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

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Wear Behavior of Fe-WC/Metal Double Layer Coatings Fabricated by Resistance Seam Weld Method
Wenqin WANG, Zhaoman WANG, Yulong LI, De WANG, Miao LI, Qing CHEN
Acta Metall Sin    2019, 55 (4): 537-546.   DOI: 10.11900/0412.1961.2018.00271
Abstract   HTML PDF (32341KB)  

Fe-WC/metal double layer coatings containing Fe-C-Si super hard alloy (SHA) particles and tungsten carbide (WC) particles were fabricated on Al7075 substrates by resistance seam welding method to improve the wear resistance of aluminum alloys. The micro-structure and phase compositions of the Fe-WC/metal double layer coatings with different WC particle sizes (fine and coarse) were investigated by SEM and EPMA. Nano-hardness of different phases in the coatings were investigated by nano-indentation test. Finally, the friction behavior of the two kinds of Fe-WC /metal double layer coatings were contrasted by ball-on-disc test using WC and SUS 304 balls. The results show that the thicknesses of Fe-WC composite/metal double layer coatings were about 600 μm. The microstructure of the coatings was: WC/Fe composite (wear resistance layer)+Fe/Al composite (metal interlayer)+Al7075 substrate. When WC ball was used as the static counterpart, the wear mechanism of the coatings with fine and coarse WC particles were severe abrasive wear and brittle fracture with little abrasive wear, respectively. When SUS304 was used as the static counterpart, the coating with fine WC powder was demonstrated difficulty to be abraded due to the protection of the iron oxide adhesive layer, and the other proved a little brittle fracture. Moreover, the wear rate of both coatings using SUS304 ball was lower than that of WC ball in the ball-on-disc test.

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Defect Induced Fatigue Behaviors of Selective Laser Melted Ti-6Al-4V via Synchrotron Radiation X-Ray Tomography
Zhengkai WU, Shengchuan WU, Jie ZHANG, Zhe SONG, Yanan HU, Guozheng KANG, Haiou ZHANG
Acta Metall Sin    2019, 55 (7): 811-820.   DOI: 10.11900/0412.1961.2018.00408
Abstract   HTML PDF (14834KB)  

As a very promising additive manufacturing (AM) technique, selective laser melting (SLM) has gained considerable attentions due to the feasibility of producing light-weight metallic components directly from virtual design data. On the other hand, high strength, low density and high corrosion resistance Ti-6Al-4V alloy has been a preferred AM used material for the aviation and military industries. However, the fatigue damage behaviors of SLMed or AMed components usually suffer from interior defects such as incomplete fusion and gas pores due to unstable process or unsuitable processing parameters. Therefore, thorough investigations on process-induced and metallurgical defects and its influence on the fatigue behavior is required for robust designs and engineering applications of high performance SLM components. As an advanced characterization approach, synchrotron radiation micro computed X-ray tomography (SR-μCT) has been recently to investigate the fatigue damage behaviors of critical components with defects. Based on self-developed in situ fatigue testing rig fully compatible with the BL13W1 at Shanghai Synchrotron Radiation Facility (SSRF), several AMed specimens were prepared for in situ fatigue SR-μCT. The Feret diameter and extreme values statistics were then adopted to characterize the defect size, morphology, population, location and the influence on fatigue life. Fatigue fractography was also examined to further identify the defect to really initiate a fatigue crack. Results show that two types of defects including gas pores and the lack of fusion can be clearly distinguished inside SLM Ti-6Al-4V alloys. Fatigue crack with a typical semi-ellipse usually initiates from the defects at the surface and near the surface. Besides, the defects less than 50 μm and sphericity of 0.4~0.65 dominate for the SLM Ti-6Al-4V alloys. It is also found that the larger the characteristic size of the defect, the lower the fatigue life. Current results can provide a theoretical basis and support to predict the fatigue performance of SLM Ti-6Al-4V alloys. Further investigations should be performed on the relationship between the critical defect and fatigue strength by introducing the Kitagawa-Takahashi diagram.

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Effect of Solution Temperature on Tensile Deformation Behavior of Mn-N Bearing Duplex Stainless Steel
Miao JIN, Wenquan LI, Shuo HAO, Ruixue MEI, Na LI, Lei CHEN
Acta Metall Sin    2019, 55 (4): 436-444.   DOI: 10.11900/0412.1961.2018.00276
Abstract   HTML PDF (19176KB)  

Advanced duplex stainless steels (DSSs) in which Ni is mostly or completely replaced by Mn and N have been developed in recent years. Such Mn-N bearing DSSs can readily achieve exceptional room-temperature tensile properties through the transformation-induced plasticity (TRIP) effect of metastable austenite. During the processing of DSSs, solution treatment is a critical step that tailors the phase fraction and the overall properties. In particular, the phase chemistry can change due to different element partitioning between two constituents, resulting in a different TRIP kinetics, when DSS is solution treated at different temperature. In this work, the effect of solution temperature on tensile deformation behavior of a new Mn-N bearing DSS was studied. The mechanical properties and work-hardening characteristic of the steels solution treated at different solution temperature (1000~1200 ℃) were investigated by thermal modeling test, and the effects of solution temperature on the deformation substructure and fracture characteristics were analyzed by OM, SEM and EBSD. The results show that as the solution temperature increases, the yield strength and tensile strength of the steels decrease, while the elongation (uniform elongation and total elongation) increases firstly and then decreases. The steel solution treated at 1100 ℃ shows the optimum uniform elongation of 46.7%, and a better combination of ultimate tensile strength and ductility of approximately 44.6 GPa·%. The work-hardening rate of the steel shows a three-stage characteristic, namely it declines firstly and then increases and subsequently declines again as the strain increases. However, the increasing extent of the work-hardening rate decreases as the solution temperature increases. The strain-induced martensitic transformation (SIMT) of metastable austenite which causes the TRIP effect has two evolution mechanisms of γεα' and γα'. But SIMT can be suppressed when the solution temperature increases. The fracture surfaces of specimens solution treated at different temperatures show a quasi-cleavage mode, in which both ferrite and strain-induced martensite exhibit cleavage fracture while the residual austenite displays a dimple-mode fracture. Furthermore, the Md30 which can characterize the stability of metastable austenite was calculated, which decreases from 81 ℃ to 38 ℃ as the solution temperature increases from 1000 ℃ to 1200 ℃, indicating that the TRIP effect gets weakening at a higher solution temperature, and the work-hardening and plasticity therefore decrease.

