Available online at www.sciencedirect.com
Journal of the European Ceramic Society 32 (2012) 3171–3185
Phase relations in the ZrO2–Nd2O3–Y2O3–Al2O3 system: Experimental study and thermodynamic modeling O. Fabrichnaya a,∗ , G. Savinykh a , G. Schreiber a , H.J. Seifert a,b a
b
Institute for Material Science, Technical University of Freiberg, Gustav-Zeuner-Str. 5, 09599 Freiberg, Germany Institute of Applied Materials IAM-AWP, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344, Eggenstein-Leopoldshafen, Germany Received 18 February 2012; received in revised form 2 April 2012; accepted 15 April 2012 Available online 9 May 2012
Abstract The Nd2 O3 –Al2 O3 and ZrO2 –Nd2 O3 –Al2 O3 systems were simultaneously re-assessed to reproduce experimental data from present study and literature. Phase relations in the Nd2 O3 –Y2 O3 –Al2 O3 system were investigated by XRD, electron microscopy and DTA for the first time. Invariant reaction in solid phases B + YAM = NdAlO3 + C was determined to occur at 1863 K. The eutectic reaction liquid = YAG + NdAlO3 + Al2 O3 was determined to occur at 2042 K and composition Al2 O3 –10 mol.% Nd2 O3 –13 mol.% Y2 O3 . The obtained data were used to assess thermodynamic parameters of this system. Isothermal sections were calculated in the range 1523–1923 K. Liquidus surface of the Nd2 O3 –Y2 O3 –Al2 O3 system was calculated. The derived databases were combined with already available descriptions of the ZrO2 –Nd2 O3 –Y2 O3 and ZrO2 –Y2 O3 –Al2 O3 systems into four-oxide database. © 2012 Elsevier Ltd. All rights reserved. Keywords: Phase diagrams; Electron microscopy; X-ray methods; Thermal analysis; Thermodynamic modeling
1. Introduction The phase relation in the ZrO2 –Nd2 O3 –Y2 O3 –Al2 O3 system is of interest because of possible new candidate materials for thermal barrier coatings (TBC) and fixation of radioactive wastes. Nowadays metastable tetragonal (t ) YSZ is used as TBC. Experimental studies indicate that change of Y to other rare earths decreases thermal conductivity of materials.1 The Nd2 O3 as a possible stabiliser of tetragonal ZrO2 /YSZ was studied in several works.2–4 However, complete substitution of Y by Nd decreases the stability of material due to monoclinic phase formation during thermal cycling.2 Therefore, it is important to find the exact concentration of Nd2 O3 which improves thermal conductivity and resistance to sintering without a marked loss of durability of the coating. The compound Nd2 Zr2 O7 with pyrochlore structure stable up to ∼2300 ◦ C is also a promising candidate for TBC due to its low thermal conductivity.5 The Nd2 O3 stabilized ZrO2 with fluorite structure presents interest as possible candidate material for
∗
Corresponding author. Tel.: +49 3731 393156; fax: +49 3731 393657. E-mail address:
[email protected] (O. Fabrichnaya).
0955-2219/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2012.04.034
nuclear wastes fixation. The amount of Nd2 O3 in the fission products is relatively high.6 Therefore, solubility of Nd2 O3 in fluorite phase and phase relations in the ZrO2 –Nd2 O3 –Y2 O3 system play important role for nuclear waste fixation. According to Xu et al.4,7 tetragonal YSZ co-doped with Nd2 O3 is of interest as advanced structural ceramics because of excellent mechanical properties and high phase stability. Neodymiumdoped yttrium aluminate with garnet structure (Nd:YAG) is most widely used solid-state laser. To produce material with laser properties reactive sintering process accompanied with liquid formation is used.8 Therefore, phase relations in the Nd2 O3 –Y2 O3 –Al2 O3 system are important for Nd:YAG processing. The ZrO2 –Nd2 O3 –Y2 O3 system was recently studied experimentally by Fabrichnaya et al.9,10 and the thermodynamic parameters were assessed based on obtained results. The thermodynamic description of the ZrO2 –Nd2 O3 –Al2 O3 system was derived by Fabrichnaya and Seifert11 based on experimental data of Lakiza and Lopato.12 However, since the thermodynamic description of the ZrO2 –Nd2 O3 system was changed by Fabrichnaya et al.10 thermodynamic parameters of the ZrO2 –Nd2 O3 –Al2 O3 system were preliminary re-assessed together with parameters of the Nd2 O3 –Al2 O3 system to
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reproduce tie-lines in ternary system. The obtained set of parameters was not published yet. The recent ab initio calculations in the Nd2 O3 –Al2 O3 system indicated that the monoclinic phase of Nd4 Al2 O9 should be stable.13 According to experimental investigations,14,15 this phase can be obtained by co-precipitation and calcination at 1073 K. However, according to Martin-Sedano et al.15 the Nd4 Al2 O9 phase completely decomposed after heat treatment at 1373 K. The experimental phase equilibrium study of the Nd2 O3 –Al2 O3 system was performed in Refs. 16–18. According to data of Couture,18 the Nd4 Al2 O9 phase became stable in narrow temperature range just below liquidus. The liquidus and sub-solidus phase relations in the Nd2 O3 –Al2 O3 system were also studied experimentally by Toropov and Kiseleva16 and Mizuno et al.17 In both studies the Nd4 Al2 O9 phase was not found. Another compound NdAl11 O18 with -alumina structure was found in samples after melting.16–19 However stability of this phase was not checked by prolonged heat treatment at temperatures below 1873 K. Additionally it should be mentioned that there is quite large differences between calculated and measured temperatures of eutectic reactions. Experimental investigations of phase relations in the Nd2 O3 –Y2 O3 –Al2 O3 system were very scarce.20,21 However phase diagram of this system was calculated by Saal et al.8 by combining thermodynamic descriptions of binary systems and taking