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

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    , Volume 55 Issue 7 Previous Issue    Next Issue
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    Defect Induced Fatigue Behaviors of Selective Laser Melted Ti-6Al-4V via Synchrotron Radiation X-Ray Tomography
    Zhengkai WU, Shengchuan WU, Jie ZHANG, Zhe SONG, Yanan HU, Guozheng KANG, Haiou ZHANG
    Acta Metall Sin, 2019, 55 (7): 811-820.  DOI: 10.11900/0412.1961.2018.00408
    Abstract   HTML   PDF (14834KB) ( 1090 )

    As a very promising additive manufacturing (AM) technique, selective laser melting (SLM) has gained considerable attentions due to the feasibility of producing light-weight metallic components directly from virtual design data. On the other hand, high strength, low density and high corrosion resistance Ti-6Al-4V alloy has been a preferred AM used material for the aviation and military industries. However, the fatigue damage behaviors of SLMed or AMed components usually suffer from interior defects such as incomplete fusion and gas pores due to unstable process or unsuitable processing parameters. Therefore, thorough investigations on process-induced and metallurgical defects and its influence on the fatigue behavior is required for robust designs and engineering applications of high performance SLM components. As an advanced characterization approach, synchrotron radiation micro computed X-ray tomography (SR-μCT) has been recently to investigate the fatigue damage behaviors of critical components with defects. Based on self-developed in situ fatigue testing rig fully compatible with the BL13W1 at Shanghai Synchrotron Radiation Facility (SSRF), several AMed specimens were prepared for in situ fatigue SR-μCT. The Feret diameter and extreme values statistics were then adopted to characterize the defect size, morphology, population, location and the influence on fatigue life. Fatigue fractography was also examined to further identify the defect to really initiate a fatigue crack. Results show that two types of defects including gas pores and the lack of fusion can be clearly distinguished inside SLM Ti-6Al-4V alloys. Fatigue crack with a typical semi-ellipse usually initiates from the defects at the surface and near the surface. Besides, the defects less than 50 μm and sphericity of 0.4~0.65 dominate for the SLM Ti-6Al-4V alloys. It is also found that the larger the characteristic size of the defect, the lower the fatigue life. Current results can provide a theoretical basis and support to predict the fatigue performance of SLM Ti-6Al-4V alloys. Further investigations should be performed on the relationship between the critical defect and fatigue strength by introducing the Kitagawa-Takahashi diagram.

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    Study on Amination Modification of Fe-BTC and Their Adsorption for Dyes and Heavy Metal Ions
    Mengwei CAO,Tao CAI,Xia ZHANG
    Acta Metall Sin, 2019, 55 (7): 821-830.  DOI: 10.11900/0412.1961.2019.00079
    Abstract   HTML   PDF (8080KB) ( 495 )

    The efficient removal of synthetic organics and metal ions from wastewater is an urgent target in the environment remedy. Metal-organic frameworks (MOFs) have received extensive attention owing to their large surface area, tunable pore size and versatile composition of metal and organic ligand, which have presented potential applications in the adsorption/separation of gas, metal ions and also organic dyes. Furthermore, the surface functionalization with some special groups has been proved to be effective in improving the adsorption activity and selectivity. In this work, the diethylenetriamine (DETA) was used to modify the Fe-BTC, and then their adsorption properties toward Congo red (CR) and Pb(II) were studied systematically. SEM, XRD, Fourier transform infrared spectroscopy (FT-IR), N2 adsorption-desorption experiments and Zeta potential measurements were employed to characterize the structure and surface properties of these modified Fe-BTC materials. The results showed that the incorporation of DETA maintained the crystal structure of Fe-BTC as while as effectively increased the surface—NH2 group and also changed the surface charge properties. In the adsorption experiments of CR and Pb(II), the adsorption capacity on the DETA-Fe-BTC was significantly increased compared to that on original Fe-BTC, the adsorption conditions were optimized and the adsorption thermodynamics were analyzed. The adsorption selectivity of DETA-BTC for CR and Pb(II) was also determined through the contrast experiments. In the cyclic adsorption experiments, the DETA-Fe-BTC also exhibited the excellent adsorption stability for CR and Pb(II).

