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

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Present Research Situation and Prospect of Multi-Scale Design in Novel Co-Based Superalloys: A Review
LIU Xingjun, CHEN Yuechao, LU Yong, HAN Jiajia, XU Weiwei, GUO Yihui, YU Jinxin, WEI Zhenbang, WANG Cuiping
Acta Metall Sin    2020, 56 (1): 1-20.   doi:10.11900/0412.1961.2019.00159
Accepted: 24 October 2019

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In recent years, the development of material genetic methods, together with multi-scale material design theory and calculation methods has provided new ideas for the alloy design of novel Co-based superalloys. Based on the published results of multi-scale design and the research work of our laboratory, this paper systematically summarizes the present research status of multi-scale design methods in the field of novel Co-based superalloys. A review of multi-scale calculation methods including first-principle calculation, CALPHAD, phase field simulation, and machine learning is presented in this paper. The development trend of multi-scale design in novel Co-based superalloys is prospected.

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Progress in Research on the Alloying of Binary Immiscible Metals
HUANG Yuan, DU Jinlong, WANG Zumin
Acta Metall Sin    2020, 56 (6): 801-820.   doi:10.11900/0412.1961.2019.00451
Accepted: 16 April 2020

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Materials based on binary immiscible metal systems are widely used in aerospace, nuclear fusion engineering, electronic packaging, anti-armor weapons and other fields. However, due to the positive formation heat and the large differences in the properties of the component, the direct alloying of binary immiscible metals and the preparation of the corresponding materials are very difficult. Varieties of methods have been developed for direct alloying of binary immiscible metals at home and abroad, and the thermodynamic and diffusion mechanism of these methods have been studied. In this review, firstly the principle and thermodynamic mechanism of mechanical alloying, physical vapor deposition and ion beam mixing, as well as their applications in binary immiscible metal powder alloys and nano-multilayer films are reviewed. Then the irradiation damage alloying (IDA) and high-temperature structure induced alloying (HTSIA) methods that are proposed and developed by our group are introduced. Besides, the principle, interfacial microstructure, thermodynamic mechanism, diffusion mechanism and application of these two methods were described in detail. Finally, the development trend of the research on alloying of binary immiscible metals is proposed.

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Tensile Properties of Selective Laser Melted 316L Stainless Steel
YU Chenfan, ZHAO Congcong, ZHANG Zhefeng, LIU Wei
Acta Metall Sin    2020, 56 (5): 683-692.   doi:10.11900/0412.1961.2019.00278
Accepted: 25 September 2019

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Selective laser melting (SLM), as the most common additive manufacturing (AM) method, is capable of manufacturing metallic components with complex shape layer by layer. Compared with conventional manufacturing technologies such as casting or forging, the SLM technology has the advantages of high degree accuracy, high material utilization rate and environmentally friendly, and has attracted great attention in the fields of aerospace, nuclear power and medicine. The 316L austenitic stainless steel is widely used in the industrial field because of the excellent corrosion resistance and plasticity. It is also one of the commonly used material systems for SLM. In this work, the tensile properties and fracture mechanism of 316L stainless steel fabricated via SLM technology were investigated. The microstructure of the SLMed 316L specimens after tensile fracture was characterized and analyzed. The results show that the SLMed 316L stainless steel has a relatively desirable combination of strength and ductility, and its tensile performance is obviously better than that of 316L stainless steel prepared by traditional methods. The nanometer-scale cell structure inside the grain contributes to the improvement of strength. Deformation twins were observed in the SLMed 316L stainless steel after tensile test. The appearance of twins is oriented-dependent, and it is easy to occur in the grain with the direction near <110>-<111>.

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Motion Characteristics of <c+a> Edge Dislocation on the Second-Order Pyramidal Plane in Magnesium Simulated by Molecular Dynamics
LI Meilin, LI Saiyi
Acta Metall Sin    2020, 56 (5): 795-800.   doi:10.11900/0412.1961.2019.00305
Accepted: 23 March 2020

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Magnesium has a hcp lattice structure, in which insufficient independent slip systems are available to accommodate applied plastic deformation at room temperature. The ductility of Mg is intimately related to the fundamental behaviors of pyramidal <c+a> dislocations, which are the major contributor to c-axis strain. In this study, the motion of <c+a> edge dislocation on the second-order pyramidal plane in Mg under external shear stress of different magnitudes and directions are simulated by molecular dynamics at 300 K, and the motion and structural evolution of dislocations are studied. The results show that the effective shear stress causing dislocation motion is lower than the external applied one and the dislocation velocity increases linearly with increasing applied shear stress. Under the same level of external shear stress, the dislocation velocity in shearing leading to c-axis tension deformation is higher than that for shearing leading to c-axis compression, and in both cases the corresponding viscous drag coefficients are significantly higher than those for basal and prismatic edge dislocations at the same temperature. The tension-compression asymmetry of dislocation motion is essentially related to the effect of applied shear stress on the extended dislocation width.

