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

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    FORMATION OF A GRADIENT NANOSTRUCTURED SURFACE LAYER ON A MARTENSITIC STAINLESS STEEL AND ITS EFFECTS ON THE ELECTRO- CHEMICAL CORROSION BEHAVIOR
    Haiwei HUANG, Zhenbo WANG, Li LIU, Xingping YONG, Ke LU
    Acta Metall Sin, 2015, 51 (5): 513-518.  DOI: 10.11900/0412.1961.2014.00556
    Abstract   HTML   PDF (3294KB) ( 1019 )

    A gradient nanostructured (GNS) surface layer was fabricated on a Z5CND16-4 martensitic stainless steel by means of surface mechanical rolling treatment (SMRT). The microstructure in the GNS surface layer was characterized by using SEM and TEM. The results showed that the mean grain size increases with depth, from about 25 nm at the topmost surface layer to the initial value in the matrix. The total thickness of the grain-refined layer is about 150 mm. The electrochemical corrosion property of the SMRT sample was compared with that of the as-received sample in a 3.5%NaCl aqueous solution. It is shown that the pitting corrosion potential increases from about 0.179 V in the as-received sample to about 0.313 V in the SMRT sample, and the self-corrosion potential also increases evidently. The formation of nanostructures, the increased structural homogeneity, and the introduction of compressive residual stresses in the GNS surface layer, as well as the decreased surface roughness, were discussed to promote the pitting corrosion resistance of the SMRT sample.

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    EFFECTS OF B ON HIGH TEMPERATURE MECHA-NICAL PROPERTIES AND THERMAL FATIGUE BEHAVIOR OF COPPER DIE-CASTING DIE STEEL
    Zhisheng WANG, Xiang CHEN, Yanxiang LI, Huawei ZHANG, Yuan LIU
    Acta Metall Sin, 2015, 51 (5): 519-526.  DOI: 10.11900/0412.1961.2014.00591
    Abstract   HTML   PDF (8296KB) ( 933 )

    Copper die-casting die steel is usually used in severe rugged environment. Liquid metal flows with high temperature and high pressure during injection and provides rapid filling of the die cavity. The copper die-casting steel should has excellent combination of the properties of high toughness, wear resistance, hardness, thermal fatigue resistance, oxidation resistance and corrosion resistance at high temperature for the cavity surface of die-casting die suffers high pressure, scour, erosion and thermal shock. A new kind of copper alloy die-casting die steel with pure austenitic matrix was conducted in this work, wherein the boride with high thermal stability and high hardness distributes in the austenitic matrix. The mechanical properties of copper alloy die-casting die steel at high temperature of 850 ℃ were studied using dynamic thermal-mechanical simulation testing machine. The thermal fatigue behavior of die steel at room temperature to 800 ℃ was performed using self-restraint Uddeholm thermal fatigue test method, and the depth extension status of surface thermal fatigue cracks and cross-sectional cracks in die steel thermal fatigue specimens was measured using stereo microscope and SEM. The effects of B content on the mechanical properties at room temperature and high temperature and on the thermal fatigue resistance were evaluated. The experimental results showed that boride distributes in austenitic matrix in the form of M2B-type boride (M represents Fe, Cr or Mn) after adding B in the tested steels, and the comprehensive performances of steel at high temperatures were effectively improved, the hardness of the steel at room temperature increased from 200 HV to 302 HV, the tensile yield strength at 850 ℃ increased from 144.3 MPa to 190.3 MPa, and the compressive yield strength increased from 139.7 MPa to 167.9 MPa. Evaluation of the degree of heat checking on 300 cyc of thermal fatigue testing at room temperature to 800 ℃ showed that the die steel containing B was rating 2~3, much better than rating 7~8 of electroslag remelting ESR-H13 steel for comparison, which mainly because the thermal fatigue cracks were blunted or deflected by boride, and then the cracks spread as scattering shapes was avoided.

