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

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Review on Research Progress of Steel and Iron Wear-Resistant Materials
WEI Shizhong, XU Liujie
Acta Metall Sin    2020, 56 (4): 523-538.   doi:10.11900/0412.1961.2019.00370
Accepted: 24 December 2019

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In this paper, the development history of iron and steel wear-resistant materials is introduced, and the composition, microstructure, wear property, antiwear mechanism and modification technology of three typical wear resistant materials, namely high manganese steel, high chromium cast iron and high vanadium high-speed steel, are mainly reviewed. The wear-resistant steel represented by high manganese steel relies on the matrix with high strength and toughness to resist wear, while the wear-resistant alloy represented by high chromium cast iron and high vanadium high-speed steel mainly relies on the wear-resistant phase with high hardness to resist wear. High vanadium high speed steel has better wear resistance than high chromium cast iron, which is related to VC characteristics with high hardness and good shape. It is proposed that high performance wear-resistant materials should have three elements: high strength and toughness matrix, multi-scale synergistic action of high quality wear-resistant phase with high hardness and good morphology, as well as good bonding interface between wear-resistant phase and matrix.

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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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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
Accepted: 25 September 2018

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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
Accepted: 07 September 2018

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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
Accepted: 19 July 2018

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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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MICROSTRUCTURE, MECHANICAL PROPERTIES AND INTERPHASE PRECIPITATION BEHAVIORS IN V-Ti MICROALLOYED STEEL
CHEN Jun , Lü Mengyang , TANG Shuai , LIU Zhenyu , WANG Guodong
Acta Metall Sin    2014, 50 (5): 524-530.   doi:10.3724/SP.J.1037.2013.00681
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The microstructure, mechanical properties and precipitation behaviors in a low carbon V-Ti microalloyed steel were investigated using thermal simulation. The microstructural characteristics of tested steel were analyzed using OM and TEM. The results show that the larger volume fraction of ferrite can be obtained for different isothermal temperatures. The ferrite volume fraction is increased and ferrite grain size is reduced as the isothermal temperature is lowered. The planar interphase precipitation can be observed for different isothermal temperatures, and both sheet spacing and precipitates size are refined by lowering isothermal temperature. Moreover, the nanometer-sized carbides have a NaCl-type crystal structure with a lattice parameter of about 0.436 nm and they can obey one variant of Baker-Nutting (B-N) orientation relationship of (100)carbide//(100)ferrite and [011]carbide//[001]ferrite. The precipitation hardening for the specimen treated at 680 ℃ for 30 min can reach 360.6 MPa.
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EFFECT OF ALLOY ELEMENTS PARTITIONINGBEHAVIOR ON RETAINED AUSTENITE ANDMECHANICAL PROPERTY IN LOW CARBONHIGH STRENGTH STEEL
TIAN Yaqiang , ZHANG Hongjun , CHEN Liansheng , SONG Jinying , XU Yong , ZHANG Shihong
Acta Metall Sin    2014, 50 (5): 531-539.   doi:10.3724/SP.J.1037.2013.00709
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The C content in high strength steel must be controlled at a lower level for the good weldability. However, the lower level of C content will reduce the C partitioning efficiency and influence the stability of retained austenite, which leads to the decrease of the product of tensile strength and elongation of high strength steel. A novel preparation mechanism of high strength steel is to employ some kind of substitutional alloying elements, for example Mn, instead of C to partitioning to enhance the austenitic stability, which would not remarkably reduce the weldability of the steel. One low alloy C-Si-Mn steel was used in present work. The Mn partitioning behavior and its effect on the stability of the retained austenite and the mechanical property were studied by means of intercritical annealing, subsequent austenitizing, then quenching and partitioning process (I&Q&P). The results show that in the process of intercritical annealing at 760 ℃, by extending the annealing time, austenite volume fraction increases gradually until it reaches the saturation, meanwhile the Mn partitioning behavior occurs and Mn content increases gradually from ferrite to austenite until it reaches the chemical potential balance in two phases. The sample is heated to 930 ℃ for 120 s, then rapidly quenching to 220 ℃, the carbon diffuses from martensite to austenite phase in the process of partitioning. After I&Q&P process, the tensile strength of experimental steel is 1310 MPa, elongation up to 12%, the product of strength and elongation up to more than 15000 MPa·%. The steel only contains a small amount of retained austenite by only C partitioning after traditional Q&P process, while the steel contains more Mn-rich retained austenite after I&Q&P process. Hence, the content and stability of retained austenite of steel can be improved significantly, which enhance the formability at room temperature.
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MECHANICAL BEHAVIORS AND MICRO-SHEAR STRUCTURES OF METALS WITH DIFFERENT STRUCTURES BY HIGH-SPEED COMPRESSION
SUN Xiurong,WANG Huizhen,YANG Ping,MAO Weimin
Acta Metall Sin    2014, 50 (4): 387-394.   doi:10.3724/SP.J.1037.2013.00634
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Dynamic compression tests on high manganese TRIP steel, pure copper, IF steel and armor steel were conducted on Hopkinson bar at the strain rate of 103~104 s-1 to make comparisons of impact resistance and microstructural features. Results show that under dynamic compression, adiabatic shear bands (ASBs) do not occur easily on pure copper and IF steel. In addition, both pure copper and IF steel show a weak resistance to impact loading due to the poor work hardening capability. The ASB occurs quickly in armor steel containing martensite and the steel shows higher residual strength, which renders it suitable application in the condition of high speed deformation. TRIP steel consisting mainly of austenite has the highest work hardening rate and the α′-M induced by deformation can delay the ASBs formation and prevent the crack extension, manifesting that it is suitable for the use at high speed deformation. Elongated subgrains and low angle grain boundaries are found within the shear bands in pure copper and IF steel with weak microtextures, whereas the ASBs in both TRIP steel and armor steel demonstrate small equiaxed grains and high angle grain boundaries. Strong fcc shearing-type microtexture of {111}-{112}<110> and weak bcc shearing-type microtexture of {110}<111> are formed within ASBs of TRIP steel and armor steel respectively.
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EFFECT OF Nb ON TRANSFORMATION ANDMICROSTRUCTURE REFINEMENT INMEDIUM CARBON STEEL
WU Si,LI Xiucheng,ZHANG Juan,SHANG Chengjia
Acta Metall Sin    2014, 50 (4): 400-408.   doi:10.3724/SP.J.1037.2013.00538
