CERAMICS INTERNATIONAL ELSEVIER Ceramics International 25(1999)395-408 Review: High temperature deformation of Al2O3-based ceramic particle or whisker composites Q. Tai*, A. Mocellin LSG2M, UMR 7584, Ecole des Mines, Parc de saurupt, F-54042 Nancy Cedex, france Received 6 September 1997; accepted 17 November 1997 Abstract The major theoretical models for creep and the creep rate equations of ceramic materials and their dispersed phase composites e briefly reviewed. Then the literature on high temperature deformation behaviours of Al2O3-based oxide ceramic particle com- osites(Al2O3-ZrO2, Al2O3-Y3Al5O12, Al2O3-Tio2) and Al2O3-based non-oxide ceramic particle or whisker composites(Al2O3- SiC(w), Al2O3-SiC(p). Al2O3-TiCx Ni-x)since the mid 1980s is reviewed. Most studies have been concerned with the Al2O3-ZrO2 and Al2O3-SiC systems. The influences of various factors on the creep behaviours, the changes of the microstructure in the deformed specimens and the creep mechanisms of these composites are summarised and analysed. c 1999 Elsevier Science Limited and Techna S.r. l. all rights reserved 1. Introduction and reliabilities. Great emphasis is placed on their high temperature creep behaviours. In recent years, structural ceramic materials have The purpose of this paper is to recall briefly the major attracted much attention, because of their excellent theoretical models for creep and then to review the mechanical properties such as high strength, hardness, available information on the plastic deformation beha- anti-abrasion, chemical stability and heat resistance. viours of Al2O3-based ceramic composites. Composites There has been a recognition of the potential of struc- reinforced by long fibres are not discussed here since tural ceramics for use both in high temperature appli- their fabrication procedures markedly differ from those cations in advanced heat engine and heat exchangers based on powder processing which yield materials with and in ambient temperature applications in cutting dispersed phases tools, and wear parts. The disadvantage of ceramics is their low fracture toughness and poor mechanical relia- bility which so far have limited their practical applica 2. Deformation mechanisms s. me o Improve toughness and retain their high-temperature creep 2. 1. The rate equations for plastic deformation properties as well as enhance their mechanical reliability are a major challenge. One way to achieve these goals is The high temperature creep of single phase crystalline through the development of composite structures. That materials may be expressed by a relationship of the fol is to the ceramic matrices are added dispersed ceramic lowing form: particles, whiskers or fibres which reinforce the matrices and improve their mechanical properties Alumina-based ceramic composites such as Al_O3- E=A Zr02, Al O3-Y3AlsO12, Al2O3-SiC, Al,O3-TiC, Al,O TiCNI-x composites are widely studied, as to their where E is the steady state creep rate, A is a dimension ambient and high temperature mechanical properties less constant, D is the appropriate diffusion coefficient, G is the shear modulus, b is the magnitude of the Bur esponding author at Nanjing University of Chemical Tech- gers vector, k is Boltzmanns constant, Tis the absolute nology, 210009, Nanjing, People's Republic of China. temperature, d is the grain size, o is the applied stress, 0272-8842/99/$20.00@ 1999 Elsevier Science Limited and Techna S.r. L. All rights reserved PII:S0272-8842(98)00017-0
Review: High temperature deformation of Al2O3-based ceramic particle or whisker composites Q. Tai *, A. Mocellin LSG2M, UMR 7584, Ecole des Mines, Parc de Saurupt, F-54042 Nancy Cedex, France Received 6 September 1997; accepted 17 November 1997 Abstract The major theoretical models for creep and the creep rate equations of ceramic materials and their dispersed phase composites are brie¯y reviewed. Then the literature on high temperature deformation behaviours of Al2O3-based oxide ceramic particle composites (Al2O3-ZrO2, Al2O3-Y3Al5O12, Al2O3-Tio2) and Al2O3-based non-oxide ceramic particle or whisker composites (Al2O3- SiC(w), Al2O3-SiC(p), Al2O3-TiCxN1-x) since the mid 1980s is reviewed. Most studies have been concerned with the Al2O3-ZrO2 and Al2O3-SiC systems. The in¯uences of various factors on the creep behaviours, the changes of the microstructure in the deformed specimens and the creep mechanisms of these composites are summarised and analysed. # 1999 Elsevier Science Limited and Techna S.r.l. All rights reserved. 