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

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    , Volume 44 Issue 9 Previous Issue    Next Issue
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    Research Articles
    Modeling of nanoscale friction using molecular dynamics simulation
    Xiao-Ming Liu; Xiaochuan You; Zhuo Zhuang
    Acta Metall Sin, 2008, 44 (9): 1025-1030 . 
    Abstract   PDF (1695KB) ( 1228 )
    The nano scratch process of a rigid diamond tip into Ni substrate has been studied by using molecular dynamcs simulation with EAM potential. Simulations are carried out to investigate the scratch depth effect on the friction force in the scrath process. Furthermore, microstructures around the tip are strongly depended on the scratch depth and dislocation loops can be formed with enough penetration depth. Also, present study reveals that stick-slip phenomenon results from dislocation emission and phonon dissipation. The sawtooth phenomena can be explained from the point that elastic energy stored in the stick process transforms to the dislocations beneath the tip, and then dissipates in the form of phonons, and finally forms the surface defects. Finally, effect of the sliding velocity is studied, which can be deduced from the simulation. The scratch velocity is the critical factor on the dislocation loop nucleation and evolution process. At the higher velocity, dislocation loop glide along slip lane downward to the bulk material. While at the lower velocity, dislocation loops beneath the tip will reaction with each other, and finnaly a large loop will be formed under the subsurface of the material. Plastic deformation will focus on the subsurface of the bulk material under low scratch velocity.
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    Texture and magnetostriction in rolled Fe-Ga based alloy
    Gao Xue-xu
    Acta Metall Sin, 2008, 44 (9): 1031-1034 . 
    Abstract   PDF (916KB) ( 1228 )
    The addition of Nb was used to improve ductility during rolling process. The (Fe81Ga19)+1at.%Nb sheet with thickness of 0.6 mm was prepared by conventional rolling, including hot rolling and cold rolling. Results that demonstrate the rolled texture, recrystallization texture and magnetostriction of (Fe81Ga19)+1at.%Nb sheets are presented in this work. The as-rolled sheet had {111} and near {001}<110> rolling-texture grains, and had magnetostriction only as 26×10-6. A near {011}<100> texture and a {001}<100> texture were formed in recrystallized sheets which were water quenched after two hours annealed at 1250 °C and1300 °C, and their magnetostriction (3/2)s were 106×10-6and136×10-6 respectively.
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    Bone Induction of a Biodegradable Magnesium Alloy
    Acta Metall Sin, 2008, 44 (9): 1035-1041 . 
    Abstract   PDF (3585KB) ( 1430 )
    AZ31B magnesium alloy samples with two dimensions (stabilization splint and dictyo-plate) were implanted on the submaxilla surface of the New-Zealand rabbits, with comparison of Ti-6Al-4V titanium alloy as a control. Two kinds of bone coloboma were made on the submaxilla surface in order to model the different areas of bone defect. It was found from the experiments, after 3 and 6 months, that newly formed bone around the AZ31B magnesium alloy stabilization splint was much more than the control group. After 6 months, both newly formed bone and osteolysis were found on the surface of the dictyo-plate implants. It can be concluded that magnesium alloy implantation is beneficial to the new bone formation, but overdose of magnesium ions from biodegradation can also lead to the increase of osteoclast activity.
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    Dendrite Primary Spacing Selection Simulation by the Cellular Automaton Model
    SHAN Bo-Wei; Weidong Huang; Xin Lin
    Acta Metall Sin, 2008, 44 (9): 1042-1050 . 
    Abstract   PDF (2067KB) ( 946 )
    A cellular automaton model was developed to simulate the primary spacing selection of dendritic array during directional solidification. A simplified growth kinetics was adopted, which could relax the computing complexity, and a strict method to determine the stable state of the system was proposed. Based on two type of primary spacing adjustment mechanisms in the simulation: branching-instability and submerging-instability, in order to determine the allowable range of primary spacing of dendritic arrays for given growth conditions, two different methods of tests were adopted, in one way the seeds number was fixed with a step-varying pulling velocity, and in another way the pulling velocity was constant with different seeds number. The simulated results showed that the allowable range is independent from test methods. The upper limit, λmax,and the lower limit, λmin, of the allowable range as the function of pulling velocity, V, can be generally expressed as the power function of the pulling velocity. During the simulation of the SCN-2.5%ethanol dendrite growth, the parameter of the power function were in good agreement with Huang’s experiments. The simulated lower limit was also in good agreement with Hunt-Lu model.
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    Effect of withdrawal velocity on dendritic interfacial characteristic for Al-Cu alloy during directional solidification
    Min Qu; Lin LIU
    Acta Metall Sin, 2008, 44 (9): 1051-1056 . 
