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Fabrication of metallic alloy powder (Ni3Fe) from Fe–77Ni scrap
Journal of Alloys and Compounds 670 (2016) 356e361
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Fabrication of metallic alloy powder (Ni3Fe) from Fee77Ni scrap Inseok Seo a, Shun-Myung Shin b, Sang-An Ha c, Jei-Pil Wang d, * a
ES Materials Research Center, Research Institute of Industrial Science and Technology, Incheon 406-840, South Korea Extractive Metallurgy Department, Korea Institute of Geoscience and Mineral Resources, Deajeon 305-350, South Korea c Department of Environmental Engineering, Silla University, Busan 46958, South Korea d Department of Metallurgical Engineering, Pukyong National University, Busan 608-739, South Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 October 2015 Accepted 21 January 2016 Available online 4 February 2016
The oxidation behavior of Fee77Ni alloy scrap was investigated at an oxygen partial pressure of 0.2 atm and temperatures ranging from 400 C to 900 C. The corresponding oxidation rate increased with increasing temperature and obeyed the parabolic rate law, as evidenced by its linear proportionality to the temperature. In addition, surface morphologies, cross-sectional views, compositions, structural properties were examined by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). Diffusion through either the spinel structure or the NiO layer, which were both present in the alloy during oxidation at elevated temperatures, was deemed the rate-limiting step of the reaction. The oxide powder less than 10 mm was obtained from Fee77Ni alloy scrap was obtained using ball-milling and sieving processes. In fact, 15 h of milling yielded a recovery ratio of 97%. Using hydrogen gas, the oxide powder was successfully reduced to an alloy powder of Ni3Fe and reduction rates of ~97% were achieved after 3 h at 1000 C. © 2016 Elsevier B.V. All rights reserved.
Keywords: Fee77Ni Oxidation rate Rate-limiting step Oxide powder Alloy powder
1. Introduction High-nickel-based alloys exhibit outstanding corrosion and high temperature resistance and are therefore used to resist extremely corrosive environments in the energy, power, chemical, and petrochemical industries. In particular, Ni-based superalloys exhibit excellent strength, creep, and corrosion resistance at elevated temperatures. As such, aircraft engine manufacturers are continually striving to increase the capabilities of such hightemperature structural materials in order to improve both engine performance and efficiency [1e5]. Ni80Fe20 Permalloy (Py) thin films are soft magnetic materials, that exhibit high anisotropic magnetoresistance and low magnetostriction. Therefore, Py is one of the most commonly used materials for magnetic data storage and can be found in various magnetic micro and nanostructures [6,7]. The high-temperature oxidation behavior of nickel alloys has been extensive investigated. These investigations have been performed, at low pressure and temperatures above room temperature [8,9], as well as under air atmosphere or during electrochemical
* Corresponding author. E-mail address: [email protected] (J.-P. Wang). http://dx.doi.org/10.1016/j.jallcom.2016.01.173 0925-8388/© 2016 Elsevier B.V. All rights reserved.
oxidation [10,11] and plasma formation [12]. Liu et al. [13], studied the oxidation behavior of a single-crystal Ni-base superalloy at 900 and 1000 C in air, and reported that two oxidation steps occurred at both temperatures. The first step was controlled by the growth of NiO and the second by the growth of Al2O3, until a continuous Al2O3 layer formed under the previously grown NiO layer. Li et al. [14] investigated the oxidation behavior of a NiCrAlYSi overlayer with and without a diffusion barrier that was deposited by one-step arc ion plating. They showed that the duplex coating system provided excellent protection to the substrate, when thin and continuous scales were adhered to the overlayer surface, which underwent only limited oxidation and interdiffusion attacks. Zhou et al. [15] studied the oxidation behavior, in air at 1000 C, of pure and doped nickel alloys that have different Co contents. This study revealed that the mass oxidation gain of the coatings increased with increasing Co content. In the present study, the effect of temperature on the oxidation behavior of Fee77Ni alloy was determined via thermal gravimetric analysis (TGA), performed at an oxygen partial pressure of 0.2 atm. The cross-sectional morphologies of the oxide layer resulting from isothermal oxidation were characterized by (SEM-EDS) as well as XRD. The mechanism governing high-temperature oxidation was discussed, based on the kinetic data and the results of microstructural analysis. In addition, the particle size distribution of the
I. Seo et al. / Journal of Alloys and Compounds 670 (2016) 356e361
2
pO2: 0.2 atm o
900 C 6
o
800 C o 700 C
4
o
600 C o
500 C o 400 C 2
0 0
5
10
15
20
25
30
Time, hr Fig. 1. Weight change per area with time on the effect of temperatures.
dx=dt ¼ kp x x2 ¼ 2kp t þ C where kp and C are the parabolic rate constant and integration constant, respectively; the plot of x as a function of time varies with values of kp and C. Parabolic oxidation at elevated temperatures occurs via the diffusion of ions or electrons through the oxide scale [16]. The temperature-dependence of the reaction is best described by an Arrhenius-type equation, which is given as follows:
kp ¼ Ae
DE* RT
where DE* is the apparent energy of activation and A is the frequency factor. The significance of DE* derived from elevated temperatures in the Arrhenius plot, must be interpreted with caution [16]. The Arrhenius plot of the parabolic rate constants is shown in Fig. 2. As the figure shows, the constants are almost linearly proportional to the temperature, which indicates that they increase proportionally with increasing temperatures. An activation energy of 17.98 kJ/mol was determined experimentally, for the oxidation of Fee77Ni at an oxygen partial pressure of 0.2 atm.
