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

About the Journal

  Current Issue
    , Volume 45 Issue 7 Previous Issue    Next Issue
    For Selected: View Abstracts
    论文
    ESSENTIAL CHARACTERISTICS AND INFLUENTIAL FACTORS FOR VERY--HIGH--CYCLE FATIGUE BEHAVIOR OF METALLIC MATERIALS
    HONG Youshi ZHAO Aiguo QIAN Gui'an
    Acta Metall Sin, 2009, 45 (7): 769-780. 
    Abstract   PDF (2194KB) ( 1837 )

    The research on very–high–cycle fatigue (VHCF) of metallic materials has become a new horizon in the field of metal research since 1980s. The behaviors of crack initiation and propagation, and the characteristics of S–N curve for metallic materials in the VHCF regime all differ from those in the low cycle and high cycle fatigue regimes. For VHCF, the cyclic stress is below the level of conventional fatigue limit and the crack initiation tends to shift from surface to interior. The defects of material, including inclusions, grain–boundary, phase interface and other micro–inhomogeneities may become interior crack initiation site. The S-N curve containing VHCF regime may present "duplex" or "step-wise" shape. The behaviors of VHCF for metallic materials are substantially affected by the strength of material, loading frequency, loading environment, etc. This paper attempts to review the research progress of essential characteristics and influential factors for VHCF behavior of metallic materials. In addition, the aspects for further research on VHCF of metallic materials are proposed, which are the process and mechanism of fatigue crack initiation and early growth, the effects of loading frequency and the environment on VHCF property, and development of quantitative model for VHCF.

    References | Related Articles | Metrics
    SOME CRITICAL ISSUES IN CREEP AND FRACTURE ASSESSMENT AT HIGH TEMPERATURE
    TU Shandong(TU Shan--Tung) XUAN Fuzhen WANG Weize
    Acta Metall Sin, 2009, 45 (7): 781-787. 
    Abstract   PDF (1285KB) ( 1406 )

    The paper briefs the current trends of the construction of high temperature plants. The need of higher efficiency and lower consumption of resources has led to higher operation parameters of the plants. It is thus believed that the safety of high temperature installations is a critical issue that could hardly be circumvented in the period of post--industrial civilization. In order to achieve a reliable design and manufacture and safe operation of the high temperature plants, some fundamental issues concerning life prediction and failure assessment should be studied. Being aware of some very slow chemical reactions occurred in the high temperature materials after a certain period of service time, it is suggested that the physico--chemical kinetics of the high temperature materials during the slow creep process should be established so that the inaccuracy of life extrapolation techniques based on the conventional Arrhenius equation could be avoided. As the actual material in a component is generally subjected to a complex stress state and the machining of a standard fracture specimen from the component is normally not possible, high temperature fracture theory under constraints should be developed to allow the estimation of fracture properties of the material. Furthermore, a unified failure assessment diagram that includes the local fracture property, the limit loading capacity and the damage law is proposed in order to assess the structural safety under complex loading and environment cases.

    References | Related Articles | Metrics
    INTERFACIAL EFFECTS OF FATIGUE CRACKING IN METALLIC MATERIALS
    ZHANG Zhefeng ZHANG Peng TIAN Yanzhong ZHANG Qingke QU Shen ZOU Hefei DUAN Qiqiang LI Shouxin WANG Zhongguang
    Acta Metall Sin, 2009, 45 (7): 788-800. 
    Abstract   PDF (2306KB) ( 1675 )

    Interfacial fatigue cracking behaviors along large--angle grain boundaries (GBs), twin boundaries (TBs), phase boundaries (PBs) and joint interfaces in metallic materials were summarized. It is found that the resistance to fatigue crack initiation decreases in the order of low--angle GBs, persistent slip bands and the large--angle GBs in pure Cu. For annealing TBs, fatigue cracking initiation strongly depends on the stacking fault energy (SFE) in Cu alloys. With decreasing SFE, fatigue cracking along TBs becomes easy. In Cu--Ag binary alloys, the misorientation across GBs or PBs plays an important role in the fatigue cracking, and large misorientation often makes the final fatigue cracking. For the Cu/solder joint interface, the interfacial fatigue cracking modes are affected by the solders and aging time. In Sn--Ag/Cu solder joints, fatigue crack normally nucleates along the interface between the Sn--Ag solder and the intermetallics compounds (IMCs); however, for Sn--Bi/Cu solder joints, brittle interfacial fatigue cracking always occurs along the interface between Cu and the IMCs due to the Bi segregation after aging for a long time.