    Abstract   PDF (1810KB) ( 1452 )
    Directional solidification experiments were carried out on Al-4wt%Cu alloy under different withdrawal velocities. It is showed that the primary dendrite arm spacing decreases with increasing withdrawal velocity which is rapid. The solid/liquid interface of dendrite is planar-convex-planar-convex, and it is reinforced first and then weakened and finally almost faded. Dendrite tip temperature is also first increased and then decreased with increasing withdrawal velocity. The KGT model and non-equilibrium effect were used to research the dendrite tip temperature, tip undercooling and composition. This explains the change of dendrite interface morphology well.
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    Effect of Transverse Convection Induced by Density Differences on Bidirectional Solidification of Metal-Gas Eutectic
    Acta Metall Sin, 2008, 44 (9): 1057-1062 . 
    Abstract   PDF (1473KB) ( 852 )
    By use of the bidirectional solidification process of metal-gas eutectic, at an atmosphere of high-pressure hydrogen or gas mixture of hydrogen and argon, a special type of porous material—porous metal with radial pore distribution, can be fabricated. During the solidification of metal-gas eutectic, the volume of the solidifying metal expands due to the evolution of gas pores. This volume expansion leads to a severe transverse convection in front of the solidification interface in the bidirectional solidification process. Experimental results show that during the solidification, the transverse convection in front of the solidifying interface will affect the growth direction of the gas pores, promote gas bubble escaping, and degrade the uniformity of gas pore distribution. By properly designing the structure of the casting mould and choosing appropriate processing parameters, the severe harmful transverse convection in front of the solidification interface can be depressed or limited to a lower level, and high quality radial-type porous magnesium with uniform pore distribution can be obtained.
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    The phase transformation residual stress in granular structure
    Xu Feng-yun
    Acta Metall Sin, 2008, 44 (9): 1063-1068 . 
    Abstract   PDF (2071KB) ( 1461 )
    The phase transformation residual stress caused by the volume expansion of transformation from austenite to martensite was calculated by finite element method based on the assumption of specific distribution and shape of martensite island. The effect of residual stress on the macroscopic strain-stress relationship is also discussed. The results show that tension hydrostatic stress exists in ferrite matrix, condensation stress exists in martensite island. The residual stress increases with the increase of the martensite amount, ferrite yield strength and tangent modulus. There is critical martensite amount for ferrite yielding entirely. The residual stress contributes to the continuous yielding of air cooling granular structure steel but effects the yield strength of granular structure little
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    Dynamic Phase Transformation and Spheroidization of Cementite of Hypereutectoid Steel Containing Aluminum during Deformation
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    Acta Metall Sin, 2008, 44 (9): 1069-1075 . 
    Abstract   PDF (2882KB) ( 1061 )
    The effect of Al on microstructure evolution of a hypereutectoid steel(0.97%C) during deformation of undercooled austenite was investigated by hot uniaxial compression tests. The results indicated that the dynamic transformation during deformation of undercooled austenite included dynamic phase transformation, spheroidization of cementite and dynamic recrystallization of ferrite. The proeutectoid cementites were formed during equilibrium transformation, while the proeutectoid cementites were restrained by deformation of undercooled austenite and the interlamellar spacing of pearlite was reduced. With the addition of Al of about 1 wt%, the proeutectoid cementites were restrained during equilibrium transformation and dynamic phase transformation, the stability of undercooled austenite during deformation was increased, retarding the formation of pearlite and the interlamellar spacing of pearlite was decreased. During the spheroidization of cementite and dynamic recrystallization of ferrite, the coarsening of cementite was restrained and the size of cementite and ferrite was refined by the addition of Al of about 1 wt%.
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    Transformation and deformation characteristics of Ti-50.8Ni-0.3Cr superelastic alloy
    Zhi-rong HE
    Acta Metall Sin, 2008, 44 (9): 1076-1080 . 
    Abstract   PDF (775KB) ( 1142 )
    Effects of annealing and deforming temperatures, and the stress-strain cycle on the transformation, deformation and stress cycling characteristics of Ti-50.8Ni-0.3Cr superelastic (SE) alloy wires and coil springs were investigated with differential scanning calorimetry, X-ray diffraction, tensile test, and cycling test. The 350—600 ℃ annealing Ti-50.8Ni-0.3Cr alloy shows SE property at the room temperature, and the microstructure at the room temperature consists of parent phase. The annealing temperatures influence remarkably transformation type of the alloy. With increasing the annealing temperature the martensitic (M) transformation temperature increases, the R transformation temperature increases firstly then decreases, and the stress-induced M stress decreases firstly then increases. With increasing the deforming temperature the stress-induced M critical shear stress of the SE spring increases. The decline of recovery ratio is quick at the beginning of the stress-strain cycle, and the ratio is slow with the increasing of cycle number. The prior cyclic training can enhance the SE stability of the alloy spring. To obtain stable SE property for the alloy spring, the annealing temperature should be 400—550 ℃, and the using temperature should be over room temperature.