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Medium carbon steel is widely used in structural steels because of its favorable strength, but lack of toughness is a limitation in industrial applications. Among the different strengthening mechanism, grain refinement is the only method to improve both strength and toughness simultaneously. The toughness of steels can be affected by micro-alloying elements and microstructures, for medium carbon wheel steel, the fracture toughness is proportional to the square root of ferrite fraction and inversely proportional to the cube root of prior austenitic grain size. In this work, Nb micro-alloying is used to improve mechanical properties of medium carbon steel. Microstructures and mechanical properties of Nb-bearing medium carbon steel were studied in contrast with traditional Nb-free steel. The continuous cooling transformation (CCT) curves of investigated steels were drawn by adopting dilatometry and metallographic method. The typical microstructures were observed by OM and SEM with EDS. The morphologies of precipitates were obtained by TEM. The effects of cooling rates on microstructure and hardness of the steel were studied with the above experimental methods. The results showed that the typical microstructure of medium carbon steel was ferrite and pearlite and the volume fraction of ferrite was increased from 4% to 24% by adding 0.06%Nb with refined microstructure. The yield strength of Nb-bearing steel was improved from 385 to 455 MPa and the Charpy V-notch energy at -20 ℃ was increased from 7 to 19 J in the condition of almost no reduction in tensile strength. It is because of Nb addition, which makes the transformation products of medium carbon steel be composed of ferrite and pearlite in a wider region of cooling rates (≤10 ℃/s) and a broader temperature range (530~690 ℃), with the hardness lower than 300 HV. With the calculation of Thermal-Calc software and solid solubility formula, Nb exists in medium carbon steel in the form of precipitate. The result of observation by TEM indicates the size of Nb precipitates was distributed in 20~50 nm. To sum up, the grain refining and precipitation strengthening are the main mechanism of Nb to promote the ferrite-pearlite transformation and improve toughness in medium carbon steel.
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FINITE ELEMENT SIMULATION OF THE EFFECT OF STRESS RELAXATION ON STRAIN-INDUCED MARTENSITIC TRANSFORMATION
FENG Rui,ZHANG Meihan,CHEN Nailu,ZUO Xunwei,RONG Yonghua
Acta Metall Sin    2014, 50 (4): 498-506.   doi:10.3724/SP.J.1037.2013.00559
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Near 50 years ago, transformation induced plasticity (TRIP) effect was proposed and TRIP steels as an advanced high strength one are widely investigated. However, the mechanism of TRIP effect can be only qualitatively explained, and has not been experimentally and theoretically verified so far. In this work, a strain equivalent model for strain-induced martensitic transformation was built in a microstructure-based finite element model of novel quenching-partitioning-tempering (Q-P-T) steel. With the model, the TRIP effect under the condition of uniaxial tension was simulated, from which the micro-mechanism of TRIP effect is revealed. Stress relaxation from TRIP relieves the stresses within untransformed retained austenite and its adjacent martensite and blocks the formation of cracks, meanwhile, a considerable retained austenite still exists at higher strain level, which is the origin of TRIP effect. Compared with original (thermal-induced) martensite, fresh (strain-induced) martensite bears higher stress. Therefore, it could be predicted that cracks form at first in fresh martensite or its boundaries. Moreover, stress relaxation makes strain-induced martensite formed in intermittent and slow way, and this is consistent with experimental results. However, in stress-free relaxation state fresh martensite appears in successive and quick way, not consistent with experiments, and thus this verifies in opposite way that TRIP effect inevitably produces stress relaxation.
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EFFECTS OF WELDING THERMAL CYCLE AND AGING TREATMENT ON THE MICROSTRUCTURE AND MECHANICAL PROPERTY OF A Ni-Fe BASE SUPERALLOY
WU Dong ),WANG Xin,),DONG Wenchao ),LU Shanping )
Acta Metall Sin    2014, 50 (3): 313-322.   doi:10.3724/SP.J.1037.2013.00355
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Increasing the steam temperature and pressure of boilers in super-ultracritical power plant is an important approach to increase the plant efficiency. The steam temperature of the most efficient coal power plant is now around 620 ℃, representing an increase of about 80 ℃ in the past 40 years, which owes to the high temperature properties improvement of boiler components, such as the superheater and the reheater. Nickel base superalloy, for example Inconel 740 and Inconel 617, is being developed by some countries for the material requirement of 700 ℃ super-ultracritical power plants. Meanwhile, weldability investigation is necessary for the developing materials since welding plays a key role on the construction of coal power plant boilers. In this work, the weldability of a kind of Ni-Fe base superalloy, one of the candidate materials for the high temperature components of 700 ℃ ultra-supercritical coal plant is studied. By welding thermal simulator (Gleeble 1500) experiments, the variation and evolution of mechanical properties and microstructure were analyzed for this Ni-Fe base superalloy, under welding thermal cycle treatment condition and aging treatment condition after welding thermal cycle. After the welding thermal cycle with a peak temperature of 1249 ℃, both the yield strength and tensile strength for solutioned Ni-Fe base superalloy at 25 and 700 ℃ were decreased, along with the increasing of ductility. After aging treatment to the Ni-Fe base superalloy experienced a welding thermal cycle, the yield strength and tensile strength at 25 ℃ were similar with those of the aged base metal. At 700 ℃, the strength of the heat affected zone (HAZ) after aging treatment is higher than that of the aged Ni-Fe base superalloy. Microstructure analysis showed that the γ' phase and MC carbide in Ni-Fe base superalloy dissolved during the high temperature welding thermal simulation experimental process. The solution of carbides in the grain boundaries caused a loss of a pinning effect on the migration of grain boundary and a decreasing of the strength. After the aging treatment to the Ni-Fe base superalloy experienced a high temperature welding thermal cycle, γ' and M23C6 carbide were precipitated. The precipitation of M23C6 at the grain boundaries during aging treatment was mainly due to the supply of the carbon from the MC which had been dissolved partially during former welding thermal cycle.
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EFFECT OF TEMPERATURE AND CONCENTRATION RATIO ON PITTING RESISTANCE OF 316L STAINLESS STEEL IN SEAWATER
XIN Sensen, LI Moucheng, SHEN Jianian
Acta Metall Sin    2014, 50 (3): 373-378.   doi:10.3724/SP.J.1037.2013.00314
Abstract225)   HTML14)    PDF (5138KB)(570)      
Due to a serious shortage of natural fresh water in many areas all over the world, the seawater desalination has emerged as an effective compensation way to meet the consumption requirements. Due to the good corrosion resistance and low cost, stainless steels have been used extensively to construct the multi effect distillation (MED) plants, especially type 316L stainless steel for the evaporation chambers. However, with the application and development of low temperature MED, there is increasingly need of higher temperature distillation and higher brine concentration in the desalinators to reduce the drainage of hot brine and increase the water production ratio, which may cause more serious corrosion on the stainless steel components in the plants. Pitting corrosion of 316L stainless steel was studied in the concentrated environments of seawater with different temperatures (25, 50, 63, 72, 85 and 95 ℃) and concentration ratios (1, 1.5, 2, 2.5 and 3 times) by using cyclic anodic polarization measurement and SEM surface observation. The results show that both pitting potential and repassivation potential of 316L stainless steel decrease linearly with temperature in the concentration ratio range of 1 to 3 times for seawater, but the change of pitting potential is very slight when the solution temperature is higher than 85 ℃ in the case of concentration ratio larger than 2 times. Both pitting potential and repassivation potential reduce linearly with the logarithm of the concentration ratio of seawater in the range of 25 to 95 ℃. It is apparent that increasing temperature and concentration ratio of seawater will deteriorate the pitting resistance of 316L stainless steel noticeably. The influence of temperature and concentration ratio is analyzed on the basis of the point defect model. Nevertheless, the concentration ratio of seawater has a weaker influence on pitting resistance of 316L stainless steel in comparison with temperature as revealed by the pitting potential changes resulted from the concentration ratio around 1.5 times and solution temperature around 72 ℃. Therefore, compared with temperature, the corrosion resistance of 316L stainless steel for low temperature MED plants may be relatively tolerant of the adjustment or fluctuation of seawater concentration.
