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Effect of Al additions on the oxidation behavior of Laves phase NbCr2 alloys at 1373 K and 1473 K
Int. Journal of Refractory Metals & Hard Materials 27 (2009) 659–663
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Short Communication
Effect of Al additions on the oxidation behavior of Laves phase NbCr2 alloys at 1373 K and 1473 K Haizhong Zheng a,b,*, Shiqiang Lu a, Zhu Jianye a, Liu Guangming a a b
School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, PR China School of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China
a r t i c l e
i n f o
Article history: Received 23 July 2008 Accepted 14 September 2008
Keywords: NbCr2 Laves phase alloys Oxidation behavior Al addition Mechanical alloying and hot pressing
a b s t r a c t Effect of Al on oxidation behavior of the Laves phase NbCr2 based alloys, which are fabricated by hot pressing and mechanical alloying, has been investigated at 1373 K and 1473 K, respectively. The oxidation resistance of the alloyed NbCr2 alloys increases with Al concentration increasing from 2 to 12 at.%. However, the alloyed NbCr2 has higher oxidation rate than unalloyed NbCr2 at 1473 K. When they are oxidized at 1373 K, NbCr2–12 at.% Al alloy shows lower oxidation rate than unalloyed NbCr2. The mechanism of oxidation has been analyzed using thermodynamic and kinetic. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction NbCr2 based intermetallic compounds have high melting temperature (1730 °C), relatively low density (7.7 g/cm3) and appreciable creep resistance [1–4]. In addition, Laves phase NbCr2 alloys have high strength (rs > 600 MPa) at 1200 °C [5]. Therefore, these excellent properties project it as a promising candidate material for high-temperature structural applications. An important property for high-temperature structural materials is the oxidation resistance. However, most of the researches [6,7] reported previously are focused on the oxidation temperature of Laves phase NbCr2 below 1150 °C. Fewer researchers have studied the oxidation behavior of Laves phase NbCr2 at or above 1200 °C. Previous studies [8] have shown that the oxide scale of NbCr2 oxidized at 1473 K had a structure of two layers: the outer layer is a porous, loosely Cr2O3 layer; the inner layer is consisted of CrNbO4. There are one or more breakdowns and wrinkling in the scale. This is attributed to the volume expansion caused by the formation of Nb2O5 and Cr2O3 at grain/particle boundaries, which produced large internal residual stresses. Thus one of challenges in NbCr2 is to control the breakdowns and spallation by adding proper alloying elements. Effect of Al additions on oxidation of other alloy system has been studied by some researchers [9–11]. It is found Al additions can significantly improve spallation resistance. * Corresponding author. Address: School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, PR China. Tel./fax: +86 7913863039. E-mail address: [email protected] (H. Zheng). 0263-4368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2008.09.007
