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

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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
Abstract   HTML PDF (11253KB)  
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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THE EFFECT OF MECHANICAL ATTRITION ON THE GRAIN GROWTH OF ELECTRODEPOSOTION Ni COATINGS
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Acta Metall Sin    2008, 44 (6): 751-756 .  
Abstract   PDF (9906KB)  
The mechanical attrition (MA) action was supplied in a traditional electroplating process by the vertical movement of glass balls on the sample surface with a special vibrating frequency. The effect of vibrating frequency and the diameter of glass balls on the grain refinement were observed. The effect of MA on grain growth was also studied. It is demonstrated from FESEM results that the new coating shows smooth and refinement surface morphology by supplying MA. The further studies showed that the grain refinement and slow grain growth could be obtained by supplying MA. The nucleation of new grains increased with the increasing the macro and micro defects on the surface of coating induced by MA. The deposited atom was forced to slide to valley when the glass balls bumped on big grains, which made the coating surface smooth. The uniformity current line could be obtained on the smooth surface, which can increase the nucleation rate and hinder the grain growth.
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THE PRESENT CONDITION AND PROSPECTS ON METAL MATRIX COMPOSITES
WU Renjie (Shanghai Jiaotong University; Shanghai 200030)
Acta Metall Sin    1997, 33 (1): 78-84.  
Abstract   PDF (870KB)  
This article presents a review on the development condition of metal matrix composites (MMCs), includes its recent situations, varieties, production techniques, applications and existent problems. Some researching topics with great demand such as interface structure and behaviours, solidification, manufacturing science on some popular varieties of MMC and in situ technologies are also involved. On the base of these situations, a future development view is prospected.
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High Temperature Creep Behavior Of Cast Ni Base Superalloy K44
HOU Jieshan; ZHANG Yulong; GUO Jianting; JI Guang; ZHOU Lanzhang; YE Hengqiang
Acta Metall Sin    2004, 40 (6): 579-null.  
Abstract   PDF (24617KB)  
High temperature tensile creep behaviors of cast nickel—based superalloy K44 in the temperature range of 850---900℃ and under the applied stress range of 225---380 MPa have been studied. The results indicate that all of the creep curves of test alloys have similar shape: a short primary creep and a dominant accelerated creep stage, and the long accelerated stage is due to the interaction between rafting r’ particles and dislocations. As the creep strain is a softening stage, the accelerated creep can be approximately described by expression. The creep fracture data follow the Monkman--Grant relationship. Cracks originate from the cavities at grain boundary or interdendritic.
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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
Abstract   HTML PDF (18013KB)  
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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EFFECT OF INDUCTION TEMPERING ON CARBIDE PRECIPITATION BEHAVIOR AND TOUGHNESS OF A 1000 MPa GRADE HIGH STRENGTH LOW ALLOY STEEL
FANG Yupei, XIE Zhenjia, SHANG Chengjia
Acta Metall Sin    2014, 50 (12): 1413-1420.   DOI: 10.11900/0412.1961.2014.00306
Abstract   HTML PDF (11158KB)  
By comparing induction tempering with conventional tempering, the effect of induction reheating tempering on carbide precipitation behavior and toughness of a 1000 MPa grade high strength low alloy steel was investigated. Microstructures of the steel in different heat treatment stages were characterized using SEM and TEM (with EDS), mechanical properties inclusive of Vickers hardness and toughness were tested. The results showed that microstructure of quenched samples consisted of lath martensite and lower bainite, needle like carbides were observed in lower bainitic lath. With tempering temperature increasing from 400 ℃ to 550 ℃, the shape of carbides located within the bainitic lath gradually changed from needle like to short rod like type. Carbides were fine and well distributed using induction tempering. When the tempering temperature was 550 ℃, the long axis length of short rod like carbides located within the bainitic lath by conventional reheating tempering was 200 nm, whereas the long axis length of short rod like carbides located within the bainitic lath by induction reheating tempering was about 60 nm. When tempering by conventional reheating, carbides mainly precipitated along martensite lath boundaries, while carbides were more dispersed in the matrix lath by induction reheating, the size of these dispersed carbides was less than 100 nm when tempering temperature was 550 ℃. As a result, a superior of mechanical properties with 344 HV and Charpy impact energy of 133 J at -20 ℃ was obtained with induction reheating tempering at 550 ℃.
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Refinement of Grain and Enhancement of Impact Energy Absorption for Low-Alloy Ultra-High Strength Bainite Steels Containing Silicon
Acta Metall Sin    2007, 43 (3): 315-320 .  
Abstract   PDF (974KB)  
Through judiciously using the silicon, manganese and nickel as the alloying elements and effectively controlling the transformation temperature, cooling and tempering parameters for a series of low alloy ultrahigh strength bainitic steels (LUHSBS), the combination of strength and toughness are very excellent and the impact energy absorption (AKV ≥185 J) has been tripled compared to the previously advanced martensitic steel 23MnNiCrMo with the same strength level (>1500 MPa). The basic reasons giving rise to the enhancement of strength and toughness consist in the increase of carbon content and the refinement of sublaths in the bainitic ferrite (BF) and the very thin films of retained austenite (AR). It is confirmed by the atomicforce microscope (AFM) and scanning tunneling microscope (STM) that there was no any large bulky areas of AR in the structure which is less stable and will reduce the impact toughness of bainitic steels. Not only is the sublaths is significantly refined, but also the average size of subgrains is less than 20 nm and the average thickness of shear units in a BF lath is only about 1.6 nm. The refinement of microstructure, the enrichment of interstitial atom carbon in BF and the increase of both the dislocation density and the volume fraction of AR are some critical factors that affect the ultimate tensile strength, the stability of AR and the impact energy absorption. Furthermore, the physical mechanism on the improvement of the combination of strength and toughness is in depth discussed.
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INFLUENCE OF PARTICLES WITH DIFFERENT SIZES ON MICROSTRUCTURE, TEXTURE AND MECHAN-ICAL PROPERTIES OF Al-Mg-Si-Cu SERIES ALLOYS
PENG Xiangyang, GUO Mingxing, WANG Xiaofeng, CUI Li, ZHANG Jishan, ZHUANG Linzhong
Acta Metall Sin    2015, 51 (2): 169-177.   DOI: 10.11900/0412.1961.2014.00276
Abstract   HTML PDF (11081KB)  
To reduce the weight of car body, Al-Mg-Si-Cu alloys have been used to produce outer body panels of automobiles due to their relatively good formability in the solution treated condition and high strength in the age hardened condition. However, their formability is significantly poor compared to that of steels, which are the major drawbacks to wide-scale application of aluminum in the automotive industry. The microstructural characteristics developed during recrystallization, most notably grain size and crystallographic texture, play a dominant role in controlling the mechanical properties and formability of sheet in the T4 condition. In this work, the effect of particles with different sizes on the mechanical properties, microstructure and texture of Al-Mg-Si-Cu alloys was studied through tensile test, OM, SEM, TEM and EBSD measurement. The results reveal that with increase of solute concentration, the average plastic strain ratio, yield strength and ultimate tensile strength increase, but the elongation decreases and with different extents in the three directions. In addition, the number of observed particles with different sizes in the alloy matrix such as Mg2Si, Al15Mn3Si2 and α-Al(Fe, Mn)Si phases also increases. When the size and concentration of these particles are controlled appropriately, lots of finer recrystallized grains can form during solution treatment due to the particle stimulated nucleation (PSN) effect of coarse particles and pinning effect of finer particles. The main texture components include CubeND18, Goss{011}<100>, P{011}<122> and Cu{112}<111> for the alloy with fine-grained structure. At last, according to the relationship among alloy composition, thermomechanical processing and microstructure, the model of nucleation and growth of recrystallized grains affected by the particles with different sizes was also proposed。
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EFFECT OF SOLIDIFYING RATE ON INTEGRALLY DIRECTIONALLY SOLIDIFIED MICROSTRUCTURE AND SOLID/LIQUID INTERFACE MORPHOLOGY OF AN Nb–Ti–Si BASE ALLOY
WANG Yong GUO Xiping
Acta Metall Sin    2010, 46 (4): 506-512.   DOI: 10.3724/SP.J.1037.2009.00696
Abstract   PDF (4027KB)  