    References | Related Articles | Metrics
    FATIGUE DEFORMATION FEATURES OF Fe--Cr ALLOY SINGLE CRYSTALS CONTAINING Cr--RICH PRECIPITATES
    LI Xiaowu CAO Xinming MA Chaoqun
    Acta Metall Sin, 2009, 45 (7): 801-807. 
    Abstract   PDF (1569KB) ( 858 )

    Although some knowledges about the fatigue deformation mechanisms of fcc alloy
    single crystals containing precipitates have been obtained in the past several
    decades, few relevant research findings have been reported on precipitates
    containing bcc alloy single crystals. In the present work, a single--slip--oriented
    bcc Fe-35%Cr alloy (mass fraction) single crystal containing Cr--rich precipitates
    was prepared as the target material, and its fatigue deformation features were
    investigated under constant plastic strain amplitude control. Experimental results
    and analyses demonstrate that, when the plastic strain amplitude
    εpl≧2.5×10-3, the Cr--rich precipitates can be
    readily sheared by the moving dislocations during deformation, leading to an
    obvious stress softening phenomenon observed at the tensile loading stage of the
    first cycle, and subsequently to a slight cyclic softening phenomenon at a very
    early stage of cycling. In addition, the tension--compression stress asymmetry was
    found during cyclic deformation of the crystals, and this enhanced stress asymmetry
    should be related to the deformation instability of Cr--rich precipitates. The slip
    deformation features were mainly manifested by the formation of coarse slip bands
    comprising a quantity of fine slip lines and also by the formation of the kink band
    at high εpl(e.g., 5.0×10-3). The primary crack
    develops roughly along the primary slip plane (101) and the crystal
    finally cracks along this plane, accompanied with some secondary cracks having
    various morphologies being formed on the crystal surface. Microstructural
    observations indicate that persistent slip band (PSB) ladder--like structures
    can be found at a low εpl of 5.0×10-4, and the
    volume fraction of them increases with increasing εpl.
    As εpl is raised to 5.0×10-3, the microstructural
    features are primarily characterized by the formation of dislocation cells.

    References | Related Articles | Metrics
    EFFECT OF TWIN LAMELLAR THICKNESS ON THE FATIGUE PROPERTIES OF NANO--TWINNED Cu
    TANG Lian LU Lei
    Acta Metall Sin, 2009, 45 (7): 808-814. 
    Abstract   PDF (1151KB) ( 1349 )

    The fatigue property of
    metals is one of the most important concerns in industrial design.
    It is affected by various factors, such as the microstructure,
    mechanics and the environment. For the polycrystalline metallic
    materials, grain boundaries (GBs) usually play an important role and
    affect the fatigue behaviors significantly. GBs could strengthen
    materials by blocking the motion of dislocations; meanwhile, the
    stress concentration which is caused by the dislocations pile--up in
    the vicinity of GBs would result in the initial fatigue crack
    easily. As a special interface of low--energy, twin boundaries (TBs)
    can strengthen materials by blocking the motion of dislocations in a
    manner similar to that of GBs. Our studies have indicated that a
    high density of nano--scale twin lamellae can provide the high
    strength without significantly compromising ductility, which is
    fundamentally different from that of GB strengthening. So far, most
    studies of the TB--related fatigue and cracking behaviors are
    concentrated on the twins with a thickness of few or tens
    micrometers. The study of the TB--related fatigue behavior in
    nanometer scale is rare. In this work, high--purity Cu specimens
    with high density of nano--scale coherent TBs were synthesized by
    means of the pulsed electro--deposition (PED). The twin lamellar
    thickness dependence of fatigue life and fatigue endurance limit of
    the nano--twinned Cu (nt--Cu) were studied by conducting
    tension--tension fatigue tests under constant stress amplitude
    control at room temperature. It is found that both the fatigue life
    and fatigue endurance limit increase with the decrease of the twin
    lamellar thickness. Postmortem SEM observations suggest a transition
    in crack initiation site from shear bands (SBs) to TBs, when the
    twin lamellar thickness is reduced from 85 to 32 nm. For the nt--Cu
    samples with thick twin lamellae, the lattices accommodate the
    plastic strain, which results in the SB cracking. For the nt--Cu
    samples with thin twin lamellae, the abundant TBs accommodate the
    plastic strain. The stress concentration along TBs which is caused
    by the interactions of dislocation--TBs facilitates the fatigue cracking along TBs.

