Availableonlineatwww.sciencedirect.col e Science direct Science ELSEVIER Technology Aerospace Science and Technology 12(2008)499-50 www.elseviercom/locatelaescte New microstructures in ceramic materials from the melt for high temperature applications Leo Mazerolles Nicolas Piquet, Marie-France Trichet, Loic Perriere a Denis Boivin, Michel parlier c,* Centre dEnudes de Chimie Metallurgique(CNRS), 15 rue G. Urbain, F-94407 Vitry-sur-Seine cedex, france b Office National d Etudes et de Recherches Aerospatiales(ONERA). Departement Materiaux Metalliques et Procedes,France Office National d Etudes et de Recherches Aerospatiale(ONERA), Departement Materiaux et Systemes Composites, 29 avenue de la Division Leclerc, 92322 Chatillon cede Received 18 July 2006: received in revised form 1l December 2007: accepted 1l December 2007 Abstract The development of new ultra high temperature structural materials in the aerospace field and in particular for gas turbine applications is a real challenge nowadays. In fact, the use of super-alloys at temperatures beyond 1150C will be difficult despite the different studies performed in order to increase their heat-resistance. For higher temperatures, ceramic oxides offer many advantages compared to nickel-based super-alloys resistance to oxidation and abrasion, lower density. Unfortunately, sintered ceramics are brittle and their failure strength decreases when the mperature increases. Ceramic composites prepared by unidirectional solidification from the melt add new potentialities to the advantages of ntered ceramics: a higher strength almost constant up to temperatures close to the melting point(no secondary phase at the grain boundaries), good creep resistance, stability of the microstructure and no chemical reaction between the constituent phases. Synthesis at a eutectic composition usually gives rise to oriented microstructures. Recently, studies on binary eutectics between alumina and rare-earth oxides led to microstructures consisting of two entangled phases in a three-dimensional and continuous network. After solidification, the eutectic phases are alumina and either a perovskite phase LnAlO3 (Ln: Gd, Eu) or a garnet phase Ln3AlsO12(Ln: Y, Yb, Er, Dy). In the case of ternary systems, zirconia was added as a third element. Mechanical properties at room temperature were studied in relation with microstructural features. In particular, it was established that fracture toughness of termary systems is higher than that of binary systems e 2008 Elsevier Masson SAS. All rights reserved Resume Le developpement de nouveaux materiaux structuraux pour des applications a haute temperature dans le domaine de l'aerospatial et en par ticulier pour des applications moteurs est un veritable enjeu de nos jours. En effet, Utilisation de superalliages a des temperatures superieures a 1150"C est difficile, malgre les differentes etudes realisees pour augmenter leurs performances. Pour des temperatures plus levees, les cera miques oxyde frittees offrent de nombreux avantages, compares aux superalliages base nickel resistance a I'oxydation, a I'abrasion, densites plus faibles Malheureusement, les ceramiques frittees sont fragiles et leur resistance a la rupture baisse avec Augmentation de la temperature. Par contre, les ceramique oxyde prepares par solidification dirigee presentent les memes avantages que les ceramique frittees, ainsi que d'autres potentialities: une resistance a la rupture plus grande et constante jusqu'a des temperatures proches du point de fusion(absence de phase vitreuse aux joints de grains ), une bonne resistance au fluage, une stabilite de la microstructure dans le temps et pas de reactivite entre les phases. Lela- boration a des compositions eutectiques conduit generalement a des microstructures orientees. Recemment, des microstructures interconnectees ont ete obtenues sur des systemes eutectiques binaires entre Ialumine et des oxyde de te res. Apres solidification, les phases eutectiques ont Ialumine et, soit une phase perovskite LnAlO3(Ln: Gd, Eu), soit une phase grenat Ln3 Als O12(Ln: Y, Yb, Er, Dy) Pour les systemes ter- naires, une phase zircon est ajoutee comme troisieme element. Les proprietes mecaniques ont ete etudiees en correlation avec les caracteristiques microstructurales. En particulier, il a ete etabli que la tenacite des systemes ternaires est plus levee que celle des systemes binaires c 2008 Elsevier Masson SAS. All rights Eomespodrng michel. parlier@onera. fr(M. Parlier 1270-9638/S-see front matter C 2008 Elsevier Masson SAS. All rights reserved. doi:10.l016jast.200712.00
Aerospace Science and Technology 12 (2008) 499–505 www.elsevier.com/locate/aescte New microstructures in ceramic materials from the melt for high temperature applications Léo Mazerolles a , Nicolas Piquet a , Marie-France Trichet a , Loïc Perrière a,c , Denis Boivin b , Michel Parlier c,∗ a Centre d’Etudes de Chimie Métallurgique (CNRS), 15 rue G. Urbain, F-94407 Vitry-sur-Seine cedex, France b Office National d’Etudes et de Recherches Aérospatiales (ONERA), Département Matériaux Métalliques et Procédés, France c Office National d’Etudes et de Recherches Aérospatiales (ONERA), Département Matériaux et Systèmes Composites, 29 avenue de la Division Leclerc, F-92322 Châtillon cedex, France Received 18 July 2006; received in revised form 11 December 2007; accepted 11 December 2007 Available online 23 December 2007 Abstract The development of new ultra high temperature structural materials in the aerospace field and in particular for gas turbine applications is a real challenge nowadays. In fact, the use of super-alloys at temperatures beyond 1150 ◦C will be difficult despite the different studies performed in order to increase their heat-resistance. For higher temperatures, ceramic oxides offer many advantages compared to nickel-based super-alloys: resistance to oxidation and abrasion, lower density. Unfortunately, sintered ceramics are brittle and their failure strength decreases when the temperature increases. Ceramic composites prepared by unidirectional solidification from the melt add new potentialities to the advantages of sintered ceramics: a higher strength almost constant up to temperatures close to the melting point (no secondary phase at the grain boundaries), good creep resistance, stability of the microstructure and no chemical reaction between the constituent phases. Synthesis at a eutectic composition usually gives rise to oriented microstructures. Recently, studies on binary eutectics between alumina and rare-earth oxides led to microstructures consisting of two entangled phases in a three-dimensional and continuous network. After solidification, the eutectic phases are alumina and either a perovskite phase LnAlO3 (Ln: Gd, Eu) or a garnet phase Ln3Al5O12 (Ln: Y, Yb, Er, Dy). In the case of ternary systems, zirconia was added as a third element. Mechanical properties at room temperature were studied in relation with microstructural features. In particular, it was established