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Microstructure and Corrosion Behavior of Fe-15Mn-5Si-14Cr-0.2C Amorphous Steel
ZHAO Yanchun, MAO Xuejing, LI Wensheng, SUN Hao, LI Chunling, ZHAO Pengbiao, KOU Shengzhong, Liaw Peter K.
Acta Metall Sin    2020, 56 (5): 715-722.   doi:10.11900/0412.1961.2019.00275
Accepted: 23 December 2019

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Amorphous steels exhibit ultra-high strength but room-temperature brittleness and strain-softening behavior as loading, which restricted the application of amorphous steels as high-performance structural material. Developing in situ crystals is an effective way to toughen the amorphous alloys. However, the crystals may sacrifice the corrosion resistance of amorphous steels. In this work, austenite and ferrite duel phases were introduced to the amorphous phase, via transformation induced plasticity (TRIP) of the austenite as loading, to enhance the ductility and improve the work-hardening behavior; and via the synergy of ferrite and amorphous phase to ensure the corrosion resistance. A novel amorphous steel Fe-15Mn-5Si-14Cr-0.2C was fabricated by magnetic suspension melting in a water-cooled copper crucible, and negative pressure suction casting into a copper mold. The microstructure and mechanical properties of the amorphous steel were characterized by XRD, EBSD and the electronic universal testing machine. The corrosion behavior in artificial seawater was studied on an electrochemical work station with a three-electrode system, and the corrosion morphology and corrosion products were characterized by SEM with EDS analysis. The results showed that the as-cast amorphous steel consisted of the amorphous matrix, CFe15.1 super-cooled austenite and Fe-Cr ferrite phases. From surface to inner, amorphous phases mainly exist in the margin, while crystalline phases are abundantly distributed in the center. The amorphous steel exhibited excellent comprehensive mechanical properties at room temperature, and its yield strength, fracture strength and plastic strain were up to 978 MPa, 2645 MPa and 35.8%, respectively. In artificial seawater, compared with 304 stainless steel, the amorphous steel showed high self-corrosion potential, low self-corrosion current density and high polarization resistance, large resistance arc radius, only one high frequency resistance arc and low corrosion kinetic rate. Moreover, the stable and dense passivation film was observed on the corrosion surface. Their excellent corrosion resistance and mechanical properties endow the amorphous steel with the potential to become a novel corrosion-resistant structural material for marine engineering.

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Recent Progress in Research and Development of Nickel-Based Single Crystal Superalloys
ZHANG Jian,WANG Li,WANG Dong,XIE Guang,LU Yuzhang,SHEN Jian,LOU Langhong
Acta Metall Sin    2019, 55 (9): 1077-1094.   doi:10.11900/0412.1961.2019.00122
Accepted: 02 July 2019

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Single crystal superalloy is the key material used in the hot section of aeroengines and industry gas turbines. The research, development and application of these alloys is generally a mirror of the industry base of a country. The recent progress in research and development of single crystal superalloys is briefly reviewed in the present paper. Some new ideas in alloy development and the design methods are summarized. The deformation behaviors, damage and failure mechanisms of single crystal superalloys during creep, fatigue, oxidation and hot corrosion have been overviewed. The role of typical defects such as low angle grain boundary, recrystallization and micro-porosity is also discussed. The recent progress in the directional solidification processes and typical parameters of high rate solidification, gas cooling casting, liquid metal cooling and fluidized bed cooling are introduced. Fundamental correlations of processing parameters to defect formation and microstructure evolution during manufacture of single crystal blade is discussed. Additionally, the future opportunities and challenges are also explored.

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Recent Progress of Microstructure Evolution and Performance of Multiphase Ni3Al-Based Intermetallic Alloy with High Fe and Cr Contents
WU Jing,LIU Yongchang,LI Chong,WU Yuting,XIA Xingchuan,LI Huijun
Acta Metall Sin    2020, 56 (1): 21-35.   doi:10.11900/0412.1961.2019.00137
Accepted: 20 August 2019

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Owing to the high temperature resistance, excellent high temperature oxidation and corrosion resistance, low density and production cost, Ni3Al-based intermetallic alloys have broad applications and attract much attention. In order to widen the application field of the Ni3Al-based superalloy, it is urgently important to improve the high-temperature performance on the basis of good weldability. Under this background, in the composition design of Ni3Al alloy, the high Fe and Cr contents can effectively enhance the phase composition and weldability of Ni3Al-based intermetallic alloys. Based on this, the microstructural characterization and phase separation sequences during solidification of a newly designed multiphase Ni3Al-based intermetallic alloy modified with high Fe and Cr elements are analyzed. On account of the typical solidification structure of the multiphase Ni3Al-based intermetallic alloy comprising γ'+γ dendrite, interdendritic β and γ'-envelope, etc., the microstructural evolutions of the alloy under different solution cooling rates, high temperature annealing, and long-term ageing processes are summarized. The effects of its corresponding complex microstructural variables (size of primary γ' phase, morphology of β, phase evolution in the interior of β, widening of γ'-envelope) on the creep behaviors of the multiphase Ni3Al-based intermetallic alloy are systematically discussed. Recent advances in welding and joining of multiphase Ni3Al-based intermetallic alloy are summarized, and the development of multiphase Ni3Al-based intermetallic alloy is also prospected.

