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MgO SECONDARY ELECTRON EMISSION FILM PREPARED BY RADIO-FREQUENCY REACTIVE SPUTERRING
Bin WANG,Liangyin XIONG,Shi LIU
Acta Metall Sin    2016, 52 (1): 10-16.   doi:10.11900/0412.1961.2015.00189
Accepted: 06 September 2015

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High, stable and durable secondary electron emission is an essential property for the application of dynodes of electron multipliers and photomultiplier tubes. The MgO film have been widely used as dynode materials for the applications owing to its good secondary electron emission properties. In this work, MgO and CoO doped MgO films, as secondary electron emission films, were prepared by radio-frequency reactive sputtering deposition on the stainless steel substrate, and also another MgO film at the surface of activated AgMg alloy was prepared. The effect of preparation processes on the secondary electron emission properties of the films was focused. It was found that the film thickness significantly affected the resistance to electron beam bombardment. With the increase of film thickness, the resistance to electron beam bombardment was significantly enhanced. Radio-frequency reactive sputtering deposition could control the film thickness by varying deposition time. The surface quality of MgO film is quite sensitive to the oxygen partial pressure of the deposition atmosphere. Higher oxygen partial pressure caused higher surface roughness, which was harmful to the secondary electron emission. After doping with CoO, the surface of MgO films were much flatter and smoother, resulting in the improvement of the secondary electron emission coefficient. The CoO doping also reduced of the sensitivity of film surface quality to the oxygen partial pressure. The secondary electron emission coefficient of CoO doped MgO film sharply decreased after heated at 550 ℃ for 1 h due to the surface quality degrading and the thermal decomposition induced loss of oxygen. Elevating the substrate temperature or oxygen partial pressure during deposition accounted for the presence of metallic Mg in film and the degrading of surface quality, which finally lead to lower secondary electron emission coefficient.

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CYCLIC OXIDATION AND HOT CORROSION BEHAVIORS OF A GRADIENT NiCoCrAlYSi COATING
Xin PENG, Sumeng JIANG, Xudong SUN, Jun GONG, Chao SUN
Acta Metall Sin    2016, 52 (5): 625-631.   doi:10.11900/0412.1961.2016.00013
Accepted: 16 March 2016

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MCrAlY (M=Ni and/or Co) coatings are widely used as overlays or bond coats for thermal barrier coatings due to their good performance against high temperature oxidation and hot corrosion. Usually, high Al content in the MCrAlY coatings can benefit the performance and lifetimes of the coatings. However, MCrAlY coatings usually contain only restricted Al content because high Al content might lead to brittleness and potential crack. Design of gradient coating can be used to solve the problem, since it can provide a balance between high Al content and high stress bearing ability. Therefore, much attention has been paid to coatings with gradient structures, and these coatings show good oxidation and corrosion resistance. In this work, a gradient and a conventional NiCoCrAlYSi coating were prepared by arc ion plating technique and subsequent annealing treatment. Cyclic oxidation tests of the two coatings were carried out between room temperature and 1000 ℃. The hot corrosion tests of the coatings were performed in two different mixed salts of 75%Na2SO4+25%K2SO4 and 75%Na2SO4+25%NaCl (mass fraction) at 900 ℃. The results indicated that the gradient coating possessed a graded distribution of Al-rich outer layer and Cr-rich inner layer after annealing treatment, and it showed better performance of re-healing alumina scale due to its possession of more β phase as Al reservoir during the cyclic oxidation. The degradation process of the gradient coating was favorably retarded by the formation of Cr(W, Re)-rich precipitates in the interdiffusion zone. In sulphates, the two coatings showed good corrosion resistance. The presence of NaCl aggravated the corrosion extent of the two coatings. Compared with the conventional coating, the gradient coating postponed the formation of internal oxidation and sulfidation, resulting from the gradient distribution of Al-enriched outer layer and Cr-enriched inner layer.