In Nb–Cr–Al alloy system, the formation of Al2O3 is preferred to that of Cr2O3 and Nb2O5, involving volume expansion of only 4.9%, as compared to 107% in the case of Cr2O3 and 169% in the case of Nb2O5 [12]. This will reduce the oxide layer residual stresses and breakdowns. It has also been suggested that Al can play a remarkable role in decreasing the initial crack density in the sample and lowering the oxygen flux toward the oxide/substrate interface [10]. However, Liu [1] has reported that Al addition of 8 at.% has little beneficial effect on the oxidation behavior of Cr–10Nb at 950 °C in air. In the reports by Liu, the base alloy was Cr–10Nb, and Nb–Cr alloys were conventionally fabricated by arc-melting. However, only limited information is available on the elevated temperature oxidation behavior and mechanisms of NbCr2–Al alloys fabricated by mechanical alloying and hot pressing. In this study, NbCr2–Al alloys were prepared by mechanical alloying and hot pressing, and their oxidation behaviors at 1373 K and 1473 K were investigated. As a comparison, oxidation study was also performed on the unalloyed NbCr2 alloy. This investigation is an attempt to understand the effect of Al addition on the oxidation behavior of NbCr2 alloys. 2. Experimental procedures The raw materials used in the present study are Cr (100 mesh), Nb (100 mesh) and Al (100 mesh) powders. The nominal purity of Cr, Nb and Al powders are 99.5%, 99.7% and 99.9%, respectively. In order to obtain more Laves phase NbCr2, the base alloy composition used in this study was 2:1 mole ratio of Cr/Nb. Various amounts of Al (0, 2, 4 and 12 at.%) were added to the base alloy, and the Cr/Nb ratio is unchanged. The elemental powders were
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mixed and charged into a stainless steel vial with stainless steel balls under Ar atmosphere. The ball-to-material weight ratio was to 13:1. The milling was performed in a Spex QM-ISP2-CL laboratory ball mill for 20 h with a speed of 400 rpm. Based on the previous study, the mechanically alloyed powders were consolidated by hot pressing in a vacuum furnace with the following cycle: (a) heated from room temperature to sintering temperature (1250 °C) with a heating rate of about 120 °C/min; (b) kept the sintering temperature for 30 min; (c) cooled down from the sintering temperature to 600 °C at about 150 °C/min, and then furnace cool from 600 °C to room temperature. The pressure in the die was kept at 45 MPa and the vacuum degree was 80 Pa in the furnace. The hot pressed compacts were 14 mm in diameter and about 7 mm in thickness. The hot pressed specimens were polished to remove the surface layers containing graphite. Microstructures of NbCr2 and NbCr2–12 at.% Al alloys were characterized using scanning electron microscopy (SEM). Oxidation tests were performed in air at 1373 K and 1473 K, respectively. The samples were placed in an alumina crucible (30 mm diameter), and the weight changes were monitored by a balance of Sartorius Type BT224S (sensitivity of 0.1 mg). Before oxidation of the specimens, their surfaces were abraded on sufficiently fine SiC paper, then washed in running water and ultrasonically cleaned with alcohol. Cross-section of the oxidized specimens was observed by SEM with an energy dispersive spectrometer (EDS). Oxidized coupons were characterized with X-ray diffraction (XRD). 3. Results and discussion 3.1. Microstructure
Fig. 1. SEM images of as-HPed NbCr2 alloys with different Al addition levels: (a) 0 at.% Al and (b) 12 at.% Al.
Fig. 1a and b shows the back-scattered electron images of the as-HPed samples with 0 at.% Al and 12 at.% Al, respectively. According to the XRD analyses (Fig. 2) and the compositional analysis through EDS (Table 1), the gray phases are NbCr2, and the Al is present in the form of solid solution in NbCr2. It can also be seen from Fig. 1, the microstructure of NbCr2–12 at.% Al alloy is more
Fig. 2. XRD pattern of as-HPed NbCr2 alloys with different levels: (a) 0 at.% Al, (b) 12 at.% Al.