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    EFFECT OF Mn PRE-PARTITIONING ON C PARTITIONING AND RETAINED AUSTENITE OF Q&P STEELS
    Liansheng CHEN, Jianyang ZHANG, Yaqiang TIAN, Jinying SONG, Yong XU, Shihong ZHANG
    Acta Metall Sin, 2015, 51 (5): 527-536.  DOI: 10.11900/0412.1961.2014.00462
    Abstract   HTML   PDF (12060KB) ( 541 )

    The chemical compositions of C and Mn have a strong influence on the stability of the metastable retained austenite at room temperature. In the intercritical annealing process, Mn element improves the stability of the austenite by partitioning from ferrite to austenite and the enrichment of Mn in austenite can also impact on the diffusion of C element from martensite to retained austenite in partitioning process. Based on C partitioning, Mn partitioning can further improve the product of strength and elongation, and has no negative effect on weldability of the low carbon high strength steel. Thus, it can effectively solve the contradiction between mechanical property and weldability of low carbon high strength steel in traditional quenching-partitioning (Q&P) process. In this case, it is of great significance to study the Mn pre-partitioning mechanism and its influence on C partitioning and retained austenite of the low carbon high strength steel. Therefore, one low alloy C-Si-Mn steel was studied in the present work. The Mn pre-partitioning behavior and its effect on C partitioning and the stability of the retained austenite were investigated by means of intercritical heating-quenching (IQ) process, Q&P and intercritical heatingaustenitizing-quenching-partitioning (I&Q&P) process. The results showed that during the process of phase transformation in the intercritical reheating, C and Mn elements constantly diffused from ferrite to austenite. When this process ended, C and Mn elements enriched in austenite. While Mn element in microstructure at room temperature was still enrichment and C element enriched regularly between the martensite laths in the I&Q&P treated steel. With the increase of C partitioning time in both Q&P and I&Q&P process, the tensile strength of steel was decreased constantly, while the elongation showed an increasing fristly and then decreasing trend. The product of strength and elongation of the steel treated by I&Q&P process reached 23478 MPa·% with the C partitioning time of 90 s. The more austenite in martensite phase would be obtained after the first quenching with the Mn pre-partitioning. It was important to prompt more C diffusing into austenite during C partitioning process to stabilize more retained austenite at room temperature of the steel after the second quenching. With the same experimental conditions, the retained austenite of the combined effects of C and Mn partitioning during I&Q&P process would be increased 2.4% than the effect of C partitioning during Q&P process.

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    EFFECT OF ONE STEP Q&P PROCESS ON MICRO- STURCTURE AND MECHANICAL PROPERTIES OF A DUAL MARTENSITE STEEL
    Xiaolin LI, Zhaodong WANG
    Acta Metall Sin, 2015, 51 (5): 537-544.  DOI: 10.11900/0412.1961.2014.00566
    Abstract   HTML   PDF (5898KB) ( 1015 )

    In accordance with the demand for reduced fuel consumption and CO2 emissions in automobiles, and with the increasing high demand for vehicle lightweight and safety, advanced high- strength steels (AHSS) have received more attentions in recent years. The recent trend for the development of AHSS has been concentrated on the complex microstructure with multiphase. Quenching and partitioning (Q&P) steel with carbon-enriched austenite within martensitic matrix as a competitive candidate of AHSS have been developed widely. It has high strength and good ductility depending on the multiphase microstructure. Therefore, the relationship of the mechanical property and the microstructure of the Q&P steels should be studied in detail. In the present work, the microstructure characterization and mechanical properties of the experimental steel treated by one step Q&P process were investigated, as well as the direct quenching and Q&T processes. The results show that the microstructure of the steel treated by one step Q&P process mainly consists of lath martensite, plate martensite and residual austenite films between martensite laths. With a increase in the holding time, the fraction of the plate martensite firstly increases and then reduces, while that of the retained austenite firstly increases and then becomes constant. The combination of strength and elongation of the steel processed by one step Q&P is much better than the one processed by the other two, that is to say, the former one can possess good strength and ductility at the same time. The product of tensile strength and elongation, the tensile strength and the elongation can achieve 21774.2 MPa·%, 1442 MPa and 15.1%, respectively. Along with the holding time increasing, tensile strength decreases but elongation rises and finally be stable.

