analyzed in order to explain that the directional lamellar structure retains a better ductility at room temperature after thermal exposure. The results show that room temperature tensile ductility is still retained above 2.0% and 1.0% after exposure for 150 and 300 h at 750 ℃, respectively. The embrittlement of the directional lamellar microstructure caused by thermal exposure is much less than that of duplex microstructure and the other lamellar microstructures. At a stress of 430 MPa, the microcrack forms at the Al-depleted brittle layer and propagates into the substrate during subsequent loading. Just as the sharp notch, the microcrack can constrain the plastic deformation, which is the main mechanism of the brittlement for TiAl alloy by thermal exposure. The directional lamellar microstructure with the lamellae interface parallel to the substrate surface is obtained, which is good for restraining the micro-crack propagation into the substrate and retaining higher ductility at room temperature after thermal exposure.

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    INFLUENCE OF TEMPERATURE ON LOW-CYCLE FATIGUE BEHAVIOR OF INCONEL 625 NICKEL-BASED SUPERALLOY WELDING JOINT
    WANG Yuanyuan, CHEN Lijia, WANG Baosen
    Acta Metall Sin, 2014, 50 (12): 1485-1490.  DOI: 10.11900/0412.1961.2014.00241
    Abstract   HTML   PDF (3876KB) ( 697 )

    The low-cycle fatigue tests of Inconel 625 nickel-based superalloy welding joints at 25 and 760 ℃ were performed . The strain-fatigue life data and cyclic stress-strain data were analyzed to determine the strain fatigue parameters of Inconel 625 superalloy welding joints. The result showed that the relationship between elastic strain amplitude, plastic strain amplitude and reversals to failure could be described by Basquin and Coffin-Manson equations, respectively. Under the strain control, the continuous cyclic softening was observed at 25 ℃, however, the cyclic hardening appeared at 760 ℃. At 25 ℃, the fatigue crack of Inconel 625 superalloy welding joints initiated transgranularly at the specimen surface and propagated in the transgranular mode. Differently, at 760 ℃, the fatigue crack initiated transgranularly at the specimen surface, but propagated in mixed transgranular and intergranular modes.

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    SERRATED YIELDING OF 5456 ALUMINIUM MAGNE- SIUM ALLOY BASED ON THREE DIMENSIONAL DIGITAL IMAGE CORRELATION
    CAI Yulong, FU Shihua, WANG Yuhui, TIAN Chenggang, GAO Yue, CHENG Teng, ZHANG Qingchuan
    Acta Metall Sin, 2014, 50 (12): 1491-1497.  DOI: 10.11900/0412.1961.2014.00251
    Abstract   HTML   PDF (1683KB) ( 705 )

    One of the key issues to deeply understand the intrinsic mechanism of Portevin-Le Chatelier (PLC) phenomenon is to analyze the spatial distribution and the evolution of the deformation in PLC bands, associated with serrated yielding. In this work, an investigation was carried out with 5456 aluminium magnesium alloy sheet specimens of different thicknesses at room temperature by using three dimensional digital image correlation (3D-DIC) method, which focused itself on the deformation analysis within serrated yielding under different loading procedures. After statistically analyzing loading curves, it is found that the average amplitude and average period increase with increasing strain. Moreover, there is a positive correlation between the average period and the thickness of specimen, while the average amplitude and the thickness of specimen are independent to some degree. In addition, the experimental result also indicates that the width of the PLC band increases with increasing thickness of the specimen. However, the spatial analysis in PLC bands proves that the angle between the PLC band and the tensile direction is about 60°, which is irrelevant to the specimen thickness and the serration amplitude. Furthermore, it is also found that PLC bands of out-of-plane displacement coincide with bands of strain in the tensile direction by temporal analysis. Most importantly, the experimental result shows that the serration amplitude is proportional to the maximum strain in PLC band when PLC band occurs.