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EFFECTS OF Mo CONTENT ON THE MICRO-STRUCTURE AND TRIBOLOGICAL PROPERTIES OF CrMoAlN FILMS
Baiyang LOU,Yuxing WANG
Acta Metall Sin    2016, 52 (6): 727-733.   doi:10.11900/0412.1961.2015.00493
Accepted: 21 March 2016

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In recent decades, CrAlN coatings have been widely used for cutting tools due to their high hardness, good wear resistance, especially excellent thermal stability and oxidation resistance. However, the rapid development in high speeds and dry cutting applications demands further improvement in hardness and wear properties of CrAlN coatings. Mo nitrides coatings are commonly used as protective surface layers against wear and corrosion. The combination of CrAlN and Mo may lead to the development of new composite coatings with superior wear properties. In this study, the CrMoAlN multilayer coatings with different Mo contents were deposited on M2 tool steel and silicon wafers substrates by closed-field unbalanced magnetron sputtering ion plating (CFUMSIP) technique in a gas mixture of Ar+N2. The chemical composition, surface and cross sectional morphologies, microstructure, mechanical and tribological properties of coatings were studied by EDS, SEM, XRD, XPS, nano-indentation and pin-on-disk tribometer, respectively. The results indicate that the CrMoAlN coatings exhibit fcc structure. Mo atoms substitute Cr and/or Al atoms in CrAlN lattice forming the solid solution CrMoAlN coatings. The surface and cross-sectional morphologies of the CrMoAlN coatings show that the grain size and the column width decrease with the increasing of Mo content. Nano-indentation result reveals a promoted hardness and elastic modulus of the CrMoAlN coatings with enhanced Mo content from 0 to 19.47% (atomic fraction) due to the solid solution strengthening and grain size diminishment. A maximum hardness and elastic modulus of the coatings are found to be 29.70 GPa and 427.53 GPa when the Mo content reached to 19.47%. The average friction coefficient and wear rate were observed to decrease with the increase of Mo content and the lowest values were 0.271 and 1.2×10-16 m3/(Nm), respectively, at 19.47%Mo.

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PREPARATION OF Ti1-xAlxN COATING IN CUTTING TITANIUM ALLOY AND ITS CUTTING PERFORMANCE
Xudong SUI,Guojian LI,Qiang WANG,Xuesi QIN,Xiangkui ZHOU,Kai WANG,Lijian ZUO
Acta Metall Sin    2016, 52 (6): 741-746.   doi:10.11900/0412.1961.2015.00454
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High-strength lightweight titanium alloy structural materials have been widely used in aerospace and other industry. However, the titanium is hard to machine due to its characteristics of low thermal conductivity, high chemical affinity and low elastic modulus. Coating tools provide a solution to overcome the problem of cutting titanium alloy. Ti1-xAlxN coating is one of the most popular candidates in cutting titanium alloy. However, the cutting performance and wear mechanism of the sputtering Ti1-xAlxN coating should be studied further in order to meet the demands of cutting titanium alloy. In this work, Ti1-xAlxN coatings with different Al contents have been prepared by magnetron sputtering. Microstructure and mechanical properties of the coatings were examined by XRD, SEM, EDX and nanoindenter. Results show that the coatings is a single fcc structure with a (111) preferred orientation when x is in the range of 0.50~0.58 (atomic fraction). When the Al content is 0.63, the hexagonal AlN is formed in the coating and the hardness declines. In addition, the surface particle size of Ti1-xAlxN coatings increases and the coating density decreases with increasing the Al content. The results of titanium cutting experiment indicate that the tool wear is mainly adhesive wear and chipping. The cutting performances of Ti0.50Al0.50N coated tool is slightly better than uncoated tool and are much better than those of Ti0.42Al0.58 and Ti0.37Al0.63N coated tools at a lower cutting speed (65 m/min). The cutting performance of Ti0.50Al0.50N coated tool is the best at a higher cutting speed of 100 m/min and is four times larger than that of uncoated tool. The excellent cutting performance of Ti0.50Al0.50N coating is mainly due to its high surface density and high hardness, which lead to the formation of regular and dense built-up edge during titanium cutting. Therefore, Ti0.50Al0.50N coating with a (111) preferred orientation, dense surface and relatively low Al content is recommended in high speed turning titanium.

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EFFECT OF CeO2 ON CORROSION BEHAVIOR OF WC-12Co COATINGS BY HIGH VELOCITY OXYGEN FUEL
Shengbo CEN,Hui CHEN,Yan LIU,Yuanming MA,Ying WU
Acta Metall Sin    2016, 52 (11): 1441-1448.   doi:10.11900/0412.1961.2016.00031
Accepted: 18 August 2016