into account solubility of Nd in garnet structure obtained by reactive sintering of Nd–YAG.22 It should be mentioned that authors8 used thermodynamic parameters of the Nd2 O3 –Al2 O3 system assessed by Saal et al.13 assuming stability of Nd4 Al2 O9 phase based on ab initio calculations. The aim of present work is to derive thermodynamic database for the ZrO2 –Nd2 O3 –Y2 O3 –Al2 O3 system. Since there is uncertainty in the phase relations in the Nd2 O3 –Al2 O3 system experimental study of this system will be performed to verify stability of the Nd4 Al2 O9 phase and to determine temperatures and compositions of eutectic reactions. The phase relations in the Nd2 O3 –Y2 O3 –Al2 O3 system will be experimentally investigated using preliminary calculations based on binary extrapolations into ternary system. The thermodynamic parameters of the ZrO2 –Nd2 O3 –Al2 O3 system will be re-assessed together with parameters of the Nd2 O3 –Al2 O3 system. 2. Experimental 2.1. Sample preparation Samples were synthesized from precursor solution in a similar way to that described by Fabrichnaya et al.10 The starting chemical were zirconium acetate solution in acetic acid, Zr(CH3 COO)4 (99.99%, Sigma–Aldrich), Nd(NO3 )3 ·6H2 O (99.99%, Alfa Aesar), Y(NO3 )3 ·6H2 O (99.9%, Alfa Aesar) and Al(NO3 )3 ·6H2 O (99.9%, Alfa Aesar). In the first step, the Nd(NO3 )3 ·6H2 O and Y(NO3 )3 ·6H2 O were dissolved in distilled water and the initial zirconium acetate solution was diluted. For synthesis of the Nd2 O3 –Y2 O3 –Al2 O3 samples aqueous solutions of the Nd(NO3 )3 ·6H2 O, Y(NO3 )3 ·6H2 O and Al(NO3 )3 ·6H2 O were prepared. Concentration of the prepared
solutions was determined by inductively coupled plasma (ICP) spectrometry. Initial solutions were mixed according to the selected ratios. The obtained precursor solution was dropped from the buret at a low speed (around 1 ml min−1 ) into a big beaker containing about 500 ml of deionized water with the pH value maintained above 9.0 by adding ammonium hydrate. The precipitation occurred during dropping and stirring. The obtained suspension was heated up and held at 333 K for 1–2 h. The precipitate was filtered and dried at 353 K. During pyrolysis at 1073 K proceeding for 3 h in air, hydroxides transform to oxides releasing water. Filtrates and samples dissolved in diluted solution of H2 SO4 were analyzed by ICP, with an accuracy of ±2%. 2.2. Sample treatment and characterization The pyrolyzed powder was pressed into cylindrical pellets and sintered in air at temperatures of 1523, 1673 and 1873 K in Pt-crucibles to obtain the equilibrium assemblage. The duration of heat treatments was 336 h at 1523 K, 192 h at 1673 K and 96 h at 1873 K. The samples were then analyzed by XRD and SEM/EDX. The XRD patterns of powdered specimen were recorded using the Präzisionsmechanic diffractometer (CuK␣ radiation; Freiberg, Germany). Lattice parameters for phases, their volume fractions and grain size were calculated by Rietveld analysis using BGMN program23 and MAUD.24 The microstructures of sintered samples were examined by SEM (Leo1530 GEMINI) and energy dispersive X-ray spectroscopy (EDX Bruker AXS Mikroanalysis GmbH) was employed to measure the phase compositions with accuracy ±3 mol% REO1.5 . For most of samples SEM images were made in the regime of back scattered electrons, while for Al2 O3 rich samples both secondary electron and back scattered images were considered to distinguish between Al2 O3 phase and pores. The samples where XRD indicated change of phase assemblage at 1523–1873 K were additionally investigated by DTA to determine temperature of transformation. Selected samples heat treated at 1523 K for 336 h were studied using SETARAM instrument SetSys evolution 1750 (TG-DTA) in Pt crucible in Ar (or He) atmosphere in the temperature range 288–1973 K with heating rate of 10 K min−1 and cooling rate of 30 K min−1 . Samples were also investigated using another SETATAM instrument SETSYS EVOLUTION 2400 (TG-DTA) in W crucibles in He atmosphere at temperatures up to 2173 K. The heating rate in both instruments was of 20 K min−1 up to 1473 K and then of 10 K min−1 ; cooling rate was of 30 K min−1 in both instruments. Temperature calibration of SETSYS EVOLUTION 1750 was made using melting points of Al, Ag, Au, Cu and Ni. Temperature calibration of SETSYS EVOLUTION 2400 was made using melting points of Al, Au and Al2 O3 as well as transformation temperature of LaYO3 from perovskite into monoclinic structure.25 It was found that temperatures according literature data and measured value can be expressed as linear function Tcorr (K) = 23 + 0.967 Tmeas (K). This equation was used for temperature correction.
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3. Thermodynamic modeling
Table 1 Phases in the ZrO2 –Nd2 O3 –Y2 O3 –Al2 O3 system and their models.
Solid solutions were modeled by compound energy formalism.26 The ZrO2 based solid solutions with fluorite structure, tetragonal and monoclinic phases were described by two-sublattice model. One sublattice is filled by cations Zr4+ , Y3+ , Nd3+ , Al3+ and another is filled by O2− or vacant to keep electro-neutrality due to substitution of Zr4+ by three-valent cations. Therefore the formula (Zr4+ ,Nd3+ ,Y3+ ,Al3+ )(O2− ,Va)2 was used to describe the ZrO2 based solid solutions. The Rare Earth based solids were modeled with three sublattices: one filled by cations, second one completely filled by oxygen and third one is mostly empty partially occupied by oxygen to balance Rare Earth substitution by Zr4+ . The pyrochlore phase with narrow homogeneity range in the ZrO2 –Nd2 O3 system was modeled by five sublattices.10 In the ternary system ZrO2 –Nd2 O3 –Y2 O3 , solubility of the Y2 O3 in