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

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    Orginal Article
    Research on Hot Working Behavior of Low-NickelDuplex Stainless Steel 2101
    Yusen SU, Yinhui YANG, Jianchun CAO, Yuliang BAI
    Acta Metall Sin, 2018, 54 (4): 485-493.  DOI: 10.11900/0412.1961.2017.00151
    Abstract   HTML   PDF (7433KB) ( 578 )

    The thermal deformation difference of two phases for duplex stainless steel (DSS) makes hot working difficult, 2101 DSS substitute Mn, N for Ni to stabilize austenite phase, which will significantly affect hot deformation behavior. Hot compression tests in the temperature ranging from 1123 to 1423 K and strain rate ranging from 0.001 to 10 s-1 were carried out on a Gleeble-3800 thermal simulator for 2101 DSS. At the same strain rate, the flow curve characteristics of 2101 DSS changed from dynamic recrystallization (DRX) to dynamic recovery with increasing deformation temperature. Increasing deformation stain rate from 0.001 s-1 to 0.01 and 0.1 s-1 increased DRX temperature range, but higher strain rate of 1 and 10 s-1 is not beneficial to DRX occurrence. In the deformation temperature region of 1253~1323 K and low strain rate of 0.001~0.1 s-1, the smaller strain value corresponding to the peak stress, the austenite DRX is more likely to occur, which is beneficial to the equiaxed recrystallized grains formation. At low strain rate, the recrystallization grain grows up with the increase of deformation temperature, the worse effect of austenite DRX is related to weakened austenite stabilized ability of Mn substitution for Ni at high Zener-Hollomon parameter values. Based on the thermal deformation equation, the apparent activation energy Q was calculated as 464.49 kJ/mol, which is slightly higher than that of 2205 DSS, and the constitutive equation of the peak flow stress was established. By combining with flow curve and microstructure analysis, the processing map exhibits the optimum processing conditions are in deformation temperature ranging from 1220 to 1350 K and strain rate ranging from 0.001 to 0.1 s-1 with high power dissipation coefficient of 0.40~0.47, under which the austenite DRX obviously occurred.

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    Study on Anisotropic Mechanical Properties of Cold Drawn Pearlitic Steel Wire
    Peibei JI, Lichu ZHOU, Xuefeng ZHOU, Feng FANG, Jianqing JIANG
    Acta Metall Sin, 2018, 54 (4): 494-500.  DOI: 10.11900/0412.1961.2017.00274
    Abstract   HTML   PDF (7898KB) ( 620 )

    Cold drawn pearlitic steel wires with ultra-high strength are widely applied in industrial fields such as bridge cables, automobile tire and springs rope. In recent years, the strengthening mechanism and microstructure evolution have been profoundly studied. In order to investigate the influence of microstructure evolution on mechanical properties, the anisotropic mechanical properties of cold drawn pearlitic steel wires were investigated by tensile test, SEM and TEM. Results indicated that the distinctions of tensile strength between three directions (parallel to the tensile axis, inclined to the tensile axis (45°), vertical to the tensile axis) were amplified with increasing strain. The effect of strain strengthening was observed in parallel and inclined directions while the vertical direction remained strength stability in 1320 MPa. The wire rod was isotropic and the fracture mode was transgranular fracture; After cold drawing, the tensile strength reached peaks in parallel direction and valleys in vertical direction. The fracture mechanism of inclined and vertical directions remained transgranular or intergranular fracture while the fracture mechanism of parallel direction was converted into microvoid accumulation fracture. In TEM, the phenomenon was discovered that due to non-homogeneous distribution in vertical direction, dislocations piled up at the boundaries resulting in stress concentration. On the contrary, the dislocations were uniformly distributed which led to homogeneous transformation in parallel direction.

