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Dissolved oxygen and the partial thermodynamic properties of γ′-Ni3Al + β-NiAl alloys
Scripta Materialia 57 (2007) 21–24 www.elsevier.com/locate/scriptamat
Dissolved oxygen and the partial thermodynamic properties of c 0-Ni3Al + b-NiAl alloys Evan Copland* Case Western Reserve University, Cleveland, OH 44106, United States Received 4 January 2007; revised 6 March 2007; accepted 9 March 2007 Available online 11 April 2007
The effect of dissolved O on the solution behavior and phase transformations in c 0 -Ni3Al + b-NiAl alloys was investigated by comparing the activities of Al, Al2O and Ni with the addition of Y, as Y2O3, to the Ni–Al–O system. In simple terms, this changed the stable oxide from Al2O3 (in Ni–Al–O) to YAlO3 (in Ni–Al–Y–O), and the activities of O and Al2O3 were reduced by about a factor of 5 and 100, respectively. There was, however, no observable change in the solution behavior or phase transformations of the Ni–Al phases. 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Knudsen effusion cell mass spectrometry; Thermodynamics; Nickel aluminides; O dissolution
Thermodynamic property measurements only have meaning if equilibrium is closely approached and the state of the system is established. For many experimental methods only some of the independent variables are measured, but regardless the state of the system still needs to be described accurately; the unmeasured variables must be identified and all assumptions clearly stated. A typical approach, however, is to only consider the sample (alloy or compound) that is identified as the focus of an investigation while the material used to contain the sample is ignored. The Knudsen effusion method [1,2] provides a good example of how this simplification results in an inaccurate description of the state of the system. In this study two Ni–Al alloys were placed in Al2O3 and Y2O3 effusion cells and the relative partial pressures of Al(g), Al2O(g) and Ni(g) were measured over a temperature range of T = 1600–1740 K. Ignoring the container implies the boundary of the system is in the vicinity of the alloy surface and only the {alloy + vapor} equilibrium is considered. The real boundary of this system is the inner surface of the effusion cell and therefore the container material must be included together with the alloy sample. For an Al2O3 effusion cell the {vapor +Al2O3} equilibrium also exists and the {alloy + Al2O3 + vapor} equilibrium accurately * Tel.: +1 216 433 3738; fax: +1 216 433 5544; e-mail: Evan.Copland@ grc.nasa.gov
describes the state of the system [3,4]. The alloy phases c 0 -Ni3Al, b-NiAl and L are all saturated with O and the Al2O3, at the inner surface of the effusion cell, is saturated with both Al and Ni from the alloy. The concentration of O in the alloy phases, Ni and Al in Al2O3 and the vapor composition are the unknown variables. Only considering the {alloy + vapor} equilibrium implicitly assumes dissolved O does not influence the solution behavior of the alloy phases and measurements made in the Ni–Al–O system can be applied directly to the Ni–Al system. Initially these assumptions appear to be acceptable because Al2O3 is in equilibrium with most Ni–Al alloys [5], the solubility limit of O is very low and Al2O3 has very limited stoichiometry, but the observed behavior of the Al–O and Ni–O systems raise some questions. According to the Al–O phase diagram [6], a L = Al(s) + Al2O3(s) eutectic is suspected at an O concentration < 3 · 108 at.% in L and T = 933 K (the freezing temperature of pure-Al in graphite is defined [7] as T = 933.473 K). The Ni–O phase diagram [8] has the L = Ni(s) + NiO(s) eutectic, at an O concentration of 0.9 at.% in L and T = 1713 ± 4 K (the freezing temperature of pure-Ni is T = 1728 ± 2 K [9]). While eutectic solidification indicates that dissolved O stabilizes L relative to the solid phases, it provides very little information about whether dissolved O influences the solution behavior (i.e., the activities of Al and Ni: a(Al) and a(Ni) in these phases). A better test of this
1359-6462/$ - see front matter 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.03.015
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E. Copland / Scripta Materialia 57 (2007) 21–24
assumption is to check if a(Al) and a(Ni) in c 0 -Ni3Al, bNiAl and L change significantly with O activity (or p(O)1) below the saturation limit (i.e., in the composition region between the Ni–Al binary and where Al2O3 becomes stable). While it is impossible to study O-free Ni–Al alloys with an effusion cell made from an oxide, it is easy to compare a(Al) and a(Ni) for a single composition at different p(O) by placing an identical alloy sample in adjacent effusion cells made from oxides with different stabilities, i.e., Al2O3 and Y2O3 (a multiple effusion-cell mass spectrometer (multi-cell KEMS) is ideal for this measurement). Component activities were compared for two alloys: Ni–27.3Al and Ni–29.4Al (at.%). These alloys were prepared by arc-melting proportions of the pure elements (at least 99.995 wt.