that fracture toughness of ternary systems is higher than that of binary systems. © 2008 Elsevier Masson SAS. All rights reserved. Résumé Le développement de nouveaux matériaux structuraux pour des applications à haute température dans le domaine de l’aérospatial et en particulier pour des applications moteurs est un véritable enjeu de nos jours. En effet, l’utilisation de superalliages à des températures supérieures à 1150 ◦C est difficile, malgré les différentes études réalisées pour augmenter leurs performances. Pour des températures plus élevées, les céramiques oxydes frittées offrent de nombreux avantages, comparés aux superalliages base nickel : résistance à l’oxydation, à l’abrasion, densités plus faibles. Malheureusement, les céramiques frittées sont fragiles et leur résistance à la rupture baisse avec l’augmentation de la température. Par contre, les céramiques oxydes préparées par solidification dirigée présentent les mêmes avantages que les céramiques frittées, ainsi que d’autres potentialités : une résistance à la rupture plus grande et constante jusqu’à des températures proches du point de fusion (absence de phase vitreuse aux joints de grains), une bonne résistance au fluage, une stabilité de la microstructure dans le temps et pas de réactivité entre les phases. L’élaboration à des compositions eutectiques conduit généralement à des microstructures orientées. Récemment, des microstructures interconnectées ont été obtenues sur des systèmes eutectiques binaires entre l’alumine et des oxydes de terres rares. Après solidification, les phases eutectiques sont l’alumine et, soit une phase pérovskite LnAlO3 (Ln : Gd, Eu), soit une phase grenat Ln3Al5O12 (Ln : Y, Yb, Er, Dy) Pour les systèmes ternaires, une phase zircone est ajoutée comme troisième élément. Les propriétés mécaniques ont été étudiées en corrélation avec les caractéristiques microstructurales. En particulier, il a été établi que la ténacité des systèmes ternaires est plus élevée que celle des systèmes binaires. © 2008 Elsevier Masson SAS. All rights reserved. * Corresponding author. E-mail address: michel.parlier@onera.fr (M. Parlier). 1270-9638/$ – see front matter © 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ast.2007.12.002
Mots-cles: Ceramiques eutectiques; Alumine: Oxyde de terre rare; Interconnecte 1. Introduction 2. Experimental procedures The improvement of energy efficiency and the reduction of 2.. Crystal g/owth polluting emission such as CO2 and NOx in some fields such Under controlled conditions. solidification from the melt the use of new high-temperature structural materials exhibiting leads to materials free of porosity and with a very low amount of a tensile strength of at least a few hundreds MPa at temper. grain boundaries which are generally at the origin of brittleness atures above 1500C, in air. For example, turbine blades and in sintered ceramics. Furthermore, directional solidification of- nozzles in the future engines would be made of lightweight, ten results in highly textured materials with well-defined crys- high-strength, creep and oxidation resistant materials at temper tallographic orientation relationships between the constituent atures where metal super-alloys, even coated by zirconia, would phases [12]. U phases [12]. Unidirectional solidification of oxide systems have already melted. The ceramic matrix composites consisting eutectic composition usually leads to composites displaying an of silicon carbide fibres -or whiskers -embedded in a ceramic o ganized microstructure ranging from fibrous to lamellar, de- matrix, which were developed in the twenty last years, do not pending upon the volume fractions of the eutectic phases (2,51 remain stable at temperatures higher than 1300C in oxidiz rmary ing atmosphere. Sintered ceramic oxides could be potentially phase generally requires planar front growth conditions.Such conditions are met by using methods which allow high G/R good candidates but, up to now, the presence of grain bound- values(where G is the thermal gradient at the solid-liquid in- ries limits their use. Melt growth composites(MGC)of oxides terface and R the growth rate)[10]. These aligned fibre-type or seemed to be promising materials by coupling crystal phases lamellar microstructures give rise to a very strong anisotropic in a microstructure almost free of grain boundaries in addition behaviour of these"in situ composites which does not always to their intrinsic resistance to oxidation. However, these com- constitute an optimum for good thermomechanical properties posites being prepared from the melt, the thermal expansion Recent works have shown that other microstructures can be mismatch between phases is a major drawback for their brit- obtained consisting in continuous and isotropic networks of in- tleness. A large review of these eutectic ceramics prepared by terpenetrated eutectic phases. Each phase is a single-crystal (i.e encloses no grain boundary)and interfaces between the two et al. [7]. Concerning this class of materials, the challenge phases adopt low energy configurations. Synthesis of this class for future applications will be () to prepare, from binary or of materials as monoliths or fibres respectively in the Al2O arising from the thermal expansion mismatch between the con- recent papers (4,8, 13, 14]. The equipments that we used so far stituent phases and (ni) to maintain a high flexural strength, a for the growth of oxide-oxide eutectics are high temperature temperatures. Waku et al. [13, 14] have recently developed com- ent rods of oriented eutectics, about 8 mm in diameter. were posites with microstructures consisting of interconnected net- grown using two different techniques works of eutectic phases and exhibiting such properties. In this paper,we will present results on similar microstructures ob- (i)the floating-zone translation using an arc image furnace tained by unidirectional solidification in different Al2O3 and perating with a 6 kw xenon lamp as radiation source. Ln2O3 based systems(Ln corresponds to lanthanide elements) Solidification runs were achieved at various rates ranging In this study we report, in the case of binary eutectics, the ef- from 2 to 30 mmh[11]: fect of solidification conditions on microstructure using two (ii) the Bridgman method consisting in lowering a molybde different crystal growth methods, the crystalline homogeneity num cylindrical crucible containing the charge, through a of the as-prepared materials, and the relative orientations of the RF heated graphite susceptor. The experiments were car- constituent phases. Furthermore, contrary to grain boundaries ried out under a pressure of 10- Pa of argon at a growth in sintered ceramics. the coherent interfaces between eutec rate of 14 mmh-l tic phases give rise to a rather strong cohesion which how- ever allows some crack deflection leading to slightly improved 2.2. Microstructural and crystallographic characterization fracture toughness. In order to improve the toughness of these materials we prepared new eutectic compositions in ternary sys- Microstructural analysis was performed on sections of rods tems by addition of zirconia to the Al2O3-Ln2 O3 eutectics and parallel and perpendicular to the growth direction. These sec- e studied the influence of this addition on the resulting mi- tions were cut using a diamond saw and their surfaces were crostructure and its effect on the fracture toughness at room polished to the micron scale using diamond paste Samples after Au-Pd coating, were examined by Scanning Electron M