y = -2.1637x + 41.2908 R2 = 0.9869
24
20
3. Results and discussion The TGA (Fig. 1) was used to measure the time-dependent change in the weight of the specimens during 24 h of oxidation. These measurements were performed at temperatures ranging from 400 C to 900 C under a gas mixture with an oxygen partial pressure of 0.2 atm. As shown in Fig. 1, the weight increased with increasing temperature, and was highest at 900 C. The oxidation rate of ironenickel alloys is described by a parabolic rate equation, which stipulates that the oxide grows at a continuously decreasing oxidation rate. The diffusional and integral forms of this equation are given as follows:
2
4
Scrap plate of Fee77Ni alloy containing 77 wt.% of Ni was obtained from a recycling company. This plate was cut into 1.5 cm in (width) 2 cm (length) coupon specimens. Prior to oxidation, the specimens were sequentially polished with 320, 500, 800, 1200, 2400, and 4000 grit SiC paper and subsequently polished with 1 mm diamond polishing suspension. The specimens were then cleaned with acetone in an ultrasonic bath, rinsed with ethanol and water, and dried under a hood. The continuous oxidation of the alloy was investigated at elevated temperatures and an oxygen partial pressure of 0.2 atm, by using the TGA. The oxidizing gas mixtures, oxygen and argon, were passed through drierites, in order to remove the moisture contained therein, and subsequently input into the reactor tube. A copper turning furnace was installed to reduce oxygen impurities in the argon gas, prior to its passage through the reaction zone. The oxidizing gas mixture flowed from the bottom of the quartz tube to the reactor zone, whereas the flow of purified argon gas was maintained through the electrobalance to prevent contamination during the oxidation. The coupon specimen was suspended by a platinum wire in the reaction zone of the quartz tube and then heated under an ultra-high-purity argon atmosphere. Once the desired temperatures of 400 Ce900 C were reached, a mixture of oxidizing gases was introduced into the reaction tube until oxidation was completed. The oxygen partial pressure was controlled by mixing argon and oxygen using each flowmeter. In addition, the change in the specimen weight during oxidation was continuously monitored and recorded for up to 24 h. The cross-sectional area of the oxidized specimens and chemical composition of each layer were examined by SEM, EDS, and XRD. Hydrogen gas reduction of the oxide powder obtained from the Fee77Ni scrap was performed at elevated temperatures of 800 C, 900 C, and 1000 C. For this reduction, the oxide specimen was placed on an alumina tray that, in turn, was placed in the center of the tube-type reactor. A copper turning furnace was used to dehumidify the ultra-high-purity (UHP) argon gas. Two flask bottles located at the end of the reactor were used for the safe handling of the wasted gas and the extra hydrogen gas was completely burned out. The sample tray containing the oxide particles was placed in the reactor and the flow of the UHP argon gas flow into the reactor was maintained until the desired temperature was reached. Hydrogen gas was subsequently introduced into the reactor, which was then cooled to room temperature under the UHP argon atmosphere.
2 Weight change area, mg/cm Weight change perper area, mg/cm unit area, mg/cm2
2. Experimental
8
lnkp, mg /cm -hr
oxide powder was determined; various physical processes were used in an attempt to fabricate a <10 mm-sized oxide powder from the Fee77Ni alloy scrap. Finally, reduction by hydrogen gas was successfully used to transform the oxide powder to metallic powder at elevated temperatures.
357
16
12
8
8
9
10
11
12 4
13
14
15
-1
10 /T, K
Fig. 2. Parabolic rate constants with elevated temperatures.
16
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Oxide particles of Fee77Ni alloy scrap were formed after 24 h of oxidation at 700 C and 900 C, at this partial pressure. The constituent phases and the morphology of these particles were identified and examined via XRD and SEM, respectively. Similar XRD patterns were obtained and are shown in Fig. 3 and the same oxide phases, i.e., NiO, Fe2O3, and Ni1.43Fe1.7O4, occurred at both temperatures. Moreover, the oxide particles were irregular-shaped and exhibited significant variations from ~10 mm to ~200 mm in size. Previous XRD results indicated that, the formation of benseniteNiO (B), spinel (S), and hematite (H) phase could be expected. As shown in Fig. 4, hematite, spinel, bunsenite, and complex phases are formed, depending on the temperature and value of the Ni/ (Ni þ Fe) molar ratio. In fact, the pseudo-binary phase diagram shows that spinel (Fe, Ni)3O4 and bunsenite (Ni, Fe)O are formed under the current experimental conditions, as previously indicated by the XRD results. This formation is attributed to the outward migration of iron ions and electrons through grain boundaries, microcracks, and voids and the inward migration of oxygen anions (mainly) through spinel and bunsenite structures. The inward migration of anions is facilitated by pores and voids in the spinel structure, which is a non-stoichiometric compound of Ni1.43Fe1.7O4. The cross-section of the oxide layer formed during 24 h of oxidation at 900 C was examined via EDS. As shown in Fig. 5, the microstructure consists of two adjacent oxide layers, whose interface is punctuated with cracks and voids. The outer layer, labeled as (1), consists mainly of nickel and nickel oxide (NiO). Diffusion through either the spinel structure or the NiO layer, which are both formed in this alloy during oxidation at elevated temperatures [16], was deemed the rate-limiting step. The inner oxide layer, (2), consists of Ni, Mn, and Fe, and three oxide phases, namely, NiO, Fe2O3, and Ni1.43Fe1.7O4. In addition, this layer contains ~15 wt.% of Mo, which may influence the physical properties of the film. Mo plays a significant role in oxidation processes, especially when thermal cycling is involved [17]. The substrate, (3) and (4), contained only Ni and Fe, which occurred in the form of a metallic alloy. An oxide powder with particle sizes of <10 mm was fabricated and is shown in Fig. 6, based on the oxidation kinetic data of the Fee77Ni alloy scrap. Control of the particle size is crucial to achieving excellent properties such as compaction, density, and sinterability of the powder. Therefore, final particle sizes of <10 mm were targeted for powders used in the metal injection molding (MIM) process. As such, scrap foils were oxidized in a muffle furnace for 2 h at 900 C, under air atmosphere, since the oxidation rate at this temperature was found to be relatively high. The initial
Fig. 3. XRD patterns of oxide scales conducted at 700 C and 900 C for 24 h.