% pure) 2–5 times. For the Al2O3 effusion cell the system is described by the {alloy + Al2O3 + vapor} equilibrium, as discussed above, where the stable alloy phases are c 0 -Ni3Al + b-NiAl, b-NiAl, L + b-NiAl and L with increasing T. The state of the system inside the Y2O3 effusion cell is more difficult to describe because of the extra component, Y, and because the alloys react with Y2O3 as shown in Figure 1. Below T = 1640 K, the bulk alloy consists of c 0 -Ni3Al (light phase) + b-NiAl (dark phase), but close to the effusion cell there were regions depleted in Al that consist of c 0 -Ni3Al and an NixY compound (white), and also regions where the c 0 -Ni3Al + b-NiAl structure is in contact with the effusion cell. In all regions, continuous layers of YAlO3 (YAP) and Y4Al2O9 (YAM) formed on the inner surface of the Y2O3 effusion cell, which agrees with the current Al2O3–Y2O3 quasi-binary [10]. As seen in Figure 1, the NixY phase exists at the interface between the bulk c 0 -Ni3Al + b-NiAl and Aldepleted zone, in spheres (in what appears to be a eutectic) in the Al-depleted zone, and also on the reacted effusion cell wall (also as a eutectic). The NixY eutectic structure suggests two liquid phases exist in the Ni– Al–Y–O system: one, LI, close to the Ni–Al binary and the other, LII, close to the Ni–Y binary. Phase compositions were determined by electronprobe analysis (EPMA) and are listed in Table 1. The small cross-section of the NixY phase (<1 lm) made it impossible to determine an accurate composition, but it did not appear to contain O and only a small amount of Al. While these alloys react with Y2O3, the measured relative partial pressures of Al(g), Ni(g) and Al2O(g) were independent of time at all temperatures, which indicates a steady-state condition existed and presumably a state of equilibrium was closely approached. Accordingly the state of the system in the Y2O3 effusion cell is best described with the {alloy + NixY + YAlO3 + vapor} equilibrium, where the alloy consists of c 0 -Ni3Al + b-NiAl, b-NiAl, LI + b-NiAl and LI with increasing temperature. Above some unknown temperature NixY melts and a second liquid, LII, is also present. Therefore, in summary, an Y2O3 effusion cell introduces both O and Y to the Ni–Al alloys and the system attains equilibrium by forming the reaction products NixY, 1
p(O) is identified because O(g) it is the dominant O vapor species under these conditions.
Figure 1. Backscattered electron images (at ·500 and ·2000) showing the microstructure of the c 0 -Ni3Al + b-NiAl/Y2O3 interface observed for Ni–29.4Al after measurement. The bulk alloy consists of c 0 -Ni3Al and b-NiAl phases with a region of Al-depletion close to the effusion cell wall. This region contains an NixY compound (white phase in alloy) and two oxide compound layers, YAlO3 and Y4Al2O9, on the inner surface of the Y2O3 effusion cell. The state of the system inside the Y2O3 effusion cell is best described by the {alloy + NixY + YAlO3 + vapor} equilibrium.
YAlO3 and Y4Al2O9. In combination, this reduces the activity of O (or p(O)) by a factor of about 5 and reduces a(Al2O3) by a factor of about 100 for {c 0 -Ni3Al + b-NiAl + NixY + YAlO3} relative to {c 0 -Ni3Al + bNiAl + Al2O3}, as shown in Figure 2. Unfortunately, these results gave no information about the activities of either Y or Y2O3 in the {alloy + NixY + YAlO3 + vapor} equilibrium. The activities a(Al), a(Al2O) and a(Ni) in these {alloy + oxide + vapor} equilibria were determined by the vapor pressure technique with a multiple Knudsen effusion cell vapor source (three effusion cells) configured for use with a Nuclide/MAAS/PATCO 12–90-HT single focus 90 permanent sector mass spectrometer, multi-cell KEMS [2,3,11]. At each temperature the activities were determined by comparing the measured partial pressure ratio of the vapor species in equilibrium with the alloy and a pure-Au reference, p(i)/p(Au), to the tabulated partial pressure ratio [p(Au)/p(i)] for pureAu and the reference states, according to: aðiÞ ¼ pðiÞ=p ðAuÞ ½p ðAuÞ=p ðiÞ ¼ I i =I Au S Au =S i ½p ðAuÞ= p ðiÞ; where i = Al(g), Al2O(g) and Ni(g). In this case, Ii is the measured ion intensity and SAu/Si is measured calibration factor that relate p(Au), in equilibrium with pure-Au(s,l), to p(Al) and p(Al2O) in equilibrium with {Al(l) + Al2O3(s)} and also p(Ni) in equilibrium
E. Copland / Scripta Materialia 57 (2007) 21–24
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Table 1. EPMA measured phase compositions (in at.%) across the c 0 -Ni3Al + b-NiAl/Y2O3 interface Phase
Ni
Al
O
Y
c 0 -Ni3Al (bulk) b-NiAl (bulk) c 0 -Ni3Al (interface) NixY YAlO3 (YAP) Y4Al2O9 (YAM) Y2O3
73.0 ± 0.3 62.1 ± 0.3 74.5 ± 0.1 75–80 1.5 ± 0.4 2.5 ± 0.7 1.3 ± 0.2
26.6 ± 0.3 37.5 ± 0.2 25.2 ± 0.1 1–3 20.3 ± 0.4 14.6 ± 0.8 –
0.3 ± 0.1 0.4 ± 0.1 0.27 ± 0.01 – 58.0 ± 0.4 57.4 ± 0.9 57.3 ± 0.4
0.01 ± 0.02 0.01 ± 0.02 0.01 ± 0.01 20–25 20.2 ± 0.3 25.5 ± 0.7 41.4 ± 0.3
Figure 2. Relative-a(O) and relative-a(Al2O3) versus T for the {alloy + NixY + YAlO3} equilibrium relative to the {alloy + Al2O3} equilibrium. These measurements were determined directly from the measured ion intensities I Al2 O and IAl according to Eqs. (1) and (2).