Since temperatures of airfoil surfaces in advanced turbine engines are approaching the limit of nickel base superalloys, Nb–Ti–Si base alloys as their potential materials have attracted much attention recently. Nb–Ti–Si base alloys have high melting temperature, suitable densities, good
elevated temperature creep strength and acceptable room temperature fracture toughness, therefore, they are expected to be employed in the temperature range of 1200—1450 ℃as structural materials. Alloying and directional solidification are generally used to obtain a better combination of room temperaure fracture toughness with high temperature creep strength and oxidation resistance for an elevated–temperature alloy. In this paper, the master alloy ingot with a nominal composition of Nb–20Ti–16Si–6Cr–5Hf–4Al–2B–0.06Y (atomic fraction, %) was prepared by using vacuum consumable arc–melting. The integrally directional solidification of this alloy was conducted in a high vacuum and ultrahigh temperature directional solidification furnace with the use of a ceramic crucible at melt temperature of 2050. The integrally directionally solidified microstructure, preferred orientation of constituent phases and solid/liquid (S/L) interface morphology at different solidifying rate (2.5, 5, 10, 20, 50 and 100 μm/s) for this alloy have been investigated by XRD, SEM and EDS, and the growth mechanism of Nbss/(Nb, X)5Si3 (where Nbss denotes Nb solid solution, X represents Ti, Hf and Cr elements) eutectic in it has been discussed. The results show that the directionally solidified microstructure of the alloy is mainly composed of hexagonally cross–sectioned primary (Nb, X)5Si3 columns and coupled grown lamellar Nbss/(Nb, X)5Si3 eutectic colonies both aligned straight and uprightly along the growth direction. When the solidifying rate varies from 2.5 μm/s to 100 μm/s, the
solid/liqid interface of the alloy undergoes an evolution from coarse cellular, fine cellulaand finally to cellular dendrite morphologies. Both the average diameter of eutectic cells and lamellar spacing in hem decrease with the increase in solidifying rate. The formatin of a regular bss/(Nb, X)5Si3 euecic morphology is attributable to a large kinetic undercoling and a low fusion entropies of alloy phases.