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High velocity oxygen fuel (HVOF) sprayed WC-Co coating has been widely used in the surface protection of components for excellent corrosion resistance and wear resistance. However, with the increasing deteriorated service environment, higher comprehensive properties of WC-Co coating are required. Addition of rare earth elements into WC-Co powder is expected to be an effective way. In this work, the micro WC-12Co, nano modified WC-12Co and CeO2 modified WC-12Co coatings were prepared by HVOF on the Q345 steel substrate. The microstructure, corrosion morphology and phase structure of coatings were observed by SEM and XRD, and the micro-hardness is measured. The corrosion behavior of the coatings in 1 mol/L H2SO4 solution was investigated by polarization test and immersion corrosion test. The results show that the addition of nano-sized CeO2 in the WC-12Co coating not only purifies the grain boundary and increases the micro hardness, but also significantly reduces the porosity of the coating, which can effectively decrease the occurrence of local corrosion. Meanwhile, the addition of nano CeO2 can make the electrode potential of coatings shift positively, reduce the corrosion current density and passivation current density, and then improve the corrosion resistance of the coating. The corrosion mechanism of nano CeO2 modified WC-12Co coating is local corrosion which induced by the pore. Co bonding phase at the pore is constantly being corroded, causing WC particles to lose the support function and to fall off, which promotes the corrosion of the coating, so that the pores are enlarged to form corrosion pits. For the micro WC-12Co coating and nano modified WC-12Co coating, not only the outermost layer of the Co bonding phase is corroded, but also serious local corrosion occurred in the pores.

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STUDY ON ATOMIC-SCALE STRENGTHENING MECHANISM OF TRANSITION-METAL NITRIDE MNx (M=Ti, Zr, Hf) FILMS WITHIN WIDE COMPOSITION RANGES
Kechang HAN,Yiqi LIU,Guoqiang LIN,Chuang DONG,Kaiping TAI,Xin JIANG
Acta Metall Sin    2016, 52 (12): 1601-1609.   doi:10.11900/0412.1961.2016.00078
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Transition-metal nitrides have long attracted considerable attention among researchers and ubiquitous applications in various fields due to their renowned mechanical properties. However almost all the discussions of the strengthening mechanism were on conventional meso scale. For further understanding on the atomic scale strengthening mechanism of transition-metal nitrides, three groups of MNx (M=Ti, Zr, Hf) films with different nitrogen contents were synthesized on the Si substrates by magnetic filtering arc ion plating. The morphologies and thickness of the as-deposited films were characterized by FESEM, the microstructures and the residual stresses were characterized by XRD, the XPS and Nano Indenter were used to measure the chemical states and hardness (also the elastic modulus) of as-deposited films, respectively. The results show that all three groups MNx films perform the B1-NaCl single-phase structure within the large composition ranges. The preferred orientation, thickness, grain size and residual stress of the MNx films with different nitrogen contents were not changed so much. While the nanohardness and elastic modulus of MNx both first increased and then decreased with the rise of nitrogen content, and the peak values all existed when x near to 0.82. The strengthening mechanism was discussed and the decisive factor of composition dependent hardness enhancement was found from the atomic-scale chemical bonding states and electronic structure in this work, rather than the conventional meso-scale factors, such as preferred orientation, grain size and residual stress.

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Effect of N Doping on Microstructure, Mechanical and Tribological Properties of V-Al-C Coatings
Xin WANG,Zhenyu WANG,Zaixin FENG,Peiling KE,Aiying WANG
Acta Metall Sin    2017, 53 (6): 709-718.   doi:10.11900/0412.1961.2016.00523
Accepted: 27 February 2017

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The crises of resource shortage have prompted ocean exploitation to spring up all over the world. Some crucial frictional components of marine equipment have to be directly faced with the conjoint action of wear and corrosion. Transition metal nitrides or carbides hard coatings have been widely used to improve tribological performance in various applications. However, the poor toughness, wear and corrosion resistance of coatings cannot meet the harsher marine environment, which needs to obtain multi-functional hard coatings providing complex properties. The nanocomposite structure coatings containing nanocrystalline phase embedded in an amorphous matrix allow tailoring their properties to desired value by designing chemical composition and nanostructure. In this work, V-Al-C and V-Al-C-N coatings were deposited on silicon and high speed steel (HSS) substrates by magnetron sputtering. The crystal microstructure, chemical composition, surface morphology, cross-sectional structure, mechanical property and friction behavior of the coatings under different contact conditions (air, distilled water and artificial seawater) were studied by XRD, XPS, SEM, nano-indentation and ball-on-disc tribometer. The results showed that the V-Al-C coating displayed columnar structure with coarse grain. When the nitrogen was incorporated, the coating structure evolved into nanocomposite structure composed of nanocrystallite and amorphous carbon. The hardness increased from (14±0.48) GPa to (24.5±0.8) GPa, and the toughness was significantly improved (H/E>0.1). In air condition, the friction coefficient decreased from 0.70 to 0.42, owing to the synergy interaction between V2O5 and amorphous carbon during sliding. The friction coefficients of the both coatings in distilled water and artificial seawater were lower than those in air owing to the boundary lubrication forming lubricative film by absorbed water. The friction coefficient in seawater was lower than those in distilled water, resulting from the formation of Mg(OH)2 and CaCO3 during sliding. However, the wear rates of the both coatings in artificial seawater were larger than that in distilled water, which demonstrated a synergism between corrosion and wear in artificial water. The V-Al-C coating was all worn out under different contact conditions owing to severe abrasive wear, while the V-Al-C-N coating showed better wear resistance, with a wear rate of 3.0×10-16 m3/(Nm) in air and 1.4×10-15 m3/(Nm) in artificial water, respectively.

