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Synthesis of Mg–Zn–Ca metallic glasses by gas-atomization and their excellent capability in degrading azo dyes
Materials Science and Engineering B 181 (2014) 46–55
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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Synthesis of Mg–Zn–Ca metallic glasses by gas-atomization and their excellent capability in degrading azo dyes Y.F. Zhao a , J.J. Si a , J.G. Song a , Q. Yang b , X.D. Hui a,∗ a b
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China School of Water Resources and Environment, China University of Geoscience, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 21 August 2013 Received in revised form 25 October 2013 Accepted 19 November 2013 Available online 2 December 2013 Keywords: Azo dyes degradation Mg–Zn–Ca Metallic glasses Gas-atomization
a b s t r a c t Mg–Zn–Ca powders of Mg63+x Zn32−x Ca5 (x = 0, 3, 7 and 10) with the diameter from 2 m to 180 m were synthesized by gas-atomization. The relationship among powder morphology, the composition, glass forming ability, thermal stability, corrosion resistance and the capacity in degrading azo dyes for these powders was investigated. It is shown that fully glass powders with the particle diameter < 150 m can be atomized for alloys with x ≤ 7. These Mg–Zn–Ca metallic glass powders exhibit remarkably superior corrosion resistance and degradation capacity in Direct Blue 6 solution to their crystalline counterparts and Fe powders. Nano-whiskers were formed uniformly and loosely on the reacted surface of the Mg70 Zn25 Ca5 glassy powder, which is considered as the mechanism of high degrading capacity for these Mg–Zn–Ca glassy alloys. This work will contribute to the development of massive production of high quality metallic glass powders with excellent capability in degrading azo dyes. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Synthetic organic dyestuffs have been widely used as important ingredients in textile, food, paper making and printing, leather and cosmetic industries. Most of the dyes contain benzene and heterocyclic macromolecular compounds with the azo functional groups ( N N and N O) and/or polar groups ( SO3 Na, OH, NH2 ). It is estimated that approximately 12% of the synthetic textile dyes were discharged as dye wastes [1]. The environment and human health will be seriously threatened if these organic dyes were discharged into the surperfacial and ground water without sufficient degradation. Therefore, it is urgent to explore efficient physical/chemical method for organic dyes treatment in industrial wastes. The degradation mechanism of azo dyes can be divided two steps. (1) The first step is to have the azo dyes reduced and the bonds of N N and N O broken. Aromatic amines will be generated at this step, which are generally more toxic than the parent compounds. From the views of physiology, biochemistry and bioenergetics, the essence of the reduction process is a respiration process, so called azorespiration [2]. The second step of degradation process is the further decomposition of aromatic amines and mineralization. Azo dyes in aqueous solution can be degrade by ozone through destroying most of the double bonds, such as
∗ Corresponding author. Tel.: +86 10 62333066; fax: +86 10 6233447. E-mail address: [email protected] (X.D. Hui). 0921-5107/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2013.11.019
C C , C N , and N N [3,4]. Based on these recognitions, a variety of physical, biological, and photochemical methods for remediating azo dyes in wastewater have been developed to date. The activated carbon adsorption method is the conventional physical process which transfers the dyes from one phase to another but cannot degrade them [5]. The biotic degradation methods are usually limited to degrading some special toxic azo dyes but fail to act in the complex environment containing various chemicals [6–8]. It has been demonstrated that metals can effectively reduce a broad array of organic compounds such as chlorinated aliphatics [9], nitro aromatics [10], polychlorinated biphenyls (PCBs) [11], pesticides and related compounds [12–15]. Zero-valent Fe has been widely used to initiate remediating more complex anthropogenic chemicals by the reduction of critical functional groups [10,16–18,19]. Pesticides that may be subject to this treatment include DDT, DDD, DDE [20], and atrazine [21]. It is especially noticed that zero-valent Mg has been also employed for degrading various organic water pollutants with its high capacity. The very low corrosion resistance of pure Mg in air and water, however, reduces its endurance and results in the consumption of a large amount of Mg by water. Therefore, it is necessary to search for the novel Mg bear materials for degradation of azo dyes. Recently, Mg–Zn–Ca bulk metallic glasses (BMGs) with biodegradation properties, biocompatibilities of released alloying elements and excellent mechanical properties have been developed [22,23]. All the three components are environmental friend, and the degrading products will not cause the secondary pollution. The first Mg–Zn–Ca bulk metallic glass was reported by Gu
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et al. in 2005 [24]. Then Mg–Zn–Ca BMGs with critical diameter of 5 mm were prepared by using conventional casting techniques [25,26]. One of the most important features for the structure of metallic glasses is that metallic glasses have not grain boundaries, which results in the obvious improvement of corrosion resistance. Wang et al. [27] synthesized Fe particle reinforced Mg69 Zn27 Ca4 BMG composites by copper mold injection casting with industrial raw materials. It has been shown that the corrosion resistance of this BMG based composite in 3.5 wt.% NaCl solution has been remarkably improved compared with that of AZ31 and pure Mg. The compressive fracture strength of Mg96−x Znx Ca4 (x = 30, 25) BMGs exhibit surprisingly high uniformity, indicating that they are reliable as structural materials [25]. The feasibility as biodegradable metallic materials has been evaluated by investigating their cytotoxicity and corrosion properties. The experimental results show that these BMGs have higher cell viabilities than that of asrolled pure Mg. By animal experiments, Zberg et al. revealed that Mg–Zn–Ca BMGs exhibit great reduction in hydrogen evolution below Zn-alloying threshold (∼28 at.%) and the same good tissue compatibility as seen in crystalline Mg implants [28]. As for the degradation capacity of Mg–Zn–Ca amorphous alloys, Wang et al. [29] reported that the ball-milled glassy Mg-based alloys have remarkably superior capacity in degrading azo dyes to that of commercial crystalline Fe powders, Mg–Zn alloy crystalline counterparts and Fe-based metallic glasses powders. Since metallic glasses are formed by far-from-equilibrium process, the constituent atoms of metallic glasses do not reside at the thermodynamic equilibrium positions. Therefore, the metastable nature imparts amorphous alloys the good catalytic and special chemical properties [30,31]. Zhang et al. revealed that there are a depressed valence band maximum and a widened empty band in the amorphous ribbon, which could reduce the activation energy of the degradation process and enhance the activity of the electrons, thus accelerating the degradation process [32]. The large glass forming ability (GFA), excellent capability in degrading azo dyes, good biocompatibility, low cost, and easy recycling ability of Mg–Zn–Ca BMGs make them promising as biomedical and structural materials [33–35]. However, the productivity of current preparation technologies of Mg–Zn–Ca amorphous alloys is relatively low which will limit the mass application for the degradation of azo dyes. Moreover, the composition dependence of degrading azo capability for this kind of metallic glass has not been investigated. In this work, we report for the first time the synthesis of Mg–Zn–Ca glassy powder with excellent capability in degrading azo dyes by gas-atomization technology. The advantages of this technology lie in the mass production of high quality powders without surface contamination. Moreover, powder metallurgy processing in the supercooled liquid state enables the production of bulk metallic glassy foams with various shapes. Since the glass forming and degradation ability of the atomized powders are affected by the compositions and sizes, four Mg–Zn–Ca alloys with Mg content in the range from 63 at.% to 73 at.% are designed. The powders of these alloys with the diameters from 2 m to 180 m are prepared. The correlations among the composition, particle size, glass forming ability, thermal stability, corrosion resistance and the capacity in degrading azo dyes for this kind of metallic glass are investigated.
2. Experimental methods The atomization processing was carried out in a close-coupled nozzle ultrasonic atomization system, as shown in Fig. 1. After removing the oxidation film from the surfaces, we put the mixture of pure metals of Mg (99.9 wt.%), Zn (99.9 wt.%), Ca (99.5 wt.%)
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Fig. 1. Schematic of close-coupled nozzle ultrasonic atomization system: (1) melting chamber, (2) induction coil, (3) nozzle system, (4) atomization chamber and (5) cyclone separator.
with the nominal chemical compositions (at.%) of Mg63 Zn32 Ca5 , Mg66 Zn29 Ca5 , Mg70 Zn25 Ca5 and Mg73 Zn22 Ca5 (hereafter denoted as M63, M66, M70, M73 for convenience) in graphite crucible in the vacuum furnace. The furnace was firstly pumped to high vacuum (5.0 × 10−3 Pa) and then backfilled with high-purity argon (99.999 wt.%) gas for protection. Then we melted the alloy by using medium frequency power supply. The melt was then cooled down by turning off the power. After holding for 10 min, the alloy was remelted by the power supply up to a superheat state above the melting point by approximately 200 K in the graphite crucible under 0.4 MPa. Then the melt was delivered through a draft tube with the diameter of 1 mm, and atomized by jetting the high-purity argon (99.999 wt.%) through the close-coupled nozzle at ultrasonic speed of 6 MPa. The Mg–Zn–Ca powders were cooled down to ambient temperature under the inert gas atmosphere in the atomizing chamber. Afterwards, the powders with diameters in the range from 2 m to 180 m were collected and separated by different sizes in the range of <25 m, 25–48 m, 48–75 m, 75–150 m and 150–180 m. In this work, we also prepared the Mg–Zn–Ca ribbons by using melt spinning. The Mg–Zn–Ca master alloy was remelted in a quartz tube by high frequency power supply in vacuum chamber, then injected onto a rotating copper wheel under the pressure of 0.2 MPa. The line velocity of the copper wheel is about 10 m/s. Under these conditions, ribbons with the size of 0.025 mm × 5 mm were obtain ribbon samples. Because the cooling rate of melt spinning is higher than that of gas-atomization, it is reasonable to infer that the four Mg–Zn–Ca ribbon samples are more easily to form glassy structures those by using atomization. The electrochemical polarization properties of the four kinds of glassy alloys were investigated by using the Gill-AC electrochemical measuring instrument. The working electrode was made by attaching a Cu wire to one end of the ribbon samples which was closely
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Fig. 2. SEM images of M66 atomized powders with different size ranges: (a) 150–180 m, (b) 75–150 m, (c) 48–75 m and (d) 10–48 m.