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    Microstructure and Impact Toughness of Welding Heat-Affected Zones of a Fe-Cr-Ni-Mo High Strength Steel
    Mingyue WEN, Wenchao DONG, Huiyong PANG, Shanping LU
    Acta Metall Sin, 2018, 54 (4): 501-511.  DOI: 10.11900/0412.1961.2017.00331
    Abstract   HTML   PDF (13660KB) ( 594 )

    Marine engineering steel is the key material for the construction of major marine infrastructure projects. Due to the harsh environment in the deep sea, the mechanical properties such as strength, low temperature toughness and so on of the marine steel are required to be higher. In this work, the weldability of a Fe-Cr-Ni-Mo high-strength steel was studied, and the microstructure and impact toughness of the steel after welding thermal cycling at different peak temperatures were analyzed. The results show that the average impact toughness of characteristic heat affected zone under different temperatures increases first and then decreases with the increase of peak temperature (Tp). The microstructures of coarse grain heat-affected zone (CGHAZ, Tp=1320 ℃) and fine grain heat-affected zone (FGHAZ, Tp=1020 ℃) are quenched martensite. Because of the coarse grain size, the impact toughness of CGHAZ is poor, which is lower than that of FGHAZ. The microstructure of inter-critical heat-affected zone (ICHAZ, Tp=830 ℃ and Tp=760 ℃) is composed of quenched martensite and tempered martensite. Due to the randomness of the proportion of the interfaces between the mixed microstructures near the V-notch, the impact energy values of ICHAZ fluctuates greatly. The homogeneous fine grain structure in ICHAZ (Tp=830 ℃) has a crack arrest effect during the impact deformation, which makes the characteristic zone have the best impact toughness. Although the grain size in ICHAZ (Tp=760 ℃) is also fine, the existence of the ultra-fine grain zones (the grain size in which is only 1~2 μm) benefits the formation of secondary voids under the impact load. The undissolved M2C and MC precipitations in matrix promote the connecting of secondary voids and then form the secondary cracks. As a result, the impact toughness of the characteristic zone is poor, and becomes the weak region of HAZ.

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    Positron Annihilation Investigation of Embrittlement Behavior in Chinese RPV Steels after Fe-Ion Irradiation
    Tianci ZHANG, Haitao WANG, Zhengcao LI, Henk SCHUT, Zhengming ZHANG, Ming HE, Yuliang SUN
    Acta Metall Sin, 2018, 54 (4): 512-518.  DOI: 10.11900/0412.1961.2017.00471
    Abstract   HTML   PDF (860KB) ( 340 )

    The reactor pressure vessel (RPV) is the key component in the nuclear power plant, which is considered irreplaceable and can be the life-limiting feature of the operation of nuclear power plant if its mechanical properties degrade sufficiently. High temperature gas-cooled reactor (HTGR) has perfect inherent safety, which is intended to be one of the fourth generation advanced nuclear reactors. However, HTGR has different service temperature with pressurized water reactor (PWR), that the service temperature of HTGR is 250 ℃ and that of PWR is 290 ℃. So the irradiation behaviour of RPV in HTGR is expected to be investigated. In this wok, 3 MeV Fe-ion irradiation was performed on Chinese A508-3 reactor pressure vessel steel which is employed by high-temperature gas-cooled reactors and pure Fe under room temperature (about 25 ℃) and high temperature (250 ℃). The ion doses were 0.1, 0.5 and 1.0 dpa for both room temperature irradiation and high temperature irradiation. SRIM modeling was performed before irradiation experiments to guide the experimental details. Positron annihilation Doppler broadening (PADB) spectroscopy experiments and nano-indentation tests (to study embrittlement behavior) were conducted for characterization. It is found that after both room temperature irradiation and high temperature irradiation, the densities of defects in the reactor pressure vessel steel and pure Fe increase, and the type of defects could be vacancy-type and solute cluster type from PADB results. The vacancy-type defect density under high temperature irradiation is lower than that under room temperature irradiation. That is because high temperature can recover the defects formed during irradiation. The hardness test results show that for both the reactor pressure vessel steel and pure Fe, the irradiation hardening increases with increasing dose. Compared to room temperature irradiation, high temperature irradiation can produce more solute clusters and fewer vacancy-type defects in the reactor pressure vessel steel. So the irradiation hardening of the reactor pressure vessel steel might be caused mainly by the formation of solute clusters.