500 L. Mazerolles et al. / Aerospace Science and Technology 12 (2008) 499–505 Keywords: Eutectic ceramics; Alumina; Rare-earth oxide; Interconnected Mots-clés : Céramiques eutectiques ; Alumine ; Oxyde de terre rare ; Interconnecté 1. Introduction The improvement of energy efficiency and the reduction of polluting emission such as CO2 and NOx in some fields such as gas turbine and thermal power generation systems, require the use of new high-temperature structural materials exhibiting a tensile strength of at least a few hundreds MPa at temperatures above 1500 ◦C, in air. For example, turbine blades and nozzles in the future engines would be made of lightweight, high-strength, creep and oxidation resistant materials at temperatures where metal super-alloys, even coated by zirconia, would have already melted. The ceramic matrix composites consisting of silicon carbide fibres – or whiskers – embedded in a ceramic matrix, which were developed in the twenty last years, do not remain stable at temperatures higher than 1300 ◦C in oxidizing atmosphere. Sintered ceramic oxides could be potentially good candidates but, up to now, the presence of grain boundaries limits their use. Melt growth composites (MGC) of oxides seemed to be promising materials by coupling crystal phases in a microstructure almost free of grain boundaries in addition to their intrinsic resistance to oxidation. However, these composites being prepared from the melt, the thermal expansion mismatch between phases is a major drawback for their brittleness. A large review of these eutectic ceramics prepared by unidirectional solidification was recently performed by Llorca et al. [7]. Concerning this class of materials, the challenge for future applications will be (i) to prepare, from binary or ternary systems, materials displaying minimal residual stresses arising from the thermal expansion mismatch between the constituent phases and (ii) to maintain a high flexural strength, a good creep resistance and a high fracture toughness at high temperatures. Waku et al. [13,14] have recently developed composites with microstructures consisting of interconnected networks of eutectic phases and exhibiting such properties. In this paper, we will present results on similar microstructures obtained by unidirectional solidification in different Al2O3 and Ln2O3 based systems (Ln corresponds to lanthanide elements). In this study we report, in the case of binary eutectics, the effect of solidification conditions on microstructure using two different crystal growth methods, the crystalline homogeneity of the as-prepared materials, and the relative orientations of the constituent phases. Furthermore, contrary to grain boundaries in sintered ceramics, the coherent interfaces between eutectic phases give rise to a rather strong cohesion which however allows some crack deflection leading to slightly improved fracture toughness. In order to improve the toughness of these materials we prepared new eutectic compositions in ternary systems by addition of zirconia to the Al2O3–Ln2O3 eutectics and we studied the influence of this addition on the resulting microstructure and its effect on the fracture toughness at room temperature. 2. Experimental procedures 2.1. Crystal growth Under controlled conditions, solidification from the melt leads to materials free of porosity and with a very low amount of grain boundaries which are generally at the origin of brittleness in sintered ceramics. Furthermore, directional solidification often results in highly textured materials with well-defined crystallographic orientation relationships between the constituent phases [12]. Unidirectional solidification of oxide systems at eutectic composition usually leads to composites displaying an organized microstructure ranging from fibrous to lamellar, depending upon the volume fractions of the eutectic phases [2,5]. The regular alignment of eutectic structures free of primary phase generally requires planar front growth conditions. Such conditions are met by using methods which allow high G/R values (where G is the thermal gradient at the solid-liquid interface and R the growth rate) [10]. These aligned fibre-type or lamellar microstructures give rise to a very strong anisotropic behaviour of these “in situ” composites which does not always constitute an optimum for good thermomechanical properties. Recent works have shown that other microstructures can be obtained consisting in continuous and isotropic networks of interpenetrated eutectic phases. Each phase is a single-crystal (i.e. encloses no grain boundary) and interfaces between the two phases adopt low energy configurations. Synthesis of this class of materials as monoliths or fibres respectively in the Al2O3– Gd2O3 and Al2O3–Y2O3 (or Er2O3) systems was reported in recent papers [4,8,13,14]. The equipments that we used so far for the growth of oxide–oxide eutectics are high temperature single-crystal growth devices that display a high thermal gradient. Rods of oriented eutectics, about 8 mm in diameter, were grown using two different techniques: (i) the floating-zone translation using an arc image furnace operating with a 6 kW xenon lamp as radiation source. Solidification runs were achieved at various rates ranging from 2 to 30 mm h−1 [11]; (ii) the Bridgman method consisting in lowering a molybdenum cylindrical crucible containing the charge, through a RF heated graphite susceptor. The experiments were carried out under a pressure of 10−3 Pa of argon at a growth rate of 14 mm h−1. 2.2. Microstructural and crystallographic characterization Microstructural analysis was performed on sections of rods parallel and perpendicular to the growth direction. These sections were cut using a diamond saw and their surfaces were polished to the micron scale using diamond paste. Samples, after Au–Pd coating, were examined by Scanning Electron Mi-