sealed with epoxy resin. After immersing the specimens in 0.2 g L−1 azo dye solution for about 15 min the open circuit potentials became almost steady. Then the potentio-dynamic polarization curves were recorded at a potential sweep rate of 20 mV min−1 . The capability in degrading Direct Blue 6 (DB6) was measured for these metallic glass powders. The DB6 was dissolved in distilled water to obtain the solution with the concentration of 0.2 g L−1 . 8 mL solution was put into a 10 mL centrifugal tube. In the preparative experiment, it was found that the degrading rate of DB6 by Mg–Zn–Ca glassy powders with small size is too rapid to be measured. Therefore we determined to employ different class of investment quantity for the glass particles with different size. For the particles with the size lower than 25 m and in the range of 48–75 m, 0.05 and 0.2 g metallic glass powders, respectively, were thrown into the centrifugal tubes. Then the tube was whirled with the speed of 150 rpm at the temperature of 300 K. The concentration change of azo dye solution during the degradation process was measured by 722S mode of visible spectrophotometer. During the degrading process, 4 mL solution was taken out at certain time interval from the centrifugal tube and filtered through 0.45 m membrane filter for measuring the absorbance at 580 nm wavelength. The concentration change of azo dye solution was determined according to the standard working curve (constructed calibration curve).
The phase constituent of atomized Mg–Zn–Ca powders was investigated by X-ray diffraction (XRD). The thermal stability of atomized powders against crystallization was examined with differential scanning calorimetry (DSC) at the heating rate from 20 K min−1 to 50 K min−1 under an argon atmosphere. The change of component elements on the surface was measured by using energy-dispersive X-ray (EDX) spectroscopy. The morphologies and microstructure of the powders were characterized by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). 3. Results and discussion 3.1. Morphologies of as-atomized powders In this work, the powders of M63, M66, M70 and M73 Mg–Zn–Ca alloys with the diameter in the range: <25 m, 25–48 m, 48–75 m, 75–150 m and 150–180 m, respectively, have been synthesized by atomization. Typical SEM images of M66 atomized powders with the diameters from 2 m to 180 m are shown in Fig. 2. It is seen that most of the powders with the size lower than 25 m take the shape of sphere. The surface of the powders is clean and smooth. The SEM images of the atomized powders with diameters lower than 10 m for M66 and M73 alloy are shown in Fig. 3.
Fig. 3. SEM images of atomized powders with diameters lower than 10 m in (a) M66 and (b) M73 alloy.
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Fig. 4. XRD curves of atomized powders of (a) M63, (b) M66, (c) M70 and (d) M73 alloy.
It is seen that the powders have perfect spherical shape. And the smaller the powders, the more smooth and cleaner the surface [36]. The powders with the size of 25–48 m are composed of spherical and dumbbell-shaped particulates. The particle shape of the powders with the size from 75 m to 150 m is similar to that of powders in the range of 25–48 m. It is also found that small planetary particles are attached to the large powders. For the powders with size larger than 150 m, there are some discoideus and elliptical particulate besides the spherical powders. And the surface of these large powders is rough and attached with small planetary particles. It is seen that from Figs. 2 and 3, the shape of particle is influenced by its size, which reflects that the solidification rates are different for powders with different diameters. The larger powders have lower solidification rate due to the long time needed to transfer the heat from the melts. During atomization process, the larger droplets might be still in liquid or semi-solid state when the powders are touched to the wall of chamber. Therefore, the large droplets may be collided to form plate-like discoideus or deformed in the flying to form elliptical particulate. However, the small droplets may be completely solidified in short term in the chamber. Under the role of the turbulence of jet gas, these small particles form suspended atmosphere in the chamber, and colloid with the large particle which incompletely solidified. As result, the small powders are adhered by the large particulates to form the planetary particles.
3.2. Phase constituents of as-atomized powders The XRD patterns of four kinds of alloy powders with different size are shown in Fig. 4. It is seen that both the composition and the size affect the phase structure of atomized powders. All the XRD patterns of M63 particles with the size up to 180 m only consists of a broad diffusion peak located at 2 ≈ 38◦ , indicating the formation of fully glassy phase. For M66 and M70, the XRD patterns of
as-atomized particles with the size of 150–180 m shows a broad halo, which indicates the presence of glassy phase, and overlapped with the crystalline peaks of ␣-Mg. The XRD patterns of the particles of M66 and M70 with size lower than 150 m exhibit only a broad halo of fully glassy phase. For the as-atomized M73 (Fig. 4d) alloy, the XRD patterns show that some weak crystalline peaks are supposed on the broad halo when the particle size is lower than 75 m, indicating the coexistence of glass structure and crystalline. As the particle size of the powders is larger than 75 m, the XRD patterns exhibit a weak halo and a set of diffraction peaks of ␣-Mg, indicating that the microstructure is almost crystalline. Our previous experimental results have shown that M66 alloy has high glass forming ability with the critical diameter up to 5 mm. The XRD patterns of Fig. 4 are in accordance with those results as shown in supplementary Figure. Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mseb.2013. 11.019. The glassy feature of the structure of the atomized powders in this work was also studied by using TEM. Fig. 5 shows the bright-field TEM image and selected-area diffraction pattern (SAD) of the powders of M70 with the diameter lower than 10 m when exposed to the electronic beam for different time. When exposed for the electronic beam for 10 min, as shown in Fig. 5a, the SAD exhibits diffuse halo without diffraction spots, indicating the structure of fully glassy phase. However, as the sample exposed for 30 min, as shown in Fig. 5b, nanoparticles were disintegrated from the powder due to the heating of high energy electronic beam. The SAD of remained powder is composed of diffuse halo and several weak spots, indicating that partial crystallization took place during the TEM observation. 3.3. The thermal stability of atomized powders It is important to understand their thermal stability and phase transformation because the glass state is essentially metastable in
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Fig. 5. (a) Bright-field TEM image and selected-area diffraction pattern of M70 powders alloy and (b) the disintegrated nanoparticles from the powder during the TEM observation due to the effect of electronic beam.
thermodynamics. Fig. 6 shows the DSC curves of the as-atomized Mg–Zn–Ca powders with different sizes when heated at 20 K min−1 . Generally, the DSC curves of the four kinds of as-atomized Mg–Zn–Ca alloys contain an undercooled liquid region and three crystallization peaks in the range from 400 K to 550 K. However, the widths of the supercooled liquid region for the four kinds of alloys are not same. And the secondary peaks on the DSC curves of M63 and M73 are not obviously, just form the left shoulder of the third peaks, indicating the internal structure of these atomized alloys are different. The glass transition temperature, Tg , onset temperatures of crystallization events, Tx , Tp1 , Tp2 , Tp3 , and corresponding latent heats, H1 , H2 , H3 , respectively, for the four kinds of alloys are presented in Table 1. It is seen that these characteristic parameters
are mainly affected by the composition. The crystallization temperature decreases with the increase of the content of Mg in the alloys. For the powders with the same composition but different particle sizes, the values of thermal parameters of Tg and Tx are similar indicating that the size effect of the powders is slight. The M66 atomized powders have the largest supercooled liquid regions among the four kinds of synthesized alloys. The crystallization behaviors of M63, M66 and M70 during continuous heating were investigated by DSC measurement. Fig. 7 shows the typical DSC traces for M66 powders with the diameter lower than 25 m at the heating rate from 20 K min−1 to 50 K min−1 . As the heating rate is increased, all the three exothermic peaks are shifted to higher temperature. The variation of observed
Fig. 6. DSC curves of atomized powders of (a) M63, (b) M66, (c) M70 and (d) M73 alloy heated at 20 K min−1 .