1. Introduction In recent years, structural ceramic materials have attracted much attention, because of their excellent mechanical properties such as high strength, hardness, anti-abrasion, chemical stability and heat resistance. There has been a recognition of the potential of structural ceramics for use both in high temperature applications in advanced heat engine and heat exchangers and in ambient temperature applications in cutting tools, and wear parts. The disadvantage of ceramics is their low fracture toughness and poor mechanical reliability which so far have limited their practical applications. Thus, methods to improve their fracture toughness and retain their high-temperature creep properties as well as enhance their mechanical reliability are a major challenge. One way to achieve these goals is through the development of composite structures. That is to the ceramic matrices are added dispersed ceramic particles, whiskers or ®bres which reinforce the matrices and improve their mechanical properties. Alumina-based ceramic composites such as Al2O3- Zr02, Al2O3-Y3Al5O12, Al2O3-SiC, Al2O3-TiC, Al2O3- TiCxN1-x composites are widely studied, as to their ambient and high temperature mechanical properties and reliabilities. Great emphasis is placed on their high temperature creep behaviours. The purpose of this paper is to recall brie¯y the major theoretical models for creep and then to review the available information on the plastic deformation behaviours of Al2O3-based ceramic composites. Composites reinforced by long ®bres are not discussed here since their fabrication procedures markedly dier from those based on powder processing which yield materials with dispersed phases. 2. Deformation mechanisms 2.1. The rate equations for plastic deformation The high temperature creep of single phase crystalline materials may be expressed by a relationship of the following form: "_ A DGb kT b d p G n 1 where "_ is the steady state creep rate, A is a dimensionless constant, D is the appropriate diusion coecient, G is the shear modulus, b is the magnitude of the Burgers vector, k is Boltzmann's constant, T is the absolute temperature, d is the grain size, is the applied stress, Ceramics International 25 (1999) 395±408 0272-8842/99/$20.00 #1999 Elsevier Science Limited and Techna S.r.l. All rights reserved PII: S0272-8842(98)00017-0 * Corresponding author at Nanjing University of Chemical Technology, 210009, Nanjing, People's Republic of China
Q. Tai. A. Mocellin/Ceramics International 25(1999)393-408 and p and n are constants termed the inverse grain size and exponent and the stress exponent, respectively. The dif- fusion coefficient D may be expressed as Do exp(-Q/ RD, where Do is a frequency factor, Q is the apparent PI=P2 PI activation energy, and R is the gas constant Vi is the volume fraction of phase i; ni is the viscosity For two phase composites, there are several equations undergoing Newtonian viscous flow which may express or predict their high temperature n= Vinl +v2772: qi is the phase'stress-concentration reep behaviours factor, Vig1+ V292=1; pi is internal stress caused by the In composites, where the second phase can be mismatch in creep strains between the phases sidered rigid, Raj and Ashby model [1] assumes that the VIPI+V2P2=0. hard second phase particles in the grain boundary of The Eqn. (5)is also valid for the case wherein one of matrix limit the grain boundary sliding and gives he phases is nondeformable by creep if diffusional mass transport around the purely elastic phase is taken into account R 2. 2. Theoretical models for plastic deformation where V is the second phase volume content, r is second phase grain radius, q and n are phenomenological There are several theoretical models for creep defor exponents and C is a constant mation. In general, they can be divided into two broad Chen model [2] considers the composites as a model categories: boundary mechanisms [5-ll and lattice system of a soft matrix containing equiaxed and rigid mechanisms [5, 12]. Boundary mechanisms rely on the inclusions. Based on a phenomenological constitutive presence of grain boundaries and occur only in poly- equation and a second phase continuum mechanics crystalline materials. They are associated with some model, his model gives: dependence on grain size so that p> l. Lattice mechan- isms are independent of the presence of grain bound E=(1-V)2+n2 () aries and occur both in single crystal and polycrystalline materials. They occur within the grain interiors and are where V is the second phase volume content, Eo is the independent of grain size, so p=0 strain rate of the reference matrix, n is the stress expo The boundary mechanisms can be subdivided into nent of the matrix four categories: diffusion creep, [5-7] interface reaction Ravichandran and Seetharaman model [3] considers controlled diffusion creep [8], grain boundary sliding that a rigid and noncreeping second phase distributes and grain rearrangement [5,6,9, 101, and cavitation creep uniformly in a continuous creeping matrix, and they and microcracking [11]. In diffusion creep where vacan- develop a simple continuum mechanics model to predict