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H. Zheng et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 659–663 Table 1 EDS results of Laves phase in the matrix Nominal Al content in alloys (at.%)
Laves phase (grey) Cr
Nb
0
Point 1 Point 2 Point 3
65.04 59.70 61.36
34.96 40.30 38.64
12
Point 2 Point 2 Point 2
54.66 53.24 54.90
34.08 35.32 33.87
uniform and more compact than that of the unalloyed NbCr2. Moreover, density measurements show that as-HPed samples with 12 at.% Al have 98.14% of the theoretical density, whereas the unalloyed NbCr2 samples have about 97%. 3.2. Oxidation kinetics The oxidation kinetics at different temperatures has been evaluated in terms of mass change per unit area with respect to exposure time. Fig. 3a and b shows the comparison of plots displaying the oxidation kinetics of the NbCr2–x at.% Al (x = 0, 2, 4 and 12 at.%) alloys, exposed for 2530 min at the temperatures of
Al – – – 11.26 11.45 11.23
1473 K and 1373 K, respectively. It can be seen from Fig. 3 that NbCr2–x at.% Al compacts have continuous mass change. Isothermal oxidation at 1473 K, seen in Fig. 3a, shows that the oxidation resistance of NbCr2 becomes worse on the alloying with Al, when compared to that of unalloyed NbCr2. The weight gained per unit area by NbCr2–12 at.% Al is slight higher than that gained by unalloyed NbCr2. The weight gain values of 4 at.% Al alloys are about 10% higher than that of NbCr2–12 at.% Al alloys, and the gap increases with the increasing of exposure time. Especially, the weight gain of NbCr2–2 at.% Al alloys are two to three times that of unalloyed NbCr2 alloys. The oxidation behaviors of all alloys at 1373 K are shown in Fig. 3b. The trend shows that the weight gain of NbCr2–2 at.% Al alloys is higher than that of NbCr2. However, NbCr2–4 at.% Al alloys and unalloyed NbCr2 alloys have identical oxidation rate for 1450 min, after which the latter shows a slightly higher oxidation resistance. When the content of Al is up to 12 at.%, the alloyed NbCr2 alloys show better oxidation resistance than the unalloyed NbCr2. 3.3. Oxidation products Fig. 4a and b shows cross-sections of NbCr2–x at.% Al (x = 0, 12) compacts exposed at 1473 K. It can be clearly seen in Fig. 4a, for unalloyed NbCr2, there are more breakdowns and wrinkling in the scale. As for Fig. 4b, fast growth of the oxidized layer was observed, and Al2O3, Cr2O3 and Nb2O5 were observed in the oxidized layer formed on the Al-added NbCr2 compacts exposed at 1473 K. The phase is confirmed by XRD analysis. As shown in Fig. 5, XRD indicates that the phases of the oxide layer were mainly Al2O3, Cr2O3 and CrNbO4. Fig. 4c shows cross-sections of NbCr2–12 at.% Al compact exposed at 1373 K. It is found that a thin protective Al2O3 scale was formed on the surface. We note that Al addition up to 12 at.% is very beneficial for a good adhesion of the oxide layer on the metal. 3.4. Discussion
Fig. 3. Weight change versus time for NbCr2–x at.% Al (x = 0, 2, 4, 12) alloys isothermally oxidized in air at (a) 1473 K and (b) 1373 K for 2530 min.
In this study, NbCr2–2 at.% Al showed the worst oxidation resistance, followed by NbCr2–4 at.% Al and 12 at.% Al alloys when alloys were oxidized at 1473 K, all of which exhibited poorer oxidation resistance as compared to unalloyed NbCr2. However, the oxidation resistance at 1373 K of NbCr2–12 at.% Al was higher than unalloyed NbCr2. It is worth noting that the oxidation resistance of NbCr2–Al alloys increased with Al concentrations increasing. This can be explained as follows: For an intermetallic compound NbCr2, Cr2O3 can form on the outer layer of the oxide scale [8]. It is known from the EDS and XRD analysis that Al was present in the form of solid solution within NbCr2. Because oxide of Al has a lower free energy of formation and a lower partial pressure of oxygen requirement as compared to that of Cr, oxidation of Al will be preferred to that of Cr. With the progress of oxidation, Al would be depleted and the activity of Cr would increase, and
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Fig. 4. SEM (secondary electron) cross-sections of oxidized (a) NbCr2 exposed at 1473 K, (b) NbCr2–12 at.% Al exposed at 1473 K, and (c) NbCr2–12 at.% Al exposed at 1373 K for 2530 min.