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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.  
Abstract   HTML PDF (16445KB)  

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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EFFECTS OF DOUBLE EXTRUSION ON THE MICROSTRUCTURE AND TENSILE PROPERTY OF THE PM PROCESSED SiCp/2009Al COMPOSITES
LIU Zhenyu, WANG Quanzhao, XIAO Bolv, MA Zongyi, LIU Yue
Acta Metall Sin    2010, 46 (9): 1121-1127.   DOI: 10.3724/SP.J.1037.2010.00207
Abstract   PDF (815KB)  

Powder metallurgy (PM), as an important method of fabricating SiC particle reinforced aluminum matrix (SiCp/Al) composites, has advantages in obtaining good interfacial bonding and enhancing tensile strength over casting or infiltrating method. The primary process of the PM method involves mixing, compaction and subsequent secondary plastic deformation. Especially, secondary plastic deformation is an important process to destroy the oxidation film on the Al particle surfaces and enhance Al-SiC bonding. However, the incorporation of the SiC particles restricts plastic flow ability of the composite and makes it difficult to be subjected to heavy single-step plastic deformation, such as single hot extrusion, hot rolling or hot forging. Instead, multi-step deformation is a critical processing approach for the SiCp/Al composites with low deformability. However, the previous attentions were mostly focused on single-step processing, multi--step plastic deformation of the SiCp/Al composites was seldom discussed. In this paper, 20%SiCp/2009Al (volume fraction) composite was fabricated using a common PM method and the effects of double extrusion on the microstructure and tensile property of the composite were investigated. It is indicated that the double extrusion could refine the SiCp size, align the SiCp and increase the aspect ratio of the SiCp, but did not exert critical effects of the grain size and <111> texture of the matrix. However, the average distance between the SiCp along the extrusion direction increased after the double extrusion, more easily inducing a SiCp-poor band which aligned approximately perpendicularly to the extrusion direction. This resulted in a decrease in the strength and an increase in the elongation along the extrusion direction.

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Acta Metall Sin    2000, 36 (8): 785-789 .  
Abstract   PDF (210KB)  
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HOT CORROSION RESISTANCE OF FILLER ALLOY BCo46
JING Yanhong,LIU Enze,ZHENG Zhi,TONG Jian,Ning Likui,HE Ping
Acta Metall Sin    2014, 50 (1): 79-87.   DOI: 10.3724/SP.J.1037.2013.00216
Abstract   HTML PDF (10964KB)  