    References | Related Articles | Metrics
    EFFECT OF P CONTENT ON MECHANICAL PROPERTIES OF FINE--GRAINED GH761 ALLOY
    YANG Shulin SUN Wenru WANG Jianxi WANG Keliang GUO Shouren HU Zhuangqi
    Acta Metall Sin, 2009, 45 (7): 815-819. 
    Abstract   PDF (904KB) ( 1432 )

    Effect of P content on the microstructure and mechanical properties of fine--grained GH761 alloy was studied. The results showed that no obvious effect on the tensile properties of fine--grained GH761 alloy was observed when P content in GH761 alloy was below 0.040% (mass fraction). However, P influenced the stress rupture and creep properties greatly. The peak value of the life at 650 ℃and 690 MPa appeared around 0.023%P. In comparison with that of the alloy with 0.0007%P, the life increased more than 2 times. At the same time, the creep rate of the alloy with 0.023%P is also the lowest in all of alloys. P is beneficial to the mechanical properties of fine--grained GH761 alloy by improving the precipitation at grain boundaries and impeding the disclocation movement. The overall properties of GH761 alloy can be improved by grain refining and appropriate amount of P addition.

    References | Related Articles | Metrics
    THERMO--MECHANICAL FATIGUE BEHAVIOR OF Ni--BASED SUPERALLOY COATED WITH Ni--Cr--Al--Y
    CHEN Zhubing HUANG Zhiwei WANG Zhongguang
    Acta Metall Sin, 2009, 45 (7): 820-825. 
    Abstract   PDF (976KB) ( 934 )

    Gas turbine engine is widely used in industry, marine and aerospace. Its efficiency can be enhanced by increasing inlet gas temperature and reducing heat transfer. To achieve this goal it is important to develop materials with higher melting points. Ni--based superalloy is suitable for vane and blade materials. The properties of superalloy have been improved by directionally solidified and single crystal in last 50 years. But it is limited to develop superalloy with much higher temperature, because the operating temperature approaches the melting point of superalloy. In this rigorous environment, high temperature oxidation and hot corrosion impair the alloy property. MCrAlY coating is commonly used as oxidation-- and corrosion--resistant material. Hitherto, it is clearly validated that the oxidation-- and corrosion--resistance of hot parts at high temperature can be greatly improved by MCrAlY coating. The difference of materials properties between MCrAlY coating and superalloy may cause interactions which influence the mechanical properties and life of MCrAlY--coated component. A few of studies have conducted on mechanical properties of $M$CrAlY--coated superalloy. The results revealed that the present of coating affected the performance of superalloy. In service, thermo--mechanical fatigue provides a closer simulation of the actual strain--temperature in an engine environment. Only a few studies were published though their results were not consistency; because the experiment is hard to perform and many factors influence the experiment results such as temperature--strain cycle shape, strain range magnitude, the ductility and the strength of the bond between the coating and the substrate. In this paper, thermo--mechanical fatigue (TMF) behaviors of a Ni--based superalloy M963 coated with Ni--Cr--Al--Y have been investigated at 450---900 ℃ in air under mechanical strain control with a strain ratio of -1 and a period of 200 s. The coating was produced either by air plasma spraying (APS) or by high velocity oxyfuel fuel (HVOF). It was shown that under the same mechanical strain, the out of phase (OP) TMF life is shorter than the in phase (IP) TMF one in both APS and HVOF coated specimens. The TMF life depends on the coating spraying technology under the same testing conditions. Observations of fracture surfaces and longitudinal sections revealed that crack initiation has great impact on the TMF life.