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Microstructure Analysis of Initial Alumina of Pt-Modified Aluminide Coatings on Ni-Based Alloy
Peng SONG, Rong CHEN, Jing FENG, Jianguo Lü, Jiansheng LU
Acta Metall Sin    2017, 53 (11): 1504-1510.   doi:10.11900/0412.1961.2017.00093
Accepted: 16 August 2017

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NiPtAl coatings are widely used as overlaying coatings besides bondcoats for thermal barrier coating (TBC) systems within high temperature environment. Oxidaiton behavior of NiPtAl coatings is mainly contribution for the failure of TBC systems or overlaying coatings. An initial oxide layer growth characteristics play a key role in extending lifetime of TBC system or overlaying coatings. In this work, the oxidation experiments of the Pt modified aluminide coating on CMSX-4 Ni-based alloy were carried out at 1150 ℃ for 1 h in 80%Ar+20%O2. The microstructures of oxide on the NiPtAl coatings are studied by OM, SEM, TEM and Raman spectroscopy. The results indicated that the oxide layer on the NiPtAl coatings included stable and met-stable Al2O3 after 1 h oxidation, and part of spalled oxide layer as well as pores within the oxide layer. The 0.5 μm thickness whisker-like θ-Al2O3 could form on NiPtAl coating during the initially oxidation stage. At the initial oxidation stage θ-Al2O3 fastly grew which resulted β-NiAl to γ'-Ni3Al transformation. The Pt particles formed on the inter-surface between α-Al2O3 and θ-Al2O3 layer due to a less Pt solid solubility in γ'-Ni3Al compared to β-NiAl in the coating. Fast growth of initial Al2O3 could induce pores formation within the alumina layer. The pores and stress due to oxidation and phase transformation could decrease the alumina adherence, and at last result in the oxide spallation.

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Effect of Al Film on the Electromagnetic Properties of Glass Fiber Reinforced Resin Matrix Composite
Yuqiu CHEN, Yapei ZU, Jun GONG, Cao SUN, Chen WANG
Acta Metall Sin    2017, 53 (11): 1511-1520.   doi:10.11900/0412.1961.2017.00178
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Metallic thin films have many properties that bulk metals do not possess, such as high impedance. Recently, increasing attention has been paid to high impedance surface in the design of antennas and absorbers. Metallic thin films used in composite materials can realize the perfect matching of electromagnetic wave in different materials. The use of metallic thin films in electromagnetic functional materials results in significant increase of the absorbing intensity and operating bandwidth. But it usually needs to pay a huge amount of manpower, material resources and a longer period of time to design excellent electromagnetic functional materials with metallic films. So it is greatly significant to understand clearly the electromagnetic influence of metallic film for designing excellent performance materials and saving costs by simulation software. Al film is a typical non-magnetic metal film. In this work, the electromagnetic reflectivity of Al films and glass fiber reinforced resin matrix composite had been studied. High frequency electromagnetic field calculation software FEKO was employed to calculate the reflection coefficient of the composites. The effect of composites' real part of permittivity εr, dielectric loss tangent tanδε, permeability μr and magnetic loss tangent tanδμ on microwave reflectivity had been discussed. The equivalent electromagnetic parameters of glass fiber reinforced resin matrix composite had been obtained through a comparison between simulation and experimental results. Due to resonance phenomena of the embedded Al film in the glass fiber reinforced resin matrix composite with certain thickness, there is an optimum resistance value of Al film that makes the composite structure have minimum reflection. Through the calculation of Al film and glass fiber reinforced resin matrix composite with different structure, the thickness relationship between Al films in calculation and Al films prepared by magnetron sputtering had been obtained. According to the theory of transmission line, the resistance of resonance is analyzed by MATLAB. This method is also applicable to the resistance solution of the homogeneous metal films at any position in the composite or frequency selective surfaces. The equivalent electromagnetic parameters of Al film and glass fiber reinforced resin matrix composite in simulation had been ascertained, and the simulation results agree well with the experimental results.