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Effect of Cross Rolling Cycle on the Deformed and Recrystallized Gradient in High-Purity Tantalum Plate
Jialin ZHU,Shifeng LIU,Yu CAO,Yahui LIU,Chao DENG,Qing LIU
Acta Metall Sin    2019, 55 (8): 1019-1033.   DOI: 10.11900/0412.1961.2018.00470
Abstract   HTML PDF (28078KB)  

Cross rolling plays an important role in the production of high-quality tantalum (Ta) sputtering targets, which are crucial in achieving thin films for micro-electronic components. However, the effect of the cross rolling cycle on the microstructure homogeneity is always ignored. Therefore, 1 and 2 cycle samples were obtained by a new approach named a 135° cross rolling. The deformation and recrystallization behavior of high-purity Ta plate then was systematically compared between 1 and 2 cross rolling cycles, aiming to elucidate why the increase of cross rolling cycles can effectively ameliorate the microstructure gradient along the thickness direction. XRD results showed that the 2 cycle sample through the thickness consisted of a relatively homogenous {111}<uvw> ([111]//normal direction (ND)) and {100}<uvw> ([100]//ND) fibers while texture distribution was extremely uneven for the 1 cycle sample. The stored energy was quantitatively analyzed by X-ray line profile analysis (XLPA) and it was found that the stored energy across the thickness distributed more homogeneously for the 2 cycle sample. Misorientation characteristics of deformed grains with different rolling cycles were analyzed in detail by visualizing the misorientation angle based on an electron backscatter diffraction dataset. Many well-defined microbands and microshear bands occurred in the {111} grain at the center layer for the 1 cycle sample, while it can be effectively destroyed with the increase of the cross rolling cycle and few peaks occurred in the "point to point" plot. Kernel average misorientation (KAM) and grain reference orientation deviation-hyper (GROD-Hyper) further confirmed their differences. Then, micorband and microshear bands were detailedly characterized by TEM, and the analysis based on relative Schmid factor suggested that the primary slip system activated in the {111} grains led to the formation of microbands in the 1 cycle sample, while multiple slip systems appeared to be activated in the 2 cycle sample and deformation was more uniform. Upon annealing, the remarkably reduced stored energy gap between the {111} and {100} grain as well as the relatively homogeneous deformation microstructure between the surface and center layer for the 2 cycle sample was conductive to synchronous recrystallization together, while the high stored energy as driving force and preferential nucleation sites at the center region led to faster recrystallization for the 1 cycle sample. The recrystallization microstructure was relatively uniform and smaller variation in grain size for the 2 cycle sample through the thickness, which was beneficial to the application of Ta sputtering target. Therefore, the increase of cross rolling cycle can ameliorate the recrystallized kinetics and microstructure of high purity Ta plate.

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Oxidation Behavior of GH984G Alloy in Steam at 700
Changshuai WANG,Lili GUO,Liying TANG,Rongcan ZHOU,Jianting GUO,Lanzhang ZHOU
Acta Metall Sin    2019, 55 (7): 893-901.   DOI: 10.11900/0412.1961.2018.00440
Abstract   HTML PDF (18649KB)  

To produce abundant and cheap electricity in a cleaner way, the next generation of advanced ultra-supercritical (A-USC) coal-fired power plants with higher thermal efficiency will operate at service temperatures at 700 ℃ and steam pressures up to 35 MPa. However, the temperature capacity of the currently used ferritic or austenitic steels in USC plants at 600 ℃ cannot meet the requirements. GH984G is a newly developed Ni-Fe-Cr base alloy designed for A-USC, but its oxidation behavior in steam at 700 ℃ is unclear. In this work, the oxidation kinetics of GH984G alloy in steam at 700 ℃ was investigated by weighting specimens at intervals. Morphology, composition and phase constituent of the steam oxide scale were characterized using SEM, EDS and XRD. The results show that the oxidation of GH984G alloy follows a parabolic law with a rate constant of 0.00521 mg/(cm2·h1/2) and steady weight gain rate of 8×10-4 g/(m2·h). The oxide scale mainly consists of Cr2O3 and Al2O3. Meanwhile, a small amount of TiO2 was observed. The oxide scale of Cr2O3 forms on the alloy surface and then the internal oxidation of Al to be Al2O3 occurs along the grain boundaries of the matrix alloy and TiO2 forms at the surface of the external oxide scale. The morphology of Cr2O3 at the surface of the oxide scale is needle-like at the initial oxidation stage and then the agglomeration of Cr2O3 was observed and the cellular shape forms. Finally, the coalescence of the cellular Cr2O3 appears and the flat surface of the external oxide scale forms. The excellent oxidation resistant of GH984G alloy in steam at 700 ℃ can be attributed to the compact external oxide scale of Cr2O3 and the root-like internal oxide scale of Al2O3.

