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

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    , Volume 41 Issue 11 Previous Issue    Next Issue
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    Research Articles
    Effect of tin content on microstructures and mechanical properties of Ti(C,N)/NiCr cermets
    WANG Quanzhao; LIU Yue; GUAN Dehui; YU Baohai; CHEN Zhiliang; BI Jing
    Acta Metall Sin, 2005, 41 (11): 1121-1126 . 
    Abstract   PDF (668KB) ( 1079 )
    Ti(C,N)/NiCr cermets were vacuum sintered by powder metallurgy technology. The effects of TiN additions on microstructures and mechanical properties of Ti(C, N)/NiCr cermets were investigated. Results revealed that the hard phase particles were refined and the morphology of the particles changed from a circular to a polygonal shape by the additions of TiN because of the formation of Ti(C,N) solid solution; With increasing TiN additions, the bonding strength increased to the maximum values at 4%TiN content during low sintering temperature and at 6%TiN content during high sintering temperature, respectively, and then decreased. Hardness changed little when TiN addition was less than 10%, but decreased dramatically when TiN addition exceeded 10%. Fracture photograph showed that trans-granular fracture was the main failure mode because of the good interface bonding strength between binding phase and ceramic hard phase.
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    Investigation on deformation induced ferrite (a kind of martensite) transformation above Ae3 temperature in a low carbon steel
    LIU Zhaoxia; LI Dianzhong; QIAO Guiwen
    Acta Metall Sin, 2005, 41 (11): 1127-1135 . 
    Abstract   PDF (1144KB) ( 1381 )
    A series of unidirectional compression tests of a low carbon steel Q235 were performed on a Gleeble 3500 thermal simulator,and the influences of strain, strain rate and deformation temperature (above the isothermal transformation temperature Ae3 from austenite to ferrite) on deformation induced ferrite transformation (DIFT) have been examined. The microstructure, nanoindentation hardness and the elastic modulus of deformation induced ferrite (DIF) and proeutectoid ferrtie were determined by optical microscope, scanning electron microscope (SEM), X-ray diffraction (XRD) and nanoindentation techniques. The results show that DIFT can take place above Ae3 temperature, the higher the strain rate and the strain, the more favorable for DIFT. When ε=80% and ε=20 s-1, the upper limit temperature of DIFT can be elevated to 945 ℃ (Ae3+98 ℃). An interesting phenomenon is found that when the deformation temperature is between 870-920℃, the total stress is increased with deformation temperature decreasing. However, when deformed between 830-870℃, the total stress is decreased with deformation temperature decreasing. Compared with the diffraction peak of proeutectoid ferrite, the diffraction peak of DIF was shift to a low angle in XRD analysis, and both the nanoindentation hardness and elastic modulus of DIF are much higher than those of proeutectoid ferrite, which proved DIF to be a kind of martensite.
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    Estimation of the critical size of inclusion in high strength steel under high cycle fatigue condition
    YANG Zhenguo; ZHANG Jiming; LI Shouxin; LI Guangyi; WANG Qingyuan HUI Weijun; WENG Yuqing
    Acta Metall Sin, 2005, 41 (11): 1136-1142 . 
    Abstract   PDF (566KB) ( 1303 )
    The critical inclusion sizes (CIS) of some steels with given matrix hardness or tensile strength and surface roughness under high cycle fatigue condition were estimated mainly on the basis of Murakami's theory, i.e., the inclusion equivalent projected area model. The results show that the CIS decreases with increasing Vickers hardness (or tensile strength) and decreasing surface roughness of specimen. The estimated CIS is supported indirectly by the experimental data from present study and previous publications available.
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    Micro-mechanisms of fatigue damage in copper crystals
    ZHANG Zhefeng; DUAN Qiqiang; WANG Zhongguang
    Acta Metall Sin, 2005, 41 (11): 1143-1149 . 
