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CN 21-1139/TG
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 Select MICROSTRUCTURAL CHARACTERIZATION OF A Ru-CONTAINING SINGLE CRYSTAL NICKEL-BASED SUPERALLOY TAN Xipeng, LIU Jinlai, SONG Xiaoping, JIN Tao, SUN Xiaofeng, HU Zhuangqi Acta Metall Sin, 2012, 48 (5): 569-574.  DOI: 10.3724/SP.J.1037.2011.00629 Abstract   PDF (2414KB) ( 1101 ) The microstructure of a Ru--containing single crystal nickel-based superalloy, which is a potential fourth generation single crystal nickel-based superalloy heat treated, has been investigated. The X--ray diffraction (XRD) results indicate that this single crystal superalloy is not a perfect monocrystal, which is comprised of many subgrains with an average neighboring misorientation difference about 0.5° and the maximum misorientation deviation about 3°; and each subgrain includes large numbers of mosaic structures with misorientation difference of 0.1°---0.2° between each other. It is found that the microstructure of the heat treated alloy is of heterogeneity, where the volume fraction of γ' phase increases and the γ' lattice misfit decreases. It is shown that the average size of γ' phase is 0.39 μm, the average γ' volume fraction 70% and the average γ' lattice misfit --2.0×10$-3. The evolution of this microstructure was also briefly described.  Select PREPARATION AND ELECTRICAL PROPERTIES OF Sc--DOPED CaZrO3 LI Ying, DING Yushi, CUI Shaogang, WANG Changzhen Acta Metall Sin, 2012, 48 (5): 575-578. DOI: 10.3724/SP.J.1037.2011.00776 Abstract PDF (1051KB) ( 679 ) CaZrO3 solid state electrolyte displays proton conductivity and high chemical stability. The Sc doped CaZrO3 proton conductors were prepared by solid state reaction in order to improve the conductivity in this study. XRD analysis suggest that CaZr1-xScxO3-α(x=0, 0.1, 0.15) samples were synthesized completely. The electrochemical impedance spectra were applied to study the conductivities and the activation energy for proton diffusion of the CaZr1-xScxO3-α(x=0, 0.1, 0.15) in the temperature range of 610---850℃, and the electric conductivities of CaZr1-xScxO3-α were compared with those of CaZr1-xInxO3-α. The experiment results show that the electric conductivities of CaZrO3, CaZr0.9Sc0.1O3-α, CaZr0.85Sc0.15O3-α, CaZr0.9In0.1O3-α and CaZr0.85In0.15O3-α are 4.3×10-19---1.4×10-6 S/cm (610---850℃), 1.16×10-4---4.6×10-4 S/cm (690---850℃), 1.8×10-4---1.4×10-3 S/cm (610---850℃), 0.34×10-4---4.30×10-4 S/cm (741---847℃) and 0.57×10-4---4.33×10-4 S/cm (585---814℃), respectively. These conductivities results show that the conductivity of CaZrO3 proton conductor can be significantly improved by doping. The conductivities of CaZrO3 increase with the Sc doping content and temperature increasing. The results reveal that the conductivities of CaZr1-xScxO3-α are higher than those of CaZr1-xInxO3-α. Sc--doping is more beneficial for increasing the conductivity of CaZrO3 solid state electrolyte.  Select HIGH TEMPERATURE OXIDATION RESISTANCE AND MECHANICAL PROPERTIES OF NiCrAlY/Al--Al2O3 COATINGS ON AN ORTHORHOMBIC Ti2AlNb ALLOY Acta Metall Sin, 2012, 48 (5): 579-586. DOI: 10.3724/SP.J.1037.2012.00087 Abstract PDF (2832KB) ( 823 ) The orthorhombic Ti2AlNb alloys have received significant attentions because of their good physical and mechanical properties. However, these orthorhombic alloys face problems of oxidation at high temperature, especially above 700 ℃. To solve these problems, the use of surface coatings is an efficient way. However, when single coating was applied on the orthorhombic Ti2AlNb alloys, problems of serious interdiffusion and interfacial reaction were encountered, which resulted in worse oxidation behavior and deteriorated mechanical properties. To obtain good oxidation protection of NiCrAlY coating on the orthorhombic Ti2AlNb alloy, an efficient diffusion barrier should be added. In this study, NiCrAlY/Al--Al2O3 double--coatings were deposited on the orthhombic--Ti2AlNb alloy by arc ion plating. NiCrAlY coating acted as oxidation resistance coating and Al--Al2O3 coating acted as diffusion barrier. By introducing metallic Al in the Al2O3 film, the problem of coefficient of thermal expansion (CTE) mismatch between film and alloy substrate might be mitigate. Also metallic Al in the Al2O3 film can act as diffusion path which permits proper interdiffusion to improve the interface adhesion. The oxidation and interdiffusion behavior of specimens with and without diffusion barriers were investigated by oxidation tests at 900 ℃. The results indicated that substantial interdiffusion and rapid oxidation degradation occurred in the coated specimens without diffusion barrier. With Al--Al2O3 diffusion barriers, deferred interdiffusion and improved oxidation resistance were observed. Different contents of metallic Al in the Al2O3 coatings had different efficiency of diffusion barrier, and also affected interfacial mechanical properties. Among these NiCrAlY/Al--Al2O3 coatings, double--coating containing 1Al--Al2O3 diffusion barrier exhibited best performance. Coefficient of diffusion hindering was used to compare and quantify the efficiency of the diffusion barriers.  