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    INFLUENCE OF TEMPERATURE ON QUASI-STATIC MECHANICAL PROPERTIES OF Zr-45Ti-5Al-3V ALLOY
    LIU Dingming, ZHANG Bo, WANG Jie, WANG Aimin, WANG Yandong, ZHANG Haifeng, HU Zhuangqi
    Acta Metall Sin, 2014, 50 (12): 1498-1504.  DOI: 10.11900/0412.1961.2014.00310
    Abstract   HTML   PDF (8161KB) ( 648 )

    Zr alloys are widely used in pressurized-water reactors as fuel cladding materials due to their low neutron absorption cross section and excellent radiation resistance. Aside from the aforementioned properties, Zr alloys have high strength, relative low density and many other excellent physical and chemical properties that make them promising structural materials used in the aerospace environment. Zr-45Ti-5Al-3V alloy is a high strength zirconium alloy which is newly developed for use in aerospace environment. The temperature in space environment can change from -100 ℃ to more than 100 ℃, so it is necessary to study the mechanical behavior of Zr-45Ti-5Al-3V alloy under different temperatures. In this work, mechanical properties of Zr-45Ti-5Al-3V alloy under different temperatures (-100, 25, 100 and 200 ℃) and strain rates (10-4, 10-3 and 10-2 s-1) were investigated. The microstructure of the Zr-45Ti-5Al-3V alloy is characterized by SEM and XRD. It is shown that the alloy is comprised of two phases: a lath-like a phase with hcp structure is distributed uniformly in the matrix comprised of a b phase with bcc structure. Quasi-static mechanical properties of Zr-45Ti-5Al-3V alloy were studied in temperature range of -100~200 ℃ under various strain rates (10-4, 10-3 and 10-2 s-1) using the Instron 5528 electric universal material testing machine. The results showed that the alloy possessed yield strength of more than 1355 MPa at room temperature and higher fracture strength in tensile test, but the elongation was small. With increasing temperature, the yield strength and the fracture strength of the alloy decreased, while the amount of plastic deformation increased. Under the condition of compression test, the yield strength also decreased with temperature increasing, while the plasticity and fracture strength reached maximum at room temperature. The influence of strain rate on the mechanical properties of the alloy was not significant under both tensile and compression tests.

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    RESEARCH ON INTERNAL CRACK SUSCEPTIBILITY OF CONTINUOUS-CASTING BLOOM BASED ON MICRO-SEGREGATION MODEL
    DOU Kun, QING Jiasheng, WANG Lei, ZHANG Xiaofeng, WANG Bao, LIU Qing, DONG Hongbiao
    Acta Metall Sin, 2014, 50 (12): 1505-1512.  DOI: 10.11900/0412.1961.2014.00317
    Abstract   HTML   PDF (1643KB) ( 745 )

    The solidification and cooling of liquid steel in continuous casting process is a complicated non-equilibrium phenomenon. During steel solidification process, the micro-segregation of solute elements between liquid steel and solidified shell will vary with their temperature-dependent diffusion coefficients and equilibrium distribution coefficients. Due to non-uniform cooling pattern in the continuous casting process of steel blooms, the fluctuation of cooling rate in bloom will have a great influence on micro-segregation degree of the elements. The micro-segregation behavior of solute elements in steel solidification process is responsible for the variation of characteristic temperatures such as zero strength temperature (ZST), zero ductility temperature (ZDT) and liquid impenetrable temperature (LIT), which make up the brittle temperature range in steel solidification. During continuous casting process of steel, internal cracks created by thermal and mechanical deformation tend to occur in this range. To prevent the occurrence of these cracks in continuous casting bloom, it is essential to better understand about the internal crack susceptibility concerning micro-segregation behavior in the non-uniform cooling process. In this work, a micro-segregation analytical model for YQ450NQR1 steel continuous casting bloom is established to study the inter-dendritic segregation behavior of main solute elements C, Si, Mn, P and S at various cooling rates, the results show that P and S are more likely to segregate compared with C, Si and Mn and the increase of cooling rate weakens the micro-segregation degree of C, Si, Mn, P and S. Based on the micro-segregation model established above, ZST, ZDT and LIT for YQ450NQR1 steel are calculated and the influences of cooling rate on ZST, ZDT and LIT are analyzed. It reveals that ZST, ZDT and LIT of YQ450NQR1 steel bloom decrease accordingly with the increase of cooling rate. On this basis, the index of internal crack susceptibility (IICS) is defined to quantitatively characterize the internal crack susceptibility of the bloom. The results show that the internal crack susceptibility becomes larger while the IICS value approaches to 1. Furthermore, an internal crack susceptibility model is obtained concerning IICS and cooling rate (CR) and the validation is performed to certify the model′s suitability in quantitatively predicting internal crack susceptibility of YQ450NQR1 steel continuous casting bloom in the non-uniform cooling process.