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Comparative Study of Thermal Shock Behavior of the Arc Ion Plating NiCrAlY and the Enamel Based Composite Coatings
Min FENG, Minghui CHEN, Zhongdi YU, Zhenbo LV, Shenglong ZHU, Fuhui WANG
Acta Metall Sin    2017, 53 (12): 1636-1644.   doi:10.11900/0412.1961.2017.00192
Accepted: 23 October 2017

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From the view of material point, high-temperature protective coatings are divided into the following two categories: ceramic coating and metallic coating. Metallic coating possesses higher toughness and bond strength to the alloy substrate than ceramic coating does. Its protectiveness relies on the formation of a slow-growing and adherent oxide scale at high temperatures. However, with increasing the oxidation time, the oxide scale will experience cracking and spalling as it has grown to the critical thickness. Ceramic coating due to its chemical inertness has been used in many corrosive environments for protection. But the weak interfacial bond and big mismatch of coefficient of thermal expansion with the alloy substrate limit its application in thermal shock environments. Since glass-ceramics combine the generally superior properties of crystallite ceramics with the easy processing of glasses, it is expected that glass-ceramic coating should show a higher spallation resistance than ceramic one under thermal shock. Cast K444 superalloy is widely used in advanced aircraft engine and gas turbine. Its protection from high-temperature oxidation under thermal shock becomes a critic issue. In this work, NiCrAlY and enamel based composite coatings on the K444 superalloy substrate by arc ion plating and spray-firing methods were prepared, respectively. Thermal shock behavior from 900 ℃ to room temperature of these two coatings was studied comparatively. One cycle of thermal shock contained the holding of samples at 900 ℃ for 1.5 h and the following cooling down in air or water. Results indicated that thermal shock resistance of the NiCrAlY coating was low. As the NiCrAlY coating was thermal shocked by water, its oxide scale cracked severely after 30 cyc, and certain crack had already transported the scale and penetrated into the interior of the underlying metallic coating; for the enamel based composite coating, however, its thermal shock resistance was high. No cracks were detected at the coating surface or interior after thermal shock test. Besides, the enamel coating still adhered well with the alloy substrate. The high resistance to thermal shock of the enamel based composite coating originated from: (1) the coefficient of thermal expansion of the enamel based composite coating matched well with that of the alloy substrate; (2) the addition of nano-sized Ni and NiCrAlY metallic particles improved the toughness of the enamel coating, in addition to enhancing its coefficient of thermal expansion.

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Wettability Improvement and Brazing of Si3N4by Sputtered Al
Geliang LIU, Bingyang MA, Hailong SHANG, Fan CHEN, Rongbin LI, Geyang LI
Acta Metall Sin    2017, 53 (12): 1645-1650.   doi:10.11900/0412.1961.2017.00026
Accepted: 15 May 2017

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In the present technology, the brazing of Si3N4 needs a reactive transition layers to resolve the non-wetting problem of usual metal fillers. Aluminum could wet Si3N4 without reaction but the brazing is very difficult due to wetting temperature above 1000 ℃. In this work, the wetting effect of sputtered Al films on Si3N4 and its physics essence were revealed. Based on this, the brazing of Si3N4 ceramic with Al or Al-Ni film fillers was realized near their melting temperature. The results showed that the seams of brazing joints with direct sputtered Al on Si3N4 film were well-stacked and less defects, and well metallurgically bonded to ceramic without reactive transition layers. The shear strength of pure Al/Si3N4 joint reached 106 MPa. The strength increased to 148 MPa with adding 1.0%Ni into film filler due hypoeutectic structure in the seam. With further increasing Ni content to 3.0%, the eutectic structure of the seam slightly decreased the strength of joint to 132 MPa. These joints above all fractured in joint seams. Moreover, the Al-1.0%Ni film filler first sputtered Ni layer was compared. Its brazing joint fractured at the interface between seam and ceramic and the shear strength decreased to only 81 MPa. This comparsion revealed the "wetting" effect of the bombardment of energetic sputtered Al particles. This effect still existed after filler melting and the direct brazing of Si3N4 ceramic without reactive transition layers was realized.