    References | Related Articles | Metrics
    EXPERIMENTAL STUDY ON UNIAXIAL RATCHETING OF 20 STEEL WITH HIGH NUMBER OF CYCLES AT ROOM TEMPERATURE
    DONG Yawei KANG Guozheng LIU Yujie WANG Hong CHENG Xiaojuan
    Acta Metall Sin, 2009, 45 (7): 826-829. 
    Abstract   PDF (803KB) ( 998 )

    Ratcheting is a cyclic accumulation of inelastic strain occurred under the stress--controlled cyclic loading with< non--zero mean stress, it is very important in assessing the fatigue life and safety of the structure subjected to cyclic loading. The existed results showed that different materials presented different ratcheting features, and the results were obtained from the cyclic tests with a small number of cycles (less than 200 cyc). Therefore, based on the uniaxial strain--controlled and stress--controlled cyclic tests with high number of cycles (more than 2000 cyc), the cyclic deformation features of 20 steel were observed at room temperature. The effects of the cyclic softening/hardening feature and yielding plateau of the steel and the applied mean stress, stress amplitude and stress ratio on the ratcheting of the steel with high number of cycles were studied. The results show that the 20 steel presents a weak and stain--amplitude--dependent cyclic hardening, and the ratcheting of the steel depends greatly on the applied mean stress, stress amplitude and stress ratio. The ratcheting strain increases monotonically with the increase of applied mean stress and stress amplitude, but decreases with the increase of stress ratio. A re--speeded ratcheting will occur at the end of cycling with high number of cycles when the applied stress levels are high enough. More importantly, the yielding plateau of the steel greatly influences the evolution of the ratcheting. It is concluded that the effect of yielding plateau on the ratcheting should be reasonably considered in constructing the cyclic constitutive model of the ratcheting for the 20 steel.

    References | Related Articles | Metrics
    STUDY ON STRAIN--INDUCED MARTENSITE TRANSFORMATION OF 304 STAINLESS STEEL DURING RATCHETING DEFORMATION
    CHENG Xiaojuan WANG Hong KANG Guozheng DONG Yawei LIU Yujie
    Acta Metall Sin, 2009, 45 (7): 830-834. 
    Abstract   PDF (988KB) ( 1088 )

    Ratcheting behavior of the materials has been extensively investigated and simulated in the last two decades, since it is important in the design and assessment of engineering structure components subjected to asymmetrical stress--controlled cyclic loading. However, most of the referable literature concerned only the macroscopic phenomenon of ratcheting and its phenomenological models. Therefore, the microstructure evolution of SS304 stainless steel during uniaxial stress--controlled cyclic loading with the ratcheting deformation was observed by using OM, SEM and XRD methods. Several specimens with the same applied stress level were first tested under the stress--controlled cyclic loading with different numbers of cycles macroscopically, and then the thin--filmed samples obtained from the specimens subjected to different numbers of cycles were microscopically observed. The results show that when the ratcheting strain reaches a certain value high enough, the strain--induced martensite transformation occurs in the process of ratcheting deformation. The resultant phase is the lath α--martensite and the amount of the induced martensite increases gradually with the number of cycles, which is proved by the quantitative analysis of XRD. It is also concluded that the plastic deformation caused by the martensite transformation has a un--negligible contribution to the total value of ratcheting strain. The total ratcheting deformation should be considered as a superimposition of two parts, i.e., the one caused by applied cyclic stress and the other caused by the strain--induced martensite transformation.

    References | Related Articles | Metrics
    STATISTICAL ANALYSIS AND RELIABILITY PREDICTION OF CREEP RUPTURE PROPERTY FOR T91/P91 STEEL
    ZHAO Jie LI Dongming FANG Yuanyuan
    Acta Metall Sin, 2009, 45 (7): 835-839. 
    Abstract   PDF (743KB) ( 2020 )

    Life prediction is an important issue for components serviced at high temperature from both economic and safety reasons. For the purpose to correlate creep rupture data, the time--temperature parameter (TTP) method is often used to condense all the rupture data obtained at various stresses and temperatures to a narrow band. Because of uncertainty factors in materials and experiment conditions, creep rupture data generally exhibit dispersity which can be described using a so--called Z--parameter method. The current paper performs a statistical analysis for creep rupture data of T91/P91 (9Cr--1Mo--V--Nb) steel, a kind of ferritic steels widely used in power plants recently by Z--parameter method. The results indicate that all the creep rupture data distribute in a narrow band in stress σ--TTP plot. The distribution of Z--parameter which is used to represent the dispersion of the creep rupture data follows normal distribution. σ--TTP--reliability R, σ--rupture time tr--R and allowable stress [σ]--temperature T--R curves are proposed based on Z--parameter method. The results exhibit a good agreement with the experimental data promulgated by NRIM and ECCC.