Select THREE-LAYER CO-FIRING FABRICATION OF LaCrO3-BASED CERAMIC INTERCONNECT, COMPOSITE ANODE SUPPORT AND YSZ ELECTROLYTE WANG Songlin, FENG Yi, WANG Dongsheng, WANG Jingwen Acta Metall Sin, 2012, 48 (5): 587-592. DOI: 10.3724/SP.J.1037.2012.00097 Abstract PDF (2736KB) ( 1092 ) Developing cost-effective methods to prepare dense ceramic interconnect membrane for solid oxide fuel cell (SOFC) stacks is currently considered as a major technical obstacle. In order to improve the co-firing compatibility of LaCrO3-based interconnects with the traditional YSZ-based SOFC anode support NiO/YSZ, interconnect material of La0.7Ca0.3Cr0.97O3-δ(LCC) was introduced to NiO/YSZ anode. Triple-phase composite NiO/YSZ/LCC was prepared, and then examined as novel anode support. Sintering character, microstructure, electrical conductivity, and thermal expansion coefficient of the composite anode were investigated in detail as a function of LCC addition, respectively. Results indicated that the NiO/YSZ/LCC composite anode had excellent overall performance. Furthermore, by using a simple drop-coating process, LCC and YSZ wet membranes were prepared on the opposite surfaces of NiO/YSZ/LCC support, respectively. Followed by three-layer co-firing at 1400℃ in air for 4 h, dense La0.7Ca0.3Cr0.97O3-δ interconnect and YSZ electrolyte thin membranes were both successfully prepared on the porous NiO/YSZ/LCC anode support. This work presents a simple drop-coating/three-layer co-firing technical route for developing dense interconnect and electrolyte membranes for YSZ-based SOFC stacks.  Select EFFECT OF STRAIN RATE ON MICROSTRUCTURE EVOLUTION AND MECHANICAL BEHAVIOR OF A LOW C HIGH Mn TRIP/TWIP STEELS WU Zhiqiang, TANG Zhengyou, LI Huaying,ZHANG Haidong Acta Metall Sin, 2012, 48 (5): 593-600. DOI: 10.3724/SP.J.1037.2011.00590 Abstract PDF (3123KB) ( 2554 ) The microstructure and mechanical properties of Fe--18Mn low carbon high manganese TRIP/TWIP steels during tensile tests in the range of initial strain rate of 1.67×10-4---103 s-1 at room temperature were studied. The inverse effect of strain rate on strength of steel was produced, the strength and ductility of steels decreased with increasing strain rate in the range of quasi--static tensile strain rate of 1.67×10-4---1.67×10-1 s-1. While inverse effect of strain rate on ductility of steels was produced in the range of dynamic tensile strain rate of 101---103 s-1, the strength and ductility of materials increased significantly with increasing strain rate. The tensile strength of high manganese TRIP/TWIP steels was 957 MPa and their elongation was 55.8%. These results indicated that Fe--18Mn steel had excellent mechanical properties and good fracture resistance. The higher the strain rates applied, the less martensite, the more directions of deformation twins. The microstructure evolution of the specimen was analyzed by SEM, TEM and XRD, martensitic transformation and deformation twins were produced during the tensile deformation, and adiabatic temperature rise effect made the matrix softening during the high--speed deformation.  Select EFFECT OF GRAIN SIZE ON ATMOSPHERIC CORROSION RESISTANCE OF ULTRA--LOW CARBON IF STEEL WANG Bing, LIU Qinyou, WANG Xiangdong Acta Metall Sin, 2012, 48 (5): 601-606. DOI: 10.3724/SP.J.1037.2012.00108 Abstract PDF (2438KB) ( 922 ) Three kinds of ultra-low carbon IF steel with different grain sizes, and same chemical composition were prepared by different rolling and heat treat process. The relationship between grain size and atmospheric corrosion resistance of IF steel was investigated by immersion corrosion test, cyclic immersion corrosion test, AFM/SEM micro-analysis and electrochemical test. The results show that the local corrosion in grain boundary increases after immersion corrosion test, the depth of crack in grain boundary becomes deeper and the width of crack becomes wider with grain sizes of IF