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Progress in Thermal Fatigue of Micro/Nano-ScaleMetal Conductors
Guangping ZHANG, Honglei CHEN, Xuemei LUO, Bin ZHANG
Acta Metall Sin    2018, 54 (3): 357-366.   doi:10.11900/0412.1961.2017.00371
Accepted: 22 December 2017

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The world has gradually entered the industrial 4.0 Era, which is dominated by the Internet of Things (IOT) and intelligent manufacturing. Especially, strong requirement for artificial intelligence and big data processing, the development and preparation of micro/nano electronic devices is becoming increasingly active, and much more concerns have been attracted to small-scale materials. Because of the constraint effect of geometric and microstructural dimensions of these materials, the thermal fatigue damage behavior is different from that of the bulk counterparts. At the same time, the change of the material scale from microns to nanometers also results in the transformation of the deformation mechanism, so that the materials exhibit different damage behaviors and significant size effects. In this paper, thermal fatigue testing methods, thermal fatigue damage and evolution, and the factors influencing thermal fatigue properties of metal film/line are reviewed, the corresponding mechanism of thermal fatigue and the size effect of the micro/nano-scale metals are discussed. The prospective research of this field in the future is addressed.

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In Vitro Corrosion Resistance of Ta2N Nanocrystalline Coating in Simulated Body Fluids
Jiang XU, Xike BAO, Shuyun JIANG
Acta Metall Sin    2018, 54 (3): 443-456.   doi:10.11900/0412.1961.2017.00246
Accepted: 19 December 2017

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Due to its combination of outstanding characteristics, such as superior biocompatibility, excellent mechanical properties as well as good corrosion resistance, Ti-6Al-4V alloy has gained much attention as one of the most popular load-bearing biomedical metals in the area of orthopedic and dental. Unfortunately, Ti-6Al-4V alloy suffers from the localized corrosion damage in human body ?uids containing high chloride ion concentrations, which leads to the release of metal ions into the human body. The released ions (e.g., Al and V) are found to not only cause allergic and toxic reactions but also exhibit potential negative effects on osteoblast behavior. To improve the corrosion resistance of Ti-6Al-4V alloy in simulated body ?uids, a 40 μm thick Ta2N nanocrystalline coating with an average grain size of 12.8 nm was engineered onto a Ti-6Al-4V substrate using a double cathode glow discharge technique. The hardness and elastic modulus of the Ta2N coating were determined to be (32.1±1.6) GPa and (294.8±4.2) GPa, respectively, and the adhesion strength of the coating deposited on Ti-6Al-4V substrate was found to be 56 N. There is no evidence of crack formation within the coating under loads ranging from 0.49 N to 9.8 N, implying that the Ta2N nanocrystalline coating has a high contact damage resistance. Moreover, the corrosion resistance of the Ta2N nanocrystalline coating is significantly greater than that of Ti-6Al-4V alloy when tested in naturally aerated Ringer's solution at 37 ℃. This is due to that the passive film developed on the coating has superior compactness compared with that formed on the uncoated Ti-6Al-4V alloy. XPS analysis indicated that at a low polarized potential, the passive film consisted of TaOxNy, which would be converted to Ta2O5 at a higher polarized potential. The analysis of Mott-Schottky curves suggested that the passive film formed on the coating exhibits n-type semiconductor properties and, as such, the density and diffusivity of carrier for the coating was considerably lower than that for the uncoated Ti-6Al-4V alloy.

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Magnetic Viscosity of Anisotropic Rare Earth Permanent Films
Yachao SUN, Minggang ZHU, Rui HAN, Xiaoning SHI, Nengjun YU, Liwei SONG, Wei LI
Acta Metall Sin    2018, 54 (3): 457-462.   doi:10.11900/0412.1961.2017.00211
Accepted: 13 September 2017