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A Review of Current State and Prospect of the Manufacturing and Application of Advanced Hot Stamping Automobile Steels
JIN Xuejun,GONG Yu,HAN Xianhong,DU Hao,DING Wei,ZHU Bin,ZHANG Yisheng,FENG Yi,MA Mingtu,LIANG Bin,ZHAO Yan,LI Yong,ZHENG Jinghua,SHI Zhusheng
Acta Metall Sin    2020, 56 (4): 411-428.   doi:10.11900/0412.1961.2019.00381
Accepted: 09 March 2020

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Ultrahigh strength steels are highly competitive materials for vehicles to concurrently meet the increasing demand of the weight reduction and passenger safety. Hot stamping is the key forming technology to manufacture automobile components with high strength. Hot stamping steel and its manufacturing technology experienced a fast development in the past decade. This paper reviewed the state of the art of the manufacturing and applications of hot stamping steels/components in the following aspects: (1) hot stamping steels (from traditional MnB steels to recently newly developed hot stamping steels); (2) forming technologies (from traditional hot stamping process to industry 4.0 intelligent production); (3) novel hot stamping + quenching & partitioning (Q&P) process and fundamentals of deformation assisted heat treatments; (4) simulation techniques for hot stamping process (modeling of the temperature-stress field, microstructure field and simulation of the manufacturing process); (5) the assessments of in-service performance of hot stamped components. Finally, the trends of the development of hot stamping steels and related forming technologies in the future will be discussed.

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A Review of Research Status of Hydrogen Embrittlement for Automotive Advanced High-Strength Steels
LI Jinxu,WANG Wei,ZHOU Yao,LIU Shenguang,FU Hao,WANG Zheng,KAN Bo
Acta Metall Sin    2020, 56 (4): 444-458.   doi:10.11900/0412.1961.2019.00427
Accepted: 15 January 2020

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This paper overviewed the current research status and important results of the hydrogen embrittlement (HE) of the representative steel types from 1st to 3rd generation advanced high-strength steel (AHSS): transformation induced plasticity (TRIP) steel, twinning-induced plasticity (TWIP) steel, quenching & partitioning (QP) steel and medium manganese steel. The main conclusions are as follows: the HE sensitivity of TRIP steel is mainly reflected in the reduction of plasticity and the small loss of strength. The HE sensitivity of TWIP steel depends heavily on the strain rate, i.e., the HE susceptibility is significantly increased as the strain rate decreases. Deformation twin boundaries and ε/γ phase interfaces are generally prone to hydrogen-induced cracking, while Σ3 annealing twin boundaries are not. However, the ε/γ phase interfaces with Nishiyama-Wassermann orientation relationship, which is similar to the Σ3 twin boundaries, could hinder the propagation of hydrogen-induced cracks. HE sensitivity of QP steel is similar to that of TRIP steel. For medium manganese steel containing a large volume fraction of austenite phase, which result in a strong TRIP effect during deformation, the HE susceptibility represented by plasticity loss and strength loss is very high. For TRIP steel, QP steel and medium manganese steel with austenite structure, the main strategy to improve their hydrogen embrittlement is to control the morphology and distribution of austenite structure; for TWIP Steel, the measures to improve hydrogen embrittlement can be taken by controlling the prestrain rate and Al Alloying.

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Effect of Cold Deformation and Solid Solution Temperature on σ-phase Precipitation Behavior in HR3C Heat Resistant Steel
CAO Tieshan, ZHAO Jinyi, CHENG Congqian, MENG Xianming, ZHAO Jie
Acta Metall Sin    2020, 56 (5): 673-682.   doi:10.11900/0412.1961.2019.00267
Accepted: 24 October 2019

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HR3C steel, widely applied in ultra-supercritical power plant, suffers an intergranular embrittlement problem during long-term high-temperature ageing or service, which will be enhanced by the precipitation of σ phase. Research has showed that the precipitation behaviors of σ phase are different significantly as the difference of manufacturers, which relates to the preparation process of cold-deformation & solid-solution treatment. In this work, the effects of cold deformation and solution treatment on the precipitation kinetics of σ phase and related mechanical properties for HR3C steel during the ageing process were studied. The results show that cold-deformation and solid solution temperature both have a significant influence on the precipitation of σ phase in the steel. The increase of cold-deformation will promote the precipitation of σ phase, and rising solution temperature helps to inhibit the growth of σ phase but increase the grain size. The precipitation kinetics study of σ phase in HR3C steel with different pre-treatment shows that σ phase growths slowly at first, and then gets into a rapid precipitation period, and finally reaches a steady-state with a value of about 5.7% (volume fraction). The impact toughness analysis shows that the increase of cold-deformation would lower down the impact toughness of HR3C steel during the ageing procedure, while the rise of the solid-solution temperature increases the impact toughness before ageing and reduces it during ageing.

