Experimental investigation and thermodynamic modeling of the ZrO2–MgO–Al2O3 system

Available online at www.sciencedirect.com

ScienceDirect Journal of the European Ceramic Society 34 (2014) 1397–1408

Experimental investigation and thermodynamic modeling of the ZrO2–MgO–Al2O3 system D. Pavlyuchkov ∗ , G. Savinykh, O. Fabrichnaya Technical University of Freiberg, Institute of Materials Science, D-09599 Freiberg, Germany Received 21 October 2013; received in revised form 19 November 2013; accepted 22 November 2013 Available online 6 January 2014

Abstract Solid state equilibria of the ZrO2 –MgO–Al2 O3 system as well as the equilibria including liquid were investigated in the whole compositional range using high temperature DTA, X-ray diffraction (XRD) and scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM/EDX). Isothermal sections at 1523, 1873 and 2023 K were constructed and the formation of the ternary X-phase in the MgO-rich region was confirmed. According to our results it forms by the peritectic reaction MgO + Spinel + L  X at 2114 K and decomposes by the X  F + MgO + Spinel reaction at 1894 K. The ternary mixing parameter of liquid was assessed using the results of melting experiments. The obtained liquid thermodynamic description was used to derive enthalpy and entropy values of the X phase. The calculated liquidus and solidus representing 5 invariant processes as well as calculated isothermal and vertical sections show good consistency with the results obtained in this work and those available in the literature. © 2013 Elsevier Ltd. All rights reserved. Keywords: ZrO2 ; TRIP steel composite; Phase diagram; CALPHAD

1. Introduction The ZrO2 -based ceramics are of scientific and industrial importance. In Ref.1 magnesia partially stabilized ZrO2 (MgPSZ) was proposed to use as a reinforced component of the steel-ceramic composite (SCC). Our present study is part of an ongoing project which is aimed at developing a thermodynamic database for simulation of reactions occurring during processing of the above mentioned composite material. Commercially available Mg-PSZ powders which are going to be used in SCC usually contain different additives such as Al2 O3 , SiO2 , HfO2 , CaO, TiO2 , etc. Recently we have already showed that even a small amount of the Al2 O3 additive results in the formation of the intergranular MgAl2 O4 phase, Ref.2 In order to assess the influence of the Al2 O3 additive on the phase transitions occurring in the commercial Mg-PSZ material phase diagram of the ZrO2 –MgO–Al2 O3 system is necessary, as well as thermodynamic parameters of the phases stable in the system.

∗

Corresponding author. Tel.: +49 3731 393641; fax: +49 3731 393657. E-mail address: [email protected] (D. Pavlyuchkov).

0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.11.038

The first attempts to construct the ZrO2 –MgO–Al2 O3 phase diagram were performed in the mid-sixties by Berezhnoj and Kordyuk.3 The authors presented a tentative variant of the melting diagram consisting of four primary phase fields: ZrO2 (fluorite), MgO, MgAl2 O4 and Al2 O3 . They observed the existence of two ternary eutectics as well as one eutectic maximum and revealed their temperatures and compositions, while solid state equilibria were not reported. Subsolidus phase relations in the ZrO2 –MgO–Al2 O3 system were studied by Tassot et al.,4 where two isothermal sections were constructed at 1873 and 2073 K. In contrast to the previous work where no ternary compounds were observed the authors4 revealed a ternary compound named X. It was found that this phase formed around the Mg5+x A12.4−x Zr1.7+0.25x O (−0.4 ≤ x ≤ 0.4) composition and the phase formation occurred at temperatures between 1873 and 2073 K. Later a crystal structure of phase X was determined by Bissert and Tassot.5 It should be mentioned that most likely the same phase was obtained some time before by Reynen et al.6 at the composition very similar to that reported in Refs.4,5 However the authors6 did report neither its crystal structure nor its temperature interval of existence. The ZrO2 –MgAl2 O4 isopleth was constructed by Shevchenko et al.7 The authors showed that additions of MgAl2 O4 stabilize the fluorite polytype of ZrO2