oxidation of Cr would take place. A mixture of oxides of Cr and Al would form in the oxide scale. This is analogous to the oxidation products in NbCr2–2 at.% Al alloy. Long term stability of the protective scale requires that the flux of solute, Al to the alloy–scale interface remains large enough to prevent the oxide of Nb or Cr from forming. Pettit [13], for example, found that there were two critical concentrations for the formation of alumina scales on Ni–Al alloys: one concentration required to form the protective scale and a larger concentration of the same to maintain its stability. Without meeting this condition, a continuous and impervious film will not form from either Al or Cr. Wagner [14] has derived an approximate equation for the concentration of solute required to maintain the growth of an external scale as
Nc ¼ V M =16Z C ðpkP =DC Þ1=2 ;
ð1Þ
where KP is the parabolic rate constant for the growth of the protective scale, VM is the molar volume of the alloy, ZC is the valence of C
and DC is the diffusivity of C in the oxide. In the present experiment, the solute C in NbCr2 is represented by Al. Al concentration in solid solution in NbCr2 was insufficient for the formation of a continuous and stable Al2O3 layer in NbCr2–2 at.% Al and NbCr2–4 at.% Al. Hence, Cr2O3 and Nb2O5 also formed. In NbCr2–12 at.% Al alloys, Al concentration was sufficient to form an Al2O3 film, but it was insufficient to maintain a steady supply of Al ions to the oxide–metal interface. This induced to the formation of Nb2O5 and Cr2O3 takes place in the samples. As a result, substantial internal stresses develop. These internal stresses may arise in two different ways: (1) volume expansion caused by the formation of Nb2O5 and Cr2O3 at grain/particle boundaries; (2) internal pressure build-up caused by the volatilization of Cr2O3 at 1149 °C [6]. In terms of volume expansion, the molar volume change from NbCr2 to Cr2O3 + Nb2O5 is about 376%. Such a significant molar volume expansion can generate large internal stresses at grain-boundaries. When exposed at 1473 K, the internal pressure can also be caused by the volatiliza-
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Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 50474009), the Aeronautical science foundation of China (Grant No. 05G56003), the Natural Science Foundation of Jiangxi province (Grant No. 0350045) and the Materials Science and Engineering Research Center of Jiangxi province (Grant No. ZX200401001). References
Fig. 5. XRD pattern of the oxide layer formed on NbCr2 and NbCr2–12 at.% Al alloys oxidized at 1473 K.
tion of Cr2O3. This would expose the internal structure of NbCr2. Once the internal structure of a sample is exposed and more interfaces are produced and exposed to oxygen. 4. Conclusions The effect of Al additions on the oxidation resistance of NbCr2 at 1373 K and 1473 K was studied. The oxidation resistance of the alloyed NbCr2 alloys increase with Al concentration between 2 and 12 at.%. However, the alloyed NbCr2 has higher oxidation rate than unalloyed NbCr2 at 1473 K. When they are oxidized at 1373 K, NbCr2–12 at.% Al alloy shows lower oxidation rate than unalloyed NbCr2. Al addition is very beneficial for a good adhesion of the oxide layer on the metal.
[1] Liu CT, Tortorelli PF, Horton JA, Carmichael CA. Effects of alloy additions on the microstructure and properties of Cr2Nb alloys. Mater Sci Eng A 1996;214:23–32. [2] Jiang C. Site preference of early transition metal elements in C15 NbCr2. Acta Mater 2007;55(5):1599–605. [3] Zhu JH, Pike LM, Liu CT, Liaw PK. Point defects in binary Laves phase alloys. Acta Mater 1999;47:2003–18. [4] Zhua JH, Pikeb LM, Liub CT, Liaw PK. Point defects in binary NbCr2 Laves-phase alloys. Scripta Mater 1998;39:833–8. [5] Takasugi T, Hanada S, Yoshida M. High temperature mechanical properties of C15 Laves phase Cr2Nb intermetallics. Mater Sci Eng A 1995;192–193:805–10. [6] Brady MP, Zhu JH, Liu CT, Tortorelli PF, Walker LR. Oxidation resistance and mechanical properties of Laves phase reinforced Cr in