《复合材料 Composites》课程教学资源（学习资料）第五章陶瓷基复合材料_Al2O3-SiC蠕变-19.pdf

Acta mater. Vo blished by Elsevier Science Ltd. Al served Printed PI!:Sl359-6454(98)00331-0 1359-6454/98s19.00+0.00 MICROSTRUCTURAL CONSTRAINTS FOR CREEP IN SiC-WHISKER-REINFORCED Al,O3 A R. DE ARELLANO-LOPEZ DOMINGUEZ-RODRIGUEZ and L L ROUTBORTA Departamento de Fisica de la Materia Condensada, Universidad de Sevilla, 41080 Seville, Spain and Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439.4838, U.S.A Received 17 July 1998; accepted 12 September 1998) a tract-New and published creep data obtained on a Sic-whisker-reinforced Al2O3 composite have been yzed in terms of an effective grain size and a threshold /critical stress. These concepts allow the for- mation of a consistent picture of the high-temperature deformation of these composites. For low volume actions of whiskers, before the formation of a point-contact percolative limit is reached, defo ceeds via grain-boundary sliding after the applied stress exceeds a temperature-dependent threshold stress. In this regime, the nominal grain size is the most important microstructural feature. For larger volume actions of whiskers, up to itical volume fraction for formation of facet-to-facet whiskers nhibit grain-boundary sliding and deformation proceeds by means of pure diffusion. In this regime, the most important microstructural feature is an effective grain size, i. e the spacing between the whiskers. De- formation proceeds until the stress reaches a temperature-dependent critical stress. At this point, damage ccurs by unaccommodated grain-boundary sliding and creep is no longer in a steady state. C 1998 Act Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved 1 INTRODUCTION Successful fabrication of SiCw/ Al2O3 composites A recent review [l] has pointed out evidence containing from 0 to 30 vol. of SiC has been three distinct volume-fraction-dependent (o) achieved by laboratories such as Argonne National regimes of mechanical behavior of rigid-particle- Laboratory(AND) [2 and Oak Ridge National regimes are separated by two critical volume frac- panies such as Advanced Ceramic Composites Inc (ARCO)[4]. In all he ed whiskers fraction is that of the formation of a point-contact which have a typical diameter of 0.6-1 um, signifi- percolative network (@pep). The second critical cantly reduced their aspect ratio during processing. volume fraction is that of the formation of a facet- from more than 50 to an average of 210, while the facet contact network(rep). The three regions of matrix grain sizes ranged between 1.2 and 3.3 um behavior are defined by o< opep, ppep <o drcn, Using equation(I), the critical percolation limit andφ>φrcp henφpep≤7%. Anelastic creep recovery exper The characteristic volume fraction for the onset iments tend to support the formation of a connect- of these networks depends on the morphology of ing whisker network over this limit[14] reinforcements In the case of the point-contact per- Several studies have reported the conditions under colation of high-aspect-ratio randomly oriented which SiCw/Al2O creeps [1, 4,, 10, 11, 13, 16).It whiskers it has been shown that [2] is well established that. the typical creep rates of the composites are (1) lower than those of the monolith: and the reduction in creep rates is due to a partial or The system SiC(whisker)/Al2O3 has been the subject complete inhibition of grain-boundary sliding numerous studies [3-15]. Whisker volume frac- because of rigid whiskers located at the Al2O ions as high as 50% have been used in some of th grain boundaries. opposites [101, but 30 vol. is a practical lim Generally, the stress exponent, n, and activation over which the whiskers are difficult to pacc energy, 2, of the composite are equal to those efficiently, forming agglomerates that inhibit full of alumina, although the complete scope of values of n≈ I and c≈400-500kJ/ mol have been d To whom all correspondence should be addressed and the parameters have been related to a diffusion-