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    Solidification of Al-Bi Alloy and Influence of Microalloying Element Sn
    Wang LI,Qian SUN,Hongxiang JIANG,Jiuzhou ZHAO
    Acta Metall Sin, 2019, 55 (7): 831-839.  DOI: 10.11900/0412.1961.2018.00450
    Abstract   HTML   PDF (16658KB) ( 439 )

    Al-Bi alloy has a low friction coefficient and high wear-resistant properties and is a good self-lubricating material for advanced bearings in automotive applications if the soft Bi-rich phase is dispersedly distributed in the comparatively harder Al-based matrix. However, Al-Bi alloy is a typical immiscible alloy. When cooling a homogeneous single phase liquid of Al-Bi alloy in the miscibility gap, it transforms into two liquids. The liquid-liquid phase transformation generally leads to the formation of a phase segregated microstructure. In the last decades, considerable efforts have been made to study the solidification behavior of Al-Bi alloy. It is demonstrated that the microstructure evolution during the liquid-liquid decomposition is a result of concurrent actions of the nucleation, growth, Ostwald ripening and motions of the Bi-rich droplets. The nucleation and the immigration of the Bi-rich droplets show a dominant influence on the solidification microstructure of Al-Bi alloy. Enhancing the nucleation rate and reducing the Marangoni migration velocity of the Bi-rich droplets promote the formation of a well dispersed microstructure. Considering that addition of surface active element to the alloy may result in a reduction in the liquid-liquid interface energy, and thus reduce the nucleation energy barrier and Marangoni migration velocity of the Bi-rich droplets, the possibility to control the solidification process and microstructure of Al-Bi alloys by adding micro-alloying element Sn was investigated. The experimental results show that microalloying element Sn can cause an effective refinement of the Bi-rich particles. The refining effect increases with the increase of Bi content up to 0.10%Sn (mass fraciton). A model was developed to calculate the microstructure formation. The numerical results demonstrate that Sn can act as an effective surface active element for Al-Bi alloys and promote the formation of a well dispersed microstructure.

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    Effect of Mo Element and Heat Treatment on Corrosion Resistance of Ni2CrFeMox High-Entropy Alloyin NaCl Solution
    Lin WEI,Zhijun WANG,Qingfeng WU,Xuliang SHANG,Junjie LI,Jincheng WANG
    Acta Metall Sin, 2019, 55 (7): 840-848.  DOI: 10.11900/0412.1961.2018.00558
    Abstract   HTML   PDF (15275KB) ( 846 )

    As a new alloy design concept, the high-entropy alloy (HEA) and the formation of simple solid solution introduce excellent properties such as high hardness, high strength and corrosion resistance. Investigations have shown that the single solid solution CrCoFeNi alloy possesses good corrosion resistance. The addition of Mo is beneficial to the corrosion resistance of the HEAs for potential industrial applications in 3.5%NaCl (mass fraction) simulating seawater type environments. The major effect of Mo is to promote the pitting potential of the alloy and inhibit the dissolution of the passivation film by forming and retaining molybdenum oxyhydroxide or molybdates (MoO42-). Considering that the cost of pure Co is higher, Ni and Co elements have similar atomic size and valence electron concentration, and the corrosion resistance of pure Ni is higher than that of pure Co, Ni2CrFeMox HEA was designed by replacing Co element with Ni element in CoCrFeNiMox HEA. As the Mo content increases in the Ni2CrFeMox HEAs, the interdendrite is a Cr and Mo rich σ phase, and the dendrite is a Cr and Mo depleted fcc phase. The potential difference between interdendrites and dendrites leads to galvanic corrosion, which accelerates the localized corrosion of alloys. Here, a solution heat treatment process is selected to reduce the precipitation phase and improve the corrosion resistance of the alloy. The effects of Mo element and heat treatment on the corrosion resistance of Ni2CrFeMox HEA in 3.5%NaCl solution were tested. The results show that the corrosion resistance of as-cast Ni2CrFeMox HEA is obviously higher than that of 316L stainless steel. The Ni2CrFeMo0.2 alloy has the best corrosion resistance because of its minimum dimensional passive current density and corrosion current density. However, the addition of excessive Mo leads to the precipitation of σ phase and galvanic corrosion, which reduces the corrosion resistance of the alloy. After solution treatment, the uniformity of alloy structure and element distribution weakens galvanic corrosion, and the corrosion resistance is obviously improved.

