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

About the Journal

  Current Issue
    , Volume 51 Issue 4 Previous Issue    Next Issue
    For Selected: View Abstracts
    INTERNAL FRICTION STUDY OF MECHANISM OF BAKE-HARDENING ON LOW CARBON STEEL
    LI Weijuan, ZHANG Hengyi, FU Hao, ZHANG Jianping, QI Xiangyu
    Acta Metall Sin, 2015, 51 (4): 385-392.  DOI: 10.11900/0412.1961.2014.00434
    Abstract   HTML   PDF (3639KB) ( 635 )

    High strength steel plates are being applied more extensively in automobile industry under higher demands of weight reduction, safety and environmental protection. As one of the high strength steel plates for automobile body panel, bake-hardening steel plate is featured by low yield strength and good formability during stamping, and is particularly featured by improved yield strength through following paint baking process, exhibiting higher strength and anti-dent ability in service. Bake-hardening (BH) is closely related to interactions between interstitial atoms and crystal defects during baking process. In this work, BH mechanisms in low carbon steel are studied under different annealing temperatures by measuring and analyzing stress-strain curves and BH values in baked conditions, and internal friction curves in both deformed and baked conditions. The results show that when the annealing temperature increases gradually from 750 to 880 ℃, the stress-strain curves exhibit discontinuous yielding behavior with stronger serration of yield platform and continuous elongation of yield point. With the increase of the annealing temperature from 750 to 780 ℃, BH value decreases, difference between Snoek peak values in deformed and baked conditions increases, SKK peak value decreases, and Kê peak value does not change significantly, which indicates that solid solution strengthening dominates the BH. When the annealing temperature increases from 780 to 880 ℃, BH value continuously increases, difference between Snoek peak values in deformed and baked conditions gradually decreases, relaxation strength of SKK peak gradually increases, and Kê peak is stable, which indicates that Cottrell atmosphere strengthening is playing an increasing role in the BH. The BH is due to a combined mechanism contributed by solid solution strengthening Cottrell atmosphere strengthening and precipitation strengthening.

    Figures and Tables | References | Related Articles | Metrics
    CARBON DIFFUSION AND ITS EFFECT ON HIGH TEMPERATURE CREEP LIFE OF Cr5Mo/A302 DISSIMILAR WELDED JOINT
    JIANG Yong, ZHANG Zuo, GONG Jianming
    Acta Metall Sin, 2015, 51 (4): 393-399.  DOI: 10.11900/0412.1961.2014.00435
    Abstract   HTML   PDF (3698KB) ( 300 )

    Based on the consideration of economy and being easy to construct on site, Cr5Mo/A302 dissimilar welded joints are widely used in elevated temperature applications such as coal-fired power station, nuclear plant and petrochemical industry. Because of the difference in carbide forming elements on both sides of welded joint fusion line, carbon diffusion will happen in service progress and induce premature invalidation. The calculation methods of carbon diffusion and its harm to creep life of dissimilar welded joints have been investigated in the past decades. Theoretically, at a certain temperature, creep damage mechanism changes according to stress levels. However, the previous works paid little attention on the effect of carbon diffusion to creep life at different stress levels and few studies have been done focusing on relationships among carbon diffusion degree, stress level and creep life. In this work, carbon diffusion behavior of Cr5Mo/A302 dissimilar welded joint was first studied by employing aging treatment test, micro-hardness measurement and Fick's second law. Subsequently, creep tests were performed to investigate the effect of 200 mm wide decarburized zone on the joint creep life at different stress levels. The results showed that carbon diffusion in Cr5Mo/A302 dissimilar welded joints became serious with the increase of aging treatment time. The widths of carbon rich zone and decarburized zone were both consistent with the parabola distribution law and could be simulated by Fick's second law. The decarburised zone, which formed in the process of aging, reduced the creep life of the joint greatly at high stress levels. However, with the decline of testing load, its effect became much smaller. When the stress level dropped to about 36% of the yield stress, its effect was negligible. Meanwhile, the relationship of stress and carbon diffusion impact factor S was established to determine the critical value of the stress below which the decarburised zone would not affect the high-temperature creep life of the dissimilar joint.