cies may flow from the zones experiencing tension to the steady state creep rates of composites those in compression either through the crystalline lat tice(Nabarro-Herring creep) or along the grain (1+C2 boundaries( Coble creep), the individual grains become (1+C) (4) elongated along the tensile axis Fig. 1). When grain +(1+C boundaries do not act as perfect sources or sinks for vacancies, the process of creating or annihilating point where C=l-I, v is the second phase volume content, n is the stress exponent of the matrix, A is the constant of the matrix For a two phase composite in which each phase undergoes diffusional creep, Wakashima and Liu give a viscoelastic constitutive equation corresponding to SUND/AR pring- dashpot model [4: A1-p(-分) where △EV1 Fig. 1. Diffusion flow by lattice(Nabarro-Herring creep)or by grain boundaries( Coble creep)
and p and n are constants termed the inverse grain size exponent and the stress exponent, respectively. The diffusion coecient D may be expressed as Do exp (ÿQ/ RT), where Do is a frequency factor, Q is the apparent activation energy, and R is the gas constant. For two phase composites, there are several equations which may express or predict their high temperature creep behaviours. In composites, where the second phase can be considered rigid, Raj and Ashby model [1] assumes that the hard second phase particles in the grain boundary of matrix limit the grain boundary sliding and gives: "_ C n dprqV exp ÿ Q RT 2 where V is the second phase volume content, r is second phase grain radius, q and n are phenomenological exponents and C is a constant. Chen model [2] considers the composites as a model system of a soft matrix containing equiaxed and rigid inclusions. Based on a phenomenological constitutive equation and a second phase continuum mechanics model, his model gives: "_ "_o 1 ÿ V 2n=2 3 where V is the second phase volume content, "_o is the strain rate of the reference matrix, n is the stress exponent of the matrix. Ravichandran and Seetharaman model [3] considers that a rigid and noncreeping second phase distributes uniformly in a continuous creeping matrix, and they develop a simple continuum mechanics model to predict the steady state creep rates of composites: "_ A 1 C 2 1C 1=n C 1 C 2 ÿ 1 2 4 3 5 n 4 where C 1 V 1=3 ÿ1, V is the second phase volume content, n is the stress exponent of the matrix, A is the constant of the matrix. For a two phase composite in which each phase undergoes diusional creep, Wakashima and Liu give a viscoelastic constitutive equation corresponding to a spring-dashpot model [4]: " E Eu 1 ÿ exp ÿ t n o 1 5 where E Eu V1 p1 q1 ÿ 1 2 V2 p2 q2 ÿ 2 u 2 and 1 p1 ÿ p2 1 p1 1 ÿ V1 1 ÿ 2 p2 1 ÿ V2 2 Vi is the volume fraction of phase i; i is the viscosity of phase i undergoing Newtonian viscous ¯ow, V11 V22; qi is the phase `stress-concentration' factor, V1q1+V2q2=1; pi is internal stress caused by the mismatch in creep strains between the phases, V1p1 V2p2 0. The Eqn. (5) is also valid for the case wherein one of the phases is nondeformable by creep if diusional mass transport around the purely elastic phase is taken into account. 2.2. Theoretical models for plastic deformation There are several theoretical models for creep deformation. In general, they can be divided into two broad categories: boundary mechanisms [5±11] and lattice mechanisms [5,12]. Boundary mechanisms rely on the presence of grain boundaries and occur only in polycrystalline materials. They are associated with some dependence on grain size so that p51. Lattice mechanisms are independent of the presence of grain boundaries and occur both in single crystal and polycrystalline materials. They occur within the grain interiors and are independent of grain size, so p=0. The boundary mechanisms can be subdivided into four categories: diusion creep, [5±7] interface reaction controlled diusion creep [8], grain boundary sliding and grain rearrangement [5,6,9,10], and cavitation creep and microcracking [11]. In diusion creep where vacancies may ¯ow from the zones experiencing tension to those in compression either through the crystalline lattice (Nabarro±Herring creep) or along the grain boundaries (Coble creep), the individual grains become elongated along the tensile axis Fig. 1). When grain boundaries do not act as perfect sources or sinks for vacancies, the process of creating or annihilating point Fig. 1. Diusion ¯ow by lattice (Nabarro±Herring creep) or by grain boundaries (Coble creep). 396 Q. Tai. A. Mocellin / Ceramics International 25 (1999) 395±408
