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Effects of Intercritical Annealing Temperature on the Tensile Behavior of Cold Rolled 7Mn Steel and the Constitutive Modeling
Feng YANG, Haiwen LUO, Han DONG
Acta Metall Sin    2018, 54 (6): 859-867.   doi:10.11900/0412.1961.2017.00315
Accepted: 19 October 2017

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Medium Mn steel is composed of sub-micron grained ferrite and austenite, the unstable austenite may transform to martensite during plastic straining. Although the mechanical properties of medium Mn steel could be easily tested by tensile test, it is quite difficult to directly measure the influences of different constituent phases on the tensile and work hardening behavior. Thus, at the present work, EBSD, TEM, XRD and a constitutive model based on dislocation density have been used to study the effects of intercritical annealing (IA) temperature on the tensile properties and work hardening behavior of a newly designed medium Mn steel, Fe-7%Mn-0.3%C-2%Al (mass fraction). Experimental results showed that with the increase of IA temperature, the mechanic stability of reverted austenite decreased gradually and the kinetics of strain induced martensite rose rapidly. The stability of the reverted austenite was moderate when intercritically annealed at 700 ℃, this led to the best plasticity and the optimal mechanical properties. Simulated results exhibited that the mechanic stability of austenite has a decisive influence on the tensile behavior of the material. The austenite stability will be too high if the IA temperature is lower, and this will lead to the lower work hardening rate and uniform elongation; when the IA temperature is moderate, the stability of austenite will be optimum, consequently strain-induced martensite would be progressively produced during straining and result in the higher work hardening rate and prolonged uniform elongation; the stability of austenite will be too lower if the IA temperature is higher, thus larger volume fraction of strain-induced martensite would be formed in a short period, and this would result in the higher tensile strength but the inferior uniform elongation.

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Theoretical Calculation of Schmid Factor and Its Application Under High Strain Rate Deformation in Magnesium Alloys
Yanyu LIU, Pingli MAO, Zheng LIU, Feng WANG, Zhi WANG
Acta Metall Sin    2018, 54 (6): 950-958.   doi:10.11900/0412.1961.2017.00398
Accepted: 22 January 2018

Abstract145)   HTML20)    PDF (3152KB)(621)      

As an important parameter, the Schmid factor has been widely applied to analyze the deformation modes in metals. In order to analyze the deformation mechanisms of magnesium alloys under high strain rate, the Schmid factors of four slip modes (basal, prismatic, pyramidal <a> and pyramidal <c+a> slips) and two twinning systems ({101?2} tension and {101?1} contraction twinnings) were systematically calculated in this work. The experimental values of Schmid factor of as-received AZ31 rolling magnesium alloy sheets were obtained by electron backscatter diffraction (EBSD) technique, and then the theoretical calculated values were compared with those values. The high strain rate compression test of AZ31 rolling magnesium sheets was conducted by using split Hopkinson pressure bar at the strain rate of 1600 s-1, and the microstructures after compression were observed by optical microscopy. The Schmid factors and microstructures are combined to discuss the predominant deformation mechanisms for different orientation samples under different loading directions. The results showed that the theoretical calculated values of Schmid factors are in good agreement with their experimental values. Therefore, the Schmid factor, owing to its simplicity and conveniene, could be used to analyze the predominant deformation mechanism and interpret the unique characteristics of "true stress-true strain" curves in magnesium alloys. Furthermore, since the Schmid factor and its variation trend associated with deformation behavior in magnesium alloys are related, the calculation result of Schmid factor can provide a theoretical analytic approach to understand anisotropic phenomena caused by strong texture in magnesium alloys.

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Hydrogen Embrittlement of Intercritically AnnealedCold-Rolled 0.1C-5Mn Steel
Xiaoli ZHAO, Yongjian ZHANG, Chengwei SHAO, Weijun HUI, Han DONG
Acta Metall Sin    2018, 54 (7): 1031-1041.   doi:10.11900/0412.1961.2017.00435
Accepted: 19 April 2018