    Figures and Tables | References | Related Articles | Metrics
    RESEARCH ON HIGH-CYCLE FATIGUE BEHAVIOR OF FV520B STEEL BASED ON INTRINSIC DISSIPATION
    GUO Qiang, GUO Xinglin, FAN Junling, WU Chengwei
    Acta Metall Sin, 2015, 51 (4): 400-406.  DOI: 10.11900/0412.1961.2014.00546
    Abstract   HTML   PDF (873KB) ( 367 )

    Systemic experimental research was carried out on high-cycle fatigue behavior of FV520B steel based on the theory and calculation model of intrinsic dissipation. The experiment results show that the intrinsic dissipation of FV520B steel increases with the increase of the applied stress amplitude. Generally, the inflection point of intrinsic dissipation corresponds to the transition of the generation mechanism of intrinsic dissipation: from the reversible motion of the microstructure (the swing of dislocation lines between strong pinning points) to the combined effects of the reversible and irreversible motion of the microstructure (the generation of permanent slip, the unpinning from strong points and the multiplication of dislocation). And the stress amplitude corresponding to the inflection point is just the critical stress value inducing fatigue damage accumulation. Moreover, the results also indicate that FV520B steel subjected to constant stress amplitude keeps a relatively steady rate related to the applied stress amplitude and independent of the loading sequences. Additionally, the loading frequency has no effect on the fatigue damage per loading cycle. Fatigue failure will occur once the amount of the intrinsic dissipation, due to the irreversible motion of the microstructure, accumulates to a threshold value. And the energy threshold is found to be independent of the loading history.

    Figures and Tables | References | Related Articles | Metrics
    REGULATION OF MULTI-PHASE MICROSTRUCTURE AND MECHANICAL PROPERTIES IN A 700 MPa GRADE LOW CARBON LOW ALLOY STEEL WITH GOOD DUCTILITY
    ZHOU Wenhao, XIE Zhenjia, GUO Hui, SHANG Chengjia
    Acta Metall Sin, 2015, 51 (4): 407-416.  DOI: 10.11900/0412.1961.2014.00576
    Abstract   HTML   PDF (12510KB) ( 574 )

    Low carbon and low alloy steels require good combination of strength and ductility to ensure safety and stability of structures. Heat treatment in intercritical area can not only produce multi-phase microstructure, but also lead to the redistribution of alloying elements in different phases. Multi-step intercritical heat treatment is favorable to obtain retained austenite that is stabilized by repeated enrichment of alloying elements in reversed austenite and nanometer-sized precipitate that are primarily formed during tempering. Excellent mechanical properties are contributed by transformation-induced-plasticity effect of retained austenite and precipitation hardening effect of nanometer-size precipitates. In this work, the microstructural evolution and relative mechanical properties were investigated in a low carbon low alloy steel processed by a three-step heat treatment, namely, intercritical annealing, intercritical tempering and tempering. The microstructure was a typical dual-phase microstructure consisting of intercritical ferrite and bainite/martensite after intercritical annealing, and primarily comprised of intercritical ferrite, tempered bainite/martensite and retained austenite after intercritical tempering. Retained austenite with volume fraction of 29% distributed at the ferrite/bainite (martensite) boundaries and betweent bainitic/martensitic laths. Retained austenite was stabilized by enrichment of C, Mn, Ni and Cu in reversed austenite during the reversion transformation process. NbC precipitates with average size of 10 nm was formed in ferrite matrix and bainite/martensite, while Cu-containing particles in size range of 10~30 nm precipitated in ferrite and retained austenite during intercritical tempering and tempering process. The morphology of NbC precipitates was spherical, elliptical and irregular, and copper precipitates were spherical. With the combination of transformation-induced-plasticity (TRIP) effect of retained austenite and precipitation hardening, the steel possessed outstanding mechanical properties: yield strength > 700 MPa, tensile strength > 900 MPa, uniform elongation > 20%, and total elongation > 30%.