{Ni(s,l) + Al2O3(s)}. For the {Al(l) + Al2O3(s)} reference state, p(Al) and p(Al2O) are defined by the reactions Al(l) = Al(g) and 4/3Al(l) + 1/3Al2O3(s) = Al2O(g), respectively. For the {Ni(s,l) + Al2O3(s)} reference state, p(Ni) is defined by Ni(s,l) = Ni(g). A more detailed description of the technique and procedures are found in Refs. [3,12]. The activity of O (or p(O)) and a(Al2O3), relative to {Al(l) + Al2O3(s)}, can be determined indirectly from the measured values of a(Al) and a(Al2O) via the reactions 2Al(g) + O(g) = Al2O(g) and 4Al(g) + Al2O3(s) = 3Al2O(g), respectively. For this discussion, however, the ‘‘relative activities’’ of O and Al2O3 in {c 0 + b + NixY + YAlO3} (identified as A) relative to {c 0 + b + Al2O3} (identified as R) were determined directly by comparing the measured ion intensity ratios of I Al2 O and IAl according to Eqs. (1) and (2). The results of this determination are shown in Figure 2.
Figure 3. Combined a(Al) and a(Al2O) measured for alloys Ni–27.3Al and Ni–29.4Al in the Al2O3 and Y2O3 effusion cells, plotted as the logarithm of activity versus 1/T.
alloys in the Al2O3 and Y2O3 effusion cells are shown in Figure 4. These results also show that there is no measurable difference in a(Ni) resulting from the change in effusion cell material. According to these results, changing the container material (from Al2O3 to Y2O3) introduced Y and reduced the O activity by a factor of about 5 and a(Al2O3) by a factor of about 100, but produced no measurable change in either a(Al) and a(Ni) in either alloy. This suggests that dissolved O and Y do not affect the solution behavior of ‘‘Ni–Al’’
2
relative-aðOÞ ¼ ðI Al2 O ðAÞ=I Al2 O ðRÞÞ ðI Al ðRÞ=I Al ðAÞÞ
ð1Þ relative-aðAl2 O3 Þ ¼ ðI Al2 O ðAÞ=I Al2 O ðRÞÞ3 ðI Al ðRÞ=I Al ðAÞÞ4 ð2Þ
The measured a(Al) and a(Al2O), in Ni–27.3Al and Ni–29.4Al in equilibrium with the Al2O3 and Y2O3 effusion cells, are shown together in Figure 3. These results indicate, for both alloys, that there is no measurable difference in a(Al), while a(Al2O) is very sensitive to the effusion cell material. The measured a(Ni) for both
Figure 4. Combined a(Ni) measured for alloys Ni–27.3Al and Ni– 29.4Al in the Al2O3 and Y2O3 effusion cells, plotted as the logarithm of activity versus 1/T.