the pyrochlore phase was modelled by five sublattices (Nd3+ ,Y3+ ,Zr4+ )2 (Zr4+ ,Y3+ ,Nd3+ )2 (O2− ,Va)6 (O2− )(Va,O2− ) assuming that Y3+ can substitute both Zr4+ and Nd3+ . The thermodynamic parameters of pyrochlore, fluorite and other phases in ternary system were assessed in the work of Fabrichnaya et al.10 and this description was accepted in the present work. Aluminate solid solutions in the system Nd2 O3 –Y2 O3 –Al2 O3 having garnet structure YAG (Y3 Al5 O12 ), orthorhombic perovskite structure YAP (YAlO3 ), rhombohedral perovskite structure NdAP (NdAlO3 ) and monoclinic structure YAM (Y4 Al2 O9 ) were modeled by taking into account mutual substitution of Y3+ and Nd3+ in Nd and Y aluminates without substitution in Al3+ positions. Therefore YAM, YAP, YAG and NdAP were considered as limited ideal solutions. The ␦-Zr3 Y4 O12 , NdAl11 O18 (-alumina structure) and Al2 O3 corundum were described as stoichiometric phases. Thermodynamic descriptions of oxides were accepted from following works: for the ZrO2 polymorphs and liquid,27 for the Nd2 O3 and Y2 O3 polymorphs and liquid28 and for the Al2 O3 corund and liquid.29 The thermodynamic parameters for binary systems ZrO2 –Nd2 O3 , Nd2 O3 –Y2 O3 , ZrO2 –Y2 O3 , Y2 O3 –Al2 O3 and ZrO2 –Al2 O3 as well as for two ternary systems ZrO2 –Nd2 O3 –Y2 O3 and ZrO2 –Y2 O3 –Al2 O3 were accepted from previous works of Fabrichnaya et al.10,30 Preliminary corrections of the published description the ZrO2 –Nd2 O3 –Al2 O3 system11 was mentioned in Ref.10 In the present paper the thermodynamic description of this system together with the description Nd2 O3 –Al2 O3 binary system will be re-considered based on experimental investigations. Phases and their thermodynamic models are listed in Table 1. Assessment of thermodynamic parameters and phase diagram calculations were performed using THERMO-CALC software package.31
Phase
Model
Fluorite (F) Tetragonal (T) Monoclinic (M) A H X C B Pyrochlore (Pyr)
(Zr4+ ,Nd3+ ,Y3+ ,Al3+ )1 (O2− ,Va)2
NdAlO3 (NdAP) YAlO3 (YAP) Y4 Al2 O9 (YAM) Y3 Al5 O12 (YAG) ␦-Zr3 Y4 O12 (␦) Corundum Al2 O3 NdAl11 O18 () Liquid (L)
3173
(Nd3+ , Y3+ ,Zr4+ )2 (Va,O2− )1 (O2− )3
(Nd3+ ,Y3+ ,Zr4+ )2 (Zr4+ ,Y3+ ,Nd3+ )2 (O2− ,Va)6 (O2− )1 (Va,O2− )1 (Nd3+ ,Y3+ )1 (Al3+ )1 (O2− )3 (Y3+ ,Nd3+ )1 (Al3+ )1 (O2− )3 (Y3+ ,Nd3+ )4 (Al3+ )2 (O2− )9 (Y3+ ,Nd3+ )5 (Al3+ )3 (O2− )12 (Y3+ )4 (Zr4+ )3 (O2− )12 (Al3+ )2 (O2− )3 (Nd3+ )1 (Al3+ )11 (O2− )18 (Nd3+ ,Y3+ ,Zr4+ )P (O2− ,AlO1.5 )Q
4. Results and discussions The XRD investigation of sample ANB1 in the Nd2 O3 –Al2 O3 system with composition corresponding to Nd4 Al2 O9 phase did not indicate formation of this phase at 1523–1873 K; only NdAP and Nd2 O3 (A) were present.
Fig. 1. Calculated isothermal sections of the Nd2 O3 –Y2 O3 –Al2 O3 system based on binary extrapolations. a – 1673 K, b – 1873 K.
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Fig. 2. Results of XRD and SEM/EDX investigation sample #13 (Al2 O3 –36 mol.%Y2 O3 –36 mol.%Nd2 O3 ) heat treated at 1673 and 1873 K. a – XRD of sample#13 heat treated at 1673 K; b – at 1873 K; c – SEM/EDX of sample #13 heat treated at 1673 K; dark phase is C, light phase is NdAP, black – pores; d – SEM/EDX of sample#13 heat treated at 1873 K: light phase is B, dark phase is YAM, black – pores.
Therefore our results do not confirm ab initio calculations of Saal et al.13 about stability of Nd4 Al2 O9 phase. As it was mentioned in the work,10 introduction of new description of the ZrO2 –Nd2 O3 system into thermodynamic database of ZrO2 –Nd2 O3 –Al2 O3 system11 required correction of thermodynamic parameters of the Nd2 O3 –Al2 O3 system and preliminary corrections were done to reproduce both phase diagram of the Nd2 O3 –Al2 O3 system and tie-lines in the ZrO2 –Nd2 O3 –Al2 O3 system. Therefore the corrected Nd2 O3 –Al2 O3 thermodynamic description was combined with descriptions of the Nd2 O3 –Y2 O3 system from the work10 and Y2 O3 –Al2 O3 system from the work of Fabrichnaya et al.30 The calculated phase diagram sections at temperatures 1523, 1673 and 1873 K were used to select sample compositions for experimental studies. The results of XRD investigation and calculations are presented in Table 2. The calculated phase diagrams at 1673 and 1873 K based on binary extrapolations are presented in Fig. 1a and b along with experimental data. It should be mentioned that phase diagrams calculated in present work based on binary extrapolations are very similar to that calculated by Saal et al.8 The difference between our results and is Saal et al.8 is that that they considered the Nd4 Al2 O9 as stable phase and took into account solubility of Nd in the garnet structure. However, several inconsistencies were found between calculations and experimental results obtained in present work. Solubility of Nd was found in YAM, YAP and YAG phases and solubility of Y was found in NdAP phase. This was not taken into
account in our preliminary calculations. Phase with -alumina structure was not indicated in the Al2 O3 -rich compositions at 1523–1873 K. Experiment also indicated change of tie-lines in the temperature range of 1673–1873 K. This can be clearly seen for composition #13: at 1673 K the C + NdAP phase assemblage was stable while at 1873 K B + YAM assemblage became stable (Table 2). The XRD patterns of sample #13 heat treated at 1673 and 1873 K are shown in Fig. 2 a and b. The results of SEM/EDX investigation of these samples are in agreement with
Fig. 3. DTA heating curve of sample #13 heat treated at 1523 K.
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Table 2 Phase assemblages in the Nd2 O3 –Y2 O3 –Al2 O3 system identified by XRD after heat treatments at 1523–1873 K and calculated using thermodynamic datasets. No.