    Figures and Tables | References | Related Articles | Metrics
    INTERPHASE PRECIPITATION BEHAVIORS OF NANOMETER-SIZED CARBIDES IN A Nb-Ti-BEARING LOW-CARBON MICROALLOYED STEEL
    LI Xiaolin, WANG Zhaodong
    Acta Metall Sin, 2015, 51 (4): 417-424.  DOI: 10.11900/0412.1961.2014.00582
    Abstract   HTML   PDF (4902KB) ( 962 )

    High strength low alloy steels utilize chemical composition design of low carbon content and are microalloyed with Nb, V and Ti, or other additions, such as Mo and B, etc. The increase of strength is attributed to grain refinement strengthening, solid-solution strengthening, dislocation strengthening and precipitation hardening. Moreover, the precipitation hardening attracts more and more attentions. However, the detailed results on the sheet spacing, inter-particle spacing, crystallography, composition and the nucleation site of the interphase precipitation carbides in Nb-Ti containing steels have not been reported as yet. In this work, the microstructure, mechanical properties and precipitation behaviors in a low carbon Nb-Ti microalloyed steel were investigated using the dilatometer and TEM. The results show that the interphase precipitation can be observed for different isothermal temperatures and the sheet spacing, inter-particle spacing and size of the interphase precipitation carbides are refined by lowing isothermal temperature. The characteristic sheets of interphase precipitation carbides were identified as planar and curved. The planar sheets of interphase precipitation carbides have been analyzed and found to be parallel with {011},{012},{013}and {035} planes of ferrite. Moreover, the interphase precipitation carbides which have been determined to be (Nb, Ti)C have a NaCl-type crystal structure with a lattice parameter of 0.434 nm and obey the Baker-Nutting (B-N) orientation relationship with respect to ferrite matrix. The contribution of the interphase precipitation hardening to the yield strength of the experiment steel has been estimated above 300 MPa, based on the Orowan mechanism.

    Figures and Tables | References | Related Articles | Metrics
    MICRO-CHARACTERIZATION OF DISSIMILAR METAL WELD JOINT FOR CONNECTING PIPE- NOZZLE TO SAFE-END IN GENERATION III NUCLEAR POWER PLANT
    DING Jie, ZHANG Zhiming, WANG Jianqiu, HAN En-Hou, TANG Weibao, ZHANG Maolong, SUN Zhiyuan
    Acta Metall Sin, 2015, 51 (4): 425-439.  DOI: 10.11900/0412.1961.2014.00299
    Abstract   HTML   PDF (20355KB) ( 457 )

    The dissimilar metal weld joint (DMWJ) in primary water system of pressurized water reactors (PWRs) has been proven to be a vulnerable component owing to its proneness to different type of flaws. Thus, maintaining integrity of such joint in case of defect presence is of great importance to the design and safe management of nuclear power plants (NPPs). For a reliable integrity analysis of DMWJ, it is essential to understand the microscopic characteristics in all regions of the joint. In this work, OM, TEM, SEM, durometer, AFM, MFM and SKPFM were utilized to investigate the microstructure, micro-hardness and the distribution of main elements, grain boundary characteristic and residual strain in the A508/52M/316L DMWJ that used for connecting the pipe safe-end and the nozzle of reactor pressure vessel in PWRs, and a comparative analysis about the microstructure and property along the radical direction of the DMWJ was obtained. The results showed that there was no region that differed from the other part of the weldment in terms of the microstructure and micro-hardness dramatically. A layer of fine grain resulting from unmelted filler metal was found in the backing weld part of the joint. The residual strain in the heat affected zone (HAZ) of 316L was higher than that in other regions. Meanwhile, drastic variations in the microstructure, chemical composition distribution and grain boundary character distribution (GBCD) in both the 316L/52Mw and the 52Mb/A508 interface regions were observed. The analyses using TEM and MFM test showed that a large number of chromium and molybdenum-rich precipitates particles distributed both along the grain boundaries and inside grains in the 316L base metal, which were identified to be precipitates with complex elementary composition rather than the normal string delta ferrite in 316L austenitic stainless steel. The SKPFM test result indicated that these precipitates were more prone to be corroded than the base metal. Therefore, further investigation about the cause of deformation and the impacts to the corrosion resistance, particularly the stress corrosion cracking (SCC) sensitivity of the precipitates needs to be carried out.