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down to 1853 K. The eutectic L  F + MgAl2 O4 and the eutectoid T  M + MgAl2 O4 reactions were determined to occur at 2088 and 1233 K respectively. Thermodynamic descriptions of the binary ZrO2 –MgO, MgO–Al2 O3 and ZrO2 –Al2 O3 systems were recently reassessed in Refs.2,8,9 correspondingly. However a thermodynamic database of the ternary ZrO2 –MgO–Al2 O3 system has not been developed yet. Therefore in this contribution we reinvestigated the phase diagram of the ZrO2 –MgO–Al2 O3 system in whole composition range in order to check reliability of the available literature: to confirm the existence of the ternary phase X and specify temperature of its formation and melting. Melting relations was studied for most crucial compositions of the system. Additionally the Calphad method was used to develop the corresponding ternary dataset reproducing the experimental data. 2. Experimental Nine samples (Table 1) were prepared by co-precipitation route. The starting chemicals were zirconium acetate solution in acetic acid, Zr(CH3 COO)4 (99.99%, Sigma–Aldrich), magnesium nitrate Mg(NO3 )2 ·6H2 O and aluminum nitrate Al(NO3 )3 ·6H2 O (99.9%, Alfa Aesar). In the first step Mg(NO3 )2 ·6H2 O and Al(NO3 )3 ·6H2 O were dissolved in the distilled water and the initial zirconium acetate solution was diluted. The initial solutions were mixed according to the selected ratio. Concentration of the prepared solution was examined by inductively coupled plasma spectroscopy (ICP-OES). The obtained precursor solution was dropped from the buret at a low speed (around 1 ml/min) into a big beaker containing about 500 ml of deionized water with the pH value maintained above 9.0 by adding ammonium hydrate (reverse precipitation). 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 then dried at 353 K. During pyrolysis proceeding at 1073 K for 3 h in air, hydroxides transformed to oxides releasing water. Filtrates and parts of samples before pyrolysis dissolved in diluted H2 SO4 were subsequently analyzed by ICP-OES with an accuracy of ±2%. According to ICP-OES analysis, the concentrations of Zr, Mg and Al in filtrates were found to be less than 10−5 mol/l that indicates completeness of the co-precipitation procedure. Additionally the sample compositions measured using ICP-OES were found to deviate in the limits of less than 1% from nominal compositions. The obtained powders were pelletized at 250 MPa and subsequently annealed in Pt-crucibles at 1523, 1873 and 2023 K. The heat treatments were performed at 1873 K and 2023 K for 7 days and 6 h respectively to reach equilibrium. The heat treatment at 1523 K was performed for 10–14 days because of sluggish kinetics. The samples were cooled by switching off the furnace. Afterwards the samples were analyzed by X-ray diffraction (XRD) and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM/EDX). The XRD patterns of powdered or polished specimens were recorded by URD63 diffractometer using Cu K␣ radiation (Seifert, FPM, Freiberg, Germany). The microstructures

of sintered samples were examined by SEM (Leo1530 GEMINI) equipped with EDX analyzer (Bruker AXS Mikroanalysis GmbH) to establish the compositions of the phases present in the samples. The Rietveld analysis of the measured XRD patterns was done using MAUD program.10 The temperatures of the transformations were registered using DTA SETSYS-EVOLUTION-1750 (SETARAM, France) instrument. During the experiment the specimens were analyzed up to 2003 K in the Pt crucibles under Ar atmosphere at heating and cooling rates of 20 K/min. Temperature calibration was made using melting points of pure Al, Ag, Au, Cu and Ni. Temperatures of transformations were determined as on-set points which were the intersections of tangent drawn at point of greatest slope in the leading edge of peak with extrapolated baseline. DTA experiments were performed for the samples preliminary heat treated at 1523 K. The melting experiments were performed using DTA SETSYS-EVOLUTION-2400 (SETARAM, France) instrument. During the experiment the specimens were analyzed up to 2273 K in the W crucibles under He atmosphere at heating and cooling rates of 20 K/min. Sample were preliminary heat treated at 2023 K. Temperature correction of this device was done using melting points of Al, Al2 O3 and solid state transformation in LaYO3 obtained by co-precipitation and heat treated at 1523 K. In order to use the transformation temperature of orthorhombic perovskite to monoclinic phase with structure B-Sm2 O3 in the LaYO3 for calibration, the sample of the same composition LaYO3 was first measured in SETSYS-EVOLUTION-1750 where temperature determination was more exactly. For the details of temperature correction procedure see Ref.11 3. Modeling The crystal structures of the cubic, tetragonal and monoclinic modifications of ZrO2 are well known. Therefore their solid solutions can be described by two sublattice model using the compound energy formalism suggested by Hillert.12 The first sublattice is occupied by Zr+4 ,Mg+2 and Al+3 cations whereas oxygen anions as well as vacant positions are present in the second sublattice. The corresponding model (Zr+4 , Mg+2 , Al+3 )(O−2 , Va0 )2 was used to treat all three ZrO2 -based solid solutions. The neutral vacancies were introduced in the second sublattice in order to compensate for the less positive cation charges than Zr+4 in the first sublattice. The four sublattice description of the non-stoichiometric binary spinel MgAl2 O4 phase, (Al+3 , Mg+2 )(Al+3 , Mg+2 , Va0 )2 (Mg+2 , Va0 )2 (O−2 )4 , was accepted in accordance with Zienert and Fabrichnaya8 neglecting possible solubility of ZrO2 . The description of solid solutions based on Al2 O3 (corundum) and MgO were accepted from Ref.8 The ternary phase X was treated as stoichiometric compound (Mg)4.68 (Al)2.64 (Zr)1.68 (0)12 which was modeled using the following temperature dependence of the Gibbs energy β

g

GAx By Oz (T ) − x0 HAα − y0 HB − z0 HO = xAx Od GAx Od (T ) + xBy Oz−d GBy Oz−d (T ) + a + bT,

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Table 1 Phase assemblages identified by SEM–EDX, XRD and calculated in the ZrO2 –MgO–Al2 O3 system. Sample, nominal composition, (mol fra.) ZrO2

MgO

Al2 O3

0.886

#1 0.101

0.013

T (K)