Composition, mole fraction Nd2 O3
Y2 O3
Al2 O3
1
0.85
0.05
0.1
2
0.75
0.15
0.1
3
0.4
0.5
0.1
4
0.3
0.6
0.1
5
0.2
0.7
0.1
6
0.1
0.8
0.1
7
0.2
0.6
0.2
8
0.1
0.5
0.4
9
0.25
0.25
0.5
10
0.05
0.4
0.55
11
0.1
0.25
0.65
12
0.3
0.1
0.6
13
0.36
0.36
0.28
14
0.27
0.42
0.31
15
0.18
0.38
0.44
16
0.16
0.29
0.55
17
0.13
0.74
0.13
18
0.6
0.3
0.1
19
0.5
0.4
0.1
20
0.1
0.1
0.8
ANB1
0.6667
0
0.3333
T (K)
XRD
Dataset-0
Dataset-1
1523 1673 1873 1523 1673 1873 1523 1673 1873 1523 1673 1873 1523 1673 1873 1523 1673 1873 1523 1673 1873 1523 1673 1873 1523 1673 1873 1523 1673 1873 1523 1673 1873 1523 1673 1873 1523 1673 1873 1523 1673 1873 1523 1673 1873 1523 1673 1873 1523 1673 1873 1523 1673 1873 1523 1673 1873 1523 1673 1873 1523 1673
NdAP + B + A NdAP + B + A NdAP + B + A NdAP + B NdAP + B NdAP + B NdAP + C NdAP + B + C B + C + YAM NdAP + C + YAM NdAP + C + YAM C + YAM NdAP + C + YAM C + YAM C + YAM C + YAM C + YAM C + YAM NdAP + C + YAM C + YAM C + YAM YAP + YAM YAP + YAM YAP + YAM NdAP + YAP + YAM NdAP + YAP + YAM NdAP + YAP + YAM YAG + YAP YAG + YAP YAG + YAP NdAP + YAG + Al2 O3 NdAP + YAG + Al2 O3 NdAP + YAG + Al2 O3 NdAP + YAG + Al2 O3 NdAP + YAG + Al2 O3 NdAP + YAG + Al2 O3 NdAP + C NdAP + C B + YAM NdAP + C + YAM NdAP + C + YAM C + YAM NdAP + YAM NdAP + YAM NdAP + YAM NdAP + YAG + YAP NdAP + YAG + YAP NdAP + YAG + YAP YAM + C YAM + C YAM + C NdAP + B NdAP + B NdAP + B + YAM NdAP + B + C NdAP + B + C B + YAM NdAP + YAG + Al2 O3 NdAP + YAG + Al2 O3 NdAP + YAG + Al2 O3 NdAP + A NdAP + A
NdAP + B + A NdAP + A NdAP + A NdAP + B NdAP + B NdAP + B + A NdAP + B + C NdAP + B + C NdAP + B + C NdAP + C NdAP + C NdAP + C NdAP + C NdAP + C + YAM NdAP + C + YAM NdAP + C + YAM C + YAM C + YAM NdAP + C + YAM NdAP + C + YAM NdAP + C + YAM NdAP + YAP + YAM NdAP + YAP + YAM NdAP + YAP + YAM NdAP + YAP NdAP + YAP NdAP + YAP NdAP + YAG + YAP NdAP + YAG + YAP NdAP + YAG + YAP NdAP + YAG +  NdAP + YAG +  NdAP + YAG +  NdAP + YAG +  NdAP + YAG +  NdAP + YAG +  NdAP + C NdAP + C NdAP + C + YAM NdAP + C + YAM NdAP + C + YAM NdAP + C + YAM NdAP + YAM + YAP NdAP + YAM + YAP NdAP + YAM + YAP NdAP + YAG + YAP NdAP + YAG + YAP NdAP + YAG + YAP NdAP + C + YAM NdAP + C + YAM NdAP + C + YAM NdAP + B NdAP + B NdAP + B NdAP + B NdAP + B NdAP + B NdAP + YAG +  NdAP + YAG +  NdAP + YAG +  NdAP + A NdAP + A
NdAP + B + A NdAP + A NdAP + A NdAP + B NdAP + B NdAP + B + A NdAP + B + C NdAP + B + C NdAP + B + YAM NdAP + C NdAC + C C + YAM NdAP + C C + YAM C + YAM C + YAM C + YAM C + YAM NdAP + C + YAM C + YAM C + YAM YAP + YAM YAP + YAM YAP + YAM NdAP + YAP + YAM NdAP + YAP + YAM NdAP + YAP + YAM YAG + YAP YAG + YAP YAG + YAP NdAP + YAG + Al2 O3 NdAP + YAG + Al2 O3 NdAP + YAG + Al2 O3 NdAP + YAG + Al2 O3 NdAP + YAG + Al2 O3 NdAP + YAG + Al2 O3 NdAP + C NdAP + C NdAP + B + YAM NdAP + C + YAM NdAP + C + YAM NdAP + B + YAM NdAP + YAM + YAP NdAP + YAM + YAP NdAP + YAM + YAP NdAP + YAG + YAP NdAP + YAG + YAP NdAP + YAG + YAP YAM + C YAM + C YAM + C NdAP + B NdAP + B NdAP + B NdAP + B NdAP + B NdAP + B NdAP + YAG + Al2 O3 NdAP + YAG + Al2 O3 NdAP + YAG + Al2 O3 NdAP + A NdAP + A
3176
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Table 2 (Continued) No.
ANB2
Composition, mole fraction Nd2 O3
Y2 O3
Al2 O3
0.3
0.0
0.7
T (K)
XRD
Dataset-0
Dataset-1
1873 1523 1673 1873 1923
NdAP + A NdAP + Al2 O3 NdAP + Al2 O3 NdAP + Al2 O3 + (tr.) NdAP + 
NdAP + A NdAP +  NdAP +  NdAP +  NdAP + 
NdAP + A NdAP + Al2 O3 NdAP + Al2 O3 NdAP + Al2 O3 NdAP + 
The thermodynamic descriptions: dataset 0 is based on extrapolations from binary systems, dataset 1 is obtained by optimization in present work.
XRD data. Microstructures of sample #13 are presented in Fig. 2 c and d. The invariant reaction was confirmed by DTA investigation of sample #13. The heat effect was registered at 1863 K. The DTA heating curve is shown in Fig. 3. No transformation was registered during cooling because of slow kinetics of solid
state reaction. XRD study of sample #13 after DTA indicated presence of B and YAM phases. The melting relations were experimentally studied by DTA. The samples #20 and 11 were experimentally studied up to 2373 K. The obtained curves on cooling and heating are
Fig. 4. DTA heating and cooling curves of samples #20 (Al2 O3 –10 mol.%Y2 O3 –10 mol.%Nd2 O3 ) and #11 (Al2 O3 –25 mol.%Y2 O3 –10 mol.%Nd2 O3 ) Sample #20: a – heating; b – cooling; Sample #11: c – heating; d – cooling.
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Table 3 Phase assemblages in the ZrO2 –Nd2 O3 –Al2 O3 system identified by XRD after heat treatments at 1523–1873 K and calculated using thermodynamic dataset. No.