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Medium-Mn steel typically alloyed with (3%~10%)Mn (mass fraction) has recently regained significant interest as one of the most promising candidates for the third-generation automobile steel due to its excellent combination of ultra-high strength and ductility as well as relatively low material cost and industrial feasibility. Considering the ever increasing strength level as well as the comparatively high amount of reverted austenite (RA) of medium-Mn steel, special attention began to be given to its hydrogen embrittlement (HE) behavior for ensuring the safety service of components made of this kind of steel. However, the effect of RA on HE of medium-Mn steel has not been fully understood. For this purpose, the susceptibility to HE of a cold-rolled medium-Mn steel 0.1C-5Mn intercritically annealed at 650 ℃ for different time to obtain different amounts of RA was investigated by using electrochemical hydrogen charging, thermal desorption spectrometry (TDS), slow strain rate test (SSRT) and SEM. The results show that the annealed samples exhibit a dual-phase microstructure of reverted globular shaped RA and ferrite. The ultimate tensile strength (σb) increases while the yield strength decreases with increasing annealing time, and both the total elongation (δ) and the product of σb to δ (σb×δ) initially increase and then decrease with increasing annealing time. That is to say, an excellent combination of strength and ductility could be obtained when the tested steel was annealed at 650 ℃ for 10 min. However, the results of TDS and SSRT show that both the absorbed diffusible hydrogen concentration and the susceptibility to HE increase with increasing annealing time, and the latter is more significant. SEM analysis of the fracture surfaces of fractured samples revealed that the hydrogen-charged annealed sample was fractured to leave both dimples filled with grains and empty dimples while the uncharged annealed specimen was ductile fractured to leave only empty dimples. The dimples filled with grains were basically a brittle intergranular cracking occurring along the boundaries of RA and/or martensite (formerly RA) grains by the hydrogen-assisted cracking mechanism. It is thus concluded that the HE behavior of intercritically annealed cold-rolled medium-Mn steel is primarily controlled by both the amount and mechanical stability of RA.

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Effect of Si and Mn Contents on the Microstructure and Mechanical Properties of Ultra-High Strength Press Hardening Steel
Kuanhui HU, Xinping MAO, Guifeng ZHOU, Jing LIU, Zhifen WANG
Acta Metall Sin    2018, 54 (8): 1105-1112.   doi:10.11900/0412.1961.2017.00487
Accepted: 19 April 2018

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It is very important to find out the mechanism of composition in steel. Many efforts have been put on the study of the effect of Si and Mn elements on the microstructure and mechanical properties of middle Mn steel, transformation induced plasticity (TRIP) steel and quenching and partitioning (Q&P) steel. But fewer studies were focused on the mechanism of Si and Mn contents in a press hardening steel. In this work, the microstructures after hot rolled and the fine martensite structure after hot stamping in ultra-high strength press hardening steel (PHS) with different Si and Mn contents were studied by OM, SEM, EBSD and TEM. The results showed that there are a great influence of Si and Mn contents on the microstructure and mechanical properties of PHS after hot rolled. The yield strength of the PHS increases from 552 MPa to 751 MPa, the ultimate tensile strength (UTS) increases from 757 MPa to 1124 MPa, and the microstructures are different with the Mn content rose from 0.57% to 1.21% and the other components remained the same. The UTS of the steels goes up as the Si content goes up from 0.25% to 0.38%, and the yield strength and the elongation show a fluctuation trend. After simulating hot stamping process at 950 ℃ and holding 5 min, the microstructure of the steels with different compositions is martensite, but it is different in the fine martensite structure and the average size of sub-grain; after hot stamping process, the comprehensive mechanical properties of the steel B with 0.30%C, 0.34%Si and 1.21%Mn are the most outstanding, the yield strength is 1161 MPa, the UTS is 1758 MPa, and the elongation is 6.5%; after hot stamping process, the microstructure of the steel B is fine lath martensite, and there is a large amount of dislocation in the martensite lath, and precipitates a small number of carbide. The mechanical properties of the ultra-high strength press hardening steels designed in this work is not obvious correlation before and after hot stamping process, and it is just a slight difference in martensite fine structure which is beneficial to controlling the performance stability of the mass industrial production.

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The Influence of Rolling Process on the Microstructure, Texture and Magnetic Properties of Low Grades Non-Oriented Electrical Steel After Phase Transformation Annealing
Chen GU, Ping YANG, Weimin MAO
Acta Metall Sin    2019, 55 (2): 181-190.   doi:10.11900/0412.1961.2018.00187
Accepted: 24 October 2018