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Relationship of Inclusions and Rolling Contact Fatigue Life for Ultra-Clean Bearing Steel
SUN Feilong, GENG Ke, YU Feng, LUO Haiwen
Acta Metall Sin    2020, 56 (5): 693-703.   doi:10.11900/0412.1961.2019.00337
Accepted: 20 November 2019

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The cleanliness of bearing steels produced in China has been greatly improved due to the significant progress in the steelmaking technologies in the past decade, leading to their total oxygen (T.O.) contents lowered to no more than 6×10-6. Under such a high cleanliness, it is then expected that the influence of non-metallic inclusions on fatigue property should be different from the previous knowledge, because both the size and quantity of inclusions are reduced greatly. Therefore, both inclusions and fatigue properties for three ultra-clean GCr15 (100Cr6) bearing steels containing T.O. around 6×10-6, which were manufactured via different industrial production processes, were studied for this purpose. First, inclusions were characterized by ASPEX SEM and then statically analyzed by the statistics of extreme values (SEV) and the generalized Pareto distribution (GPD). Next, their rolling contact fatigue lives (RCF) L10 and L50 were measured by flat washer tests. Only the largest inclusion in each sample is required for predicting the characteristic sizes of maximum inclusion (CSMI) for the three steels using the SEV method. The calculated CSMIs, however, are not consistent with the variation of either L10 or L50, indicating they are not relevant. In contrast, the types of inclusions above threshold (u) size can be classified and their number density of inclusions quantified when the GPD method is employed. In particularly, the CSMIs of different types of inclusions can be determined. In this case, it has been found that the CSMI of TiN inclusion, which is the most dangerous for initiating cracking, is in a good agreement with the low probability rolling fatigue life (L10), suggesting that they are very correlated. This, however, cannot explain the variation of high-probability fatigue life (L50). Instead, the density of total inclusions also played an important role on the L50 of ultra-clean bearing steels in addition to the CSMI of TiN inclusions. This is reasonable because cracking shall be initiated at not only the most dangerous TiN inclusion during the early failure but also some other highly dense inclusions particularly during the late failure. Therefore, it is then concluded that the L10 is much more related to the CSMI of most dangerous TiN inclusion; whilst the L50 is strongly affected by the number density of total inclusions.

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Progress and Perspective of Ultra-High Strength Steels Having High Toughness
LUO Haiwen,SHEN Guohui
Acta Metall Sin    2020, 56 (4): 494-512.   doi:10.11900/0412.1961.2019.00328
Accepted: 15 November 2019

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Ultra-high strength steels have been widely used in the critical engineering structures in both military and civilian applications due to the combination of ultra-high strength and excellent toughness. In this paper, firstly, the typical ultra-high strength steel grades that have been employed were introduced, and their compositions, mechanical properties, application and histories of development were summarized with the emphasis on their microstructures and strengthening/toughening mechanism; secondly, the latest progress on the emerging ultra-high strength steel grades was reviewed, including their compositions, microstructures, strengthening mechanism and mechanical properties; thirdly, the newly emerging demands on replacing the currently employed ultra-high strength steels in China were defined, including steels for low-density but ultra-strong armors, the large ball grinding mill, cutters of tunnel boring machine and high pressure fracturing pump; finally, recent research results on ultra-high strength and high-toughness medium Mn steel were presented, which overcame the trade-off of strength and toughness to a greater extent; on this basis, some suggestions were put forward for the future development of these steel grades to meet the urgent national demands.

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Strength, Ductility and Fracture Strain ofPress-Hardening Steels
YI Hongliang,CHANG Zhiyuan,CAI Helong,DU Pengju,YANG Dapeng
Acta Metall Sin    2020, 56 (4): 429-443.   doi:10.11900/0412.1961.2020.00003
Accepted: 11 March 2020