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Effect of Mo Element and Heat Treatment on Corrosion Resistance of Ni2CrFeMox High-Entropy Alloyin NaCl Solution
Lin WEI,Zhijun WANG,Qingfeng WU,Xuliang SHANG,Junjie LI,Jincheng WANG
Acta Metall Sin    2019, 55 (7): 840-848.   DOI: 10.11900/0412.1961.2018.00558
Abstract   HTML PDF (15275KB)  

As a new alloy design concept, the high-entropy alloy (HEA) and the formation of simple solid solution introduce excellent properties such as high hardness, high strength and corrosion resistance. Investigations have shown that the single solid solution CrCoFeNi alloy possesses good corrosion resistance. The addition of Mo is beneficial to the corrosion resistance of the HEAs for potential industrial applications in 3.5%NaCl (mass fraction) simulating seawater type environments. The major effect of Mo is to promote the pitting potential of the alloy and inhibit the dissolution of the passivation film by forming and retaining molybdenum oxyhydroxide or molybdates (MoO42-). Considering that the cost of pure Co is higher, Ni and Co elements have similar atomic size and valence electron concentration, and the corrosion resistance of pure Ni is higher than that of pure Co, Ni2CrFeMox HEA was designed by replacing Co element with Ni element in CoCrFeNiMox HEA. As the Mo content increases in the Ni2CrFeMox HEAs, the interdendrite is a Cr and Mo rich σ phase, and the dendrite is a Cr and Mo depleted fcc phase. The potential difference between interdendrites and dendrites leads to galvanic corrosion, which accelerates the localized corrosion of alloys. Here, a solution heat treatment process is selected to reduce the precipitation phase and improve the corrosion resistance of the alloy. The effects of Mo element and heat treatment on the corrosion resistance of Ni2CrFeMox HEA in 3.5%NaCl solution were tested. The results show that the corrosion resistance of as-cast Ni2CrFeMox HEA is obviously higher than that of 316L stainless steel. The Ni2CrFeMo0.2 alloy has the best corrosion resistance because of its minimum dimensional passive current density and corrosion current density. However, the addition of excessive Mo leads to the precipitation of σ phase and galvanic corrosion, which reduces the corrosion resistance of the alloy. After solution treatment, the uniformity of alloy structure and element distribution weakens galvanic corrosion, and the corrosion resistance is obviously improved.

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Effect of Nd Content on the Structure and Magnetic Properties of Si(111)/Cr/Nd-Co/Cr Thin Films Prepared by Magnetron Sputtering
Xianmei HE, Liuniu TONG, Cheng GAO, Yichao WANG
Acta Metall Sin    2019, 55 (10): 1349-1358.   DOI: 10.11900/0412.1961.2018.00490
Abstract   HTML PDF (19580KB)  

A series of Si(111)/Cr(10 nm)/NdCox(400 nm)/Cr(10 nm) thin films with the atomic ratio of Co/Nd (x) varying from 2.5 to 7.2 were prepared by radio frequency magnetron sputtering. The influence of Nd content on the structure and magnetic properties of the as-prepared and post annealed films were investigated by XRD, SEM, VSM and AFM/MFM. Magnetic measurements at room temperature show that the compositional variation of the perpendicular anisotropy energy (Ku) exhibits a broad peak around x=5.2 with maximum of Ku=(80±5) kJ/m3 for the as-prepared Nd-Co amorphous films. MFM characterization shows that the root mean square deviation of phase shift in MFM images (Δ?rms) also have a compositional dependence which is similar to that of Ku-x. The experimental results show that the stress induced magneto-elastic anisotropy is the primary origin of the perpendicular magnetic anisotropy (PMA) in the as-prepared Nd-Co amorphous film. After rapid thermal annealing (RTA) process in a vacuum atmosphere at 600 °C, intermetallic compounds such as Nd2Co17, Nd4Co3 and NdCo2 are precipitated in all the studdied films, while NdCox nanocrystals accompanied by the precipitation of Nd2Co7 symbiotic phase were observed only in the films with x=2.5 and 3.8 (the atomic fraction of Nd excesses at least 4%). The in-plane coercivity of the films with x=2.5 and 3.8 was significantly enhanced (Hc-in=54, 51 kA/m) due to the precipitation of NdCox and Nd2Co7 nanocrystals, while that of the samples with x>4.4 remained low value (Hc-in=4~8 kA/m).

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Research Progress in Irradiation Damage Behavior of Tungsten and Its Alloys for Nuclear Fusion Reactor
Yucheng WU
Acta Metall Sin    2019, 55 (8): 939-950.   DOI: 10.11900/0412.1961.2018.00405
Abstract   HTML PDF (10703KB)  

Controlled thermonuclear fusion energy, regarded as the ultimate and ideal energy source, is considered as the principle way to effectively solve the future energy problem because of its cleaning and abundant raw materials. In the actual fusion reaction process, plasma facing materials (PFMs) will have to face the extremely harsh and severe environment. W and its alloys are the most promising PFMs candidate materials for the present reference design. However, due to its low-temperature brittleness, recrystallization brittleness, radiation-reduced brittleness and other disadvantages, they are still far from all the requirements of PFMs. In this paper, the principles of damage behavior under different irradiation particles were described in detail, and the research progress in related fields in recent years was also reviewed, in order to provide references for the research on the irradiation of W-based materials in the future.

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Influence of Two-Step Bainite Transformation on Toughness in Medium-Carbon Micro/Nano-Structured Steel
WAN Xiangliang, HU Feng, CHENG Lin, HUANG Gang, ZHANG Guohong, WU Kaiming
Acta Metall Sin    2019, 55 (12): 1503-1511.   DOI: 10.11900/0412.1961.2019.00065
Abstract   HTML PDF (13850KB)  

