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EFFECTS OF SECOND PHASES ON MICROARC OXIDATION PROCESS OF MAGNESIUM BASE MATERIALS
Yanqiu WANG,Kun WU,Fuhui WANG
Acta Metall Sin    2016, 52 (6): 689-697.   doi:10.11900/0412.1961.2015.00500
Accepted: 06 April 2016

Abstract62)   HTML1)    PDF (939KB)(484)      

The effects of second phases on microarc oxidation (MAO, also named plasma electrolytic oxidation-PEO) behavior of Mg base materials were investigated and the related mechanism was discussed. The formation of barrier layer and its influence on sparking discharge behavior were characterized and analyzed on the base of systematic selecting and designing substrate materials. The variation of second phases at the early MAO stage was observed and analyzed by SEM and EDS, and then the effect mechanism of second phases on MAO behaviors was revealed. Voltage evolution trend during MAO were recorded to study the formation state of the barrier layer on the different Mg base materials. According to the growth mechanism of MAO film, the film growth process can be simplistically considered as a repeated breakdown and reconstruction process of a capacitor. Accordingly, the growth process of MAO film on multiphase metal materials and the effects of second phases were discussed. The results show that different second phases in substrate materials have different effects on formation process of MAO films, depending on their own characteristics. For the second phases which have the characteristics of valve metals, although selective sparking discharge occurs at the early stage of MAO, the second phases will not hinder the growth of MAO film since barrier layer can form on the second phases, and they will not induce structural defects into the film-substrate interface. If the second phases have not the characteristics of valve metals, their conductivity property will be an important influencing factor to affect the MAO behaviors. For the elecinsulating second phases which have not the characteristics of valve metals, sparking discharge just occurs on Mg matrix in the substrate, while doesn't occur on the second phases; the second phases exist in the MAO film as heterogeneous phases, do not react in MAO process, and will not hinder the growth of MAO film. For the semi-conductive second phases which have not the characteristics of valve metals, they delay the growth of MAO film because they destroy the integrity of barrier layer. For the electroconductive second phases which have not the characteristics of valve metals, they seriously hinder the growth of MAO film.

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ADVANCES IN SiC FIBER REINFORCED TITANIUM MATRIX COMPOSITES
Yumin WANG, Guoxing ZHANG, Xu ZHANG, Qing YANG, Lina YANG, Rui YANG
Acta Metall Sin    2016, 52 (10): 1153-1170.   doi:10.11900/0412.1961.2016.00347
Accepted: 29 August 2016

Abstract114)   HTML16)         

SiC fibers can be used to reinforce a range of titanium base materials including alloys of the (α+β) type, metastable β type and near α type, as well intermetallic based γ-TiAl and orthorhombic Ti2AlNb. Along the fiber directions the obtained composites possess exceptional strength and stiffness, creating a large room and great flexibility for the design of higher performance components to be used in both aero engine and aircraft. The composites can be used by itself such as in sheet and bar form, or as a reinforcing module embedded in titanium alloy components, e.g., as a ring at the rim of a compressor disk. In this paper, recent progress in the development and application of SiC fiber reinforced titanium matrix composites was reviewed, emphasizing the work conducted at the Institute of Metal Research, Chinese Academy of Sciences. Five aspects of research were covered, the first is fiber manufacture and batch production, in which the influence of the chemical vapor deposition parameters on the quality of the W-core SiC fiber was discussed, and the relationship between the tungsten-SiC interface reaction and the high temperature stability of the fiber was described. The second part covers the composite interface, in which a detailed discussion was given to both the chemical and physical compatibility, followed by the design of different reaction layers between the SiC fiber and different titanium based matrixs. The mechanical property section presents tensile data of a range of composites developed in the authors' group and compares to literature reports where available, together with a comprehensive discussion of failure due to fatigue and creep. The fourth part deals with nondestructive testing, presenting new results of inspection on real size composite components using a combination of several techniques including X-ray, industrial CT and ultrasonic scanning. The limitations of each method were shown and the technical challenges were identified. The last part describes the development of structural parts and their verification testing. Titanium matrix composite sheets with [0/90] laminate prepared by both the foil-fiber-foil and matrix coated fiber methods were highlighted, followed by a description of the development of full size bladed ring and excess revolution testing. Future directions of research on SiC fiber reinforced titanium matrix composites were also discussed.