Fig. 4. Pseudo-binary phase diagram of ‘Fe2O3’eNiO in air. The closed-circle data points are from the present study; the dotted curves are calculated using FactSage. Legend: S ¼ spinel, B ¼ bunsenite, L ¼ liquid, and H ¼ hematite [17].
oxidation stage lasted for up to 2 h and subsequent cyclic oxidation was conducted in 2-h intervals, until complete oxidation of the foils was achieved. The resulting oxide particles were then ground, by using a ball-milling machine, in order to control the particle size of the final product. Powders with sizes of <10 mm, were obtained via sieving; the particle size distribution of the samples was determined by applying various levels of sieving. During the ball-milling process, size-control of the powder is achieved through careful selection of the milling parameters. A fixed milling speed of 84 revolutions per minute (RPM) was used, whereas the milling time was increased and the diameter of the tungsten carbide (WC) balls used as media, was varied; 10, 10, and 5 balls with diameters of 5 mm, 15 mm, and 50 mm, respectively, were used. As Fig. 7 shows, the recovery ratio of the oxide powder was ~70% in the first 3 h of milling, and then increased significantly after 6 h. A ratio of over 95% was obtained after 15 h; this value might correspond to a saturation and hence higher values were not expected with further milling. The particle-size distribution (PSD) of a powder has a significant effect on its physical and chemical properties, such as the particle compaction, density, and sinterability. In this work, the PSD of the oxide powder was determined by using a Mastersizer 3000 that is capable of analyzing particles with sizes ranging from 0.01 mm to 3500 mm. The morphology of the particles was examined via SEM and are shown in Fig. 8. The figure shows the distribution of average particle sizes, in terms of volume, as a function of the particle diameter; the particle size can be estimated from the fraction of the volume (of the powder) occupied by particles of some given average diameter. The powder has a mean particle size of ~7 mm and the corresponding distribution (Fig. 7) can be considered an amalgamation of two distributions, with peaks at particle sizes of ~3 and 7 mm. This implies that the size of the powder ranges possibly from 3 to 7 mm. In addition to the particle size, the morphology of the constituent particles may also have a significant effect on the performance or processing of particulate materials; an SEM image of the powder reveals the irregular and faceted shape of the particles. Multiple-step reactions for the hydrogen reduction of iron and nickel oxides were proposed. The resulting change in the Gibbs free energy (DG) was calculated, by using a thermodynamic program incorporated in the HSC Chemistry5.1 software. As shown below, hematite (Fe2O3) is directly reduced to metallic iron and water vapor, in a one-step
I. Seo et al. / Journal of Alloys and Compounds 670 (2016) 356e361
359
Fig. 5. Microstructure of cross-sectional area corresponding to chemical composition of each oxide layer conducted at 900 C for 24 h.
Fig. 6. Overall process to manufacture oxide powder from Fee77Ni alloy scrap.
100 60
90
50
85
40
80
30
75
20
70
10
2
4
6
8 10 Time, hr
12
14
Particle size, μm
Recovery ratio, %
RPM: 84
95
16
Fig. 7. Recovery ratio of oxide powder less than 10 mm with a fixed RPM.
process. Hematite is sequentially transformed into magnetite (Fe3O4) and metallic iron in a two-step process, and is reduced to wüstite (FeO) in the three-step process, which is subsequently decomposed into magnetite and iron. Magnetite is then reduced to iron, and water vapor is simultaneously generated. In the multiplestep process, hematite is transformed into magnetite and is reduced again to wüstite. The wüstite is decomposed into magnetite once again, thereby revealing the cyclical nature of the
Fig. 8. Particle size distribution and shape of oxide powder of Fee77Ni alloy.
magnetite-to-wüstite phase transformation, which gives rise, eventually, to metallic iron. On the other hand, nickel oxide is directly reduced to metallic nickel and water is generated as a gas phase, in a single step [18].
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One-step process [Fe2O3 / Fe] Fe2O3 þ 3H2(g) / 2Fe þ 3H2O(g) DG600 C ¼ 1.96 kcal
Two-step process [Fe2O3 / Fe3O4 / Fe] Fe2O3 þ H2(g) / 2Fe3O4 þ H2O(g) DG700 C ¼ 23.30 kcal 2Fe3O4 þ H2(g) / 3Fe þ 4H2O(g) DG1100 C ¼ 1.54 kcal
Three-step process [Fe2O3 / FeO / Fe3O4 / Fe] Fe2O3 þ H2(g) / 2FeO þ H2O(g) DG600 C ¼ 5.62 kcal 4FeO / Fe3O4 þ Fe DG600 C ¼ 0.21 kcal Fig. 10. XRD peaks and SEM image of alloy powder after hydrogen reduction.