1523

1873

2023

0.198

#2 0.640

1523 0.162

1873

2023

0.210

#3 0.177

1523 0.613

1873

2023

0.901

#4 0.052

1523 0.047 1873

2023

0.599

#5 0.203

1523 0.198 1873

2023

0.333

#6 0.556

1523 0.111

1873

Phase assemblage, XRD data

Rietveld analysis (Vol.%)

Composition (mol fra.), SEM/EDX data ZrO2

MgO

Al2 O3

Phase assemblage, calculated

Composition (mol fra.), calculated ZrO2

MgO

Al2 O3

M(T) Spinel MgO T T F M(T) Spinel T M(T)

94.5 2.6 2.4 0.5 46.0 16.5 38.8 2.7 32.9 67.1

– – – – – 0.89 0.98 0.01 0.91 –

– – – – – 0.09 0.01 0.47 0.08 –

– – – – – 0.01 0.01 0.52 0.02 –

T Spinel MgO – – F T Spinel F –

0.987 0.000 0.000 – – 0.883 0.974 0.000 0.886 –

0.008 0.514 0.999 – – 0.108 0.010 0.502 0.101 –

0.005 0.485 0.001 – – 0.009 0.016 0.498 0.013 –

Spinel M(T) MgO T Spinel MgO F X X MgO Spinel F

49.2 25.2 31.2 1.4 42.5 30.2 23.9 3.4 86.2 6.8 4.9 2.1

– – – – 0.02 0.01 0.83 0.26 0.23 – – 0.81

– – – – 0.47 0.98 0.16 0.58 0.59 – – 0.17

– – – – 0.51 0.01 0.01 0.16 0.18 – – 0.02

Spinel T MgO – Spinel MgO F – X MgO Spinel –

0.000 0.987 0.000 – 0.000 0.000 0.833 – 0.219 0.000 0.000 –

0.514 0.008 0.999 – 0.539 0.996 0.162 – 0.609 0. 989 0.551 –

0.485 0.005 0.001 – 0.461 0.004 0.005 – 0.172 0.011 0.449 –

Al2 O3 Spinel M(T) T Al2 O3 Spinel M(T) T Al2 O3 Spinel M(T) T

52.3 27.7 16.3 3.6 52.8 29.8 16.9 0.5 51.9 34.9 12.8 0.4

– – – – 0.00 0.01 0.97 – 0.00 0.01 0.98 –

– – – – 0.00 0.41 0.01 – 0.00 0.37 0.01 –

– – – – 1.00 0.58 0.02 – 1.00 0.62 0.01 –

Al2 O3 Spinel T

0.000 0.000 0.986 – 0.000 0.000 0.967 – 0.000 0.000 0.959 –

0.000 0.261 0.001 – 0.000 0.328 0.002 – 0.000 0.231 0.002 –

1.000 0.739 0.013 – 1.000 0.672 0.031 – 1.000 0.769 0.039 –

M(T) Spinel T M(T) Spinel T F M(T) Spinel T

91.4 8.1 0.5 89.4 10.2 0.4 – 84.2 15.3 0.5

– – – 0.98 0.01 – – 0.98 0.01 –

– – – 0.01 0.46 – – 0.01 0.46 –

– – – 0.01 0.53 – – 0.01 0.53 –

T Spinel MgO T Spinel – F T Spinel

0.987 0.000 0.000 0.965 0.000 – 0.896 0.965 0.000 –

0.008 0.514 0.999 0.007 0.479 – 0.086 0.007 0.479 –

0.005 0.485 0.001 0.03 0.521 – 0.017 0.028 0.521 –

M(T) Spinel T M(T) Spinel T M(T) Spinel T

62.5 36.7 0.8 47.2 52.6 0.3 48.0 51.6 0.4

– – – 0.96 0.01 – 0.98 0.01 –

– – – 0.02 0.47 – 0.01 0.47 –

– – – 0.02 0.52 – 0.01 0.52 –

T Spinel – T Spinel F T Spinel F

0.993 0.000 – 0.973 0.000 0.883 0.965 0.000 –

0.002 0.288 – 0.010 0.502 0.108 0.007 0.479 –

0.005 0.712 – 0.017 0.498 0.009 0.028 0.521 –

Spinel M(T) MgO T F

30.2 44.2 24.2 1.4 44.1

– – – – 0.82

– – – – 0.17

– – – – 0.02

Spinel T MgO – F

0.000 0.987 0.000 – 0.832

0.514 0.008 0.999 – 0.163

0.485 0.005 0.001 – 0.005

Al2 O3 Spinel T – Al2 O3 Spinel T –

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Table 1 (Continued) T (K)

Sample, nominal composition, (mol fra.) ZrO2

MgO

Phase assemblage, XRD data

Al2 O3

2023

0.364

#7 0.424

1526 0.212

1873

2023

0.121

#8 0.667

1523 0.212

1873

2023

0.894

#9 0.101

Rietveld analysis (Vol.%)