DZ468 alloy is a promising candidate in turbine blade serving in marine atmosphere because of its superior mechanical properties and good hot corrosion resistance. And brazing technology is indispensable to produce turbine blade because of its complicated hollow structure. Therefore, for accelerating the application of DZ468 superalloy, a new type Co-based filler named BCo46 was developed. Due to serving in marine atmosphere, the attack of hot corrosion can be inevitable. Frequently, the quality of turbine blade depends on the joining region, and the joining region has a lot to do with the filler used to braze the base alloy. Therefore, in this work, the hot corrosion behavior and mechanism of BCo46 alloy in the mixture of 75%Na2SO4 + 25%NaCl (mass fraction) at 900 ℃ was investigated, and compared with a common Ni-based filler BNi-2, and the base alloy DZ468. Moreover, the hot corrosion resistance of the joint bonded with BCo46 was evaluated. XRD, SEM, EDS and ICP spectrograph were employed to study the corrosion products of the tested alloys. The results show that the hot corrosion processes of BCo46, BNi-2 and DZ468 alloys are all accompanied with the dissolution of oxide scale and the formation of sulfide, which support the model of sulfidation-(acid-based) melting model. For filler alloy BCo46, Cr2O3, Co3BO5 and (Ni, Co)Cr2O4 mainly exist in the outer corrosion layer and some CrS is formed in the inner corrosion layer; the external corrosion products of BNi-2 are NiO, (Ni, Co)Cr2O4, Ni2FeBO5 and Cr2O3, and the internal precipitation is Ni2S3. The hot corrosion resistance of filler alloy BCo46 is superior to DZ468 superalloy and BNi-2 filler, it is because (1) BCo46 contains 22% Cr which can form dense and continuous oxide scale Cr2O3; (2) the diffusion rates of O and S are slower in Co- than Ni-based alloy; (3) Co can increase the adhesion of the oxide scale-base metal; (4) collaboration dissolution doesn't occur in BCo46. Furthermore, the formation of boride can deteriorate the hot corrosion resistance of alloy by forming oxides of boron which can dissolve oxide scale, consume Cr and increase phase interfacial area to accelerate the diffusion of S and O. The joint after heat treatment can reach the level of the hot corrosion resistance of the base metal DZ468.

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MICROSTRUCTURE AND MECHANICAL PROPERTIES OF 1500 MPa GRADE ULTRA–HIGH STRENGTH LOW ALLOY STEEL
WANG Lijun CAI Qingwu YU Wei WU Huibin LEI Aidi
Acta Metall Sin    2010, 46 (6): 687-694.   DOI: 10.3724/SP.J.1037.2009.00855
Abstract   PDF (3201KB)  

A novel sort of 1500 MPa grade ultra–high strength low alloy structural steel with multi–element of Si–Mn–Cr–Ni–Mo was designed. Effects of four different processes of TMCP (thermo–mechanical controlled processing), controlled rolling+air–cooled, controlled rolling + direct quenching and controlled rolling+direct quenching+tempering at 250℃ on the microstructure and mechanical properties were investigated. The results indicate that the directly quenched steel has a maximum tensile strength of 1890 MPa, yield strength of 1280 MPa and elongation of 13%. After tempered at 250 ℃ for 30 min, the tensile strength of the steel decreased to 1820 MPa, while the yield strength increased to 1350 MPa, which is ascribed to the comprehensive effect of the softening mechanism due to the recoverof dislocation sub–structure and the strengthening mechanism due to the decomposition of retained austenite and "–carbide precipitation. Duplex phase microstructure involving lath bainite, martensite segmented by bainite, and retained austenite was obtained by the process of air–cooling and TMCP, so that it has excellent strength and plasticity. Carbon diffusion phenomenon exists in the quenching process of low–carbon steel. Both the decomposition of retained austenite and the carbon partitioning into austenite from martensite or bainite were found during tempering process. The paper demonstrates that the precipitation particles of cubic structure nucleated in austenite, growing up  and coarsning uring the whole cooling process. Futhermore, the emergence of a lage number of second–phasprecipitation cores was not found in martensite or bainite after phase tansformation.