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    Deuterium and Tritium Permeation in the Reduced Activation Ferritic/Martensitic Steel
    Dongjun FAN, Guangda LU, Guikai ZHANG, Jinchun BAO, Feilong YANG, Xin XIANG, Chang'an CHEN
    Acta Metall Sin, 2018, 54 (4): 519-526.  DOI: 10.11900/0412.1961.2017.00373
    Abstract   HTML   PDF (2778KB) ( 470 )

    Reduced activation ferritic/martensitic (RAFM) steel is the preferred structural material for test blanket module in fusion reactor. The study of the diffusion transport character of deuterium and tritium in the steel is of great significance for fuel recycling, tritium diffusion control, recovery and safety to China Fusion Engineering Test Reactor (CFETR) item. Two kind of RAFM steels have been developed in China and one of them, China low activation martensitic (CLAM) steel, is chosen to investigate its diffusive transport parameters of deuterium and tritium in this work. By gas-driven permeation experiment between 573 K and 873 K, deuterium transport parameters are measured. The permeability is: ΦD=3.41×10-8exp(-39181/(RT)) (mol/(msPa0.5)), the diffusion coefficient is DD=1.43×10-7exp(-22110/(RT)) (m2/s), the solubility constant is SD=2.38×10-1exp(-17071/(RT)) (mol/(m3Pa0.5)). Between 573 K and 823 K, the permeability of tritium is ΦT=2.50×10-8exp(-38493/(RT)) (mol/(msPa0.5)). According to the isotopes effect rule, the diffusion coefficient and solubility constant of tritium is deduced respectively: DT=1.95×10-7 exp(-22797/(RT)) (m2/s) and ST=1.28×10-1exp(-15696/(RT)) (mol/(m3Pa0.5)). A strange behavior appears in experiments with deuterium: after forced air cooling to the steel, the permeation flux through it quickly rise first and then anomaly lower for hours. The mechanism arousing the phenomenon and effects on use in engineering needs further investigation.

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    Investigation of Corrosion Behavior of Welded Joint of X70 Pipeline Steel for Deep Sea
    Ge MA, Xiurong ZUO, Liang HONG, Yinglun JI, Junyuan DONG, Huihui WANG
    Acta Metall Sin, 2018, 54 (4): 527-536.  DOI: 10.11900/0412.1961.2017.00149
    Abstract   HTML   PDF (6540KB) ( 523 )

    X70 pipeline steel with thick specifications (40.5 mm) for 3500 m deep sea reached the international advanced level in the wall thickness and service depth. Due to the high heat input during the welding process, the corrosion resistance of inside welding and outside welding would vary depending on the microstructure differences. The corrosion resistance of the welded joints of X70 pipeline for deep sea was studied by the immersion test, the weight loss test, the electrochemical test in this work. The components of the passive film were analyzed by XRD and the microstructure was observed by SEM. The results show that the corrosion resistance of the weld metal is the best. The corrosion resistance of the heat affected zone follows. The corrosion resistance of the base metal is the worst. And for the same area, the corrosion resistance of the inside welding is better than that of the outside welding. The formation of dense Fe3O4 passivation film can effectively slow down the progress of the reaction, and the corrosion products of Fe2O3, FeOOH and Fe(OH)3 which are loose in the outer layer, have no protective effect on the matrix. The microstructure of the weld metal with the best corrosion resistance is mostly the intragranular nucleation ferrite and martensite-austenite (M-A) constituent is fine and uniform. The microstructure gradient of the heat affected zone is the largest, the M-A constituent is coarse and the corrosion resistance is inferior to the weld metal. The base metal consists of ferrite and bainite, the bainite is island-like distribution and the corrosion resistance is the worst. Microstructure of the inside welding is more refined, owing to the influence of outside welding thermal cycle, and the volume fraction of M-A constituent in inside welding is higher than that of the outside welding, so the corrosion resistance is better than that of the outside welding.