    References | Related Articles | Metrics
    STRESS--STRAIN BEHAVIORS AROUND THE YIELD STRENGTH IN SEQUENTIAL ROOM TEMPERATURE CREEP TESTS
    Acta Metall Sin, 2009, 45 (7): 840-843. 
    Abstract   PDF (825KB) ( 976 )

    The deformation of metallic materials at low homologous temperature (the ratio of experimental temperature and melting point T/Tm<0.2) used to be described by time independent models. With the development of measuring technique and the improvement of required accuracy, the time dependence of deformation has been concerned about. So far, room temperature creep (RTC) has been observed in many metals and alloys, and some experiments have been performed to investigate the fundamental deformation mechanism and influencing factors. A previous paper reported that RTC increased the subsequent flow stress of an X70 pipeline steel. However, the evolution of deformation following RTC is not thoroughly understood. In this paper, sequential RTC tests and their effects on the subsequent deformation have been investigated in the X70 pipeline steel. The steel exhibits time dependent deformation at various stress levels around yield strength, and RTC is generally primary α type. Moreover, the post--yield RTC markedly enhances the subsequent flow stress. The influence of RTC on subsequence stress--strain behavior can be evaluated using strain rate--stress curve: the burst point of strain rate, due to pre--yield RTC process, is lower than elastic limit, while a distinct peak is observed after RTC has been performed at higher stress level. Local mobile dislocation model has been applied to analyze the behavior.

    References | Related Articles | Metrics
    EXPERIMENTAL INVESTIGATION ON FATIGUE BEHAVIOR OF NANOCRYSTALLINE NICKEL
    XIE Jijia HONG Youshi
    Acta Metall Sin, 2009, 45 (7): 844-848. 
    Abstract   PDF (921KB) ( 999 )

    Electrodeposited nanocrystalline (nc) metal is often used as a model material in nc material investigation. But electrodeposition typically yields only thin foils that are at most several hundred micrometers in thickness, this arouses experimental difficulties in fatigue testing. There are several investigations on fatigue of electrodeposited nc metals. However, for the lack of direct experimental evidence, the mechanism of fatigue crack nucleation for nc materials is still not clear. In addition to fatigue properties, the microstructure stability is another key point for the practice of bulk nc materials. Some research papers indicated that the grains of nc metal would grow up under quasi--static loading, but no any investigation give out results under cyclic loading. In this paper, fatigue of electrodeposited nc Ni was experimentally investigated. Fatigue testing was carried out to obtain the S--N curves. For the reason that surface is the most important site for fatigue crack initiation, atomic force microscopy (AFM) was used to scan the sample surface before and after fatigue testing, which provides a direct observation on fatigue crack nucleation mechanism. For investigation on the stability of microstructure, the AFM was also used to measure the grain size of samples after fatigue loading, and nanoindenter was used to investigate the change of mechanical properties of samples after fatigue testing. The S--N curves indicate that nanocryatalline samples have a higher fatigue limit than coarse grain ones. The AFM images indicate that cell pellet morphology with the average size of 73 nm appeared on sample surface after high cycle fatigue testing and the grain size is the same as those before the fatigue testing. From the results of nanoindentation, the mechanical properties including hardness, strain rate sensitivity and elastic modules of samples also keep no obvious change after fatigue loading. Based on these results, the fatigue crack nucleation mechanism of electrodeposited nc Ni was discussed.