    Abstract   PDF (932KB) ( 1147 )
    Micro--mechanisms of fatigue damage in copper single-, bi- and poly- crystals were summarized in the present paper. A number of investigations reveal that fatigue crack mainly initiates along persistent slip bands (PSBs) in copper single crystals at low or medium strain range, however, nucleates along coarse deformation bands (DBs) at high strain range. For copper bi-crystals, various large-angle grain boundaries (GBs) are always the preferential sites for the nucleation and propagation of fatigue cracks while the low-angle GBs do not lead to fatigue cracking during fatigue. Fatigue cracks mainly nucleated along large--angle GBs, sometimes along PSBs in polycrystalline copper, however, the initiation of fatigue crack at twin boundaries (TBs) was not observed due to the compatible slip deformation across the TBs.
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    TEM observation of annealing microstructure of deformed Al-Mg-Sc-Zr alloy
    ZHAO Weitao; YAN Desheng; RONG Lijian
    Acta Metall Sin, 2005, 41 (11): 1150-1154 . 
    Abstract   PDF (480KB) ( 1252 )
    Al-6Mg-0.4Mn-0.2Sc-0.1Zr-0.1Cr(mass fraction, %) alloy was prepared by vacuum induction melting, and the microstructures and mechanical properties of cold worked samples annealed at 350, 400, 450 and 500℃ for 1 h were investigated. TEM observation indicated that tangled dislocation cells are typical microstructure in cold worked sample; After 350℃ annealing, the dislocations arrange regularly and the density decreases sharply, the alloy recovers partly and the grain size is about 500 nm; Further annealing to 500℃, recrystallization proceeds gradually in a wide temperature range due to the Zener drag effect of Al3(Sc, Zr) phase . As a result, the mechanical property changes greatly at 350℃ but keeps steady with further temperature increasing.
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    Effects of grain boundary η phase on the mechanical properties of Fe-Ni-Cr alloy
    LI Xiuyan; RONG Lijian; LI Yiyi
    Acta Metall Sin, 2005, 41 (11): 1155-1158 . 
    Abstract   PDF (445KB) ( 1057 )
    The microstructures and mechanical properties of an Fe-Ni-Cr austenite alloy undergone different ageing treatments have been examined. Theη phase precipitated on the grain boundary of the over-aged alloy. Contrary to be crack sources and make mechanical properties of alloys deteriorate as it has been always considered, ηphase was found to have little effect on room temperature properties and furthermore, it improved the high temperature ductility of the alloy via blocking the grain boundary slip due to its coherency with the austenite matrix. SEM fractography shows that the 1023 K tensile fracture transforms from intergranular mode to microvoid coalescence mode due to the presentation ofηphase. Addition of Nb causes more dispersed distribution ofη phase along grain boundaries and therefore results in higher high temperature ductility.
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    α-Cr precipitation behavior and its effect on high Cr-containing superalloys
    DONG Jianxin; XIE Xishan
    Acta Metall Sin, 2005, 41 (11): 1159-1166 . 
    Abstract   PDF (846KB) ( 1212 )
    Theα-Cr precipitation behavior and its effects on the different Cr-containing superalloys, such as GH169, GH625, GH648 and two high-Cr tested alloys, were investigated by microstructure analysis approach in order to compare their behavior and optimize theα-Cr phase morphology. The results show that noα-Cr phase was observed after a standard heat treatment but with long-term aging theα-Cr phase precipitates and grows usually clinging to δ phase in GH169 alloy and clinging to M6C carbide andδphase in GH625 alloy. However, for high Cr-containing alloys (Cr>30%, GH648 and two tested alloys), the α-Cr phase with various morphologies precipitates directly from austenitic matrix and from short-plate to long acicular and lamellar shape. Furthermore, the optimization of α-Cr phase was initially studied by a special heat treatment in order to obtain fine and uniform α-Cr particles.
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    Microstructure and initial stage oxidation of NiCoCrAlY coatings deposited by arc ion plating technique
    SUN Chao; WANG Qimin; TANG Yongji; GUAN Qingfeng; GONG Jun; WEN Lishi
    Acta Metall Sin, 2005, 41 (11): 1167-1173 . 