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    Convective Flow and Its Effect on the Liquid-liquid Phase Transformation of Monotectic Alloys
    Acta Metall Sin, 2008, 44 (9): 1081-1084 . 
    Abstract   PDF (633KB) ( 889 )
    A two-phase convective flow model is presented. The model describing the microstructure evolution during the liquid-liquid phase transformation is developed by taking into account of the convective effect. Calculations are carried out for Al-Pb alloy. The effect of the convection on the microstructure formation during the liquid-liquid phase transformation is investigated. The results indicate that the convection changes the nucleation behavior of the minority phase droplets, enhances the maximums of the number density, the radius and the volume fraction of the droplets in front of the solidification interface. It promotes the formation of the solidification microstructure with a serious macrosegregation.
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    SIMULATIONS AND ANALYSIS OF PHASE TRANSFORMATIONS IN bcc-Fe UNDER ISOTHERMAL COMPRESSION
    Acta Metall Sin, 2008, 44 (9): 1085-1089 . 
    Abstract   PDF (1038KB) ( 1071 )
    Using EAM potential and molecular dynamics (MD), we simulate the transition (bcc to hcp) in bcc-Fe under isothermal compression (along [001] orientation). Simulated results reveal that while stress beyond transition threshold, hcp nucleations appear and form flaky grains along (110) face and the system shows an over-relaxation of stress. Mean stress and hcp mass percent increase linearly with volume compression during the growth of hcp nucleations, and longitudinal deviatonic stress reduces linearly with hcp mass percent for the whole transition process. Average potential of hcp atoms are greater than those of bcc atoms.
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    Effects of humidity on domain switching in BaTiO3 single crystals under sustained electric field
    Acta Metall Sin, 2008, 44 (9): 1090-1094 . 
    Abstract   PDF (4300KB) ( 828 )
    Effects of humidity on domain switching behaviors in BaTiO3 single crystals under electric field were investigated by polarized light microscopy (PLM). It was found that the humidity has different effect on different domain configurations. The low humidity has no effect on both a-b domain configuration and b-c domain configuration under fields. However, the high humidity can promote b domain switch to c domain under electric fields. The effects were explained by different adsorption of polar H2O molecules on different domain areas in humidity conditions.
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    EFFECTS OF GRAIN-BOUNDARY PHASES ON HYDROGEN EMBRITTLEMENT OF FE-NI-CR AUSTENITIC ALLOY BY INTERNAL FRICTION
    Jian ZHANG
    Acta Metall Sin, 2008, 44 (9): 1095-1098 . 
    Abstract   PDF (1234KB) ( 1317 )
    Dynamic mechanical analyzer (DMA) was used to investigate the hydrogen embrittlement of two Fe-Ni-Cr austenitic alloys. The effects of hydrogen and grain-boundary phases on internal friction of the alloys were analyzed. Optical microscopy and scanning electron microscopy were used to observe the microstructures and grain-boundary phases. The results show that abundant Ni3Ti phases and little carbides precipitated at grain boundaries in the alloy without boron, while and only little Ni3Ti phase and carbides precipitated at grain boundaries in the alloy with boron. Thermal hydrogen charging caused two new internal friction peaks at about 27℃ and 36℃ in the alloys with and without boron respectively, and the peaks were resulted from the interaction between hydrogen atoms and the interfaces of grain-boundary phases. Boron decreased the temperature of hydrogen-induced internal friction peaks, indicating that the diminishing in grain-boundary phase by boron reduced the ability of grain boundary to trap hydrogen atoms. As a result, boron lowered the ductility loss of the alloy.
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    The influence of size effect of conductivity on the microwave absorption properties of thin Al films
    terry liau; Hao Du; Li-shi WEN
    Acta Metall Sin, 2008, 44 (9): 1099-1104 . 
    Abstract   PDF (1850KB) ( 1164 )
    Thin Aluminum films with different thickness were deposited on PET substrate by Mid-frequency Magnetron Sputtering (MFMS). The DC conductivity of these thin films was measured using linear four probe method. The microwave absorption properties of the Al-FR4 epoxy layered structure were characterized by vector network analyzer. The results indicated that the size effect on conductivity have a pronounced influence on microwave absorption properties of the thin aluminum film, which were verified by theoretical calculation with transfer matrix method.