Q. Tai. A. Mocellin/Ceramics International 25(1999)395-408 defects may control the creep deformation. This is also been proposed by a number of authors but are not termed interface reaction-controlled diffusion creep. to be reviewed here Because this process involves diffusion of vacancies, the The lattice mechanisms can be briefly subdivided into grains are also elongated along the tensile axis In grain two categories [12]: dislocation climb and glide con- boundary sliding and grain rearrangement, there are trolled by climb on the one hand and dislocation climb several different models(Lifshitz sliding, Rachinger on the other(Fig 4). For the dislocation climb and glide sliding, Ashby and Verrall model, Gifkins model, etc. ) controlled by climb mechanism, in ceramics, the anion Lifshitz sliding occurs naturally as part of diffusion cation ratio ra/rc is < 2, and the ceramics must have five creep, to maintain grain contiguity, the grains elongate independent slip systems. While for the dislocation along the tensile axis and maintain their adjacent climb from Bardeen-Herring sources, the ra/rc ratio is neighbours, so there is no increase in the number of >2, and the ceramics are either lacking some slip sys grains lying along the tensile axis. In contrast, during tems, or if five independent systems are available, not all Rachinger sliding, the grains slide, rearrange and retain may be active simultaneously [12]. The dislocation creep there is an increase in number of grains along the tensile tensile axis as weave the elongation of grains along the their original shapes, but exchange their neighbours, so mechanisms involve axis. In Ashby and Verrall model, which is two-dimen sional, during the deformation process, grains suffer a 2.3. Methods to identify deformation mechanism transient but complex shape change by diffusional transport Fig. 2). While in the Gifkins model, which In general, several mechanisms may contribute to the may be viewed as three-dimensional, during the defor- creep deformation at elevated temperature, but creep is mation process, grains move apart by grain boundary usually controlled by only one of these mechanisms. To sliding caused by the motion of grain boundary dis- identify which mechanism is dominant, there are several locations, resulting in a gap between the grains. When methods the gap is large enough, it is filled by an emerging grain Ashby [13]constructed deformation-mechanism maps from one layer to the next(Fig. 3)[10]. In these two by using rate-equations and sufficient data on the models, the grains almost retain their original shapes, materials. These maps for example show the fields of and there is an increase in number of grains along the stress and temperature in which each independent tensile axis. In the cavitation creep and microcracking, mechanism for plastic deformation is dominant. Knowl- extensive cavities form. They grow and link up forming edge of any two of the three variables(stress, temperature microcrack by grain boundary sliding. The principal and strain-rate) locates a point on the map, identifies mode of deformation is a damage mechanism. Varia- the dominant mechanism or mechanisms and gives the tions or refinements of the previous basic models have value of the third variable. Mohamed and Langdon [14] constructed deformation-mechanism maps with grain size as a variable. based on the deformation -mechanism maps constructed by Ashby and Mohamed et al., Heuer et al. [9], for example, have devised a stress-grain size deformation mechanism map for MgO-doped Al2O3 at 1500C(Fig. 5). They suggested that diffusional defor mation dominated for most grain sizes of interest, but Fig. 2. Grain boundary sliding and grain rearrangement by diffusion shby and verrall model)[10] 双 Fig 3. Grain boundary sliding and grain rearrangement. A gap forms between four grains and is filled by an emerging grain(Gifkins model) Fig 4. Dislocations move by (a)climb, (b)climb and glide
defects may control the creep deformation. This is termed interface reaction-controlled diusion creep. Because this process involves diusion of vacancies, the grains are also elongated along the tensile axis. In grain boundary sliding and grain rearrangement, there are several dierent models (Lifshitz sliding, Rachinger sliding, Ashby and Verrall model, Gifkins model, etc.). Lifshitz sliding occurs naturally as part of diusion creep, to maintain grain contiguity, the grains elongate along the tensile axis and maintain their adjacent neighbours, so there is no increase in the number of grains lying along the tensile axis. In contrast, during Rachinger sliding, the grains slide, rearrange and retain their original shapes, but exchange their neighbours, so there is an increase in number of grains along the tensile axis. In Ashby and Verrall model, which is two-dimensional, during the deformation process, grains suer a transient but complex shape change by diusional transport Fig. 2). While in the Gifkins model, which may be viewed as three-dimensional, during the deformation process, grains move apart by grain boundary sliding caused by the motion of grain boundary