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Press-hardening steels (PHS) are increasingly used for vehicle body structure components because of their lightening potential owning to superiorly high strength, adequate ductility and fracture resistance. New PHS grades with higher strength and enhanced fracture resistance are being widely studied now for achieving further vehicle weight reduction, and the recent development in this field is reviewed in this article. Combining quenching and partitioning (Q&P) with the hot stamping process has been explored by some researchers, as well as tempering after the hot stamping using the medium-Mn steels. A certain amount of austenite could remain by the above processes and the resulted tensile strength can exceed 1500 MPa while tensile ductility of 10%~16% can be achieved utilizing the transformation-induced plasticity (TRIP) effect. A V micro-alloyed steel (34MnB5V) for hot stamping has been designed, utilizing both grain refinement and precipitation strengthening of VC. The tensile strength of the newly developed 34MnB5V exceeds 2000 MPa which is much higher than that of the most commonly used PHS 22MnB5 (1500 MPa). Meanwhile, the ductility and bending properties of the above two steels are comparable. Al-Si coated PHS is usually adopted to avoid oxidation during heating and improve its corrosion resistance after stamping. However, its bendability after forming is lower than that of the bare grade when surface decarburization is absent. The thickness of the brittle Fe2Al5 phase was reduced and the carbon enrichment at the interface of α-Fe and martensite matrix was weakened after hot stamping by thinning of the Al-Si coating. Thus, the bending property was improved. The applicability of the new designed processes for the existing production lines should be considered in future studies. The bending test should be adopted for the deformability evaluation rather than the uniaxial tensile test simply. The welding property and the mechanism of hydrogen embrittlement should also be studied for industrial application of the new developed steels.

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Numerical Simulation of Stress Evolution of Thin-Wall Titanium Parts Fabricated by Selective Laser Melting
KE Linda,YIN Jie,ZHU Haihong,PENG Gangyong,SUN Jingli,CHEN Changpeng,WANG Guoqing,LI Zhongquan,ZENG Xiaoyan
Acta Metall Sin    2020, 56 (3): 374-384.   doi:10.11900/0412.1961.2019.00198
Accepted: 18 November 2019

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Selective laser melting (SLM) is a very promising additive manufacturing (AM) technology for fabrication of thin-walled parts due to its high forming accuracy with complex shape. The higher temperature gradient in rapid heating and cooling process is prone to produce larger thermal stress, which will induce warpage deformation of SLMed parts. However, most of the current SLM stress studies focus on the residual stress, and only a few reports on the transient stress in the thermal cycle during SLM. In this work, a thermal-mechanical coupled transient dynamic finite element model was established to study the effects of laser scan rate and layer thickness on stress evolution during SLM processing. The results show that under the action of thermal cycle, the internal stress evolution in SLM of titanium alloy thin-walled parts presents a thermal stress cycle. Under the relief annealing of the thermal stress cycle, the peak thermal stress increases first and then decreases in the heating stage, and stabilizes and approaches the value of residual stress in the cooling stage. The residual stress of SLMed thin-walled parts is less than the transient peak stress during heating. After several thermal cycles with stress relief annealing effect, the peak thermal stress of SLM thin-walled parts can be reduced by more than 30%.

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Research Progress of Laser Additive Manufacturing of Maraging Steels
TAN Chaolin,ZHOU Kesong,MA Wenyou,ZENG Dechang
Acta Metall Sin    2020, 56 (1): 36-52.   doi:10.11900/0412.1961.2019.00129
Accepted: 21 June 2019

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Additive manufacture is recognized as a world-altering technology which triggered a world-wide intensive research interest. Here the research progress and application of the laser additive manufacturing maraging steel (MS) are systematically outlined. The advantages of selective laser melting (SLM) additive manufacture of MS is emphasized. The processing parameter and properties optimizations, build orientation based anisotropies, age hardening mechanism, gradient materials, and applications in die and moulds of SLM-processed MS are reviewed in detail. Achieving relative density of >99% in SLM-processed MS is effortless, owing to the wide SLM process window of MS. Mechanical properties of MS produced with optimized SLM processing parameters and post heat treatments are comparable to traditionally wrought parts. The build orientation hardly affects the property anisotropies of MS. The age hardening behaviour in MS follows Orowan bowing mechanism. MS-based gradient multi-materials (such as MS-Cu, MS-H13, etc.) with high bonding strength are fabricated by SLM, which provides a new approach to produce high-performance functionally gradient multi-materials components. Lastly, the application in conformal cooling moulds of SLM-processed MS is elucidated, and future research interests related to MS are also proposed.

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Research Status of Weldability of Advanced Steel
PENG Yun,SONG Liang,ZHAO Lin,MA Chengyong,ZHAO Haiyan,TIAN Zhiling
Acta Metall Sin    2020, 56 (4): 601-618.   doi:10.11900/0412.1961.2019.00369
Accepted: 22 February 2020

Abstract114)   HTML3)    PDF (19246KB)(290)      

New generation advanced steel has been studied with the increased requirement for high property steel by various engineering fields since the 21st century. Correspondingly, their welding materials and welding techniques are crucial for the application of the steels. In this paper, the research status and the development of the welding processes, microstructure and properties of welded joint of the advanced steel, including ultra-fine grained steel, low carbon bainitic steel, high nitrogen austenite stainless steel and high strength automotive steel are introduced. The microstructure evolution of welded joints, the microstructure and properties of welded joints, the formation of inclusions and martenite-austenite (M-A) components and its influence on properties, and the influence of alloying elements and heat input on weld properties are reviewed. Study results show that heat affected zone (HAZ) is the main area which affects the performance of welded joints, and proper welding materials and processes are required to achieve a matching welded joint. The strengthening and toughening mechanism of weld joint, mechanism of fatigue crack growth, effect of welding thermal process on microstructure and properties of steel, are also reviewed. At last, the research prospect on welding materials and welding techiques is presented.