MICROSTRUCTURAL CONSTRAINTS FOR CREEP IN SiC-WHISKER-REINFORCED Al2O3 A. R. DE ARELLANO-LOÂ PEZ1 {, A. DOMIÂ NGUEZ-RODRIÂ GUEZ1 and J. L. ROUTBORT2 1 Departamento de FõÂsica de la Materia Condensada, Universidad de Sevilla, 41080 Seville, Spain and 2 Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439-4838, U.S.A. (Received 17 July 1998; accepted 12 September 1998) AbstractÐNew and published creep data obtained on a SiC-whisker-reinforced Al2O3 composite have been analyzed in terms of an eective grain size and a threshold/critical stress. These concepts allow the formation of a consistent picture of the high-temperature deformation of these composites. For low volume fractions of whiskers, before the formation of a point-contact percolative limit is reached, deformation proceeds via grain-boundary sliding after the applied stress exceeds a temperature-dependent threshold stress. In this regime, the nominal grain size is the most important microstructural feature. For larger volume fractions of whiskers, up to the critical volume fraction for formation of facet-to-facet contact, whiskers inhibit grain-boundary sliding and deformation proceeds by means of pure diusion. In this regime, the most important microstructural feature is an eective grain size, i.e. the spacing between the whiskers. Deformation proceeds until the stress reaches a temperature-dependent critical stress. At this point, damage occurs by unaccommodated grain-boundary sliding and creep is no longer in a steady state. # 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. 1. INTRODUCTION A recent review [1] has pointed out evidence of three distinct volume-fraction-dependent (f) regimes of mechanical behavior of rigid-particlereinforced ceramic-matrix composites. These three regimes are separated by two critical volume fractions. As f increases, the ®rst critical volume fraction is that of the formation of a point-contact percolative network (fpcp). The second critical volume fraction is that of the formation of a facetto-facet contact network (ffcp). The three regions of behavior are de®ned by f < fpcp, fpcp<f < ffcp, and f>ffcp. The characteristic volume fraction for the onset of these networks depends on the morphology of reinforcements. In the case of the point-contact percolation of high-aspect-ratio randomly oriented whiskers it has been shown that [2] fpcp 0:7 aspect ratio 1 The system SiC(whisker)/Al2O3 has been the subject of numerous studies [3±15]. Whisker volume fractions as high as 50% have been used in some of the composites [10], but 30 vol.% is a practical limit over which the whiskers are dicult to pack eciently, forming agglomerates that inhibit full densi®cation. Successful fabrication of SiCw/Al2O3 composites containing from 0 to 30 vol.% of SiC has been achieved by laboratories such as Argonne National Laboratory (ANL) [12] and Oak Ridge National Laboratory (ORNL) [3], and commercially by companies such as Advanced Ceramic Composites Inc. (ARCO) [4]. In all cases, the as-received whiskers, which have a typical diameter of 0.6±1 mm, signi®- cantly reduced their aspect ratio during processing, from more than 50 to an average of 110, while the matrix grain sizes ranged between 1.2 and 3.3 mm. Using equation (1), the critical percolation limit is then fpcpR7%. Anelastic creep recovery experiments tend to support the formation of a connecting whisker network over this limit [14]. Several studies have reported the conditions under which SiCw/Al2O3 creeps [1, 4, 6±8, 10, 11, 13, 16]. It is well established that: . the typical creep rates of the composites are lower than those of the monolith; and . the reduction in creep rates is due to a partial or complete inhibition of grain-boundary sliding because of rigid whiskers located at the Al2O3 grain boundaries. Generally, the stress exponent, n, and activation energy, Q, of the composite are equal to those of alumina, although the complete scope of composite behavior is complex. In compression values of n11 and Q1400±500 kJ/mol have been reported for ®ne-grained polycrystalline alumina, and the parameters have been related to a diusionActa mater. Vol. 46, No. 18, pp. 6361±6373, 1998 # 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain PII: S1359-6454(98)00331-0 1359-6454/98 $19.00 + 0.00 {To whom all correspondence should be addressed. 6361