situ composites. Intermetallics 2000;8:1111–8. [7] Brady MP, Tortorelli PF, Walker LR. Water vapor and oxygen/sulfur-impurity effects on oxidation and nitridation in single- and two-phase Cr–Nb alloys. Oxid Met 2002;58:297–318. [8] Zheng HZ, Lu SQ, Su Q, et al. Study on scaling of mechanically alloyed and hot pressed NbCr2 Laves phase at 1200 °C in air [J]. Int J Refr Metals Hard Mater 2008;26:1–4. [9] Stergiou A, Tsakiropoulos P. The intermediate and high-temperature oxidation behaviour of (Mo, X)Si2 (X = W, Ta) intermetallic alloys. Intermetallics 1997;5(2):117–26. [10] Yanagihara K, Przybylski K, Maruyama T. The role of microstructure on pesting during oxidation of MoSi2 and Mo(Si, Al)2 at 773 K. Oxid Met 1997;47:277–93. [11] Dasgupta T, Umarji AM. Thermal properties of MoSi2 with minor aluminum substitutions. Intermetallics 2007;15:128–32. [12] Pilling NB, Bedworth RE. The oxidation of metals at high temperature. J Inst Met 1923;29:529–82. [13] Pettit FS. Oxidation mechanisms for nickel–aluminum alloys at temperatures between 900 and 1300 °C. Trans TMS-AIME 1967;237:1296–305. [14] Wagner C. Theoretical analysis of the diffusion process determining the oxidation rate of alloys. J Electrochem Soc 1952;99:369–80.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Short Communication
Effect of Al additions on the oxidation behavior of Laves phase NbCr2 alloys at 1373 K and 1473 K Haizhong Zheng a,b,*, Shiqiang Lu a, Zhu Jianye a, Liu Guangming a a b
School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, PR China School of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China
a r t i c l e
i n f o
Article history: Received 23 July 2008 Accepted 14 September 2008
Keywords: NbCr2 Laves phase alloys Oxidation behavior Al addition Mechanical alloying and hot pressing
a b s t r a c t Effect of Al on oxidation behavior of the Laves phase NbCr2 based alloys, which are fabricated by hot pressing and mechanical alloying, has been investigated at 1373 K and 1473 K, respectively. The oxidation resistance of the alloyed NbCr2 alloys increases with Al concentration increasing from 2 to 12 at.%. However, the alloyed NbCr2 has higher oxidation rate than unalloyed NbCr2 at 1473 K. When they are oxidized at 1373 K, NbCr2–12 at.% Al alloy shows lower oxidation rate than unalloyed NbCr2. The mechanism of oxidation has been analyzed using thermodynamic and kinetic. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction NbCr2 based intermetallic compounds have high melting temperature (1730 °C), relatively low density (7.7 g/cm3) and appreciable creep resistance [1–4]. In addition, Laves phase NbCr2 alloys have high strength (rs > 600 MPa) at 1200 °C [5]. Therefore, these excellent properties project it as a promising candidate material for high-temperature structural applications. An important property for high-temperature structural materials is the oxidation resistance. However, most of the researches [6,7] reported previously are focused on the oxidation temperature of Laves phase NbCr2 below 1150 °C. Fewer researchers have studied the oxidation behavior of Laves phase NbCr2 at or above 1200 °C. Previous studies [8] have shown that the oxide scale of NbCr2 oxidized at 1473 K had a structure of two layers: the outer layer is a porous, loosely Cr2O3 layer; the inner layer is consisted of CrNbO4. There are one or more breakdowns and wrinkling in the scale. This is attributed to the volume expansion caused by the formation of Nb2O5 and Cr2O3 at grain/particle boundaries, which produced large internal residual stresses. Thus one of challenges in NbCr2 is to control the breakdowns and spallation by adding proper alloying elements. Effect of Al additions on oxidation of other alloy system has been studied by some researchers [9–11]. It is found Al additions can significantly improve spallation resistance. * Corresponding author. Address: School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, PR China. Tel./fax: +86 7913863039. E-mail address: [email protected] (H. Zheng). 0263-4368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2008.09.007