2Fe3O4 þ H2(g) / 3Fe þ 4H2O(g) DG1100 C ¼ 1.54 kcal 1.68 wt.% at 900 C, and 0.72 wt.% at 1000 C, as determined by the N/O gas analyzer. The oxygen content was approximately constant after a reaction time of 1 h, indicating that the reduction of the oxide was almost complete. After a 3-h hydrogen reduction at 1000 C, the oxygen content of the sample was <1 wt.%. The powder of the metallic ironenickel alloy resulting from this 3-h reduction, was examined via XRD and SEM. As the XRD pattern shows (Fig. 10), the powder is composed only of the metallic Ni3Fe phase. This indicates that the oxide powder was almost transformed into a metallic powder of ironenickel alloy, with the same near-stoichiometric composition as that of the original sample. In addition, SEM examination of the metallic powder revealed that the ironenickel particles had a mean particle size of <10 mm. These particles had irregular shapes and, in general, small particles agglomerated to form large clusters.
Multiple-step process [Fe2O3 / Fe3O4 4 FeO / Fe] Fe2O3 þ H2(g) / 2Fe3O4 þ H2O(g) DG700 C ¼ 23.30 kcal 2Fe3O4 þ H2(g) / 3FeO þ H2O(g) DG800 C ¼ 0.16 kcal 4FeO / Fe3O4 þ Fe DG600 C ¼ 0.12 kcal In the case of the reduction of nickel oxide; One-step process [NiO / Ni] NiO þ H2(g) / Ni þ H2O(g) DG800 C ¼ 11.11 kcal The reduction of ironenickel oxide by flowing hydrogen gas at a rate of 100 cc/min, was determined as a function of the temperature and reaction time (Fig. 9). As the figure shows, the rate increased significantly after a reaction time of 1 h, but only modestly with increasing temperature. Furthermore, the oxygen content in the specimen decreased from 21.56 wt.% to 2.46 wt.% at 800 C,
100
Reduction rate, %
80 60
o
800 C o 900 C o 1000 C
40 20 0 0
Temp. Oxygen content
1
800oC 2.46
2
900oC 1.68
1000oC 0.72
3
Time, hr Fig. 9. Variation of reduction rate and oxygen content on the effect of temperature.
4. Conclusion The oxidation rate of Fee77Ni alloy scrap increased with increasing temperature, for a fixed oxygen partial pressure of 0.2 atm. The oxidation behavior of the specimen obeyed the parabolic rate law, regardless of the temperature, and an activation energy of 17.98 kJ/mol was determined experimentally. Furthermore, diffusion through either the spinel structure or the NiO layer was deemed the rate-limiting step of the reaction. The particle size distribution of the oxide powder was determined; the constituent particles of the powder had irregular and faceted shapes and a mean size of 3e7 mm. Using hydrogen gas, the oxide powder was successfully reduced to a metallic powder and the oxygen content decreased by ~0.72 wt.% after 3 h at1000 C. The metallic powder obtained, Ni3Fe, had a near-stoichiometric composition. Acknowledgment This study was supported by the R&D Center for Valuable Recycling (Global-Top Environmental Technology Development Program) funded by the Ministry of Environment. (Project No.: GT11-C-01-060-0). References [1] S.H. Choi, J.Y. Yun, H.M. Lee, Y.M. Kong, B.K. Kim, K.A. Lee, High temperature oxidation behavior of Ni based porous metal, J. Korean Powder Metall. Inst. 18 (2) (2011) 122e128.
I. Seo et al. / Journal of Alloys and Compounds 670 (2016) 356e361 [2] Y. Larring, R. Haugsrud, T. Norby, HT corrosion of Cre5 wt.%Fee1 wt.% Y2O3 alloy and conductivity of the oxide scale-effects of water vapor, J. Electrochem. Soc. 150 (2003) 374e379. [3] D.L. Douglass, P. Kofstad, A. Rahmel, G.C. Wood, International workshop on high temperature oxidation-subject area 1: the role of water vapour in the oxidation of metals and alloys, Oxid. Met. 45 (5,6) (1996) 529e545. [4] H. Buscail, S. Heinze, P.H. Dufour, J.P. Larpin, Water-vapor-effect on the oxidation of Fe 21.5 wt.% Cr, 5.6 wt.%Al at 1000 C, Oxid. Met. 47 (5/6) (1997) 445e464. [5] H. Al-Badairy, G. Tatlock, The influence of the moisture content of the atmosphere on alumina scale formation and growth during high temperature oxidation of PM2000, Mater. High Temp. 17 (2000) 133e137. [6] C. Nishimura, Y. Nagai, K. Yanagisawa, T. Toshima, Magnetic properties of NiFe films prepared using ion-beam sputtering, IEEE Trans. Magn. 23 (1987) 2728e2736. [7] W.E. Boggs, The oxidation of iron aluminium alloys from 450 C to 900 C, J. Electrochem. Soc. 118 (1977) 906e913. [8] K. Onal, M.C. Maris-Sida, G.H. Meier, F.S. Pettit, Water vapor effects on the cyclic oxidation resistance of alumina forming alloys, Mater. High Temp. 20 (2003) 81e91. [9] R. Janakiraman, G.H. Meier, F.S. Pettit, The effect of water vapor on the oxidation of alloys that develop alumina scales for protection, Metall. Mater. Trans. A 30A (1999) 2905e2913.