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MICROSTRUCTURES AND MECHANICAL PROPERTIES OF TiAl/Ti3Al MULTI-LAYERED COMPOSITE
Zaoyu SHEN,Limin HE,Guanghong HUANG,Rende MU,Jinwang GU,Weizhong LIU
Acta Metall Sin    2016, 52 (12): 1579-1585.   doi:10.11900/0412.1961.2016.00091
Accepted: 20 July 2016

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In recent years, intermetallic compounds have received a lot of considerable attentions for high temperature applications in modern aircraft manufacturers, high temperature engine components, shape memory devices and power generation industry. Among these materials, Ti-Al intermetallic compounds are fascinating materials owing to their low density, high stiffness and good creep properties. However, the structure of the metallic bonding in these intermetallics is the important reason for their insufficient ductility at room temperature. In this work, large-sized TiAl/Ti3Al multi-layered composite thin sheet with uniform chemical composition was prepared by electron beam physical vapor deposition (EB-PVD) technology. The composite and microstructure of multi-layered composite were analyzed by XRD and SEM. The results indicated that the prepared material with visible lamellar structure was composed of α2-Ti3Al and γ-TiAl phases. The densification process of composite was carried out by hot isostatic pressing. The multi-layered material was evaluated with static tensile test before and after hot isostatic pressing. The multi-layered composite after hot isostatic pressing had a higher tensile strength and a good characteristic of tensile elongation. Based on the tensile fracture morphology, the microscopic deformation mechanisms and fracture mechanism were investigated. After hot isostatic pressing, the fracture mechanism transforms to a mixed mode which consists of intergranular fracture and cleavage fracture.

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Effect of Graphite Flake Size on the Strength and Thermal Conductivity of Graphite Flakes/Al Composites
Xiaoyun LIU,Wenguang WANG,Dong WANG,Bolv XIAO,Dingrui NI,Liqing CHEN,Zongyi MA
Acta Metall Sin    2017, 53 (7): 869-878.   doi:10.11900/0412.1961.2017.00015
Accepted: 05 April 2017

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Graphite flakes reinforced Al matrix composites (Gf /Al) with low density, good machining property and high thermal conductivity are considered an excellent heat sink materials used in electronic industry. When the composites are manufactured by liquid method such as liquid infiltration, it is easy to achieve a high thermal conductivity composite. However, the Al4C3 phase would be formed in the composite, which will decrease the corrosion properties of the composites. The powder metallurgy technique could avoid the formation of the Al4C3 phase. In this work, three seized graphite flakes (150, 300, 500 μm) were used to investigate the effect of the graphite flake size on the strength and thermal conductivity of Gf/Al alloy composites. The 50%Gf /Al alloy (volume fraction) composites were fabricated by the powder metallurgy technique. The density of all the three Gf /Al alloy composites were similar to the theoretical density. The graphite flakes had a well bonding with Al alloy matrix without cracks and pores. The (001)Gf basal plane of the graphite flakes were almost parallel to the circular plane (xy plane) of the composites ingot. However, for the small graphite flakes, their (001)Gf basal plane was not well parallel to the xy plane of the composite ingot due to the powder metallurgy process. For the large graphite flakes, they exhibited a good orientation in the xy plane of the composite ingot. The strength of the Gf /Al alloy composites decreased with the increase of the graphite flake size. For the 150 μm graphite flake, the bending strength of the Gf /Al alloy composite was 82 MPa. However, for the 500 μm graphite flake, the bending strength of the composite decreased to 39 MPa. Due to the low strength between the layers of the graphite flake, the cracks were prone to expand in the graphite flake. As the size of the graphite flake increased, this phenomenon became more obviously. It is easy to observe that the graphite flakes peeled off on the fracture surfaces. When the size of the graphite flake increased from 150 μm to 500 μm, the thermal conductivity increased by 63%. The highest thermal conductivity was 604 W/(mK). The interfacial thermal conductance (hc) of the composites were calculated by the Maxwell-Garnett type effective medium approximation model. The hc of 300 and 500 μm graphite flake Gf /Al alloy composites were slightly lower than the theoretical value (calculated by the acoustic mismatch model). However, the hc of the 150 μm graphite flake Gf /Al alloy composite was lower than that of the theoretical value. Besides the size of the graphite flakes, the shape, distribution and defect of the graphite flakes also influenced the thermal conductivity of the composites.