    Figures and Tables | References | Related Articles | Metrics
    THE CORROSION BEHAVIOUR OF NiCu LOW ALLOY STEEL IN A DEAERATED BICARBONATE SOLUTION CONTAINING Cl- IONS
    LU Yunfei, YANG Jingfeng, DONG Junhua, KE Wei
    Acta Metall Sin, 2015, 51 (4): 440-448.  DOI: 10.11900/0412.1961.2014.00349
    Abstract   HTML   PDF (3504KB) ( 559 )

    The corrosion behaviour of low alloy steel containing Ni and Cu was studied because it is a promising candidate canister material for the disposal of high-level radioactive waste (HLW) in China. Due to the intensely radioactive nature of HLW, the waste has to be prevented from reaching the biosphere for many tens of thousands of years. Deep geological disposal is now considered to be the most preferable option for isolating HLW and it relies on series of natural and engineered barriers, e.g. a metallic canister. However, as soon as the waste package is settled, groundwater would seep back slowly through the outer barriers and ultimately arrive at the surface of the canister. Accordingly, there comes the groundwater-induced dissolution of the canister and subsequent transport of radionuclides through the barriers. That is to say, the effectiveness of radionuclide retention and isolation depends mostly and finally on the corrosion resistance of metallic canisters in deep groundwater environments. In this work, the test solution is deaerated 0.1 mol/L NaHCO3+0.1 mol/L NaCl, simulating the deep groundwater environment. The evolution of corrosion of NiCu low alloy steel in the test solution was investigated by electrochemical measurements. XRD was used to illustrate the composition of formed corrosion products. SEM was used to observe the electrode surface morphology and the cross section of the rust layer. The electrochemical results showed that low alloy steel has a lower corrosion rate and is less prone to localized corrosion than low carbon steel. In order to understand the mechanism of alloying elements, EDS and EPMA were used to analyse the distribution of alloying elements cross-sectional. XPS and E-pH diagram were used to estimate the possible existence form of alloying elements. By means of EDS and EPMA, it was founded that Ni is concentrated in the inner rust layer while the enrichment of Cu is not so obvious. XRD, XPS and E-pH results indicated that Ni and Cu are existed in the form of NiFe2O4 and CuFeO2 respectively.

    Figures and Tables | References | Related Articles | Metrics
    EFFECT OF NOTCH ORIENTATION AND LOCAL RECRYSTALLIZATION ON THERMAL FATIGUE PROPERTIES OF A DIREC- TIONALLY SOLIDIFIED Co-BASED SUPERALLOY
    PU Sheng, WANG Li, XIE Guang, DING Xianfei, LOU Langhong, FENG Qiang
    Acta Metall Sin, 2015, 51 (4): 449-457.  DOI: 10.11900/0412.1961.2014.00425
    Abstract   HTML   PDF (8884KB) ( 506 )