E ARELLANO. LOPEZ et aL. CREEP OF SIC-WHISKER-REINFORCED ALUMINA Table 1. Alumina nominal grain sizes(NGS)for ANL and ORNL composites Sample ANLO ANLS ANL15 ANL30 ORNLO ORNLIO ORNL20 NGS (um) 1.8 1.5 fastest path (lattice or gra 2. PREVIOUS RESULTS of which is represented by the grain size(Gs) Two important points remain unexplained In this work, we re-analyze published creep data from composites fabricated at Argonne National The creep rates are independent of the nominal Laboratory and Oak Ridge National Laboratory grain size(NGS)and only depend weakly on the The composites contained 0-30 vol. of Sic whisker volume fraction, especially for higher whiskers. The sample designations are formed by whisker content [1, 13, 15 the initials of the manufacturer, followed by a For composites containing 5-8 vol. whiskers, number representing the volume fraction of n>2 for low stress and ne l for high stress. This whiskers. The alumina grain size of each material is contrary to n a I at low stress, and n>> I at was determined by the manufacturer [10, 12, 15 high stress measured in composites containing and is included in Table 1. The same type of Sic more than 10 vol % Additionally, the acti- whiskers, of typical radius of 0.3-0.5 um, and typi- vation energies for the 5-8 vol. composites, cal aspect ratios 210, was used for all composites <700 kJ/mol, are much higher than measured Figure I shows log-log plots of the strain rates in alumina and in higher whisker-content com- of the composites (Ec), corrected by the NGs,vs posites 6, 8, 13 stress(a) for creep of ANL and ORNL composites Several models to describe the deformation of The stress exponents resulting from these exper composites are available. Wilkinson [1 proposed a iments have been previously reported [13], with rheological model for creep based on considering alues between I and 2 for the lower stresses and the composite as a creep-resistant reinforcement n>2 for higher stresses, as seen in Fig. 1. The new embedded in a plastic matrix. He acknowledges, set of experiments for the composition ORNLIO iCw/Al2O3 because of lack of knowledge of several 100 MPa. Three different groups of behavior can be parameters. Yoon and Chen [20] developed a conti- uum theory for non-Newtonian flow of a two- l. the monolithic from the two different sources phase composite containing rigid inclusions that behaves essentially the same, n=1.3. Differ- partially suppress flow. This model was applied to ences in absolute strain rates can be explained zirconia-mullite composites [20] and later to Sic- when they are corrected with d [Fig. 1(a)]. This whisker-reinforced Y-TZP [21]. However, recent correlation forms the basis of normalization by work by Parthasarathy et al. [22 on SiC-whisker d, rather than d, throughout our analysis reinforced Mg-PSZ /mullite composites showed that 2. the results for the 5 vol. composite, ANL5 the model by Yoon and Chen does not accurately can best be described by n=2.6 for lower stress, describe creep of composites containing high and n=1.3 for higher stresses. The model that aspect-ratio reinforcements. That work suggests ill explain the physical basis at a better explanation is achieved by modifi of the two-line fit: and cations to classical microscopic-based creep models. 3. samples containing more than 10 vol. of This work will apply the same microscopic-based whiskers are characterized by ns I at lower reep models as in Ref. [22] to address the questi stresses, and then n>3 at higher stress. The bove.With that purpose, a set of published data strain rates, corrected using d [Fig. I(b)]for will be re-analyzed and combined with some new the various compositions vary by approximately The nominal grain size is not a significant cr parameter. Instead, the space available between 3. EFFECTIVE GRAIN SIZE the whiskers is the significant microstructural creep parameter. When first introduced [23]. this 3.1. Development of the network of whiskers parameter was called"effective grain size"(EGS In this section we describe the determination of The results for composites having low o can be the EGS for samples containing more tha explained by means of a temperature-dependent 10 vol% whiskers. These types of composites are threshold stress. The use of a threshold stress in normally fabricated by uniaxial hot-pressing(HP). explaining the behavior of Sic-whisker-alumina so there is a preferential orientation of the whiskers composites was introduced in the past [23]. in planes perpendicular to the hP direction,within Parthasarathy et al. [22] have recently also used which the orientation of the whiskers can be con- the concept of a threshold stress. sidered random. A schematic is shown in Fig. 2