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[10] J.R. Regina, J.N. DuPont, A.R. Marder, The effect of water vapor on passivelayer stability and corrosion behavior of Fe-Al-Cr base alloys, Oxid. Met. 61 (1/2) (2004) 69e90. [11] J.R. Regina, J.N. DuPont, A.R. Marder, Gaseous corrosion resistance of Fe-Al based alloys containing Cr additions. Part II: Scale morphology, Mater. Sci. Eng. A 405 (2005) 102e110. [12] G.R. Wallwlork, The oxidation of alloys, Rep. Prog. Phys. 39 (1976) 401e410. [13] C.T. Liu, J. Ma, X.F. Sun, Mechanism of the oxidation and degradation of the aluminide coating on the nickel-base single-crystal superalloy DD32M, J. Alloys Compd. 491 (2010) 522e531. [14] W.Z. Li, Y.Q. Li, Q.M. Wang, C. Sun, X. Jiang, Oxidation of a NiCrAlYSi overlayer with or without a diffusion barrier deposited by one-step arc ion plating, Corros. Sci. 52 (2010) 1753e1762. [15] M. Oku, S. Suzuki, N. Ohtsu, T. Shishido, K. Wagatsuma, Micro-and nano pyramids of manganese-doped Indium oxide, Appl. Surf. Sci. 254 (2008) 5141e5149. [16] R.T. Foley, C.J. Guare, The oxidation of iron-nickel alloys, J. Electrochem. Soc. 106 (11) (1959) 936e940. [17] M.A. Rhamdhani, P.C. Hayes, E. Jak, Sub solidus phase equilibria of the Fe-Ni-O system, Matell. Mater. Trans. B 39B (2008) 690e701. [18] Sung-Ho Joo, Shun-Myung Shin, Dong-Ju Shin, Jei-Pil Wang, Reduction of high nickel-based oxide particles using hydrogen gas, Appl. Mech. Mater. 778 (2015) 148e151.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Fabrication of metallic alloy powder (Ni3Fe) from Fee77Ni scrap Inseok Seo a, Shun-Myung Shin b, Sang-An Ha c, Jei-Pil Wang d, * a
ES Materials Research Center, Research Institute of Industrial Science and Technology, Incheon 406-840, South Korea Extractive Metallurgy Department, Korea Institute of Geoscience and Mineral Resources, Deajeon 305-350, South Korea c Department of Environmental Engineering, Silla University, Busan 46958, South Korea d Department of Metallurgical Engineering, Pukyong National University, Busan 608-739, South Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 October 2015 Accepted 21 January 2016 Available online 4 February 2016
The oxidation behavior of Fee77Ni alloy scrap was investigated at an oxygen partial pressure of 0.2 atm and temperatures ranging from 400 C to 900 C. The corresponding oxidation rate increased with increasing temperature and obeyed the parabolic rate law, as evidenced by its linear proportionality to the temperature. In addition, surface morphologies, cross-sectional views, compositions, structural properties were examined by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). Diffusion through either the spinel structure or the NiO layer, which were both present in the alloy during oxidation at elevated temperatures, was deemed the rate-limiting step of the reaction. The oxide powder less than 10 mm was obtained from Fee77Ni alloy scrap was obtained using ball-milling and sieving processes. In fact, 15 h of milling yielded a recovery ratio of 97%. Using hydrogen gas, the oxide powder was successfully reduced to an alloy powder of Ni3Fe and reduction rates of ~97% were achieved after 3 h at 1000 C. © 2016 Elsevier B.V. All rights reserved.
Keywords: Fee77Ni Oxidation rate Rate-limiting step Oxide powder Alloy powder
1. Introduction High-nickel-based alloys exhibit outstanding corrosion and high temperature resistance and are therefore used to resist extremely corrosive environments in the energy, power, chemical, and petrochemical industries. In particular, Ni-based superalloys exhibit excellent strength, creep, and corrosion resistance at elevated temperatures. As such, aircraft engine manufacturers are continually striving to increase the capabilities of such hightemperature structural materials in order to improve both engine performance and efficiency [1e5]. Ni80Fe20 Permalloy (Py) thin films are soft magnetic materials, that exhibit high anisotropic magnetoresistance and low magnetostriction. Therefore, Py is one of the most commonly used materials for magnetic data storage and can be found in various magnetic micro and nanostructures [6,7]. The high-temperature oxidation behavior of nickel alloys has been extensive investigated. These investigations have been performed, at low pressure and temperatures above room temperature [8,9], as well as under air atmosphere or during electrochemical
* Corresponding author. E-mail address: [email protected] (J.-P. Wang). http://dx.doi.org/10.1016/j.jallcom.2016.01.173 0925-8388/© 2016 Elsevier B.V. All rights reserved.
oxidation [10,11] and plasma formation [12]. Liu et al. [13], studied the oxidation behavior of a single-crystal Ni-base superalloy at 900 and 1000 C in air, and reported that two oxidation steps occurred at both temperatures. The first step was controlled by the growth of NiO and the second by the growth of Al2O3, until a continuous Al2O3 layer formed under the previously grown NiO layer. Li et al. [14] investigated the oxidation behavior of a NiCrAlYSi overlayer with and without a diffusion barrier that was deposited by one-step arc ion plating. They showed that the duplex coating system provided excellent protection to the substrate, when thin and continuous scales were adhered to the overlayer surface, which underwent only limited oxidation and interdiffusion attacks. Zhou et al. [15] studied the oxidation behavior, in air at 1000 C, of pure and doped nickel alloys that have different Co contents. This study revealed that the mass oxidation gain of the coatings increased with increasing Co content. In the present study, the effect of temperature on the oxidation behavior of Fee77Ni alloy was determined via thermal gravimetric analysis (TGA), performed at an oxygen partial pressure of 0.2 atm. The cross-sectional morphologies of the oxide layer resulting from isothermal oxidation were characterized by (SEM-EDS) as well as XRD. The mechanism governing high-temperature oxidation was discussed, based on the kinetic data and the results of microstructural analysis. In addition, the particle size distribution of the
I. Seo et al. / Journal of Alloys and Compounds 670 (2016) 356e361
2
pO2: 0.2 atm o
900 C 6
o
800 C o 700 C
4
o
600 C o
500 C o 400 C 2
0 0
5
10
15
20
25
30
Time, hr Fig. 1. Weight change per area with time on the effect of temperatures.