    The directionally solidificated (DS) Co-based superalloys are widely used in aircraft turbine vanes due to the good stress-rupture parameters and excellent hot corrosion resistance. The cyclic change of temperatures and complex stress state thermal fatigue (TF) cracks happen frequently in vanes during service. However, most of the work are conducted in Ni-based superalloys and there is rare report concerning the TF behavior of DS Co-based superalloys. Furthermore, due to the residual strain accumulated during processing, shot peening, grinding and recrystallization (RX) frequently occur when the DS components are exposed to high temperatures. It is believed that RX may change the microstructure, especially adding more grain boundaries to DS alloys, and result in the reduction of the mechanical properties of DS superalloys. Therefore, in this work, V-notch plate specimens with notch direction perpendicular and parallel to the DS orientation are machined from the DS plate. Local RX grains are prepared (local indented and then heat treated) in the notch areas of some samples. TF test is conducted between 1000 ℃ to room temperature. The effect of DS orientation and RX on TF properties of a DS Co-based superalloy is investigated. The results indicate that the cracks propagate along the interdendritic regions in the samples with notches parallel to the DS direction, which exhibites lower TF properties than samples with notches vertical to the DS direction. TF cracks initiate and propagate along RX boundaries in samples containing RX grains. Precipitation of M23C6 carbides is found along the RX boundaries during TF tests. Due to the oxidation at the tip of crack, M23C6 desquamates and leads to the formation of micro voids, which accelerates the crack propagation and decreases TF properties of tested alloy. In samples with notches parallel to the DS direction, cracks preferentially propagate along the RX grain boundaries.

    Figures and Tables | References | Related Articles | Metrics
    CARBIDE EVOLUTION BEHAVIOR OF K416B AS-CAST Ni-BASED SUPERALLOY WITH HIGH W CONTENT DURING HIGH TEMPERATURE CREEP
    XIE Jun, YU Jinjiang, SUN Xiaofeng, JIN Tao, SUN Yuan
    Acta Metall Sin, 2015, 51 (4): 458-464.  DOI: 10.11900/0412.1961.2014.00543
    Abstract   HTML   PDF (2292KB) ( 547 )

    As-cast Ni-based superalloys with high W content are used extensively in the turbine vane of aero-engine due to their good heat resistance and temperature capability. During high temperature service, the creep deformations and microstructure evolution are occurred in the using materials, and the creep behavior mainly depends on their chemical composition and microstructure, such as size, distribution and morphology of g ' phase and carbides. Thereinto, the mophologies of carbide phases are closely related to creep resistance of the alloy. Generally, the carbide particles displaying dispersive distribution may enhance the creep resistance of the alloy, while the carbide with continuous morphologies distributed in the boundaries, they may provide easy paths for crack propagation and degrade the mechanical properties of the alloy. Besides the creep life of the alloy also depends on the microstructure evolution under high temperature. But the evolution mechanism of carbides in K416B superalloy during creep is still unclear up to now. For this reason, by means of creep property measurement and microstructure observation, the evolution behavior of precipitates in K416B Ni-based superalloy with high W content during high temperature creep has been investigated. The results show that the size of g ' phase is inhomogeneous in the as-cast alloy, and the stripe MC-carbide distribute in the inter-dendrite regions displaying Chinese structures. During high temperature creep applied stress, fine M6C carbide discontinuously precipitate in the deformed g matrix. The thermodynamics analysis indicates that the carbon element segregates in the regions of stress concentration and combines with carbide-forming elements W etc, which promoted the fine M6C carbide to precipitate from the g matrix. At the same time, the grooves are formed on the surface of stripe MC carbide, and then gradually decomposed and transformed into M6C particles. Thereinto, the additional press formed in the surface of stripe MC carbide is the main factor to promote the MC phase continuous dissolution and spheroidizing.

    Figures and Tables | References | Related Articles | Metrics
    EFFECT OF MINOR Sn AND Nb ADDITIONS ON THE THERMAL STABILITY AND COMPRESSIVE PLASTICITY OF Zr-Cu-Fe-Al BULK METALLIC GLASS
    YANG Bin, LI Xin, LUO Wendong, LI Yuxiang
    Acta Metall Sin, 2015, 51 (4): 465-472.  DOI: 10.11900/0412.1961.2014.00485
    Abstract   HTML   PDF (4851KB) ( 576 )