Composition, mole fraction ZrO2
Nd2 O3
Al2 O3
1
0.6014
0.0490
0.3496
2
0.6014
0.1608
0.2378
3
0.6014
0.2861
0.1119
4
0.4
0.056
0.544
5
0.4
0.2
0.4
6
0.1818
0.7272
0.091
7
0.1818
0.0455
0.7727
presented in Fig. 4 a–d. The temperature of eutectic reaction was calculated to be 2042 K as mean value of on-set points indicated on cooling and heating for both samples #11 and 20. The SEM/EDX results for both samples are presented in Fig. 5a and b. The composition of eutectics was measured the same for both samples 77 mol.% Al2 O3 , 10 mol.% Nd2 O3 and 13 mol.% Y2 O3 . It can be seen from microstructure of sample #20 that Al2 O3 phase was the primary phase, while crystallization path of sample #11 was different: YAG phase crystallized first. Since the phase with -alumina structure was not found in the ternary system Nd2 O3 –Y2 O3 –Al2 O3 one additional sample ANB2 (30 mol.% Nd2 O3 and 70 mol.% Al2 O3 ) was prepared. No  phase was indicated after heat treatment at 1523 and 1673 K and only traces of this phase were found together with major phases NdAP and Al2 O3 in the sample #ANB2 heat treated at 1873 K. The XRD investigation of this sample heat treated at 1923 K indicated that reaction of  phase formation was completed: no Al2 O3 was found by XRD. The XRD patterns of samples #ANB2 heat treated at 1673, 1873 and 1923 K are presented in Fig. 6a–c. The DTA heating curve of sample ANB2 up to 2003 K is shown in Fig. 6d. The heat effect was indicated at temperature of 1963 K, which is substantially higher than the beginning of -phase formation (1873 K) observed in heat treated sample. This fact can be explained by overheating effect due to slow kinetics. Both samples ANB1 and ANB2 were investigated by high temperature DTA (Setsys Evolution 2400) to check eutectic temperatures. The DTA heating and cooling curves for sample ANB2 are shown in Fig. 7 a and b. The result of SEM/EDX investigation of ANB2 sample after DTA is shown in Fig. 7c. It can be seen from SEM image that primary crystallization phase is NdAP. The eutectic temperature was determined as mean value
T (K)
XRD
Up-dated dataset
1523 1673 1873 1523 1673 1873 1523 1673 1873 1523 1673 1873 1523 1673 1873 1523 1673 1873 1523 1673 1873
F + M + Al2 O3 + NdAP(tr.) F + M + Al2 O3 F + M + Al2 O3 M + NdAP + Al2 O3 (tr) + F(tr) F + NdAP + Al2 O3 F + NdAP + Al2 O3 F + NdAP + Pyr F + NdAP + Pyr F + NdAP + Pyr F + Al2 O3 + T + M + NdAP(tr) F + M + Al2 O3 F + Al2 O3 M + NdAP + Al2 O3 + T + F(tr) F + NdAP + Al2 O3 F + NdAP + Al2 O3 Pyr + NdAP + A Pyr + NdAP + A F + NdAP + A F + NdAP + Al2 O3 F + NdAP + Al2 O3 F + NdAP + Al2 O3 + 
F + T + Al2 O3 F + T + Al2 O3 F + T + Al2 O3 F + NdAP + Al2 O3 F + NdAP + Al2 O3 F + NdAP + Al2 O3 F + NdAP + Pyr F + NdAP + Pyr F + NdAP + Pyr F + NdAP + Al2 O3 F + Al2 O3 F + Al2 O3 F + NdAP + Al2 O3 F + NdAP + Al2 O3 F + NdAP + Al2 O3 Pyr + NdAP + A Pyr + NdAP + A F + NdAP + A F + NdAP + Al2 O3 F + NdAP + Al2 O3 F + NdAP + Al2 O3
Fig. 5. SEM/EDX investigation of sample #20 and #11 after DTA. a – sample #20; b – sample #11; grey phase – YAG, black phase – Al2 O3 , white phase – NdAP.
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Fig. 6. XRD patterns of sample ANB2 in the system Nd2 O3 –Al2 O3 heat treated at 1673–1923 K and DTA results on heating up to 2003 K. a – 1673 K; b – 1873 K; c – 1923 K; DTA curve on heating up to 2003 K.
Table 4 Invariant reactions in the Nd2 O3 –Al2 O3 system. Reaction
Type
Temperature (K)
Composition, xL (Al2 O3 )
L+X=H
Peritectic p1
2473 2473 2373 2373 2360 2438 2363 2343 2143 2164 2123 2073 2113 2067 2068 2173 2062 2042 2023 1993 2023 1898 1898
0.082 0.07 0.1385 0.12 0.5 0.5 0.5 0.5 0.2647 0.25 0.2 0.25 0.275 0.8017 0.83 0.90 0.7964 0.83 0.8 0.77 0.8 – –
L+H=A L = NdAP
L = A + NdAP
L + Al2 O3 =  L = NdAP + 
 = NdAP + Al2 O3
Peritectic p2 Congruent
Eutectic e1
Peritectic p3
Eutectic e2
Eutectoid e3
Reference This work, calc. 18
This work, calc. 18
This work, calc. 17 18 16
This work, calc. This work, exp. 17 16 18
This work, calc. 17 18
This work, calc. This work, exp. 18 17 16
This work, calc. This work, exp.
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Fig. 8. Results of DTA investigation of sample ANB1 heated up to 2173 K followed by SEM/EDX investigation. a – heating; b – SEM/EDX results; dark phase is primary NdAP, white phase is A-Nd2 O3 .