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On Silver Nano Particles and Silver-Bearing Materials as Virus and Bacteria Killing Agents
HU Yemin,LIAN Xintong,DONG Han
Acta Metall Sin    2020, 56 (4): 633-641.   doi:10.11900/0412.1961.2020.00061
Accepted: 10 March 2020

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It is shown by the studies that silver nano particles and silver-bearing materials have noticeable antiviral effects. The research progresses of silver nano particles and silver-bearing materials as antiviral agents are summarized and reviewed based on the aspects of possible antiviral mechanisms as well as bio-safety. The antibacterial effects of Ag-bearing iron and steels, and the effects of Ag on mechanical properties and corrosion resistance, are also summarized. It is shown by the results that silver alloying and silver nano particles could promote the inhabitation of ferrous materials to bacteria and virus.

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Relationships Between Elastic Constants and EAM/FS Potential Functions for Cubic Crystals
DUAN Lingjie,LIU Yongchang
Acta Metall Sin    2020, 56 (1): 112-118.   doi:10.11900/0412.1961.2019.00257
Accepted: 11 September 2019

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Potential functions are extensively applied in molecular dynamics (MD) simulation of metals. Selection of them is a very important step in MD simulations due to its effects of the precision and reliability of the simulations. They are one of the most important reference data during the process of calculation. In order to cover the shortage of pairwise potentials for modelling transition metals, EAM/FS many-body potentials have been introduced since 80's of last century. For the sake of determining parameters in the EAM/FS potential functions of bcc and fcc crystals through macro mechanical properties, relations between the EAM/FS potential functions and elastic constants were investigated in this work. Expressions of the pressure (P) and the bulk modulus (B), elastic constant (C44) and shear elastic modulus (Cp=(C11-C12)/2) in terms of the embedding function, pair potential function and the electron density distribution function were deduced for bcc and fcc structures, respectively. It was found that the magnitude of the C44 and Cp depends on the distances between the considered atom and surrounding atoms, but also the configuration of surrounding atoms. Finally, by converting five fitting equations about the cohesive energy (utot) and P, B, C44, Cp into an optimization model of finding minimum value, the values of the six undetermined parameters in the cohesive energy were given for five typical bcc crystals (V, Mo, Nb, Ta and W) and three typical fcc crystals (Cu, γ-Fe, Ni), respectively. For each crystal, calculation errors show accuracy of parameter values. The obtained calculation results, for the minimum cohesive energy and the corresponding atomic distance, fit well with the reported experimental data, by adopting the above values of the parameters, which indicates the effectiveness for our method.

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M3 Microstructure Control Theory and Technology of the Third-Generation Automotive Steels with HighStrength and High Ductility
WANG Cunyu,CHANG Ying,ZHOU Fengluan,CAO Wenquan,DONG Han,WENG Yuqing
Acta Metall Sin    2020, 56 (4): 400-410.   doi:10.11900/0412.1961.2019.00371
Accepted: 07 January 2020

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An important topic is the achievement of high strength and high plasticity for the development of automotive steels. Present article reviews the M3 (multiphase, metastable and multiscale) microstructure and property control theory and technology of high-strength and high-ductility third-generation automotive steels, as well as new challenges. M3 microstructure and property-microstructure control theory provide theoretical support for the development of steels with high strength and high plasticity. Transformation induced plasticity (TRIP) effect of metastable austenite has a significant influence on properties and microstructure of steels. On the one hand, it can enhance the work-hardening rate and thereby improve strength and plasticity of steels. On the other hand, it causes some new problems, such as the increase of the shear edge crack sensitivity, the decrease of hydrogen induced delayed fracture properties, and more complex transformation behavior of metastable austenite under cyclic loading. At present, the quality consistency and basic research on application are insufficient for the high-strength and high-plasticity steels with metastable austenite. As a widely-applied product, the automotive steels need be evaluated in microstructure evolution and properties from the whole chain including composition design, microstructure control, cutting process, forming process, joining process and service performance. The evaluation results will provide the basis for the improvement of microstructure control theory and technology. Full consideration will be given in the technical applicability and cost of products.