dx=dt ¼ kp x x2 ¼ 2kp t þ C where kp and C are the parabolic rate constant and integration constant, respectively; the plot of x as a function of time varies with values of kp and C. Parabolic oxidation at elevated temperatures occurs via the diffusion of ions or electrons through the oxide scale [16]. The temperature-dependence of the reaction is best described by an Arrhenius-type equation, which is given as follows:
kp ¼ Ae
DE* RT
where DE* is the apparent energy of activation and A is the frequency factor. The significance of DE* derived from elevated temperatures in the Arrhenius plot, must be interpreted with caution [16]. The Arrhenius plot of the parabolic rate constants is shown in Fig. 2. As the figure shows, the constants are almost linearly proportional to the temperature, which indicates that they increase proportionally with increasing temperatures. An activation energy of 17.98 kJ/mol was determined experimentally, for the oxidation of Fee77Ni at an oxygen partial pressure of 0.2 atm.
y = -2.1637x + 41.2908 R2 = 0.9869
24
20
3. Results and discussion The TGA (Fig. 1) was used to measure the time-dependent change in the weight of the specimens during 24 h of oxidation. These measurements were performed at temperatures ranging from 400 C to 900 C under a gas mixture with an oxygen partial pressure of 0.2 atm. As shown in Fig. 1, the weight increased with increasing temperature, and was highest at 900 C. The oxidation rate of ironenickel alloys is described by a parabolic rate equation, which stipulates that the oxide grows at a continuously decreasing oxidation rate. The diffusional and integral forms of this equation are given as follows:
2
4
Scrap plate of Fee77Ni alloy containing 77 wt.% of Ni was obtained from a recycling company. This plate was cut into 1.5 cm in (width) 2 cm (length) coupon specimens. Prior to oxidation, the specimens were sequentially polished with 320, 500, 800, 1200, 2400, and 4000 grit SiC paper and subsequently polished with 1 mm diamond polishing suspension. The specimens were then cleaned with acetone in an ultrasonic bath, rinsed with ethanol and water, and dried under a hood. The continuous oxidation of the alloy was investigated at elevated temperatures and an oxygen partial pressure of 0.2 atm, by using the TGA. The oxidizing gas mixtures, oxygen and argon, were passed through drierites, in order to remove the moisture contained therein, and subsequently input into the reactor tube. A copper turning furnace was installed to reduce oxygen impurities in the argon gas, prior to its passage through the reaction zone. The oxidizing gas mixture flowed from the bottom of the quartz tube to the reactor zone, whereas the flow of purified argon gas was maintained through the electrobalance to prevent contamination during the oxidation. The coupon specimen was suspended by a platinum wire in the reaction zone of the quartz tube and then heated under an ultra-high-purity argon atmosphere. Once the desired temperatures of 400 Ce900 C were reached, a mixture of oxidizing gases was introduced into the reaction tube until oxidation was completed. The oxygen partial pressure was controlled by mixing argon and oxygen using each flowmeter. In addition, the change in the specimen weight during oxidation was continuously monitored and recorded for up to 24 h. The cross-sectional area of the oxidized specimens and chemical composition of each layer were examined by SEM, EDS, and XRD. Hydrogen gas reduction of the oxide powder obtained from the Fee77Ni scrap was performed at elevated temperatures of 800 C, 900 C, and 1000 C. For this reduction, the oxide specimen was placed on an alumina tray that, in turn, was placed in the center of the tube-type reactor. A copper turning furnace was used to dehumidify the ultra-high-purity (UHP) argon gas. Two flask bottles located at the end of the reactor were used for the safe handling of the wasted gas and the extra hydrogen gas was completely burned out. The sample tray containing the oxide particles was placed in the reactor and the flow of the UHP argon gas flow into the reactor was maintained until the desired temperature was reached. Hydrogen gas was subsequently introduced into the reactor, which was then cooled to room temperature under the UHP argon atmosphere.
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oxide powder was determined; various physical processes were used in an attempt to fabricate a <10 mm-sized oxide powder from the Fee77Ni alloy scrap. Finally, reduction by hydrogen gas was successfully used to transform the oxide powder to metallic powder at elevated temperatures.