Fig. 7. Results of DTA investigation of sample ANB2 heated up to 2173 K followed by SEM/EDX investigation. a – heating; b – cooling; c – SEM/EDX results; dark grey phase is -NdAl11 O18 , light phase is primary NdAP.
on heating and cooling taking into account temperature correction to be 2042 K. The eutectic composition was determined by EDX as 83 mol.% Al2 O3 and 17 mol.% Nd2 O3 . The DTA heating curve for sample ANB1 is shown in Fig. 8a. The eutectic temperature was determined as 2164 K after temperature correction. The on-set point on cooling was 10 K higher than on heating. This can be explained by second heat effect due to crystallization of primary phase which could not be separated from effect of eutectic reaction. Therefore the temperature of transformation on cooling was not taken into account in determination
of eutectic temperature. The results of SEM/EDX of sample ANB1 is shown in Fig. 8b. The eutectic composition was determined by EDX as 25 mol.% Al2 O3 and 75 mol% Nd2 O3 . The experimental results obtained in present work are in reasonable agreement with data of Couture18 for both eutectic reactions, with data of Mizuno et al.17 for L = A + NdAP and with data of Toropov and Kiseleva16 for L = NdAP + . Though the temperatures of eutectic reactions obtained in the present work are higher than found in literature,16–18 they are in a good agreement with calculations.11 It should be mentioned that isothermal section of the ZrO2 –Nd2 O3 –Al2 O3 system at 1523 K presented by Lakiza and Lopato12 indicated the presence of  phase for the Al2 O3 rich compositions. This is in contradiction with results obtained in present work for the Nd2 O3 –Al2 O3 system. Therefore several compositions in the ZrO2 –Nd2 O3 –Al2 O3 system were selected for experimental investigation. The results of XRD investigation at 1523, 1673 and 1873 K are presented in Table 3. The Al2 O3 rich samples did not indicate formation of -alumina at 1523–1873 K according to XRD study. Only traces of  phase were found in the sample #7 at 1873 K. Therefore the results for the Al2 O3 rich compositions at 1523 K obtained in the present study indicate three-phase field Al2 O3 + F + NdAP (sample #7) are in contrast to data of Lakiza and Lopato12 who found two
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Fig. 9. Calculated phase diagram of the Nd2 O3 –Al2 O3 system.
three-phase field instead Al2 O3 + F +  and  + F + NdAP. It should be mentioned that for several compositions in the Al2 O3 rich compositions the equilibrium was not reached at 1523 K and we found fluorite, monoclinic, NdAP and Al2 O3 simultaneously (samples #1, 2, 4, and 5). This can be explained by invariant reaction between fluorite, tetragonal, NdAP and Al2 O3 phases which probably occurs at temperature close to 1523 K. Tetragonal phase transforms into monoclinic phase on cooing that was also observed by Wang32 in the ZrO2 –based systems. The other tie-lines indicated in present work at 1523 K (samples #3 and 6) are consistent with Lakiza and Lopato.12 The experimental results obtained for the system Nd2 O3 –Al2 O3 were used in optimization. The parameters of  and NdAP phases were optimized first and checked for consistency in the ZrO2 –Nd2 O3 –Al2 O3 system. New binary description of the ZrO2 –Nd2 O3 system was introduced into the thermodynamic dataset of the ZrO2 –Nd2 O3 –Al2 O3 system. The parameters of NdAP and  phases were adjusted to reproduce experimental phase relations in binary and ternary system simultaneously. Therefore the obtained data for  and NdAP phases consistent with phase relations in ternary system were accepted and then mixing parameters of liquid phase in the Nd2 O3 –Al2 O3 system were optimized. The calculated phase diagram of the Nd2 O3 –Al2 O3 system is presented in Fig. 9 and calculated results for invariant reactions are compared with experimental data in Table 4. The optimized parameters of liquid in the Nd2 O3 –Al2 O3 system were introduced into thermodynamic database of the ZrO2 –Nd2 O3 –Al2 O3 system while the ternary mixing parameter for the liquid phase was kept the same as in work of Fabrichnaya and Seifert11 Liquidus and vertical sections of ternary ZrO2 –Nd2 O3 –Al2 O3 system were re-calculated. It should be mentioned that calculated liquidus surface were in agreement with experimental data of Lakiza and Lopato,12 as well as vertical sections except for phase relations in the Al2 O3 rich composition at temperatures below 1873 K due to instability of  phase. The phase assemblages calculated with the up-dated description are compared with experimental results in Table 3. The calculated isothermal sections at 1523, 1673 and 1873 K along with experimental results obtained in present work are presented in Fig. 10a–c.
Fig. 10. Calculated isothermal sections of the ZrO2 –Nd2 O3 –Al2 O3 system along with experimental data obtained in the present work. a – 1523 K; b – 1673 K; c – 1873 K.
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Table 5 Invariant equilibria in the ZrO2 –Nd2 O3 –Al2 O3 system. Reaction
Type
L = F + Pyr L+X=H+F L = NdAP + Pyr L + Pyr = F + NdAP L+H=A+F L = NdAP + F + A
Eutectic emax1 Transitional U1 Eutectic emax2 Transitional U2 Transitional U3 Eutectic E1
L + T = Al2 O3 + F
Transitional U4
Pyr + L = F + NdAP L + Al2 O3 =  + F
Transitional U5 Transitional U6
L =  + NdAP + F
Eutectic E2
Temperature (K)
2486 2330 2206 2166 2125 2092 2023 2021 2043 2006 1946 1988 1945 1948
Composition
Source
x(Al2 O3 )
x(ZrO2 )
x(Nd2 O3 )
0.1133 0.0386 0.2928 0.2286 0.1784 0.1997 0.19 0.5325 0.56 0.4763 0.5384 0.57 0.5382 0.53
0.4876 0.2556 0.2760 0.2320 0.1492 0.1382 0.21 0.3400 0.34 0.3015 0.2713 0.22 0.2703 0.26
0.3991 0.7065 0.4311 0.5594 0.6724 0.6621 0.60 0.1275 0.10 0.2222 0.1903 0.21 0.1915 0.21
calc. calc. calc. calc. calc. calc. 12
calc. 12
calc. calc. 12
calc. 12
It should be mentioned that according to calculations the invariant reaction F + Al2 O3 = T + NdAP occurs at 1504 K. That is why all four phases were simultaneously found in several samples heat treated at 1523 K. Isothermal section calculated at 1500 K shows change of tie-lines due to invariant reaction F + Al2 O3 = T + NdAP (Fig. 11a). Isothermal section calculated at 1923 K is presented in Fig. 11b; the tie-lines are consistent with results of Lakiza and Lopato.12 Calculated data on invariant reactions are compared with data12 in Table 5. Calculated liquidus surface is presented in Fig. 12. Calculated vertical sections are presented in Fig. 13a–d along with experimental data.12 New thermodynamic description of the Nd2 O3 –Al2 O3 system was introduced into the thermodynamic database for the Nd2 O3 –Y2 O3 –Al2 O3 system. The Gibbs energies of following metastable end-members of solid solutions Nd4 Al2 O9 , Nd3 Al5 O12 , NdAlO3 with structure of orthorhombic perovskite
Fig. 11. Calculated isothermal sections of the ZrO2 –Nd2 O3 –Al2 O3 system. a – 1500 K; b – 1923 K.