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EFFECTS OF Nb/Ti RATIOS ON THE MICROSTRUCTURAL EVOLUTIONS OF CAST Ni-BASED SUPERALLOYS DURING LONG-TERM THERMAL EXPOSURE
SUN Wen, QIN Xuezhi, GUO Yongan, GUO Jianting, LOU Langhong, ZHOU Lanzhang
Acta Metall Sin    2014, 50 (6): 744-752.   DOI: 10.3724/SP.J.1037.2014.00040
Abstract   HTML PDF (11214KB)  
Effects of Nb/Ti ratios on the microstructural evolutions of cast Ni-based suerpalloys during long-term thermal exposure are investigated by OM, SEM and TEM. The results show that Nb/Ti ratios have no influence on the evolution of g ' morphology and size during long-term thermal exposure. However, with decrease of Nb/Ti ratios in alloys, the volume fraction of g ' phase increases. Both parameters Nb/Ti and (Nb+Ti)/(W+Mo) of primary MC have a good linear relationship with Nb/Ti ratios in alloys. With decrease of Nb/Ti ratios in alloys, both parameters for primary MC linearly decrease and sequentially thermal stability of primary MC is weakened. However, the results also show that Nb/Ti and (Nb+Ti)/(W+Mo) ratios of primary MC are not the principle factors determining the thermal stability of primary MC. The degeneration degree of primary MC can be calculated by the volume fraction of primary MC before and after degeneration, while the thermal stability of primary MC can be quantitatively characterized by degeneration degree of primary MC. Furthermore, with decreased Nb/Ti ratios in alloys, the grain boundaries coarsen more severely during long-term thermal exposure. Meanwhile, precipitation tendency of M23C6 carbide on grain boundaries increases and that of M6C carbide on grain boundaries decreases. However, the precipitation and evolution of m phase during long-term thermal exposure is not affected by Nb/Ti ratios obviously.
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RESEARCH PROGRESS ON PLASTIC DEFORMATION MECHANISM OF Mg ALLOYS
LIU Qing
Acta Metall Sin    2010, 46 (11): 1458-1472.   DOI: 10.3724/SP.J.1037.2010.00446
Abstract   PDF (1882KB)  
Possible slip and twinning systems and their critical resolved shear stresses of Mg alloys with hcp structure were described. Research works on plastic deformation behavior and micro-mechanism of different kinds of Mg alloys were reviewed. Both microstructure and texture evolutions during different thermomechanical processes, both dynamic and static recrystallization mechanisms of Mg alloys were described and discussed. Deformation and strengthening mechanisms of precipitates hardening Mg alloys were also addressed with emphasis on the interaction between precipitates and twinning/slip.
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ANALYSIS OF THE STATIC RECRYSTALLIZATION AT TENSION TWINS IN AZ31 MAGNESIUM ALLOY
LI Xiao YANG Ping MENG Li CUI Feng’e
Acta Metall Sin    2010, 46 (2): 147-154.   DOI: 10.3724/SP.J.1037.2009.00533
Abstract   PDF (5367KB)  

Due to the poor plasticity of magnesium alloys at room temperature (about 15%), twinning plays an important role in the deformation of magnesium alloys, and twins will be the dominant recrystallization nucleation sites. There are at least two types of twinning in magnesium: the {1012}–type tension twinning and the {1011}–type compression twinning. Tension twinning proceeds much more easily than compression twinning since its volume fraction is much higher than that of compression twins, which may have a promotion effect on the recrystallization to a certain degree. Based on the previous research on the static recrystallization at compression twins, the evolution of microstructure and texture in AZ31 magnesium alloy during its static recystallization at tension twins was futher investigated; and the orientational characteristics of new grains formed at tension twins in the early stage of static recrystallization were analyzed by EBSD technique. The results showed that tension twins played only a subordinate role in recrystallization nucleation and suppressed recrystallization rate, thus failed to rfine grain size effectively. The strong basal texture waretained and weakened wih no new texture component being detectd dring annealing. New grains were observed to nucleae preferentially at the intersections of tension twin variants or the intersections between tension twins and compression twins. Their orientations were relative random and are strongly scattered from those of original tension twins or compression twins. A comparison of the recrystallization at tension twins and compression twins was made.

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GRADIENT NANOSTRUCTURED MATERIALS
Ke LU
Acta Metall Sin    2015, 51 (1): 1-10.   DOI: 10.11900/0412.1961.2014.00395
Abstract   HTML PDF (3441KB)  

In this paper, research progresses on gradient nanostructured materials in recent years is briefly reviewed. It includes classification of gradient nanostructures, properties and processing techniques of the gradient nanostructured materials. Perspectives and challenges on scientific understanding and industrial applications of gradient nanostructured materials are addressed.