    References | Related Articles | Metrics
    STUDY ON FATIGUE CRACK GROWTH BEHAVIOR OF 16MnR STEEL UNDER DIFFERENT CONDITIONS
    XIONG Ying CHEN Bingbing ZHENG Sanlong GAO Zengliang
    Acta Metall Sin, 2009, 45 (7): 849-855. 
    Abstract   PDF (1191KB) ( 1812 )

    16MnR steel is a most widely used material for pressure vessels. The study on fatigue is very important for safety and fatigue life assessment of pressure vessels. Several experimental researches for fatigue of 16MnR steel were conducted and test data have been gotten, but it was short of fatigue study of the material under various conditions at present. The fatigue crack growth tests of 16MnR hot--rolling steel were carried out at different< temperatures and R--ratios with compact tension (CT) specimens in this paper. Different sized circular notches at the tip of the slot ranging from very sharp< to blunt were used in the CT specimens and the radii of the notches were 0.1, 1 and 2 mm, and the thicknesses of CT specimens were 3.8, 12.5, 18 and 25 mm, respectively. The fatigue crack growth performances were obtained. The effects of temperatures, notch sizes, R--ratios and specimen thickness on fatigue crack growth behaviors were studied. It was shown that the fatigue crack growth rates at 150 and 300 ℃ were lower than those at 25 and 425 ℃, the lowest one was at 300 ℃ and the crack growth rate increased with temperature above 300 ℃. The great influence of notch sizes on the fatigue crack growth rate was observed at early stage of crack growth only. The results also indicated that fatigue crack growth rate increased as the specimen thickness increased, but the fatigue crack growth rate of 16MnR steel was insensitive to the R--ratios.

    References | Related Articles | Metrics
    LOW--CYCLE FATIGUE BEHAVIOR OF AS--EXTRUDED Mg--x%Al--3%Ni ALLOYS
    CHEN Lijia WANG Xin ZHI Ying XU Yanwu
    Acta Metall Sin, 2009, 45 (7): 856-860. 
    Abstract   PDF (820KB) ( 1103 )

    Due to the low density, high specific strength and stiffness, magnesium alloys are being considered for automotive and aerospace applications. The structural applications of magnesium components need a decent low--cycle fatigue performance, because cyclic loading is often encountered. In order to identify the low--cycle fatigue behavior of the newly developed Mg--x%Al--3%Ni(x=4, 5, 6, 7, mass fraction) extruded magnesium alloys with different contents of Al, the total strain--controlled low--cycle fatigue tests were performed at room temperature. The cyclic stress response, strain fatigue life and cyclic stress--strain behaviors were investigated for the hot--extruded Mg--x%Al--3%Ni alloys. The results of the low--cyclic fatigue tests show that the hot--extruded Mg--x%Al--3%Ni alloys exhibit the cyclic strain hardening during fatigue deformation. The dependences of the strain fatigue life on plastic strain amplitude and elastic strain amplitude can be described by the Coffin--Manson and Basquin equations, respectively. In the hot extruded Mg--x%Al--3%Ni alloys with different contents of Al, the extruded Mg--5%Al--3%Ni alloy gives the longest fatigue life and the highest fatigue resistance. In addition, a linear relation between the cyclic stress amplitude and cyclic strain amplitude can be noted for the hot--extruded Mg--x%Al--3%Ni alloys.

    References | Related Articles | Metrics
    FATIGUE FEATURES AND MECHANISM OF Al--7Si--0.3Mg CAST ALLOY UNDER NONPROPORTIONAL LOADINGS
    MO Defeng HE Guoqiu ZHU Zhengyu LIU Xiaoshan ZHANG Weihua
    Acta Metall Sin, 2009, 45 (7): 861-865. 
    Abstract   PDF (1145KB) ( 913 )

    Widely application of cast aluminum alloy requires an understanding of its cyclic deformation behavior which is material dependent, and it is a complex function of strain amplitude, loading path, etc. In this study, multi--axial fatigue tests were conducted on cast Al--7Si--0.3Mg alloy with the same equivalent strain amplitude of 0.22% under six multi--axial path loadings, which were proportional, circular, square, diamond, rectangle and ellipse paths. TEM was employed to investigate the dislocation structures of the fatigue failure specimens. Cyclic hardening dominates the whole fatigue process under every loading path, but the rate and extent of cyclic hardening are quite dependent on particular loading paths. The fatigue life under nonproportional loading is much lower than that under proportional loading, and it also depends on the various nonproportional loading paths. The specimen with circular path loading has the shortest life and the most severe cyclic hardening among all the loading paths. The continuously changing of direction of maximum shear--stress plane is attributed to the complicated dislocation substructures and severe stress concentration during the cyclic process. The interaction among dislocation, particle and cell boundary is the main reason for cyclic hardening. The structure and density of dislocation in fatigue failure specimens under various loading paths exhibit quite different. From double dislocation bands, multiple dislocation bands, labyrinth structure to cell structure, the dislocation mobility decreases and stress concentration degree increases.