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    Preparation of Ti(CxN1-x) thick films on titanium by plasma electrolytic carbonitriding
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    Acta Metall Sin, 2008, 44 (9): 1105-1110 . 
    Abstract   PDF (2633KB) ( 999 )
    ABSTRACT Porous nanocrystalline Ti(CxN1-x) thick films which firmly bond to the substrate were obtained on commercially pure titanium by plasma electrolytic carbonitriding. The evolutions of the microstructure and phase compositions of the PECN modified layers with the treatment time were investigated. The results show that the thickness, ratios of C/N and pores sizes of the Ti(CxN1-x) films tend to increase with the discharge time. When discharge-treated for 150 minutes, the film is about 15 冚m thick and exhibits nanocrystalline characterization with grain size of 40-60 nm. The TiH2-riched layer which was induced by the permeation of hydrogen during the PECN locates beneath the Ti(CxN1-x) film, and it can be completely removed by subsequent vacuum annealing treatment while the composition and the surface morphology of the Ti(CxN1-x) film keep unchanged.
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    Driving Force for the Nucleation from Supercooled Liquid and Thermodynamic Analysis of Glass Forming Ability for Binary Alloys
    Na Wang; Zhenmin Du
    Acta Metall Sin, 2008, 44 (9): 1111-1115 . 
    Abstract   PDF (2468KB) ( 1388 )
    Using CALPHAD technique and the reported thermodynamic parameters, the driving forces for the formations of all crystalline phases from supercooled liquid states were analyzed, and the composition ranges with low crystalline abilities but high glass forming abilities(GFA) are estimated thermodynamically for glass forming systems. The typical glass forming binary systems, Cu-Zr, Nb-Ni and Pd-Si, were selected and investigated, in which the eutectic depths of the equilibrium phase diagrams are visibly different. The compositional dependency of GFA was predicted in each alloy system, the best glass formers are obtained and are compared with the eutectic points and the experimental results. The present estimation can explain the experimental best formers for the bulk amorphous systems satisfactorily.
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    CORROSION BEHAVIOR AND VARIATION OF APPARENT MECHANICAL PROPERTY OF ONE NEWLY-DEVELOPED LOW CARBON BAINITIC STEEL IN ENVIRONMENT CONTAINING CHLORIDE ION
    Shu-Tao WANG; Kewei GAO
    Acta Metall Sin, 2008, 44 (9): 1116-1124 . 
    Abstract   PDF (2740KB) ( 1053 )
    The corrosion resistance of one newly-developed low carbon bainitic steel, ultra-low carbon ferritic steel and 09CuPCrNi steel was researched in environment containing chloride ions through alternate wet-dry accelerated corrosion test. The bainitic steel possesses both excellent mechanical property and excellent corrosion resistance. The corrosion rate of the bainitic steel was obviously lower than that of the other two steels and the gap increased with corrosion time. The phase constituents of the rust layers of the three kinds of steel were similar, while corrosion products of the bainitic steel were finest and most compact. The enrichment of Cr and Cu in the rust layer near the matrix of the bainitic steel was most obvious, while the content of chloride was lowest in the same location. The ability of rust layer in the bainitic steel to block permeation of chloride ion was higher than that of the two other steel.
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    EIS STUDY ON THE EFFECTS OF HEAT TREATMENT ON CORROSION BEHAVIOR OF NICKEL BASE ALLOY COATING
    Yong Wang
    Acta Metall Sin, 2008, 44 (9): 1125-1130 . 
    Abstract   PDF (1647KB) ( 1231 )
    NiCrBSi alloy powders were sprayed on a low carbon steel substrate using High velocity oxy-fuel (HVOF) spraying technique and then the samples with coating were heat treated in air furnace for 2h or 10min at 900°C. The corrosion failure process and corrosion resistance of coatings in 3.5% NaCl aqueous solution were investigated by using Electrochemical Impedance Spectroscope (EIS). EIS data shows that the as-sprayed coating cannot resist the penetration of corrosive solution and after 20h the steel substrate begins to be corroded. Local corrosion happens in 2h heat-treated coating if contacting coating with brine longer than 15h, while shape of EIS plots can hold stable for a long time and uniform corrosion happens in 10min heat-treated coating. Relationship of corrosion resistance with time for as-sprayed coating and heat-treated coatings in 3.5% NaCl was obtained using equivalent circuits. The corrosion resistance of 10min heat-treated coating and resistance to penetration of aqueous medium are far better than of as-sprayed coating, but the corrosion resistance of heat-treated coatings with long holding time (2h) is inferior to that of as-sprayed coating. The reasons why heat treatment can change the corrosion behavior of sprayed coating were discussed by the aid of analyzing microstructure.