1523 0.005

1873

2023

Composition (mol fra.), SEM/EDX data ZrO2

MgO

Al2 O3

ln

0.51 0.01 0.18 0.02 0.02

Spinel MgO X F MgO

0.000 0.000 0.219 0.812 0.000

0.539 0.996 0.609 0.182 0.991

0.460 0.004 0.172 0.006 0.009

Spinel M(T) MgO T Spinel F MgO Spinel F X

46.3 41.5 10.7 1.5 47.0 42.9 10.1 46.4 30.5 23.1

– – – – 0.02 0.82 – 0.02 0.81 0.25

– – – – 0.46 0.16 – 0.48 0.17 0.57

– – – – 0.52 0.02 – 0.50 0.02 0.18

Spinel T MgO – Spinel F MgO Spinel F X

0.000 0.987 0.000 – 0.000 0.832 0.000 0.000 0.823 0.219

0.514 0.008 0.999 – 0.539 0.163 0.996 0.533 0.168 0.609

0.485 0.005 0.001 – 0.460 0.005 0.004 0.466 0.008 0.172

Spinel MgO M(T) T Spinel MgO F X Spinel MgO F

53.2 29.8 14.6 2.4 49.0 36.2 14.8 52.5 32.3 13.6 1.6

– – – – 0.01 0.01 0.81 0.22 0.02 0.02 –

– – – – 0.47 0.96 0.17 0.60 0.48 0.96 –

– – – – 0.51 0.02 0.02 0.18 0.50 0.02 –

Spinel MgO T – Spinel MgO F X Spinel MgO –

0 0 0.987 – 0.832 0.000 0.000 0.219 0.000 0.000 –

0.514 0.999 0.008 – 0.163 0.539 0.996 0.609 0.551 0.989 –

0.485 0.001 0.005 – 0.005 0.460 0.004 0.172 0.449 0.011 –

M(T) MgO Spinel T T F M(T) Spinel M(T) T

97.6 0.9 0.8 0.7 52.7 20.2 26.2 0.9 60.4 39.6

– – – – – 0.90 0.97 0.03 0.90 –

– – – – – 0.09 0.02 0.47 0.08 –

– – – – – 0.01 0.01 0.50 0.02 –

T MgO Spinel – – F T – F –

0.987 0.000 0.000 – – 0.890 0.979 – 0.894 –

0.008 0.999 0.514 – – 0.105 0.011 – 0.101 –

0.005 0.001 0.485 – – 0.005 0.010 – 0.005 –

+ Gex ,

where YiS is the mole fraction of a constituent i in the sublattice S, αS is the number of sites on sublattice S per mole of formula unit of a phase and Gex is the excess Gibbs energy of mixing expressed as:   YjT Σ YiS YkS LSi,k + YiS Σ YjT YmT LTj,m + Gex,h−0 Gex = T

S

S

Al2 O3

0.47 0.92 0.59 0.18 0.97

i

S

MgO

0.02 0.07 0.24 0.81 0.01

G = Σ Σ YiS YjT Gi,j + RT Σ αS Σ YiS ln YiS YjT

ZrO2

31.1 24.8 74.6 22.2 3.2

φ

+ RT Σ αT Σ YjT T j

Composition (mol fra.), calculated

Spinel MgO X F MgO

where 0 Hi is the enthalpy of pure element i in its stable state at 298.15 K, GAx Od (T ) and GBy Oz−d (T ) are the Gibbs energies oxides. The Neumann–Kopp rule was applied for X phase to estimate its heat capacity because experimental data for CP of this phase exists so far. The Gibbs energy of formation of solution phases modeled by two-sublattices was expressed as: i j

Phase assemblage, calculated

T

Binary interaction parameters LSi,k between i and k species in the sublattice S were expressed as n

LSi,j = Σ Li,j (YiS − YjS ) n

The term Gex,h−0 is the high-order contribution to the excess Gibbs energy. In the solid solutions only binary interactions were considered. The liquid was described by a two-sublattice partially ionic model12 using the following formula (Zr+4 , Mg+2 , Al+3 )P (O−2 , AlO3/2 )Q where P and Q are the number of sites on the cation and anion sublattices, respectively. The stoichiometric factors P and Q vary with the composition in order to maintain electroneutrality. The anion sublattice additionally contains the neutral species (i.e. AlO3/2 ) and can also contain vacancies. Vacancies were not included in modeling of the liquid, because phase interactions with metal were not considered in this work. The ternary mixing parameter L(Zr+4 , Mg+2 :O−2 , AlO3/2 ) was introduced into the