dislocations, resulting in a gap between the grains. When the gap is large enough, it is ®lled by an emerging grain from one layer to the next (Fig. 3) [10]. In these two models, the grains almost retain their original shapes, and there is an increase in number of grains along the tensile axis. In the cavitation creep and microcracking, extensive cavities form. They grow and link up forming microcrack by grain boundary sliding. The principal mode of deformation is a damage mechanism. Variations or re®nements of the previous basic models have also been proposed by a number of authors but are not to be reviewed here. The lattice mechanisms can be brie¯y subdivided into two categories [12]: dislocation climb and glide controlled by climb on the one hand and dislocation climb on the other (Fig. 4). For the dislocation climb and glide controlled by climb mechanism, in ceramics, the anion/ cation ratio ra/rc is<2, and the ceramics must have ®ve independent slip systems. While for the dislocation climb from Bardeen±Herring sources, the ra/rc ratio is >2, and the ceramics are either lacking some slip systems, or if ®ve independent systems are available, not all may be active simultaneously [12]. The dislocation creep mechanisms involve the elongation of grains along the tensile axis as well. 2.3. Methods to identify deformation mechanism In general, several mechanisms may contribute to the creep deformation at elevated temperature, but creep is usually controlled by only one of these mechanisms. To identify which mechanism is dominant, there are several methods. Ashby [13] constructed deformation-mechanism maps by using rate-equations and sucient data on the materials. These maps for example show the ®elds of stress and temperature in which each independent mechanism for plastic deformation is dominant. Knowledge of any two of the three variables (stress, temperature and strain-rate) locates a point on the map, identi®es the dominant mechanism or mechanisms and gives the value of the third variable. Mohamed and Langdon [14] constructed deformation-mechanism maps with grain size as a variable. Based on the deformation-mechanism maps constructed by Ashby and Mohamed et al., Heuer et al. [9], for example, have devised a stress-grain size deformation mechanism map for MgO-doped Al2O3 at 1500C (Fig. 5). They suggested that diusional deformation dominated for most grain sizes of interest, but Fig. 2. Grain boundary sliding and grain rearrangement by diusion (Ashby and Verrall model) [10]. Fig. 3. Grain boundary sliding and grain rearrangement. A gap forms between four grains and is ®lled by an emerging grain (Gifkins model) [10]. Fig. 4. Dislocations move by (a) climb, (b) climb and glide. Q. Tai. A. Mocellin / Ceramics International 25 (1999) 395±408 397
some basal slip could easily occur. Furthermore, when composite theory: isostrain and isostress model [15,16 grain size of Al_O3 was between 50 and 500 um and the (Fig. 6). Isostrain and isostress prediction diagrams can imposed stress was large enough, then dislocation climb be constructed by calculation. By comparing the and glide could occur. But in practice, the theory and experimental data with the isostrain and isostress pre- the experimental data used to construct the maps are diction, the model fitting the creep deformation can be poor or insufficient for many ceramic materials, thus determined. Since the isostress model is dominated by limiting their use in applications he least creep resistant phase and highest creep rate and At present, the main method to identify the possible the isostrain model is dominated by the most creep ate controlling mechanism is to compare the values of resistant phase and lowest creep rate, the phase con np and o obtained from experiments with theoretical trolling the creep behaviour can be determined predictions. Chokshi et al. [12 indicated that many For the two phase composites in which both phases ceramics exhibit stress exponents of N5, N3 or l deform inelastically, a self-consistent model was devel which appeared to be associated with dislocation glide oped [17, 18], which predicts the deformation behaviour and climb, climb from Bardeen-Herring sources, and of the composites when the viscoplastic laws of diffusion creep, respectively. The stress exponent of 2 phase are known. By self-consistent calculations, effec- might be due to the presence of a partially wetting grain tive strain rate sensitivity parameter and effective pre- boundary glassy phase or to control by an interface factor which are characteristic of the composite reaction[ 12] Besides, the stress exponent between I and behaviour can be obtained. Stress and strain rates in might be associated with grain boundary sliding and each phase are also attainable. From the comparison grain rearrangement, and a higher stress exponent between the model