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    Crevice Corrosion of X70 and 3Cr Low Alloy Steels Under Supercritical CO2 Condition
    Jianan ZOU, Xiaolu PANG, Kewei GAO
    Acta Metall Sin, 2018, 54 (4): 537-546.  DOI: 10.11900/0412.1961.2017.00353
    Abstract   HTML   PDF (15422KB) ( 340 )

    With the exploitation of high pressure gas fields and the development of carbon capture and storage (CCS) techniques, the corrosion problem of steels under CO2 environment has been paid more and more attention. To transportation easier and cost reduction, CO2 in pipelines and containers is usually pressured to a high pressure, such as supercritical state. The supercritical CO2 corrosion environment includes the CO2-saturated aqueous phase and the water-saturated supercritical CO2 (SC CO2) phase. Moreover, corrosive ions such as Cl- usually exist in CO2 corrosion environment, which could accelerate the occurrence of corrosion. Low alloy steels, widely used as pipelines and construction materials in oil/gas and CCS industries, are susceptible to corrosion in the aggressive environment that contains high-concentration ions and acidic gases, especially to severe localized corrosion. In this work, the crevice corrosion behavior of 3Cr and X70 steels exposed in supercritical CO2-saturated 3.5%NaCl solution and NaCl solution-saturated supercritical CO2 phase was investigated. SEM, EDS and 3D laser microscopy were used to analyze the corrosion product scale on the steel surface. The results show that both the steels occurred crevice corrosion on the edge of crevice, but slightly occurred corrosion inside the crevice. The crevice corrosion occurred due to the galvanic effect of areas inside and outside the crevice. In supercritical CO2 phase, 3Cr steel exhibited a higher uniform corrosion resistance than X70 steel, while the crevice corrosion resistance of 3Cr steel was lower than that of X70 steel. The different crevice corrosion behaviors between X70 and 3Cr steels might be attributed to the synergistic effect of elements Cr and Cu on enhancing the crevice corrosion.

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    Plasma-MIG Hybrid Welding Hot Cracking Susceptibility of 7075 Aluminum Alloy Based on Optimum of Weld Penetration
    Yingkai SHAO, Yuxi WANG, Zhibin YANG, Chunyuan SHI
    Acta Metall Sin, 2018, 54 (4): 547-556.  DOI: 10.11900/0412.1961.2017.00357
    Abstract   HTML   PDF (10406KB) ( 401 )

    The 7075 aluminum alloys have major applications in commercial, transportation industry and military air carriers, owing to their associated light weight, high strength, good machinability, high fracture toughness and low fatigue crack growth. Several welding techniques, such as metal inert gas (MIG) welding, tungsten inert gas (TIG) welding, laser welding and friction stir welding (FSW), have been applied to weld the 7075 aluminum alloys. However, their applications are limited because of the lower weld strength, slower welding speed and other significant limitations of them. Among the different welding techniques, plasma-MIG hybrid welding is a new fabrication technique with many advantages such as stable welding process, no weld spatter, the decreased pores, small grain size and high joint quality. Up to now, the study mainly focuses on coaxial plasma-MIG hybrid welding, and it is rare in dealing with the hot cracking susceptibility of 7000 series aluminum alloys welded by paraxial plasma-MIG hybrid welding. In this work, the paraxial plasma-MIG hybrid welding system was used to weld 7075-T6 aluminum alloy plates. The quantitative relationship between plasma-MIG hybrid welding parameters of 7075 aluminum alloy and weld penetration was established by linear regression orthogonal test. Hot ductility tests were studied by using the thermal simulated test to determine the brittleness temperature range of the alloy. Welding hot cracking susceptibility tests were conducted by using the fish bone method, and the type and cause of the hot cracking were analyzed by SEM, EDS and OM. The results indicated that the brittleness temperature range of 7075 aluminum alloy was 470~620 ℃. When the heat inputs of plasma-MIG hybrid welding were 2.52, 2.95 and 3.42 kJ/cm respectively, the welding hot cracking susceptibility decreased and then increased with the heat input increasing. The type of cracking in partially melted zone of base metal was liquation cracking, and that of weld zone was solidification cracking. When the heat input was 2.95 kJ/cm, the welding hot cracking sensitivity was the least, and the welding cracking was solidification cracking. Compared to MIG welding joints, the hot cracking susceptibility of plasma-MIG hybrid welding joints decreased by 47% under the same conditions.

