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

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Deformation Behavior and Toughening of High-Entropy Alloys
Zhaoping LU, Zhifeng LEI, Hailong HUANG, Shaofei LIU, Fan ZHANG, Dabo DUAN, Peipei CAO, Yuan WU, Xiongjun LIU, Hui WANG
Acta Metall Sin    2018, 54 (11): 1553-1566.   DOI: 10.11900/0412.1961.2018.00372
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A new alloy design concept, high-entropy alloys (HEAs), has attracted increasing attentions and becomes a new research highlight recently. Different from traditional alloy design strategy which usually blends with one or two elements as the principal constituent and other minor elements for the further optimization of properties, HEAs are multicomponent alloys containing several principle elements (usually ≥5) in equiatomic or near equiatomic ratio. Due to their unique atomic structure, HEAs possess a lot of distinguished properties. Since the discovery of HEAs, a variety of HEA systems have been developed and shown unique physical, chemical and thermodynamic properties, especially the promising mechanical properties such as high strength and hardness, abrasion resistance, corrosion resistance and softening resistance. Here in this short review manuscript, starting from the research challenges for understanding the deformation mechanism of HEAs, this work briefly summarized the mechanical properties and deformation behavior of HEAs, reviewed the proposed strengthening-toughening strategies and their corresponding deformation mechanism in HEAs. A brief perspective on the research directions of mechanical behavior of HEAs was also proposed.

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Friction Stir Welding of Magnesium Alloys: A Review
Zongyi MA, Qiao SHANG, Dingrui NI, Bolv XIAO
Acta Metall Sin    2018, 54 (11): 1597-1617.   DOI: 10.11900/0412.1961.2018.00392
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In recent years, the increasing application demand for Mg alloys in automobile, rail transport, aviation and aerospace industries brings about the growing prominence of seeking reliable techniques to join Mg alloys. As a solid state welding method, friction stir welding (FSW) exhibits unique advantages in joining Mg alloys, and thus arouses widespread research interest. This paper emphatically reviewed the research status of conventional friction stir butt-welding of Mg alloys, and highlighted the welding process, microstructure evolution, texture characteristics, mechanical behavior and their interaction mechanisms. It was indicated that the texture plays a vital role in FSW joint performance of wrought Mg alloys, which is quite different from that in the FSW Al alloy joints. The specific strong texture formed in the weld is the main factor that gives rise to the impediment to achieving equal-strength joints to base materials. At the same time, some focuses like the weldability and the factors that influence joint performance in other types of FSW like lap welding, spot welding and double-sided welding; the weldability, interface bonding mechanism, joint performance and its affecting factors and optimization methods in dissimilar FSW between Mg alloys and other materials like Mg alloys of other grades, Al alloys and steels, were summarized and discussed. Finally, the future research and development directions in FSW of Mg alloys were prospected.

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The Dynamic Behavior Hidden in the Long Time Scale of Metallic Glasses and Its Effect on the Properties
Weihua WANG, Peng LUO
Acta Metall Sin    2018, 54 (11): 1479-1489.   DOI: 10.11900/0412.1961.2018.00247
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Metallic glasses (MGs) have disordered microstructure and no defects like in crystalline materials and possess a suite of outstanding mechanical and functional properties, showing thus promising potential for wide applications. Due to the lack of long range structural order, it is fraught with difficulties to construct the structure-property relationship in amorphous materials. The study of relaxation dynamics provides a very important approach to understand MGs, and is vital to understand their stability and deformation behavior and remains a core issue in the field of condensed matter physics and materials science. In recent years, with the use of more advanced research methods and the deepening of research, it was found that there exists rich dynamics covered by the extremely wide time scale and the different length scales of glassy state. Different dynamic modes not only correlate with each other but also show distinction. This article reviews recent progress in the study of relaxation dynamics in MGs, and its role in understanding and modifying material properties and optimizing material preparation.

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EFFECT OF ALUMINUM ON THE SOLIDIFICATION MICROSTRUCTURE OF M2 HIGH SPEED STEEL
ZHOU Xuefeng , FANG Feng , TU Yiyou , JIANG Jianqing , XU Huixia , ZHU Wanglong
Acta Metall Sin    2014, 50 (7): 769-776.   DOI: 10.3724/SP.J.1037.2013.00621
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The effect of aluminum on the solidification microstructure of M2 high speed steel, particularly the morphology and microstructure of eutectic carbides, has been investigated by OM, TEM, SEM, EBSD and XRD. The results show that the as-cast microstructure consists of dislocation martensite and M2C eutectic ledeburite. Excessive amount of aluminum, 1.2%, favors the formation of ferrite and needle-like carbides. After the addition of aluminum, eutectic carbides are distributed more homogeneously. Additionally, the morphology of M2C eutectic carbides transforms from the fibrous to the plate-like, and their microstructure also changes significantly. The plate-like M2C has crystal defects, such as micro-twins and stacking faults, and different growing orientation between adjacent plates whereas the fibrous carbides have few defects and single crystal orientation. Compared to fibrous carbides, the plate-like carbides are much difficult to get spheroidized at high temperature, which is unfavorable for carbide refinement. The ferrite, formed by adding excessive amount of aluminum, cannot be eliminated by ordinary heat treatments, decreasing remarkably the hardness of high speed steel after quenching.