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Application and Research of Typical Intermetallics-Based High Temperature Structural Materials in China
GONG Shengkai, SHANG Yong, ZHANG Ji, GUO Xiping, LIN Junpin, ZHAO Xihong
Acta Metall Sin    2019, 55 (9): 1067-1076.   doi:10.11900/0412.1961.2019.00148
Accepted: 05 July 2019

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Intermetallics is composed of two or more metals or of a metal and a nonmetal. The coexistence of covalent and metal bond makes the intermetallic compound have long-term ordered superlattice structure, which greatly reduces dislocation mobility at high temperature, thus exhibiting good high-temperature strength. Typical structural intermetallics such as Ti-Al, Ni-Al and Nb-Si, have the advantages of excellent high-temperature strength and low density, which are very suitable for high-temperature structural parts of aerospace. However, the application of such materials is limited by low fracture toughness at room temperature and poor oxidation resistance at high temperature, which attracts more and more attentions and brings challenges in this field. In this paper, the research and application status in high-temperature strengthening, toughening, oxidation resistance and preparation technology of Ti-Al, Ni-Al, Nb-Si intermetallics-based alloys are introduced.

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Mechanical Properties of AlSiMg Alloy Specifically Designed for Selective Laser Melting
GENG Yaoxiang, FAN Shimin, JIAN Jianglin, XU Shu, ZHANG Zhijie, JU Hongbo, YU Lihua, XU Junhua
Acta Metall Sin    2020, 56 (6): 821-830.   doi:10.11900/0412.1961.2019.00306
Accepted: 03 January 2020

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Using complex shapes and precise structural parts is becoming a strong trend in modern advanced manufacturing. However, traditional manufacturing technology hardly achieves the complex geometric parts directly. Selective laser melting (SLM) is an advanced manufacturing technology for metallic materials, enables production parts with complex geometry combined with the enhancement of design flexibility. The cooling rate of molten pool can reach 103~106 K/s during the SLM process. In this case, the solid solubility of the alloying elements in the matrix can be greatly enhanced. Aluminum alloy has been widely used in industry. At present, the strength of SLM-formed aluminum alloys is far lower than that of high-strength aluminum alloys obtained from a traditional process. It is necessary to develop high-strength aluminum alloy composition based on SLM technical characteristics. The present study is devoted to design high-strength AlSiMg1.5 aluminum alloy specifically for SLM using the local structure model based on the liquid-solid structural compatibility of the alloy and the technical characteristics of the liquid quenching in SLM. The effect of the ageing treatment on the microstructure, the hardness, and the compressive properties of the SLM-formed AlSiMg1.5 alloy was systematically studied. Almost completely dense samples were obtained by adjusting the parameters of SLM process. When the ageing temperature was 300 ℃, the super-solid solution Si precipitated and grew in the island-like Al-rich structure, and the reticular Si-rich structure decomposed and spheroidized gradually with the increases of ageing time of SLM-formed AlSiMg1.5 samples. In this case, the hardness and the strength of the samples decreased, but the elongation increased significantly. The microstructures of the SLM-formed AlSiMg1.5 samples did not change obviously when the ageing temperature was 150 ℃. But the hardness and yield strength of the samples significantly increased first and then decreased slightly. The maximum microhardness and compressive yield strength of SLM-formed AlSiMg1.5 samples aged at 150 ℃ were (169±1) HV and (453±4) MPa, respectively, and the elongation of samples exceeds 25%. In this study, a special Al91.0Si7.5Mg1.5 (mass fraction, %) aluminum alloy specifically for SLM with excellent formability and mechanical properties was designed.

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A Method to Calculate the Dislocation Density of a TWIP Steel Based on Neutron Diffraction and Synchrotron X-Ray Diffraction
LI Yizhuang,HUANG Mingxin
Acta Metall Sin    2020, 56 (4): 487-493.   doi:10.11900/0412.1961.2020.00016
Accepted: 16 March 2020

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The modified Williamson-Hall method, which has been widely used to calculate dislocation densities of high-strength steels and other structural alloys, is re-examined in this work, and is further applied to calculate the dislocation density of a deformed twinning-induced plasticity (TWIP) steel by using its neutron diffraction patterns and synchrotron X-ray diffraction patterns. This paper aims not only to promote the proper use of the method but also to shed light on its underlying pre-requisites and assumptions, and is thus expected to help avoid any errors during its usage.