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Non-oriented electrical steel sheets are important metallic functional materials for the iron cores in transformers and electrical motors, which require the performance characteristics of low iron loss and high magnetic induction. The magnetic properties of electrical steel critically depend on the microstructure and the occurring texture components. In addition, alloy elements can affect the magnetic properties by altering the electrical resistivity, microstructure and texture. At present, the quality of commercial non-oriented electrical steels are mainly optimized by the control of deformation, recrystallization parameters and chemical composition. And the microstructure, texture and magnetic properties are significantly influenced by the rolling process before recrystallization annealing. The favorite {100} texture in such condition takes at maximum only about 20% in volume fraction. In contrast, phase transformation combined with deformation can lead to nearly 80% volume fraction of {100}-oriented grains. In this work, the influence of rolling process on the microstructure, texture and magnetic properties of low grades non-oriented electrical steel after phase transformation annealing was studied by means of EBSD, XRD and magnetism measuring techniques. The starting material is a columnar-grained industrial low grades electrical steel cast slab. Five different initial microstructures are obtained after different rolling processes, the αγα phase transformation annealing of samples is conducted in a tube furnace under H2 atmosphere. The results show that phase transformation annealing can significantly coarsen grains and reduce the iron loss of non-oriented electrical steels compared with traditional recrystallization annealing. And the phase transformation texture is influenced by texture memory. Compared with hot rolling-cold rolling process, more {100}-oriented grains are obtained and the magnetic properties of non-oriented electrical steels are improved significantly after phase transformation in the directly cold rolling process. The proportion of non-{111} oriented grains increases and more initial {100}-oriented grains are retained after phase transformation in the process with lower hot rolling temperature, which improve the magnetic properties of final sample. In addition, the presence of P and Al elements in commercial electrical steels may affect the microstructure, texture and magnetic properties of non-oriented electrical steels due to segregation and oxidation after phase transformation.

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Microstructure and Mechanical Properties of a Novel Cold Rolled Medium-Mn Steel with Superior Strength and Ductility
Chengwei SHAO, Weijun HUI, Yongjian ZHANG, Xiaoli ZHAO, Yuqing WENG
Acta Metall Sin    2019, 55 (2): 191-201.   doi:10.11900/0412.1961.2018.00081
Accepted: 11 December 2018

Abstract131)   HTML8)    PDF (7409KB)(562)      

Recently, energy conservation, environmental protection and security are the main factors considered by the automotive manufacturers. Medium-Mn steel with excellent combination of specific strength and ductility have been regarded as the potential candidates for automotive applications. The excellent combination of specific strength and ductility depends on the microstructure under different heat treatment processes of the steels. Therefore, the relationship of the combination of specific strength and ductility and microstructure should be studied in detail. A new alloy system of aluminum-containing medium-Mn steel was developed in this study. The addition of aluminum stabilizes α-ferrite, and facilitates the presence of δ-ferrite during solidification. The addition of Mn and C compensates the effect of aluminum on phase stability and ensures austenite formation. In this investigation, the effects of intercritical annealing temperature on the microstructure and tensile properties of a newly designed cold-rolled aluminum-containing medium-Mn steel (0.2C-5Mn-0.6Si-3Al, mass fraction, %) were investigated by SEM, XRD and uniaxial tensile tests. The tensile results show that an excellent combination of ultimate tensile strength (σb) of 1062 MPa, total elongation (δ) of 58.2% and σb×δ of 61.8 GPa% could be obtained after annealing at 730 ℃. The inverted austenite of the cold-rolled steel coarsenes and gradually changes its morphology from mainly lamellar to mainly equiaxed with increasing intercritical annealing temperature, and a duplex microstructure consisting of multi-scale retained austenite could be obtained at 730 ℃, which possesses suitable mechanical stability and thus presents prolonged transformation-induced plasticity (TRIP) effect during tensile deformation. This kind of sustainable TRIP effect and the cooperative deformation of ferrite are responsible for the superior mechanical properties. The investigation of tensile fracture behavior shows that the nucleation and growth of voids occurred mainly at the interfaces between soft ferrite and hard martensite induced by deformation.