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    LASER IN SITU SYNTHESIZED TITANIUM DIBORIDE AND NITINOL REINFORCE TITANIUM MATRIX COMPOSITE COATINGS
    LIN Yinghua, LEI Yongping, FU Hanguang, LIN Jian
    Acta Metall Sin, 2014, 50 (12): 1513-1519.  DOI: 10.11900/0412.1961.2014.00185
    Abstract   HTML   PDF (6322KB) ( 754 )

    Laser cladding is a technique in which a laser beam is used as the heating source to melt the alloy powder to be clad on the surface of titanium alloy substrate. Currently, the surface of many titanium alloy components needs repairing after a period of service in order to extend their service life. TiB and TiB2 are considered as the excellent ceramic reinforced particle for their compatible physical and thermodynamic properties, high hardness and Young's modulus of elasticity. The intermetallic compound NiTi, well-known for its shape memory effect and pseudo-elasticity, is one of the rarely few intermetallic compounds having excellent combination of high strength, ductility and toughness as well as excellent wear resistance and fabrication processing properties. An in-situ TiB/TiB2 structured ceramic materials as the reinforcing phase and NiTi intermetallic phase as the matrix would be expected to have an outstanding combination of high hardness and toughness. NiTi alloy, TiB short fiber and TiB2 particulate reinforced titanium matrix composite coatings were prepared by laser in situ synthesis on titanium surface with different ratios of Ni powder and TiB2 powder mixture as a preset level. Synthesis of titanium matrix composite coating was analyzed by XRD, SEM and EPMA. The results show that the surface quality of the coating increases with increasing laser power density and the amount of Ni powder. Whereas, the new phase of NiTi2 and coarse diameter of TiB short fiber are found in the coating when the amount of Ni added is improved. The reaction mechanism is discussed based on thermodynamic calculations. The reaction driving force size to Ni3Ti>NiTi2>NiTi>TiB order arrangement are found by thermodynamic calculation, and reaction mechanism of competition between the different elements is discussed based on phase variation of the type and content in the coating.

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    EFFECT OF Ni ADDITION ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF TiB2/TiB TITANIUM MATRIX COMPOSITE COATINGS
    LIN Yinghua, LEI Yongping, FU Hanguang, LIN Jian
    Acta Metall Sin, 2014, 50 (12): 1520-1528.  DOI: 10.11900/0412.1961.2014.00263
    Abstract   HTML   PDF (10890KB) ( 357 )

    Titanium alloys have been known as useful materials for their superior mechanical properties, low density and high specific strength. However, the application of conventional titanium alloys on engine parts of airplane is limited by their poor wear resistance, low fatigue strength and low hardness. Particles reinforced titanium matrix composites have attracted extensive investigation in material science and engineering. Mechanical properties can be improved by reinforcing the loaded outer layer of Ti with ceramic particles. TiB and TiB2 are considered as the excellent ceramic reinforced particles for their compatible physical and thermodynamic properties, high hardness and Young's modulus of elasticity. However, TiB2 has high brittleness. The intermetallic compound NiTi, well-known for its shape memory effect and pseudo-elasticity, is one of the rarely few intermetallic compounds having excellent combination of high strength, ductility and toughness as well as excellent wear resistance and fabrication processing properties. An in situ TiB/TiB2 structured ceramic materials as the reinforcing phase and NiTi intermetallic phase as the matrix would be expected to have an outstanding combination of high hardness and toughness. To investigate the microstructure and properties of the cladded layers, two types of composites were prepared by laser cladding powders containing TiB2 and Ni+TiB2 as a preset level on the surface of titanium alloy. The composite coatings were analyzed by XRD, SEM, EPMA, micro hardness tester and brinell hardness. The results showed that TiB2 particulate and TiB short fiber reinforced titanium matrix composite coating were obtained, which had poor quality of coating shape when Ni was not added. The coating was mainly composed of TiB2, TiB, Ti and NiTi phase when Ni was added and surface coating quality was improved and the bcc structure of NiTi alloy was filled with TiB2 particulate and TiB short fiber surrounding. The coating was coarse with particle size of TiB2 at 3~5 μm when Ni was not added, while it contained fine particles of TiB2 with particle size of 0.5~3 μm and b-Ti base appeared when Ni was added. The micro-hardness of the coating was reduced when Ni was added, but the fracture toughness of the coating increased. The mechanism of toughening was discussed based on fracture behaviors. Fracture toughness of titanium matrix composite coatings were improved mainly through particle debonding and short fiber breakage by the offset resulting in crack deflection.