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    Anisotropy and Deformation Mechanisms ofAs-Extruded Mg-3Zn-1Y Magnesium AlloyUnder High Strain Rates
    Xudong LI, Pingli MAO, Yanyu LIU, Zheng LIU, Zhi WANG, Feng WANG
    Acta Metall Sin, 2018, 54 (4): 557-565.  DOI: 10.11900/0412.1961.2017.00147
    Abstract   HTML   PDF (5031KB) ( 857 )

    As a very important design principle, the dynamic properties of materials attracted extensive attention in resent years and a bunch of works have been done concerning with the materials deformation behaviors under high strain rates. However, the dynamic behaviors of magnesium alloys are not through understood, especially the rare earth based magnesium alloys. In order to investigate the dynamic and anisotropic behavior under high strain rates deformation of as-extruded Mg-3Zn-1Y magnesium alloy, the split Hopkinson pressure bar (SHPB) apparatus was used to testing the true stress-true strain curves under the high strain rates of 1000, 1500 and 2200 s-1 of as-extruded Mg-3Zn-1Y magnesium alloy. The OM and SEM were used to analysis the micorstructure evolution and fracture surface morphology of the alloy. The true reason behind the anisotropic phenomenon was revealed based on the deformation mechanism of highly basal-textured magnesium alloy. The results demonstrate that the as-extruded Mg-3Zn-1Y magnesium alloy exhibits pronounced anisotropy during compression according to the loading direction. The anisotropy of the as-extruded Mg-3Zn-1Y magnesium alloy are arised from the variety of the deformation mechanisms. When the loading direction is along extrusion direction, the predominant deformation mode changes from extension twinning at a lower strain to prismatic slip at a higher strain. While compressed along extrusion radial direction (ERD), the predominant deformation mode changes from contraction twinning to a coordination of basal and second order pyramidal slip with the increasing of strain.

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    Oxidation and Microstructure Evolution of CoAl Coating on Directionally Solidified Ni-Based Superalloys DZ466
    Weipeng REN, Qing LI, Qiang HUANG, Chengbo XIAO, Limin HE
    Acta Metall Sin, 2018, 54 (4): 566-574.  DOI: 10.11900/0412.1961.2017.00240
    Abstract   HTML   PDF (4959KB) ( 568 )

    Aluminide coatings are widely employed to protect internal cooling channels of high grades blades and buckets in gas turbines have always been in severe conditions including high temperature oxidation and hot corrosion. There is a major concern for the application of aluminide coatings that refer to the inter-diffusion between aluminide coating and superalloy substrate at high temperatures. Diffusion of Al from the coating to the underlying substrate usually leads to depletion of Al in the coating, resulting in inferior oxidation resistance of the coating. Accordingly, Ni declines to diffuse counter currently from the substrate into the coating, as well as other refractory elements, such as Cr, Mo and W etc.. The inter-diffusion between aluminide coating and superalloy substrate results in degradation or various evolution behaviors of aluminide coatings, in other words, substrate composition significantly affects the properties of aluminide coatings. CoAl coating was prepared on directionally solidified superalloy DZ466 by low pressure chemical vapour deposition (LP-CVD). Oxidation behavior and microstructure evolution of CoAl coating was investigated during long term (about 5000 h) exposure at 900 ℃. Results suggested that, high concentration of aluminum did help to form Al2O3 on the surface of coating, improving oxidation resistance of DZ466 at 900 ℃. Evolution of matrix phase and precipitates in the CoAl coating during exposure was displayed, β-NiAl/CoAl phase in the coating transformed gradually to γ'-Ni3Al phase, higher transformation rate for the γ' phase closed to the substrate due to the diffusion between the coating and the sub strate superalloy. During exposure, α-Cr phase precipitated in the middle layer, which inclined to form close to carbides and grow by consuming them. Needle like TCP phase (μ phase) grew in the inner layer that arranged in order, which was due to the cubic microstructure of γ/γ'. Heredity-effect was in company with the precipitates evolution.

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    Brazing of Coated Al Foil Filler to AlN Ceramic
    Xiaoyi ZHANG, Hailong SHANG, Bingyang MA, Rongbin LI, Geyang LI
    Acta Metall Sin, 2018, 54 (4): 575-580.  DOI: 10.11900/0412.1961.2017.00159
    Abstract   HTML   PDF (4878KB) ( 400 )