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Effect of Hot Band Annealing Processes on Texture and Formability of 19Cr2Mo1W Ferritic Stainless Steel
Houlong LIU,Mingyu MA,Lingling LIU,Liangliang WEI,Liqing CHEN
Acta Metall Sin    2019, 55 (5): 566-574.   doi:10.11900/0412.1961.2018.00540
Accepted: 04 March 2019

Abstract71)   HTML3)    PDF (13885KB)(385)      

Low-cost ferritic stainless steels with excellent oxidation resistance and anti-corrosion ability are widely used in the fields of household appliances, hardware decoration, architectural structures, fuel cells and automobile exhaust systems. In order to achieve good formability of the ferritic stainless steel, the annealing process of hot-rolled sheet is crucial. As a newly developed 444-type heat-resistant ferritic stainless steel containing W and Ce, however, the influence of hot band annealing process of 19Cr2Mo1W ferritic stainless steel on its formability is not clear and need to have a deep understanding. In this work, the effect of annealing temperature of hot band on the microstructure, texture and formability of this steel was studied by means of XRD, EBSD, roughness measurement and formability test. The results indicated that although annealing processes were carried out at different temperatures after hot rolling, the characteristic of texture in the hot-rolled and annealed sheet was inherited to the cold-rolled sheet to some extent. The increased intensities of {223}<11ˉ0> and {111}<01ˉ1> texture components in the hot-rolled and annealed sheet were beneficial to improvement of the γ-fiber texture in the cold-rolled and annealed sheet. The extent of deviation from γ-fiber texture in the cold-rolled and annealed sheet was increased with increasing the intensities of {001}<11ˉ0>~{115}<11ˉ0> texture components in the cold-rolled sheet. An increased annealing temperature of the hot-rolled sheet could effectively weaken the intensities of {001}<11ˉ0>~{115}<11ˉ0> texture components in the cold-rolled sheet. In addition, the banded microstructures in the hot-rolled and annealed sheet were significantly reduced by increasing annealing temperature of the hot-rolled sheet, which improved the microstructure uniformity and formability of the cold-rolled and annealed sheet.

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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
Accepted: 27 March 2019

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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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Ring Rolling Forming and Properties of Ti2AlNb Special Shaped Ring Prepared by Powder Metallurgy
Zhengguan LU,Jie WU,Lei XU,Xiaoxiao CUI,Rui YANG
Acta Metall Sin    2019, 55 (6): 729-740.   doi:10.11900/0412.1961.2019.00015
Accepted: 21 March 2019

Abstract82)   HTML3)    PDF (23413KB)(373)      

Ti2AlNb alloy was considered as the candidate material to replace superalloys such as GH4169 in gas turbine engine applications due to higher strength-weight ratio at elevated temperatures. Powder metallurgy (PM) offers the potential for solving many of the problems associated with the large ingots, such as center-line porosity and chemical inhomogeneity. In order to study the feasibility of preparing Ti2AlNb special shaped ring with large size, PM + ring rolling combined process is considered as a potential method and discussed in this work. PM Ti2AlNb alloy and special shaped ring (D>800 mm) with a nominal composition of Ti-22Al-24.5Nb-0.5Mo (atomic fraction, %) were prepared from pre-alloyed powder using hot isostatic pressing (HIP). Hot compression tests of PM Ti2AlNb alloy and wrought alloy with the same chemical composition were conducted on Gleeble-3800 testing machine under 930~1050 ℃ and 0.001~1 s-1 conditions. Ring rolling was conducted on PM Ti2AlNb special shaped ring by horizontal rolling machine, and the microstructure evolution and properties performance of PM ring after rolling forming process were studied. The results show that the processing window for PM Ti2AlNb alloy is broader than that for wrought alloys, and wrought Ti2AlNb alloy is easier to crack at low temperature or relative high strain rate. PM Ti2AlNb alloy has more homogeneous chemical composition and uniform α2 phase distribution. Stress instability phenomenon of PM Ti2AlNb alloy is more obvious than that of wrought alloy which is related to phase transition of Ti2AlNb alloy. Optimized deformation temperature for PM Ti2AlNb special shaped ring was set as 1030~1045 ℃ with reference to the hot compression results. Ti2AlNb special shaped ring after two rolling steps has no any kinds of defects presented by X-ray testing, ultrasonic testing and fluorescence detection. O laths inside PM Ti2AlNb alloy become shorter and narrow, and α2 phase tends to be a coarser and spherical structure due to the hot deformation. After a typical heat treatment (980 ℃, 2 h, AC+830 ℃, 24 h, AC), nearly B2+O microstructure is obtained in Ti2AlNb special shaped ring. Compared with the undeformed alloy, tensile ductility at room temperature and 650 ℃ of Ti2AlNb ring after hot deformation improves due to the refined O phase structure.