L Mazerolles et al. / Aerospace Science and Technology 12(2008)499-505 Table 1 Unit-cell parameters of eutectic phases of the various directionally solidified composites Corundum struct hexagonal symmetry, R-3c a=0.47591nmc=1.29916nm LnAlO3 perovskite-type struct, orthorhombic symmetry, Pnma Ln=Gd GAP 0.53063 0.74452 0.52580 Ln= Eu EAP 0.52877 0.74599 0.52775 Lnz Alsol garnet-type struct. cubic symmetry. Ia3d EAG Ln= D 签了 g rographs of the transverse sections of eutectics showing the continuous three-dimensional interconnected microstructure consisting ontrast) and single-crystal lanthanide and aluminum oxide- based compounds. (a) GAP= GdAlO3, (b) EAP= EuAlO3, (c)EAG= Er3 Als O12 d) YAG= croscopy(SEM) using a Leo 1530(Leo, Germany) he other elements of the lanthanide series. LnAlO3 has a with a Princeton Gamma Tech(USA)EDX system torted perovskite structure (orthorhombic symmetry, Pnma) tered electron contrast was used for these observatio and Ln3 Al5O12 crystallizes with a garnet-type structure(cu- bution of elements and compositions of eutectic phases were bic symmetry, Ia3d). The unit-cell parameters of eutectic determined by EDX microanalysis. The structural analysis of phases of various grown composites were determined from were achieved from X-Ray powder patterns recorded with a SEM images of the microstructures corresponding to cross- PW 1830 Philips diffractometer(using the A=0. 17889 nm Ka sections perpendicular to the solidification direction are shown radiation of cobalt). Growth directions and local relative ori- in Fig. 1. In every case, we observe continuous networks entations of phases were established from Transmission Elec- of two single-crystal phases: Al2O3( dark contrast)and a tron Microscopy(TEM) observations carried out on a Jeol lanthanide and aluminum oxide compound(white contrast) 2000EX microscope(operating at 200 kV). Thinned foils of (Fig. 1:(a)GAP= GdAlO3, (b)EAP= EuAlO3, (c)EAG transverse sections were prepared by mechanical dimpling and Er3AlsO12, (d) YAG= Y3AlsO12). These micrographs were ion-milling. Electron backscattered diffraction(EBSD) spec- obtained from eutectics prepared by the floating-zone method troscopy was used to map the crystallographic orientations dis- at a 20 mmh- growth rate. Similar microstructures were ob- tribution and to determine the crystalline homogeneity of spec- tained by the Bridgman method by using slower solidification imens. EBSD patterns were collected from a Zeiss DSM 960 rates(typically 5 mmh-) because of a lower thermal gra- equipped with a tungsten filament and a TSL analysis system. dient inherent to the technique. The same morphology was A step size of 0.5 um was generally used. Measurements of observed on sections parallel to the growth direction reveal the ratio of the crack length to the Vickers indent size provided ing the three-dimensional configuration of the microstructure estimates of fracture toughness using the calibration curve de- in these systems veloped by Marshall and Evans 3] The two phases interpenetrate without grain boundaries, pores or colonies. These complex microstructures are consistent 3. Results with other observations reported in literature[7, 13, 14]. The do- main mean size of each phase (length of the shortest axis)varies 3. 1. Binary systems very little with the different Al2O3-Ln2O3 eutectics(Fig. 1a, b, c)except in the case of the Al2O3-YAG eutectic that displays The eutectic composites were prepared from the Al2O3- a faceted larger microstructure(Fig. Id). For all the compos- Ln2O3 systems. The phase diagrams of these systems all dis- ites, the microstructure size does not change on a large central composition at a temperature close to 1800c part of rods when solidification rates are lower than 20 mm h the rich side. Depending on the added rare-earth This domain size decreases towards the edge of the rod when ases consist in an Al2O3 phase associated specimens are prepared using the arc image furnace technique. to either LnAlO3(Ln= Sm, Eu, and Gd) or Ln3AlsO12 for This decrease is directly related to the heating method that in
L. Mazerolles et al. / Aerospace Science and Technology 12 (2008) 499–505 501 Table 1 Unit-cell parameters of eutectic phases of the various directionally solidified composites Al2O3 Corundum struct. hexagonal symmetry, R − 3c a = 0.47591 nm c = 1.29916 nm LnAlO3 perovskite-type struct., orthorhombic symmetry, P nma a (nm) b (nm) c (nm) Ln = Gd GAP 0.53063 0.74452 0.52580 Ln = Eu EAP 0.52877 0.74599 0.52775 Ln3Al5O12 garnet-type struct. cubic symmetry, I a3d a (nm) Ln = Y YAG 1.20065 Ln = Er EAG 1.19867 Ln = Dy DAG 1.20594 Fig. 1. Back-scattered SEM micrographs of the transverse sections of eutectics showing the continuous three-dimensional interconnected microstructure consisting of Al2O3 (dark contrast) and single-crystal lanthanide and aluminum oxide-based compounds. (a) GAP = GdAlO3, (b) EAP = EuAlO3, (c) EAG = Er3Al5O12, (d) YAG = Y3Al5O12. croscopy (SEM) using a Leo 1530 (Leo, Germany) equipped with a Princeton Gamma Tech (USA) EDX system. Backscattered electron contrast was used for these observations. Distribution of elements and compositions of eutectic phases were determined by EDX microanalysis. The structural analysis of the different phases and determination of the cell parameters were achieved from X-Ray powder patterns recorded with a PW 1830 Philips diffractometer (using the λ = 0.17889 nm Kα radiation of cobalt). Growth directions and local relative orientations of phases were established from Transmission Electron Microscopy (TEM) observations carried out on a Jeol 2000EX microscope (operating at 200 kV). Thinned foils of transverse sections were prepared by mechanical dimpling and ion-milling. Electron backscattered diffraction (EBSD) spectroscopy was used to map the crystallographic orientations distribution and to determine the crystalline homogeneity of specimens. EBSD patterns were collected from a Zeiss DSM 960 equipped with a tungsten filament and a TSL analysis system. A step size of 0.5 µm was generally used. Measurements of the ratio of the crack length to the Vickers indent size provided estimates of fracture toughness using the calibration curve developed by Marshall and Evans [3]. 3. Results 3.1. Binary systems The eutectic composites were prepared from the Al2O3– Ln2O3 systems. The phase diagrams of these systems all display a eutectic composition at a temperature close to 1800 ◦C on the alumina rich side. Depending on the added rare-earth oxide, eutectic phases consist in an Al2O3 phase associated to either LnAlO3 (Ln = Sm, Eu, and Gd) or Ln3Al5O12 for the other elements of the lanthanide series. LnAlO3 has a distorted perovskite structure (orthorhombic symmetry, Pnma) and Ln3Al5O12 crystallizes with a garnet-type structure (cubic symmetry, I a3d). The unit-cell parameters of eutectic phases of various grown composites were determined from X-Ray diffraction diagrams and are reported in Table 1. SEM images of the microstructures corresponding to crosssections perpendicular to the solidification direction are shown in Fig. 1. In every case, we observe continuous networks of two single-crystal phases: Al2O3 (dark contrast) and a lanthanide and aluminum oxide compound (white contrast) (Fig. 1: (a) GAP = GdAlO3, (b) EAP = EuAlO3, (c) EAG = Er3Al5O12, (d) YAG = Y3Al5O12). These micrographs were obtained from eutectics prepared by the floating-zone method at a 20 mm h−1 growth rate. Similar microstructures were obtained by the Bridgman method by using slower solidification rates (typically 5 mm h−1) because of a lower thermal gradient inherent to the technique. The same morphology was observed on sections parallel to the growth direction revealing the three-dimensional configuration of the microstructure in these systems. The two phases interpenetrate without grain boundaries, pores or colonies. These complex microstructures are consistent with other observations reported in literature [7,13,14]. The domain mean size of each phase (length of the shortest axis) varies very little with the different Al2O3–Ln2O3 eutectics (Fig. 1a, b, c) except in the case of the Al2O3–YAG eutectic that displays a faceted larger microstructure (Fig. 1d). For all the composites, the microstructure size does not change on a large central part of rods when solidification rates are lower than 20 mm h−1. This domain size decreases towards the edge of the rod when specimens are prepared using the arc image furnace technique. This decrease is directly related to the heating method that in-