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Anisotropy of Elasticity of a Ni Base Single Crystal Superalloy
LIU Jinlai, YE Lihua, ZHOU Yizhou, LI Jinguo, SUN Xiaofeng
Acta Metall Sin    2020, 56 (6): 855-862.   doi:10.11900/0412.1961.2019.00355
Accepted: 17 March 2020

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The anisotropy of elasticity of single crystal superalloy is essential to understand its mechanical behavior, e.g. calculating the vibration frequency of turbine blade and avoiding resonance during operation. However, it's difficult to calculate the stiffness constants of single crystal superalloy by theory methods. In this work, one simple experimental method is employed to determine the stiffness constants. The slabs of a first generation single crystal superalloy in two orientations 〈001〉〈100〉 and 〈011〉〈110〉 are employed to measure the Young's modulus and shear modulus of this alloy. The Young's modulus and shear modulus of the first specimen and the shear modulus of the second specimen are measured by resonance method from room temperature to 1100 ℃. The three stiffness constants C11, C12 and C44 of this superalloy are calculated from the measured moduli. The Young's modulus and shear modulus in any orientation can be calculated based on the stiffness constants. Further, the 3D distribution map of Young's modulus and maximum and minimum of shear modulus related to primary orientation can be drawn, so the distribution feature of modulus in 3D space can be acquired conveniently. When the primary orientations are along 〈001〉 and 〈111〉, the shear modulus in plane is isotropy with secondary orientation. When the primary orientation is along 〈011〉, the shear modulus demonstrates significant anisotropy with secondary orientation, the shear modulus reaches minimum with secondary orientation 〈110〉, while the maximum is obtained in secondary orientation 〈100〉.

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Solidification Structure Refinement in TWIP Steel by Ce Inoculation
LI Gen, LAN Peng, ZHANG Jiaquan
Acta Metall Sin    2020, 56 (5): 704-714.   doi:10.11900/0412.1961.2019.00288
Accepted: 06 January 2020

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Twinning-induced plasticity (TWIP) steel represents a novel grade of advanced high strength and ductility with significant potential for automotive industry. However, high alloying in TWIP steel leads to the inhomogeneous solute distribution and anisotropic local deformation. It is well known that the refinement of solidification structure is an effective solution to the above defects. Much attention has been paid to heterogeneous nucleation by Ce particles, acting as nucleating sites in liquid steel. The present work focuses on how Ce content and casting parameters affect the refinement of solidification structure in Fe-22Mn-0.65C TWIP steel, aiming to provide an effective technology in high alloy steel production. The reaction products of Ce inoculation were predicted by thermodynamics software FactSage 7.0 and their effectiveness of heterogeneous nucleation was estimated by lattice misfit model. The solidification structure refinement by Ce inoculation under different conditions was experimentally studied by OM, SEM, EBSD and EPMA. The results show that, with increasing Ce content the reaction products transferred from Ce2O3 to Ce2O3+a small amount of Ce2O2S, and both kinds of particles can act as heterogeneous nucleation cores theoretically. For as-cast solidification structure, the ratio of equiaxed grain area increased from 25% to 72%, average equiaxed grain size decreased from 480 μm to 130 μm and the segregation ratio of Mn decreased from 1.61 to 1.41. Meanwhile, the tendency of particle agglomeration was weakened by lowering inoculation temperature, resulting in the improvement structure refinement. In this work, the recommended inoculation parameters are concluded as (0.02%~0.04%)Ce with superheat of 20 ℃.

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Precipitation σ Phase Evoluation and Mechanical Properties of (CoCrFeMnNi)97.02Mo2.98 High Entropy Alloy
YAO Xiaofei, WEI Jingpeng, LV Yukun, LI Tianye
Acta Metall Sin    2020, 56 (5): 769-775.   doi:10.11900/0412.1961.2019.00330
Accepted: 24 December 2019

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Mo in the form of solid solution atom or compound phase is distributed in CoCrFeMnNi high entropy alloy, which has the effect of solution strengthening or second phase strengthening. The method of annealing was used to heat treated (CoCrFeMnNi)97.02Mo2.98 high entropy alloy to investigate effects of σ phase on mechanical properties of (CoCrFeMnNi)97.02Mo2.98 high entropy alloy. SEM, EDS and XRD were used to analyze effects of annealing temperature on precipitation σ phase (CrMo phase) in (CoCrFeMnNi)97.02Mo2.98 high entropy alloy. The mechanical properties were tested by microhardness and tensile test, and the influencing mechanism of σ phase on the mechanical properties was investigated. The results show that with increase of the annealing temperature, the quantity of precipitation σ phase increases in (CoCrFeMnNi)97.02Mo2.98 high entropy alloy, and the σ phase is first precipitated at the grain boundary, and is after precipitated in intracrystalline. The morphologies of σ phase at the grain boundary are changed gradually from tiny strips of discontinuous distribution to thick strip of continuous distribution. With the annealing temperature increases further, the morphologies of σ phase are changed from strip of continuous distribution to granular of continuous distribution. The precipitation σ phases in (CoCrFeMnNi)97.02Mo2.98 high entropy alloy by annealing have the effect of second phase reinforcement, with the annealing temperature increase, the numbers of precipitation σ phase increase, and the hardness and strength both increase, which is obviously at temperature higher than 900 ℃. The σ phase precipitation in intracrystalline, and its refinement, can improve the strength and plasticity of (CoCrFeMnNi)97.02Mo2.98 high entropy alloy synchronously.

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