    References | Related Articles | Metrics
    EFFECTS OF LOADING RATE, NOTCH GEOMETRY AND LOADING MODE ON THE CLEAVAGE FRACTURE BEHAVIOR OF 16MnR STEEL
    WANG Guozhen WANG Yuliang XUAN Fuzhen TU Shantung WANG Zhengdong
    Acta Metall Sin, 2009, 45 (7): 866-872. 
    Abstract   PDF (1256KB) ( 912 )

    The changes in loading rate and test temperature influence deformation (yield and flow) and fracture behavior of steel. The most significant effect of increasing loading rate is to shift the quasi--static fracture toughness transition curve toward higher temperatures, that is, to raise the temperatures at which cleavage may occur. This is very critical for structural steel components. The high strain rate sensitivity of these steels makes the safety design on the basis of quasi--static behavior rather poor. Therefore, the reliability of structures possibly loaded at higher loading rates requires the knowledge of the corresponding material fracture behavior. However, the effects of loading rate on the cleavage fracture mechanism, fracture stress and toughness of various steels have not been understood completely. Specifically, the prediction of the cleavage fracture behavior in notched specimens and actual structures and components loaded at various loading rates remain to be learned. In engineering structures and components possibly loaded at various loading rates, notch defects may be difficult to avoid and some notch--like geometries are necessary for structure design. Therefore, the understanding of notch toughness at various loading rates is also very important. In this work, the effects of loading rate, notch geometry and loading mode on the cleavage fracture behavior of 16MnR steel were studied by experiments and FEM calculations. The results show that the cleavage fracture mechanism of this steel, the corresponding local cleavage fracture stress σf and macroscopic cleavage fracture stress σF do not change with the above three factors. The change of the notch toughness of the notched specimens with different notch geometries and loading modes with loading rates can be predicated by the criterion σyymaxσF, where σyymax is the maximum normal stress ahead of a notch and can be obtained by FEM calculations. The σF can be regarded as an engineering notch toughness parameter, and may be used for assessing the integrity of structures with notch defects. The σF values of steels can be measured by the Griffiths--Owen notched specimen at prescribed test temperature and loading rate.

    References | Related Articles | Metrics
    MICROSTRUCTURAL INSTABILITY OF ULTRAFINE--GRAINED COPPER UNDER ANNEALING AND HIGH--TEMPERATURE DEFORMING
    JIANG Qingwei LIU Yin WANG Yao CHAO Yuesheng LI Xiaowu
    Acta Metall Sin, 2009, 45 (7): 873-879. 
    Abstract   PDF (1507KB) ( 1034 )

    The thermal stability and hardness behavior of ultrafine--grained (UFG) copper produced by equal channel angular pressing (ECAP) under the condition of annealing were studied by differential scanning calorimeter (DSC) and micro--hardness tests, and the microstructural changes of this material under uniaxial compression or cyclic deformation at temperatures ranging from room temperature to 300 ℃ were examined by electron channeling contrast (ECC) technique in scanning electron microscopy (SEM) and by transmission electron microscopy (TEM). It was found that under annealing even at a certain temperature below recrystallization temperature, UFG copper would exhibit a structural evolution, i.e., recrystallization and grain coarsening, the process of which may happen gradually at a low developing rate, so that the DSC response curve as a functional of annealing time cannot detect such a process. The grain coarsening behavior of UFG copper under high--temperature compression is related to the strain rate, i.e., the higher the strain rate, more remarkable the localization of grain coarsening becomes; the lower the strain rate, many more grains become coarsened integrally. Comparatively speaking, the grain coarsening induced by high--temperature cyclic deformation takes place more notably and uniformly, and some typical dislocation arrangements, like dislocation walls and dislocation cells etc., can be observed in some coarsened grains. Also, the inhomogeneity of grain coarsening under high--temperature deformation was quantitatively discussed in terms of a ratio V of maximum grain size (Dmax) to average grain size (Daver), which is available for the coarsened grains.