D. Pavlyuchkov et al. / Journal of the European Ceramic Society 34 (2014) 1397–1408

liquid phase to obtain better reproduction of the experimental data of the melting experiments. 4. Results and discussion Compositions of samples were selected in a way to determine or confirm all possible phase equilibria existing in the system at the selected temperatures. It should be mentioned that chemical compositions of the samples measured using EDX were found to be in good agreement with the nominal compositions. Therefore this method we use for determination of chemical composition of the phases coexisting in the samples.1 The results of investigation of the annealed samples by XRD and SEM/EDX analyses are summarized in Table 1. The temperatures of heat effects observed by DTA analysis on heating and cooling are presented in Table 2. 4.1. Isothermal sections The samples annealed at the lowest temperature (1523 K) applied in this work exhibited a very fine structure. The grain sizes of the phases present in the samples were found to be smaller than 2 ␮. Therefore the EDX analysis was only used to determine the overall composition of the samples and the phase assemblages were determined using only XRD analysis. It is well known that the tetragonal ZrO2 phase existing at elevated temperatures changes its crystal structure through martensite transformations to monoclinic during cooling. Therefore the monoclinic ZrO2 phase detected in X-ray diffraction patterns was associated with transformed tetragonal ZrO2 phase marked in Table 1 as M(T). Besides of this some of the remaining untransformed T phase was observed in all samples annealed at 1523 K and its volume fraction reached up to 4% according to Rietveld analysis. According to the obtained results the corresponding isothermal section consists of one two-phase region, T + Spinel, and two three phase regions T + MgO + Spinel and T + Spinel + Al2 O3 (Fig. 1a). Additionally the phase diagram was calculated at the corresponding temperature using the developed dataset based on binary extrapolation into ternary region (Fig. 1b). The calculated phase equilibria exhibit good agreement with those observed in the studied samples. The phase equilibria experimentally determined at 1873 K are shown in Fig. 2a. In contrast to the previous case, the grains of the phases coexisting in equilibria at this temperature were large enough (<10 ␮m) to measure successfully their chemical compositions using SEM-EDX technique. These compositions are shown on the corresponding isothermal section with opened squares (Fig. 2a) and listed in Table 1. The observed phase assemblage was also confirmed by XRD analysis (Table 1). According to the obtained results the formation of the ZrO2 based solid solution with fluorite structure (F) causes existence of additional two three-phase equilibria regions: MgO + F + Spinel, F + T + Spinel and respectively

1 The chemical compositions of phases and overall composition of samples were determined analyzing only the X-ray spectra of metallic elements.

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one two-phase equilibria region F + Spinel. Additionally the ternary T + Spinel + Al2 O3 region and the binary T + Spinel region observed at 1523 K were also detected at 1873 K. The calculated isothermal section at 1873 K was obtained by binary extrapolation into ternary region (Fig. 2b). A comparison of both experimentally constructed and calculated isothermal sections reveals very similar character of the phase equilibria. It should be noted that our results are well consistent to those presented by Tassot et al.4 However the experimentally constructed two-phase field T + MgAl2 O4 is visibly narrower to those shown in Ref.4 and calculated in this work. According to EDX measurements the corresponding spinel homogeneity range lies between 52 and 58 mol.% of Al2 O3 , whereas calculations indicate the following compositional range – 50–67 mol.% of Al2 O3 . Such a disagreement can be explained by influence of ZrO2 dissolved in MgAl2 O4 which was not taken into account in modeling. The spinel phase at 1873 K was found to dissolve about 1–2 mol.% of ZrO2 . It can be assumed that the Zr+4 ions substitute Mg+2 in the tetrahedral sublattice of spinel. The amount of Mg+2 subsequently increases in the second sublattice in order to compensate the charge increase forming fictive compound Mg2 ZrO4 with spinel structure. At the same time the amount of Al+3 decreases in the second sublattice. Thus the spinel phase would contain less of Al2 O3 in equilibria with T and Al2 O3 phases than that shown in the calculated isothermal section at 1873 K. On the other hand the compositional range of the fluorite solid solution was well reproduced by developed database. The experimentally constructed isothermal section at 2023 K is presented in Fig. 3a. In contrast to the previous temperatures the formation of a ternary compound was observed in the MgO-rich region of the phase diagram. According to XRD analysis (Fig. 4a) this phase has a crystal structure identical to that presented in Ref.5 The EDX measurements revealed a phase composition very similar to that of the X-phase reported in Ref. 4 . In order to specify its lower temperature limit the sample #2 annealed at 1523 K was analyzed by DTA up to 2023 K. The XRD patterns before and after the DTA are shown in Fig. 4b and c, correspondingly. The DTA heating and cooling curves of sample #2 are presented in Fig. 5 and in Table 2. It should be mentioned that the transformation temperatures were determined as on-set points, because temperature calibration was also done in the same way. The obtained heating curve (Fig. 5a) indicates three thermal effects. At 1382 K the initial phase assemblage M + MgO + Spinel (Fig. 4a) transforms into T + MgO + Spinel. The latter subsequently transforms into F + MgO + Spinel at 1703 K. The final thermal effect represents formation of the X-phase at 1894 K according to reaction F + MgO + Spinel  X. The cooling curve indicates only one heat effect at 1887 K which was substantially smaller than on heating and therefore can be associated with partial decomposition of the ternary phase. The remaining fraction of X phase was detected using XRD (Fig. 4b) along with the F, MgO and Spinel phases. The upper temperature limit of the X-phase was determined using high temperature DTA of the sample #2 close to pure X-phase composition, as well as samples #6, #7 and #8 locating in the surrounding three-phase fields: X + MgO + F, X + F + Spinel and X + Spinel + MgO, respectively. Its highest

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Fig. 1. Isothermal sections of the ZrO2 –MgO–Al2 O3 phase diagram at 1523 K: (a) experimentally determined, (b) calculated.

Fig. 2. Isothermal sections of the ZrO2 –MgO–Al2 O3 phase diagram at 1873 K: (a) experimentally determined, (b) calculated.