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    EVOLUTION OF TEXTURES OF COLUMNAR GRAINS IN Fe-3%Si ELECTRICAL STEEL SLABS
    Yongjun FU, Ping YANG, Qiwu JIANG, Xiaoda WANG, Wenxu JIN
    Acta Metall Sin, 2015, 51 (5): 545-552.  DOI: 10.11900/0412.1961.2014.00573
    Abstract   HTML   PDF (8290KB) ( 446 )

    Different numbers of columnar grains in a Fe-3%Si electrical steel slabs with their major axes being parallel to the rolling direction were used for cold rolling and recrystallization annealing. The evolution of the texture of columnar grains was followed by EBSD technique. The results show that, in the case of single columnar grain with cube orientation, cube texture is adverse to be retained in condition of primary cold rolling with high reduction and recrystallization annealing, while strong cube texture can form after secondary cold rolling with low reduction and recrystallization annealing. But the cube texture hinders strongly the abnormal growth of Goss grains. For the sample containing two columnar grains with Goss and cube orientation, the initial Goss orientation rotates to {111}<112> orientation quickly and cube texture is retained effectively during cold rolling by high reduction. The interaction between the Goss and cube columnar grains is not strong. For the multi-columnar grains with different orientations, the grain boundaries between columnar grains promote g-texture and weaken the cube texture, which is in favor of abnormal growth of Goss grains.

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    EFFECT OF TEMPERING TIME ON MICROSTRUC- TURE AND MECHANICAL PROPERTIES OF HIGH Ti MICROALLOYED QUENCHED MARTENSITIC STEEL
    Ke ZHANG, Xinjun SUN, Qilong YONG, Zhaodong LI, Gengwei YANG, Yuanmei LI
    Acta Metall Sin, 2015, 51 (5): 553-560.  DOI: 10.11900/0412.1961.2014.00470
    Abstract   HTML   PDF (5388KB) ( 546 )

    With the development of Ti microalloying technology, the application and theory research of Ti in microalloyed steels are becoming more deeply and widely. However, the effect of tempering time on the microstructure and mechanical properties of high Ti microalloyed quenched martensitic steel has been rarely touched upon, meanwhile, it has long been inconclusive whether precipitated phases coarsening or the recovery and softening of martensitic matrix is the dominant role resulting in the decrease of hardness along with long time tempering of microalloyed steel. In this work, the effect of tempering time on the microstructure and mechanical properties of high Ti microalloyed quenched steel was systemactically investigated by TEM, XRD and Vickers-hardness test, and the interaction between precipitation hardening and microstructural softening of the high Ti microalloyed steel was also studied. The results indicate that the hardness increases for Ti microalloyed steel with tempering time 10~300 s, which is attributed to the fact that the precipitation hardening by nano-sized TiC particles is greater than the recovery and softening of matrix. With the tempering time from 300 s to 10 h, nano-sized TiC particles precipitate more and more and the mass fraction of TiC with the size less than 5 nm increases, owning to the precipitation hardening produced by tiny TiC which offsets the hardness decrease due to the gradual softening with recovery of matrix, and therefore, the hardness can keep a long platform; in addition, with the tempering time 10~20 h, the hardness decreases significantly and the deacreasing rate of hardening for steel with Ti microalloying is higher than that for steel without Ti microalloying. The average particle size of TiC increases from 2.76 nm at 10 h to 3.15 nm at 20 h. Calculation results show that the decrease of hardness caused by coarsening of TiC is 11.94 HV, while caused by recovery of matrix is 24.56 HV. It is shown that the recovery of matrix is the dominating factor for reduction in hardness, but coarsening of tiny TiC speeds the decrease of hardness and is also an important factor resulting in the decrease of hardness.

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    TEXTURE OF Ti60 ALLOY PRECISION BARS AND ITS EFFECT ON TENSILE PROPERTIES
    Zibo ZHAO, Qingjiang WANG, Jianrong LIU, Zhiyong CHEN, Shaoxiang ZHU, Bingbing YU
    Acta Metall Sin, 2015, 51 (5): 561-568.  DOI: 10.11900/0412.1961.2014.00451
    Abstract   HTML   PDF (9741KB) ( 895 )