    The wettabilities of molten metals on ceramics are poor normally. In order to improve the wettability, all existing ceramic brazing methods introduce a compound transition layer formed by the reaction of active metal and ceramic. The transition layer between brazing seam and ceramic however creates negative effect on the properties of brazing joints. Although Al is the scarce metal which can wet some ceramics such as AlN and Al2O3 without reaction, the difficulty of removing oxide layer on surface prevents it being ceramic brazing filler. This work proposed a kind of coated Al foil filler able to remove its own Al2O3 film and an elevating temperature brazing process to enhance Al/AlN joint strength. Removing Al2O3 film effect of vapor deposited Ni/Al double layer film on Al foil and the effect of brazing temperature on improving joint strength were studied. The results showed that due to buried by Ni/Al double layer film, Al2O3 film on Al foil original surface broken and was swept in Al-1%Ni (atomic fraction) alloy liquid during heating and melting process. As a result, the direct brazing of Al to AlN without interface reaction transition layer was realized. The joint strength was significantly enhanced by elevating brazing temperature. When brazing at 680 ℃, the joint fractured along the interface between Al seam and AlN and the sheer strength was 79 MPa because of Al liquid not wetting AlN. With the elevating of brazing temperature, the wettability and interface strength of Al/AlN improved. The fracture gradually transferred to brazing seam from interface. The joint strength increased and reached to the maximum value of 146 MPa at 840 ℃.

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    Preparation and Enhanced Hot Corrosion Resistance of aZr-Doped PtAl2+(Ni, Pt)Al Dual-Phase Coating
    Chengyang JIANG, Yingfei YANG, Zhengyi ZHANG, Zebin BAO, Shenglong ZHU, Fuhui WANG
    Acta Metall Sin, 2018, 54 (4): 581-590.  DOI: 10.11900/0412.1961.2017.00376
    Abstract   HTML   PDF (7127KB) ( 413 )

    Prior to practical service, hot-section components (e.g. airfoils and vanes) of a gas turbine engine are necessarily coated by a protective metallic coating (such as aluminide diffusion coating, modified aluminide coating and MCrAlY overlay etc.) to resist high temperature oxidation and hot corrosion. Among the modified aluminide coatings, the coating with Pt-modification has attracted great attention and is widely used in applications requiring high reliability and extended service life since it possesses superior oxidation/corrosion resistance at high temperature. The presence of Pt in aluminide coating is favorable for increasing bonding strength of oxide scale, enlarging phase region of β-NiAl and confining detrimental effect of sulphur etc. Although Pt-modification has exhibited visible benefits for acquiring better high-temperature performance, it is far from satisfaction to develop an ideal aluminide diffusion coating. Reactive elements such as Y, Hf, Zr or their oxides have been employed to modify the nickel aluminide coating system, with an aim to further improve scale adhesion and promote exclusive formation of α-Al2O3 simultaneously. In this work, a Zr-doped PtAl2+(Ni, Pt)Al dual-phase aluminide coating was prepared on a Ni-based single crystal superalloy by co-deposition of Pt-Zr through electroplating and subsequent aluminization treatments. The coating was mainly composed of three layers: the outmost layer consisted of double phases with PtAl2 particles dispersed in β-(Ni, Pt)Al domain, while the interlayer comprised β-(Ni, Pt)Al with small amount of Cr-precipitates, and the bottom layer was an inter-diffusion zone (IDZ). Zirconium was mainly distributed inside β-(Ni, Pt)Al solid solution in both the outmost layer and the interlayer. Compared with normal PtAl2+(Ni, Pt)Al dual-phase coating, the hot corrosion behavior of the Zr-doped PtAl2+(Ni, Pt)Al coating was assessed in a salt mixture of Na2SO4/NaCl (75:25, mass ratio) at 850 ℃ in static air. The results indicated that the Zr-doped PtAl2+(Ni, Pt)Al dual-phase coating exhibited superior hot corrosion resistance since Zr was confirmed able to capture and fix S and Cl to diminish their detrimental effects. Meanwhile, a pre-oxidation treatment did not effectively improve the overall hot corrosion resistance of normal PtAl2+(Ni, Pt)Al coating because the thin alumina scale formed during pre-oxidation was unable to prohibit the sustained inward-invasion of the mixed salt.