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Table 1 Thermal stability parameters of M63, M66, M70 and M73 at the heating rate of 20 k min−1 . Samples
H1 (J/g)
Tp2 (K)
H2 (J/g)
Tp3 (K)
H3 (J/g)
498 499 500 501 502
16.7 16.61 15.32 14.91 16.66
Size (m)
Tg (K)
Tx (K)
Tp1 (K)
M63
<25 25–48 48–75 75–150 150–180
417 418 417 416 416
431 431 430 430 429
441 440 440 440 439
7.872 7.668 7.975 7.431 7.271
477 476 477 477 477
M66
<25 25–48 48–75 75–150 150–180
408 408 407 407 408
423 422 422 423 421
431 431 430 430 430
7.997 7.807 7.573 7.541 5.427
476 476 477 478 479
14.8 14.68 14.37 14.17 11.37
524 524 523 523 523
7.085 6.983 6.969 6.542 6.071
M70
<25 25–48 48–75 75–150 150–180
396 399 399 400 400
407 409 409 410 408
417 419 418 419 418
7.861 7.5 7.158 7.41 6.526
454 455 454 454 454
16.54 14.98 13.34 13.91 12.54
526 526 525 525 525
6.015 5.672 5.528 5.748 5.26
M73
<25 25–48 48–75 75–150 150–180
396 396 396 397 396
406 405 408 408 408
417 416 416 415 416
20.44 20.11 19.72 8.208 2.205
488 487 488 487 487
509 508 509 509 508
3.833 3.771 3.667 2.489 1.048
0.2652 0.2169 0.226 0.1649 0.03531
0.3704 0.3192 0.3564 0.1423 0.03051
Fig. 8. Electrochemical polarization dynamics curves of the four kinds of metallic glass ribbons and crystallized M73 ribbon in 0.2 g L−1 direct blue 6 solution. Fig. 7. DSC traces of M66 powders with the diameter lower than 25 m at the heating rate from 20 K min−1 to 50 K min−1 .
characteristic temperatures with the applied heating rate can be described by the Kissinger equations [37,38]
ln
T2 ˇ
=
E +A RT
(1)
where ˇ is heating rate, T is peak temperature, R is the gas constant, and A is constant. The apparent activation energy, E, can be obtained by the slope of −ln(T2 /ˇ) versus 1/RT linear. The activation energies calculated from the slope of the DSC plots for M63, M66 and M70 glassy alloys are listed in Table 2. It is seen that M66 glassy alloy has the largest Eg , Ex and Ep1 , meaning that M66 has the best thermal stability to resist the crystallization among the three kinds of glassy powders.
Table 2 Activation energies for glass transition, crystallization, and first thermal peak in M63, M66 and M70 powders. Samples
Eg (kJ/mol)
Ex (kJ/mol)
Ep1 (kJ/mol)
M63 M66 M70
238 242 240
245 247 245
242 245 239
3.4. The corrosion resistance properties The corrosion resistance behavior is critical to the application of these powders for decoloration process. The electrochemical polarization dynamics curves of the four kinds of glassy ribbons in 0.2 g L−1 DB6 solution are shown in Fig. 8. Some corrosion data, including the passive potential (E0 ) and passive current density (ip ) derived from the curves are summarized in Table 3. It is shown that the corrosion resistance of Mg–Zn–Ca metallic glasses in azo dye solutions was directly influenced by the composition. It is obvious that for glassy structures, the lower the content of Mg, the higher the potential of pitting corrosion and the lower the passive curent density, indicating that the decrease of Mg content increases the corrosion resistance of the glassy alloys. From Fig. 8, it is also seen that the corrosion resistance of glassy structure is far superior to that of crystalline structure. For M73 alloy, the pitting potential of Table 3 Passive potential (E0 ) and passive current density (ip ) derived from the curves of Fig.8 for the five kinds of alloys. Sample
ip (A cm−2 )
E0 (mV)
M63 M66 M70 M73 Crystallized M73
0.21 0.40 4.57 6.03 30.90
−1065.9 −1134.3 −1224.2 −1316.8 −1440.5
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Fig. 9. Visual images of DB6 solution after degraded for different time by (a) Fe powders and (b) the presented atomized M70 powders with diameter of 48–75 m and the investment quantity of 0.2 g.
crystalline structure is lower than that of glassy structure by about 120 mV, and the the passive curent density is increased by about 4 times. According to Ref. [39], Zn is the least reactive element in neutral pH, in Mg–Zn–Ca ternary system. As the Ca content is constant, the decrease of Mg content corresponds to the increase in Zn content. In the glassy structure, all the elements exist homogeneously in the atomistic form. Therefore, it is easy to understand that the observed increase in corrosion resistance is actually due to the increase of Zn. In other words, the appropriate increase of Mg may increase the activity in azo dye solutions although it may result in the decrease of the service life of Mg–Zn–Ca metallic glass powders. 3.5. The degradation capacity The degradation process of DB6 solution by the presented atomized M66 and Fe powders which have the same particle size in the range from 48 m to 75 m was visually observed. As shown in Fig. 9, it is seen that the degradation capacity of Mg based glassy powders is remarkably improved compared with that of Fe powders. The azo dye in the solution containing atomized M66 powders was degraded very fast in the first minute of exposure. The blue color of solution began to decay after 1 min. After 4 min of the exposure, the color of the solution disappears completely, indicating that the azo bonding is almost degraded. For the solution containing Fe powders, the color did not change obviously even after 20 min. The blue color in this kind of solution still holds after 60 min. The above phenomenological variation can be described by the dependence of the concentration of DB6 in the solution on the time. From Fig. 10a, it is seen that the concentration of DB6 was decreased to 0.01 g L−1 in 2 min by M66 glassy powders with the diameters lower than 25 m. With the diameters of glassy powders increased to 48–75 m, the degrading rate is decreased. After 5 min, the concentration is decreased to 0.04 g L−1 . It is found that the decomposed azo dye by the commercial Fe powders has not reached 10% after an hour. The compositional and size dependence of degradation rate of the glassy powders is shown in Fig. 10b and c. Given the same azo
dye concentration and investment quantity, the reaction rates are influenced by the content of Mg and the structure. Here we compare the degradation process of the DB6 solution by using the four kinds of Mg based atomized powders which are in the same size ranges. It can be concluded that the degradation rate follows the order of M70 > M73 > M66> M63 under the same experimental conditions. This result means that the degradation rate of the DB6 solution by glassy alloys is determined by the content of Mg. From the Xray results, it is seen that M73 is of partially glassy structure. The existence of crystalline phase decreases the reaction rate although the content of Mg is most among the four interested alloys. From Fig. 10, it is also seen that the size of the particles affects the degradation rate of the DB6. In the degradation experiments with the investment quantity of 0.2 g glassy powders which have the diameters lower than 25 m, the degradation process of the DB6 by glassy powders with diameters of lower than 25 m is too fast to be recorded. Therefore, we have to decrease the investment quantity to 0.05 g for these fine powders. This result illustrates that the particles with smaller size has higher degrading capacity. It is well known that the degradation reaction of azo dyes by using metallic glasses occurs on the surface of powders. When effective collision between dye molecules and powders takes place [40,41], Mg and Ca lose their electrons while the dye molecule accepts electrons, the azo bond N N is then broken into two NH2 . The degradation process can be described by the pseudofirst-order kinetic model [42,43], C = A + Be−t/t0
(2)
where C is the concentration of the solution which is taken from the centrifugal tube, A and B are fitting constants, t is the degradation time, and t0 is the time when the characteristics concentration decreases to e−1 of the initial state derived by fitting the experimental data. The reaction rate can be estimated by the value of t0 , as shown in Table 4. For atomized M66 alloy shown in Fig. 10a, it can be calculated that the capacity of particles with the diameters lower than 25 m (t0 = 0.48 min) is about 23 times faster than that of the powders with 48–75 m (t0 = 11.05 min) with the investment quantity of 0.2 g. It is also found that the capacity of M70 powders
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Fig. 11. (a) XRD patterns of as-atomized and annealed M70 powders with diameters of 45–80 m and (b) decolorization process of DB6 solution with the concentration 0.2 g L−1 at the ambient temperature by as-atomized and crystallized powders of M70 alloy.
Fig. 10. Decolorization process of DB6 solution with the concentration 0.2 g L−1 at the ambient temperature by (a) M66 (d < 25 m and 48–75 m) and Fe powders (d = 48–75 m), (b) M63, M66, M70 and M73 (d = 48–75 m) and (c) M63, M66, M70 and M73 (d < 25 m) powders, with the investment quantity of 0.2, 0.2 and 0.05 g, respectively.
Table 4 The characteristic time (t0 ) of degradation process value of M63, M66, M70 and M73 atomized powders from the curves of Fig. 8b and c. Sample
M63
M66
M70
M73
t0 (48–75 m) t0 (<25 m)
11.05 21.54
5.09 13.25
0.98 7.15
3.26 9.28
(t0 = 0.98 min) with 48–75 m is about 11 times faster than that of the M63 glass powders (t0 = 11.05 min) when the investment quantity is 0.2 g. When the investment quantity is decreased to 0.05 g for the powders with the size lower 25 m, the capacity of M70 powders (t0 = 7.15 min) is about 3 times faster than that of the M63 glass powders (t0 = 21.54 min).