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Rare earth permanent thin films are useful for magnetic microdevices such as micromotors, since its excellent magnetic properties are able to raise the performance of the devices. In order to judge the reliability of permanent magnet materials, it is quite theoretical and practical to study the time dependence behavior of magnetization, that is, magnetic viscosity or magnetic after-effect. In this work, NdFeB, CeFeB and NdFeB/CeFeB films were fabricated on the Si substrates by direct current (DC) magnetron sputtering. A Ta underlayer of 50 nm and a coverlayer of 40 nm were sputtered at room temperature to align the easy axis of the RE2Fe14B grains perpendicular to the film plane and to prevent oxidation of the magnetic films, respectively. NdFeB and CeFeB magnetic films were deposited at 903 and 883 K, respectively, and submitted to an in-situ rapid thermal annealing at 948 K for 30 min. The microstructure and magnetic properties of the films were characterized by XRD and physical property measurement system (PPMS). The results indicate that the films show excellent perpendicular anisotropy. A coercivity Hc of 1377.4 kA/m is obtained for NdFeB monolayer film at room temperature. The magnetic viscosity coefficient (S) of the films was studied over a range of temperatures (5~300 K). It is found that the values of S for all films are less than 1, and are quite similar at low temperature (5 K). Both weakened thermal agitation and strengthened anisotropy energy barriers are supposed to decrease transition frequency (f) and prolong relaxation time (τ) at low temperature, which lead to S decreasing. The magnetic viscosity of NdFeB/CeFeB thin film is as similar as that of the CeFeB monolayer thin film, and both are much smaller than that of the NdFeB film. It is shown that the dual-hard magnetic layer structure can effectively reduce the viscosity coefficient and improve the time stability of the NdFeB/CeFeB thin film. Furthermore, the temperature dependence of the initial decay rates (dM/dt) from 0 s to 10 s was discussed. The initial magnetic decay of the film demonstrates a similar temperature behavior as the magnetic viscosity coefficient S.

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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
Accepted: 14 December 2017

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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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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
Accepted: 29 January 2018

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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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Influences of Target Peak Current Density on the Microstructure and Mechanical Properties of TiN Films Deposited by Dual Pulsed Power Magnetron Sputtering
Huiying SHI, Chao YANG, Bailing JIANG, Bei HUANG, Di WANG
Acta Metall Sin    2018, 54 (6): 927-934.   doi:10.11900/0412.1961.2017.00406
Accepted: 09 April 2018

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The low kinetic energy and low ionization rate of deposited particle of traditional magnetron sputtering led to low density and poor adhesion of TiN film. The peak current density between cathodic target and anodic chamber was increased several times through the adoption of pulsed power supply mode with low duty cycle, which further enhanced kinetic energy and ionization rate of deposited particle. But the average deposition rate of thin film was significantly reduced. Therefore, a design concept of dual pulsed electric field mode was proposed, which allowed to adjust duration time and target peak current density of the dual pulses. It not only enhanced kinetic energy and ionization rate of deposited particle to satisfy the demand of fabrication of high performance film, but also increased the duration time of pulse to achieve high average deposition rate. In the manuscript, TiN films were deposited by dual pulsed power magnetron sputtering with different target peak current densities of the second pulse stage. The microstructure and mechanical properties of TiN films were characterized using XRD, SEM, nanoindentation and microscratch test. It was found that the TiN film deposited under target peak current density of 0.87 A/cm2 exhibited finely dense microstructure with average grain size of 17 nm. Additionally, the hardness and film-substrate adhesion of such film were as high as 29.5 GPa and 30.0 N, respectively.

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Effect of Thickness on the Surface and Electronic Properties of Bi Film
Caihong DONG, Yongli LIU, Yang QI
Acta Metall Sin    2018, 54 (6): 935-942.   doi:10.11900/0412.1961.2017.00422
Accepted: 01 February 2018