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    EFFECTS OF STRUCTURE AND INTERNAL STRESSES IN OXIDE FILMS ON CORROSION MECHANISM OF NEW ZIRCONIUM ALLOY
    ZHANG Haixia, LI Zhongkui, ZHOU Lian, XU Bingshe, WANG Yongzhen
    Acta Metall Sin, 2014, 50 (12): 1529-1537.  DOI: 10.11900/0412.1961.2014.00261
    Abstract   HTML   PDF (2484KB) ( 786 )

    The corrosion resistance of new zirconium alloys containing Nb, used as the fuel cladding materials in water-cooled nuclear power reactors, is closely related to the characteristics of the oxide films, including the internal stresses and the crystal structure. However, the relation of the corrosion kinetics to the internal stresses and the crystal structure of the oxide films has not been well understood, also the corrosion mechanism of new zirconium alloys has not been confirmed. Therefore, it is helpful to solve the above problems, furthermore improve the corrosion resistance of new zirconium alloys, to characterize the internal stresses and the crystal structure of the oxide films accurately. The internal stresses and the crystal structure of the oxide films of NZ2 zirconium alloy, corroded in 360 ℃, 18.6 MPa lithiated water and 400 ℃, 10.3 MPa steam, were tested by XRD and Raman spectroscopy, and the microstructure of the oxide films was investigated by SEM. The results of the crystal structure show that tetragonal ZrO2 (t-ZrO2) content in the oxide films of NZ2 alloy decreases, monoclinic ZrO2 (m-ZrO2) content increases with the prolongation of the corrosion time, t-ZrO2 transforms into m-ZrO2. And cubic ZrO2 (c-ZrO2) appears in the oxide films when the thickness of the oxide films reaches 2 mm. Corrosion resistance of NZ2 alloy is improved when the content of t-ZrO2 in the oxide films increases. The results of the internal stresses and the microstructure of the oxide films indicate that the high compressive stresses exist in the oxide films. At the beginning of the corrosion, the compressive stresses in the oxide films increase with the corrosion time. When the thickness of the oxide films reaches 2 mm, the compressive stresses exceed the critical value and the stresses are released. The stress relaxation leads to the formation of the cracks, which reduces the protection of the oxide films, therefore the corrosion transition occurs. After the transition, the compressive stresses of the oxide films are constantly low. So the corrosion transition is closely related to the relaxation of the compressive stresses. The high compressive stresses and t-ZrO2 content are corresponding to the good corrosion resistance. Also the stabilization mechanisms of t-ZrO2 and c-ZrO2 are explored, finally the corrosion mechanism of new zirconium alloys is established.

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    EFFECT OF OXIDATION TIME UNDER HIGH MAG- NETIC FIELD ON THE MICROSTRUCTURE AND OPTICAL PROPERTIES OF OXIDIZED Co-DOPED ZnO FILMS
    LI Guojian, WANG Zhen, WANG Qiang, WANG Huimin, DU Jiaojiao, MA Yonghui, HE Jicheng
    Acta Metall Sin, 2014, 50 (12): 1538-1542.  DOI: 10.11900/0412.1961.2014.00376
    Abstract   HTML   PDF (2569KB) ( 635 )

    The effects of the oxidation time under high magnetic field on the microstructure and optical properties of oxidized Co-doped Zn prepared by vacuum evaporation were studied by FESEM, XRD and ultraviolet visible spectrophotometer. The results show that the growth of coral dendritic structures of ZnO films is inhibited by high magnetic field. Spherical particles of ZnO film oxidized for 60 min without magnetic field transforms to leaf like morphologies with the application of a 6 T magnetic field. With increasing oxidation time under high magnetic field, the preferred orientation of (101) transforms to (002); distribution of magnetic Co atoms in ZnO is influenced; the band gap is decreased to the range of 2.95~3.13 eV. The transmittance is decreased by the spherical morphologies. However, it is increased by the application of high magnetic field. These results give a new method to tune the microstructure and optical properties of surface morphology, preferred orientation, magnetic atomic distribution, transmittance, and band gap.

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