Metallic glasses are metastable, and the constituent atoms do not reside at the thermodynamic equilibrium positions (corresponding to crystalline state) but locate at far-from-equilibrium positions [29,30]. The metastable nature imparts metallic glasses a variety of excellent properties which are unachievable for crystalline alloys, such as the good catalytic and chemical properties. These merits of glassy alloys can be observed by comparing the degrading capacity of the atomized powders with their crystalline counterparts. As shown in Fig. 11a, M70 powders have been completely crystallized after annealed at 573 K for 1 h. Mg solid solution, Ca2 Mg5 Zn13 and MgZn2 compounds are formed. The solid solution reserves partial decolorization capacity of azo dye. But Ca2 Mg5 Zn13 and MgZn2 compounds completely lose their catalytic capacity, leading to the decrement of decolorization rate. From Fig. 11b, it is clearly shown that the crystallized powders exhibit much lower decolorization capacity than that of glassy structure. The concentration of the azo dyes only decreases 25% after 20 min when degraded by the crystallized alloy. To explore the decolorization mechanism of Mg based glassy alloys, we studied the surface morphologies and compositions of the powders after reacted with the DB6 solution. Fig. 12 shows that nano-whiskers are distributed uniformly and loosely on the reacted surface of the M70 glassy powder. The uniform distribution without aggregation also denotes that the powders have high activity in this reaction. The loose structure of the reaction products on the surface assures high reaction rate. It is also observed that the surface
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Fig. 12. (a) and (b) The surface morphologies after reacted with DB6 solution for an hour and (c) and (d) the EDX patterns of the initial surface of as-atomized and reacted surface of M70 powders after reacted with DB6 solution.
beneath the nano-whiskers is still clean and densely, indicating the glassy structure without crystalline boundary is indeed beneficial to the homogenous corrosion and the saving of service life. The EDX spectroscopy results, as shown in Fig. 12c and d, of the as-atomized powders and the sample reacted with DB6 solution for half an hour illustrate the variation of the composition during the decolorization process. It is seen that the initial concentration on the surface of sample is close with the nominal chemical composition. After reaction with azo dye solution, the concentration of Mg increases by 4.6%, while the amount of Zn and Ca is decreased by 4 and 0.6%, respectively. This means that more corroded Zn and Ca were dissolved into the azo dye solution than Mg.
(2)
(3)
(4)
4. Conclusions (1) Mg–Zn–Ca glassy powders with Mg content in the range from 63 at.% to 73 at.% and the diameter from 2 m to180 m were synthesized by gas-atomization technology. Most of the powders with the size lower than 25 m take the shape of sphere.
(5)
With the particle size increasing, dumbbell, discoideus and elliptical particulates are increased. The phase structure of atomized powders was affected by the composition and the size. Under the present conditions, the powders of M63, M66 and M70 with the size lower than 150 m are of fully glassy structure. And M73 powders exhibit the coexistence of glassy structure and crystallines. The DSC curves for the four kinds of as-atomized Mg–Zn–Ca alloys contain an undercooled liquid region and three crystallization peaks in the range from 400 K to 550 K. M66 glassy alloy exhibits the largest Eg , Ex and Ep1 . The corrosion resistance of Mg–Zn–Ca metallic glasses in azo dye solutions is directly influenced by the composition. The lower the content of Mg, the higher the potential of pitting corrosion and the lower the passive curent density. It has been verified that the corrosion resistance of glassy structure is far superior to that of crystalline structure. The degradation capacity of Mg based glassy powders is remarkably higher than that of Fe powders. The degradation capacity of the glassy powders is affected by the composition and particle size of powders. Especially, it has been confirmed
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that the degrading capacity of M70 powders with 48–75 m is about 11 times faster than that of the M63 glass powders when the investment quantity is 0.2 g. Nano-whiskers were formed uniformly and loosely on the reacted surface of the M70 glassy powders, denoting that the powder particles have high activity in this reaction. Acknowledgment The authors are grateful for the financial support of National Natural Science Foundation of China (Nos. 51071018, 51271018). References [1] H. Lachheb, E. Puzenat, A. Houas, M. Ksibi, E. Elaloui, C. Guillard, J. Herrmann, Appl. Catal. B: Environ. 39 (2002) 75–90. [2] Y.P. Guo, J.Q. Fang, L.W. Xie, X.F. Wu, Environ. Eng. Z2 (2012) 32–35. [3] P.R. Gogate, A.B. Pandit, Adv. Environ. Res. 8 (2004) 501–551. [4] A.de O. Martins, V.M. Canalli, C.M.N. Azevedo, M. Pires, Dyes Pigment 68 (2006) 227–234. [5] S.K. Aksu, S. Gucer, Turk. J. Eng. Environ. Sci. 34 (2010) 275–279. [6] T. Eggena, A. Majcherczyk, Chemosphere 7 (2006) 1116–1125. [7] I.K. Konstantinou, T.A. Albanis, Appl. Catal. B: Environ. 49 (2004) 1–14. [8] J. Singh, P.J. Shea, L.S. Hundal, S.D. Comfort, T.C. Zhang, D.S. Hage, Weed Sci. 46 (1998) 381–388. [9] X. He, F. Liu, Y. Huang, X. Li, M. Tang, J. He, J. Environ. Sci.-China 24 (2003) 139–142. [10] A. Agrawal, P.G. Tratnyek, Environ. Sci. Technol. 30 (1995) 153–160. [11] F.W. Chuang, R.A. Larson, M.S. Wessman, Environ. Sci. Technol. 29 (1995) 2460–2463. [12] K.Y. Hwa, W.W. Bernd, I.F. Cheng, G.D. John, M.W. Chien, Environ. Sci. Technol. 33 (1999) 1307–1310. [13] H.L. Lien, W.X. Zhang, Colloids Surf. A: Physicochem. Eng. Aspects 191 (2001) 97–105. [14] S.S. Chen, H.D. Hsu, C.W. Li, J. Nanopart. Res. 6 (2004) 639–647.
55
[15] B. Gu, L. Liang, M.J. Dickey, X. Yin, S. Dai, Environ. Sci. Technol. 32 (1998) 3366–3373. [16] N. Sangkil, G.T. Paul, Water Res. 34 (2000) 1837–1845. [17] C. Noubactep, A. Schoner, P. Woafo, Clean-Soil Air Water 37 (2009) 930–937. [18] J. Cao, L. Wei, Q. Huang, L. Wang, S. Han, Chemosphere 38 (1999) 565–571. [19] C.Q. Zhang, Z.W. Zhu, H.F. Zhang, Z.Q. Hu, J. Environ. Sci.-China 24 (2012) 1021–1026. [20] G.D. Sayles, G. You, M. Wang, M.J. Kupferle, Environ. Sci. Technol. 31 (1997) 3448–3454. [21] S.J. Monson, L. Ma, D.A. Cassanda, R.F. Spalding, Anal. Chim. Acta 373 (1998) 153–160. [22] X.N. Gu, Y.F. Zheng, S.P. Zhong, T.F. Xi, J.Q. Wang, W.H. Wang, Biomaterials 31 (2010) 1093–1103. [23] Y.F. Zheng, B. Liu, X.N. Gu, Mater. Rev. 1 (2009) 1–6. [24] X. Gu, G.J. Shiflet, F.Q. Guo, S.J. Poon, J. Mater. Res. 20 (2005) 1935–1938. [25] Y.Y. Zhao, E. Ma, J. Xu, Scripta Mater. 58 (2008) 496–499. [26] Y.Y. Zhao, X. Zhao, J. Alloys Compd. 515 (2012) 154–160. [27] J.F. Wang, H. Song, Y.Y. Wei, S.F. Guo, F.S. Pan, Mater. Lett. 91 (2003) 311–314. [28] B. Zberg, P.J. Uggowitzer, J.F. Löffler, Nat. Mater. 11 (2009) 887–891. [29] J.Q. Wang, Y.H. Liu, M.W. Chen, D.V. Louzguine-Luzgin, A. Inoue, J.H. Perepezko, Sci. Rep. (2012), http://dx.doi.org/10.1038/srep00418. [30] J.Q. Wang, Y.H. Liu, M.W. Chen, G.Q. Xie, D.V. Louzguine-Luzgin, A. Inoue, J.H. Perepezko, Adv. Funct. Mater. 22 (2012) 2567–2571. [31] B. Lin, X. Bian, P. Wang, G. Luo, Mater. Sci. Eng. B 177 (2012) 92–95. [32] C.Q. Zhang, Z.W. Zhu, H.F. Zhang, Z.Q. Hu, Chin. Sci. Bull. 56 (2011) 3988–3992. [33] C.M. Zhang, X. Hui, Z.G. Li, G.L. Chen, Mater. Sci. Eng. A 491 (2008) 470–475. [34] C.M. Zhang, X. Hui, Z.G. Li, G.L. Chen, Mater. Lett. 62 (2008) 1129–1131. [35] X. Hui, W. Dong, G.L. Chen, K.F. Yao, Acta Mater. 55 (2007) 907–920. [36] E.R. Wang, X.D. Hui, S.S. Wang, Y.F. Zhao, G.L. Chen, Mater. Sci. Eng. A 528 (2011) 5764–5771. [37] M. Gogebakan, P.J. Warren, B. Cantor, Mater. Sci. Eng. A 226 (1997) 168–172. [38] G.J. Fan, J.F. Loffler, R.K. Wunderlich, H.J. Fecht, Acta Mater. 52 (2004) 667–674. [39] J.D. Cao, K.J. Laws, N. Birbilis, M. Ferry, Corrosion Eng. Sci. Technol. 47 (2012) 329–334. [40] C. Zhang, Z. Zhu, H. Zhang, Z. Hu, J. Non-Cryst. Solids 358 (2012) 61–64. [41] S.D. Kalme, G.K. Parshetti, S.U. Jadhav, S.P. Govindwar, Bioresour. Technol. 98 (2007) 1405–1410. [42] R. Patel, S. Suresh, J. Hazard. Mater. 137 (2006) 1729–1741. [43] H. Shu, M. Chang, H. Yu, W. Chen, J. Colloid Interface Sci. 314 (2007) 89–97.