    New Ni-free Zr61.5Cu21.5-xFe5Al11Sn1Nbx (x=0,1, 2, atomic fraction, %) and Zr61.5Cu21.5Fe5Al12 bulk metallic glasses (BMGs) rods with diameters of 2 and 3 mm were fabricated by copper mold casting. In order to improve the plasticity of the Zr61.5Cu21.5Fe5Al12 BMG, minor Sn and Nb with lower thermal neutron cross-sections was added into the Zr-Cu-Fe-Al alloy. The experimental results showed that the glass-forming abilities of the BMGs with Sn and Nb elements were reduced slightly. Among them with Sn and Nb elements, however, Zr61.5Cu19.5Fe5Al11Sn1Nb2 BMG exhibits high compressive strength, high ductility together with extensive “work hardening”. HRTEM study verifies the glassy states of both Zr61.5Cu19.5Fe5Al11Sn1Nb2 and Zr61.5Cu21.5Fe5Al12 alloys samples. The difference between the microstructures of the BMGs samples with and without Sn and Nb elements is that the atomic arrangement in Zr61.5Cu19.5Fe5Al11Sn1Nb2 BMG is more closely than that in Zr61.5Cu21.5Fe5Al12 BMG. Positron annihilation lifetime spectroscopy study showed further that the Zr61.5Cu19.5Fe5Al11Sn1Nb2 BMG has more closely atomic arrangement than the Zr61.5Cu21.5Fe5Al12 BMG. The structural free-volume size of the former BMG is smaller than that of the latter BMG. And the total free-volume amount of the former BMG is obviously higher than that of the latter BMG. Uniformly distributed free volume is beneficial to improve the shear band formation, branching, and interactions of the Zr61.5Cu19.5Fe5Al11Sn1Nb2 BMG, which increases finally the compressive ductility of the BMG.

    Figures and Tables | References | Related Articles | Metrics
    TEXTURE FORMATION AND GRAIN BOUNDARY CHARACTERISTIC OF Al-4.5Cu ALLOYS DIRECTIONALLY SOLIDIFIED UNDER HIGH MAGNETIC FIELD
    ZHONG Hua, REN Zhongming, LI Chuanjun, ZHONG Yunbo, XUAN Weidong, WANG Qiuliang
    Acta Metall Sin, 2015, 51 (4): 473-482.  DOI: 10.11900/0412.1961.2014.00496
    Abstract   HTML   PDF (12586KB) ( 2649 )

    Directional solidification of Al-4.5Cu alloy refined by adding Al-5Ti-1B has been carried out to investigate the texture formation and grain boundary characteristic of the paramagnetic crystal under a high magnetic field. OM and EBSD were applied to analyze the microstructures solidified at different temperature gradients (G) and magnetic field intensities (B). The results show that at the temperature gradient of 27 K/cm, the orientations of fcc a-Al grains without magnetic field are random. However, as a high magnetic field is imposed, the easy magnetization axes 〈310〉 of the a-Al grains are aligned parallel to the direction of the magnetic field leading to 〈310〉 texture. Meanwhile, the ratio of coincidence site lattice (CSL) grain boundaries increases with the increment of magnetic field intensity and reaches its maximum value at 4 T, but decreases as the magnetic field enhances further. On the other hand, when the temperature gradient is elevated, columnar dendrite morphology is exhibited without magnetic field; while a 6 T high magnetic field is introduced, the columnar dendrites are broken and equiaxed grains of random orientations are obtained. The alignment behavior of the free crystals in melt could be attributed to the magnetic crystalline anisotropy of a-Al. Moreover, the influence of fluid flow on the texture formation and CSL grain boundary development under magnetic field is discussed. The absence of convection is benefit for grain reorientation and CSL boundary formation. The application of high static magnetic field will inhibit the macro-scale convection. However, the interaction between thermoelectric current and magnetic field will cause micro-scale fluid flow, i.e., thermoelectric magnetic convection (TEMC). The TEMC will give rise to perturbation near the solid-liquid interface leading to the appearance of freckles as well as the decreasing of the ratio of CSL boundary. Moreover, it is proposed that the formation of CSL boundary is associated with the rotation of the free grains in melt along specific crystallographic axes by magnetic torque.