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MICROSTRUCTURE AND MECHANICAL PROPERTY OF LASER WELDED JOINT FOR HYPOEUTECTOID U-Nb ALLOY
LI Yubin,WANG Wei,HE Jianjun,ZHANG Zhiqiang,ZHANG Tongyan
Acta Metall Sin    2014, 50 (3): 379-386.   doi:10.3724/SP.J.1037.2013.00493
Abstract223)   HTML9)    PDF (12526KB)(756)      
The microstructures and mechanical properties of hypoeutectoid U-Nb alloy laser welded joint were investigated by optical microscopy (OM), X-ray diffractometer (XRD), transmission electron microscopy (TEM), split hopkinson pressure bar (SHPB) and other analysis apparatus. The results show that the microstructure of hypoeutectoid U-Nb alloy base metal is α-U+γ-U lamellar pearlite under isothermal heat treatment, while the laser welding seam is composed of α' lath martensite for pre-heated or α' twin martensite for no pre-heated with orthogonal crystal structure. The quasi-static tensile strength of welded joint (about 400 MPa) is much less than base metal and microstructures of weld, for the main reason of incomplete penetration weld and low fracture toughness. Between dynamic impact loading for base and welded joint, the strain rate of welded joint is lower than base metal, and the yield strength of welded joint is higher. Also, the compressive stress-strain curves indicated that the flow stresses for welded joint increased with the increase of strain rate and the obvious effect of strain rate hardening has been observed. At strain rate of 2000 s-1, selected plastic deformation taking place in welded joint is due to the tremendous difference mechanic properties between weld seam and base metal, and the adiabatic shear band(ASB) only appears in the rest of welded joint.
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PLASTIC DEFORMATION MECHANISMS IN NANOTWINNED METALS
LU Lei,YOU Zesheng
Acta Metall Sin    2014, 50 (2): 129-136.   doi:10.3724/SP.J.1037.2013.00697
Abstract376)   HTML27)    PDF (2381KB)(2045)      
A brief overview is provided about the plastic deformation mechanisms in nanotwinned metals. The unique two-dementional nanoscale twin lamellae lead to different dislocation slip systems activated during plastic deformation. It has been revealed that there are three distinctly different dislocation-mediated deformation mechanisms in nanotwinned metals, namely dislocation pile-up against and slip transfer across twin boundaries, Shockley partials gliding on twin boundaries leading to twin boundary migration, and threading dislocations slip confined by neighboring twin boundaries. It is further demonstrated that these three dislocation-mediated mechanisms are switchable upon changing in the loading direction with respect to twin boundaries.
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PREPARATION TECHNIQUES FOR NANO-STRUCTURED METALLIC MATERIALS VIA PLASTIC DEFORMATION
TAO Nairong,LU Ke
Acta Metall Sin    2014, 50 (2): 141-147.   doi:10.3724/SP.J.1037.2013.00803
Abstract235)   HTML47)    PDF (2183KB)(1228)      
This work summarized the deformation techniques of preparing the nanostructured metallic materials, including large-strain deformation techniques (clod rolling, accumulative cold-bonding, equal channel angular pressing, high pressure torsion), high-strain-rate deformation technique (dynamic plastic deformation), and high-strain-gradient deformation techniques (surface mechanical attrition treatment and surface mechanical grinding treatment). The effects of deformation modes and deformation parameters on grain refinement are analyzed. Future trends and challenges of the deformation techniques for preparing nanostructured metallic materials are discussed.
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INFLUNECE OF STACKING FAULT ENERGY ON THE MICROSTRUCTURES, TENSILE AND FATIGUE PROPERTIES OF NANOSTRUCTURED Cu-Al ALLOYS
AN Xianghai,WU Shiding,ZHANG Zhefeng
Acta Metall Sin    2014, 50 (2): 191-201.   doi:10.3724/SP.J.1037.2013.00591
Abstract257)   HTML22)    PDF (11089KB)(2063)      
Influences of stacking fault energy (SFE) on the microstructures, tensile properties and fatigue behaviors of nanostructured (NS) Cu-Al alloys prepared by severe plastic deformation (SPD) were systematically summerized. With the reduction of SFE, it is found that the dominant formation mechanism of nanostructures gradually transformed from the dislocation subdivision to the twin fragmentation and the grain sizes also decrease; while microstructural homogeneity is achieved more readily in the materials with either high or low SFE than in the materials with medium SFE. The strength of NS Cu-Al significantly increases with decreasing the SFE, while there is an optimal SFE for the ductility of these materials. More significantly, the strength-ductility synergy of Cu-Al alloys is prominently enhanced with reducing the SFE. Finally, simultaneous improvements of low-cycle fatigue and high-cycle fatigue properties of NS Cu-Al alloys were achieved with decreasing the SFE. This can be attributed to the enhanced microstructure stability and the reduced strain localization in shear bands. With the reduction of SFE, the fatigue damage micro-mechanism was also transformed from grain boundary (GB) migration to other GB activities such as, atom shuffling, GB sliding and GB rotation.
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MICROSTRUCTURE AND MECHANICAL PROPERTIES OF FRICTION STIR PROCESSED ULTRAFINE-GRAINED AND NANOSTRUCTURED Cu-Al ALLOYS
XUE Peng,XIAO Bolü,MA Zongyi
Acta Metall Sin    2014, 50 (2): 245-251.   doi:10.3724/SP.J.1037.2013.00661
Abstract194)   HTML26)    PDF (4287KB)(708)      
Ultrafine-grained (UFG) and nanostructured (NS) materials have attracted considerable interest due to their special microstructure and mechanical properties. Severe plastic deformation is one of the optimum approaches to fabricate bulk, dense and contamination-free UFG and NS metallic materials. However, high density of dislocations and unstable microstructure were usually induced in these UFG and NS metallic materials, resulting in poor tensile plasticity and fatigue properties. In this study, bulk UFG and NS Cu-Al alloys were successfully prepared via friction stir processing (FSP) with additional forced water cooling. FSP Cu-Al alloys exhibited uniform recrystallized microstructure with equiaxed ultrafine grains, and the grain sizes reduced gradually as the stacking fault energy (SFE) decreased. Abundant nano-twin layers formed in the ultrafine grains of FSP Cu-Al alloys with low SFEs, which further refined the ultrafine grains and NS microstructure was achieved. The strength of the FSP Cu-Al alloys increased clearly with decreasing the SFEs due to the gradually refined microstructure, but the uniform elongation increased initially and then decreased in the Cu-Al alloy with the lowest SFE.
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MICROSTRUCTURES AND PROPERTIES OF 0Cr32Ni7Mo4N DUPLEX STAINLESS STEEL AFTER VARIOUS FORMING PROCESSES
HE Hong,LI Jingyuan,QIN Liyan,WANG Yide,FANG Fei
Acta Metall Sin    2014, 50 (1): 1-10.   doi:10.3724/SP.J.1037.2013.00393
Abstract477)   HTML12)    PDF (17483KB)(1756)      