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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Synthesis of Mg–Zn–Ca metallic glasses by gas-atomization and their excellent capability in degrading azo dyes Y.F. Zhao a , J.J. Si a , J.G. Song a , Q. Yang b , X.D. Hui a,∗ a b
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China School of Water Resources and Environment, China University of Geoscience, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 21 August 2013 Received in revised form 25 October 2013 Accepted 19 November 2013 Available online 2 December 2013 Keywords: Azo dyes degradation Mg–Zn–Ca Metallic glasses Gas-atomization
a b s t r a c t Mg–Zn–Ca powders of Mg63+x Zn32−x Ca5 (x = 0, 3, 7 and 10) with the diameter from 2 m to 180 m were synthesized by gas-atomization. The relationship among powder morphology, the composition, glass forming ability, thermal stability, corrosion resistance and the capacity in degrading azo dyes for these powders was investigated. It is shown that fully glass powders with the particle diameter < 150 m can be atomized for alloys with x ≤ 7. These Mg–Zn–Ca metallic glass powders exhibit remarkably superior corrosion resistance and degradation capacity in Direct Blue 6 solution to their crystalline counterparts and Fe powders. Nano-whiskers were formed uniformly and loosely on the reacted surface of the Mg70 Zn25 Ca5 glassy powder, which is considered as the mechanism of high degrading capacity for these Mg–Zn–Ca glassy alloys. This work will contribute to the development of massive production of high quality metallic glass powders with excellent capability in degrading azo dyes. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Synthetic organic dyestuffs have been widely used as important ingredients in textile, food, paper making and printing, leather and cosmetic industries. Most of the dyes contain benzene and heterocyclic macromolecular compounds with the azo functional groups ( N N and N O) and/or polar groups ( SO3 Na, OH, NH2 ). It is estimated that approximately 12% of the synthetic textile dyes were discharged as dye wastes [1]. The environment and human health will be seriously threatened if these organic dyes were discharged into the surperfacial and ground water without sufficient degradation. Therefore, it is urgent to explore efficient physical/chemical method for organic dyes treatment in industrial wastes. The degradation mechanism of azo dyes can be divided two steps. (1) The first step is to have the azo dyes reduced and the bonds of N N and N O broken. Aromatic amines will be generated at this step, which are generally more toxic than the parent compounds. From the views of physiology, biochemistry and bioenergetics, the essence of the reduction process is a respiration process, so called azorespiration [2]. The second step of degradation process is the further decomposition of aromatic amines and mineralization. Azo dyes in aqueous solution can be degrade by ozone through destroying most of the double bonds, such as
∗ Corresponding author. Tel.: +86 10 62333066; fax: +86 10 6233447. E-mail address: [email protected] (X.D. Hui). 0921-5107/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2013.11.019
C C , C N , and N N [3,4]. Based on these recognitions, a variety of physical, biological, and photochemical methods for remediating azo dyes in wastewater have been developed to date. The activated carbon adsorption method is the conventional physical process which transfers the dyes from one phase to another but cannot degrade them [5]. The biotic degradation methods are usually limited to degrading some special toxic azo dyes but fail to act in the complex environment containing various chemicals [6–8]. It has been demonstrated that metals can effectively reduce a broad array of organic compounds such as chlorinated aliphatics [9], nitro aromatics [10], polychlorinated biphenyls (PCBs) [11], pesticides and related compounds [12–15]. Zero-valent Fe has been widely used to initiate remediating more complex anthropogenic chemicals by the reduction of critical functional groups [10,16–18,19]. Pesticides that may be subject to this treatment include DDT, DDD, DDE [20], and atrazine [21]. It is especially noticed that zero-valent Mg has been also employed for degrading various organic water pollutants with its high capacity. The very low corrosion resistance of pure Mg in air and water, however, reduces its endurance and results in the consumption of a large amount of Mg by water. Therefore, it is necessary to search for the novel Mg bear materials for degradation of azo dyes. Recently, Mg–Zn–Ca bulk metallic glasses (BMGs) with biodegradation properties, biocompatibilities of released alloying elements and excellent mechanical properties have been developed [22,23]. All the three components are environmental friend, and the degrading products will not cause the secondary pollution. The first Mg–Zn–Ca bulk metallic glass was reported by Gu
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et al. in 2005 [24]. Then Mg–Zn–Ca BMGs with critical diameter of 5 mm were prepared by using conventional casting techniques [25,26]. One of the most important features for the structure of metallic glasses is that metallic glasses have not grain boundaries, which results in the obvious improvement of corrosion resistance. Wang et al. [27] synthesized Fe particle reinforced Mg69 Zn27 Ca4 BMG composites by copper mold injection casting with industrial raw materials. It has been shown that the corrosion resistance of this BMG based composite in 3.5 wt.% NaCl solution has been remarkably improved compared with that of AZ31 and pure Mg. The compressive fracture strength of Mg96−x Znx Ca4 (x = 30, 25) BMGs exhibit surprisingly high uniformity, indicating that they are reliable as structural materials [25]. The feasibility as biodegradable metallic materials has been evaluated by investigating their cytotoxicity and corrosion properties. The experimental results show that these BMGs have higher cell viabilities than that of asrolled pure Mg. By animal experiments, Zberg et al. revealed that Mg–Zn–Ca BMGs exhibit great reduction in hydrogen evolution below Zn-alloying threshold (∼28 at.%) and the same good tissue compatibility as seen in crystalline Mg implants [28]. As for the degradation capacity of Mg–Zn–Ca amorphous alloys, Wang et al. [29] reported that the ball-milled glassy Mg-based alloys have remarkably superior capacity in degrading azo dyes to that of commercial crystalline Fe powders, Mg–Zn alloy crystalline counterparts and Fe-based metallic glasses powders. Since metallic glasses are formed by far-from-equilibrium process, the constituent atoms of metallic glasses do not reside at the thermodynamic equilibrium positions. Therefore, the metastable nature imparts amorphous alloys the good catalytic and special chemical properties [30,31]. Zhang et al. revealed that there are a depressed valence band maximum and a widened empty band in the amorphous ribbon, which could reduce the activation energy of the degradation process and enhance the activity of the electrons, thus accelerating the degradation process [32]. The large glass forming ability (GFA), excellent capability in degrading azo dyes, good biocompatibility, low cost, and easy recycling ability of Mg–Zn–Ca BMGs make them promising as biomedical and structural materials [33–35]. However, the productivity of current preparation technologies of Mg–Zn–Ca amorphous alloys is relatively low which will limit the mass application for the degradation of azo dyes. Moreover, the composition dependence of degrading azo capability for this kind of metallic glass has not been investigated. In this work, we report for the first time the synthesis of Mg–Zn–Ca glassy powder with excellent capability in degrading azo dyes by gas-atomization technology. The advantages of this technology lie in the mass production of high quality powders without surface contamination. Moreover, powder metallurgy processing in the supercooled liquid state enables the production of bulk metallic glassy foams with various shapes. Since the glass forming and degradation ability of the atomized powders are affected by the compositions and sizes, four Mg–Zn–Ca alloys with Mg content in the range from 63 at.% to 73 at.% are designed. The powders of these alloys with the diameters from 2 m to 180 m are prepared. The correlations among the composition, particle size, glass forming ability, thermal stability, corrosion resistance and the capacity in degrading azo dyes for this kind of metallic glass are investigated.
2. Experimental methods The atomization processing was carried out in a close-coupled nozzle ultrasonic atomization system, as shown in Fig. 1. After removing the oxidation film from the surfaces, we put the mixture of pure metals of Mg (99.9 wt.%), Zn (99.9 wt.%), Ca (99.5 wt.%)
47
Fig. 1. Schematic of close-coupled nozzle ultrasonic atomization system: (1) melting chamber, (2) induction coil, (3) nozzle system, (4) atomization chamber and (5) cyclone separator.
with the nominal chemical compositions (at.%) of Mg63 Zn32 Ca5 , Mg66 Zn29 Ca5 , Mg70 Zn25 Ca5 and Mg73 Zn22 Ca5 (hereafter denoted as M63, M66, M70, M73 for convenience) in graphite crucible in the vacuum furnace. The furnace was firstly pumped to high vacuum (5.0 × 10−3 Pa) and then backfilled with high-purity argon (99.999 wt.%) gas for protection. Then we melted the alloy by using medium frequency power supply. The melt was then cooled down by turning off the power. After holding for 10 min, the alloy was remelted by the power supply up to a superheat state above the melting point by approximately 200 K in the graphite crucible under 0.4 MPa. Then the melt was delivered through a draft tube with the diameter of 1 mm, and atomized by jetting the high-purity argon (99.999 wt.%) through the close-coupled nozzle at ultrasonic speed of 6 MPa. The Mg–Zn–Ca powders were cooled down to ambient temperature under the inert gas atmosphere in the atomizing chamber. Afterwards, the powders with diameters in the range from 2 m to 180 m were collected and separated by different sizes in the range of <25 m, 25–48 m, 48–75 m, 75–150 m and 150–180 m. In this work, we also prepared the Mg–Zn–Ca ribbons by using melt spinning. The Mg–Zn–Ca master alloy was remelted in a quartz tube by high frequency power supply in vacuum chamber, then injected onto a rotating copper wheel under the pressure of 0.2 MPa. The line velocity of the copper wheel is about 10 m/s. Under these conditions, ribbons with the size of 0.025 mm × 5 mm were obtain ribbon samples. Because the cooling rate of melt spinning is higher than that of gas-atomization, it is reasonable to infer that the four Mg–Zn–Ca ribbon samples are more easily to form glassy structures those by using atomization. The electrochemical polarization properties of the four kinds of glassy alloys were investigated by using the Gill-AC electrochemical measuring instrument. The working electrode was made by attaching a Cu wire to one end of the ribbon samples which was closely
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Fig. 2. SEM images of M66 atomized powders with different size ranges: (a) 150–180 m, (b) 75–150 m, (c) 48–75 m and (d) 10–48 m.