Fig. 3. Isothermal sections of the ZrO2 –MgO–Al2 O3 phase diagram at 2023 K: (a) experimentally determined, (b) calculated.

D. Pavlyuchkov et al. / Journal of the European Ceramic Society 34 (2014) 1397–1408

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Table 2 The DTA results for selected samples of the ZrO2 –MgO–Al2 O3 system. Sample

Heat treatment

Registered onsets (K) Heating

Cooling

#1

1523 K for 10 days

1867 1712 1410

1846 1666 1151

#2

1523 K for 10 days

1894 1703 1382

1887

1873 K for 7 days

2114

2057 1577

#3

1523 K for 10 days

2080

2103 2066

#4

1523 K for 10 days

2139 1843 1435

2107 1233

#5

1523 K for 10 days

2156 2145 1845 1389

2148 2129 1837 1164

#6

2023 K for 6 h

2130 2113

2110 2078

#7

2023 K for 6 h

2187a 2103

2156a 2081

#8

2023 K for 6 h

2194 2112

2140 2058

a

Referred to liquidus temperature.

melting point of 2114 K was observed for the X + Spinel + MgO equilibrium. The upper and lower temperature limits of the Xphase were used to optimize its enthalpy and entropy values of the X phase. The thermodynamic description of liquid with introduced ternary mixing parameter (see below) was used to derive parameters of X phase. The procedure is described below along 1382 1894

Heat flow, a.u.

1703

Exo 1887 1400

Fig. 4. XRD for sample #2 annealed at: (a) 2023 K for 6 h; (b) 1523 K for 10 days; (c) after DTA shown in Fig. 5.

1600

Temperature, K

1800

Fig. 5. The DTA heating and cooling curves of sample #2 annealed at 1523 K for 10 days.

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D. Pavlyuchkov et al. / Journal of the European Ceramic Society 34 (2014) 1397–1408

Fig. 6. The SEM back scattered electron images and the corresponding XRD patterns of sample #9 annealed at: (a and c) 1873 K for 6 days; (b and d) 2023 K for 6 h.

with solid-liquid equilibria. The corresponding Gibbs energy function of X phase was assessed as: 0

GX = 1.32GCORUND + 1.68GZRO2M + 4.68GMGOSOL + 135380.9 − 100T

where GCORUND and GMGOSOL taken from Hallstedt13 and GZRO2M taken from Wang et al.14 The calculated isothermal section at 2023 K (Fig. 3b) shows good consistency with the experimentally constructed diagram. In comparison with phase relations at 1873 K both the calculated and experimental isothermal sections demonstrate extension of binary homogeneity ranges of the F and Spinel phases with temperature increase. However, similarly to the phase equilibria observed at 1873 K, the experimentally determined Spinel + T phase-field was found to be narrower of that present on the calculated isothermal section at 2023 K. Additionally the fluorite phase shows certain extension into ternary region with temperature increase. This is clearly visible when comparing SEM micrographs taken from sample #9 annealed at 1873 and 2023 K. At 1873 K the sample consists of F, T and a small amount of Spinel (Fig. 6a). At 2073 K the microstructures exhibit single phase structure (see Fig. 6b) indicating the ternary extension of the fluorite homogeneity range. Such a transition from the F + T + Spinel equilibria to the fluorite single phase region observed in this sample can be perfectly reproduced by calculations using the developed dataset

(Table 1). The same transition was as well observed for sample #1. It should be mentioned that the phase assemblage observed in SEM for 1873 K was confirmed by XRD. The corresponding XRD pattern presented in Fig. 6c exhibits presence of fluorite, tetragonal and monoclinic phases which can be also observed in microstructures (Fig. 6a): the fluorite phase along with the tetragonal phase precipitated on cooling inside the fluorite and monoclinic phase (transformed tetragonal phase). However, the Rietveld analysis of the XRD pattern of the sample annealed at 2023 K reveals no fluorite phase. Instead the pattern consists of broadened peaks of major monoclinic and of the rest of the tetragonal phases (Fig. 6d). A close inspection of samples #1 and #9 using SEM shows the following microstructure presented in Fig. 7. Possibly such drastic precipitation of the metastable nano-scaled tetragonal lens-like particles occurred in the fluorite solid solution supersaturated with magnesium and aluminum ions at 2023 K during cooling.2 The quantity of the fluorite matrix decreases and becomes too small to retain tetragonal structure of the precipitates. Finally a major fraction of the intragranular precipitates transforms to monoclinic symmetry and produces broad monoclinic peaks (Fig. 6d).

2 More detailed information about the formation and stability of the intragranular tetragonal particles inside fluorite grains in the ZrO2 –MgO system can be found in Ref.15 .

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L  Spinel + F and sample #3 (2080 K) corresponding to eutectic reaction E2 L = Spinel + Al2 O3 + T. The ternary parameter was used to assess the Gibbs energy of phase X. After that melting points of samples #2 (2114 K), #6 (2113 K) and #7 (2103 K) were used as temperatures of invariant reactions P (L + MgO + Spinel  X), U1 (L + MgO  X + F) and E1 (L  F + X + Spinel), respectively, to refine ternary parameter of liquid. This parameter was assessed as: 0

Fig. 7. The SEM back scattered electron image (25,000×) of sample #9 annealed at 2023 K for 6 h. Some nano-domains of the tetragonal precipitates are outlined with ellipses.