steel increase from 15 μm to 220 μm. The crack and cavity in the rust after cycle immersion corrosion test are increased and the atmospheric corrosion resistance is decreased with IF steel grain size coarsing from 15 μm to 46 μm. As grain size increase from 15 μm to\linebreak 220 μm, the whole compactness of rust are increased, the rust resistance and the atmospheric corrosion resistance are increased. The effect of grain size on the corrosion current density of local grain boundary was analysed and the mechanics of corrosion was discussed. The total quantity of corrosion surface defect is decreased due to the decrease of grain boundary energy with the increase of grain size and the atmospheric corrosion resistance is increased. Meanwhile, the local corrosion near the grain boundary is increased duo to the increase of local corrosion current density with the increase of grain size and the atmospheric corrosion resistance is decreased. The atmospheric corrosion resistance is influenced by the two factors simultaneously.  Select PRECIPITATION BEHAVIOR OF AZ91 MAGNESIUM ALLOYS WITH DIFFERENT La CONTENTS ZHANG Jinling1,2,3, FENG Zhiyong1, HU Lanqing1,2,3,WANG Shebin1,2,3, XU Bingshe1,2,3 Acta Metall Sin, 2012, 48 (5): 607-614. DOI: 10.3724/SP.J.1037.2012.00014 Abstract PDF (3322KB) ( 1386 ) The changes of microhardness, microstructures and precipitation phase morphologies of as--cast and aged AZ91 alloys with different La were characterized by TEM, OM, SEM and Vickers. The influences of discontinuous β phase and Al11La3 on AZ91 magnesium alloy were discussed and the diffusion coefficient of Al atom in the aging process was calculated. The results showed that the alloy with 0.16%La aged at 170 ℃ for 24 h has the largest hardness (138 HV). By extending aging time and adding La element, the discontinuous precipitation phases was transformed into continuous lath--sharped phases and the diffusion coefficient of Al atom was decreased, the volume and distribution of β phase were changed. During the process of heat treatment, the topography of Al11La3 in grain boundary was not changed, but the anchoring effect of Al11La3 restricted the growth of β phase. By controlling the content of La and parameters of aging treatment, the volume and size of β phase were changed, in the mean time the strengthening and toughness properties of alloys can been improved effectively.  Select EFFECT OF FORCED FLOW ON THREE DIMENSIONAL DENDRITIC GROWTH OF Al-Cu ALLOYS Acta Metall Sin, 2012, 48 (5): 615-620. DOI: 10.3724/SP.J.1037.2012.00069 Abstract PDF (1657KB) ( 902 ) The dendrite morphology is determined by the interaction between the capillarity effect and the transports of heat and solute, and is significantly altered by the presence of fluid flow during solidification. A lot of numerical models have been developed to investigate the effect of fluid flow on the dendritic growth of pure materials. But up to date, only a few researches were carried out on the effect of fluid flow on the dendritic growth of alloys. The effect of fluid flow on three dimensional (3D) dendrite tip selection parameter of alloys remains an unsolved scientific problem. A 3D cellular automaton (CA) model for dendritic growth of alloys was developed in this paper. 3D CA is solved in coupling with a momentum transport model in order to predict the evolution of dendritic morphology during solidification of alloys in the presence of flow. The dendrite growth with a forced flow in an undercooled melt of an Al-4%Cu (mass fraction) alloy was simulated. The effect of forced flow on dendritic growth was investigated. The results show that a forced flow affect the three dimensional dendritic growth of an alloy significantly. The growth of the primary and secondary arm in the upstream direction is much greater than that in the downstream direction. The growth direction of the primary arm perpendicular to the flow direction tilted into the upstream direction. The dendrite tip of the primary arm perpendicular to the flow direction shows an asymmetric morphology. The degree of the tilt and the asymmetry of the tip become stronger with the increase of the forced flow velocity. With the increase of the flow velocity the growth velocity of the upstream dendrite tip increases, the radius and the selection parameter of the upstream dendrite tip decrease. For a given undercooling, the effect of forced flow on the selection parameter of the upstream dendrite tip becomes stronger with the increase of the anisotropy of the interfacial energy. For a given alloy, the effect of forced flow on the selection parameter of the upstream dendrite tip also becomes stronger with the increase of undercooling.  