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Bismuth and its alloys exhibit a number of peculiarities and mysterious features due to its three-dimensional (3D) hexagonal crystal, and have attracted the interest of many researchers for many years. Currently, the trivial-to-topological and semimetal-semiconductor transitions have been focused, as the result of its semi-metallic and large spin-orbit coupling. The binary compounds of Bi2M3 and binary alloys BixM1-x (M=Se, Sb and Te) are found to be 3D topological insulators, as the result of small band gap and large spin-orbit coupling in Bi crystals and Bi compounds, which make these crystals topologically important. In the case of Bi films, strong spin-orbit (SO) coupling interaction is also a fundamental mechanism to induce the Z2 topology. Recently, ultrathin Bi films have also been theoretically predicted to be an elemental two-dimensional topological insulator. And, all the ultrathin Bi(111) films are characterized by a nontrivial Z2 number independent of the film thickness. In the past few years, ultrathin films of Bi with a thickness down to several BLs (bilayers) on Si substrate have been prepared in experiments, finding that thicknesses have an effect on the properties of Bi films. However, the effect of thickness on films had not be studied for microscopic mechanism experimentally in detail. In this work, the effects of thickness on the surface and electronic properties of (00Ɩ) and (012) oriented films of Bi using the first-principles method were studied. With the increase of thickness, (00Ɩ) oriented Bi films became more stable, and the film of the even-numbered layers was more stable than that of the odd-numbered layer. However, the (012) oriented Bi films presented totally different behavior comparing with the (00Ɩ) oriented Bi film. The stabilities of (012) oriented film became less stable as the thickness increased, and possessed the approximated surface energy of even-numbered layers (00Ɩ) oriented Bi films when their layer numbers were closed to four. Further analysis of the cohesive energy, geometry structure and electronic band structures showed that, all the thin films presented the transition from semi-conductors to semi-metal or metal as the thickness increases.

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Cracking Behavior of Sol-Gel Derived YAlO3/Ti2AlC Composite Coatings During Drying and Pyrolysis
Chao FAN, Qing JIA, Yuyou CUI, Rui YANG
Acta Metall Sin    2018, 54 (7): 991-998.   doi:10.11900/0412.1961.2017.00325
Accepted: 01 December 2017

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Sol-gel derived YAlO3/MAX composite coatings were designed as protective coatings for γ-TiAl base intermetallic compounds which exhibit insufficient oxidation resistance at temperatures above 800 ℃. However, at present, it's still a big challenge to achieve crack-free surfaces while preparing YAlO3/MAX composite coatings via sol-gel processing, especially during drying and low temperature heat treatment. Hence, cracking behavior of YAlO3/Ti2AlC composite coatings, which were derived from nanoparticles-gel system, was studied in this work by means of in situ techniques such as high-temperature optical microscopy (HTOM). According to this work, cracking of YAlO3/Ti2AlC composite coatings during drying and pyrolysis mainly occurred in stage 3, i.e., the pyrolysis stage of slurry, in which the maximum stress that coating system can tolerate decreased gradually as a result of pyrolysis of the gel network and was eventually exceeded by the increasing internal stresses generated owing to heating and volume change of coating system. Coating thickness, which varied in the plane of coatings and was affected by the difference of drying rate during stage 1, was a critical factor that determined the positions where cracks may be initiated. It was observed that cracks were more easily formed on those sites with thicker coatings, where often produced great stress concentration. Both crack width and spacing can be decreased by applying fast heating rate, since large-scale non-homogeneous distribution of internal stress concentration in coatings was reduced in this way and cracking behavior of coatings was consequently confined into very small region. In this work, a heating rate of 5 ℃/min was the best choice to obtain YAlO3/Ti2AlC composite coatings with acceptable surface quality.

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Preparation and Properties of Biological Functional Magnesium Coating on Ti6Al4V Substrate
Xiaoming YU, Lili TAN, Zongyuan LIU, Ke YANG, Zhonglin ZHU, Yangde LI
Acta Metall Sin    2018, 54 (6): 943-949.   doi:10.11900/0412.1961.2017.00285
Accepted: 09 November 2017

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Currently, metallic biomaterials used in orthopedics are normally bioinert which is hard to integrate with the bone tissue inducing aseptic loosening and easy to get infection, which is the main reason of implantation failure. Mg base metals are considered to be a new generation of revolutionary metallic biomaterials due to its similar density and mechanical properties with natural bone, good biocompatibility, degradability in the body as well as the biological functional ability to promote new bone tissue formation. In addition, the degradation of Mg may increase the local pH which can inhibit the growth of bacteria. In this work, pure Mg coating was deposited on Ti6Al4V substrate by arc ion plating. The effects of different working pressures on the surface quality and properties of Mg coating were investigated. The degradation, antibacterial and biosafety properties were studyied. The results showed that the pure Mg coating can be deposited on the surface of Ti6Al4V substrate and the coating was uniform and smooth. The immersion test in vitro showed that the degradation was very fast because of galvanic corrosion, and the whole process was finished in about one week. The results of antimicrobial experiments showed that the Mg coating can kill staphylococcus aureus and showed good antibacterial function. The results of cytotoxicity test showed that Mg coating promoted rabbit bone marrow mesenchymal stem cells (rBMSCs) growth and proliferation.

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