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Molecular Dynamics Modeling and Studying of Micro-Deformation Behavior in Metal Roll-Bonding Process
Qingdong ZHANG,Shuo LI,Boyang ZHANG,Lu XIE,Rui LI
Acta Metall Sin    2019, 55 (7): 919-927.   doi:10.11900/0412.1961.2018.00524
Accepted: 23 April 2019

Abstract123)   HTML1)    PDF (13275KB)(498)      

Stainless steel/carbon steel bimetallic products, which have the characteristic corrosion resistance of stainless steel as well as the characteristics of high strength and low cost of carbon steel, have been widely used in petrochemical, aviation, shipping and other industries. Roll-bonding is an efficient solid-phase joining method for industrial production of bimetallic products. Different from diffusion bonding and friction welding process, the atoms bond and diffuse at interface while the base metal undergoes severe plastic deformation in the process of roll-bonding. In present work, the micro-deformation behavior in the interfacial area of stainless steel/carbon steel during roll-bonding process is studied based on the molecular dynamics method. Firstly, the applicability of the potential function for the bimetallic composite models with different lattice structures was discussed. Then bimetallic model of FeCrNi/Fe and single metal model of FeCrNi, Fe were established. The effect of non-coherent interface on the deformation behavior was revealed by comparing the deformation process of three models. The results show that the mechanical properties and deformation processes of bimetal and single metal are different in the process of deformation bonding. Due to the existence of non-coherent interface, the dislocation in pure Fe matrix is accumulated at the interface during deformation. The local shear effect of interface atoms makes the dislocation formation in FeCrNi matrix easier, thus reducing the yield strength of FeCrNi matrix. The effect of interface on dislocation propagation during alternation makes the dislocation density change alternately in the two matrixes, which improves the ability of material to resist plastic deformation. In addition, the alternately change of the dislocation density within the two matrixes during the deformation process leads to the special phenomenon that the deformation of the two matrixes is alternately changed.

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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
Accepted: 15 April 2019

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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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Generation and Interaction Mechanism of Tension Kink Band in AZ31 Magnesium Alloy
ZHOU Bo, SUI Manling
Acta Metall Sin    2019, 55 (12): 1512-1518.   doi:10.11900/0412.1961.2019.00149
Accepted: 23 August 2019

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The deformation structures, such as deformation twins, dislocations and kink bands, play an important role in the plasticity of magnesium alloys during the deformation process. However, due to the complexity of hcp structure, the deformation structures of the magnesium alloys, especially the interactions between deformation structures are still not well understood. Thus, it is of great scientific significance to study the microstructure of magnesium alloys, especially to characterize their structural characteristics of the interaction areas, which plays a significant role in understanding the structure and performance relationships of magnesium alloys. In this work, a combination of TEM and SAED pattern was applied to study the interaction mechanism associated with different kinds of deformation structures in Mg-Al-Zn (AZ31) alloy. When the applied external force is not beneficial for deformation twins and dislocations, kink bands act as a supplementary deformation mode to coordinate the asymmetry of hcp structure. According to crystallographic analysis, it is found that under the action of tensile stress nearly lie on basal plane in hcp structures, the basal dislocation pairs form and move to the opposite directions, forming tension kink band with the interface of {101ˉ2} plane. The angle between the tension kink band interface and the basal plane is about 43°. The tension kink bands can further contribute to the strength and toughness of the material. These results will open a new insight into the understanding of interaction mechanism of deformation structures and greatly promote the development of Mg alloys.

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Preparation of Steel/Aluminum Laminated Composites by Differential Temperature Rolling with Induction Heating
XIAO Hong,XU Pengpeng,QI Zichen,WU Zonghe,ZHAO Yunpeng
Acta Metall Sin    2020, 56 (2): 231-239.   doi:10.11900/0412.1961.2019.00150
Accepted: 06 September 2019