sealed with epoxy resin. After immersing the specimens in 0.2 g L−1 azo dye solution for about 15 min the open circuit potentials became almost steady. Then the potentio-dynamic polarization curves were recorded at a potential sweep rate of 20 mV min−1 . The capability in degrading Direct Blue 6 (DB6) was measured for these metallic glass powders. The DB6 was dissolved in distilled water to obtain the solution with the concentration of 0.2 g L−1 . 8 mL solution was put into a 10 mL centrifugal tube. In the preparative experiment, it was found that the degrading rate of DB6 by Mg–Zn–Ca glassy powders with small size is too rapid to be measured. Therefore we determined to employ different class of investment quantity for the glass particles with different size. For the particles with the size lower than 25 m and in the range of 48–75 m, 0.05 and 0.2 g metallic glass powders, respectively, were thrown into the centrifugal tubes. Then the tube was whirled with the speed of 150 rpm at the temperature of 300 K. The concentration change of azo dye solution during the degradation process was measured by 722S mode of visible spectrophotometer. During the degrading process, 4 mL solution was taken out at certain time interval from the centrifugal tube and filtered through 0.45 m membrane filter for measuring the absorbance at 580 nm wavelength. The concentration change of azo dye solution was determined according to the standard working curve (constructed calibration curve).
The phase constituent of atomized Mg–Zn–Ca powders was investigated by X-ray diffraction (XRD). The thermal stability of atomized powders against crystallization was examined with differential scanning calorimetry (DSC) at the heating rate from 20 K min−1 to 50 K min−1 under an argon atmosphere. The change of component elements on the surface was measured by using energy-dispersive X-ray (EDX) spectroscopy. The morphologies and microstructure of the powders were characterized by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). 3. Results and discussion 3.1. Morphologies of as-atomized powders In this work, the powders of M63, M66, M70 and M73 Mg–Zn–Ca alloys with the diameter in the range: <25 m, 25–48 m, 48–75 m, 75–150 m and 150–180 m, respectively, have been synthesized by atomization. Typical SEM images of M66 atomized powders with the diameters from 2 m to 180 m are shown in Fig. 2. It is seen that most of the powders with the size lower than 25 m take the shape of sphere. The surface of the powders is clean and smooth. The SEM images of the atomized powders with diameters lower than 10 m for M66 and M73 alloy are shown in Fig. 3.
Fig. 3. SEM images of atomized powders with diameters lower than 10 m in (a) M66 and (b) M73 alloy.
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Fig. 4. XRD curves of atomized powders of (a) M63, (b) M66, (c) M70 and (d) M73 alloy.
It is seen that the powders have perfect spherical shape. And the smaller the powders, the more smooth and cleaner the surface [36]. The powders with the size of 25–48 m are composed of spherical and dumbbell-shaped particulates. The particle shape of the powders with the size from 75 m to 150 m is similar to that of powders in the range of 25–48 m. It is also found that small planetary particles are attached to the large powders. For the powders with size larger than 150 m, there are some discoideus and elliptical particulate besides the spherical powders. And the surface of these large powders is rough and attached with small planetary particles. It is seen that from Figs. 2 and 3, the shape of particle is influenced by its size, which reflects that the solidification rates are different for powders with different diameters. The larger powders have lower solidification rate due to the long time needed to transfer the heat from the melts. During atomization process, the larger droplets might be still in liquid or semi-solid state when the powders are touched to the wall of chamber. Therefore, the large droplets may be collided to form plate-like discoideus or deformed in the flying to form elliptical particulate. However, the small droplets may be completely solidified in short term in the chamber. Under the role of the turbulence of jet gas, these small particles form suspended atmosphere in the chamber, and colloid with the large particle which incompletely solidified. As result, the small powders are adhered by the large particulates to form the planetary particles.
3.2. Phase constituents of as-atomized powders The XRD patterns of four kinds of alloy powders with different size are shown in Fig. 4. It is seen that both the composition and the size affect the phase structure of atomized powders. All the XRD patterns of M63 particles with the size up to 180 m only consists of a broad diffusion peak located at 2 ≈ 38◦ , indicating the formation of fully glassy phase. For M66 and M70, the XRD patterns of
as-atomized particles with the size of 150–180 m shows a broad halo, which indicates the presence of glassy phase, and overlapped with the crystalline peaks of ␣-Mg. The XRD patterns of the particles of M66 and M70 with size lower than 150 m exhibit only a broad halo of fully glassy phase. For the as-atomized M73 (Fig. 4d) alloy, the XRD patterns show that some weak crystalline peaks are supposed on the broad halo when the particle size is lower than 75 m, indicating the coexistence of glass structure and crystalline. As the particle size of the powders is larger than 75 m, the XRD patterns exhibit a weak halo and a set of diffraction peaks of ␣-Mg, indicating that the microstructure is almost crystalline. Our previous experimental results have shown that M66 alloy has high glass forming ability with the critical diameter up to 5 mm. The XRD patterns of Fig. 4 are in accordance with those results as shown in supplementary Figure. Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mseb.2013. 11.019. The glassy feature of the structure of the atomized powders in this work was also studied by using TEM. Fig. 5 shows the bright-field TEM image and selected-area diffraction pattern (SAD) of the powders of M70 with the diameter lower than 10 m when exposed to the electronic beam for different time. When exposed for the electronic beam for 10 min, as shown in Fig. 5a, the SAD exhibits diffuse halo without diffraction spots, indicating the structure of fully glassy phase. However, as the sample exposed for 30 min, as shown in Fig. 5b, nanoparticles were disintegrated from the powder due to the heating of high energy electronic beam. The SAD of remained powder is composed of diffuse halo and several weak spots, indicating that partial crystallization took place during the TEM observation. 3.3. The thermal stability of atomized powders It is important to understand their thermal stability and phase transformation because the glass state is essentially metastable in
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Fig. 5. (a) Bright-field TEM image and selected-area diffraction pattern of M70 powders alloy and (b) the disintegrated nanoparticles from the powder during the TEM observation due to the effect of electronic beam.
thermodynamics. Fig. 6 shows the DSC curves of the as-atomized Mg–Zn–Ca powders with different sizes when heated at 20 K min−1 . Generally, the DSC curves of the four kinds of as-atomized Mg–Zn–Ca alloys contain an undercooled liquid region and three crystallization peaks in the range from 400 K to 550 K. However, the widths of the supercooled liquid region for the four kinds of alloys are not same. And the secondary peaks on the DSC curves of M63 and M73 are not obviously, just form the left shoulder of the third peaks, indicating the internal structure of these atomized alloys are different. The glass transition temperature, Tg , onset temperatures of crystallization events, Tx , Tp1 , Tp2 , Tp3 , and corresponding latent heats, H1 , H2 , H3 , respectively, for the four kinds of alloys are presented in Table 1. It is seen that these characteristic parameters
are mainly affected by the composition. The crystallization temperature decreases with the increase of the content of Mg in the alloys. For the powders with the same composition but different particle sizes, the values of thermal parameters of Tg and Tx are similar indicating that the size effect of the powders is slight. The M66 atomized powders have the largest supercooled liquid regions among the four kinds of synthesized alloys. The crystallization behaviors of M63, M66 and M70 during continuous heating were investigated by DSC measurement. Fig. 7 shows the typical DSC traces for M66 powders with the diameter lower than 25 m at the heating rate from 20 K min−1 to 50 K min−1 . As the heating rate is increased, all the three exothermic peaks are shifted to higher temperature. The variation of observed
Fig. 6. DSC curves of atomized powders of (a) M63, (b) M66, (c) M70 and (d) M73 alloy heated at 20 K min−1 .