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Effects of MgO Modified HA Nanoparticles on the Microstructure and Properties of Mg-Zn-Zr/m-HA Composites
Haoran ZHENG, Minfang CHEN, Zhen LI, Chen YOU, Debao LIU
Acta Metall Sin    2017, 53 (10): 1364-1376.   doi:10.11900/0412.1961.2017.00249
Accepted: 04 August 2017

Abstract59)   HTML10)    PDF (19180KB)(559)      

Magnesium metal matrix composites (MMCs) are hot research spots in recent years because of their adjustable mechanical and corrosion properties. However, the agglomerate particles in MMCs limit its applications in many areas. In order to solve this problem, MgO surface modified hydroxyapatite ceramic nanorods (m-HA) were prepared and added as reinforcement in this work. Mg-3Zn-0.8Zr alloy (MZZ), Mg-3Zn-0.8Zr composites with unmodified (MZZH) and modified (MZZMH) nanorods were produced by high shear mixing technology. Effects of m-HA nanorods on the microstructure, mechanical properties and corrosion properties of Mg-Zn-Zr/m-HA composites were investigated. The results showed that the addition of HA nanorods refined the grain size of MZZ alloy and gave a raise to the mechanical properties and electrochemical corrosion resistance of MZZ alloy. The grain size of MZZMH was smaller than that of MZZH and the distribution of m-HA nanorods in the matrix was more uniform than that of HA nanorods. Moreover, the as-extruded MZZMH composite exhibited a yield tensile strength of 291 MPa and ultimate tensile strength of 325 MPa, greater than that of MZZH. The corrosion potential of MZZMH was approximately 59 mV greater than that of MZZH. The corrosion rate of MZZMH was 5 mm/a after immersion 7 d in SBF, lower than that of MZZH. The corrosion resistance of MZZMH was better than that of MZZH due to the different corrosion mechanism. Surface corrosion products of MZZMH was alternating Mg(OH)2 and Ca-P compound at the early stage of immersion, but surface corrosion layer of MZZH specimen was always Mg(OH)2. The mechanical properties and corrosion resistance of Mg-Zn-Zr/m-HA composites were improved by the addition of m-HA.

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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
Abstract53)   HTML0)    PDF (2167KB)(351)      

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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Numerical Simulation of Temperature Field and Thermal Stress in ZTAp/HCCI Composites DuringSolidification Process
Xiaoyu CHONG, Guangchi WANG, Jun DU, Yehua JIANG, Jing FENG
Acta Metall Sin    2018, 54 (2): 314-324.   doi:10.11900/0412.1961.2017.00351
Accepted: 19 December 2017

Abstract143)   HTML10)    PDF (6993KB)(680)      