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    Corrosion Behaivour of X65 Low Carbon Steel During Redox State Transition Process of High LevelNuclear Waste Disposal
    Canshuai LIU,Zhaohui TIAN,Zhiming ZHANG,Jianqiu WANG,En-Hou HAN
    Acta Metall Sin, 2019, 55 (7): 849-858.  DOI: 10.11900/0412.1961.2018.00481
    Abstract   HTML   PDF (20380KB) ( 348 )

    Domestic and foreign researches on the corrosion behavior of low carbon steel canister in high level nuclear waste geological repositories focus on the initial aerobic stage and the later anaerobic stage, while few researches have been reported on the corrosion behavior during the disposal transition period. The long term electrochemical corrosion behavior of X65 low carbon steel in 80 ℃ Gaomiaozi bentonite saturated with anaerobic Beishan groundwater has been studied by electrochemical measurement system in anaerobic glovebox constructed independently. The results indicated that the open circuit potential of X65 low carbon steel decreased gradually during 150 d, while the electrochemical impedance of the corrosion film increased with immersion time. Pitting corrosion occurred at the beginning of immersion tests, and finally transformed into general corrosion. Morphologies, compositions, and phases of the corrosion film formed on the carbon steel surface were examined by SEM, EDS and μ?XRD. The results showed that the corrosion film was mainly composed of blocks, slices, rods and swellings. The elemental distribution in the corrosion film was uniform, and the phases were composed of magnetite and hematite. The average corrosion rates were detected by weight loss measurement, which decreased from 195.88 μm/a to 20.58 μm/a. The corrosion rates (V) followed a power function pattern V=8.34t-0.88, indicating that the film growth process was controlled by oxygen diffusion.

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    Influence of Hydrostatic Pressure on Corrosion Behavior of Ultrapure Fe
    Rongyao MA,Changgang WANG,Xin MU,Xin WEI,Lin ZHAO,Junhua DONG,Wei KE
    Acta Metall Sin, 2019, 55 (7): 859-874.  DOI: 10.11900/0412.1961.2019.00044
    Abstract   HTML   PDF (16786KB) ( 424 )

    Hydrostatic pressure is part of the crucial factors affecting deep sea corrosion. At present, there have been a lot of studies on the pitting behavior of metallic materials under hydrostatic pressure, but most of them take passive metallic materials as the research object, and the influence rule of hydrostatic pressure on the pitting behavior of metallic materials also presents diversity. People not only have no clear understanding of its mechanism, but also have some disputes. The generation and growth of pitting corrosion are dependent on the structure of materials, chemical composition and service environment. Inclusion, passivation ability and surface roughness can all affect the pitting behavior of metal materials. Due to the single composition and simple structure of ultrapure Fe, the influence of phase, inclusion and other factors on corrosion behavior under hydrostatic pressure can be avoided, which is more conducive to elucidate the mechanism of hydrostatic pressure on metal corrosion behavior. In addition, the influence of hydrostatic pressure on the corrosion behavior of ultrapure Fe is rarely reported. So, the effect of hydrostatic pressure on the corrosion behavior of ultrapure Fe exposed to 3.5%NaCl aqueous solution is investigated by potentiodynamic polarization curves and electrochemical noise method. The noise signals are analyzed by shot noise theory, stochastic analysis and Hilbert-Huang transform. Besides, the surface morphology of the corrosion sample is observed by SEM. The results of weight loss test and potentiodynamic polarization study show that increasing hydrostatic pressure accelerated the corrosion rate of ultrapure Fe exposed to 3.5%NaCl. The results of electrochemical noise study show that increasing hydrostatic pressure promotes the development of pitting corrosion and increases the tendency of local corrosion throughout the immersion. At the beginning of soaking, local corrosion (such as pitting nucleation, metastable pitting and stable pitting) mainly occurred in ultrapure Fe, increasing of hydrostatic pressure inhibits the pitting nucleation process, but promotes the development of metastable pitting and steady pitting, and increases the growth probability of pitting. With the immersion time prolonging, the uniform corrosion gradually changed into the principal corrosion type, increasing hydrostatic pressure still promotes the development of metastable pitting and stable pitting and improves the growth probability of pitting corrosion, but relatively inhibits the uniform corrosion process.