Duplex stainless steels consist of a two phase microstructure involving α-ferrite and γ-austenite. These alloys have a remarkable combination of mechanical properties together with good corrosion resistance under critical working conditions and are suitable for marine and petro-chemical applications. However, the poor hot workability of these materials makes the industrial processing of flat products particularly critical. Many investigations focus on the mechanisms and behaviors of hot deformation on these materials. Several factors are frequently reported give rise to hot cracking: phase proportions, size and morphology of both phases, softening mechanisms in constituting phases, microstructural evolution during hot work, and strain partitioning between α and γ. On the contrary, few studies have been carried on cold rolling performance. Hot cracking should be avoid during forming process of duplex stainless steel, the more effective way of manufacturing in such materials is also needs research. In this work, the formability of 0Cr32Ni7Mo4N duplex stainless steel was studied in the hot rolling and directly cold rolling processes. The deformation mechanism of α and γ phase at room temperature, the microstructure evolution after hot rolling, cold rolling and solution treatment were investigated. Mechanical properties and corrosion resistance of two kinds of cold-rolled sheets were tested. The metallography and corrosion morphology were observed by OM and SEM. The results show that cracks emerged along the edge of hot-rolled plate even it was reheated three times, and it has good cold rolling formability after cutting edge of the plate. On the other hand the as-cast billet solution-treated at 1100 ℃ has good cold rolling performance. Deformation mechanism of α phase at room temperature is that multi-slip system form dislocation cell structure, while single slip model and mechanical twins appear in γ phase. As the temperature of heat-treatment raised, microstructure became more homogeneous and the amount of precipitate particles decreased. The experimental results show that the tensile strength of cold-rolled sheet after heat-treatment reaches 1082.9 MPa and the elongation is 29.3%. Critical pitting potential of the specimen in 3.5%NaCl liquor is 1060 mV; weight loss after intergranular corrosion in 65%HNO3 solution is 0.05 g/(m2·h).