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E. Copland / Scripta Materialia 57 (2007) 21–24
phases, which supports this common assumption. It is important to recognize that this is not surprising because the expected change in concentration of dissolved O and Y in the Ni–Al alloys phases considered in this study is negligibly small. The relationship between O activity and O concentration in c 0 -Ni3Al, b-NiAl and L phases was not determined; without this information there is some uncertainty in relating these measurements to the binary Ni–Al system. This issue will be further investigated by considering the effect of a graphite effusion cell and comparing the behavior of the {alloy + Al2O3 + vapor} and {alloy + C + vapor} equilibria. A graphite effusion cell should remove O from the system and replace it with C.2 In addition to component activities, these measurements provide accurate information about equilibrium phase transformations in the Ni–Al–O and Ni–Al–Y– O systems. The slope changes in log-a(i) versus 1/T plots shown in Figures 3 and 4 represent a change in phase stability or a phase transformation. In line with the binary Ni–Al phase diagram [13], the phase boundaries and transformation temperatures observed in this study are listed in Table 2. For the measurements made in Al2O3 effusion cells the c 0 -Ni3Al + b-NiAl/L + b-NiAl phase boundary observed with Ni–27.3Al appear to represent the {c 0 -Ni3Al + b-NiAl + L + Al2O3 + vapor} invariant equilibrium in the Ni–Al–O system and the eutectic reaction: LðþAl2 O3 Þ ¼ c0 -Ni3 Al þ b-NiAlðþAl2 O3 Þ. The reaction temperature determined in this study, T = 1640 ± 1 K, is close to the assessed value of T = 1642 K [13] and agrees with the most recent experimental determination of T = 1639 ± 2 K from Ref. [14]. In the study by Battezzati et al. [14], the reaction temperature was determined by high-temperature differential scanning calorimetry of a series of binary Ni–Al alloys in Al2O3 cells, which is the same {alloy + Al2O3 + vapor} equilibrium considered in this study. For Ni–29.4Al the eutectic reaction was bypassed by entering the {b-NiAl + Al2O3 + vapor} phase field at T = 1605 ± 5 K and subsequently the {L + b-NiAl + Al2O3 + vapor} phase field at T = 1642 ± 3 K. For the Y2O3 effusion cell there is an extra component, Y, but as the ‘‘Ni–Al’’ alloys are in equilibrium with both NixY and YAlO3 the degree of freedom in the Ni–Al–Y–O system is the same as the Ni–Al–O system. The extra component and stable phases make it more difficult to understand the nature of the phase transformation in the Ni–Al–Y–O system, but presumably the c 0 -Ni3Al + b-NiAl/L + b-NiAl boundary represents the {c 0 -Ni3Al + b-NiAl + L + NixY + YAlO3 + vapor} invariant equilibrium and the reaction: LðþNix Yþ YAlO3 Þ ¼ c0 -Ni3 Al þ b-NiAlðþNix Y þ YAlO3 Þ. There is no observable difference in the transformation temperatures of these alloys, whether the measurements were made in the Y2O3 or in the Al2O3 effusion cell. This suggests that dissolved O and Y have no influence on
2
Initially O-free Ni–Al alloys were thought to be impossible to study, but a graphite effusion cell could remove O from the system (suggestion from G. Rossenblatt). This will be considered in a future study.
Table 2. Measured phase transformation temperatures in Al2O3 and Y2O3 effusion-cells Phase boundary 0
c + b/b c 0 + b/L + b b/L + b L + b/L
Ni–27.3Al
Ni–29.4Al
– 1640 ± 1 K – 1672 ± 1 K
1605 ± 5 K – 1642 ± 3 K 1704 ± 2 K
the phase transformation behavior of Ni–Al alloys, and similar transformations identified for the {alloy + Al2O3 + vapor} equilibrium could be assumed to occur for bulk c 0 -Ni3Al + b-NiAl alloys in the Y2O3 effusion cells. In summary, changing the container material from Al2O3 (Ni–Al–O system) to Y2O3 (Ni–Al–Y–O system) introduced Y and equilibrium was obtained with the formation of NixY, YAlO3 and Y4Al2O9. This change reduced the O activity by a factor of 5 and a(Al2O3) by a factor of about 100, but there was no measurable change in either a(Al), a(Ni) or a change in the observed phase transformations of the Ni–Al alloys. These results support the assumption of ignoring the container material, but also highlight the attention needed to accurately describe the state of a system typically encountered in thermodynamic property measurements. In general, the nature of an experiment dictates the system that is studied; for example, the Ni–Al binary is not studied directly but, rather, measurements are made in higher-order systems (e.g., Ni–Al–O or Ni– Al–Y–O). The thermodynamic data need to be considered in terms of these additional components and phases. [1] M. Knudsen, Ann. der Phys. 28 (1909) 75; M. Knudsen, Ann. der Phys. 28 (1909) 999. [2] M. Inghram, J. Drowart, in: High Temperature Technology, McGraw Hill, New York, 1960, pp. 219–240. [3] C. Chatillon, M. Allibert, A. Pattoret, in: J. Hastie (Ed.), Proc. of the 10th Materials Res. Symp. on Characterization of High Temperature Vapors and Gases, National Bureau of Standards, Gaithersburg, MD (1978) 181– 210. [4] E. Copland, in: E. Opila, J. Fergus, T. Maruyama, J. Mizusaki, T. Narita, D. Shifler, E. Wuchina (Eds.), High Temperature Corrosion and Materials Chemistry V, The Electrochemical Society, Pennington, NJ, 2005, pp. 361– 369. [5] F. Elrefaie, W. Smeltzer, J. Electrochem. Soc. 128 (1981) 2237. [6] H. Wriedt, Bull. Alloy Phase Diag. 6 (1985) 548–553, pp. 567–568. [7] M.L. McGlashan, J. Chem. Thermodyn. 22 (1990) 653 (ITS-90). [8] J. Taylor, A. Dinsdale, Z. Metallkd. 81 (1990) 354. [9] R. Bedford, G. Bonnier, H. Maas, F. Pavese, Metrologia 33 (1996) 133. [10] J. Gro¨bner, H. Lukas, F. Aldinger, Z. Metallkd. 87 (4) (1996) 268. [11] O. Kubaschewski, C.B. Alcock, Metallurgical Thermochemistry, fifth ed., Pergamon Ress, Oxford, 1979. [12] E. Copland, J. Phase Equilib. Diffusion, in press. [13] W. Huang, Y. Chang, Intermetallics 6 (1998) 487. [14] L. Battezzati, M. Baricco, L. Pascale, Scripta Mater. 39 (1998) 87.