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INTEGRATED SIMULATION OF THE FORGING PROCESS FOR GH4738 ALLOY TURBINE DISK AND ITS APPLICATION
LI Linhan, DONG Jianxin, ZHANG Maicang, YAO Zhihao
Acta Metall Sin    2014, 50 (7): 821-831.   DOI: 10.3724/SP.J.1037.2013.00675
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In order to control the grain size of forged turbine disk of wrought superalloy like GH4738 more effectively, constitutive equations and grain structure evolution models of GH4738 alloy are used in Deform 3DTM for achieving integrated simulation of whole forging process of GH4738 alloy turbine disk (from preheating billet for upsetting to die forging). By using of integrated simulation, the variation of temperature, average grain size, etc., during the whole forging process has been explored, making it possible to control these parameters quantitatively. Comparing with traditional simple stage simulation, results of integrated simulation are more consistent with corresponding experimental results of forged turbine disk (300 mm in diameter). Therefore, the reliability of the integrated simulation is verified. Finally, with the application of integrated simulation, GH4738 alloy turbine disk with a diameter of 1450 mm has been successfully forged by 8×104 t forging press. This work provides a more practical simulation method for helping the process design of forging large turbine disk.
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MICROSTRUCTURE AND MECHANICAL PROPERTIESOF A Ni-BASED SUPERALLOY WITH REFINED GRAINS
YANG Jinxia, SUN Yuan, JIN Tao, SUN Xiaofeng, HU Zhuangqi
Acta Metall Sin    2014, 50 (7): 839-844.   DOI: 10.3724/SP.J.1037.2013.00745
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A new Ni-based superalloy with the refined grains is to be used in industrial and aircraft turbines because of its high strength and excellent fatigue resistance at lower and medium temperatures (500~800 ℃). The grains with six different sizes have been made by decreasing the pouring temperature from 1460 to 1480 ℃ then 1500 ℃ and adding refiner to alloy and planting seed on the surface of mold. The size of equiaxed crystal grain is reduced to 0.5 mm in the center part of specimen with the columnar crystals in the outside of specimen made by the refining process which is finer than those of traditional process. It has been found that g' phase and carbide are finer in refined grains than those in the coarse grains made by decreasing the pouring temperature. The room-temperature tensile properties and high cycle fatigue properties of tested alloy are improved with decreasing grain size. The stress-rupture properties are increased under the conditions of 760 ℃ and 662 MPa while are decreased with decreasing the grain size. The grain structure and size are refined by the refining process that dominated the excellent mechanical properties of tested alloy at lower and medium temperatures. However, it is not good for the mechanical properties at high temperatures.
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PRECIPITATION BEHAVIOR AND PRECIPITATION STRENGTHENING OF NANOSCALE CEMENTITE IN CARBON STEELS DURING ULTRA FAST COOLING
WANG Bin, LIU Zhenyu, Feng Jie, ZHOU Xiaoguang, WANG Guodong
Acta Metall Sin    2014, 50 (6): 652-658.   DOI: 10.3724/SP.J.1037.2013.00584
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In recent years, the precipitation strengthening by cementite, which is a common and economical second phase constituent in steels, has drawn renewed attention in the context of precipitation strengthening, because if cementites can be effectively refined to the scale of a few nanometers, they can induce significant precipitation strengthening effect. Therefore, nanoscale cementite is viewed as a viable option to replace precipitates of microalloying elements for reducing alloy costs in steel products. Given that cementites are usually to form lamellar pearlite structure in a traditional cooling process and generally tend to coarsen at relatively high temperatures, the thermodynamic feasibility for the formation of nanoscale cementite precipitates during cooling has been determined in the previous study, and the non-equilibrium precipitation of nanoscale cementite can be realized by increasing the cooling rate after hot rolling. Thus, the ultra fast cooling (UFC) technology was applied after the hot strip rolling for the research of precipitation behavior and precipitation strengthening of nanoscale cementite in carbon steels. The experimental results demonstrated that the UFC technology shows the unique effects on strengthening in carbon steels and a large number of dispersed nanoscale cementite precipitates with the size of 10~100 nm have been formed in 0.17%C and 0.33%C steels. The nanoscale precipitation of cementite was realized in the microstructure by UFC without the microalloy elements addition. Both the yield strength and tensile strength of the steels increased gradually with the stop temperature of UFC decreasing, and the yield strength increments of 0.17%C and 0.33%C steel were more than 100 MPa, when the stop temperature of UFC decreased from 890 ℃ to 600 ℃. Besides, thermomechanical treatment (TMT) process was introduced after UFC to explore uniform nucleation of cementite in hot-rolled carbon steels, and it is a feasible way to realize the uniform precipitation of nanoscale cementite in the entire miscrostructure for the further strengthening improvement. This was accomplished by subjecting the UFC cooled steel to a small degree of plastic deformation, with the aim to increase the dislocation density evidently. By combining UFC and TMT processing, the yield strength of 0.17%C steel is greater than 600 MPa, leading to a superior strengthening effect.