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Table 1 Thermal stability parameters of M63, M66, M70 and M73 at the heating rate of 20 k min−1 . Samples
H1 (J/g)
Tp2 (K)
H2 (J/g)
Tp3 (K)
H3 (J/g)
498 499 500 501 502
16.7 16.61 15.32 14.91 16.66
Size (m)
Tg (K)
Tx (K)
Tp1 (K)
M63
<25 25–48 48–75 75–150 150–180
417 418 417 416 416
431 431 430 430 429
441 440 440 440 439
7.872 7.668 7.975 7.431 7.271
477 476 477 477 477
M66
<25 25–48 48–75 75–150 150–180
408 408 407 407 408
423 422 422 423 421
431 431 430 430 430
7.997 7.807 7.573 7.541 5.427
476 476 477 478 479
14.8 14.68 14.37 14.17 11.37
524 524 523 523 523
7.085 6.983 6.969 6.542 6.071
M70
<25 25–48 48–75 75–150 150–180
396 399 399 400 400
407 409 409 410 408
417 419 418 419 418
7.861 7.5 7.158 7.41 6.526
454 455 454 454 454
16.54 14.98 13.34 13.91 12.54
526 526 525 525 525
6.015 5.672 5.528 5.748 5.26
M73
<25 25–48 48–75 75–150 150–180
396 396 396 397 396
406 405 408 408 408
417 416 416 415 416
20.44 20.11 19.72 8.208 2.205
488 487 488 487 487
509 508 509 509 508
3.833 3.771 3.667 2.489 1.048
0.2652 0.2169 0.226 0.1649 0.03531
0.3704 0.3192 0.3564 0.1423 0.03051
Fig. 8. Electrochemical polarization dynamics curves of the four kinds of metallic glass ribbons and crystallized M73 ribbon in 0.2 g L−1 direct blue 6 solution. Fig. 7. DSC traces of M66 powders with the diameter lower than 25 m at the heating rate from 20 K min−1 to 50 K min−1 .
characteristic temperatures with the applied heating rate can be described by the Kissinger equations [37,38]
ln
T2 ˇ
=
E +A RT
(1)
where ˇ is heating rate, T is peak temperature, R is the gas constant, and A is constant. The apparent activation energy, E, can be obtained by the slope of −ln(T2 /ˇ) versus 1/RT linear. The activation energies calculated from the slope of the DSC plots for M63, M66 and M70 glassy alloys are listed in Table 2. It is seen that M66 glassy alloy has the largest Eg , Ex and Ep1 , meaning that M66 has the best thermal stability to resist the crystallization among the three kinds of glassy powders.
Table 2 Activation energies for glass transition, crystallization, and first thermal peak in M63, M66 and M70 powders. Samples
Eg (kJ/mol)
Ex (kJ/mol)
Ep1 (kJ/mol)
M63 M66 M70
238 242 240
245 247 245
242 245 239
3.4. The corrosion resistance properties The corrosion resistance behavior is critical to the application of these powders for decoloration process. The electrochemical polarization dynamics curves of the four kinds of glassy ribbons in 0.2 g L−1 DB6 solution are shown in Fig. 8. Some corrosion data, including the passive potential (E0 ) and passive current density (ip ) derived from the curves are summarized in Table 3. It is shown that the corrosion resistance of Mg–Zn–Ca metallic glasses in azo dye solutions was directly influenced by the composition. It is obvious that for glassy structures, the lower the content of Mg, the higher the potential of pitting corrosion and the lower the passive curent density, indicating that the decrease of Mg content increases the corrosion resistance of the glassy alloys. From Fig. 8, it is also seen that the corrosion resistance of glassy structure is far superior to that of crystalline structure. For M73 alloy, the pitting potential of Table 3 Passive potential (E0 ) and passive current density (ip ) derived from the curves of Fig.8 for the five kinds of alloys. Sample
ip (A cm−2 )
E0 (mV)
M63 M66 M70 M73 Crystallized M73
0.21 0.40 4.57 6.03 30.90
−1065.9 −1134.3 −1224.2 −1316.8 −1440.5
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Fig. 9. Visual images of DB6 solution after degraded for different time by (a) Fe powders and (b) the presented atomized M70 powders with diameter of 48–75 m and the investment quantity of 0.2 g.
crystalline structure is lower than that of glassy structure by about 120 mV, and the the passive curent density is increased by about 4 times. According to Ref. [39], Zn is the least reactive element in neutral pH, in Mg–Zn–Ca ternary system. As the Ca content is constant, the decrease of Mg content corresponds to the increase in Zn content. In the glassy structure, all the elements exist homogeneously in the atomistic form. Therefore, it is easy to understand that the observed increase in corrosion resistance is actually due to the increase of Zn. In other words, the appropriate increase of Mg may increase the activity in azo dye solutions although it may result in the decrease of the service life of Mg–Zn–Ca metallic glass powders. 3.5. The degradation capacity The degradation process of DB6 solution by the presented atomized M66 and Fe powders which have the same particle size in the range from 48 m to 75 m was visually observed. As shown in Fig. 9, it is seen that the degradation capacity of Mg based glassy powders is remarkably improved compared with that of Fe powders. The azo dye in the solution containing atomized M66 powders was degraded very fast in the first minute of exposure. The blue color of solution began to decay after 1 min. After 4 min of the exposure, the color of the solution disappears completely, indicating that the azo bonding is almost degraded. For the solution containing Fe powders, the color did not change obviously even after 20 min. The blue color in this kind of solution still holds after 60 min. The above phenomenological variation can be described by the dependence of the concentration of DB6 in the solution on the time. From Fig. 10a, it is seen that the concentration of DB6 was decreased to 0.01 g L−1 in 2 min by M66 glassy powders with the diameters lower than 25 m. With the diameters of glassy powders increased to 48–75 m, the degrading rate is decreased. After 5 min, the concentration is decreased to 0.04 g L−1 . It is found that the decomposed azo dye by the commercial Fe powders has not reached 10% after an hour. The compositional and size dependence of degradation rate of the glassy powders is shown in Fig. 10b and c. Given the same azo
dye concentration and investment quantity, the reaction rates are influenced by the content of Mg and the structure. Here we compare the degradation process of the DB6 solution by using the four kinds of Mg based atomized powders which are in the same size ranges. It can be concluded that the degradation rate follows the order of M70 > M73 > M66> M63 under the same experimental conditions. This result means that the degradation rate of the DB6 solution by glassy alloys is determined by the content of Mg. From the Xray results, it is seen that M73 is of partially glassy structure. The existence of crystalline phase decreases the reaction rate although the content of Mg is most among the four interested alloys. From Fig. 10, it is also seen that the size of the particles affects the degradation rate of the DB6. In the degradation experiments with the investment quantity of 0.2 g glassy powders which have the diameters lower than 25 m, the degradation process of the DB6 by glassy powders with diameters of lower than 25 m is too fast to be recorded. Therefore, we have to decrease the investment quantity to 0.05 g for these fine powders. This result illustrates that the particles with smaller size has higher degrading capacity. It is well known that the degradation reaction of azo dyes by using metallic glasses occurs on the surface of powders. When effective collision between dye molecules and powders takes place [40,41], Mg and Ca lose their electrons while the dye molecule accepts electrons, the azo bond N N is then broken into two NH2 . The degradation process can be described by the pseudofirst-order kinetic model [42,43], C = A + Be−t/t0
(2)
where C is the concentration of the solution which is taken from the centrifugal tube, A and B are fitting constants, t is the degradation time, and t0 is the time when the characteristics concentration decreases to e−1 of the initial state derived by fitting the experimental data. The reaction rate can be estimated by the value of t0 , as shown in Table 4. For atomized M66 alloy shown in Fig. 10a, it can be calculated that the capacity of particles with the diameters lower than 25 m (t0 = 0.48 min) is about 23 times faster than that of the powders with 48–75 m (t0 = 11.05 min) with the investment quantity of 0.2 g. It is also found that the capacity of M70 powders
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Fig. 11. (a) XRD patterns of as-atomized and annealed M70 powders with diameters of 45–80 m and (b) decolorization process of DB6 solution with the concentration 0.2 g L−1 at the ambient temperature by as-atomized and crystallized powders of M70 alloy.
Fig. 10. Decolorization process of DB6 solution with the concentration 0.2 g L−1 at the ambient temperature by (a) M66 (d < 25 m and 48–75 m) and Fe powders (d = 48–75 m), (b) M63, M66, M70 and M73 (d = 48–75 m) and (c) M63, M66, M70 and M73 (d < 25 m) powders, with the investment quantity of 0.2, 0.2 and 0.05 g, respectively.
Table 4 The characteristic time (t0 ) of degradation process value of M63, M66, M70 and M73 atomized powders from the curves of Fig. 8b and c. Sample
M63
M66
M70
M73
t0 (48–75 m) t0 (<25 m)
11.05 21.54
5.09 13.25
0.98 7.15
3.26 9.28
(t0 = 0.98 min) with 48–75 m is about 11 times faster than that of the M63 glass powders (t0 = 11.05 min) when the investment quantity is 0.2 g. When the investment quantity is decreased to 0.05 g for the powders with the size lower 25 m, the capacity of M70 powders (t0 = 7.15 min) is about 3 times faster than that of the M63 glass powders (t0 = 21.54 min).