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    Influence of Electromagnetic Swirling Flow in Nozzle on Solidification Structure and Macrosegregation of Continuous Casting Square Billet
    Chunlei WU,Dewei LI,Xiaowei ZHU,Qiang WANG
    Acta Metall Sin, 2019, 55 (7): 875-884.  DOI: 10.11900/0412.1961.2018.00487
    Abstract   HTML   PDF (9338KB) ( 405 )

    During continuous casting production of square billet, the quality of steel billet is determined by equiaxed grain rate and defect grade of centerline segregation. The application of mold electromagnetic stirrer (M-EMS) can improve the quality, but it also brings some negative effects. Some researchers have attempted to make a swirling flow generated in submerged entry nozzle keep rotating in the mold to replace M-EMS. In this work, the influence of electromagnetic swirling flow in nozzle (EMSFN) on morphology of solidification structure and macrosegregation characteristics of carbon and sulfur was studied under different industrial test conditions, and the results were compared with those obtained by M-EMS. The results show that when the current frequency of EMSFN device is 50 Hz, with the current intensity increasing from 200 A to 600 A, the quantity of equiaxed grains increases gradually, while the severity of centerline segregation decreases first and then increases. The optimum value of centerline segregation was obtained when solidification structure was dominated by fine columnar crystals. Therefore, the swirling flow intensity of molten steel in submerged entry nozzle can be changed by adjusting the current parameters of EMSFN device, and thus the billets with different morphology of solidification structure and severity of macrosegregation can be obtained. Under the experimental condition, when the current parameters of EMSFN device reach certain values, EMSFN can achieve the same or even better effect as M-EMS in improving the quality of billet.

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    Study on Interface of Linear Friction Welded Joint Between TC11 and TC17 Titanium Alloy
    Suigeng DU,Man GAO,Wanting XU,Xifeng WANG
    Acta Metall Sin, 2019, 55 (7): 885-892.  DOI: 10.11900/0412.1961.2018.00512
    Abstract   HTML   PDF (22210KB) ( 671 )

    As a solid-state welding technology, linear friction welding has unique advantages in machining dissimilar titanium alloy blade disc. However, there still lacks sufficient support in basic applied research, and the mechanism of interface formation is still under study. In this work, the microstructure of the welded joint between TC11 and TC17 titanium alloys was analyzed by OM, SEM and TEM, respectively. The results showed that common grains and common grain boundaries are formed at the weld interface. In the common grain, a phase boundary is formed in the weld interface. Elements diffusion is observed on both sides of the common grain boundary and the phase boundary in the common grain. Under the action of rejection, adsorption and towing of solute elements in the formation of common grains and common grain boundary, the observed diffusion distance of elements in the phase boundary of the common grain is longer than the one in the common grain boundary. The composition change at the phase boundary of the weld zone is greater than the one inside the phase. A large number of small needle-like α phases are formed at the weld interface that has a large number of deformed twins. The structure of the interface in common grains consists of two interfaces (recrystallization growth interfaces of both sides) and two growth regions (ordered and disordered). The dynamic recrystallization also has an ordered and disordered crystallization process similar to that of solidification crystallization.

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    Oxidation Behavior of GH984G Alloy in Steam at 700
    Changshuai WANG,Lili GUO,Liying TANG,Rongcan ZHOU,Jianting GUO,Lanzhang ZHOU
    Acta Metall Sin, 2019, 55 (7): 893-901.  DOI: 10.11900/0412.1961.2018.00440
    Abstract   HTML   PDF (18649KB) ( 485 )