Micro/nano-structured bainitic steel provides a unique combination of ultra-high strength and high ductility due to their structure consisting of micro/nano-scale bainitic-ferrite and retained austenite, but the toughness is a little bit low. The retained austenite plays a leading role for the toughness, and it can significantly increase the toughness of micro/nano-structured bainitic steel by refining the size of blocky retained austenite and improving the content of film retained austenite. Simultaneously, the structure of retained austenite affects the stability of retained austenite, and even can change the micro-deformation and determine the toughness. This work has been refined retained austenite of medium-carbon bainitic steel by using two-step bainitic transformation to study phase transformation of retained austenite through heat treatment. The effect of retained austenite on the impact toughness in medium-carbon micro/nano-structured steels was analyzed by one (300 ℃ for 6 h) and two-step (300 ℃ for 2 h, then 250 ℃ for 24 h) bainitic transformation processes. The microstructure, phase fraction, misorientation, crystallographic grain size and impact energy of different heat treatment steels were observed, detected and analyzed. The results showed that the impact property of two-step bainitic transformation was significantly higher than that of one-step bainitic transformation in medium-carbon steel, which the impact energy in -40 ℃ increased from 31 J to 42 J. The main reason is the new bainitic ferrite was formed in two-step bainitic transformation, the untransformed retained austenite was divided and refined by new bainitic-ferrite, reducing the formation of massive martensite during water quenching after isothermal bainite process. It significantly improve the toughness of the steel because the fracture energy was increased, owing to making crack bifurcation and even preventing the propagation of cracks in the impact process. Through the above-mentioned studies, this research not only precisely refines the retained austenite structure, reveals the effect of retained austenite stability on deformation mechanism and resolves toughness mechanism, but also provides the theoretical guidance for the production of micro/nano-structured bainitic steels in combination with good toughness.

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Surface Characteristics and Stochastic Model of Corroded Structural Steel Under General Atmospheric Environment
WANG Youde,XU Shanhua,LI Han,ZHANG Haijiang
Acta Metall Sin    2020, 56 (2): 148-160.   DOI: 10.11900/0412.1961.2019.00156
Abstract   HTML PDF (18377KB)  

Steel structures exposed to corrosive atmospheres for a long time are highly susceptible to corrosion damage. The safety assessments of existing corroded steel structures rely heavily on the quantification of corrosion itself. In order to study the corrosion characteristics of structural steel in general atmospheric environment, 6 batches of artificial accelerated corrosion experiments and 8 a of natural exposure experiments were carried out. The surface characteristic parameters and evolution rules of corroded structural steel were studied by the surface morphology tests and self-programmed morphology analysis program. The distribution characteristics of corrosion depth, pit depth and aspect ratio were clarified, and the changing laws of statistical parameters (such as mean value and standard deviation) and pitting shapes were revealed. The results indicated that the corrosion depth of structural steel in general atmospheric environment obeyed the normal distribution, and the pit depth and aspect ratio obeyed the lognormal distribution. With the increase of corrosion degree, the mean value and standard deviation of corrosion depth, the peak value of power spectrum density of corrosion depth, and the logarithmic mean value of pit depth gradually increased, and the logarithmic mean value of pit aspect ratio decreased. Meanwhile, the shape of pits was gradually changed from a cylinder or hemisphere to a cone. Finally, based on the statistical analysis results of corrosion depth parameters and pit parameters, and taking the variation laws and internal relationships of characterization parameters into consideration, the stochastic field model of corrosion depth and the random distribution model of corrosion pits were established, which achieved the accurate simulation and reconstruction of surface characteristics of corroded steel under general atmospheric environment.

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Formation of Sliver Defects in Single Crystal Castings of Superalloys
Acta Metall Sin    DOI: 10.11900/0412.1961.2019.00287
Development of Numerical Simulation in Nickel-Based Superalloy Turbine Blade Directional Solidification
XU Qingyan,YANG Cong,YAN Xuewei,LIU Baicheng
Acta Metall Sin    2019, 55 (9): 1175-1184.   DOI: 10.11900/0412.1961.2019.00126
Abstract   HTML PDF (16856KB)  

Ni-based superalloy turbine blades have been widely used in aerospace and industrial engine. Numerical simulation techniques can optimize the superalloy directional solidification process and enhance the rate of finished products. This paper summarized the existing macroscopic and microscopic numerical models in the superalloy blade directional solidification process. Simulations have been done on the temperature field evolution, grain structure and dendrite morphology in typical HRS and LMC directional solidification conditions, and the resulting microstructure features were investigated. In particular, the application of varying withdrawal rate in directional solidification of the superalloy blade was introduced. And the advantages of the varying withdrawal rate technique were emphasized by comparing it with the constant withdrawal rate method. The simulation results indicate that by applying varying withdrawal rate, the convex or concave shape of the mushy zone can be change to flat shape, so that parallel columnar grains can be obtained with enhanced high-temperature performance of the turbine blade.

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Microstructure and High-Temperature Deformation Behavior of Dissimilar Superalloy Welded Joint of DD407/IN718
LIU Yang,WANG Lei,SONG Xiu,LIANG Taosha
Acta Metall Sin    2019, 55 (9): 1221-1230.   DOI: 10.11900/0412.1961.2019.00097
Abstract   HTML PDF (30541KB)  

Welding is an important joining method to fabricate the dissimilar welding integral blisk structure of single crystal and polycrystalline superalloy. The microstructure and properties of the welded joint are the key factors to determine the reliability of the integral blisk structure of dissimilar superalloys. The single crystal superalloy of DD407 and polycrystalline superalloy of IN718 were butt welded by continuous fiber laser system. The evolution of microstructure and composition segregation of the welded joints fabricated under the optimized welding parameters as-welded (AW) and after post weld heat treatment (PWHT) were investigated. The high temperature tensile deformation behavior of the welded joint after PWHT was also examined. The results show that the microstructures of fusion zone (FZ) in the welded joint consist of planar crystal, cellular crystal, columnar crystal and equiax crystal. The difference of the dendrite microstructures between the two sides of the weld centerline is very obvious. In terms of the joint as-welded, the microhardness of the FZ is low and there exists obvious micro-segregation. After PWHT, the micro-segregation has been improved and the microhardness increases significantly in the FZ which is much more than those of both base metals (BMs) of DD407 and IN718 alloys. There exists local hardening zone in the heat-affected zone (HAZ) of DD407 single crystal alloy and narrow softening zone and grain boundary liquation phenomenon in the HAZ of IN718 polycrystalline alloy. The ultimate tensile strength and elongation of the welded joint after tensile test at 650 ℃ are 1111 MPa and 9.42%, respectively. And the tensile specimen of the welded joint fails in the BM of IN718 polycrystalline alloy. The main deformation mode of the laser welded joint at high temperature includes the multi-slips of dislocation in the BM and FZ of single crystal alloy, and the dislocation slip and grain-boundary sliding in the BM of polycrystalline alloy. The tensile fracture surface is characterized by multi-source cracking, and the dimple and crystal sugar shaped facture surface exist simultaneously in the crack source area, which is a mixed fracture of microvoid aggregation and intergranular fracture. So the tensile fracture mechanism contains micro-void accumulation fracture and inter-granular fracture. The grain boundary liquefaction in the HAZ of IN718 polycrystalline does not affect the short-time high temperature mechanical properties of the welded joints.