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EFFECT OF GRAIN BOUNDARY ANGLE ON STRESS RUPTURE PROPERTIES OF A Ni-BASED BICRYSTAL SUPERALLOY
CAO Liang,ZHOU Yizhou,JIN Tao,SUN Xiaofeng
Acta Metall Sin    2014, 50 (1): 11-18.  
Abstract304)   HTML16)    PDF (16445KB)(3122)      

Bicrystal slabs with different grain boundary angles were cast to study the effect of varied grain boundary angle on stress rupture properties of a Ni-based bicrystal superalloy. It was found that the stress rupture lives of single crystal specimens were superior to those with grain boundaries. With the increase of grain boundary angle, the stress rupture life was decreased and the fracture type was transferred from trans-granular to inter-granular fracture. The reduced rupture properties was attributed to the inhabitation of grain boundary on slip deformation. With the rise of temperatures, the effect of grain boundaries on rupture properties was enhanced and the critical value of grain boundary angle from trans-granular to inter-granular fracture was decreased. Inter-granular fracture occurred from 12° grain boundary in the rupture test of 871 ℃ and 552 MPa, and it occurred from 4.5° grain boundary in the rupture test of 1100 ℃ and 120 MPa. Since the grain boundary became weaker at higher temperature, the angle of low-angle boundary in single crystal superalloys should be controlled strictly.