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    A novel method for solidification rate measurement of weld pool surface
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    Acta Metall Sin, 2008, 44 (9): 1131-1135 . 
    Abstract   PDF (1024KB) ( 806 )
    A novel method for calculating the solidification rate of laser spot weld pool surface is presented. The surface temperature of weld pool during solidification process is measured by a point infrared sensor. Based on the difference in emissivity of the solid-liquid surface, the output data of infrared sensor, which can accurately reveal the liquid-solid transformation, is demonstrated. Therefore the solidification rate at different locations of the molten pool surface is obtained. Experiment result shows that the solidification rate is related to the weld geometry.
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    Study on Vacuum Brazing of 25Cr3MoA/YG6 Using Cu-based Microcrystalline Brazing Alloy
    QiuYa Zhai; Cunkun Liang; Junping LIU; Qunsheng XUE
    Acta Metall Sin, 2008, 44 (9): 1136-1140 . 
    Abstract   PDF (1712KB) ( 1199 )
    The vacuum brazing of cemented carbide YG6 and 25Cr3MoA steel was conducted by using a Cu-11%Sn-2%Ni microcrystalline brazing alloy. The microstructure of joint, behavior of atom mutual diffusion and distributions of alloy elements within the joint were analysed. The shearing strength of joints was tested using a special tester. The experimental results show that the Cu-based microcrystalline braze performs an excellent wettability and spreadability during vacuum brazing of 25Cr3MoA/YG6, and the brazing seam obtained is permeated soundly, thus the joint has a higher welding-on rate; the microstructure of brazing seam consists mainly of Cu-rich Cu13.7Sn phase and Ni-rich Cu9NiSn3 phase distributing within the Cu-rich phase. The diffusion range of Cu atoms from brazing seam to base metals reaches to 2.5μm. Therefore, the diffusion brazing of 25Cr3MoA and YG6 has be realized and the shearing strength of joint is high up to 169MPa.
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    The interfacial behavior of molten steel and liquid slag in a slab continuous casting mold with electromagnetic brake and argon gas injection
    YU Hai-Qi
    Acta Metall Sin, 2008, 44 (9): 1141-1148 . 
    Abstract   PDF (6983KB) ( 993 )
    It described a numerical simulation study for the interfacial behaviour between molten steel and liquid slag layer with different flow-control technologies in the slab continuous casting mold, considering the effects of argon gas flow rate, casting speed and current intensity of coils on the interfacial behaviour of molten steel and liquid slag with the coupling action of electromagnetic brake (EMBr) and argon gas injection. The relationship between the level fluctuation index of F value and level fluctuation of free surface was also investigated. For a given casting speed with EMBr, increasing the argon gas flow rate can aggravate the local fluctuation of interface so much as lead to the breakup of the steel-slag interface, F value increases with the increasing argon gas flow rate and the thickness of liquid slag near the meniscus increases linearly with F value. For a given argon gas flow rate with EMBr, increasing the casting speed has helpful for restraining the interfacial fluctuation near the nozzle, and F value also increasing, the thickness of liquid slag near the meniscus reduces linearly with F value. On the contrary, for a certain casting speed and argon gas flow rate, increasing the current intensity of coils can aggravate the steel/slag interfacial fluctuation near the nozzle.
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    PREPARATION AND CHAIACTERISTICS OF (Y,Gd)2O3:Eu3+ NANOPARTICLES
    WeiMin Ma
    Acta Metall Sin, 2008, 44 (9): 1149-1152 . 
    Abstract   PDF (669KB) ( 822 )
    In this paper, the (Y,Gd)2O3:Eu3+ superfine powders were prepared by co-precipitation method and ammonia was employed as precipitating agent. The means of XRD 、TEM、 SEM and DSC/TG were used to analyze the morphology and characteristics of the powders. The excitation spectrum and emission spectrum of the samples were analysed by the fluorophotometer. The results show that the powders are spherical particles with uniform size distribution of 20nm in the case of the powders were calcined at 800℃ for 2h. The sample has two excitation peaks at 235nm and 250nm when the monitoring wavelength is 612nm. The two peaks are caused by the Gd2O3 medium absorption and the CTS absorption of Eu3+ respectively. The intensity of the emission spectrum with the exciting at 235nm is higher than excited at 250nm. When the molar concentration of corporation Eu3+ is 3%, the emission peak of 5D0→7F2 transition states of the emission spectrum is the strongest, and when the corporation Eu3+ is 7%, intensity of the emission peak decreased, which is due to the concentration quenching.
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