    To produce abundant and cheap electricity in a cleaner way, the next generation of advanced ultra-supercritical (A-USC) coal-fired power plants with higher thermal efficiency will operate at service temperatures at 700 ℃ and steam pressures up to 35 MPa. However, the temperature capacity of the currently used ferritic or austenitic steels in USC plants at 600 ℃ cannot meet the requirements. GH984G is a newly developed Ni-Fe-Cr base alloy designed for A-USC, but its oxidation behavior in steam at 700 ℃ is unclear. In this work, the oxidation kinetics of GH984G alloy in steam at 700 ℃ was investigated by weighting specimens at intervals. Morphology, composition and phase constituent of the steam oxide scale were characterized using SEM, EDS and XRD. The results show that the oxidation of GH984G alloy follows a parabolic law with a rate constant of 0.00521 mg/(cm2·h1/2) and steady weight gain rate of 8×10-4 g/(m2·h). The oxide scale mainly consists of Cr2O3 and Al2O3. Meanwhile, a small amount of TiO2 was observed. The oxide scale of Cr2O3 forms on the alloy surface and then the internal oxidation of Al to be Al2O3 occurs along the grain boundaries of the matrix alloy and TiO2 forms at the surface of the external oxide scale. The morphology of Cr2O3 at the surface of the oxide scale is needle-like at the initial oxidation stage and then the agglomeration of Cr2O3 was observed and the cellular shape forms. Finally, the coalescence of the cellular Cr2O3 appears and the flat surface of the external oxide scale forms. The excellent oxidation resistant of GH984G alloy in steam at 700 ℃ can be attributed to the compact external oxide scale of Cr2O3 and the root-like internal oxide scale of Al2O3.

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    Preparation and Performance of a Novel Wear-Resistant and High Temperature Oxidation-Resistant NiCrAlSiC Composite Coating
    Mingyu ZHAO,Huijuan ZHEN,Zhihong DONG,Xiuying YANG,Xiao PENG
    Acta Metall Sin, 2019, 55 (7): 902-910.  DOI: 10.11900/0412.1961.2019.00034
    Abstract   HTML   PDF (19875KB) ( 711 )

    MCrAl (M=Ni, Co, or their combinations) coatings have been widely used as high temperature oxidation protection coatings on turbine blades, as they can thermally grow stable, dense and well adherent Al2O3 protective scales. Due to the particulate nature of the exhaust, MCrAl coatings often fail owing to severe high-temperature wear. To improve the anti-wear resistance of the MCrAl coatings, NiCrAlSiC composite coatings were designed and fabricated by the combination of electrophoretic deposition (EPD) and electrodeposition (ED). The compositions, morphologies and structures of the as-deposited composite coatings were characterized by XRD, SEM, EPMA and TEM. A Ni7.4Cr6.2Al14.3SiC (mass fraction, %) coating, as well as a contrast SiC-free Ni7.2Cr6.2Al coating, was prepared. No cracks or micro pores were found either at the coating/substrate interface or in the coating, and elements distributed uniformly in the coating. Compared to the SiC-free coating, oxide scale on the NiCrAlSiC coating transformed from a three-layered structure (NiO, NiAl2O4 and Al2O3) to a thinner two-layered structure (NiAl2O4 and Al2O3), showing better high temperature oxidation resistance. And microhardness of the NiCrAlSiC coating increased 26%, together with the wear rate reduced 52%. Wear mechanism of the NiCrAl coating was abrasive wear, while that of the NiCrAlSiC coating switched to adhesive wear. These results indicate that the addition of SiC improves both high temperature oxidation resistance and wear resistance of the NiCrAl composite coating obviously.

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    Multi-Scale Study on the Fracture Behavior of Hot Compression B4C/6061Al Composite
    Li ZHOU,Pengfei ZHANG,Quanzhao WANG,Bolü XIAO,Zongyi MA,Tao YU
    Acta Metall Sin, 2019, 55 (7): 911-918.  DOI: 10.11900/0412.1961.2018.00453
    Abstract   HTML   PDF (17474KB) ( 576 )