    Abstract   PDF (731KB) ( 1155 )
    The microstructure of the NiCoCrAlY coatings deposited by arc ion plating (AIP) and its oxidation mechanism at initial stage were investigated by XRD, SEM, TEM and photostimulated luminescence spectroscopy (PSLS). The results showed that as-deposited NiCoCrAlY coatings were composed of nano-grains and possessed apparent layered structure. After vacuum annealing, the layered microstructure disappeared and the coating became much denser. The nano-grains in the coatings became much larger and twinned grains appeared. In addition, a very thin oxide scale mainly of α-Al2O3 formed on the coating surface after annealing. In the subsequent initial oxidation stages, transient θ-Al2O3 increased and then turned into stable α-Al2O3. The related mechanism was also discussed.
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    Influence of strain rate on near-neutral pH environmentally assisted cracking of pipeline steels
    FANG Bingyan; HAN Enhou; WANG Jianqiu; KE Wei
    Acta Metall Sin, 2005, 41 (11): 1174-1182 . 
    Abstract   PDF (834KB) ( 1146 )
    Environmentally assisted cracking (EAC) of X-70 pipeline steels in near-neutral pH environments, including the standard solution, NS4, and an actual soil solution, was investigated using constant loading tests, slow strain rate tests (SSRT) and cyclic loading tests. It was revealed that local strain rate is a key factor that builds a bridge between different tests, different samples and different cracking processes, and dominates fracture mode. Therefore, local strain rate is one of the most important parameters in the EAC of X-70 pipeline steels. Stress corrosion cracking (SCC) can occur only if the local strain rate is smaller than the upper susceptible local strain rate for SCC, i.e. 5×10-5 s-1. As for cyclic loading tests, when the local strain rate is higher than this upper local strain rate for SCC, corrosion fatigue (CF) will occur. Further more, mechanical facture will take place if the local strain rate is high enough. Meanwhile, it is shown that as far as the EAC of X-70 pipeline steels in near-neutral pH environments is concerned, the cracking is dominated by the local strain rate no matter which the mechanism is, dissolution or (and) hydrogen related process. Thus the mechanism of the near-neutral pH cracking for X-70 in-service pipelines in the field is SCC instead of CF. In addition, it is proposed that the term of near-neutral pH SCC is inappropriate, which should be replaced by near-neutral strain assisted corrosion cracking to better describe the EAC of X-70 pipeline steels that is, in fact, local-strain-rate-based environmentally cracking.
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    High strength nano-structured Ti-Nb-Zr-Sn alloy
    HAO Yulin; YANG Rui
    Acta Metall Sin, 2005, 41 (11): 1183-1189 . 
    Abstract   PDF (644KB) ( 1279 )
    To obtain fully dense nano-structured metallic materials, severe plastic deformation technique usually has to be used because normal deformation mechanisms, such as dislocation interaction, deformation twins and/or martensitic transformation, are ineffective in grain refinement. Recently we found localized unstable plastic deformation in a $\beta$ type Ti-Nb-Zr-Sn titanium alloy that is very effective in grain refinement.By taking advantage of this deformation mechanism, grain size less than 50 nm can be easily achieved in conventionally cold-rolled sheet with thickness of 1.5 mm. In this paper we examine the conditions under which such an unusual deformation mechanism can be realised and report further strengthening of the nano-structured alloy by aging treatment. The origin of this deformation mechanism will be discussed.
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    Liquid structure and melt treatment of M963 CAST Ni base superalloy
    YIN Fengshi; GUAN Hengrong; SUN Xiaofeng$; HU Zhuangqi
    Acta Metall Sin, 2005, 41 (11): 1190-1194 . 
    Abstract   PDF (543KB) ( 923 )
    The liquid structure and melt treatment of M963 Ni base superalloy have been studied by using high temperature X-ray diffraction. The results show that there exist Ni3Al-like cluster and residual MC carbide in the alloy melt superheated at 1520℃, and the liquid structure of the M963 superalloy has a close relationship to the thermal history of the alloy melt. Both the rupture life and elongation of M963 superalloy at 975℃ under 225 MPa are all prolonged obviously by melt treatment at 1550-1650℃. The commercial MgO crucible is unsuitable for high temperature treatment of melt at 1850℃, but by using a new developed crucible such treatment can carry out and can obviously increase the rupture property of the recycled M963 superalloy at 975℃ and 225 MPa.