In Nb–Cr–Al alloy system, the formation of Al2O3 is preferred to that of Cr2O3 and Nb2O5, involving volume expansion of only 4.9%, as compared to 107% in the case of Cr2O3 and 169% in the case of Nb2O5 [12]. This will reduce the oxide layer residual stresses and breakdowns. It has also been suggested that Al can play a remarkable role in decreasing the initial crack density in the sample and lowering the oxygen flux toward the oxide/substrate interface [10]. However, Liu [1] has reported that Al addition of 8 at.% has little beneficial effect on the oxidation behavior of Cr–10Nb at 950 °C in air. In the reports by Liu, the base alloy was Cr–10Nb, and Nb–Cr alloys were conventionally fabricated by arc-melting. However, only limited information is available on the elevated temperature oxidation behavior and mechanisms of NbCr2–Al alloys fabricated by mechanical alloying and hot pressing. In this study, NbCr2–Al alloys were prepared by mechanical alloying and hot pressing, and their oxidation behaviors at 1373 K and 1473 K were investigated. As a comparison, oxidation study was also performed on the unalloyed NbCr2 alloy. This investigation is an attempt to understand the effect of Al addition on the oxidation behavior of NbCr2 alloys. 2. Experimental procedures The raw materials used in the present study are Cr (100 mesh), Nb (100 mesh) and Al (100 mesh) powders. The nominal purity of Cr, Nb and Al powders are 99.5%, 99.7% and 99.9%, respectively. In order to obtain more Laves phase NbCr2, the base alloy composition used in this study was 2:1 mole ratio of Cr/Nb. Various amounts of Al (0, 2, 4 and 12 at.%) were added to the base alloy, and the Cr/Nb ratio is unchanged. The elemental powders were
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mixed and charged into a stainless steel vial with stainless steel balls under Ar atmosphere. The ball-to-material weight ratio was to 13:1. The milling was performed in a Spex QM-ISP2-CL laboratory ball mill for 20 h with a speed of 400 rpm. Based on the previous study, the mechanically alloyed powders were consolidated by hot pressing in a vacuum furnace with the following cycle: (a) heated from room temperature to sintering temperature (1250 °C) with a heating rate of about 120 °C/min; (b) kept the sintering temperature for 30 min; (c) cooled down from the sintering temperature to 600 °C at about 150 °C/min, and then furnace cool from 600 °C to room temperature. The pressure in the die was kept at 45 MPa and the vacuum degree was 80 Pa in the furnace. The hot pressed compacts were 14 mm in diameter and about 7 mm in thickness. The hot pressed specimens were polished to remove the surface layers containing graphite. Microstructures of NbCr2 and NbCr2–12 at.% Al alloys were characterized using scanning electron microscopy (SEM). Oxidation tests were performed in air at 1373 K and 1473 K, respectively. The samples were placed in an alumina crucible (30 mm diameter), and the weight changes were monitored by a balance of Sartorius Type BT224S (sensitivity of 0.1 mg). Before oxidation of the specimens, their surfaces were abraded on sufficiently fine SiC paper, then washed in running water and ultrasonically cleaned with alcohol. Cross-section of the oxidized specimens was observed by SEM with an energy dispersive spectrometer (EDS). Oxidized coupons were characterized with X-ray diffraction (XRD). 3. Results and discussion 3.1. Microstructure
Fig. 1. SEM images of as-HPed NbCr2 alloys with different Al addition levels: (a) 0 at.% Al and (b) 12 at.% Al.
Fig. 1a and b shows the back-scattered electron images of the as-HPed samples with 0 at.% Al and 12 at.% Al, respectively. According to the XRD analyses (Fig. 2) and the compositional analysis through EDS (Table 1), the gray phases are NbCr2, and the Al is present in the form of solid solution in NbCr2. It can also be seen from Fig. 1, the microstructure of NbCr2–12 at.% Al alloy is more
Fig. 2. XRD pattern of as-HPed NbCr2 alloys with different levels: (a) 0 at.% Al, (b) 12 at.% Al.