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DYNAMIC SOFTENING MECHANISM OF 2099 ALLOY DURING HOT DEFORMATION PROCESS
ZHANG Fei , SHEN Jian , YAN Xiaodong , SUN Jianlin , JIANG Na , ZHOU Hua
Acta Metall Sin    2014, 50 (6): 691-699.   DOI: 10.3724/SP.J.1037.2013.00718
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Dynamic softening mechanism of 2099 alloy was investigated by isothermal compression tests, thermal activation parameters calculation and comparison, EBSD and TEM techniques. On the basis of Zener-Hollomon parameter (Z) and deformation temperature (T), combining thermal activation parameters and microstructures analysis, the softening mechanism of 2099 alloy during hot deformation has been proposed. Dislocation cross slip plays a dominant role under the conditions of lnZ≥35.5 and T≤380 ℃. While, deformation mechanisms such as cross slip, climb of dislocation and unzipping of attractive junction play a joint role when lnZ≤37.4 and T≥340 ℃. Particularly, dynamic recrystallization occurred in the range of lnZ≤35.1 and T≥420 ℃, cross slip, climb and unzipping of dislocation and dynamic recrystallization are the main softening mechanisms in this condition. Dynamic recrystallization nucleation mechanisms are constituted of grain boundaries bulging nucleation and subgrain rotated induced nucleation, and the latter becomes more significant with increasing temperature and decreasing strain rate.
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INVESTIGATION OF MICROSTRUCTURE ANDTEXTURE OF b PHASE IN A FORGED TC18TITANIUM ALLOY BAR
LI Kai , YANG Ping , SHA Aixue , YAN Mengqi
Acta Metall Sin    2014, 50 (6): 707-714.   DOI: 10.3724/SP.J.1037.2014.00003
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To understand the differences in the mechanical properties between the center and surface regions of large size forged TC18 titanium alloy bar, electron backscatter diffraction (EBSD) technique is applied to reveal the differences in textures and strains in b phase in addition to the microstructures observation. The influence of the states of b phase on the deviation to the Burgers orientation relationship (OR) between a /b phases is also analyzed according to the acquired EBSD information. It is found that the b phases in different positions of forged bar are in different states of strain, textures and grain sizes. The b phase in the center shows strong <100> texture and coarse and inhomogeneous grain sizes which all contribute to the difference in properties. The spheroidization of ap phase proceeded mainly within b grains at subgrain boundaries and the OR between two phases changed slightly as the lamella ap phase transforms into block-like ap phase, but changed strongly during transforming to globular morphology. The misorientation in b phase is an effective parameter to evaluate the contribution of work-hardening state for the enhancement of strength, whereas the level of the deviation to Burgers OR is an effective parameter to evaluate the spheroidizing rate and the recovery extent.
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EFFECT OF ULTRA-FAST CONTINIOUS ANNEALING ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF LOW Si GRADE Nb-Ti MICROALLOYING TRIP STEEL
LUO Zongan, LIU Jiyuan, FENG Yingying, PENG Wen
Acta Metall Sin    2014, 50 (5): 515-523.   DOI: 10.3724/SP.J.1037.2013.00623
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Si-containing transformation induced plasticity (TRIP) steel is noted for good balance of excellent formability and high strength as the advanced high strength steel (AHSS). The advantage of this steel can be attributed to the TRIP effect, which is the transformation of the retained austenite. Furthermore, the local increase in specific volume caused by the TRIP effect can help to close propagating cracks. It is favorable for the automotive structural components based on the high work hardening rate and energy absorption behavior. Low Si-containing can optimize the galvanized performance of the cold rolling TRIP steel, and the ferrite stabilization can be compensated by adding Al. Microalloying with Nb and Ti may provide effective means for further strengthening via grain refinement and precipitation strengthening. The ultra-fast continuous annealing comprised of rapid heating and short austempering is a new-style process for grain refinement and precipitation solidifying. However, the influences of the process on the cold rolling low Si TRIP steel, especially the austenite transformation characteristics and their effects on microstructure and mechanical properties, were rarely reported. Therefore, in this work the microstructures of low Si grade Nb-Ti microalloying TRIP steel under different ultra-fast continuous annealing conditions were observed via EBSD and TEM, and the tensile properties were discussed. The results show that the polygonal ferrite is refined by heating rate of 100 ℃/s and short asutempering procedure. The dispersive and fine microalloyed carbonitrides formed during the hot-rolling stage are reserved. Therefore, the strength and ductility are enhanced simultaneously. The slow cooling procedure can effectively contribute to eliminate the yield point, while the strength is slightly decreased. As the annealing temperature increasing, the strength is enhanced. When the annealing temperature is 830 ℃, the morphology of retained austenite consists of alternated film and bainite-ferrite plates, resulting in optimal combination of strength and ductility: tensile strength 748 MPa, yield strength 408 MPa, uniform elongation 21.3%, work hardening exponent 0.27, balance of strength and ductility is 15932.4 MPa·%.
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