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Research Progress in Powder Metallurgy Superalloys and Manufacturing Technologies for Aero-Engine Application
ZHANG Guoqing,ZHANG Yiwen,ZHENG Liang,PENG Zichao
Acta Metall Sin    2019, 55 (9): 1133-1144.   DOI: 10.11900/0412.1961.2019.00119
Abstract   HTML PDF (20457KB)  

The research progress in powder metallurgy (PM) superalloys and manufacturing technologies are reviewed. The key control factors of Ar gas atomization (AA) powder manufacturing are introduced, including the aspects of the equipment development, atomization process, particle size, oxygen content, powder morphology and inclusion control. For the turbine disk manufacturing technology, the research progress of dual-property turbine disk, dual-alloy integral turbine wheel technologies and isothermal forging die materials are summarized. In the field of basic research, high-throughput experiment, advanced characterization and creep behavior of PM superalloys were introduced. According to the current major demand for aero-engines and 3D printing, the future of PM superalloys manufacturing technology is prospected.

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Research Progress of Wrought Superalloys in China
DU Jinhui,LV Xudong,DONG Jianxin,SUN Wenru,BI Zhongnan,ZHAO Guangpu,DENG Qun,CUI Chuanyong,MA Huiping,ZHANG Beijiang
Acta Metall Sin    2019, 55 (9): 1115-1132.   DOI: 10.11900/0412.1961.2019.00142
Abstract   HTML PDF (38676KB)  

Wrought superalloys are high temperature alloys produced by casting-forging-hot rolling-cold drawing, including disc, plate, bar, wire, tape, pipe etc. These products are widely used in aviation, aerospace, energy, petrochemical, nuclear power and other industrial fields. In this paper, domestic progress of wrought superalloys in recent ten years was reviewed, including advances in fabrication process, research in new alloys (GH4169G, GH4169D, GH4065 and GH4068 alloy et al.) and new techniques (deforming of FGH4096 alloy, nitriding of NGH5011 alloy and 3D printing of In718 alloy et al.).

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High Cycle Fatigue Behavior of Second Generation Single Crystal Superalloy
LI Jiarong,XIE Hongji,HAN Mei,LIU Shizhong
Acta Metall Sin    2019, 55 (9): 1195-1203.   DOI: 10.11900/0412.1961.2019.00110
Abstract   HTML PDF (29783KB)  

Ni-based single crystal superalloys have excellent comprehensive properties and become the preferred material for advanced aeroengine turbine blades. DD6 alloy which has been widely used in China and DD5 alloy are the second generation single crystal superalloy, and their chemical compositions and mechanical properties are quite different. In the past few decades, high cycle fatigue failure has become one of the main causes of turbine blade failure. More and more attention has been paid to the high cycle fatigue properties of single crystal superalloys. Therefore, it is important to study the high cycle fatigue behavior of single crystal superalloys, especially the second generation single crystal superalloys. In order to compare high cycle fatigue performance, two typical second generation single crystal (SC) superalloys DD6 and DD5 with [001] orientation were subjected to high cycle fatigue (HCF) loading at temperatures of 760 and 980 ℃ in ambient atmosphere. The results demonstrate that the fatigue limit of DD6 alloy is 414 and 403 MPa at temperatures of 760 and 980 ℃, respectively. DD6 alloy exhibits an excellent HCF performance under a condition of stress ratio of -1 regardless of medium or high temperature. Analysis on fracture surfaces of DD6 and DD5 alloys at 760 and 980 ℃ demonstrate that quasi-cleavage mode is observed. In addition, different types of dislocation structures were developed during the cyclic deformation. When the stress amplitude is low, dislocation movement in the γ matrix by bowing and cross slip is the main deformation mechanism and shearing γ' particles by dislocation pairs occurs occasionally under high stress level. The analysis shows that the carbon content of DD5 alloy is eight times than that of DD6 alloy, which makes the carbide content much higher than DD6 alloy, and there are significant differences in carbide morphology. In the process of fatigue fracture, carbide plays two roles of secondary crack initiation position and crack propagation channel, which greatly accelerates the fatigue crack growth rate. In the end, the fatigue resistance of DD5 alloy is reduced.