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Both cold-rolled and hot-rolled steel/aluminum laminated composites exhibited obvious strain-hardening of steel layer because the rolling temperature, limited by the melting point of aluminum (about 660 ℃), was lower than dynamic recrystallization temperature of steel (about 710 ℃). This led to poor deformation ability of composite plates and subsequent processing cracks. And the initial bonding of cold-rolled steel/aluminum composite plates usually required more than 50% highly first pass reduction, which resulted in high requirement for rolling mill capacity, especially for medium or thick size composite plates. To solve above two problems simultaneously, in this study, the steel/aluminum composite plates were prepared by differential temperature rolling (DTR) with induction heating in an argon atmosphere. The bonding properties and microstructure of the steel/aluminum laminated composites were studied, and the effect of DTR process on the bonding properties was analyzed compared with the cold rolling process. The results show that dynamic recovery and recrystallization occurred with equiaxed grains appearing in the structure of the rolled carbon steel due to the higher heating temperature of the steel layer, and an equiaxed fine grain zone with an average grain size of approximately 5 μm was formed near the interface of the steel side, which greatly reduced the hardening phenomenon of the laminated composites compared with the cold rolled clad plate. The micro-interface of DTR steel/aluminum clad plate was tightly bonded without holes and gaps. The diffusion width of Al and Fe elements across the interface reached 2.4 μm, indicating the clad plate achieved a good metallurgical bonding state, and the fine grained zone near the interface improved the properties of the sheet. The combined effect made the shear strength of the DTR clad plates much higher than that of the cold-rolled plate. At 45% reduction, the shear strength of DTR composite plate reached 85 MPa, which was 7 times of cold-rolled composite plate with the same reduction (12 MPa). The fracture of cold-rolled composite plate occurred at the steel/aluminum interface, showing brittle fracture, while the fracture of DTR clad plates occurred in the aluminum alloy matrix with a large number of dimples in the shear section, showing the characteristics of plastic fracture.

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Partially Recrystallized Structure and Mechanical Properties of CoCrFeNiMo0.2 High-Entropy Alloy
CAO Yuhan,WANG Lilin,WU Qingfeng,HE Feng,ZHANG Zhongming,WANG Zhijun
Acta Metall Sin    2020, 56 (3): 333-339.   doi:10.11900/0412.1961.2019.00274
Accepted: 11 February 2020

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In recent years, high-entropy alloys have triggered broad research interests due to their unique and intriguing mechanical properties. In general, the increase in strength is accompanied by the reduction in ductility. Therefore, strong and ductile metallic materials have always been pursued by metallurgist. Heterogeneous structure has been reported to be very useful for overcoming the strength-ductility trade-off in metallic materials. In this work, typical partially recrystallized structure has been obtained in CoCrFeNiMo0.2 high-entropy alloy by cryogenic rolling and annealing. The effect of partially recrystallized structure on the mechanical properties has been studied. After 35% cold rolling (RTR35%) and 35% cryogenic rolling (CTR35%) and annealed at 800 ℃ for 30 min, CoCrFeNiMo0.2 high-entropy alloys developed partially recrystallization microstructures featured by coarse deformed grains and fine recrystallized grains. The yield strength of the CTR35% sample is 539.3 MPa and its elongation is 46.8%, which is similar in strength but 30% higher in elongation when compared with the RTR35% sample. This can be understood from the fact that samples rolled at cryogenic temperature showed a higher volume fraction of fine recrystallized grains, resulting in better strain hardening capability.

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Fabrication and Properties of Novel Multi-LayeredMetal Composites
ZHANG Le,WANG Wei,M. Babar Shahzad,SHAN Yiyin,YANG Ke
Acta Metall Sin    2020, 56 (3): 351-360.   doi:10.11900/0412.1961.2019.00245
Accepted: 13 November 2019

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With the development of science and technology, more and more products with excellent quanlity and abundant functionalities have been exploited and provided. Inspired by the concept of "brick wall" structure or layer structure with alternated distribution of hard and soft phases discovered in nature creatures such as mother pearl shellfish, a entirely novel steel composite which not only can minimize the shortcomings of the original materials at the maximum extent, but also possess excellent mechanical performance as well as new physical properties, has been developed. Taking ultra-high strength maraging steel and 316L austenitic stainless steel as the original materials, the influence of deformation reduction under high vacuum on interfacial bonding strength and interface characteristics of heterogeneous multi-layered metal composites was studied, and the fabrication feasibility of heterogeneous multi-layered metal composites was explored. The results showed that in the vacuum hot-pressing process, the interfaces under different deformations were clear and straight. Slight mutual diffusion phenomenon occurred in the hot-pressing process. Due to the difference of rheological properties of the original materials at high temperature, dynamic recovery and dynamic recrystallization occurred in the 316L layer, while deformed microstructure was dominant in the maraging steel layer. Combined with rolling process and heat treatment, bulk metal composites with 9 layers and 11 layers were prepared, respectively. The results of the three-point bending experiment showed that the crack occurred firstly at the outermost side of the multi-layer composites which withstood the tensile stress. Due to the passivation, delamination and bridging of heterogeneous interface in the multi-layer metal composites, the propagation path of crack was greatly extended and more energy was consumed, which showed excellent ability to block the crack propagation.

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