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Oxide particles of Fee77Ni alloy scrap were formed after 24 h of oxidation at 700 C and 900 C, at this partial pressure. The constituent phases and the morphology of these particles were identified and examined via XRD and SEM, respectively. Similar XRD patterns were obtained and are shown in Fig. 3 and the same oxide phases, i.e., NiO, Fe2O3, and Ni1.43Fe1.7O4, occurred at both temperatures. Moreover, the oxide particles were irregular-shaped and exhibited significant variations from ~10 mm to ~200 mm in size. Previous XRD results indicated that, the formation of benseniteNiO (B), spinel (S), and hematite (H) phase could be expected. As shown in Fig. 4, hematite, spinel, bunsenite, and complex phases are formed, depending on the temperature and value of the Ni/ (Ni þ Fe) molar ratio. In fact, the pseudo-binary phase diagram shows that spinel (Fe, Ni)3O4 and bunsenite (Ni, Fe)O are formed under the current experimental conditions, as previously indicated by the XRD results. This formation is attributed to the outward migration of iron ions and electrons through grain boundaries, microcracks, and voids and the inward migration of oxygen anions (mainly) through spinel and bunsenite structures. The inward migration of anions is facilitated by pores and voids in the spinel structure, which is a non-stoichiometric compound of Ni1.43Fe1.7O4. The cross-section of the oxide layer formed during 24 h of oxidation at 900 C was examined via EDS. As shown in Fig. 5, the microstructure consists of two adjacent oxide layers, whose interface is punctuated with cracks and voids. The outer layer, labeled as (1), consists mainly of nickel and nickel oxide (NiO). Diffusion through either the spinel structure or the NiO layer, which are both formed in this alloy during oxidation at elevated temperatures [16], was deemed the rate-limiting step. The inner oxide layer, (2), consists of Ni, Mn, and Fe, and three oxide phases, namely, NiO, Fe2O3, and Ni1.43Fe1.7O4. In addition, this layer contains ~15 wt.% of Mo, which may influence the physical properties of the film. Mo plays a significant role in oxidation processes, especially when thermal cycling is involved [17]. The substrate, (3) and (4), contained only Ni and Fe, which occurred in the form of a metallic alloy. An oxide powder with particle sizes of <10 mm was fabricated and is shown in Fig. 6, based on the oxidation kinetic data of the Fee77Ni alloy scrap. Control of the particle size is crucial to achieving excellent properties such as compaction, density, and sinterability of the powder. Therefore, final particle sizes of <10 mm were targeted for powders used in the metal injection molding (MIM) process. As such, scrap foils were oxidized in a muffle furnace for 2 h at 900 C, under air atmosphere, since the oxidation rate at this temperature was found to be relatively high. The initial
Fig. 3. XRD patterns of oxide scales conducted at 700 C and 900 C for 24 h.
Fig. 4. Pseudo-binary phase diagram of ‘Fe2O3’eNiO in air. The closed-circle data points are from the present study; the dotted curves are calculated using FactSage. Legend: S ¼ spinel, B ¼ bunsenite, L ¼ liquid, and H ¼ hematite [17].
oxidation stage lasted for up to 2 h and subsequent cyclic oxidation was conducted in 2-h intervals, until complete oxidation of the foils was achieved. The resulting oxide particles were then ground, by using a ball-milling machine, in order to control the particle size of the final product. Powders with sizes of <10 mm, were obtained via sieving; the particle size distribution of the samples was determined by applying various levels of sieving. During the ball-milling process, size-control of the powder is achieved through careful selection of the milling parameters. A fixed milling speed of 84 revolutions per minute (RPM) was used, whereas the milling time was increased and the diameter of the tungsten carbide (WC) balls used as media, was varied; 10, 10, and 5 balls with diameters of 5 mm, 15 mm, and 50 mm, respectively, were used. As Fig. 7 shows, the recovery ratio of the oxide powder was ~70% in the first 3 h of milling, and then increased significantly after 6 h. A ratio of over 95% was obtained after 15 h; this value might correspond to a saturation and hence higher values were not expected with further milling. The particle-size distribution (PSD) of a powder has a significant effect on its physical and chemical properties, such as the particle compaction, density, and sinterability. In this work, the PSD of the oxide powder was determined by using a Mastersizer 3000 that is capable of analyzing particles with sizes ranging from 0.01 mm to 3500 mm. The morphology of the particles was examined via SEM and are shown in Fig. 8. The figure shows the distribution of average particle sizes, in terms of volume, as a function of the particle diameter; the particle size can be estimated from the fraction of the volume (of the powder) occupied by particles of some given average diameter. The powder has a mean particle size of ~7 mm and the corresponding distribution (Fig. 7) can be considered an amalgamation of two distributions, with peaks at particle sizes of ~3 and 7 mm. This implies that the size of the powder ranges possibly from 3 to 7 mm. In addition to the particle size, the morphology of the constituent particles may also have a significant effect on the performance or processing of particulate materials; an SEM image of the powder reveals the irregular and faceted shape of the particles. Multiple-step reactions for the hydrogen reduction of iron and nickel oxides were proposed. The resulting change in the Gibbs free energy (DG) was calculated, by using a thermodynamic program incorporated in the HSC Chemistry5.1 software. As shown below, hematite (Fe2O3) is directly reduced to metallic iron and water vapor, in a one-step
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Fig. 5. Microstructure of cross-sectional area corresponding to chemical composition of each oxide layer conducted at 900 C for 24 h.
Fig. 6. Overall process to manufacture oxide powder from Fee77Ni alloy scrap.
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process. Hematite is sequentially transformed into magnetite (Fe3O4) and metallic iron in a two-step process, and is reduced to wüstite (FeO) in the three-step process, which is subsequently decomposed into magnetite and iron. Magnetite is then reduced to iron, and water vapor is simultaneously generated. In the multiplestep process, hematite is transformed into magnetite and is reduced again to wüstite. The wüstite is decomposed into magnetite once again, thereby revealing the cyclical nature of the
Fig. 8. Particle size distribution and shape of oxide powder of Fee77Ni alloy.
magnetite-to-wüstite phase transformation, which gives rise, eventually, to metallic iron. On the other hand, nickel oxide is directly reduced to metallic nickel and water is generated as a gas phase, in a single step [18].