Fig. 12. Calculated liquidus surface for the ZrO2 –Nd2 O3 –Al2 O3 system.
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Fig. 13. Calculated vertical sections of the ZrO2 –Nd2 O3 –Al2 O3 system along with data of Lakiza and Lopato.12 a – NdAlO3 –Nd2 Zr2 O7 ; b – x(ZrO2 ) = 0.2; c – x(ZrO2 ) = 0.4; d – x(Al2 O3 ) = x (ZrO2 ).
and YAlO3 with structure of rhombohedral perovskite were assessed. The solid solutions were assumed to be ideal. The phase assemblages calculated with the new description are compared with calculations based on binary extrapolations and experimental results in Table 2. The calculated sections of the Nd2 O3 –Y2 O3 –Al2 O3 phase diagrams at 1523, 1673 and 1873 K are presented in Fig. 14a–c. They are in reasonable agreement with obtained experimental data (Table 2). According to calculations invariant reaction B + YAM = C + NdAP occurs at 1833 K, that is in a good agreement with value 1863 K obtained by DTA in the present work. The results obtained on melting in the present work were used to optimize ternary parameter of liquid phase. The calculated liquidus surface of the Nd2 O3 –Y2 O3 –Al2 O3 system is presented in Fig. 15. The calculated temperature and liquid compositions for invariant reactions are presented in Table 6. The calculated temperature and composition of L = YAG + Al2 O3 + NdAP reaction are in a good agreement with experimental results obtained in present work. Thermodynamic parameters of the
Nd2 O3 –Y2 O3 –Al2 O3 system optimized in the present work are shown in Table 7. The thermodynamic descriptions of the ZrO2 –Nd2 O3 –Al2 O3 and Nd2 O3 –Y2 O3 –Al2 O3 systems obtained in the present work were combined with descriptions of the ZrO2 –Y2 O3 –Al2 O3 system30 and ZrO2 –Nd2 O3 –Y2 O3 from work of Fabrichnaya et al.10 It should be mentioned that mixing parameters of fluorite (F) and B phases in the ZrO2 –Nd2 O3 –Y2 O3 system were slightly modified in present study to avoid appearance of fluorite phase at high temperatures in the Nd2 O3 –Y2 O3 system: 0
L(F, Nd3+ , Y3+ : O2− ) = 0 L(F, Nd3+ , Y3+ : Va) = 30412 − 20·T , J/mol
0
L(F, Nd3+ , Y3+ , Zr4+ : O2− ) = 0 L(F, Nd3+ , Y3+ , Zr4+ : Va) = −425352 − 196.67·T , J/mol
0
L(B, Nd3+ , Zr4+ : O2− : O2− ) = 0 L(B, Nd3+ , Zr4+ : O2− : Va) = −24595 − 13.77·T , J/mol
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Table 6 Invariant equilibria in the Nd2 O3 –Y2 O3 –Al2 O3 system. Reaction
L+H=X C + H(L) = B H(L) = B + A L = NdAP + YAP L = NdAP + YAM L = NdAP + YAP + YAM L + C = YAM + B L + YAP = YAG + NdAP L + A = B + NdAP L = NdAP + YAM + B L +  = Al2 O3 + NdAP L = Al2 O3 + YAG + NdAP
Type
pmin1 D1 D2 emax1 emax2 E1 U1 U2 U3 E2 U4 E3
Temperature (K)
2399 2244 2211 2191 2179 2178 2120 2094 2084 2071 2055 2019 2042
This adjustment did not influence the calculated phase diagrams presented in Ref.10 Calculated liquidus surface of the ZrO2 –Nd2 O3 –Y2 O3 system is presented in Fig. 16. The calculated temperatures of transitional reactions U1 (L + H = X + C) and U2 (L + C = F + X) are 2613 and 2605 K, respectively. The obtained thermodynamic database of the ZrO2 –Nd2 O3 –Y2 O3 –Al2 O3 system can be applied for modeling of phase reactions at the TBC–TGO interface. Therefore it can be estimated at which concentration of
Composition of liquid
Source
x(Al2 O3 )
x(Nd2 O3 )
x(Y2 O3 )
0.0963 0.1695 0.1832 0.5031 0.3950 0.4044 0.2401 0.6696 0.2557 0.2622 0.7927 0.7707 0.77
0.6161 0.3643 0.4833 0.1798 0.2043 0.1901 0.3504 0.3189 0.5256 0.4278 0.1877 0.1093 0.10
0.2875 0.4662 0.3335 0.3171 0.4007 0.4055 0.4095 0.1801 0.2187 0.3100 0.0196 0.1200 0.13
calc. calc. calc. calc. calc. calc. calc. calc. calc. calc. calc. calc. exp.
Nd- or Y-stabilizers formation of the NdAP, YAG or YAP phases occurs resulting in incompatibility between thermal barrier coating and thermally grown oxide. The thermodynamic database for the ZrO2 –Nd2 O3 –Y2 O3 –Al2 O3 system allows one to evaluate the thermal stability of the coating and its stability during thermal cycling. Using the derived database it is possible to calculate T0 -lines for diffusionless transformations (e.g., Fluorite = Tetragonal) and thus estimate how much stabiliser can be introduced into the tetragonal phase under quenching.