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MICROSTRUCTURE AND MECHANICAL PROPERTIES OF Mg-4Zn-2Al-2Sn ALLOYS EXTRUDED AT LOW TEMPERATURES
ZHAO Dongqing,),ZHOU Jixue ),LIU Yunteng ),DONG Xuguang ),WANG Jing ),YANG Yuansheng,)
Acta Metall Sin    2014, 50 (1): 41-48.   doi:10.3724/SP.J.1037.2013.00352
Abstract470)   HTML40)    PDF (11253KB)(13121)      
Due to the high demand of light-weight alloys in automotive applications, wrought magnesium (Mg) alloys, applied as automotive sheet and extrusions, are attracting great attention. However, some inherent disadvantages of common wrought Mg alloys have limited their application, such as poor corrosion resistance, poor creep resistance and low formability. It is well known that Sn can provide thermally stable Mg2Sn particles in the matrix of magnesium alloys. Our previous study shows that the Mg-4Zn-2Al-2Sn alloy has potential to be developed into a wrought Mg alloy. Currently, the microstructure, texture and mechanical properties of Mg-4Zn-2Al-2Sn alloy extruded at temperatures of 225, 250 and 275 ℃ have been investigated, where complete dynamic recrystallization occurred during extrusion and the average grain size was reduced to 4.4, 7.1 and 10.5 μm, respectively. The amount and morphology of the second phases were directly influenced by the extrusion temperature. Extruded at 225 ℃, irregular Mg2Sn phase in size of 20~60 nm precipitated in the grains. With the extrusion temperature increasing to 275 ℃, Mg2Sn of about 500 nm and micron-size Mg32(Al, Zn)49 precipitates were observed. The {0002} texture was formed at 225 and 250 ℃ during the extrusion. While the temperature increased to 275 ℃, due to the activation of prismatic slip system, {1010}<0002> texture of prismatic plane parallel to extrusion direction was also observed. When compressive stress loaded along the extrusion direction, the {1010}<0002> texture suppressed the activation of the tensile twinning {1012}<1011>, which leads to a decrease of asymmetry between tension and compression.
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PHASE FIELD CRYSTAL SIMULATION OF DISLOCA-TION MOVEMENT AND REACTION
GAO Yingjun,LU Chengjian,HUANG Lilin, LUO Zhirong,HUANG Chuanggao
Acta Metall Sin    2014, 50 (1): 110-120.  
Abstract420)   HTML10)    PDF (13829KB)(1127)      

Transformations of grain boundaries often strongly influence both the structure and the properties of polycrystalline and nanocrystalline materials. Thus, plastic deformation processes in fine-grained polycrystals and nanocrystalline solids are associated with transformations of grain boundaries, which crucially affect the structure and mechanical characteristics of such solids. Motion of grain boundary dislocations in plastically deformed materials is commonly considered to be the absorption of lattice dislocations by grain boundaries. In order to reveal the mechanism of motion of a low-angle symmetric tilt grain boundary (STGB) associated with the emission and absorption of lattice dislocation, the emission and evolution of a STGB under strain were simulated by phase-field crystal (PFC) model. The decay of STGB and dislocation reactions of separation, annihilation and mergence and their mechanisms were analyzed from the energy point of view, furthermore, the active energy of the dislocation separation was calculated. The research results show that the low-angle STGB is composed of pair dislocations in a line arrangement in two dimensions of triangular atomic lattice, in which there are two sets of basic Burgers vectors. The evolution process of STGB decay can be divided into six typical stages which includes the detail features as: dislocation climbs firstly along the STGB under strain, then the dislocation occurs to break up into two new dislocations after it gets enough energy to overcome the active potential barrier of dislocation, at this time the STGB emits pair dislocations to move in gliding in grain instead of climbing along STGB; gliding for while, the dislocation crosses the grain until it is annihilated by another dislocation at the STGB right in the front, i.e. the Grain boundary absorbs or merges the gliding dislocation. The remain of dislocation in the STGB can still climb along the grain boundary in which splits off again into two dislocations when it gets enough energy, at the same time it looks as if STGB emits the dislocations and changes the dislocation movement from climbing to gliding again. The dislocation continues gliding until it meets another gliding dislocation in grain to be annihilated, finally the total dislocations are annihilated and the STGB disappears. The two grain systems with STGB become one grain system. The two sets of basic Burgers vectors of lattice dislocation in triangular lattice can validly be used to express the dislocation reaction of emission, separation, mergence, absorption, annihilation, and also can reveal the creation of new Burgers vector and the annihilation of old Burgers vectors and mechanism of the directional change of Burgers vectors during the dislocation reaction.