As advanced wear-resistant materials, it is important to promote the process and application of high chromium cast iron (HCCI) matrix composite reinforced by zirconia toughened alumina ceramic particles (ZTAp/HCCI composite). For the purpose of wider applications of this kind of composite, it is urgent to optimize the process parameters of casting process for it. Based on the finite element software the temperature field and thermal stress in ZTAp/HCCI composite during casting process were simulated. The temperature fields of castings are investigated using the uniform initial temperature and the non-uniform initial temperature at the beginning of solidification. It is more appropriate to the actual situation at the end of mold filling process when the initial temperature of solidification is considered as an unstable temperature field. The influence from performs with different honeycomb shapes is considered in the calculations of temperature fields of castings. In this work, the thermo-elastic plastic model was used to accurately describe the thermal stress in the castings with different honeycomb shapes of preforms, and the results indicate that the thermal stress in them decreases with the increase of edge number of holes in preforms. Finally, the hot crack in castings is predicted and the shakeout process is optimized. It is concluded that the simulated results are in good agreement with the experimental results.

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Numerical Simulation Analysis of Continuous Casting Cladding Forming for Cu-Al Composites
Xinhua LIU, Huadong FU, Xingqun HE, Xintong FU, Yanqing JIANG, Jianxin XIE
Acta Metall Sin    2018, 54 (3): 470-484.   doi:10.11900/0412.1961.2017.00460
Accepted: 05 January 2018

Abstract109)   HTML9)    PDF (6468KB)(520)      

High performance Cu-Al composites have widely applied in aviation, aerospace and other fields, at the same time the continuous casting as one of composite forming technologies has been also developed in recent years. Obviously, it is an effective and cheap way to numerically simulate the solidification process of short process continuous casting for manufacturing Cu-Al composites before fabricating them. To meet the need of simulation, in this work, a numerical method for theoretically describing the Cu-Al composite forming in continuous casting processes was proposed. The vertical continuous casting of copper clad aluminum bar billet and the horizontal continuous casting of copper and aluminum composite plate were performed. Based on this method, the steady state temperature fields in solidification processes in the above two kinds of casting technologies were numerically simulated by using proCAST software package. In this work the effects of the theoretical parameters on the steady state temperature fields and then on the performance of Cu-Al composites fabricated by using the above two casting technologies were carefully discussed. It is found that the experimental and simulated results are in good agreement. For the cases of the copper clad aluminum bar billet with a cross section of 100 mm×100 mm, and the copper or aluminum plate with a thickness of 20 mm and a width of 75 mm (coat thicknesses of 4~7 mm), the feasible parameters for producing high performance Cu-Al composites, for examples, are as follows: for the former the temperature of copper liquid is 1250 ℃, the temperature of aluminum liquid is 750 ℃, the length of crystallizer is 200 mm, the length of graphite mandrel tube is 290 mm, the flux of the first cooling water is 1600~2000 L/h, the flux of the second cooling water is 900~1300 L/h, the distance from the second cooling water to the exit of crystallizer is 30 mm, and the withdrawing speed is 60~80 mm/min. For the latter the temperature of copper melt was 1250 ℃, the temperatures of aluminum melt are 760~800 ℃, the withdrawing speed is 40~80 mm/min, and the length of aluminum duct is 20 mm.

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Fabrication and Mechanical Characteristics of Multi-Laminated Aluminum Matrix Composites Reinforcedby Continuous Basalt Fibers
Hao DING, Xiping CUI, Changshou XU, Aibin LI, Lin GENG, Guohua FAN, Junfeng CHEN, Songhe MENG
Acta Metall Sin    2018, 54 (8): 1171-1178.   doi:10.11900/0412.1961.2017.00530
Accepted: 14 June 2018

Abstract85)   HTML0)    PDF (7595KB)(537)      