4.2. Solid–liquid equilibria Melting relations in the ZrO2 –MgO–Al2 O3 system were studied by DTA up to 2373 K and the obtained heat effects on heating and cooling are presented in Table 2. The melting temperatures were attributed to invariant reactions based on subsolidus phase relations at 2023 K. The temperatures of reactions were determined as on-set points on heating. Larger or smaller undercooling was usually indicated on cooling curves. Additionally it should be mentioned that the solidification occurred as a non-equilibrium process. The initial thermodynamic parameters of liquid obtained by extrapolation from binaries leads to stability of liquid phase at temperatures below 2073 K what is in contradiction with the experimental data obtained in the present work. Therefore a ternary mixing parameter was introduced and optimized to reproduce melting point of sample #4 (2139 K) considered as transition reaction U2 L + T  F + Spinel, sample #5 (2145 K) considered as temperature of eutectic reaction emax

L(Mg+3 , Zr +4 : O−2 , AlO1.5 ) = 129049.2 J/mol

The optimized liquid description allowed us to calculate liquidus and solidus projections presented in Fig. 8. The liquidus projection (Fig. 8a) consists of 6 primary phase fields and represents one incongruent (P), two transition (U1 , U2 ) and two congruent (E1 , E2 ) processes. In turn, the solidus projection presented in Fig. 8b Exhibits 5 isothermal plains: MgO + X +Spinel, MgO + F + X, F + T + Spinel, F + Spinel + X and T + Spinel + Al2 O3 , correspondingly. The calculated and experimental temperatures of the invariant processes are summarized in Table 3. According to the calculations the ternary X-phase forms by peritectic reaction L + MgO + Spinel  X at 2199 K. This temperature is 87 K higher than the experimentally determined one. Taking in account the uncertainty of temperature measurements in the high temperature range we consider this difference as a quite acceptable. This reaction is illustrated by the microstructure of the sample #8 after DTA presented in Fig. 9a where X-phase crystallizes around a mixture of Spinel and MgO and subsequently decomposes on cooling. The SEM observations carried out after high-temperature DTA of the sample #2 located close to the X-phase composition confirm its incongruent formation. The corresponding microstructure (Fig. 9b) exhibits large primary grains of MgO existing in equilibrium with liquid at the maximal temperature reached for this sample in DTA. The X-phase peritectically solidifies from residual liquid around the MgO grains by monovariant crystallization MgO + L  X along the PU1 curve (see Fig. 8a) and subsequently decomposes to the

Fig. 8. The liquidus (a) and solidus (b) surfaces projections of the ZrO2 –MgO–Al2 O3 phase diagram.

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Fig. 9. The SEM back scattered electron images after DTA of samples: (a) #8; (b) #2; (c)#6; (d)#3; (e)#5; (f)#7. The compositions of the samples are presented in Table 1.

eutectoid mixture on cooling. Liquid solidification finishes in eutectic point E1 . The corresponding eutectic microstructure is marked by E1 in Fig. 9b and its composition was determined by EDX. The microstructure presented in Fig. 9c exhibits result of crystallization of sample #6. According the calculations it crystallizes from the L + MgO region. The heat effect related to disappearance of X phase was observed at 2130 K (Table 2) on heating in DTA. Based on calculations it can be concluded that the complete melting of sample #6 should occur at temperatures above 2273 K and it was not reached in DTA experiment. The F-phase solidifies during cooling as a consequence of the L  F + MgO monovariant process along the e1 U1 curve (Fig. 8a). Subsequently the X-phase forms as a ternary phase around the F fluorite solid solution and MgO grains (Fig. 8b). One can see that in contrast to the previous samples where the

X-phase fully decomposes, in this sample it stays partially retained around the F grains. Similar to sample #2, solidification of liquid finishes in eutectic point E1 . The Fig. 9d shows the mutual crystallization of the T and Spinel phases occurring in sample #3 along the monovariant line U2 E2 (see Fig. 6a). The last portion of liquid crystallizes to the ternary eutectic mixture corresponding to the E2 point in the phase diagram. The calculated composition of E2 was found to be in a very good agreement with that experimentally measured from the eutectic mixture (Table 3). It should be mentioned, that the authors of3 observed a ternary eutectic in Al2 O3 -rich compositional region at the temperature very similar to that calculated in our work (Table 3). The microstructure of sample #5 shows primary crystallization of the F-ZrO2 solid solution (Fig. 9e). Afterwards liquid solidifies according to the

D. Pavlyuchkov et al. / Journal of the European Ceramic Society 34 (2014) 1397–1408

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Table 3 Invariant equilibria in the ZrO2 –Al2 O3 –MgO system. Reaction

Type

Temperature (K)

Composition

Source

x(ZrO2 )

x(Al2 O3 )

x(MgO)