and the experiments, the possible (n>3)can also be associated with cavitation creep and deformation mechanisms of each phase can be deter microcracking. The inverse grain size exponent points to mined and the phase controlling the creep behaviour either a boundary mechanism (p>1)or a lattice can also be qualitatively determined mechanism (p=0). In general, a direct observation and analysis of the microstructure of the specimens after deformation is necessary to check preliminary conclu- 3. Plastic deformation behaviours of Al2O3-based sions drawn from the experimentally determined stress- ceramic composites train rate relationship For two phase composites, especially for those with In studies of the plastic deformation behaviours of duplex microstructures, authors are not only interested Al2O3-based ceramic composites, great attention is in the dominant mechanism of deformation, but also try concentrated on three aspects: strain rates, micro- to find out which phase controls the creep behaviour. structural changes and deformation mechanisms Creep of composites can be modelled by using standard As concerns the first of these items one of the main aims is to investigate the relationship between the creep rates and operating variables(imposed stress, grain size, temperature)and to evaluate the creep parameters(n, P, Q). The deformation behaviours are critically dependent Nabarro Climb, Dp DIffusional Crae :88: 3. MOCELILIOEmGEAY STRESS, MPa Fig. 6. Idealized composite microstructures:(a) isostress and(b)iso- Fig. 5. Deformation map for MgO-doped Al2O3 at 1500C [9]. strain orientations
some basal slip could easily occur. Furthermore, when grain size of Al2O3 was between 50 and 500 mm and the imposed stress was large enough, then dislocation climb and glide could occur. But in practice, the theory and the experimental data used to construct the maps are poor or insucient for many ceramic materials, thus limiting their use in applications. At present, the main method to identify the possible rate controlling mechanism is to compare the values of n,p and Q obtained from experiments with theoretical predictions. Chokshi et al. [12] indicated that many ceramics exhibit stress exponents of 5, 3 or 1, which appeared to be associated with dislocation glide and climb, climb from Bardeen±Herring sources, and diusion creep, respectively. The stress exponent of 2 might be due to the presence of a partially wetting grain boundary glassy phase or to control by an interface reaction. [12] Besides, the stress exponent between 1 and 3 might be associated with grain boundary sliding and grain rearrangement, and a higher stress exponent (n53) can also be associated with cavitation creep and microcracking. The inverse grain size exponent points to either a boundary mechanism (p51) or a lattice mechanism (p=0). In general, a direct observation and analysis of the microstructure of the specimens after deformation is necessary to check preliminary conclusions drawn from the experimentally determined stress± strain rate relationships. For two phase composites, especially for those with duplex microstructures, authors are not only interested in the dominant mechanism of deformation, but also try to ®nd out which phase controls the creep behaviour. Creep of composites can be modelled by using standard composite theory: isostrain and isostress model [15,16] (Fig. 6). Isostrain and isostress prediction diagrams can be constructed by calculation. By comparing the experimental data with the isostrain and isostress prediction, the model ®tting the creep deformation can be determined. Since the isostress model is dominated by the least creep resistant phase and highest creep rate and the isostrain model is dominated by the most creep resistant phase and lowest creep rate, the phase controlling the creep behaviour can be determined. For the two phase composites in which both phases deform inelastically, a self-consistent model was developed [17,18], which predicts the deformation behaviour of the composites when the viscoplastic laws of each phase are known. By self-consistent calculations, eective strain rate sensitivity parameter and eective prefactor which are characteristic of the composite behaviour can be obtained. Stress and strain rates in each phase are also attainable. From the comparison between the model and the experiments, the possible deformation mechanisms of each phase can be determined and the phase controlling the creep behaviour can also be qualitatively determined. 3. Plastic deformation behaviours of Al2O3-based ceramic composites In studies of the plastic deformation behaviours of Al2O3-based ceramic composites, great attention is concentrated on three aspects: strain rates, microstructural changes and deformation mechanisms. As concerns the ®rst of these items, one of the main aims is to investigate the relationship between the creep rates and operating variables (imposed stress, grain size, temperature) and to evaluate the creep parameters (n, p, Q). The deformation behaviours are critically dependent Fig. 5. Deformation map for MgO-doped Al2O3 at 1500C [9]. Fig. 6. Idealized composite microstructures: (a) isostress and (b) isostrain orientations. 398 Q. Tai. A. Mocellin / Ceramics International 25 (1999) 395±408