L Mazerolles et al Aerospace Science and Technology 12 (2008)499-50 Table 2 Growth directions and orientation relationships of the directionally solidified composites Eutectic phases Al2 O3-LnAF Al2Oz-LnAG 0101Al2O3 directions ∥ D11] LnAF (0001)Al2O3 relationships r(100) LnAP m These results were obtained from selected Area electron Fig. 2. Colony structure observed in the Al2O3-YAG composite at a solidifica- Diffraction( SAED) patterns corresponding to analyzed regions tion rate of 30 mmh-I of some micrometers. In order to control the single-crystal homogeneity of larger areas, the EBSD technique was used 2110 ig. 4, shows the Pole Figures maps and Inverse Pole Figures (IPF) for the Al2O3-YAG eutectic. Each colour corresponds to one crystallographic direction as indicated in the reference stereographic triangle shown in inset. For example, crystals with their (111)axis normal to the surface of the sample will 100 0003 be blue, and so on. The nearly perfect alignment of (1010)and (110)crystallographic orientations with the ND direction(cen- tre of the stereo projection), normal to the observed surface and consequently parallel to the growth direction, is in good agree ment with the results reported in Table 2 The orientation maps(150 x 36 um) presented in Fig. 4 reveal the sample texture and the nearly single crystal homo- geneity of the sample. A unique colour corresponds to the YAG phase exhibiting one growth direction and the absence of grain boundary. Two different colours are visible for alumina. They tion pattern obtained on a platelet perpendicular to the correspond to the same(1010) orientation revealing two twin- [0110] growth direction of a Al03-GAP eutectic related variants of Al2O3 because the [1010] and [0110] direc- tions are not strictly equivalent and cannot be distinguished by duces a radial temperature gradient higher than the RF heating electron diffraction. Comparison of the Al2O3 and YAG pole sed in the Bridgman method. Above the 20 mm h-I solidifi- figures with electron diffraction patterns shows similar crystal cation rate, a colony microstructure begins to appear as shown lographic relationships(Table 2) Electron diffraction studies were performed on thin plates 3.2. Ternary systems cut perpendicularly to the rod axis. They reveal that growth di- rections of these eutectics follow well-defined crystallographic As shown in the previous section, microstructure of com- axes. The orientations are usually unique. Two preferred ori- posites, in different oxide binary systems, can be controlled by entations were determined for Al2O3, depending only on the unidirectional solidification. A high flexural strength, thermal used solidification process: [011o] when specimens are grown stability and creep resistance of some of these binary eutec- by the floating zone method and [1123] when the Bridgman tics have been reported in the literature. However, the quality method is employed. The electron diffraction pattern in Fig 3 of interfaces involving strong bonding between phases is not was performed on a platelet perpendicular to the [olio] growth the best property for displaying a high fracture toughness. In direction of a Al2O3-GAP eutectic. The selected area is cen- order to increase the toughness of these materials by addition ed on the interface and diffraction spots of both phases are of a third phase(ZrO2)we investigated eutectic compositio superimposed on the same pattern. Crystallographic principal in the Al2 O3/Zro2/Ln2O3 ternary phase diagrams leading directions are strictly aligned according to the following epi- the association of the alumina and zirconia phases with garnet taxial relations or perovskite phases. Such compositions are only known for the AlzO3/ZrO2/YAG eutectic. After solidification, the phases [0110]Al2O3//[011] Gd AlO identified by X-ray diffraction, are an alumina phase, a per- ovskite(or garnet) phase and a Ln-containing fully-stabilized 0001)Al2O3//(211) Gd AlO3 cubic zirconia phase. The starting eutectic compositions, the The growth directions and orientation relationships are gathered in Table 2 for the as-prepared eutectics I For colours see the web version of this article
502 L. Mazerolles et al. / Aerospace Science and Technology 12 (2008) 499–505 Fig. 2. Colony structure observed in the Al2O3–YAG composite at a solidification rate of 30 mm h−1. Fig. 3. Electron diffraction pattern obtained on a platelet perpendicular to the [0110¯ ] growth direction of a Al2O3–GAP eutectic. duces a radial temperature gradient higher than the RF heating used in the Bridgman method. Above the 20 mm h−1 solidifi- cation rate, a colony microstructure begins to appear as shown in Fig. 2. Electron diffraction studies were performed on thin plates cut perpendicularly to the rod axis. They reveal that growth directions of these eutectics follow well-defined crystallographic axes. The orientations are usually unique. Two preferred orientations were determined for Al2O3, depending only on the used solidification process: [0110¯ ] when specimens are grown by the floating zone method and [1123¯ ] when the Bridgman method is employed. The electron diffraction pattern in Fig. 3 was performed on a platelet perpendicular to the [0110¯ ] growth direction of a Al2O3–GAP eutectic. The selected area is centred on the interface and diffraction spots of both phases are superimposed on the same pattern. Crystallographic principal directions are strictly aligned according to the following epitaxial relations: [0110¯ ] Al2O3 // [011¯] Gd AlO3 (0001) Al2O3 // (21¯1¯) Gd AlO3 The growth directions and orientation relationships are gathered in Table 2 for the as-prepared eutectics. Table 2 Growth directions and orientation relationships of the directionally solidified composites Eutectic phases Al2O3–LnAP Al2O3–LnAG Growth directions [1010¯ ] Al2O3 // [011¯] LnAP [1010¯ ] Al2O3 // [110] LnAG Orientation relationships (0001) Al2O3 // (211) or (100) LnAP (0001) Al2O3 // (112¯ ) LnAG These results were obtained from Selected Area Electron Diffraction (SAED) patterns corresponding