Metallic glasses are metastable, and the constituent atoms do not reside at the thermodynamic equilibrium positions (corresponding to crystalline state) but locate at far-from-equilibrium positions [29,30]. The metastable nature imparts metallic glasses a variety of excellent properties which are unachievable for crystalline alloys, such as the good catalytic and chemical properties. These merits of glassy alloys can be observed by comparing the degrading capacity of the atomized powders with their crystalline counterparts. As shown in Fig. 11a, M70 powders have been completely crystallized after annealed at 573 K for 1 h. Mg solid solution, Ca2 Mg5 Zn13 and MgZn2 compounds are formed. The solid solution reserves partial decolorization capacity of azo dye. But Ca2 Mg5 Zn13 and MgZn2 compounds completely lose their catalytic capacity, leading to the decrement of decolorization rate. From Fig. 11b, it is clearly shown that the crystallized powders exhibit much lower decolorization capacity than that of glassy structure. The concentration of the azo dyes only decreases 25% after 20 min when degraded by the crystallized alloy. To explore the decolorization mechanism of Mg based glassy alloys, we studied the surface morphologies and compositions of the powders after reacted with the DB6 solution. Fig. 12 shows that nano-whiskers are distributed uniformly and loosely on the reacted surface of the M70 glassy powder. The uniform distribution without aggregation also denotes that the powders have high activity in this reaction. The loose structure of the reaction products on the surface assures high reaction rate. It is also observed that the surface
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Fig. 12. (a) and (b) The surface morphologies after reacted with DB6 solution for an hour and (c) and (d) the EDX patterns of the initial surface of as-atomized and reacted surface of M70 powders after reacted with DB6 solution.
beneath the nano-whiskers is still clean and densely, indicating the glassy structure without crystalline boundary is indeed beneficial to the homogenous corrosion and the saving of service life. The EDX spectroscopy results, as shown in Fig. 12c and d, of the as-atomized powders and the sample reacted with DB6 solution for half an hour illustrate the variation of the composition during the decolorization process. It is seen that the initial concentration on the surface of sample is close with the nominal chemical composition. After reaction with azo dye solution, the concentration of Mg increases by 4.6%, while the amount of Zn and Ca is decreased by 4 and 0.6%, respectively. This means that more corroded Zn and Ca were dissolved into the azo dye solution than Mg.
(2)
(3)
(4)
4. Conclusions (1) Mg–Zn–Ca glassy powders with Mg content in the range from 63 at.% to 73 at.% and the diameter from 2 m to180 m were synthesized by gas-atomization technology. Most of the powders with the size lower than 25 m take the shape of sphere.
(5)
With the particle size increasing, dumbbell, discoideus and elliptical particulates are increased. The phase structure of atomized powders was affected by the composition and the size. Under the present conditions, the powders of M63, M66 and M70 with the size lower than 150 m are of fully glassy structure. And M73 powders exhibit the coexistence of glassy structure and crystallines. The DSC curves for the four kinds of as-atomized Mg–Zn–Ca alloys contain an undercooled liquid region and three crystallization peaks in the range from 400 K to 550 K. M66 glassy alloy exhibits the largest Eg , Ex and Ep1 . The corrosion resistance of Mg–Zn–Ca metallic glasses in azo dye solutions is directly influenced by the composition. The lower the content of Mg, the higher the potential of pitting corrosion and the lower the passive curent density. It has been verified that the corrosion resistance of glassy structure is far superior to that of crystalline structure. The degradation capacity of Mg based glassy powders is remarkably higher than that of Fe powders. The degradation capacity of the glassy powders is affected by the composition and particle size of powders. Especially, it has been confirmed
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that the degrading capacity of M70 powders with 48–75 m is about 11 times faster than that of the M63 glass powders when the investment quantity is 0.2 g. Nano-whiskers were formed uniformly and loosely on the reacted surface of the M70 glassy powders, denoting that the powder particles have high activity in this reaction. Acknowledgment The authors are grateful for the financial support of National Natural Science Foundation of China (Nos. 51071018, 51271018). References [1] H. Lachheb, E. Puzenat, A. Houas, M. Ksibi, E. Elaloui, C. Guillard, J. Herrmann, Appl. Catal. B: Environ. 39 (2002) 75–90. [2] Y.P. Guo, J.Q. Fang, L.W. Xie, X.F. Wu, Environ. Eng. Z2 (2012) 32–35. [3] P.R. Gogate, A.B. Pandit, Adv. Environ. Res. 8 (2004) 501–551. [4] A.de O. Martins, V.M. Canalli, C.M.N. Azevedo, M. Pires, Dyes Pigment 68 (2006) 227–234. [5] S.K. Aksu, S. Gucer, Turk. J. Eng. Environ. Sci. 34 (2010) 275–279. [6] T. Eggena, A. Majcherczyk, Chemosphere 7 (2006) 1116–1125. [7] I.K. Konstantinou, T.A. Albanis, Appl. Catal. B: Environ. 49 (2004) 1–14. [8] J. Singh, P.J. Shea, L.S. Hundal, S.D. Comfort, T.C. Zhang, D.S. Hage, Weed Sci. 46 (1998) 381–388. [9] X. He, F. Liu, Y. Huang, X. Li, M. Tang, J. He, J. Environ. Sci.-China 24 (2003) 139–142. [10] A. Agrawal, P.G. Tratnyek, Environ. Sci. Technol. 30 (1995) 153–160. [11] F.W. Chuang, R.A. Larson, M.S. Wessman, Environ. Sci. Technol. 29 (1995) 2460–2463. [12] K.Y. Hwa, W.W. Bernd, I.F. Cheng, G.D. John, M.W. Chien, Environ. Sci. Technol. 33 (1999) 1307–1310. [13] H.L. Lien, W.X. Zhang, Colloids Surf. A: Physicochem. Eng. Aspects 191 (2001) 97–105. [14] S.S. Chen, H.D. Hsu, C.W. Li, J. Nanopart. Res. 6 (2004) 639–647.
55
[15] B. Gu, L. Liang, M.J. Dickey, X. Yin, S. Dai, Environ. Sci. Technol. 32 (1998) 3366–3373. [16] N. Sangkil, G.T. Paul, Water Res. 34 (2000) 1837–1845. [17] C. Noubactep, A. Schoner, P. Woafo, Clean-Soil Air Water 37 (2009) 930–937. [18] J. Cao, L. Wei, Q. Huang, L. Wang, S. Han, Chemosphere 38 (1999) 565–571. [19] C.Q. Zhang, Z.W. Zhu, H.F. Zhang, Z.Q. Hu, J. Environ. Sci.-China 24 (2012) 1021–1026. [20] G.D. Sayles, G. You, M. Wang, M.J. Kupferle, Environ. Sci. Technol. 31 (1997) 3448–3454. [21] S.J. Monson, L. Ma, D.A. Cassanda, R.F. Spalding, Anal. Chim. Acta 373 (1998) 153–160. [22] X.N. Gu, Y.F. Zheng, S.P. Zhong, T.F. Xi, J.Q. Wang, W.H. Wang, Biomaterials 31 (2010) 1093–1103. [23] Y.F. Zheng, B. Liu, X.N. Gu, Mater. Rev. 1 (2009) 1–6. [24] X. Gu, G.J. Shiflet, F.Q. Guo, S.J. Poon, J. Mater. Res. 20 (2005) 1935–1938. [25] Y.Y. Zhao, E. Ma, J. Xu, Scripta Mater. 58 (2008) 496–499. [26] Y.Y. Zhao, X. Zhao, J. Alloys Compd. 515 (2012) 154–160. [27] J.F. Wang, H. Song, Y.Y. Wei, S.F. Guo, F.S. Pan, Mater. Lett. 91 (2003) 311–314. [28] B. Zberg, P.J. Uggowitzer, J.F. Löffler, Nat. Mater. 11 (2009) 887–891. [29] J.Q. Wang, Y.H. Liu, M.W. Chen, D.V. Louzguine-Luzgin, A. Inoue, J.H. Perepezko, Sci. Rep. (2012), http://dx.doi.org/10.1038/srep00418. [30] J.Q. Wang, Y.H. Liu, M.W. Chen, G.Q. Xie, D.V. Louzguine-Luzgin, A. Inoue, J.H. Perepezko, Adv. Funct. Mater. 22 (2012) 2567–2571. [31] B. Lin, X. Bian, P. Wang, G. Luo, Mater. Sci. Eng. B 177 (2012) 92–95. [32] C.Q. Zhang, Z.W. Zhu, H.F. Zhang, Z.Q. Hu, Chin. Sci. Bull. 56 (2011) 3988–3992. [33] C.M. Zhang, X. Hui, Z.G. Li, G.L. Chen, Mater. Sci. Eng. A 491 (2008) 470–475. [34] C.M. Zhang, X. Hui, Z.G. Li, G.L. Chen, Mater. Lett. 62 (2008) 1129–1131. [35] X. Hui, W. Dong, G.L. Chen, K.F. Yao, Acta Mater. 55 (2007) 907–920. [36] E.R. Wang, X.D. Hui, S.S. Wang, Y.F. Zhao, G.L. Chen, Mater. Sci. Eng. A 528 (2011) 5764–5771. [37] M. Gogebakan, P.J. Warren, B. Cantor, Mater. Sci. Eng. A 226 (1997) 168–172. [38] G.J. Fan, J.F. Loffler, R.K. Wunderlich, H.J. Fecht, Acta Mater. 52 (2004) 667–674. [39] J.D. Cao, K.J. Laws, N. Birbilis, M. Ferry, Corrosion Eng. Sci. Technol. 47 (2012) 329–334. [40] C. Zhang, Z. Zhu, H. Zhang, Z. Hu, J. Non-Cryst. Solids 358 (2012) 61–64. [41] S.D. Kalme, G.K. Parshetti, S.U. Jadhav, S.P. Govindwar, Bioresour. Technol. 98 (2007) 1405–1410. [42] R. Patel, S. Suresh, J. Hazard. Mater. 137 (2006) 1729–1741. [43] H. Shu, M. Chang, H. Yu, W. Chen, J. Colloid Interface Sci. 314 (2007) 89–97.