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AGING PROCESS OPTIMIZATION FOR A HIGH STRENGTH AND TOUGHNESS OF FV520B MARTENSITIC STEEL
ZHOU Qianqing ZHAI Yuchun
Acta Metall Sin    2009, 45 (10): 1249-1254.  
Abstract   PDF (2845KB)  

Low carbon martensitic precipitation hardening stainless steels are widely utilized in many engineering applications due to their high strength with reasonable toughness, ductility and corrosion resistance. However, those properties and their combinations are not always satisfactory to their users. For further improvement of the mechanical properties of these types of steels, a fundamental understanding of the detailed microstructural features with various aging conditions is necessary. Therefore, the effects of aging temperature, aging time and cooling rate on the microstructure and mechanical poperties of a martesitic precipitation hardening stainless steel FV520B were investigated by OM, SEM, TEM and XRD methods. The results show that the steel aged at 630℃ for a short time and then furnace cooled, in which a typical lath martensitic with the proper amounts of everse austenite and fined dispersed precipitates was bservedhas a good combination of high strength and high toughness. It could be an optimized ang process for FV520B steel.

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THE CURRENT SITUATION OF APPLICATION AND DEVELOPMENT OF SUPERALLOYS IN THE FIELDS OF ENERGY INDUSTRY
GUO Jianting
Acta Metall Sin    2010, 46 (5): 513-527.   DOI: 10.3724/SP.J.1037.2009.00860
Abstract   PDF (1117KB)  

Superalloys are extensively applied in the fields of energy industry. In the high parameter ultra--supercritical boiler used for coal--fired generation, superheater/reheater tubes must be made of superalloys which meet the requirement of good creep resistant, good fire--side corrosion resistant and steam--side oxidation resistant properties. In gas turbine engine used for gas power, turbine blades and guide vanes should be made of hot--corrosion resistant superalloys which must meet the requirements with respect to excellent high temperature corrosion resistance and long--term microstructural stability. In the field of nuclear power, heat exchange tubes used for steam generator require superalloys with excellent solution corrosion resistance. In the field of coal gasification and energy conservation and pollution reduction, the superalloys with excellent hot corrosion resistant and high temperature wear resistant properties are widely applied. In petroleum exploitation, especially in deep mining, drilling tools must be made of corrosion resistant and wear resistant superalloys because of the sour environment, the temperature of 4-150 ℃ and the existences of CO2, H2S, sand and so on. This paper makes a brief introduction on the current situation of application and development of the superalloys at home and abroad in these fields.