    Figures and Tables | References | Related Articles | Metrics
    MICROSTRUCTURE EVOLUTION MECHANISM AND MECHANICAL PROPERTIES OF FeNiCrAl ALLOY REINFORCED BY COHERENT NiAl SYNTHE- SIZED BY THERMITE PROCESS
    WANG Xing, XI Wenjun, CUI Yue, LI Shujie
    Acta Metall Sin, 2015, 51 (4): 483-491.  DOI: 10.11900/0412.1961.2014.00497
    Abstract   HTML   PDF (8933KB) ( 1009 )

    The excellent thermal conductivity, low thermal expansion and high oxidation resistance of ferritic FeNiCrAl alloys, provide them with the potential to be replacements for nickel-based superalloys in high-temperature applications. However, their usage is limited, because of their poor high-temperature mechanical properties. The high melting point of NiAl intermetallic compounds, together with their excellent high temperature stability and similar lattice parameters to a-Fe, allow them to be used to coherently strengthen ferritic FeNiCrAl alloys to extend their high-temperature performance. Traditionally, these Fe(Ni, Cr)/NiAl alloys are prepared by vacuum reaction melting followed by an aging process. But the aging process has drawbacks including excessive cost, the length of aging time required and coarsening of the NiAl phase at high temperature. A more cost-effective thermite reaction process, was tried to prepare the Fe(Ni, Cr)/NiAl alloys. In this route, ferrite FeNiCrAl alloys were strengthened by a high volume fraction nanoscale-NiAl phase which was achieved without using the aging process. Several types of thermites were designed and studies were conducted to explore the transformations of the alloy microstructures and the changes of the tensile properties with the various thermite compositions. The microstructures of these thermites synthesized Fe(Ni, Cr)/NiAl alloys were investigated using XRD, SEM, EDS, TEM and SAED. The effect of Al content in the thermites on the microstructures of the alloys was studied. Experimental results showed that when the thermites contained no more than 25.4% (mass fraction) of Al, the synthesized Fe(Ni, Cr)/NiAl alloys were composed primarily of an austenite phase. The main component phase of the alloy composites was transformed into ferrite when the mass fraction of Al in the thermites was 26.6%, meanwhile the NiAl particle precipitates arose. As the Al content of the mixture was further increased, the NiAl precipitates were gradually replaced by an intertexture structure. The intertexture structure was totally dominant when the mass fraction of Al in the thermites was 31.4%. Experimental results showed that this intertexture microstructure material was composed of a ferritic FeNiCrAl matrix with a width of 80~100 nm and NiAl precipitates with a width of about 50 nm, and the two phases matched coherently. This microstructure resulted from liquid spinodal decomposition. The effect of Al content on the mechanical properties of the alloys was also investigated. The increase of the Al content in the thermites resulted in a decrease of the elongation of the alloys, which varied from 25.5% to 1.7% when the mass fraction of Al ranged from 24.2% to 29.0%. When the thermites contained 26.6% mass fraction of Al, the tensile strength of the alloy achieved its maximum value of 640.87 MPa.

    Figures and Tables | References | Related Articles | Metrics
    REAL-TIME OBSERVATION OF SOLIDIFICATION MICROSTRUCTURE IN LASER REMELTING POOL
    WANG Lilin, LIN Xin, WANG Yonghui, YU Honglei, HUANG Weidong
    Acta Metall Sin, 2015, 51 (4): 492-498.  DOI: 10.11900/0412.1961.2014.00527
    Abstract   HTML   PDF (6100KB) ( 710 )