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    Microstructural evolution of nanocrystal-line Al-Zn-Mg-Cu alloy powders by cryomilling
    YANG Bin; CHENG Junsheng; FAN Jianzhong; TIAN Xiaofeng; CHEN Hanbin ZHANG Jishan
    Acta Metall Sin, 2005, 41 (11): 1195-1198 . 
    Abstract   PDF (335KB) ( 868 )
    Nanocrystalline Al-Zn-Mg-Cu alloy powders have been successfully fabricated by mechanically milling at cryogenic temperature (cryomilling) in liquid nitrogen. Microstructural evolution of the alloy powders during cryomilling was analyzed. The experimental results show that the as-atomized Al-Zn-Mg-Cu alloy powder size was gradually reduced and approached 35 nm after 15 h cryomilling. MgZn2 phase gradually disappeared and ultimately was super-saturated into α-Al. No significant grain growth for the nanocrystalline Al-Zn-Mg-Cu alloy was found when it was heated at or below 709 K, for example, the average grain size of the milling alloy heated at 709 K for 1 h is only 71 nm. The results obtained herein suggest that the significant thermal stability of the cryomilled Al-Zn-Mg-Cu material may be attributed to the presence of nano-scale Al2O3 particles.
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    Effect of heat treatment on microstructure and properties of AZ80 magnesium alloy
    TANG Wei; HAN Enhou; XU Yongbo; LIU Lu
    Acta Metall Sin, 2005, 41 (11): 1199-1206 . 
    Abstract   PDF (849KB) ( 1548 )
    The effect of aging temperature on precipitates in AZ80 magnesium alloy was studied by using environmental scanning electron microscope (ESEM) and transmission electron microscope (TEM). The precipitate/matrix orientation relationships (ORs) were measured, the precipitation mechanism was analyzed and the effect of precipitates on properties of alloy was tested. A new precipitated way of granular β-Mg17Al12 phase was found in aged alloy, which was granular and nucleated at grain boundaries (GBs) and twin boundaries. Plate-like or graunlar-like β-Mg17Al12 precipitated continuously in the alloy aged at higher temperature (310℃). The plates were parallel to the basal plane of matrix, nucleated and grew homogeneously in grains, and had little effect on properties of alloy. Whereas the granular β-phase was harmful for the alloy property. The discontinuous precipitation of β-phase occurred at lower aged temperature (<250℃), which was lamellar structure parallel or perpendicular to the basal plane of matrix, nucleated at grain boundaries and grew into grains. The grain boundaries moved into the grains with the growth of these discontinuous precipitates. The alloy was strengthened by the discontinuous precipitates evidently.
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    Hot deformation behavior and micro-structure evolution of superalloy GH742
    ZHANG Beijiang; ZHAO Guangpu; XU Guohua; FENG Di
    Acta Metall Sin, 2005, 41 (11): 1207-1214 . 
    Abstract   PDF (1126KB) ( 1278 )
    Hot deformation behavior of superalloy GH742 was studied by isothermal compression test conducted on MTS machine. Flow stress data at various temperatures from 950℃ to 1150℃ and strain rates from 0.001 s-1 to 1 s-1 were obtained, microstructure evolution during deformation under the above conditions was also investigated. The results show that when deformation was performed at single phase region above 1075℃, rather low flow stress is exhibited, the apparent activation energy approachs the activation energy of grain boundary diffusion, the deformation mechanism is controlled by grain growth rate and the fully recrystallized structures can be achieved. In two phase region, GH742 alloy presents considerably high activation energy, with decreasing temperature and increasing strain rate the flow stress increases swiftly, while the dynamic recrystallization was strongly prohibited. At transition temperature between single and two phase regions, a discontinuous change in flow stress was observed, and the apparent activation energy sharply increased, strain induced dynamically precipitated γ phase inhibits the migration of grain boundaries, the diameter of the recrystallized grain is drastically reduced with increase of plastic strain, and the microstructure was effectively refined.
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    Creep deformation mechanism in a Ni base single crystal superalloy
    LIU Lirong; JIN Tao; ZHAO Nairen; WANG Zhihui; SUN Xiaofeng; GUAN Hengrong; HU Zhuangqi
    Acta Metall Sin, 2005, 41 (11): 1215-1220 . 