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STUDY OF ANISOTROPIC BEHAVIOR FOR ZIRCONIUM ALLOYS CORRODED IN LITHIATED WATER
SUN Guocheng ZHOU Bangxin YAO Meiyi XIE Shijing LI Qiang
Acta Metall Sin    2012, 48 (9): 1103-1108.   DOI: 10.3724/SP.J.1037.2012.00329
Abstract   PDF (2156KB)  
Zirconium alloys of a hexagonal close--packed crystal structure have prominent anisotropic characteristic in comparison with metals of a cubic crystal structure and a strong texture is produced in sheet or tubular materials during the fabrication process. The anisotropic characteristic is bound to be reflected on the corrosion behavior of zirconium alloys. In order to investigate the effect of texture and compositions on the anisotropic growth of oxide layer formed on zirconium alloys and clarify the mechanism of improving corrosion resistance by adding Nb in zirconium alloys, Zr-4, N18 and ZIRLO zirconium alloys with different contents of Nb were adopted as the experimental materials. All the plate specimens of zirconium alloys 2 mm in thickness have a similar texture. Corrosion tests were carried out in a static autoclave at 360 ℃, 18.6 MPa in lithiated water with 0.01 mol/L LiOH. The results show that the anisotropic growth of oxide layer on different surfaces of the specimens was only observed for Zr-4 specimen but not for N18 and ZIRLO specimens. The thickness of oxide layer develops much faster on the rolling surface (SN surface) than that on the surface perpendicular to the rolling direction (SR surface) and the surface perpendicular to the transversal direction (ST surface) for Zr-4 specimen after 90-100 d exposure, and the corrosion resistance on the SR and ST surfaces was much better than that on the SN surface. However, for N18 and ZIRLO specimens the anisotropic growth of oxide layer was restrained by the addition of Nb, and the oxide thickness on these three different surfaces was the same after 280 d exposure. Therefore the corrosion resistance of N18 and ZIRLO sheet or tubular specimens was superior to Zr-4 corroded in lithiated water, because the oxide layers grew mainly on the SN surface of the specimens. If making a comparison among Zr-4, N18 and ZIRLO specimens about the growth rate of oxide layers only on the SR and ST surfaces, it is shown that the growth rate of oxide layers increased with the increase of Nb content in these alloys. From a point of view for the improving corrosion resistance, the addition of Nb no more than 0.3\% (mass fraction) is recommended.
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PHASE–FIELD SIMULATION OF SOLUTE PRECIPITATIONS AROUND THE  γ PHASE IN Al–Ag ALLOY
GAO Yingjun LUO Zhirong ZHANG Shaoyi HUANG Chuanggao
Acta Metall Sin    2010, 46 (12): 1473-1480.   DOI: 10.3724/SP.J.1037.2010.00216
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Interactions between different precipitation products during phase decomposition for different alloys have been observed. Spinodal decomposition and intragranular precipitation are the two well–know mechanisms for explaining phase decomposition, both of which interaction mechanisms have been investigated experimentally in recently. A local free energy density function depending on aging temperature and composition has been proposed to describe the interaction between the Ag solute field and γ precipitates in phase–field simulation of spinodal decomposition in Al–Ag alloy. The evolution of spinodal decomposition in Al–Ag alloy with 4.2%Ag and 22%Ag has been simulated by the phase–field method using this function to represent numerically the precipitated Guinier–Preston zones (GPZ) around a γ phase. The simulated results show that PFZ around a precipitated phase is an elliptical and its width is about two times the width of γ phase. In the region far from PFZ, a pattern of Ag solute field appears due to spinodal decomposition. When Ag–depleted zones are relatively far apart with each other, spinodal decomposition is strongly affected by them. The formation of PFZ resulting from spinodal decomposition has been initiated in the central region of the supersaturated α matrix before the modulation effect of Ag solute field at edge reachs here. It is found that two or three Ag–rich bands appear around the PFZ. During aging, Ag diffuses from not only the α matrix but also the edge of Ag–depleted zones, where an accumulation of Ag occurs. After long time aging, many droplet–like Ag solute bands are formed near PFZ around γ phase. These simulated results are in beter agreement with the experimental results.
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TEXTURE FORMATION AND GRAIN BOUNDARY CHARACTERISTIC OF Al-4.5Cu ALLOYS DIRECTIONALLY SOLIDIFIED UNDER HIGH MAGNETIC FIELD
ZHONG Hua, REN Zhongming, LI Chuanjun, ZHONG Yunbo, XUAN Weidong, WANG Qiuliang
Acta Metall Sin    2015, 51 (4): 473-482.   DOI: 10.11900/0412.1961.2014.00496
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Directional solidification of Al-4.5Cu alloy refined by adding Al-5Ti-1B has been carried out to investigate the texture formation and grain boundary characteristic of the paramagnetic crystal under a high magnetic field. OM and EBSD were applied to analyze the microstructures solidified at different temperature gradients (G) and magnetic field intensities (B). The results show that at the temperature gradient of 27 K/cm, the orientations of fcc a-Al grains without magnetic field are random. However, as a high magnetic field is imposed, the easy magnetization axes 〈310〉 of the a-Al grains are aligned parallel to the direction of the magnetic field leading to 〈310〉 texture. Meanwhile, the ratio of coincidence site lattice (CSL) grain boundaries increases with the increment of magnetic field intensity and reaches its maximum value at 4 T, but decreases as the magnetic field enhances further. On the other hand, when the temperature gradient is elevated, columnar dendrite morphology is exhibited without magnetic field; while a 6 T high magnetic field is introduced, the columnar dendrites are broken and equiaxed grains of random orientations are obtained. The alignment behavior of the free crystals in melt could be attributed to the magnetic crystalline anisotropy of a-Al. Moreover, the influence of fluid flow on the texture formation and CSL grain boundary development under magnetic field is discussed. The absence of convection is benefit for grain reorientation and CSL boundary formation. The application of high static magnetic field will inhibit the macro-scale convection. However, the interaction between thermoelectric current and magnetic field will cause micro-scale fluid flow, i.e., thermoelectric magnetic convection (TEMC). The TEMC will give rise to perturbation near the solid-liquid interface leading to the appearance of freckles as well as the decreasing of the ratio of CSL boundary. Moreover, it is proposed that the formation of CSL boundary is associated with the rotation of the free grains in melt along specific crystallographic axes by magnetic torque.
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PRECIPITATES IN HR3C STEEL AGED AT HIGH TEMPERATURE
FANG Yuanyuan ZHAO Jie LI Xiaona
Acta Metall Sin    2010, 46 (7): 844-849.   DOI: 10.3724/SP.J.1037.2010.00037
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HR3C steel (Fe-25Cr-20Ni-Nb-N) is a new type of austenitic heat-resistant steel which has been widely used for super-heater and re-heater tubes in the ultra supercritical pressure (USC) boiler. The mechanical properties of HR3C steel were dependent on the stability of the microstructure, particularly the large amount of precipitates formed during service. This work was focused on the change of microstructure and the precipitations of HR3C steel after thermal aging for 500 h at 750℃. The results indicated that the major precipitates were variform M23C6 carbides and NbCrN nitrides. M23C6 carbides precipitated on grain boundaries and in intragranular. Fine and dispersed NbCrN nitrides were found to precipitate in intragranular. The hardness of HR3C steel had been improved through dispersed M23C6 and fine NbCrN precipitates after thermal aging for 500 h at\linebreak 750℃. All the M23C6 carbides, with lattice parameter of three times of the austenite matrix,  grown in a cube to cube orientation relationship with the matrix. Some of the M23C6 carbides which precipitated on grain boundaries were transformed into σ phase.