Table 7 Thermodynamic parameters of the Nd2 O3 –Y2 O3 –Al2 O3 system at 298.15–6000 K. Phase
Model/parameter
YAG
(Nd3+ ,Y3+ )3 (Al3+ )5 (O2− )12 0 GYAG (Nd3+ :Al3+ :O2− ) = 1.5
GND2O3A + 2.5 GCORUND − 1,00,000
0 GYAG (Y3+ :Al3+ :O2− ) = GYAG − 1684.33
YAP
(Nd3+ ,Y3+ )1 (Al3+ )1 (O2− )3 0 GYAP (Nd3+ :Al3+ :O2− ) = GNDALO3 + 10,000 0 GYAP (Y3+ :Al3+ :O2− ) = −24515.2904 − 6.858584·T + 0.5·GCORUND + 0.5·GY2O3C
YAM
(Nd3+ ,Y3+ )4 (Al3+ )2 (O2− )9 0 GYAM (Nd3+ :Al3+ :O2− ) = 2GND2O3A + GCORUND-64770 − 10·T 0 GYAM (Y3+ :Al3+ :O2− ) = GYAM − 9035.18728 + 2.18809722·T
NdAP
(Nd3+ ,Y3+ )1 (Al3+ )1 (O2− )3 (Nd3+ :Al3+ :O2− ) = GNDALO3 0 GNdAP (Y3+ :Al3+ :O2− ) = −24,515.2904 − 6.858584·T + 0.5·GCORUND + 0.5·GY2O3C + 10,000 (Nd3+ )1 (Al3+ )11 (O2− )18 0 G = 5.5GCORUND + 0.5GNd2O3A − 38,780.5 − 9.69756T (Nd3+ ,Y3+ )P (O2− ,AlO1.5 )Q 0 GL (Nd3+ :O2− ) = GND2O3L 0 GL (AlO ) = 0.5·GAl2O3L 1.5 0 GL (Y3+ :O2− ) = GY2O3C + 1,08779 − 40.509·T 0 LL (Nd3+ ,Y3+ :O2− ) = 0 0 LL (Nd3+ :O2− ,AlO ) = −1,38,603.374 1.5 1 LL (Nd3+ :O2− ,AlO ) = −63,151.126 1.5 0 LL (Y3+ :O2− ,AlO ) = −1,24,455 + 12.23067·T 1.5 1 LL (Y3+ :O2− ,AlO ) = 2,66,564 − 151.915942·T 1.5 0 LL (Nd3+ ,Y3+ :O2− ,AlO ) = 25,000 1.5
0 GNdAP
NdAl11 O18 () IONIC LIQ (L)
Functions GNDALO3 = −18,32,701.1 + 641.583·T − 109.1273·T·ln(T) − 0.013364·T2 + 8,90,990/T GYAG = −73,63,443 + 2714.0547·T − 438.9177·T·ln(T) − 0.034315335·T2 + 4,80,0831.7/T GYAM = −56,86,303 + 2239.26502·T − 368.158373·T·ln(T) − 0.018363228·T2 + 4310328.16/T Functions GCORUND, GAL2O3L, GHSEROO can be found in work.29 GND2O3A, GND2O3L, GY2O3C in work.28
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Fig. 15. Calculated liquidus surface of the Nd2 O3 –Y2 O3 –Al2 O3 system; circles indicate compositions of samples #20 and 11.
Fig. 16. Calculated liquidus surface for the ZrO2 –Nd2 O3 –Y2 O3 system.
5. Conclusions
Fig. 14. Calculated isothermal sections of the Nd2 O3 –Y2 O3 –Al2 O3 system along with experimental data obtained in present work. a – 1523 K; b – 1673 K; c – 1873 K.
The thermodynamic parameters were re-assessed in the Nd2 O3 –Al2 O3 system based on experimental data obtained in present work and literature data. Simultaneously isothermal sections of ternary system ZrO2 –Nd2 O3 –Al2 O3 system were calculated and checked for the consistency with experimental data obtained in present work at 1523–1873 K and with data of Lakiza and Lopato12 at 1923 K. Then binary liquid parameters were re-assessed in the Nd2 O3 –Al2 O3 system. Ternary mixing parameter of liquid phase in the ZrO2 –Nd2 O3 –Al2 O3 system derived in work of Fabrichnaya and Seifert11 was not changed because the calculations reproduced liquidus surface of Lakiza and Lopato12 reasonably well.
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The phase relations in the Nd2 O3 –Y2 O3 –Al2 O3 system have been experimentally studied for the first time. The isothermal sections were constructed at 1523, 1673 and 1873 K. Temperature and composition of eutectic reaction L = YAG + NdAP + Al2 O3 have been determined by DTA and followed by SEM/EDX investigations. The obtained experimental results were used to derive thermodynamic database of this system. The thermodynamic parameters of the Nd2 O3 –Al2 O3 system were combined with other binary descriptions to create thermodynamic database for ternary system Nd2 O3 –Y2 O3 –Al2 O3 . Mutual solubility of Nd and Y in aluminate phases was taken into account and the Gibbs energies of Nd4 Al2 O9 , Nd3 Al5 O12 , NdAlO3 (orthorhombic) and YAlO3 (rhombohedral) metastable end-members were estimated. The calculated isothermal sections are in a good agreement with experimental results. The experimentally determined temperature of solid state reaction B + YAM = C + NdAP is well reproduced by calculations. Experimental data on temperature and liquid composition for eutectic reaction L = YAG + NdAP + Al2 O3 were used to assess ternary parameter of liquid phase and to calculate liquidus surface of the Nd2 O3 –Y2 O3 –Al2 O3 system. The mixing parameters of fluorite and B monoclinic phase in the ZrO2 –Nd2 O3 –Y2 O3 system were slightly modified to avoid appearance of fluorite phase in the Nd2 O3 –Y2 O3 system at high temperatures. Finally the descriptions of four ternary systems were combined and thermodynamic database of the ZrO2 –Nd2 O3 –Y2 O3 –Al2 O3 system was derived. The obtained database could be used for calculation of the TBC stability as well as for other applications. Acknowledgment This work was financially supported by German Research Foundation (grant SE-647/9-1). References 1. Levi CG. Emerging materials and processes for thermal barrier system. Curr Opin Solid State Mater Sci 2004;8:77–91. 2. Rebollo NR, Gandhi AS, Levi CG. Phase stability issues in emerging TBC systems. In: Opila EJ, Hou P, Maruyama T, Pieraggi B, McNallan M, Shifler D, Wuchina E, editors. High temperature corrosion and materials chemistry IV, Electrochemical society proceedings, vol. PV-2003-16. 2003. p. 431–42. 3. Khor KA, Yang J. Rapidly solidified neodymia – stablised zirconia coatings prepared by DC plasma spaying. Surface Coat Technol 1997;96:313–22. 4. Xu T, Vleugels J, Van der Biest O, Kann Y, Wang P. Phase stability and mechanical properties of TZP with a low mixed Nd2 O3 /Y2 O3 stabiliser content. J Eur Ceram Soc 2006;26:1205–11. 5. Xu Q, Pan W, Wang J, Wan Ch, Qi L, Mia H. Rare Earth zirconate ceramics with fluorite structure for thermal barrier coatings. J Am Ceram Soc 2006;89:340–2. 6. Hinatsu Y, Muromura T. Phase relations in the systems ZrO2 –Y2 O3 –Nd2 O3 and ZrO2 –Y2 O3 –CeO2 . Mater Res Bull 1986;21:1343–9. 7. Xu T, Vleugels J, Van der Biest O, Wang P. Mechanical properties of Nd2 O3 /Y2 O3 -coated zirconia ceramics. Mater Sci Eng A 2004;374:239–43.
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