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H. Zheng et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 659–663 Table 1 EDS results of Laves phase in the matrix Nominal Al content in alloys (at.%)
Laves phase (grey) Cr
Nb
0
Point 1 Point 2 Point 3
65.04 59.70 61.36
34.96 40.30 38.64
12
Point 2 Point 2 Point 2
54.66 53.24 54.90
34.08 35.32 33.87
uniform and more compact than that of the unalloyed NbCr2. Moreover, density measurements show that as-HPed samples with 12 at.% Al have 98.14% of the theoretical density, whereas the unalloyed NbCr2 samples have about 97%. 3.2. Oxidation kinetics The oxidation kinetics at different temperatures has been evaluated in terms of mass change per unit area with respect to exposure time. Fig. 3a and b shows the comparison of plots displaying the oxidation kinetics of the NbCr2–x at.% Al (x = 0, 2, 4 and 12 at.%) alloys, exposed for 2530 min at the temperatures of
Al – – – 11.26 11.45 11.23
1473 K and 1373 K, respectively. It can be seen from Fig. 3 that NbCr2–x at.% Al compacts have continuous mass change. Isothermal oxidation at 1473 K, seen in Fig. 3a, shows that the oxidation resistance of NbCr2 becomes worse on the alloying with Al, when compared to that of unalloyed NbCr2. The weight gained per unit area by NbCr2–12 at.% Al is slight higher than that gained by unalloyed NbCr2. The weight gain values of 4 at.% Al alloys are about 10% higher than that of NbCr2–12 at.% Al alloys, and the gap increases with the increasing of exposure time. Especially, the weight gain of NbCr2–2 at.% Al alloys are two to three times that of unalloyed NbCr2 alloys. The oxidation behaviors of all alloys at 1373 K are shown in Fig. 3b. The trend shows that the weight gain of NbCr2–2 at.% Al alloys is higher than that of NbCr2. However, NbCr2–4 at.% Al alloys and unalloyed NbCr2 alloys have identical oxidation rate for 1450 min, after which the latter shows a slightly higher oxidation resistance. When the content of Al is up to 12 at.%, the alloyed NbCr2 alloys show better oxidation resistance than the unalloyed NbCr2. 3.3. Oxidation products Fig. 4a and b shows cross-sections of NbCr2–x at.% Al (x = 0, 12) compacts exposed at 1473 K. It can be clearly seen in Fig. 4a, for unalloyed NbCr2, there are more breakdowns and wrinkling in the scale. As for Fig. 4b, fast growth of the oxidized layer was observed, and Al2O3, Cr2O3 and Nb2O5 were observed in the oxidized layer formed on the Al-added NbCr2 compacts exposed at 1473 K. The phase is confirmed by XRD analysis. As shown in Fig. 5, XRD indicates that the phases of the oxide layer were mainly Al2O3, Cr2O3 and CrNbO4. Fig. 4c shows cross-sections of NbCr2–12 at.% Al compact exposed at 1373 K. It is found that a thin protective Al2O3 scale was formed on the surface. We note that Al addition up to 12 at.% is very beneficial for a good adhesion of the oxide layer on the metal. 3.4. Discussion
Fig. 3. Weight change versus time for NbCr2–x at.% Al (x = 0, 2, 4, 12) alloys isothermally oxidized in air at (a) 1473 K and (b) 1373 K for 2530 min.