    The final quality of parts fabricated by high energy beam (laser, electron beam and arc) processing technology is determined by solidification microstructure formation in the molten pool, which attracts lot of attention of researches. However, real-time observation of solidification microstructure formation in the molten metal pool is very difficult because of its high temperature, rapid solidification and opacity. In this work, using a transparent model alloy of succinonitrile-2.0% (mass fraction) ethanol (SCN-2.0%Eth), the solidification microstructure evolution in the molten pool during laser surface remelting (001) crystal plane of a single-crystal substrate was real-time observed as the laser scanning direction deviated different angles from [100] crystal orientation of the substrate. It was found that and dendritic columns grow symmetrically in the molten pool when the scanning direction parallels to the [100] crystal direction. Dendritic columns grow asymmetrically in the molten pool when the scanning direction deviates an angle of 20° from the [100] crystal orientation. Specifically, dendritic columns always grow at one side of the molten pool while [100] and [010] dendritic columns compete to grow alternately at the other side. [100] and dendritic columns grow perpendicular to each other in the molten pool when the scanning direction deviates an angle of 45° from the [100] crystal orientation. According to the preferential growth criterion of dendrite, a model describing the dendritic growth behavior in laser remelting pool was established. It can explain the experimental results well. The results showed that the solidification microstructure formation in laser remelting pool is influenced by both pool morphology and crystal orientation of the substrate.

    Figures and Tables | References | Related Articles | Metrics
    NUMERICAL SIMULATION OF DIRECTIONAL SOLIDIFIED MICROSTRUCTURE OF WIDE-CHORD AERO BLADE BY BRIDGEMAN PROCESS
    TANG Ning, WANG Yanli, XU Qingyan, ZHAO Xihong, LIU Baicheng
    Acta Metall Sin, 2015, 51 (4): 499-512.  DOI: 10.11900/0412.1961.2014.00345
    Abstract   HTML   PDF (7910KB) ( 422 )

    The aero turbine is spun by high-temperature and high-pressure burning gases. The practice has proven that the directional solidification (DS) turbine blade with perfect column grains has still excellent high-temperature performance in this kind of working environment. This means that the size and orientation of column grains have great influence on the high-temperature property and performance of turbine blades. On the other hand, the high-quality blade is not easy to be produced in DS process due to the difficulty of obtaining the desired temperature field needed to produce the grains with ideal morphology. In addition, the growth of columnar grains in the wide-chord hollow guide blade is obstructed by the complex camber and the platform. How to produce turbine blades with desired microstructures is the key problem in the DS process. Numerical simulation of the DS process is an effective way to investigate the growth and the morphology of the grains and hence to optimize the process. In this work, a mathematical-physical model for simulating the DS process of wide-chord blade is established in which nucleation and grain growth in the blade in the DS process are modeled by the cellular automation (CA) method with multi-scale dynamic bidirectional coupling technology. Some general analytic indicators are proposed to assess the morphology of mushy zone and grains in a blade quantitatively. Based on the simulated results by using the usual starter blocks 1, 2 and 3, a new starter block is designed considering numerically controlled cutting. Temperature fields and grains in DS processes and corresponding indicators at different withdrawal rates for above 4 starter blocks are numerically predicted to investigate the influences of varying these technological parameters, and hence to determine the influence mechanism to the DS process. For comparison, the DS validation experiments by using starter blocks 1, 2 and 3 have been carried out. The numerical and experimental results agree well, their morphologies including those faulty grains are similar. It is found that higher withdrawal rate leads to larger concavation of mushy zone, but the effect of chill is stronger than that of withdrawal rate if the contact area between casting and chill plate is large enough. Better grain structure in a blade is achieved by starter block 3 than by starter blocks 1 and 2. By starter block 4, the amount of column grains is larger and the amount of lateral grain boundaries is smaller, compared with that of starter blocks 1, 2 and 3. Therefore higher withdrawal rate could be adoptable without excessive concavation of mushy zone, resulting in parallel column grains, finer dendrites in the blade, and much higher blade productivity. Optimum withdrawal rates are also determined for starter blocks 3 and 4.

    Figures and Tables | References | Related Articles | Metrics