to analyzed regions of some micrometers. In order to control the single-crystal homogeneity of larger areas, the EBSD technique was used. Fig. 4, shows the Pole Figures maps and Inverse Pole Figures (IPF) for the Al2O3–YAG eutectic. Each colour corresponds to one crystallographic direction as indicated in the reference stereographic triangle shown in inset.1 For example, crystals with their 111 axis normal to the surface of the sample will be blue, and so on. The nearly perfect alignment of 1010¯ and 110 crystallographic orientations with the ND direction (centre of the stereo projection), normal to the observed surface and consequently parallel to the growth direction, is in good agreement with the results reported in Table 2. The orientation maps (150 × 36 µm) presented in Fig. 4 reveal the sample texture and the nearly single crystal homogeneity of the sample. A unique colour corresponds to the YAG phase exhibiting one growth direction and the absence of grain boundary. Two different colours are visible for alumina. They correspond to the same {1010¯ } orientation revealing two twinrelated variants of Al2O3 because the [1010¯ ] and [0110¯ ] directions are not strictly equivalent and cannot be distinguished by electron diffraction. Comparison of the Al2O3 and YAG pole figures with electron diffraction patterns shows similar crystallographic relationships (Table 2). 3.2. Ternary systems As shown in the previous section, microstructure of composites, in different oxide binary systems, can be controlled by unidirectional solidification. A high flexural strength, thermal stability and creep resistance of some of these binary eutectics have been reported in the literature. However, the quality of interfaces involving strong bonding between phases is not the best property for displaying a high fracture toughness. In order to increase the toughness of these materials by addition of a third phase (ZrO2) we investigated eutectic compositions in the Al2O3/ZrO2/Ln2O3 ternary phase diagrams leading to the association of the alumina and zirconia phases with garnet or perovskite phases. Such compositions are only known for the Al2O3/ZrO2/YAG eutectic. After solidification, the phases, identified by X-ray diffraction, are an alumina phase, a perovskite (or garnet) phase and a Ln-containing fully-stabilized cubic zirconia phase. The starting eutectic compositions, the 1 For colours see the web version of this article
L Mazerolles et al. /Aerospace Science and Technology 12(2008)499-505 YAG 0001 Fig 4. Inverse Pole Figures maps of Al2 O3 and YAG phases(top) and Pole Figures of 1010Al203, 0001 A203, 110YAG and 112YAG orientations(bottom). Table 3 can see that morphology of the garnet-type phase is modified Eutectic compositions, unit-cell parameters of the constituent phases and com- depending on the rare-earth oxide added to alumina and zir position of the zirconia phase conia(indicated by arrows). At the same solidification rate. 65Al2O3-19Zn02-16Y2O3(mol%) large facets are developed for the ternary eutectic with Ln=Er, Aho YAG at the contrary of eutectics containing yttrium. Secondly, the a=1.2005nm (15.5 mol% Y203) Al2O3/Y2O3/ZrO2 eutectic displays curved smooth interfaces c=1.2997 rather than planar sharp interfaces observed in the AlzO3-YAG 659Al2O3-186ZO2-15.5Er2O3(mol%) eutectic. Zirconia phase always grows at the interface between Al2O3 and YAG. In the case of Ln= Er, cubic zirconia disper a=1.1983nm (15.3 mol Er2 03) soids are observed, not only at the interfaces, but also in the C=1.2999nr alumina phase. A similar microstructure was observed for yt- 8Al2O3-23 ZrO2-19 Gd O3(mol%) trium based eutectics prepared by the Bridgman technique at Al203 (16 mol% Gd203) higher solidification rates. Electron diffraction patterns(Fig. 6) a=0.53022nm b=0.74418nr performed at the interfaces of the eutectic phases reveal the epi- c=0.52541nm taxial relationships, between AlzO3 and ZrO2(a) and, between Al2O3 and Y3AlsO12(b). We determined the following growth directions unit-cell parameters of the constituent phases and the compo- sition of zirconia phase are reported in Table 3. The amount of 001]YAG//[001ZrO2 Ln(orY)2O3 as a solid solution in ZrOz was determined from coupled with relative orientations: crystal parameter using a Vegard law [15] Typical microstructures of transverse sections for some of (0001)Al2O3//(100)ZrO2 these ternary eutectics are shown in Fig. 5. First of all, we and (0001)Al2O3//(100) YAG ψm 5业m Fig. 5. Back-scattered SEM micrographs of the transverse sections of ternary eutectics containing AlO3(dark contrast), YAG(a), EAG(b)and GAP(c)and zirconia(indicated by arrows)
L. Mazerolles et al. / Aerospace Science and Technology 12 (2008) 499–505 503 Fig. 4. Inverse Pole Figures maps of Al2O3 and YAG phases (top) and Pole Figures of 1010¯ Al2O3, 0001Al2O3, 110YAG and 112YAG orientations (bottom). Table 3 Eutectic compositions, unit-cell parameters of the constituent phases and composition of the zirconia phase 65 Al2O3–19 ZrO2–16 Y2O3 (mol%) Al2O3 YAG ZrO2 a = 0.4758 nm a = 1.2005 nm (15.5 mol% Y2O3) c = 1.2997 nm a = 0.5165 nm 65.9 Al2O3–18.6 ZrO2–15.5 Er2O3 (mol%) Al2O3 EAG ZrO2 a = 0.4759 nm a = 1.1983 nm (15.3 mol%Er2O3) c = 1.2999 nm a = 0.5159 nm 58 Al2O3–23 ZrO2–19 Gd2O3 (mol%) Al2O3 GAP ZrO2 a = 0.4759 nm a = 0.53022 nm (16 mol% Gd2O3) c = 1.2993 nm b = 0.74418 nm a = 0.5200 nm c = 0.52541 nm unit-cell parameters of the constituent phases and the composition of zirconia phase are reported in Table 3. The amount of Ln(orY)2O3 as a solid solution in ZrO2 was determined from crystal parameter using a Vegard law [15]. Typical microstructures of transverse sections for some of these ternary eutectics are shown in Fig. 5. First of all, we can see that morphology of the garnet-type phase is modified depending on the rare-earth oxide added to alumina and zirconia (indicated by arrows). At the same solidification rate, large facets are developed for the ternary eutectic with Ln = Er, at the contrary of eutectics containing yttrium. Secondly, the Al2O3/Y2O3/ZrO2 eutectic displays curved smooth interfaces rather than planar sharp interfaces observed in the Al2O3–YAG eutectic. Zirconia phase always grows at the interface between Al2O3 and YAG. In the case of Ln = Er, cubic zirconia dispersoids are observed, not only at the interfaces, but also in the alumina phase. A similar microstructure was observed for yttrium based eutectics prepared by the Bridgman technique at higher solidification rates. Electron diffraction patterns (Fig. 6) performed at the interfaces of the eutectic phases reveal the epitaxial relationships, between Al2O3 and ZrO2 (a) and, between Al2O3 and Y3Al5O12 (b). We determined the following growth directions: [0110¯ ] Al2O3 // [001] YAG // [001] ZrO2 coupled with relative orientations: (0001) Al2O3 // (100) ZrO2 and (0001) Al2O3 // (100) YAG Fig. 5. Back-scattered SEM micrographs of the transverse sections of ternary eutectics containing Al2O3 (dark contrast), YAG (a), EAG (b) and GAP (c) and zirconia (indicated by arrows)