In this study, NbCr2–2 at.% Al showed the worst oxidation resistance, followed by NbCr2–4 at.% Al and 12 at.% Al alloys when alloys were oxidized at 1473 K, all of which exhibited poorer oxidation resistance as compared to unalloyed NbCr2. However, the oxidation resistance at 1373 K of NbCr2–12 at.% Al was higher than unalloyed NbCr2. It is worth noting that the oxidation resistance of NbCr2–Al alloys increased with Al concentrations increasing. This can be explained as follows: For an intermetallic compound NbCr2, Cr2O3 can form on the outer layer of the oxide scale [8]. It is known from the EDS and XRD analysis that Al was present in the form of solid solution within NbCr2. Because oxide of Al has a lower free energy of formation and a lower partial pressure of oxygen requirement as compared to that of Cr, oxidation of Al will be preferred to that of Cr. With the progress of oxidation, Al would be depleted and the activity of Cr would increase, and
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Fig. 4. SEM (secondary electron) cross-sections of oxidized (a) NbCr2 exposed at 1473 K, (b) NbCr2–12 at.% Al exposed at 1473 K, and (c) NbCr2–12 at.% Al exposed at 1373 K for 2530 min.
oxidation of Cr would take place. A mixture of oxides of Cr and Al would form in the oxide scale. This is analogous to the oxidation products in NbCr2–2 at.% Al alloy. Long term stability of the protective scale requires that the flux of solute, Al to the alloy–scale interface remains large enough to prevent the oxide of Nb or Cr from forming. Pettit [13], for example, found that there were two critical concentrations for the formation of alumina scales on Ni–Al alloys: one concentration required to form the protective scale and a larger concentration of the same to maintain its stability. Without meeting this condition, a continuous and impervious film will not form from either Al or Cr. Wagner [14] has derived an approximate equation for the concentration of solute required to maintain the growth of an external scale as
Nc ¼ V M =16Z C ðpkP =DC Þ1=2 ;
ð1Þ
where KP is the parabolic rate constant for the growth of the protective scale, VM is the molar volume of the alloy, ZC is the valence of C
and DC is the diffusivity of C in the oxide. In the present experiment, the solute C in NbCr2 is represented by Al. Al concentration in solid solution in NbCr2 was insufficient for the formation of a continuous and stable Al2O3 layer in NbCr2–2 at.% Al and NbCr2–4 at.% Al. Hence, Cr2O3 and Nb2O5 also formed. In NbCr2–12 at.% Al alloys, Al concentration was sufficient to form an Al2O3 film, but it was insufficient to maintain a steady supply of Al ions to the oxide–metal interface. This induced to the formation of Nb2O5 and Cr2O3 takes place in the samples. As a result, substantial internal stresses develop. These internal stresses may arise in two different ways: (1) volume expansion caused by the formation of Nb2O5 and Cr2O3 at grain/particle boundaries; (2) internal pressure build-up caused by the volatilization of Cr2O3 at 1149 °C [6]. In terms of volume expansion, the molar volume change from NbCr2 to Cr2O3 + Nb2O5 is about 376%. Such a significant molar volume expansion can generate large internal stresses at grain-boundaries. When exposed at 1473 K, the internal pressure can also be caused by the volatiliza-
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Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 50474009), the Aeronautical science foundation of China (Grant No. 05G56003), the Natural Science Foundation of Jiangxi province (Grant No. 0350045) and the Materials Science and Engineering Research Center of Jiangxi province (Grant No. ZX200401001). References
Fig. 5. XRD pattern of the oxide layer formed on NbCr2 and NbCr2–12 at.% Al alloys oxidized at 1473 K.
tion of Cr2O3. This would expose the internal structure of NbCr2. Once the internal structure of a sample is exposed and more interfaces are produced and exposed to oxygen. 4. Conclusions The effect of Al additions on the oxidation resistance of NbCr2 at 1373 K and 1473 K was studied. The oxidation resistance of the alloyed NbCr2 alloys increase with Al concentration between 2 and 12 at.%. However, the alloyed NbCr2 has higher oxidation rate than unalloyed NbCr2 at 1473 K. When they are oxidized at 1373 K, NbCr2–12 at.% Al alloy shows lower oxidation rate than unalloyed NbCr2. Al addition is very beneficial for a good adhesion of the oxide layer on the metal.
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