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EFFECT OF TEMPERING TEMPERATURE ON LOW TEMPERATURE IMPACT TOUGHNESS OF A LOW CARBON Mn-SERIES BAINITIC STEEL
GAO Guhui ZHANG Han BAI Bingzhe
Acta Metall Sin    2011, 47 (5): 513-519.   DOI: 10.3724/SP.J.1037.2010.00550
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In order to reduce the cost of alloying elements, low carbon Mn series bainitic steels have been developed. The effect of tempering temperature on the microstructure and low temperature impact toughness of a low carbon Mn-series steel has been investigated in the present study. The as rolled steel plate samples with 30 mm thickness were tempered from 280 ℃ to 600 ℃ for 2 h. Metallographic microstructure show that the< microstructure of the as-rolled steel is mostly bainite laths. Bainite laths start to merge and broaden after tempering at 460 ℃, and quasi--polygonal ferrite structures could be revealed after tempering at 600 ℃. Compared with the as-rolled steel, after tempering at 460 ℃, the yield strength of the steel changes slightly, remaining 725 MPa, while the Charpy absorbed energy at $-$40 ℃ increases from 38 J to 146 J, and the ductile-brittle transition temperature (DBTT) decreases from -18 ℃ to -48 ℃. The EBSD and TEM results indicate that the improvement of low temperature impact toughness after tempering at 460 ℃ is caused by the increase of fraction of high angle boundaries and the decrease of effective grain size during the recovery process of bainite laths.
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DEVELOPMENT AND APPLICATION OF ATOM PROBE TOMOGRAPHY
LIU Wenqing, LIU Qingdong, GU Jianfeng
Acta Metall Sin    DOI: 10.3724/SP.J.1037.2013.00362
Acta Metall Sin    2002, 38 (8): 888-892 .  
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EFFECT OF STRAIN RATE ON MICROSTRUCTURE EVOLUTION AND MECHANICAL BEHAVIOR OF A LOW C HIGH Mn TRIP/TWIP STEELS
WU Zhiqiang, TANG Zhengyou, LI Huaying,ZHANG Haidong
Acta Metall Sin    2012, 48 (5): 593-600.   DOI: 10.3724/SP.J.1037.2011.00590
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The microstructure and mechanical properties of Fe--18Mn low carbon high manganese TRIP/TWIP steels during tensile tests in the range of initial strain rate of 1.67×10-4---103 s-1 at room temperature were studied. The inverse effect of strain rate on strength of steel was produced, the strength and ductility of steels decreased with increasing strain rate in the range of quasi--static tensile strain rate of 1.67×10-4---1.67×10-1 s-1. While inverse effect of strain rate on ductility of steels was produced in the range of dynamic tensile strain rate of 101---103 s-1, the strength and ductility of materials increased significantly with increasing strain rate. The tensile strength of high manganese TRIP/TWIP steels was 957 MPa and their elongation was 55.8%. These results indicated that Fe--18Mn steel had excellent mechanical properties and good fracture resistance. The higher the strain rates applied, the less martensite, the more directions of deformation twins. The microstructure evolution of the specimen was analyzed by SEM, TEM and XRD, martensitic transformation and deformation twins were produced during the tensile deformation, and adiabatic temperature rise effect made the matrix softening during the high--speed deformation.
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INFLUENCE OF CONTINUOUS ANNEALING PROCESS ON MICROSTRUCTURE AND PROPERTIES OF BORON CONTAINING ENAMEL STEEL
DONG Futao, DU Linxiu, LIU Xianghua, XUE Fei
Acta Metall Sin    DOI: 10.3724/SP.J.1037.2013.00187
Acta Metall Sin    2001, 37 (1): 29-33 .  
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