Dissolved oxygen and the partial thermodynamic properties of c 0-Ni3Al + b-NiAl alloys Evan Copland* Case Western Reserve University, Cleveland, OH 44106, United States Received 4 January 2007; revised 6 March 2007; accepted 9 March 2007 Available online 11 April 2007
The effect of dissolved O on the solution behavior and phase transformations in c 0 -Ni3Al + b-NiAl alloys was investigated by comparing the activities of Al, Al2O and Ni with the addition of Y, as Y2O3, to the Ni–Al–O system. In simple terms, this changed the stable oxide from Al2O3 (in Ni–Al–O) to YAlO3 (in Ni–Al–Y–O), and the activities of O and Al2O3 were reduced by about a factor of 5 and 100, respectively. There was, however, no observable change in the solution behavior or phase transformations of the Ni–Al phases. 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Knudsen effusion cell mass spectrometry; Thermodynamics; Nickel aluminides; O dissolution
Thermodynamic property measurements only have meaning if equilibrium is closely approached and the state of the system is established. For many experimental methods only some of the independent variables are measured, but regardless the state of the system still needs to be described accurately; the unmeasured variables must be identified and all assumptions clearly stated. A typical approach, however, is to only consider the sample (alloy or compound) that is identified as the focus of an investigation while the material used to contain the sample is ignored. The Knudsen effusion method [1,2] provides a good example of how this simplification results in an inaccurate description of the state of the system. In this study two Ni–Al alloys were placed in Al2O3 and Y2O3 effusion cells and the relative partial pressures of Al(g), Al2O(g) and Ni(g) were measured over a temperature range of T = 1600–1740 K. Ignoring the container implies the boundary of the system is in the vicinity of the alloy surface and only the {alloy + vapor} equilibrium is considered. The real boundary of this system is the inner surface of the effusion cell and therefore the container material must be included together with the alloy sample. For an Al2O3 effusion cell the {vapor +Al2O3} equilibrium also exists and the {alloy + Al2O3 + vapor} equilibrium accurately * Tel.: +1 216 433 3738; fax: +1 216 433 5544; e-mail: Evan.Copland@ grc.nasa.gov
describes the state of the system [3,4]. The alloy phases c 0 -Ni3Al, b-NiAl and L are all saturated with O and the Al2O3, at the inner surface of the effusion cell, is saturated with both Al and Ni from the alloy. The concentration of O in the alloy phases, Ni and Al in Al2O3 and the vapor composition are the unknown variables. Only considering the {alloy + vapor} equilibrium implicitly assumes dissolved O does not influence the solution behavior of the alloy phases and measurements made in the Ni–Al–O system can be applied directly to the Ni–Al system. Initially these assumptions appear to be acceptable because Al2O3 is in equilibrium with most Ni–Al alloys [5], the solubility limit of O is very low and Al2O3 has very limited stoichiometry, but the observed behavior of the Al–O and Ni–O systems raise some questions. According to the Al–O phase diagram [6], a L = Al(s) + Al2O3(s) eutectic is suspected at an O concentration < 3 · 108 at.% in L and T = 933 K (the freezing temperature of pure-Al in graphite is defined [7] as T = 933.473 K). The Ni–O phase diagram [8] has the L = Ni(s) + NiO(s) eutectic, at an O concentration of 0.9 at.% in L and T = 1713 ± 4 K (the freezing temperature of pure-Ni is T = 1728 ± 2 K [9]). While eutectic solidification indicates that dissolved O stabilizes L relative to the solid phases, it provides very little information about whether dissolved O influences the solution behavior (i.e., the activities of Al and Ni: a(Al) and a(Ni) in these phases). A better test of this
1359-6462/$ - see front matter 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.03.015
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E. Copland / Scripta Materialia 57 (2007) 21–24
assumption is to check if a(Al) and a(Ni) in c 0 -Ni3Al, bNiAl and L change significantly with O activity (or p(O)1) below the saturation limit (i.e., in the composition region between the Ni–Al binary and where Al2O3 becomes stable). While it is impossible to study O-free Ni–Al alloys with an effusion cell made from an oxide, it is easy to compare a(Al) and a(Ni) for a single composition at different p(O) by placing an identical alloy sample in adjacent effusion cells made from oxides with different stabilities, i.e., Al2O3 and Y2O3 (a multiple effusion-cell mass spectrometer (multi-cell KEMS) is ideal for this measurement). Component activities were compared for two alloys: Ni–27.3Al and Ni–29.4Al (at.%). These alloys were prepared by arc-melting proportions of the pure elements (at least 99.995 wt.