    Abstract   PDF (578KB) ( 1186 )
    The creep deformation mechanisms of a Ni base single crystal superalloy at 760℃/780 MPa and 982℃/248 MPa were studied by a transmission electron microscopy (TEM). The results show that under moderate temperature and high stress (760℃/780 MPa), the γ' phase is sheared through dissociation of matrix dislocation, leaving a stacking fault at low strain stage; while at high strain stage, the γ' phase is cut by dislocation pairs. Under high temperature and low stress (982℃/248 MPa), the deformation is based on the movement of dislocations in the matrix, bowing of the matrix dislocations between the γ' precipitates and dislocation reacting to form the dislocation networks at low strain stage; while at high strain stage, the γ' phase is cut as that of 760℃/780 MPa
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    A superheater tube superalloy GH2984 with excellent properties
    GUO Jianting; DU Xiukui
    Acta Metall Sin, 2005, 41 (11): 1221-1227 . 
    Abstract   PDF (549KB) ( 1273 )
    A superheater tube superalloy GH2984 with excellent properties had been successfully developed, which does not contains expensive element Co but has 32%-36% Fe, so it has low cost. In this paper, the characteristics of GH2984, including the chemical composition, mechanical and chemical properties, the microstructure stability after long-time thermal exposure and the processing property at hot and cold conditions have been introduced briefly. Then the superiority of this alloy used in ship boiler and ultra-supercritical coal-fired boiler as superheater was analyzed. The results from practical application to the ship boilers show that the GH2984 superalloy is suitable to the ship boiler superheater tube material, and the alloy also is very important candidate of a superheater and reheater materials in the ultra-supercritical steam boilers
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    Study of bio--corrosion of pure magnesium
    REN Yibin; HUANG Jingjing; YANG Ke; ZHANG Bingchun; YAO Zhiming WANG Hao
    Acta Metall Sin, 2005, 41 (11): 1228-1232 . 
    Abstract   PDF (457KB) ( 1350 )
    Magnesium and its alloys as bio-materials have many obvious advantages. It is possible to develop new type of bio-degradable medical magnesium alloys by use of the poor corrosion resistance of magnesium. The effect of impurity content and processing state on the bio-corrosion properties of two pure magnesiums in the physiological saline solution were investigated. The results showed that both the reduction of impurity content and the grain refinement can enhance the open circuit potential of pure magnesium and decrease the corrosion rate in the physiological saline solution. The corrosion rate of pure magnesium can be controlled through ways of changing the impurity content, grain refinement and solution treatment, which makes magnesium suitable as a new type of pre-developed bio-degradable metal material
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    the decomposition of metastable high temperature γ' precipitates in U720LI alloy
    SONG Xiping; LI Hongyu; GAI Jingfeng; CHEN Guoliang
    Acta Metall Sin, 2005, 41 (11): 1233-1236 . 
    Abstract   PDF (1953KB) ( 964 )
    The decomposition of spherical γ' phase precipitated at high temperature during continuous cooling of U720LI Ni-based alloy has been studied. Fast cooling from supersaturated γ solid solution of the alloy could not inhibit fine γ'precipitates, and the size of the γ' precipitates is about 40-60 nm.When slow cooling to 650℃ and then fast cooling to the room temperature, two kinds of γ' precipitates were found. One was precipitated at high temperature with the size about 200 nm and another was precipitated at low temperature with the size of 50-65 nm in average. Moreover, the big γ' precipitates which precipitated at high temperature were found to be decomposed to form finer γ' precipitates. Because the high temperature γ'\γ interface offers necessary condition for non-classical nucleation theory, any segregation area of γ' formation elements at the high temperature γ'/γ interface can act as pre-existed γ'nuclei and thus promote the precipitation of finer γ' from the saturated γ matrix. Through the short-distance diffusion of the γ' formation elements from the high temperature γ' to pre-existed γ' nuclei, the pre-existed γ' nuclei can grow to be 10-20 nm in size. At the same time, the high temperature γ' precipitates became instable due to loss of γ' formation elements and began to be decomposed as a result. The subsequent ageing promotes the decomposition of the big high temperature γ'.
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