Select EFFECTS OF DEFORMATION AND COOLING RATE ON NANO-SCALE PRECIPITATION IN HOT-ROLLED ULTRA-HIGH STRENGTH STEEL WANG Xiaonan, DI Hongshuang, DU Linxiu Acta Metall Sin, 2012, 48 (5): 621-628. DOI: 10.3724/SP.J.1037.2012.00082 Abstract PDF (4244KB) ( 861 ) In order to control nano-scale precipitation (Nb, Ti)C in hot-rolled 780 MPa grade C-Mn steel micro-alloyed with niobium and titanium for automobile frames, the effects of deformation and cooling rate on nano-scale precipitation were investigated by using the thermal simulation experiment technology, then through the transmission electron microscopy observation and statistical analysis. The result indicated, deformation could significantly improve density of dislocation, subgrain boundary and vacancy etc in microstructure, and promote heterogeneous nucleation of precipitation, and improve nucleation rate of precipitation and decrease the average diameter of precipitation. Deformation could improve vacancy concentration and promoted vacancy nucleation. The induction period of precipitation nucleation decrease with the increase of deformation amount and strain rate, and precipitation more easily to nucleate. Precipitation nucleation driving force was mainly supersaturation of microalloy in undeformed experimental steel, and the nucleation mechanism was mainly homogeneous nucleation. However, the nucleation mechanism was mainly heterogeneous nucleation in deformed experimental steel. In one fixed experimental deformation condition, when the cooling rate below 5℃/s, there was (Nb, Ti)C-PFZ (precipitate free zone) nearby original austenitic grain boundaries or subgrain boundaries, and the width of PFZ at cooling rate of 0.5, 1, 2 and 5℃/s were 46.9, 30.2, 28.1 and 0 nm, respectively, so the width of PFZ decreased with the cooling rate increasing. When the cooling rate reached 15℃/s, the nucleation of precipitation was totally inhibited during cooling process. The number of precipitation along with the cooling rate increases gradually decreases. With the increase of cooling rates, the nucleation zone of precipitation was transferred from austenite to ferrite or bainite, and the average diameter of precipitation was refined. Due to grain boundaries or the subgrain boundaries were main traps for supersaturated vacancy, but the diffusivity of vacancy was high, which made the vacancy concentration nearby grain boundaries or the subgrain boundaries lower than critical vacancy concentration for precipitation nucleation, so precipitate could not nucleate nearby grain boundaries or subgrain boundaries. Due to the diffusivity of vacancy was affected by temperature, when the cooling rate was slow, vacancy had enough time to diffuse and annihilate, which made wide PFZ formed. Whereas, when the cooling rate was high, the diffusivity of vacancy was reducing or disappearing, so the width of PFZ was small. In orde to ensure experimental steel had higher yield strength, austenite zone precipitation and (Nb, Ti)C-PFZ nearby boundaries should be inhabited, so the cooling rate should be more than 15 ℃/s in the practical rolling process.  Select EFFECT OF LONGITUDINAL MAGNETIC FIELD ON THE MICROSTRUCTURE OF DIRECTIONALLY SOLIDIFIED SUPERALLOY DZ417G WITH DIFFERENT SIZES XUAN Weidong, REN Zhongming, LI Chuanjun, REN Weili, CHEN Chao, YU Zhan Acta Metall Sin, 2012, 48 (5): 629-635. DOI: 10.3724/SP.J.1037.2011.00621 Abstract PDF (6218KB) ( 868 ) In order to understand the influence of a longitudinal magnetic field on the microstructures of the superalloy DZ417G with different sample sizes, the superalloy DZ417G was directionally solidified under a longitudinal magnetic field. The solidification structures of the alloy with various dimensions of specimen have been investigated. The results showed that the columnar dendrites on the edge of samples were broken and changed into equiaxed dendrites in the magnetic field at the withdrawal velocity of 5$\mu\$m/s and temperature gradient of 70℃/cm. Additionally, it was found that equiaxed dendrites area expanded from the edge to the center of sample and the number of freckle-like macrosegregation increased with the size of sample. The columnar to equiaxed transition and freckle-like macrosegregation can be attributed to the thermoelectric magnetic convection (TEMC) induced by the magnetic field in the front of solid/liquid interface during directional solidification.