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    Nickel-Based Single-Crystal Superalloys (Ni, Co)-Al-(Ta, Ti)-(Cr, Mo, W) Designed by Cluster-Plus-Glue-Atom Model and Their 1000 h Long-Term Ageing Behavior at 900 ℃
    Yu ZHANG, Qing WANG, Honggang DONG, Chuang DONG, Hongyu ZHANG, Xiaofeng SUN
    Acta Metall Sin, 2018, 54 (4): 591-602.  DOI: 10.11900/0412.1961.2017.00334
    Abstract   HTML   PDF (6577KB) ( 378 )

    It has been pointed out recently that the compositions of industrial alloys are originated from cluster-plus-glue-atom structure units in solid solutions. Specifically for nickel-based superalloys, after properly grouping the alloying elements into Al, Ni-like (, including Ni, Co, Fe, Re, Ru and Ir), γ′γ, including Ta, Ti, V, Nb), and γ-forming Cr-like (γ, including Cr, Mo and W), the optimal formula for single-crystal superalloys has been established [Al-12](Al1γ0.5γ1.5). In this work, the first generation single-crystal superalloys were investigated on the basis of the proposed formula, by using =(Ni and Co), γ=(Ta and Ti), and γ=(Cr, Mo and W). Two series of alloys were designed, formulated respectively as group A: [Al-Ni11Co1](Al1TaxTi0.5-xCr1W0.25Mo0.25), with x=0, 0.25 and 0.5 (the corresponding mass fractions of Ta and Ti are respectively 0Ta-2.65Ti, 4.82Ta-1.26Ti and 9.32Ta-0Ti), and group B: [Al-Ni12-yCoy](Al1Ta0.25Ti0.25Cr1W0.25Mo0.25), with y=1.5, 1.75, 2 and 2.5 (the corresponding mass fractions of Co are respectively 9.43Co, 11Co, 12.57Co and 15.71Co). The single-crystal superalloys were prepared using selector technique. And then they underwent the following tests of incipient melting, standard heat treatment and 1000 h long term ageing at 900 ℃. It is found that: (1) In group A, with increasing Ta content (decreasing Ti), all the incipient melting temperatures are increased to above 1330 ℃, and to the highest value is between 1335 ℃ and 1340 ℃ for alloy 9.32Ta-0Ti; the γ/γ′ lattice negative misfits after standard heat treatment are reduced from -0.262% (0Ta-2.65Ti) to -0.247% (9.32Ta-0Ti); the γ′ coarsening tendency after long-term ageing is deduced, and alloy 9.32Ta-0Ti has the lowest coarsening rate (K=5.6×10-5 μm3/h). (2) In group B, the Co content does not influence the incipient melting temperature (always above 1330 ℃) and the coarsening rate of γ′ after long-term ageing. The major role of Co is to increase the mean size of the γ′ precipitates to about 0.55 μm and the γ′ volume fraction to about 69% after the standard heat treatment. These two groups of alloys have their γ′ coarsening rates approaching the level of third-generation single-crystal superalloys (K≈(2.08~3.82)×10-5 μm3/h).

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    A First-Principles Study on Basal/Prismatic Reorientation-Induced Twinning Path and Alloying Effect in Hexagonal Metals
    Gang ZHOU, Lihua YE, Hao WANG, Dongsheng XU, Changgong MENG, Rui YANG
    Acta Metall Sin, 2018, 54 (4): 603-612.  DOI: 10.11900/0412.1961.2017.00252
    Abstract   HTML   PDF (8534KB) ( 522 )

    In hexagonal metals and alloys, deformation twinning plays an important role, because it is closely relevant to the mechanical behaviors. Recent studies have proposed a new twinning mode via direct lattice reorientation, which results in the basal/prismatic boundary, however, some important details remain unanswered, e.g., the twinning path and alloying effect. In this work, first principles calculations were employed to systematically study the reorientation process from basal to prismatic orientation in hexagonal metals and corresponding alloying effect. The result indicates that different activation energies are required to reorient in various hexagonal metals, and among them, the energy in Mg is the lowest and Os is the highest. Shear and shuffle components compose the reorientation process, where the shuffle component always contributes a significant part of the activation energy in Mg, whereas in Ti with sufficient shear strain, subsequent transition becomes energy-downhill. The pure shear was effected by alloying elements in Mg alloys, but pure shuffle in Ti alloys. Under certain shear or shuffle, subsequent activation energy has a complex dependence on alloying elements.

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