    Microstructure and texture of titanium alloy are determined by thermomechanical and heat treatments and can significantly affect the mechanical properties of the final products. In this work, the microstructure and texture evolution during the heat treatment in α/β and β phase field in Ti60 precision forging bars were investigated. The results implied that the actual deformation temperature gradually decreased during precision forging processes. The microstructure and texture of Ti60 bar were determined by the finish forging temperature and the diameter, and strong microtexture macrozones existed in the forged Ti60 bar. For the bar with diameter of 45 mm (D45), the finish forging temperature fell in the lower temperature region of the α/β phase field, and the main α textures in these bars were <0001> and < 10 1 ? 0 > fiber texture components in initial Ti60 bar. The similarity of the microstructure and texture were found after heat treatment at 950 ℃. The intensity of < 10 1 ? 0 > fiber texture gradually decreased while that of <0001> fiber texture increased with the increase of the heat treatment temperature. Heat treatments have little influence on the strength of forged Ti60 bars of D45, while their ductility was reduced after β heat treatment. For the bar with diameter of 30 mm (D30), the finish forging temperature was below the α/β phase field, and the main α texture in those bars was < 10 1 ? 0 > fiber texture component. The intensity of <0001> fiber texture in those bars increased while that of < 10 1 ? 0 > fiber texture gradually decreased with the increase of the heat treatment temperature. Their room temperature strength significantly increased with the increase of the heat treatment temperature, and yield strength and tensile strength reached to 1086 and 1144 MPa, respectively, but the elongation only 3.3% after β heat treatment.

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    TENSILE AND HIGH CYCLE FATIGUE PROPERTIES OF Ti-47Al-2Cr-2Nb DIRECTIONALLY SOLIDIFIED BY COLD CRUCIBLE METHOD
    Hongsheng DING, Zibo SHANG, Yongzhe WANG, Ruirun CHEN, Jingjie GUO, Hengzhi FU
    Acta Metall Sin, 2015, 51 (5): 569-579.  DOI: 10.11900/0412.1961.2014.00447
    Abstract   HTML   PDF (12400KB) ( 443 )

    TiAl-based alloys have recently received considerable attention as one of the promising candidates for application in aero engine blades by replacing the Ni-based superalloys because of their unique properties, such as high specific strength, high specific stiffness and good oxidation resistance. However, there are some shortcomings limiting the application of TiAl-based alloys, namely, their brittleness and poor processing properties. Nevertheless, aero engine blades usually suffer a variety of cyclic loadings during the period of services, which finally results in fatigue failure. According to statistics, fatigue failure, mainly high cycle fatigue (HCF), occupies almost 80% failure modes of gas turbine blades in aero engines. Consequently, more and more researches about fatigue behavior of blade materials have been done in the last tens of years. However, there are less relevant results about TiAl-based alloys, especially HCF properties. Recently, the advancement of directional solidification (DS) of TiAl-based alloys using cold crucible has revealed that the ductility can be enhanced at room and elevated temperature. For purpose to verify the influence of DS structures on the tensile and HCF properties, TiAl-based alloy in composition with Ti-47Al-2Cr-2Nb (atomic fraction, %) was prepared and evaluated in this work. Directionally solidified Ti-47Al-2Cr-2Nb alloy ingots with different withdrawal rates (1.0, 1.2 and 1.4 mm/min) were prepared by cold crucible method under alter electromagnetic field in a vacuum furnace. Based on these ingots, macro and microstructures have been characterized by methods of digital camera, OM, SEM and XRD. Furthermore, the tensile properties at room and high temperature (800 ℃) as well as HCF properties at room temperature have been measured respectively. So, the relationship between microstructures and mechanical properties of TiAl-based alloy, especially HCF properties, was demonstrated reasonably and mechanism in which HCF cracks propagated was discussed. The results show that the comprehensive mechanical properties of Ti-47Al-2Cr-2Nb alloy can be significantly improved after directionally solidified using cold crucible. The tensile strength reaches 652 MPa at room temperature with the maximum elongation of 1.5%. Meanwhile, the tensile strength at 800 ℃ attains 490 MPa with the elongation of 5.0%. Based on the data of HCF test at room temperature with the stress ratio of 0.1, the equations of stress amplitude-number of cycles to failure (S-N) curve at different withdrawal rates are calculated. The fatigue limits are 300 and 247 MPa with the withdrawal rates of 1.0 and 1.2 mm/min, respectively, namely, with the increase of withdrawal rate, the fatigue fracture resistance decreases. The mode of HCF fracture of directionally solidified Ti-47Al-2Cr-2Nb alloy behaves in brittle cleavage fracture. And micro-cracks which can propagate along and perpendicular to the lamellae at the same time are observed between α2/g lamellae and around B2 phases.