Continuous basalt fiber (CBF) is a new type of performance outstanding inorganic nonmetallic material. In comparison with carbon fibers, basalt fibers exhibit greater failure strain as well as better impact and fire resistance with less poisonous fumes and 50% cost reduction. It is also known that basalt fibers display higher mechanical properties, better chemical stability and superior thermal and electrical insulation as compared with glass fibers. Basalt fiber has been widely used as a reinforcing composite material for construction industry and for preparation of polymer matrix composites. As high-performance low-cost reinforcements, basalt fibers should have a great potential for strengthening metal matrix composites (MMCs) and reducing their preparation cost. However, so far, few reports focused on the investigation on metal matrix composites reinforced by continuous basalt fibers, especially for lack of feasible fabrication technologies. Thus, in the present work, two-dimensional continuous basalt fiber cloth and Al-12Si alloys foils were selected as raw materials and alternately stacked to obtain a sandwiched structure. Subsequently, vacuum pressure infiltration was utilized to fabricate aluminum matrix composites reinforced by continuous basalt fibers (CBF/Al) with volume fraction of 65% successfully. Influence of infiltration parameters on microstructure evolution of resulting aluminum matrix composites was investigated and formation mechanism of multi-layered structure of CBF/Al composite was clarified. Moreover, mechanical properties of the multi-layered CBF/Al composite were evaluated. The results showed that when the infiltration parameters were 660 ℃, 10 MPa and 10 min, fully dense CBF/Al composite could be achieved and the novel composite displayed a unique multi-layered structure, namely continuous basalt fibers in forms of cruciform crossing distributed within aluminum alloys matrix. It is noteworthy that no obvious chemical reaction happened between continuous basalt fibers and Al-12Si alloys, and sound metallurgical bonding interface between them was obtained due to the interdiffusion of Al and Si elements. Unfortunately, mechanical properties of multi-layered CBF/Al composite did not reach a desired level, which was attributed to (i) decreasing of effective load-carrying capacity due to the imperfect distribution manner of continuous basalt fibers and (ii) deteriorating of intrinsic mechanical properties at high temperature.

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Exploring Plastic Deformation Mechanism of MultilayeredCu/Ti Composites by Using Molecular Dynamics Modeling
Haifeng ZHANG, Haile YAN, Nan JIA, Jianfeng JIN, Xiang ZHAO
Acta Metall Sin    2018, 54 (9): 1333-1342.   doi:10.11900/0412.1961.2018.00009
Accepted: 06 June 2018

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Multilayered metallic composites have attracted great interest because of their excellent characteristics. In recent years, the mechanical behavior of Cu/Ti composites is described in terms of macroscopic or mesoscopic scales, but the micromechanism regarding dislocation slip, twinning and shear banding at heterogeneous interfaces remains unclear. In this work, the molecular dynamics method is used to study the uniaxial tensile and plane strain compression deformation of the Cu/Ti multilayered composites with characteristic initial crystal orientations. The simulation results show that under the tensile load, dislocations are preferentially nucleated at the heterogeneous interface between Cu and Ti, and then slip along {111} plane within the Cu layers. The corresponding mechanism is confined layer slip. With the multiplication of dislocations, dislocations interact with each other, and intrinsic stacking faults and deformation twins are formed in Cu layers. However, no dislocation slip or twinning is activated within the Ti layers at this stage of deformation. As the load increases, the stress concentration at the Cu/Ti interface leads to the fracture of the composites. For the composites under plane strain compression, the stress concentration at the Cu/Ti interface triggers the formation of shear bands in the Ti layer, and there are only very limited dislocations within the shear bands and their adjacent area. With the increase of applied strain, the common action of various deformation mechanisms causes the grains to rotate, and the disorder degree of complex atoms increases. In addition, the micro-plastic deformation mechanism and mechanical properties of Cu/Ti complex with different initial orientations and strain rates are significantly different. The results reveal the microscopic deformation mechanism of the laminated composites containing hcp metals.