% pure) 2–5 times. For the Al2O3 effusion cell the system is described by the {alloy + Al2O3 + vapor} equilibrium, as discussed above, where the stable alloy phases are c 0 -Ni3Al + b-NiAl, b-NiAl, L + b-NiAl and L with increasing T. The state of the system inside the Y2O3 effusion cell is more difficult to describe because of the extra component, Y, and because the alloys react with Y2O3 as shown in Figure 1. Below T = 1640 K, the bulk alloy consists of c 0 -Ni3Al (light phase) + b-NiAl (dark phase), but close to the effusion cell there were regions depleted in Al that consist of c 0 -Ni3Al and an NixY compound (white), and also regions where the c 0 -Ni3Al + b-NiAl structure is in contact with the effusion cell. In all regions, continuous layers of YAlO3 (YAP) and Y4Al2O9 (YAM) formed on the inner surface of the Y2O3 effusion cell, which agrees with the current Al2O3–Y2O3 quasi-binary [10]. As seen in Figure 1, the NixY phase exists at the interface between the bulk c 0 -Ni3Al + b-NiAl and Aldepleted zone, in spheres (in what appears to be a eutectic) in the Al-depleted zone, and also on the reacted effusion cell wall (also as a eutectic). The NixY eutectic structure suggests two liquid phases exist in the Ni– Al–Y–O system: one, LI, close to the Ni–Al binary and the other, LII, close to the Ni–Y binary. Phase compositions were determined by electronprobe analysis (EPMA) and are listed in Table 1. The small cross-section of the NixY phase (<1 lm) made it impossible to determine an accurate composition, but it did not appear to contain O and only a small amount of Al. While these alloys react with Y2O3, the measured relative partial pressures of Al(g), Ni(g) and Al2O(g) were independent of time at all temperatures, which indicates a steady-state condition existed and presumably a state of equilibrium was closely approached. Accordingly the state of the system in the Y2O3 effusion cell is best described with the {alloy + NixY + YAlO3 + vapor} equilibrium, where the alloy consists of c 0 -Ni3Al + b-NiAl, b-NiAl, LI + b-NiAl and LI with increasing temperature. Above some unknown temperature NixY melts and a second liquid, LII, is also present. Therefore, in summary, an Y2O3 effusion cell introduces both O and Y to the Ni–Al alloys and the system attains equilibrium by forming the reaction products NixY, 1
p(O) is identified because O(g) it is the dominant O vapor species under these conditions.
Figure 1. Backscattered electron images (at ·500 and ·2000) showing the microstructure of the c 0 -Ni3Al + b-NiAl/Y2O3 interface observed for Ni–29.4Al after measurement. The bulk alloy consists of c 0 -Ni3Al and b-NiAl phases with a region of Al-depletion close to the effusion cell wall. This region contains an NixY compound (white phase in alloy) and two oxide compound layers, YAlO3 and Y4Al2O9, on the inner surface of the Y2O3 effusion cell. The state of the system inside the Y2O3 effusion cell is best described by the {alloy + NixY + YAlO3 + vapor} equilibrium.
YAlO3 and Y4Al2O9. In combination, this reduces the activity of O (or p(O)) by a factor of about 5 and reduces a(Al2O3) by a factor of about 100 for {c 0 -Ni3Al + b-NiAl + NixY + YAlO3} relative to {c 0 -Ni3Al + bNiAl + Al2O3}, as shown in Figure 2. Unfortunately, these results gave no information about the activities of either Y or Y2O3 in the {alloy + NixY + YAlO3 + vapor} equilibrium. The activities a(Al), a(Al2O) and a(Ni) in these {alloy + oxide + vapor} equilibria were determined by the vapor pressure technique with a multiple Knudsen effusion cell vapor source (three effusion cells) configured for use with a Nuclide/MAAS/PATCO 12–90-HT single focus 90 permanent sector mass spectrometer, multi-cell KEMS [2,3,11]. At each temperature the activities were determined by comparing the measured partial pressure ratio of the vapor species in equilibrium with the alloy and a pure-Au reference, p(i)/p(Au), to the tabulated partial pressure ratio [p(Au)/p(i)] for pureAu and the reference states, according to: aðiÞ ¼ pðiÞ=p ðAuÞ ½p ðAuÞ=p ðiÞ ¼ I i =I Au S Au =S i ½p ðAuÞ= p ðiÞ; where i = Al(g), Al2O(g) and Ni(g). In this case, Ii is the measured ion intensity and SAu/Si is measured calibration factor that relate p(Au), in equilibrium with pure-Au(s,l), to p(Al) and p(Al2O) in equilibrium with {Al(l) + Al2O3(s)} and also p(Ni) in equilibrium
E. Copland / Scripta Materialia 57 (2007) 21–24
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Table 1. EPMA measured phase compositions (in at.%) across the c 0 -Ni3Al + b-NiAl/Y2O3 interface Phase
Ni
Al
O
Y
c 0 -Ni3Al (bulk) b-NiAl (bulk) c 0 -Ni3Al (interface) NixY YAlO3 (YAP) Y4Al2O9 (YAM) Y2O3
73.0 ± 0.3 62.1 ± 0.3 74.5 ± 0.1 75–80 1.5 ± 0.4 2.5 ± 0.7 1.3 ± 0.2
26.6 ± 0.3 37.5 ± 0.2 25.2 ± 0.1 1–3 20.3 ± 0.4 14.6 ± 0.8 –
0.3 ± 0.1 0.4 ± 0.1 0.27 ± 0.01 – 58.0 ± 0.4 57.4 ± 0.9 57.3 ± 0.4
0.01 ± 0.02 0.01 ± 0.02 0.01 ± 0.01 20–25 20.2 ± 0.3 25.5 ± 0.7 41.4 ± 0.3
Figure 2. Relative-a(O) and relative-a(Al2O3) versus T for the {alloy + NixY + YAlO3} equilibrium relative to the {alloy + Al2O3} equilibrium. These measurements were determined directly from the measured ion intensities I Al2 O and IAl according to Eqs. (1) and (2).