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CHARACTERISTICS OF A NiAl EUTECTIC ALLOY JJ—3 WITH EXCELLENT PROPERTIES
YUAN Chao, ZHOU Lanzhang, LI Gusong, GUO Jianting
Acta Metall Sin    doi:10.3724/SP.J.1037.2013.00472
CORROSION BEHAVIOR OF COPPER ALLOYS IN DEEP OCEAN ENVIRONMENT OF SOUTH CHINA SEA
SUN Feilong, LI Xiaogang, LU Lin, WAN Hongxia, DU Cuiwei, LIU Zhiyong
Acta Metall Sin    doi:10.3724/SP.J.1037.2013.00142
DEVELOPMENT AND APPLICATION OF ATOM PROBE TOMOGRAPHY
LIU Wenqing, LIU Qingdong, GU Jianfeng
Acta Metall Sin    doi:10.3724/SP.J.1037.2013.00362
MODELING OF MICROPOROSITY FORMATION IN AN Al-7%Si ALLOY
LI Zhengyang, ZHU Mingfang, DAI Ting
Acta Metall Sin    doi:10.3724/SP.J.1037.2013.00128
MICROSTRUCTURE, MECHANICAL PROPERTIES AND STRENGTHENING MECHANISMS OF A Cu BEARING LOW-CARBON STEEL TREATED BY Q&P PROCESS
YAN Shu, LIU Xianghua, LIU WJ, LAN Huifang, WU Hongyan
Acta Metall Sin    2013, 49 (8): 917-924.   doi:10.3724/SP.J.1037.2013.00176
Abstract274)      PDF (1074KB)(813)      

A low carbon steel containing Cu addition was treated by Q&P process using a CAS-200 continuous annealing simulator. The microstructure of the steel was characterized by means of SEM, EBSD, XRD and TEM and its mechanical properties were investigated by tensile testing at room temperature. Cu-rich precipitates formed during the Q&P process were observed as spherical particles in martensitic laths and are 9 nm to 20 nm in diameter. According to the Orowan mechanism, those fine particles may have a contribution to the yield strength of the steel about 134 MPa. Also observed are three different morphologies of the retained austenite phase in the test steel, i.e. thin film--like, fine granular and blocky, formed at different locations. The test steel has a good comprehensive mechanical properties, of which the product of tensile strength and elongation, the tensile strength and the total elongation are as high as 21.2 GPa·%, 1326 MPa and 16 %, respectively. The excellent combined properties can be attributed to the effect of transformation induced plasticity (TRIP) caused by the retained austenite.

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ANALYSIS OF FRACTURE DURING SUPERPLASTIC TENSION
GUAN Zhiping, MA Pinkui, SONG Yuquan
Acta Metall Sin    2013, 49 (8): 1003-1011.   doi:10.3724/SP.J.1037.2013.00078
Abstract496)      PDF (630KB)(713)      

The matter of fracture in tension is also the issue of fracture elongation. The ability of superplasticity of materials is mainly characterized by excellent fracture elongations. Since first superplastic phenomenon was recorded, the investigations of superplasticity have not halted. Most of the existing literatures focused on physical or microstructural mechanisms while less attention was paid to mechanical theories on the superplastic deformation. However, superplastic phenomena on large elongation in superplastic tension are closely related to the mechanical stability and are finally dependent on the special fracture mechanism. Correspondingly, in this article, the studies of fracture mechanism of the superplastic deformation are reviewed, which involved nucleation, growth and coalescence of cavities. Then, the literatures related to the mechanical stability in superplastic tension are classified and reviewed, which involved the mechanical analysis and numerical simulation of the fracture elongation or the limit strain induced from neck’s initiation and development. The conclusions indicate that there has yet been no united and confirmed opinion on the superplastic fracture mechanism which has numerous versions from the microstructural or physical view, and the superplastic fracture mechanism would have maken no significant progress unless many long-term investigations will be carried out in the future. In order to interpret the essence of large fracture elongation, the current task should be thoroughly investigate the mechanical stability in superplastic tension based on the advanced technology of numeric analysis. In numeric analysis, the precise and quantitative constitutive equation should be adopted and the deformation conditons involving strain paths should be taken into account.