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Research on the Microstructure and Properties of In Situ (TiB2-TiB)/Cu Composites
Jianqiang REN, Shuhua LIANG, Yihui JIANG, Xiang DU
Acta Metall Sin    2019, 55 (1): 126-132.   doi:10.11900/0412.1961.2017.00532
Accepted: 08 May 2018

Abstract91)   HTML0)    PDF (5229KB)(486)      

Copper matrix composites have attracted a lot of interest regarding their application as electrical materials. However, the development of copper matrix composites has suffered setbacks because of a trade-off between electrical conductivity and strength. In this work, TiB2 particles and TiB whiskers hybrid reinforced copper matrix composites were in situ fabricated by mechanical alloying and hot pressing. The microstructures of hot-pressed composites were characterized by XRD, OM, SEM and TEM. The mechanism of in situ reaction during hot pressing process and the influence of microstructures on physical properties of hot-pressed composites were analyzed. The Cu and Ti raw powders were firstly reacted at 800 ℃ by forming Cu3Ti transient phase. Then, the Cu-Ti liquid micro-zone was formed at 850 ℃, which is higher than the melting point of Cu3Ti phase. With the increasing of temperature further, TiB2 particles and TiB whiskers were formed in the liquid micro-zone by the diffusion of B atoms from copper matrix. When the reinforcing phase is consisted of mainly TiB whiskers, the hardness of composites is relatively high. But the composites reinforced mainly by TiB2 particles have a higher electrical conductivity. The combined properties of hybrid reinforced copper matrix composites were optimized due to the combination action of TiB2 particles and TiB whisker. For the case of 3%(TiB2-TiB)/Cu composites, the hardness and the electrical conductivity are 86.6 HB and 70.4% IACS, respectively.

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Protection Mechanism Study of Enamel-Based Composite Coatings Under the Simulated Combusting Gas Shock
Cean GUO, Minghui CHEN, Yimin LIAO, Bei SU, Dongbai XIE, Shenglong ZHU, Fuhui WANG
Acta Metall Sin    2018, 54 (12): 1825-1832.   doi:10.11900/0412.1961.2018.00230
Accepted: 06 July 2018

Abstract82)   HTML3)    PDF (4520KB)(327)      

High-temperature-resistant enamel coatings have been reported to be applied in non-critical hot end components of aero-engine and gas turbine recently. Although the enamel with a series of excellent properties can be as high-temperature-resistant coating material under appropriate condition, its lower soft point and inherent brittleness limit their use in broader application under severe service condition. Enamel-based composite coatings (an enamel matrix with the addition of ceramic particles and/or metal platelets) can remarkably increase the properties of the enamel coating and their protection mechanism under dynamic thermal shock needs further investigation. In this work, two kinds of enamel-based composite coatings, 70%enamel+25%Al2O3 and 70%enamel+20%Al2O3+10%NiCrAlY (mass fraction, %) abbreviated to E25A and E20A10M respectively, were designed and fired on K38G superalloy substrate, and their protection mechanism was comparatively studied at 900 ℃ under the simulated combusting gas shock. The thermal shock fire was produced by the mixture gas of C3H8+O2 and its ejecting pressure on the coating surface was 0.4 MPa. After the temperature has been stable at 900 ℃, samples were hold for 15 s and then cooling down in air for 120 s, constituting a thermal shock cycle. Results indicated that, after 150 cyc of thermal shock, both the coatings bond well with the alloy substrate, thus shows high resistance to spallation along interface. For the E25A coating, its microstructure had no obvious change after thermal shock and the surface is still intact. The addition of secondary phase Al2O3 increases the stability of enamel at high temperature. With regard to the E20A10M coating, holes and cracks form consecutively, and peeling off occurs at surface after thermal shock. Interfacial reaction between the NiCrAlY particles and enamel following Cr(NiCrAlY)+ZnO(enamel)→CrO(interface)+Zn↑ results in the formation of enamel swelling, which then, under the synergistic effect of combusting gas shear stress and interface thermal stress, leads to the peeling off of enamel and metal inclusions at surface.