on the physical and chemical properties of reinforcing grain growth or cavitation. Fridez [21] has reported particles or whiskers, their content, morphologies and that creep deformation accelerated grain growth. The distributions, and microstructures of composites change in the crystallographic texture of alumina was including grain sizes and shapes, pores, grain bound observed at higher stress range. [21, 23] The main defor aries, interfaces, as well as stress and temperature. Dur- mation mechanisms were diffusional creep, grain ing deformation, there often occur microstructural boundary sliding and sometimes basal slip. The diffu changes such as grain growth, changes of grain shape sional creep can become inter rface-controlled at low (grain elongation), texture development, formation of stresses, causing non-Newtonian creep behaviour. The ntermediate or intergranular phase, dislocation activity, cavitation was often caused by unaccommodated GBS vacancy nucleation and evolution, cavitation and evo- or basal slip and the basal slip can give rise to a defor ution, formation and development of microcracks, etc. mation texture creep deformation is an essential aspect which li The study of such microstructural changes accompar In Al2O3-based ceramic particle or whisker compo- sites, the creep behaviours of Al2O3-ZrO2 and Al_O3 ortant foundation for analysing creep deformation SiC composites have been most extensively investigated behaviours and creep mechanisms of composites In the following the creep behaviours of Al2O3-based we will first recall briefly the high oxide ceramic composites and Al2O3-based nonoxide temperature deformation behaviours of fine-grained ceramic composites will be discussed. The correspond- alumina and then review the high temperature defor- ing experimental data produced during the past 10 years mation behaviours of Al2O3-based ceramic composites. or so in both families of materials are summarised in High temperature deformation behaviours of fine- Tables I and 2, respectively grained alumina were widely investigated. [5-9][18-23] Most authors have shown that n=1-2, P=2-3, and 3. 1. Al2O3-based oxide ceramic particle composites 0=430-500 KJ mol- can represent deformation data at lower stress range. The stress exponent generally 3. 1.1. A1203-ZrOz composites decreased with increasing grain size. In some studies. Since Wakai et al. reported that a 3 mol% yttria stabi- non-steady state deformation has been reported due to lized zirconia exhibited superplasticity [24, Al2O3-ZrO Table l Deformation behaviours of Al2O3-based oxide ceramic particle composites Reference Additive Content Test Atm. a ranges e ranges T(C) Grain size (um) n p Q (vol%) type (MPa) (S-) AlO3 additiv KJ mol-) Wakai et al Zro 72.7C 10=4-10-31400-1500 Kellett et a ZrO,20Cair30-10010-4-10-21500 0.72.1 Wakai et al.(1988)[27 14.3Tair10-10010-8-10-51250-1450100.51.7-2.1 Wakai et al. (1988)[28 ZrO,727Tair10-14010-7-10-51250-14500.50.52.1 590-600 Nieh et al.(1989)[29] ZrO,72.7 r vacuun54010-5-10-21450-1650 ~0.5b Wakai et al.(1989)[301 ZrO, Tair2-10010-7-10-31250-14500.60.521-24-720-780 ZrO230.8Tair2-10010-7-10-31250-14501.00.619-24-680-740 ZrO314.3Tair2-10010-7-10-31250-14501.0 0.5172.1 540-760 Wang et al. (1991)B31 ZrO 5 C argon5-20010-5-10-41400-150028 3630 HfO25 C argon5-20010-510-41400-150027 5 C argon5-6010--10-41400-15003.8 Owen et al. (1994)[32] 7 T 4-10010-8-10-31327-14770.4 0.42.82.1 French et al. (1994)[15] TZZY 50Tair35-7510-810-61200-1350 air35-7510-810-71210-1350 Calderon-Mereno et al. (1995)[33] ZrO2 5.5C 0-15010-8-10-41300-145047 14 Calderon-Mereno et al. (1995)[33 ZrO2 5.5Cair10-15010-810-41300145026<1 Chevalier et al. (1997)[34 ZrO, 10B Bair50-20010-810-6120014002 0.5 760 Calderon-Mereno(1997)[35] 40Cair45-8510-510-41400-15002.31.61.7 Calderon-Mereno(1997)[35] Zro 649510-5-10 Calderon-Mereno(1997)[35] Zro 10Cair70-12310-5-10-415002 Flacher et al.(1997)[36 ZrO,5-20Cair20-13010-5-10-41300-14000.2 Clarisse et al. (1997)[3 ZrO, 0Cair4-20010-6-10-31275-14000.640.551-2 640-705 Clarisse et al. (1997)[37 ZrO2380Cair420010-7-10-31275-14001.120.801-2 Clarisse et al. (1997)[37] ZrO2380Cair420010-7-10-41275-14001 0.711-2 663-715 Duong et al. ( 1993)[16 YAG50Cair3-2010-8-10-51400-150010 612 YAG 3-2010-8-10-51400-15008 592 compression, T=tension, B=bendin Average grain