{Ni(s,l) + Al2O3(s)}. For the {Al(l) + Al2O3(s)} reference state, p(Al) and p(Al2O) are defined by the reactions Al(l) = Al(g) and 4/3Al(l) + 1/3Al2O3(s) = Al2O(g), respectively. For the {Ni(s,l) + Al2O3(s)} reference state, p(Ni) is defined by Ni(s,l) = Ni(g). A more detailed description of the technique and procedures are found in Refs. [3,12]. The activity of O (or p(O)) and a(Al2O3), relative to {Al(l) + Al2O3(s)}, can be determined indirectly from the measured values of a(Al) and a(Al2O) via the reactions 2Al(g) + O(g) = Al2O(g) and 4Al(g) + Al2O3(s) = 3Al2O(g), respectively. For this discussion, however, the ‘‘relative activities’’ of O and Al2O3 in {c 0 + b + NixY + YAlO3} (identified as A) relative to {c 0 + b + Al2O3} (identified as R) were determined directly by comparing the measured ion intensity ratios of I Al2 O and IAl according to Eqs. (1) and (2). The results of this determination are shown in Figure 2.
Figure 3. Combined a(Al) and a(Al2O) measured for alloys Ni–27.3Al and Ni–29.4Al in the Al2O3 and Y2O3 effusion cells, plotted as the logarithm of activity versus 1/T.
alloys in the Al2O3 and Y2O3 effusion cells are shown in Figure 4. These results also show that there is no measurable difference in a(Ni) resulting from the change in effusion cell material. According to these results, changing the container material (from Al2O3 to Y2O3) introduced Y and reduced the O activity by a factor of about 5 and a(Al2O3) by a factor of about 100, but produced no measurable change in either a(Al) and a(Ni) in either alloy. This suggests that dissolved O and Y do not affect the solution behavior of ‘‘Ni–Al’’
2
relative-aðOÞ ¼ ðI Al2 O ðAÞ=I Al2 O ðRÞÞ ðI Al ðRÞ=I Al ðAÞÞ
ð1Þ relative-aðAl2 O3 Þ ¼ ðI Al2 O ðAÞ=I Al2 O ðRÞÞ3 ðI Al ðRÞ=I Al ðAÞÞ4 ð2Þ
The measured a(Al) and a(Al2O), in Ni–27.3Al and Ni–29.4Al in equilibrium with the Al2O3 and Y2O3 effusion cells, are shown together in Figure 3. These results indicate, for both alloys, that there is no measurable difference in a(Al), while a(Al2O) is very sensitive to the effusion cell material. The measured a(Ni) for both
Figure 4. Combined a(Ni) measured for alloys Ni–27.3Al and Ni– 29.4Al in the Al2O3 and Y2O3 effusion cells, plotted as the logarithm of activity versus 1/T.
24
E. Copland / Scripta Materialia 57 (2007) 21–24
phases, which supports this common assumption. It is important to recognize that this is not surprising because the expected change in concentration of dissolved O and Y in the Ni–Al alloys phases considered in this study is negligibly small. The relationship between O activity and O concentration in c 0 -Ni3Al, b-NiAl and L phases was not determined; without this information there is some uncertainty in relating these measurements to the binary Ni–Al system. This issue will be further investigated by considering the effect of a graphite effusion cell and comparing the behavior of the {alloy + Al2O3 + vapor} and {alloy + C + vapor} equilibria. A graphite effusion cell should remove O from the system and replace it with C.2 In addition to component activities, these measurements provide accurate information about equilibrium phase transformations in the Ni–Al–O and Ni–Al–Y– O systems. The slope changes in log-a(i) versus 1/T plots shown in Figures 3 and 4 represent a change in phase stability or a phase transformation. In line with the binary Ni–Al phase diagram [13], the phase boundaries and transformation temperatures observed in this study are listed in Table 2. For the measurements made in Al2O3 effusion cells the c 0 -Ni3Al + b-NiAl/L + b-NiAl phase boundary observed with Ni–27.3Al appear to represent the {c 0 -Ni3Al + b-NiAl + L + Al2O3 + vapor} invariant equilibrium in the Ni–Al–O system and the eutectic reaction: LðþAl2 O3 Þ ¼ c0 -Ni3 Al þ b-NiAlðþAl2 O3 Þ. The reaction temperature determined in this study, T = 1640 ± 1 K, is close to the assessed value of T = 1642 K [13] and agrees with the most recent experimental determination of T = 1639 ± 2 K from Ref. [14]. In the study by Battezzati et al. [14], the reaction temperature was determined by