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    SELECTION OF THE SOLIDIFICATION PATH OF Mg-Zn-Gd TERNARY CASTING ALLOY
    Shaojun LIU, Guangyu YANG, Wanqi JIE
    Acta Metall Sin, 2015, 51 (5): 580-586.  DOI: 10.11900/0412.1961.2014.00512
    Abstract   HTML   PDF (2542KB) ( 620 )

    Mg-Zn-Gd base alloys possess much superiority, such as, high strength, light weight, low cost, etc., and favorable for the application in various airframe components. Two kinds of eutectic phases, such as, W(Mg3Zn3Gd2) and I(Mg3Zn6Gd), can be usually found in Mg-Zn-Gd alloy under the traditional casting conditions. The interface between W phase and α(Mg) is incoherent and thus weak. However, I phase has quasiperiodic lattice leading to a coherent interface between I phase and α(Mg). Therefore, compared with W phase, I phase is more effective to obstruct dislocations slipping and so efficiently strengthening the alloy. So, controlling the solidification path, i.e., controlling the relative amount of I phase and W phase, is critical for increasing the heat resistant of Mg-Zn-Gd magnesium alloy. In this work, the solidification path of Mg-4.58Zn-2.6Gd ternary casting alloy was investigated by experiments and numerical analysis. Experimental results showed that at lower cooling rate (≤0.75 K/s), α+W(Mg3Zn3Gd2) eutectic will be formed first, while at higher cooling rate (≥7.71 K/s), α(Mg)+I(Mg3Zn6Gd) eutectic will be formed first. A numerical model for predicting solidification path of the primary phase in multi-component alloy with considering the effects of solute diffusion in liquid phase and the cooling rate was developed. The thermodynamic data in the computation model was obtained by using the database of Thermo-Calc. The numerical results were in favorable agreement with the experimental ones. The numerical model established in this work provides a direct and easy way to predict solidification path of Mg-Zn-Gd alloy for different casting conditions. The validity of this model was further confirmed by other three different Mg-Zn-Gd alloys, i.e., Mg-3.8Zn-2.0Gd, Mg-5.5Zn-2.0Gd and Mg-5.5Zn-4Gd. It is also found that for Mg-Zn-Gd alloy, the higher Zn-content and the higher cooling rate will promote the formation of I phase. However, higher Gd-content and the lower cooling rate is favorable for the formation of W phase.

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    CORROSION BEHAVIOR OF PURE Mg BASED ON GENERATION/COLLECTION AND FEEDBACK MODES OF SCANNING ELECTROCHEMICAL MICROSCOPY
    Xinyin WANG, Yan XIA, Yaru ZHOU, Linlin NIE, Fahe CAO, Jianqing ZHANG, Chunan CAO
    Acta Metall Sin, 2015, 51 (5): 631-640.  DOI: 10.11900/0412.1961.2014.00602
    Abstract   HTML   PDF (5061KB) ( 965 )

    Since electrochemical impedance spectroscopy, polarization curve and hydrogen collection were the main technologies for corrosion research of Mg and its alloy. However, those methods only provide the mean information of entire surface of corrosion electrode. In this work, H2 evolution and active sites of pure Mg from localized sites (point, line and surface) in NaCl and Na2SO4 solution based on generation/collection and feedback modes of scanning electrochemical microscopy (SECM) were studied. The results indicate that both cathodic and anodic polarization are in favor of H2 evolution in NaCl and Na2SO4 solution, which is well in line with the negative difference effect by the classical H2 collection, but the SECM results show that the H2 evolution in localized sites is not uniform and stable. The H2 evolution rate increases with NaCl concentration increasing, which is opposite in Na2SO4 solution. The higher NaCl concentration, anodic polarization and lower pH value accelerate the formation of active sites on pure Mg surface.

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