L + MgO + Spinel  X

Peritectic P

2199 2114

0.269 –

0.193 –

0.538 –

calc exp

L + MgO  X + F

Transition U1

2156 2112

0.389 –

0.134 –

0.477 –

calc exp

L  F + Spinel

Eutectic emax1

2148 2145 2133 2088

0.445 0.307 0.39 0.42

0.257 0.383 0.31 0.29

0.298 0.310 0.30 0.29

calc exp [3] [7]

L  F + X + Spinel

Eutectic E1

2129 2103 2113

0.399 0.32 0.413

0.187 0.31 0.170

0.414 0.37 0.420

calc exp [3]

L + T  Spinel + F

Transition U2

2117 2139

0.441 –

0.389 –

0.170 –

calc exp

L  Al2 O3 + T + Spinel

Eutectic E2

2093 2080 2103

0.383 0.35 0.41

0.534 0.535 0.42

0.083 0.115 0.17

calc exp [3]

eutectic crystallization L  F + Spinel along the monovariant curve E1 U2 having a maximum at the invariant point emax . The calculated composition of emax was found to be in a good consistency with the results presented in3,7 (Table 3). However our measurements revealed the emax composition located approximately 14 mol.% of ZrO2 below the corresponding monovariant line E1 U2 present on the calculated liquidus (Table 3 and Fig. 8a). Such disagreement is probably caused by the uncertainty of the EDX measurements and by nonequilibrium character of crystallization. Similar inconsistency between EDX measurements and calculations of the present work have been revealed for the ternary eutectic E1 observed in samples #2, 6, 7 and 8. This eutectic was crystallized in the last portion of liquid in the form of a laminar structure (see Fig. 9b, c, f and a, respectively). However the calculated composition of E1 was surprisingly found to be in a very good agreement with

that reported in Ref.3 (Table 3) where it was probably obtained at more equilibrium conditions. The ZrO2 –MgAl2 O4 isopleth calculated in the present work and that experimentally constructed by Shevchenko et al. in Ref.7 are compared in Fig. 10. The calculated diagram exhibits much more complex structure containing lager number of phase fields. According to data presented in Ref.7 the single phase region of the fluorite solid solution is stable down to 1853 K. However our calculations show that the single F-region forms above 2400 K and possesses by a much narrower compositional range of that shown in Ref.7 The thermal effects explained by the authors of Ref.7 with eutectoid decomposition F  T + Spinel at 1853 K we associate with the temperature of transition from T + Spinel to T + Spinel + F phase assemblage. The temperature of this transition is varying in the range between 1771 and 1906 K according the calculations. However the composition of

Fig. 10. The ZrO2 –MgAl2 O4 vertical sections: (a) calculated in this work along with the DTA results of samples #4 and #5, (b) experimentally determined in Ref.7 .

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the monovariant eutectic L  F + Spinel located in the corresponding isopleth is in good agreement with that shown in Ref.7 It should be noted that the onsets observed on the heating curve in DTA for samples #4 and #5 shows good agreement with the calculated ZrO2 –MgAl2 O4 isopleth.

ZrO2 -7mass. %Y2 O3 (7YSZ) and therefore creation of thermodynamic database of the ZrO2 –Y2 O3 –CaO–MgO–Al2 O3 –SiO2 system along with experimental studies is especially important for understanding and modeling of processes occurring between CMAS and TBC.

5. Conclusions

Acknowledgments

The subsolidus and melting phase relations in the ZrO2 –MgO–Al2 O3 system were investigated in the present work using equilibration technique, DTA and SEM/EDX microstructure. The obtained results were used to assess thermodynamic parameters of ternary phase X, which stability limits were established in the present work, and ternary mixing interaction parameter in the liquid phase. Therefore the thermodynamic database of the ZrO2 –MgO–Al2 O3 system was derived based on binary descriptions and parameters assessed in the present work. Isothermal sections at 1523, 1873 and 2023 K, liquidus and solidus surfaces as well as the ZrO2 –MgAl2 O4 vertical section were calculated and compared with experimental data obtained in the present work and available in Refs.3,4,7 It can be concluded that thermodynamic description reproduces experimental data within uncertainty limits. Further improvement of the derived database can be recommended: taking into account solubility of ZrO2 in spinel phase and more advanced modeling of the X-phase which would require more advanced crystallographic study including determination of site occupancies in the spinel phase dissolving ZrO2 and the X-phase. The derived thermodynamic database can be applied to model phase transformations in ceramic component of TRIP-steel matrix composite. The ceramic component is ZrO2 stabilized by 10 mol.% MgO which contains also noticeable addition of Al2 O3 . As it was shown in the present study the Al2 O3 addition influences substantially temperature and nature of phase transformations in the ZrO2 –MgO–Al2 O3 system. Beside this application the thermodynamic description obtained in the present work can be included into database for special applications. The obtained database would be an important part of the larger database including iron oxides and MnO to model chemical reactions at the interface between TRIP steel and ceramic component. The other possible application of the derived description is related with the necessity to model phase relation between calcium-magnesium alumosilicates (CMAS) and thermal barrier coatings (TBC) in gas turbines. Attacks of siliceous minerals (as dust, sand, volcanic ash and runway debris) caused erosive wear or local spallations of TBC at low temperatures and formation of glassy melts destroying TBC at high temperatures.16 Ceramic material of TBC is

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