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One-step process [Fe2O3 / Fe] Fe2O3 þ 3H2(g) / 2Fe þ 3H2O(g) DG600 C ¼ 1.96 kcal
Two-step process [Fe2O3 / Fe3O4 / Fe] Fe2O3 þ H2(g) / 2Fe3O4 þ H2O(g) DG700 C ¼ 23.30 kcal 2Fe3O4 þ H2(g) / 3Fe þ 4H2O(g) DG1100 C ¼ 1.54 kcal
Three-step process [Fe2O3 / FeO / Fe3O4 / Fe] Fe2O3 þ H2(g) / 2FeO þ H2O(g) DG600 C ¼ 5.62 kcal 4FeO / Fe3O4 þ Fe DG600 C ¼ 0.21 kcal Fig. 10. XRD peaks and SEM image of alloy powder after hydrogen reduction.
2Fe3O4 þ H2(g) / 3Fe þ 4H2O(g) DG1100 C ¼ 1.54 kcal 1.68 wt.% at 900 C, and 0.72 wt.% at 1000 C, as determined by the N/O gas analyzer. The oxygen content was approximately constant after a reaction time of 1 h, indicating that the reduction of the oxide was almost complete. After a 3-h hydrogen reduction at 1000 C, the oxygen content of the sample was <1 wt.%. The powder of the metallic ironenickel alloy resulting from this 3-h reduction, was examined via XRD and SEM. As the XRD pattern shows (Fig. 10), the powder is composed only of the metallic Ni3Fe phase. This indicates that the oxide powder was almost transformed into a metallic powder of ironenickel alloy, with the same near-stoichiometric composition as that of the original sample. In addition, SEM examination of the metallic powder revealed that the ironenickel particles had a mean particle size of <10 mm. These particles had irregular shapes and, in general, small particles agglomerated to form large clusters.
Multiple-step process [Fe2O3 / Fe3O4 4 FeO / Fe] Fe2O3 þ H2(g) / 2Fe3O4 þ H2O(g) DG700 C ¼ 23.30 kcal 2Fe3O4 þ H2(g) / 3FeO þ H2O(g) DG800 C ¼ 0.16 kcal 4FeO / Fe3O4 þ Fe DG600 C ¼ 0.12 kcal In the case of the reduction of nickel oxide; One-step process [NiO / Ni] NiO þ H2(g) / Ni þ H2O(g) DG800 C ¼ 11.11 kcal The reduction of ironenickel oxide by flowing hydrogen gas at a rate of 100 cc/min, was determined as a function of the temperature and reaction time (Fig. 9). As the figure shows, the rate increased significantly after a reaction time of 1 h, but only modestly with increasing temperature. Furthermore, the oxygen content in the specimen decreased from 21.56 wt.% to 2.46 wt.% at 800 C,
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4. Conclusion The oxidation rate of Fee77Ni alloy scrap increased with increasing temperature, for a fixed oxygen partial pressure of 0.2 atm. The oxidation behavior of the specimen obeyed the parabolic rate law, regardless of the temperature, and an activation energy of 17.98 kJ/mol was determined experimentally. Furthermore, diffusion through either the spinel structure or the NiO layer was deemed the rate-limiting step of the reaction. The particle size distribution of the oxide powder was determined; the constituent particles of the powder had irregular and faceted shapes and a mean size of 3e7 mm. Using hydrogen gas, the oxide powder was successfully reduced to a metallic powder and the oxygen content decreased by ~0.72 wt.% after 3 h at1000 C. The metallic powder obtained, Ni3Fe, had a near-stoichiometric composition. Acknowledgment This study was supported by the R&D Center for Valuable Recycling (Global-Top Environmental Technology Development Program) funded by the Ministry of Environment. (Project No.: GT11-C-01-060-0). References [1] S.H. Choi, J.Y. Yun, H.M. Lee, Y.M. Kong, B.K. Kim, K.A. Lee, High temperature oxidation behavior of Ni based porous metal, J. Korean Powder Metall. Inst. 18 (2) (2011) 122e128.
I. Seo et al. / Journal of Alloys and Compounds 670 (2016) 356e361 [2] Y. Larring, R. Haugsrud, T. Norby, HT corrosion of Cre5 wt.%Fee1 wt.% Y2O3 alloy and conductivity of the oxide scale-effects of water vapor, J. Electrochem. Soc. 150 (2003) 374e379. [3] D.L. Douglass, P. Kofstad, A. Rahmel, G.C. Wood, International workshop on high temperature oxidation-subject area 1: the role of water vapour in the oxidation of metals and alloys, Oxid. Met. 45 (5,6) (1996) 529e545. [4] H. Buscail, S. Heinze, P.H. Dufour, J.P. Larpin, Water-vapor-effect on the oxidation of Fe 21.5 wt.% Cr, 5.6 wt.%Al at 1000 C, Oxid. Met. 47 (5/6) (1997) 445e464. [5] H. Al-Badairy, G. Tatlock, The influence of the moisture content of the atmosphere on alumina scale formation and growth during high temperature oxidation of PM2000, Mater. High Temp. 17 (2000) 133e137. [6] C. Nishimura, Y. Nagai, K. Yanagisawa, T. Toshima, Magnetic properties of NiFe films prepared using ion-beam sputtering, IEEE Trans. Magn. 23 (1987) 2728e2736. [7] W.E. Boggs, The oxidation of iron aluminium alloys from 450 C to 900 C, J. Electrochem. Soc. 118 (1977) 906e913. [8] K. Onal, M.C. Maris-Sida, G.H. Meier, F.S. Pettit, Water vapor effects on the cyclic oxidation resistance of alumina forming alloys, Mater. High Temp. 20 (2003) 81e91. [9] R. Janakiraman, G.H. Meier, F.S. Pettit, The effect of water vapor on the oxidation of alloys that develop alumina scales for protection, Metall. Mater. Trans. A 30A (1999) 2905e2913.
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