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Recent Progress on the Fabrication of TiAl-Based Composites Sheet by Reaction Annealingof Elemental Foils
Lin GENG, Hao WU, Xiping CUI, Guohua FAN
Acta Metall Sin    2018, 54 (11): 1625-1636.   doi:10.11900/0412.1961.2018.00308
Accepted: 23 July 2018

Abstract79)   HTML0)    PDF (6585KB)(423)      

This paper reviews the current progresses on the fabrication of TiAl-based composites produced by reaction annealing of elemental Ti and Al matrix composite foils. This technique includes deformation and reaction annealing of the multilayer Ti/Al metal matrix composite (MMC) sheet, which prevents traditionally direct deformation of brittle TiAl intermetallic, and TiAl-based composites sheets with good strength-ductility synergy have been produced. The research on microstructure evolution and forming mechanism of the TiAl-based composites sheet during reaction annealing has been summarized, with the focus on the reaction mechanism between Al-MMCs and Ti during reaction annealing, and the method to eliminate Kirkendall voids is proposed. A feasible proposal is provided to fabricate large scale TiAl-based composite sheets.

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High Temperature Deformation Behavior of High Strength and Toughness Ti-Ni Base Bulk Metallic Glass Composites
Yanchun ZHAO, Hao SUN, Chunling LI, Jianlong JIANG, Ruipeng MAO, Shengzhong KOU, Chunyan LI
Acta Metall Sin    2018, 54 (12): 1818-1824.   doi:10.11900/0412.1961.2018.00256
Accepted: 23 July 2018

Abstract81)   HTML1)    PDF (3456KB)(363)      

Room-temperature brittleness and strain-softening during deformation of bulk metallic glasses, and limited processability of shape memory alloys have been stumbling blocks for their advanced functional structural applications. To solve the key scientific problems, a new shape memory bulk metallic glass based composite, through the approach using transformation-induced plasticity (TRIP) effect of shape memory alloys to enhance both ductility and work-hardening capability of metallic glasses, and superplasticity of bulk metallic glass in supercooled liquid region to realize near net forming, was developed in this work. And the Ti-Ni base bulk metallic glass composites (BMGCs) rods were prepared by the levitation suspend melting-water cooled Cu mold process. Microstructure, thermal behavior, mechanical properties and high temperature deformation behavior of the alloy were investigated. The results show that the as-cast alloy microstructure consists of amorphous matrix, undercooled austenite and thermally-induced martensite. Besides, the size of the crystal phase precipitated on the amorphous matrix increases from the surface to the inside. The alloy exhibits excellent comprehensive mechanical properties at room temperature. The yield strength, fracture strength and the plastic strain of alloy are up to 1286 MPa, 2256 MPa and 12.2%, respectively. Under compressive loading in the supercooled liquid region, the composite exhibits approximate Newtonian behavior at lower strain rate in higher deformation temperature, and the optimum deformation temperature is T>480 ℃ and the intersection part with supercooled liquid region (SLR). When the temperature is 560 ℃ and the strain rate is 5×10-4 s-1, the stress sensitivity index m and the energy dissipation rate ψ are 0.81 and 0.895, respectively. Furthermore, the volume of activation is quantified to characterize the rheological behavior.

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Research on Single SiC Fiber Reinforced TC17 CompositesUnder Transverse Tension
Jialin LIU, Yumin WANG, Guoxing ZHANG, Xu ZHANG, Lina YANG, Qing YANG, Rui YANG
Acta Metall Sin    2018, 54 (12): 1809-1817.   doi:10.11900/0412.1961.2018.00124
Accepted: 16 August 2018

Abstract61)   HTML1)    PDF (5177KB)(336)      