Table 4 Hardness (Vickers indenter) and fracture toughness of the investigated eutec- tics. The symbols L and // correspond to sections perpendicular and parallel to the growth direction System Al2O3-YAG(⊥) Al2O3-EAG(1) 2110 Al2O3-ZrO2-YAG (1) Fig6 Electron diffraction patterns at the Al2O3-YAG (left)and Al2O3-ZrO2 Al2O3-GAP (L) Al2O3-7rO2-GAP(⊥) 1010 0001 Because the eutectic samples are not large enough for con- ventional fracture toughness testing the vickers indentation technique was employed. Fracture toughness was calculated from the equation proposed by Evans for median radial cracks Although the obtained values are an approximation of the"true fracture toughness, they can be considered for comparison be tween similar ceramic eutectics tested with the same method These measurements, gathered in Table 4, led to the following features binary eutectics display a fracture toughness higher than Al O3 and close to known values for Al2O3-ZrO2 com- posites [6] the lower the mean size of the microstructure, the higher the KIc value(Al2O3-YAG< Al2O3-EAG< Al2O3-GAP); YAG ZrO,(Y,O,) addition of zirconia induces a noticeable toughening effect This effect is more important for the systems associatin Fig 7 Pole Figures corresponding to the three phases in the Al2O3-YAG-ZrO alumina with a garnet type structure. The estimated value of Kic increases from 6 to 10 MPa m/2 The orientation relationships between alumina and cubic zirco- Kic values measured perpendicularly or parallel to growth nia are strictly similar to those determined in previous papers directions are very similar, which is in good agreement with the isotropy of the microstructure; they do not seem to de- for the Al2O3-ZrO2 binary eutectic [9]. Crystallographic ori entations of the three phases are verified for large areas of the pend on the crystallographic orientation specimen as shown on IPF images which reveal unique god lows increasing the fracture toughness Similar Kic values have directions on 2x 2 mm2 zones and relative orientations in These results clearly show that the addition of zirconia al agreement with electron diffraction studies(Fig. 7) been measured by other authors on rapidly solidified eutec tics [1]. In the case of Al2O3-YAG the increase of toughnes 3.3. Mechanical properties is more significant. It could be attributed to both effects: re- finement of the microstructure in the ternary eutectic and also dispersion of zirconia at the interfaces giving rise to residual As previously reported, addition of zirconia to binary e- stresses higher than for the binary eutectic. Such stresses at the tectics modifies the solidification conditions and consequently interfaces involve crack deflection mechanisms around zirconia the final microstructure of the composite. However, the single- particles as shown in Fig. 8. That hypothesis has been con crystal homogeneity of the material is not changed. The orien- firmed by recent measurements of residual stresses from Raman tation relationships between zirconia and the two other eutectic spectroscopy on binary and ternary We investigated the infuence of that addition on hardness 4. Conclusion and fracture toughness of these materials at room temperature Hardness values measured with a Vickers indenter are reported Interconnected microstructures were obtained from oxide- in Table 4. The I and// symbols correspond to measurements based binary and ternary systems using two different growth carried out on sections perpendicular and parallel to the growth methods. Whatever the used method, morphology of m direction. The mean value of hardness varies very little with the crostructure does not change. The floating zone method al considered oxide systems lows growing composites at solidification rates higher than
504 L. Mazerolles et al. / Aerospace Science and Technology 12 (2008) 499–505 Fig. 6. Electron diffraction patterns at the Al2O3–YAG (left) and Al2O3–ZrO2 (right) interfaces. Fig. 7. Pole Figures corresponding to the three phases in the Al2O3–YAG–ZrO2 eutectic. The orientation relationships between alumina and cubic zirconia are strictly similar to those determined in previous papers for the Al2O3–ZrO2 binary eutectic [9]. Crystallographic orientations of the three phases are verified for large areas of the specimen as shown on IPF images which reveal unique growth directions on 2×2 mm2 zones and relative orientations in good agreement with electron diffraction studies (Fig. 7). 3.3. Mechanical properties As previously reported, addition of zirconia to binary eutectics modifies the solidification conditions and consequently the final microstructure of the composite. However, the singlecrystal homogeneity of the material is not changed. The orientation relationships between zirconia and the two other eutectic phases involve low-energy interfaces. We investigated the influence of that addition on hardness and fracture toughness of these materials at room temperature. Hardness values measured with a Vickers indenter are reported in Table 4. The ⊥ and // symbols correspond to measurements carried out on sections perpendicular and parallel to the growth direction. The mean value of hardness varies very little with the considered oxide systems. Table 4 Hardness (Vickers indenter) and fracture toughness of the investigated eutectics. The symbols ⊥ and // correspond to sections perpendicular and parallel to the growth direction System HV (GPa) KIC (MPa m1/2) Al2O3–YAG (⊥) 21.8 5.2 Al2O3–EAG (⊥) 16.9 6.2 Al2O3–ZrO2–YAG (⊥) 18.4 9.8 Al2O3–ZrO2–YAG (//) 19.8 8.9 Al2O3–ZrO2–EAG (⊥) 18 10 Al2O3–GAP (⊥) 15.9 7.5 Al2O3–ZrO2–GAP (⊥) 17.9 8.5 Because the eutectic samples are not large enough for conventional fracture toughness testing, the Vickers indentation technique was employed. Fracture toughness was calculated from the equation proposed by Evans for median radial cracks. Although the obtained values are an approximation of the “true” fracture toughness, they can be considered for comparison between similar ceramic eutectics tested with the same method. These measurements, gathered in Table 4, led to the following features: – binary eutectics display a fracture toughness higher than Al2O3 and close to known values for Al2O3–ZrO2 composites [6]; – the lower the mean size of the microstructure, the higher the KIC value (Al2O3–YAG < Al2O3–EAG < Al2O3–GAP); – addition of zirconia induces a noticeable toughening effect. This effect is more important for the systems associating alumina with a garnet type structure. The estimated value of KIC increases from 6 to 10 MPa m1/2; – KIC values measured perpendicularly or parallel to growth directions are very similar, which is in good agreement with the isotropy of the microstructure; they do not seem to depend on the crystallographic orientation. These results clearly show that the addition of zirconia allows increasing the fracture toughness. Similar KIC values have been measured by other authors on rapidly solidified eutectics [1]. In the case of Al2O3–YAG the increase of toughness is more significant. It could be attributed to both effects: re- finement of the microstructure in the ternary eutectic and also dispersion of zirconia at the interfaces giving rise to residual stresses higher than for the binary eutectic. Such stresses at the interfaces involve crack deflection mechanisms around zirconia particles as shown in Fig. 8. That hypothesis has been con- firmed by recent measurements of residual stresses from Raman spectroscopy on binary and ternary eutectics. 4. Conclusion Interconnected microstructures were obtained from oxidebased binary and ternary systems using two different growth methods. Whatever the used method, morphology of microstructures does not change. The floating zone method allows growing composites at solidification rates higher than