    References | Related Articles | Metrics
    EFFECT OF GRAIN SIZE ON ULTRA--HIGH--CYCLE FATIGUE PROPERTIES OF 42CrMoVNb STEEL
    ZHANG Yongjian HUI Weijun XIANG Jinzhong DONG Han WENG Yuqing
    Acta Metall Sin, 2009, 45 (7): 880-886. 
    Abstract   PDF (1257KB) ( 1452 )

    For low and medium strength steels, grain size has significant effects on their fatigue properties, whereas non--metallic inclusion has no or little effect. In previous work, the effects of grain size on high--cycle fatigue fracture behaviors of 42CrMoVNb high strength steel were studied and illustrated that grain refining has a complicated influence on its fatigue properties. In this paper, the ultra--high--cycle fatigue properties of 42CrMoVNb high strength steel with three kinds of prior austenite grain sizes produced by different heat treatment procedures were studied. Experimental results show that both fatigue strength and fatigue strength ratio don't increase monotonically with the decrease of gain size, and fairly better fatigue properties could be obtained at a medium grain size of 15 μm. SEM observations of fatigue fracture surface reveal that most of fatigue cracks initiated from inclusions and a granular bright facet (GBF) was found in the vicinity around inclusion at cycles beyond about 1×106. Further investigation shows that the stress intensity factor range at crack initiation site of inclusion ΔKinc trends to decrease gradually with increasing the fatigue life Nf, while the stress intensity factor range at GBF boundary ΔKGBF keeps almost constant with varying Nf. ΔKGBF of coarse grain size is higher than that of fine grain size. It could conclude that the effect of grain size on ultra--high--cycle fatigue properties is rather complicated and an appropriate size of prior austenite might be existed.

    References | Related Articles | Metrics
    HOT COMPRESSION BEHAVIOR AND FLOW STRESS PREDICTION OF ZK60 MAGNESIUM ALLOY
    QIN YinJiang PAN Qinglin HE Yunbin LI Wenbin LIU Xiaoyan FAN Xi
    Acta Metall Sin, 2009, 45 (7): 887-891. 
    Abstract   PDF (1048KB) ( 1185 )

    In order to study the workability and establish the flow stress constitutive equation for ZK60 magnesium alloy, hot compressive deformation behavior of the magnesium alloy was investigated at the temperature range from 523 to 673 K and strain rate range from 0.001 to 1 s-1 on Gleeble--1500 thermal simulator. The results show that flow stress of ZK60 magnesium alloy decreases with the increase of deformation temperature and the decrease of strain rate. The flow stress curves obtained from experiments are composed of four different stages, i.e., work hardening, transition, softening and steady stages. While for the relative high temperature and low strain rate, transition stage is not very obvious. A method to predict flow stress considering the effect of true strain was presented. Flow stress model is expressed by nine independent parameters and they are obtained by Least--Square method. The predicted stress--strain curves are in good agreement with the experimental results, which confirmed that the developed model can give a reasonable estimate of the flow stress for ZK60 magnesium alloy.

    References | Related Articles | Metrics
    EFFECT OF PRECIPITATE ON PLC EFFECT IN 2024 Al ALLOY
    XIONG Shaomin ZHANG Qingchuan CAO Pengtao XIAO Rui
    Acta Metall Sin, 2009, 45 (7): 892-896. 
    Abstract   PDF (1255KB) ( 1041 )

    It is normally accepted that the interaction among solute atoms, dislocations
    and precipitate leads to Portevin--Le Chatelier (PLC) effect during the plastic
    deformation of alloys. Precipitate is directly responsible for some inverse behavior of
    PLC effect, such as the inversion of the temperature dependence of critical strain;
    because these behaviors appear only when precipitate exist in the alloys. In this paper,
    the solute concentration in matrix and the fraction of the precipitate in 2024 Al alloy
    are changed by heat treatment. Subsequently, tensile experiments are conducted at
    room temperature (25 ℃) and low temperature (-100 ℃) on these treated specimens.
    The magnitude of serration and the critical strain of the serrated flow are analyzed and
    the results show that the diffusing solute atoms are necessary for the appearance of
    PLC effect while the cutting of the precipitate particles alone can not lead to this
    phenomenon. The mobile dislocations will be blocked and piled up strongly in the front
    of precipitate and thus precipitate will have an influence on PLC effect. This influence
    is obvious during tensile tests at medium strain rate.

    References | Related Articles | Metrics