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THE ω PHASE IN A LOW ALLOY MARTENSITIC STEEL
PING Dehai, YIN Jiang, LIU Wenqing, SU Yanjing, RONG Lijian, ZHAO Xinqing
Acta Metall Sin    2013, 49 (7): 769-774.   doi:10.3724/SP.J.1037.2013.00206
Abstract554)      PDF (4567KB)(1349)      

The microstructure of a low alloy martensitic steel has been investigated using TEM. It was indicated that the as-quenched plate and lath martensites consist of ferrite matrix and high density of nanometer-scaled ultrafine particles embedded in the matrix. These particles were designated to beω phase with a primitive hexagonal crystal structure. Theω particles exhibit an orientation relationship with the ferrite (α-Fe) matrix as follows: [113]α//[2113]ω,(110)α //(1101)ω and (211)α//(0110)ω, with lattice parameters of aω=21/2aα , cω=31/2/2aα. The results of the present study suggested that the carbon atoms in the steel are not homogenously distributed in the martensites. The ferrite matrix possesses very low content of carbon, and most of the carbon atoms are concentrated in the ω phase.

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NUMERICAL ANALYSIS OF THE WELD BEAD PROFILES IN UNDERWATER WET FLUX-CORED ARC WELDING
ZHAO Bo, WU Chuansong,JIA Chuanbao, YUAN Xin
Acta Metall Sin    2013, 49 (7): 797-803.   doi:10.3724/SP.J.1037.2013.00061
Abstract411)      PDF (1617KB)(870)      

Underwater wet flux-cored arc welding (FCAW) has great potential prospects of wide application in ocean engineering due to its easiness of automation and high weld quality. However, the thermal process of underwater wet welding is more complicated: the arc energy distribution is more concentrated in high-pressure environment of underwater, the convection heat transfer coefficient of the weldment under water is much larger than that in air. This study focuses on establishing the numerical model for analyzing the thermal process and the temperature field in underwater wet FCAW by using the FEM software SYSWELD. Both the generalities and peculiarities of the conventional GMAW (gas metal arc welding) in air and underwater wet FCAW processes are taken into consideration, especially the two remarkable characteristics of underwater wet welding, i.e., the water compressing action to the arc, and the enhanced heat losses caused by the surrounding water. Based on the calculated temperature profiles, the weld bead shape and sizes are predicted in underwater FCAW, which lays the foundation for the process optimization. It is found that under 4 groups of typical welding conditions the calculated weld bead dimensions are in agreement with the experimental ones, which validated the energy distribution pattern of the heat source and the numeric model for underwater wet welding. Experiments showed that the weld bead was thinner and deeper in underwater wet welding than that in conventional GMAW under the same welding parameters, while the variation regularity of weld bead profile is similar.

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MICROSTRUCTURE AND TENSILE PROPERTIES OF Ti-45.5Al-2Cr-2Nb-0.15B ALLOY PROCESSED BY HOT EXTRUSION
LIU Renci, WANG Zhen, LIU Dong, BAI Chunguang, CUI Yuyou, YANG Rui
Acta Metall Sin    2013, 49 (6): 641-648.   doi:10.3724/SP.J.1037.2012.00762
Abstract678)      PDF (4305KB)(889)      

The near isothermal canned hot extrusion at a temperature close to α transus temperature was used to fabricate Ti-45.5Al-2Cr-2Nb-0.15B alloy rod. Microstructures and tensile properties of samples taken from different locations of the extrudate were compared with each other, and the formation mechanism of extrusion microstructure was investigated in combination with the finite element simulation. It was found that lamellar grains were significantly refined by hot extrusion. Microstructure and tensile elongation were homogeneous along the axial direction of extruded rods, but heterogeneous along the radial direction. The center of rods with coarse fully-lamellar microstructure had low tensile elongation, and the edge of rods with fine near lamellar microstructure had high tensile elongation. Such heterogeneities could not be eliminated in subsequent α solid solution treatment. Lamellar grain size decreased with increasing effective strain. There existed the refined homogeneous microstructure in the regions with effective strain larger than 2.25. The difference of microstructure type was mainly due to different temperatures of different parts of rods during extrusion process. In the edge of rod tails, the γ phase lamellar structure precipitated from α phase was formed due to the chilling effect caused by contacting with the cold die, then the lamellar structure with tortuous boundary was formed in subsequent deformation. Tensile elongation was found to decrease with increasing lamellar grain size, but the poor tensile elongation in the center was mainly attributed to the existence of lamellar grains which lamellar boundaries were nearly perpendicular to the extrusion direction.

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