Crack propagation and stress distribution in binary and ternary directionally solidified eutectic ceramics

https://doi.org/10.1016/j.jeurceramsoc.2010.09.016Get rights and content

Abstract

The development of directionally solidified eutectic (DSE) ceramics for gas turbine applications necessitates improving their strength and toughness. The early stage of crack propagation is investigated in either binary (Al2O3/Y3Al5O12, Al2O3/GdAlO3 and Al2O3/Er3Al5O12) or ternary (Al2O3/Y3Al5O12/ZrO2, Al2O3/GdAlO3/ZrO2 and Al2O3/Er3Al5O12/ZrO2) DSE ceramics. Post-mortem scanning electron microscopy (SEM) examination of biaxial flexure induced cracks revealed crack deflection and branching in the various phases and in the phase boundaries. These observations are correlated to analytical and finite element (FE) internal stress calculations, FE determination of the axial shear stress component in the interfaces in the vicinity of the specimen surface (free-edge effect) and FE calculations of the stress distribution resulting from an applied loading. Results from ruby (Cr3+) fluorescence piezo-spectroscopy measurements are analyzed, taking into account the hydrostatic and plane stress hypotheses. Moreover, transmission electron microscopy (TEM) examinations have confirmed the role of interfaces in the crack nucleation and propagation modes.

Introduction

The development of new ultra high temperature structural materials in the aerospace field and in particular for gas turbine applications is a real challenge. Despite the various studies performed to increase the heat-resistance of nickel-based superalloys, their use at temperatures beyond 1400 K remains difficult. For higher temperatures, sintered ceramic oxides offer many advantages compared to superalloys: resistance to oxidation and abrasion, lower density. Unfortunately, sintered ceramics are brittle and their failure strength decreases when the temperature increases. Ceramic materials prepared from oxides by unidirectional solidification from the melt (melt growth composites (MGC)) add new potentialities to the advantages of sintered ceramics: a higher strength, almost constant up to temperatures close to the melting point (no amorphous phases at the interfaces), good creep resistance, stability of the microstructure and no chemical reaction between the constituent phases.1, 2, 3 The microstructure of directionally solidified eutectic (DSE) ceramics consists in three-dimensional (3-D) and continuous interconnected networks of single-crystal eutectic phases. After solidification of binary eutectics, the eutectic phases are alumina and either an LnAlO3 perovskite phase (Ln, lanthanide element: Gd, Eu) or an Ln3Al5O12 garnet phase (Ln: Y, Yb, Er, Dy). In the case of ternary systems, zirconia is added as a third phase in order to refine the microstructure and to promote energy dispersive crack deflection modes acting in favour of a better toughness. In the present case, the directionally solidified eutectic ceramics under investigation are either binary (Al2O3/Y3Al5O12 (YAG), Al2O3/Er3Al5O12 (EAG) and Al2O3/GdAlO3 (GAP)) or ternary (Al2O3/YAG/ZrO2, Al2O3/EAG/ZrO2 and Al2O3/GAP/ZrO2) eutectics. Studies to control the microstructure of directionally solidified eutectic ceramics have been performed, acting on the processing parameters of the floating-zone method (arc image furnace).4, 5, 6, 7, 8 The mechanical properties have thus been investigated on the small specimens manufactured through this process. A biaxial testing disc flexure device9 has been used to investigate the early stage of crack propagation in the interconnected microstructure of the DSE ceramics. Post-mortem scanning electron microscopy (SEM) examination of the biaxial flexure induced cracks is focused on the possibility of crack deflection in the various phases and in the phase boundaries, a phenomenon which may markedly improve the toughness of these eutectic composites. However, the mechanical properties and the crack propagation modes depend on the level of the internal thermal stresses. In this context, internal stresses measurements have already been performed in eutectic ceramics using either X-ray10, 11 or neutron12 diffraction techniques or fluorescence piezo-spectroscopy.3, 13, 14, 15 Analytical and finite element (FE) calculations of the thermal mismatch stresses can also be performed; they however require the prior knowledge of the thermomechanical parameters of the various phases16, 17, 18, 19, 20, 21, 22, 23, 24 and of the eutectic composites.25

Consequently, complementary measurements have been performed, not only on the eutectic composites, but also on the individual garnet and perovskite phases. Even if analytical calculations had already provided a good estimate of the residual stress level in the vicinity of the interfaces in the simple configuration of infinite concentric cylinders,9 additional FE stress calculations have been performed in order to map the stress components in more complex geometrical configurations. Moreover, concerning the free-edge effect in the vicinity of the specimen surface, FE calculations have permitted to determine the axial shear stress component in the interfaces. Internal stress measurements through ruby (Cr3+) fluorescence piezo-spectroscopy have provided an average value of the stress state in the alumina phase of the various investigated DSE composites. The fluorescence signal being emitted from a small depth interaction volume, FE calculations, taking into account the presence of the neighbouring surface, have allowed comparing these experimental results with those of analytical and FE calculations representative of the bulk material. Furthermore, transmission electron microscopy (TEM) examinations were performed in order to investigate, at a finer scale, the role of the interfaces in crack nucleation and propagation. Finally, a comparative analysis of all these complementary experimental and calculated results was aimed at a better understanding of the role of the interfaces in the fracture modes and on the toughness of the DSE composites.

Section snippets

Materials

Eutectic samples are prepared from high purity powders (Al2O3: Baikowski Chimie, France; Y2O3, Er2O3, Gd2O3: Rhodia, formerly Rhône Poulenc, France; ZrO2: Th. Goldschmidt Industriechemikalien, Germany) mixed at the ratios corresponding to the eutectic compositions.4, 26, 27, 28, 29, 30 Rods, isostatically pressed at room temperature, are then sintered at 1675 K for 10 h in order to improve their handling strength. Directional solidification is performed in air using the floating-zone translation

Crack propagation modes

The essential propagation mode in the six investigated directionally solidified eutectics is transgranular crack propagation.9 Following the zig–zag propagation of a crack over a long distance in the Al2O3/YAG binary eutectic composite confirms these previous observations (Fig. 2a). This type of crack propagation does not only result from deflections of the cleavage crack inside each phase or when crossing phase boundaries, but also from crack deflection in these interfaces themselves. The

Discussion

The various crack propagation modes observed using the biaxial bending tests have to be correlated to the level of the thermal mismatch stresses, to the stress distribution resulting from the applied loading and to the nature of the interfaces between the various phases.

Conclusion

The crack propagation modes observed in directionally solidified eutectic ceramics seem to be drastically influenced by several factors: the thermal mismatch stresses, the stress concentration factors resulting from the Young's modulus ratio of the various phases and the nature of the interfaces.

Concerning the internal stresses, fluorescence piezo-spectroscopy has allowed an experimental measurement of the internal stresses in the alumina phase of the binary and ternary eutectics. The level of

Acknowledgements

The authors would like to thank M.-H. Ritti (dilatometric measurements), A. Mavel (compression tests), M. Raffestin (FEG-SEM investigation), K. Makaoui (fluorescence piezo-spectroscopy measurements), Drs. O. Lavigne, P. Beauchêne and C. Huchette for their kind cooperation and fruitful discussions.

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