    B4C/Al composites possess excellent physical and mechanical properties, especially the capacity of neutron absorption, and therefore are increasingly used in nuclear industry for storage and transportation of spent fuels. However, very little study has reported the fracture behavior of B4C/Al composite under hot compression. Therefore, at the present work, the hot compression fracture behavior of B4C/6061Al composite was studied by combining experimental and simulation methods, and the fracture model and damage parameters were determined. A unidirectional multi-scale finite element model was established to analyze the meso damage mechanism of B4C/6061Al composite. The results show that the shear damage model cannot predict the fracture behavior of B4C/6061Al composite because of the inhomogeneous microstructure, and the GTN damage model can accurately predict the hot compression fracture behavior of B4C/6061Al composite. At the same time, by comparing with the experimental results, the GTN damage parameters of 31%B4C/6061Al composite were determined, and then by applying the damage parameters, the calculated crack depth and load-displacement curves agree well with the experimental results. In addition, the micro-damage mechanism of B4C/6061Al composite during hot compression process was analyzed accurately with the unidirectional multi-scale finite element method, which was caused by brittle fracture of particles, debonding between matrix and interface, and ductile damage of matrix.

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    Molecular Dynamics Modeling and Studying of Micro-Deformation Behavior in Metal Roll-Bonding Process
    Qingdong ZHANG,Shuo LI,Boyang ZHANG,Lu XIE,Rui LI
    Acta Metall Sin, 2019, 55 (7): 919-927.  DOI: 10.11900/0412.1961.2018.00524
    Abstract   HTML   PDF (13275KB) ( 710 )

    Stainless steel/carbon steel bimetallic products, which have the characteristic corrosion resistance of stainless steel as well as the characteristics of high strength and low cost of carbon steel, have been widely used in petrochemical, aviation, shipping and other industries. Roll-bonding is an efficient solid-phase joining method for industrial production of bimetallic products. Different from diffusion bonding and friction welding process, the atoms bond and diffuse at interface while the base metal undergoes severe plastic deformation in the process of roll-bonding. In present work, the micro-deformation behavior in the interfacial area of stainless steel/carbon steel during roll-bonding process is studied based on the molecular dynamics method. Firstly, the applicability of the potential function for the bimetallic composite models with different lattice structures was discussed. Then bimetallic model of FeCrNi/Fe and single metal model of FeCrNi, Fe were established. The effect of non-coherent interface on the deformation behavior was revealed by comparing the deformation process of three models. The results show that the mechanical properties and deformation processes of bimetal and single metal are different in the process of deformation bonding. Due to the existence of non-coherent interface, the dislocation in pure Fe matrix is accumulated at the interface during deformation. The local shear effect of interface atoms makes the dislocation formation in FeCrNi matrix easier, thus reducing the yield strength of FeCrNi matrix. The effect of interface on dislocation propagation during alternation makes the dislocation density change alternately in the two matrixes, which improves the ability of material to resist plastic deformation. In addition, the alternately change of the dislocation density within the two matrixes during the deformation process leads to the special phenomenon that the deformation of the two matrixes is alternately changed.

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    Crystal Plasticity Finite Element Method Investigation of the High Temperature Deformation Consistency in Dual-Phase Titanium Alloy
    Xuexiong LI,Dongsheng XU,Rui YANG
    Acta Metall Sin, 2019, 55 (7): 928-938.  DOI: 10.11900/0412.1961.2018.00380
    Abstract   HTML   PDF (13091KB) ( 824 )

    Based on the rate-dependent crystal plasticity constitutive model considering all slip systems, a series of dual-phase polycrystalline models were established using 3D Voronoi tessellation to investigate the high temperature plastic deformation of Ti-6Al-4V alloy with different microstructure features. The spatial distributions and evolution of stress and strain in various grains and phases were calculated in detail, and a new method was proposed to evaluate quantitatively the deformation consistency in the alloy with two phases. Simulations show that grain boundary region responds preferentially in the early stage of deformation. The encircling structure formed between β and α grains can enhance the differences in the local strain distribution. Increasing the aspect ratio of grains and the fractions of heterogeneous phase interface can reduce the local compatibility of deformation. The stress frequency statistics of both α and β phases show a double peak form, with α phase higher in average strain, and β phase higher in stress distribution. Increasing of the volume fractions of α phase may reduce the tensile yield strength, and cause the stress consistency coefficient to decrease, while the strain consistency coefficient decreases first and then increases. As initial α-basal texture intensity increases, both tensile yield strength and stress consistency coefficient increase, while the strain consistency coefficient decreases first and then increases.

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