L Mazerolles et al. /Aerospace Science and Technology 12(2008)499-505 YAG [2] DJ.S. Cooksay, D Munson, M P. Wilkinson, A H. Hellawell, Freezing of ome continuous binary eutectic mixtures, Phil. Mag. 10(1964)745-769. zrO 3]AG. Evans, E.A. Charles, Fractures toughness determination by indenta- tion,. Am. Ceram Soc. 59(1976)371-372. YAG [4] C.S. Frazer, E.C. Dickey, A Sayir, Crystallographic texture and orienta- tion variants in Al2O3-Y3AlsO12 directionally solidified eutectic cry tals, J. Crystal Growth 233(2001)187-19% [5] J.D. Hunt, K.A. Jackson, Binary eutectic solidification, Trans. AIME 236 (1966)843-852 Al2 O3 [6] FF Lange, Transformation toughening, part 4, J Mater. Sci. 17(1982) Il 247-254 [7 J. Llorca, V.M. Orera, Directionally solidified eutectic ceramic oxides, Fig 8. Al2O3-YAG-ZrO eutectic- TEM (left) and SEM(right) images of Prog. Mater. Sci. 51(2006)711-809 deflection of cracks at the interfaces between zirconia particles and the other [8]J. Martinez Fernandez, A Sayir, S.C. Farmer, High temperature creep de- formation of directionally solidified Al2O3-Er3AlsO12, Acta Mater. 51 (2003)1705-1720. the Bridgman method, due to a higher thermal gradient; how [91L Mazerolles, D. Michel, R Portier, Interfaces in oriented AlO3-ZrO ever, the microstructure displays a lower homogeneity in the (Y2O3)eutectics, J Am Ceram Soc. 69(1986)252-255 external zones of the as-prepared rods. Preferred growth di- [10] E.R. Mollard, M C Flemings, Growth of composites from the melt,Trans AIME239(1967)1526-1546 rections and crystallographic orientation relationships between [11] A. Revcolevschi, Are image fumace for X-ray diffraction studies to onstituent phases were revealed for all the investigated eutec- 3000C and high temperature crystal growth, Rev. Int. Hautes Temp. Re- tics. The EBSD technique allowed showing the single-crystal fract.7(1970)73-90. feature of grown composites. New ternary eutectic compo- 12)A. Revcolevschi G. Dalhenne, D. Michel, Interfaces in direct solidified oxide-oxide eutectics. in: External and Intermal Interf sitions were experimentally determined corresponding to the Metal Oxides, Materials Science Forum, Trans. Tech. Publications, 1988 addition of zirconia to binary eutectic compositions. This addi- tion led to a significant increase of fracture toughness at room [13. Waku, N Nakagawa, T. Wakamoto, H Ohtsubo, K Shimizu, Y oku, A ductile ceramic eutectic composite with high strength at 1873 K, Nature389(1997)49-52 [14] Y. Waku, N. Nakagawa, T. Wakamoto, H. Ohtsubo, K. Shimizu,YKo- References cu, High temperature strength and stability of unidirectionally solidi fied Al,O3/YAG eutectic composite, J Mater. Sci. 33 (1998)1217-1225 [1] J.M. Calderon-Moreno, M. Yoshimura, Al203-Y3AlsO12(YAG)-ZrO [15] M. Yashima, N. Ishisawa, M. Yoshimura, Application to an ion-packing ternary composite rapidly solidified from the eutectic melt, J. Eur. Ceram. on defects clusters to zirconia solid solutions: Il Applicability of Vegards's Soc.25(2005)1365-1368. law,J.Am. Ceram Soc.75(1992)1550-1557
L. Mazerolles et al. / Aerospace Science and Technology 12 (2008) 499–505 505 Fig. 8. Al2O3–YAG–ZrO2 eutectic – TEM (left) and SEM (right) images of deflection of cracks at the interfaces between zirconia particles and the other two phases. the Bridgman method, due to a higher thermal gradient; however, the microstructure displays a lower homogeneity in the external zones of the as-prepared rods. Preferred growth directions and crystallographic orientation relationships between constituent phases were revealed for all the investigated eutectics. The EBSD technique allowed showing the single-crystal feature of grown composites. New ternary eutectic compositions were experimentally determined corresponding to the addition of zirconia to binary eutectic compositions. This addition led to a significant increase of fracture toughness at room temperature. References [1] J.M. Calderon-Moreno, M. Yoshimura, Al2O3–Y3Al5O12 (YAG)–ZrO2 ternary composite rapidly solidified from the eutectic melt, J. Eur. Ceram. Soc. 25 (2005) 1365–1368. [2] D.J.S. Cooksay, D. Munson, M.P. Wilkinson, A.H. Hellawell, Freezing of some continuous binary eutectic mixtures, Phil. Mag. 10 (1964) 745–769. [3] A.G. Evans, E.A. Charles, Fractures toughness determination by indentation, J. Am. Ceram. Soc. 59 (1976) 371–372. [4] C.S. Frazer, E.C. Dickey, A. Sayir, Crystallographic texture and orientation variants in Al2O3–Y3Al5O12 directionally solidified eutectic crystals, J. Crystal Growth 233 (2001) 187–195. [5] J.D. Hunt, K.A. Jackson, Binary eutectic solidification, Trans. AIME 236 (1966) 843–852. [6] F.F. Lange, Transformation toughening, part 4, J. Mater. Sci. 17 (1982) 247–254. [7] J. Llorca, V.M. Orera, Directionally solidified eutectic ceramic oxides, Prog. Mater. Sci. 51 (2006) 711–809. [8] J. Martinez Fernandez, A. Sayir, S.C. Farmer, High temperature creep deformation of directionally solidified Al2O3–Er3Al5O12, Acta Mater. 51 (2003) 1705–1720. [9] L. Mazerolles, D. Michel, R. Portier, Interfaces in oriented Al2O3–ZrO2 (Y2O3) eutectics, J. Am. Ceram. Soc. 69 (1986) 252–255. [10] F.R. Mollard, M.C. Flemings, Growth of composites from the melt, Trans. AIME 239 (1967) 1526–1546. [11] A. Revcolevschi, Arc image furnace for X-ray diffraction studies to 3000 ◦C and high temperature crystal growth, Rev. Int. Hautes Temp. Refract. 7 (1970) 73–90. [12] A. Revcolevschi, G. Dalhenne, D. Michel, Interfaces in directionally solidified oxide–oxide eutectics, in: External and Internal Interfaces of Metal Oxides, Materials Science Forum, Trans. Tech. Publications, 1988, pp. 173–197. [13] Y. Waku, N. Nakagawa, T. Wakamoto, H. Ohtsubo, K. Shimizu, Y. Kohtoku, A ductile ceramic eutectic composite with high strength at 1873 K, Nature 389 (1997) 49–52. [14] Y. Waku, N. Nakagawa, T. Wakamoto, H. Ohtsubo, K. Shimizu, Y. Kohtoku, High temperature strength and stability of unidirectionally solidi- fied Al2O3/YAG eutectic composite, J. Mater. Sci. 33 (1998) 1217–1225. [15] M. Yashima, N. Ishisawa, M. Yoshimura, Application to an ion-packing on defects clusters to zirconia solid solutions: II Applicability of Vegards’s law, J. Am. Ceram. Soc. 75 (1992) 1550–1557