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Controllable Preparation and Self-Lubricating Mechanism Analysis of Bilayer Porous Iron-Based Powder Metallurgy Materials
ZHANG Guotao , YIN Yanguo , TONG Baohong , ZHANG Xingquan
Acta Metall Sin    2019, 55 (11): 1448-1456.   DOI: 10.11900/0412.1961.2019.00083
Abstract   HTML PDF (12531KB)  

The bilayer porous material with dense substrate layer and variable porosity surface layer was prepared by powder metallurgy technology. TiH2 was used as the pore former to improve the oil content in the surface layer, and amide wax was used as a dense agent to increase the density and strength of the substrate. The microstructure, phases distribution and the worn surface morphology were characterized by SEM, EDS, XRD, etc. The tribological properties under boundary lubrication conditions were tested by end-face friction tester. The self-lubricating mechanism of single and bilayer sintered materials under different load conditions was analyzed by comparing their friction coefficients under the progressive loading friction test. Results show that adding TiH2 in the surface layer can effectively improve the porosity and oil ratio of the bilayer materials. Meanwhile, the hard particles TiC generated by the in-situ synthesis reaction have a hard reinforcing effect on the pore channel, which will improve the wear resistance and maintain steady the contact interface and lubrication state of the friction pair. The composite material containing 3.5%TiH2 has better mechanical and tribological properties. The looser surface layer of the composite material has a better oil self-lubricating property, and the dense substrate can effectively prevent the oil moving downward and keep the lubricant between the friction surfaces. So the comprehensive tribological and mechanical properties of the composite material are better than that of the single-layer material, which is suitable for heavy load or complex lubrication conditions.

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Research Status of Weldability of Advanced Steel
Acta Metall Sin    DOI: 10.11900/0412.1961.2019.00369
Simulation of gas-liquid two-phase flow in metallurgical process
Acta Metall Sin    DOI: 10.11900/0412.1961.2019.00385
Effect of Na on Early Atmospheric Corrosion of Al
Xingchen CHEN, Jie WANG, Deren CHEN, Shuncong ZHONG, Xiangfeng WANG
Acta Metall Sin    2019, 55 (4): 529-536.   DOI: 10.11900/0412.1961.2018.00280
Abstract   HTML PDF (6254KB)  

Aluminum and aluminum alloy are widely used in every field of modern life. It is especially important to understand the detailed mechanisms of aluminum atmospheric corrosion. Traditional studies only consider the role of oxygen reduction and focus on anions such as Cl, SO42- in the environment, ignoring the effects of cations such as Na+ on the atmospheric corrosion. However, recent studies have shown that the effect of Na element on the corrosion of aluminum can not be ignored. In this work, single-shot laser-induced breakdown spectroscopy (LIBS) was used to measure the aluminum atomic lines after corrosion for 35 d in the atmospheric environment, and combined with a three-dimensional tomography measurement, to study the depth profiling of Na on the aluminum surface. The results show that the Na element on the surface of the aluminum originates from the atmospheric environment, and Na is involved in the formation of corrosion product NaAlCO3(OH)2. The content of NaAlCO3(OH)2 decreases as the depth increases following an exponential power function. The content decrease of NaAlCO3(OH)2 in different depths can be transformed into the change of cathode area. Combined with the measured polarization curve of aluminum, the atmospheric corrosion model of aluminum including the presence of oxygen reduction and the change of cathode area was established using COMSOL software. The calculated corrosion depth is 6.155 μm, which is consistent with the depth of Na element measured by LIBS experiments. By studying the distribution of Na cations and corrosion products, a simulation model was established to reveal the influence on corrosion mechanism, which is of great significance for the study of early atmospheric corrosion of aluminum.

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The Strengthening Mechanism of Cu Bearing High Strength Steel As-Quenched and Tempered and Cu Precipitation Behavior in Steel
Zhengyan ZHANG,Feng CHAI,Xiaobing LUO,Gang CHEN,Caifu YANG,Hang SU
Acta Metall Sin    2019, 55 (6): 783-791.   DOI: 10.11900/0412.1961.2018.00485
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High strength low alloy (HSLA) steels are widely used in the construction of ship structures, oil pipelines, offshore platforms and so on because of their good strength, toughness and weldability. HSLA steel is generally designed with low carbon and Cu alloying. Tempered lath bainite or martensite and nano-precipitate phase of Cu can be obtained by quenching and ageing process after rolling to ensure the excellent matching of strength, low temperature toughness and weldability of HSLA steel. At present, increasing attention has been focused on the precipitation behavior and strengthening mechanism of Cu particles in HSLA steel which was aged at the peak hardness of ageing curve. However, in practical engineering applications, overageing heat treatment is generally used to make HSLA steel achieve a good match of strength and toughness. In this work, the microstructure and nano-sized Cu precipitates of an industrial production HSLA steel plate with thickness of 35 mm were characterized by SEM, EBSD, HRTEM and APT. Meanwhile, the strengthening mechanism of the tested steel was investigated. The results show that Cu precipitates in the tested steel processed by overageing are mainly in the range of 6~50 nm, Cu particles exhibiting short rod or spherical shape within 30 nm are 9R structure, and other particles size larger than 30 nm exhibiting long rod or spherical shape are fcc structure. The segregation of trace Mn and Ni in rod particles on the interface between Cu particles and matrix is more obvious. After ageing at a higher temperature range, the yield strength of the tested steel decreases linearly with the increase of tempering temperature. The main strengthening mechanism of the HSLA steel is fine grain strengthening, followed by dislocation strengthening and precipitation strengthening. The calculated results show that every 1%Cu added in the tested steel can produce about 90 MPa precipitation strengthening increment under the condition of overageing heat treatment. The strength difference between the surface and the center of the tested steel plate is about 40 MPa, which is mainly due to the difference of grain size and dislocation density of steel.