on the physical and chemical properties of reinforcing particles or whiskers, their content, morphologies and distributions, and microstructures of composites including grain sizes and shapes, pores, grain boundaries, interfaces, as well as stress and temperature. During deformation, there often occur microstructural changes such as grain growth, changes of grain shape (grain elongation), texture development, formation of intermediate or intergranular phase, dislocation activity, vacancy nucleation and evolution, cavitation and evolution, formation and development of microcracks, etc. The study of such microstructural changes accompanying creep deformation is an essential aspect which is an important foundation for analysing creep deformation behaviours and creep mechanisms of composites. In this chapter, we will ®rst recall brie¯y the high temperature deformation behaviours of ®ne-grained alumina and then review the high temperature deformation behaviours of Al2O3-based ceramic composites. High temperature deformation behaviours of ®negrained alumina were widely investigated. [5±9] [18±23] Most authors have shown that n=1±2, p=2±3, and Q=430±500 KJ molÿ1 can represent deformation data at lower stress range. The stress exponent generally decreased with increasing grain size. In some studies, non-steady state deformation has been reported due to grain growth or cavitation. Fridez [21] has reported that creep deformation accelerated grain growth. The change in the crystallographic texture of alumina was observed at higher stress range. [21,23] The main deformation mechanisms were diusional creep, grain boundary sliding and sometimes basal slip. The diusional creep can become interface-controlled at low stresses, causing non-Newtonian creep behaviour. The cavitation was often caused by unaccommodated GBS or basal slip and the basal slip can give rise to a deformation texture. In Al2O3-based ceramic particle or whisker composites, the creep behaviours of Al2O3-ZrO2 and Al2O3- SiC composites have been most extensively investigated. In the following the creep behaviours of Al2O3-based oxide ceramic composites and Al2O3-based nonoxide ceramic composites will be discussed. The corresponding experimental data produced during the past 10 years or so in both families of materials are summarised in Tables 1 and 2, respectively. 3.1. Al2O3-based oxide ceramic particle composites 3.1.1. Al2O3-ZrO2 composites Since Wakai et al. reported that a 3 mol% yttria stabilized zirconia exhibited superplasticity [24], Al2O3-ZrO2 Table 1 Deformation behaviours of Al2O3-based oxide ceramic particle composites Reference Additive Content (vol%) Test type a Atm. ranges (MPa) ranges (Sÿ1 ) T ( C) Grain size (m) Al2O3 additive np Q (KJ molÿ1 ) Wakai et al. (1986) [25] ZrO2 72.7 C air ± 10ÿ4 ±10ÿ3 1400±1500 ± ± 1.2±2.0 ± 620 Kellett et al. (1986) [26] ZrO2 20 C air 30±100 10ÿ4 ±10ÿ2 1500 1.1 0.7 2.1 ± ± Wakai et al. (1988) [27] ZrO2 14.3 T air 10±100 10ÿ8 ±10ÿ5 1250±1450 1.0 0.5 1.7±2.1 ± 750 Wakai et al. (1988) [28] ZrO2 72.7 T air 10±140 10ÿ7 ±10ÿ5 1250±1450 0.5 0.5 2.1 ± 590±600 Nieh et al. (1989) [29] ZrO2 72.7 T vacuum 5±40 10ÿ5 ±10ÿ2 1450±1650 0.5b 2± ± Wakai et al. (1989) [30] ZrO2 50 T air 2±100 10ÿ7 ±10ÿ3 1250±1450 0.6 0.5 2.1±2.4 ± 720±780 ZrO2 30.8 T air 2±100 10ÿ7 ±10ÿ3 1250±1450 1.0 0.6 1.9±2.4 ± 680±740 ZrO2 14.3 T air 2±100 10ÿ7 ±10ÿ3 1250±1450 1.0 0.5 1.7±2.1 ± 640±760 Wang et al. (1991) [31] ZrO2 5 C argon 5±200 10ÿ5 ±10ÿ4 1400±1500 2.8 ± ± 3 630 HfO2 5 C argon 5±200 10ÿ5 ±10ÿ4 1400±1500 2.7 ± ± 3 685 TiO2 5 C argon 5±60 10ÿ5 ±10ÿ4 1400±1500 3.8 ± ± 2 570 Owen et al. (1994) [32] ZrO2 72.7 T air 4±100 10ÿ8 ±10ÿ3 1327±1477 0.4 0.4 2.8 2.1 585 French et al. (1994) [15] ZrO2 50 T air 35±75 10ÿ8 ±10ÿ6 1200±1350 2.3 1.8 ± 633 YAG 50 T air 35±75 10ÿ8 ±10ÿ7 1210±1350 2.0 2.6 ± 695 Calderon-Mereno et al. (1995) [33] ZrO2 5.5 C air 10±150 10ÿ8 ±10ÿ4 1300±1450 4.7 ± 1.4 ± 580 Calderon-Mereno et al. (1995) [33] ZrO2 5.5 C air 10±150 10ÿ8 ±10ÿ4 1300±1450 2.6 <1 1.8 ± 540 Chevalier et al. (1997) [34] ZrO2 10 B air 50±200 10ÿ8 ±10ÿ6 1200±1400 2 0.5 2.5 ± 760 Calderon-Mereno (1997) [35] ZrO2 40 C air 45±85 10ÿ5 ±10ÿ4 1400±1500 2.3 1.6 1.7 ± ± Calderon-Mereno (1997) [35] ZrO2 20 C air 64±95 10ÿ5 ±10ÿ4 1500 2.3 1.6 1.4 ± ± Calderon-Mereno (1997) [35] ZrO2 10 C air 70±123 10ÿ5 ±10ÿ4 1500 2.3 1.6 1.2 ± ± Flacher et al. (1997) [36] ZrO2 5±20 C air 20±130 10ÿ5 ±10ÿ4 1300±1400 0.2Ä 2 ± 650 Clarisse et al. (1997) [37] ZrO2 80 C air 4±200 10ÿ6 ±10ÿ3 1275±1400 0.64 0.55 1±2 ± 640±705 Clarisse et al. (1997) [37] ZrO2 80 C air 4±200 10ÿ7 ±10ÿ3 1275±1400 1.12 0.80 1±2 ± 642±723 Clarisse et al. (1997) [37] ZrO2 80 C air 4±200 10ÿ7 ±10ÿ4 1275±1400 1.40 0.71 1±2 ± 663±715 Duong et al. (1993) [16] YAG 50 C air 3±20 10ÿ8 ±10ÿ5 1400±1500 10 3 1.1 ± 612 YAG 75 C air 3±20 10ÿ8 ±10ÿ5 1400±1500 8 3 1.1 ± 592 a C=compression, T=tension, B=bending. b Average grain size. Q. Tai. A. Mocellin / Ceramics International 25 (1999) 395±408 399