Transverse mechanical properties of titanium matrix composites (TMCs) play an important role during its engineering service. Although SiCf / TC17 composite is one of the most promising TMC candidates for aeroengine, as we know its transverse properties have not been reported yet until now. In this work, the transverse strength of single SiC fiber reinforced TC17 composite was evaluated using cruciform specimen. The surface and cross-section of fractured specimen were investigated by SEM to determine the failure position during tensile test. Finite element simulation method was also used to analyze the mechanism of interfacial failure and crack propagation. During the transverse tensile test of single fiber specimen, the initial non-linearity in the stress-strain curve occurred at the stress of (271±12) MPa, which indicated the beginning of fiber-matrix interface failure. SEM observation showed that the crack in the center of sample appeared at the interface of reaction layer and carbon coating with a 24°~68° angle to the applied loading direction and its length extended with the increase of the applied stress. The finite element simulation results based on bilinear cohesive element model showed that transverse fracture of composite interface was shear failure mode, which agreed well with the test results. Before the occurrence of non-linearity in the stress-strain curve, the crack initiated at the circular interface between reaction layer and carbon coating with a 40°~50° angle to the applied loading direction. Crack initiation locations in test samples were different with those in simulation samples, because the actual composite interface was rough and some micro-flaws formed in the interface, whereas it was assumed to be an ideal rigid interface for simulation. Then the crack propagated along both circumferential and axial directions because of the shear stress. With the crack growing, the interface close to 0°angle to the applied loading direction failed first caused by the radial tensile stress, whereas the interface near 90° failed later as a result of circumferential shear stress. After complete failure of the interface, stress redistribution occurred around the SiC fiber and the interface separation increased with the increasing of the applied load, which gave rise to the yielding and deforming of the matrix near fiber until the final fracture of the composite.

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Progress in Interface Modification and Nanoscale Study of Diamond/Cu Composites
Di ZHANG, Mengying YUAN, Zhanqiu TAN, Ding-Bang XIONG, Zhiqiang LI
Acta Metall Sin    2018, 54 (11): 1586-1596.   doi:10.11900/0412.1961.2018.00355
Accepted: 21 August 2018

Abstract95)   HTML2)    PDF (1860KB)(697)      

Due to the superiority in high thermal conductivity, low thermal expansion and good resistance from heat and corrosion, diamond/Cu composites show great prospect in thermal management applications. However, the thermal properties of diamond/Cu composites are impeded by their interface incompatibility. Interface modification is an effective method to enhance interfacial bonding and reduce interfacial thermal resistance. Based on the principles and factors related with interface design, this paper briefly reviewed some hot topics in diamond/Cu composites, including the main research progress, issues remained to be solved and nanoscale interface design with layer thickness lower than 200 nm, and its prospect of the future development.

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Comparative Study of Hot Corrosion Behavior of theEnamel Based Composite Coatings and the ArcIon Plating NiCrAlY on TiAl Alloy
Yimin LIAO, Min FENG, Minghui CHEN, Zhe GENG, Yang LIU, Fuhui WANG, Shenglong ZHU
Acta Metall Sin    2019, 55 (2): 229-237.   doi:10.11900/0412.1961.2018.00293
Accepted: 27 August 2018

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TiAl intermetallic alloys have attracted great attention for its potential application in preparing low pressure turbine blades in aircraft engine. However, its poor oxidation and corrosion resistance becomes a challenge at temperatures above 800 ℃, which leads to the developing of protective coatings. Enamel coating is considered as one of the candidates that match the TiAl alloy well, meanwhile provide corrosion protection. Enamel coating has many advantages such as high thermochemical stability, adjustable thermal expansion coefficient and simple preparation process. This study comparatively investigates hot corrosion behavior of the Ti-45Al-2Mn-2Nb alloy, the traditional NiCrAlY coating and the enamel based composite coating in (75%Na2SO4+25%NaCl, mass fraction) melted salt. Results indicate that after 80 h of hot corrosion, the bare alloy has completely destroyed. For the NiCrAlY coating, it protects the underlying alloy well by forming a protective Al2O3 scale initially. However, serious interdiffusion between coating and substrate results in the degeneration of the coating as well as the scale. At the same time, the basic dissolution of Al2O3 film accelerates corrosion. So obvious spallation takes place after 60 h corrosion. The enamel based composite coating shows excellent thermal stability and low corrosion rate. During the whole hot corrosion test, it still retains its original blue glazing color and luster. Furthermore, the enamel coating suppresses the inward diffusion of oxygen and corrosive ions into the alloy substrate, and thus, it protects the TiAl alloy well from corrosion of the molten (75%Na2SO4+25%NaCl, mass fraction) salt.

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