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Performance Research of Magnesium Base Lanthanum Hexaaluminate Prepared by Co-Precipitation
Ying LI,Chao SUN,Jun GONG
Acta Metall Sin    2019, 55 (5): 657-663.   DOI: 10.11900/0412.1961.2018.00448
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Thermal barrier coatings are widely used on turbine blades to provide high temperature insulation, oxidation and corrosion protection. Thermal barrier coatings are composed of matrix, oxide layer, bonding layer and ceramic layer. The lanthanum magnesium hexaaluminate with magnetoplumbite structure have a high aspect ratio, large specific surface area and strong resistance to high temperature sintering, and it can be used as ceramic layer of thermal barrier coatings. In this work, the former powders of lanthanum magnesium hexaaluminate was prepared at synthesized temperature of 60 ℃, pH=11.5. Comparing with the conventional chemical co-preparation synthesis, the synthesized temperature was raised and the pH value for synthesizing was reduced, which resulted in improving production efficiency of former powders. And the lanthanum magnesium hexaaluminate powders for ceramic layer of thermal barrier coating was prepared after the precursor powders were calcinated at 1500 ℃ for 5 h. The phase structure of reaction products, morphology of the powders, full width at half maximum and spectral intensity were analyzed by XRD, SEM, TEM and XPS. The results showed that the magnetoplumbite structured formation generation was elevated much more efficiency if the former powders were precipitated at higher temperature. Since the bonding energy of La, Mg, Al and O atoms increased, the kinetic energy of electrons decreased, therefore, chemical composition of the reaction products were steady near 1500 ℃. During the crystallization process of lanthanum magnesium hexaaluminate, the spinel layer was generated firstly and mirror layer was consequently produced. In the process of producing pure magnetoplumbite phase powders, the full width at half maximum of LaAlO3 formula and MgAl2O4 formula were increased and the activation energy of the crystal structure was higher than that before reaction, which was beneficial to improve anti-sintering performance and thermal stability of the ceramic coating.

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Anomalous Thermal Expansion Behavior of Cold-RolledTi-35Nb-2Zr-0.3O Alloy
Chunbo LAN,Jianeng LIANG,Yuanxia LAO,Dengfeng TAN,Chunyan HUANG,Xianzhong MO,Jinying PANG
Acta Metall Sin    2019, 55 (6): 701-708.   DOI: 10.11900/0412.1961.2018.00347
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Thermal expansion behavior is one of the intrinsic properties of most materials, which is very difficult to control their thermal expansion behavior. Metallic material with ultra-low coefficient of thermal expansion named Invar effect was first found in Fe-Ni alloys. Recently, a multifunctional titanium alloy termed Gum metal (the typical composition is Ti-36Nb-2Ta-3Zr-0.3O, mass fraction, %; three electronic parameters: electron per atom ratio e/a≈4.24, bond order Bo≈2.87 and d electron orbital energy level Md≈2.45 eV) has been developed, and the alloy exhibits Invar effect after severe cold working. It is well known that the Invar effect of Fe-Ni alloys is related to the magnetic transition. However, titanium and its alloys are paramagnetic, and thus this mechanism cannot be used to explain Invar effect of Gum metal. In addition, the Invar effect of Gum metal is related to a dislocation-free plastic deformation mechanism. So far, there is still some controversy about this mechanism. In this study, a new β-type Ti-Nb base alloy Ti-35Nb-2Zr-0.3O (mass fraction, %) was developed whose three electronic parameters are different from those of the above mentioned Gum metal. The alloy was melted under high-purity argon atmosphere in an electric arc furnace, and the effects of cold rolling on microstructures and thermal expansion behaviors were characterized by OM, XRD, SEM, TEM and thermal mechanical analyzer (TMA). Results showed that the stress-induced martensitic α" (SIM α") phase transformation occurs after cold rolling, and the dominant <110> texture forms after severe plastic deformation. The equiaxed grains of Ti-35Nb-2Zr-0.3O alloy exhibit ordinary positive thermal expansion behavior and the thermal expansion rate increases with the increase of temperature. After cold deformation, negative thermal expansion occurs along rolling direction, and normal thermal expansion higher than solution treated sample occurs along transverse direction. The abnormal thermal expansion extent of the alloy increases with the increase of deformation reduction. The 30% cold deformed alloy along rolling direction possesses Invar effect between room temperature to 250 ℃, which is possibly related to SIM α" phase transformation, lattice distortion and <110> texture formation. The anomalous thermal expansion of the cold deformed samples in a temperature range from 25 ℃ to 110 ℃ is attributed to the lattice transition of SIM α" to β phase, while above 110 ℃ is attributed to the precipitation of ω and α phases.

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Current Research and Future Prospect on the Preparation and Architecture Design of Nanomaterials Reinforced Light Metal Matrix Composites
Huiyuan WANG,Chao LI,Zhigang LI,Jin XU,Hongjiang HAN,Zhiping GUAN,Jiawang SONG,Cheng WANG,Pinkui MA
Acta Metall Sin    2019, 55 (6): 683-691.   DOI: 10.11900/0412.1961.2018.00517
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In recent years, nanomaterials reinforced light metal matrix composites (LMMCs) have been researched widely, due to the enhancement in strength and ductility at room temperature, good wear resistance, excellent high temperature performance and structural-functional integration. However, there remain many challenges in developing high-performance nanomaterials reinforced LMMCs to date. The challenges mainly concentrate in the attainment of homogeneous dispersion or a controlled inhomogeneous microstructure of nanomaterials reinforcements, and the formation of the strong interfacial bonding. In the present review, therefore, current developments in fabrication, multi-scale hybrid reinforcement, novel architecture design and new processing method have been addressed. Moreover, further research interests related to the designs of nanomaterials reinforced LMMCs exhibiting high strength and plasticity, optimal architecture design and structural-functional integration have been proposed.