high-temperature differential scanning calorimetry of a series of binary Ni–Al alloys in Al2O3 cells, which is the same {alloy + Al2O3 + vapor} equilibrium considered in this study. For Ni–29.4Al the eutectic reaction was bypassed by entering the {b-NiAl + Al2O3 + vapor} phase field at T = 1605 ± 5 K and subsequently the {L + b-NiAl + Al2O3 + vapor} phase field at T = 1642 ± 3 K. For the Y2O3 effusion cell there is an extra component, Y, but as the ‘‘Ni–Al’’ alloys are in equilibrium with both NixY and YAlO3 the degree of freedom in the Ni–Al–Y–O system is the same as the Ni–Al–O system. The extra component and stable phases make it more difficult to understand the nature of the phase transformation in the Ni–Al–Y–O system, but presumably the c 0 -Ni3Al + b-NiAl/L + b-NiAl boundary represents the {c 0 -Ni3Al + b-NiAl + L + NixY + YAlO3 + vapor} invariant equilibrium and the reaction: LðþNix Yþ YAlO3 Þ ¼ c0 -Ni3 Al þ b-NiAlðþNix Y þ YAlO3 Þ. There is no observable difference in the transformation temperatures of these alloys, whether the measurements were made in the Y2O3 or in the Al2O3 effusion cell. This suggests that dissolved O and Y have no influence on
2
Initially O-free Ni–Al alloys were thought to be impossible to study, but a graphite effusion cell could remove O from the system (suggestion from G. Rossenblatt). This will be considered in a future study.
Table 2. Measured phase transformation temperatures in Al2O3 and Y2O3 effusion-cells Phase boundary 0
c + b/b c 0 + b/L + b b/L + b L + b/L
Ni–27.3Al
Ni–29.4Al
– 1640 ± 1 K – 1672 ± 1 K
1605 ± 5 K – 1642 ± 3 K 1704 ± 2 K
the phase transformation behavior of Ni–Al alloys, and similar transformations identified for the {alloy + Al2O3 + vapor} equilibrium could be assumed to occur for bulk c 0 -Ni3Al + b-NiAl alloys in the Y2O3 effusion cells. In summary, changing the container material from Al2O3 (Ni–Al–O system) to Y2O3 (Ni–Al–Y–O system) introduced Y and equilibrium was obtained with the formation of NixY, YAlO3 and Y4Al2O9. This change reduced the O activity by a factor of 5 and a(Al2O3) by a factor of about 100, but there was no measurable change in either a(Al), a(Ni) or a change in the observed phase transformations of the Ni–Al alloys. These results support the assumption of ignoring the container material, but also highlight the attention needed to accurately describe the state of a system typically encountered in thermodynamic property measurements. In general, the nature of an experiment dictates the system that is studied; for example, the Ni–Al binary is not studied directly but, rather, measurements are made in higher-order systems (e.g., Ni–Al–O or Ni– Al–Y–O). The thermodynamic data need to be considered in terms of these additional components and phases. [1] M. Knudsen, Ann. der Phys. 28 (1909) 75; M. Knudsen, Ann. der Phys. 28 (1909) 999. [2] M. Inghram, J. Drowart, in: High Temperature Technology, McGraw Hill, New York, 1960, pp. 219–240. [3] C. Chatillon, M. Allibert, A. Pattoret, in: J. Hastie (Ed.), Proc. of the 10th Materials Res. Symp. on Characterization of High Temperature Vapors and Gases, National Bureau of Standards, Gaithersburg, MD (1978) 181– 210. [4] E. Copland, in: E. Opila, J. Fergus, T. Maruyama, J. Mizusaki, T. Narita, D. Shifler, E. Wuchina (Eds.), High Temperature Corrosion and Materials Chemistry V, The Electrochemical Society, Pennington, NJ, 2005, pp. 361– 369. [5] F. Elrefaie, W. Smeltzer, J. Electrochem. Soc. 128 (1981) 2237. [6] H. Wriedt, Bull. Alloy Phase Diag. 6 (1985) 548–553, pp. 567–568. [7] M.L. McGlashan, J. Chem. Thermodyn. 22 (1990) 653 (ITS-90). [8] J. Taylor, A. Dinsdale, Z. Metallkd. 81 (1990) 354. [9] R. Bedford, G. Bonnier, H. Maas, F. Pavese, Metrologia 33 (1996) 133. [10] J. Gro¨bner, H. Lukas, F. Aldinger, Z. Metallkd. 87 (4) (1996) 268. [11] O. Kubaschewski, C.B. Alcock, Metallurgical Thermochemistry, fifth ed., Pergamon Ress, Oxford, 1979. [12] E. Copland, J. Phase Equilib. Diffusion, in press. [13] W. Huang, Y. Chang, Intermetallics 6 (1998) 487. [14] L. Battezzati, M. Baricco, L. Pascale, Scripta Mater. 39 (1998) 87.