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Characteristics and Evolution of the Spot Segregations and Banded Defects in High Strength Corrosion Resistant Tube Steel
Bo LI,Zhonghua ZHANG,Huasong LIU,Ming LUO,Peng LAN,Haiyan TANG,Jiaquan ZHANG
Acta Metall Sin    2019, 55 (6): 762-772.   DOI: 10.11900/0412.1961.2018.00557
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C110 casing tube is one of the high strength corrosion resistant steel products for deep well oil exploration. Due to the co-existence of acidic media such as H2S and the high pressure, there are frequently sulfide stress corrosion cracking (SSC) failures produced in the tubes, which are supposed to be closely connected with their banded segregation defects. The relationship between the as-cast spot segregation and the following as-rolled banded defects, together with the impacts of quenching and tempering (QT) treatment have been revealed. The banded defects in high strength corrosion resistant oil tube have been studied experimentally from its very beginning of as-cast state. With aids of OM, SEM, EDS and EPMA observation and analysis, the various spot like segregations in round casting were revealed along with their following banded structure in both as-rolled and QT tubes. The mechanism and appearance of the segregation induced banded defects were investigated comparatively of the both tubes. It is pointed out that there are normally two kinds of spot like segregations in steel castings, speckle type and porosity type, respectively. There are not only severe positive segregations of solutes, such as C, Cr, Mo and Mn etc., in the macro-etched spot like areas, a finer dendritic sub-structure has also been observed in the speckle type spot segregation zones. It has been found that the width of the banded defects in the as-rolled tubes is closely related to the types of segregations, and the severe banded defects, which are difficult to remove by heat treatment, are recognized to originate directly from the spot like segregations. Solute segregations are found in the microstructure of banded defects of the both as-rolled and QT tubes but with different existences. A kind of pearlite plus bainite banded structure is present in the former tube, while the banded defect of latter is composed of concentrated granular carbides, which explains the difference of their hardness behavior.

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Microstructure and Tensile Property of TC4 Alloy Produced via Electron Beam Rapid Manufacturing
Zheng LIU,Jianrong LIU,Zibo ZHAO,Lei WANG,Qingjiang WANG,Rui YANG
Acta Metall Sin    2019, 55 (6): 692-700.   DOI: 10.11900/0412.1961.2019.00007
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Electron beam rapid manufacturing (EBRM) is one of the 3D printing technologies. The main attractions of EBRM technology are its high efficiency and economy in fabricating large, complex near net shape components dielessly and only needing limited machining. In general, the microstructure and texture of titanium alloy can play a significant role in determining its mechanical behaviors. In the present work, the microstructure, texture and tensile property of TC4 alloy produced by electron beam rapid manufacturing (EBRM) are investigated. Results show that the microstructure is comprised of columnar prior β grains that orient parallel to the building direction. The width of the columnar β grains increased rapidly at the initial several build layers, and the subsequent increase rate of the width of the columnar β grains tends to slow down. Fine α lamellae with gradient size are observed inside the columnar prior β grains, which occur because the alloy experiences different complex thermal histories during the EBRM-produced process. The size of α lamellae tends to decrease with the increase of build layers. The XRD result shows that the TC4 alloy has a typical α phase texture, (the c-axes are either concentrated at about 45° or are perpendicular to the building direction). At the same time, the <$10\bar{1}0$> poles are relative to random distribution. For the tensile samples along the electron beam scanning direction, the yield strengths do not show significant change with the increase of build layers, but the tensile strengths increase. The ductility of the alloy also has an upward trend, despite of a slightly decreasing ductility in the top sample. The tensile samples at the bottom of the alloy (10 mm and 20 mm away from the substrate) have similar work hardening exponents, which are lower than the top sample. The top sample shows the highest work hardening exponent. This difference in the tensile properties can be highly attributed to the gradient microstructure. The alloy also presents obvious anisotropy in tensile strength. The tensile sample along the 45° direction has a higher strength than the sample along the X direction, while the tensile sample along the Z direction shows the lowest strength. This anisotropic strength is strongly associated with the α phase texture. When the loading direction is 45° to the building direction, most of the c-axes of α phase are about parallel to the loading direction, showing a "hard" orientation, leading to a higher strength than other oriented samples. Conversely, when the loading direction is along the building direction, most of the α phase present a "soft" orientation, resulting in lower strength compared to the tensile samples along the 45° or the X direction.

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Study on the Evolution of Residual Stress During Ageing Treatment in a GH4169 Alloy Disk
Hailong QIN,Ruiyao ZHANG,Zhongnan BI,Lee Tung Lik,Hongbiao DONG,Jinhui DU,Ji ZHANG
Acta Metall Sin    2019, 55 (8): 997-1007.   DOI: 10.11900/0412.1961.2018.00428
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GH4169 alloy, a precipitation-strengthened nickel-iron base superalloy, has been widely used in aerospace and energy industries due to its excellent high-temperature strength which derived from the coherent phases (γ″ and γ'). To form these precipitates, the manufacturing process of GH4169 usually involves solid solution heat treatment followed by rapid cooling and double ageing heat treatment. Significant residual stresses are induced during rapid cooling and then partially relieved during the subsequent ageing treatment. However, the reduced residual stress after ageing are still large enough to affect the final machining operations, resulting in the component exceeding the dimensional tolerances if they are not well considered. Furthermore, residual stresses in the final components may lead to further distortion beyond estimation during service, which could deteriorate the engine performances. In the present study, the evolution of residual stresses at heating, isothermal ageing, and air-cooling stages of ageing heat treatment in a GH4169 alloy disk was characterized by in situ neutron diffraction. Considering the effect of residual stresses on the precipitation behavior of γ″, two different types of stress-free samples were used as the basis for the stress analysis. The results show that significant residual stresses were induced during water quenching, which were found to be 340.62 MPa tensile in hoop/radial directions and 33.34 MPa compressive in axial direction in the center of the disk. Subsequently, an in situ ageing heat treatment was undertaken at 720 ℃ for 8 h. During the heating stage, the yield strength of the material decreases with increasing temperature, leading to residual stress relaxation through plastic deformation from 340.62 MPa to 227.67 MPa in hoop/radial direction in the disk center. At the isothermal ageing stage, residual stresses relieved apparently by about 40 MPa during the first 100 min, later on a slower linear relaxation remained for the rest of the ageing heat treatment. The strength of the alloy increased and the creep rate decreased due to the formation of γ″ and γ′ strengthening phases, indicating that most of stress relaxation occurred as a result of creep deformation at the early stage of isothermal ageing. The magnitude of residual stress was almost invariable in the subsequent air-cooling stage.

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