fastest path (lattice or grain boundary), the length of which is represented by the grain size (GS). Two important points remain unexplained. . The creep rates are independent of the nominal grain size (NGS) and only depend weakly on the whisker volume fraction, especially for higher whisker content [1, 13, 15]. . For composites containing 5±8 vol.% whiskers, n>2 for low stress and n11 for high stress. This is contrary to n11 at low stress, and n >> 1 at high stress measured in composites containing more than 10 vol.%. Additionally, the activation energies for the 5±8 vol.% composites, 1700 kJ/mol, are much higher than measured in alumina and in higher whisker-content composites [6, 8, 13]. Several models to describe the deformation of composites are available. Wilkinson [1] proposed a rheological model for creep based on considering the composite as a creep-resistant reinforcement embedded in a plastic matrix. He acknowledges, however, the diculty of applying such a model to SiCw/Al2O3 because of lack of knowledge of several parameters. Yoon and Chen [20] developed a continuum theory for non-Newtonian ¯ow of a twophase composite containing rigid inclusions that partially suppress ¯ow. This model was applied to zirconia±mullite composites [20] and later to SiCwhisker-reinforced Y-TZP [21]. However, recent work by Parthasarathy et al. [22] on SiC-whiskerreinforced Mg-PSZ/mullite composites showed that the model by Yoon and Chen does not accurately describe creep of composites containing highaspect-ratio reinforcements. That work suggests that a better explanation is achieved by modi®- cations to classical microscopic-based creep models. This work will apply the same microscopic-based creep models as in Ref. [22] to address the questions above. With that purpose, a set of published data will be re-analyzed and combined with some new data, making two assumptions. . The nominal grain size is not a signi®cant creep parameter. Instead, the space available between the whiskers is the signi®cant microstructural creep parameter. When ®rst introduced [23], this parameter was called ``eective grain size'' (EGS). . The results for composites having low f can be explained by means of a temperature-dependent threshold stress. The use of a threshold stress in explaining the behavior of SiC-whisker±alumina composites was introduced in the past [23]. Parthasarathy et al. [22] have recently also used the concept of a threshold stress. 2. PREVIOUS RESULTS In this work, we re-analyze published creep data from composites fabricated at Argonne National Laboratory and Oak Ridge National Laboratory. The composites contained 0±30 vol.% of SiC whiskers. The sample designations are formed by the initials of the manufacturer, followed by a number representing the volume fraction of whiskers. The alumina grain size of each material was determined by the manufacturer [10, 12, 15], and is included in Table 1. The same type of SiC whiskers, of typical radius of 0.3±0.5 mm, and typical aspect ratios r10, was used for all composites. Figure 1 shows log±log plots of the strain rates of the composites (e_c), corrected by the NGS, vs stress (s) for creep of ANL and ORNL composites. The stress exponents resulting from these experiments have been previously reported [13], with values between 1 and 2 for the lower stresses, and n>2 for higher stresses, as seen in Fig. 1. The new set of experiments for the composition ORNL10, resulted in n = 1.620.2 for stresses below 100 MPa. Three dierent groups of behavior can be described: 1. the monolithic from the two dierent sources behaves essentially the same, n = 1.3. Dierences in absolute strain rates can be explained when they are corrected with dÿ3 [Fig. 1(a)]. This correlation forms the basis of normalization by dÿ3 , rather than dÿ2 , throughout our analysis; 2. the results for the 5 vol.% composite, ANL5, can best be described by n = 2.6 for lower stress, and n = 1.3 for higher stresses. The model that will be developed will explain the physical basis of the two-line ®t; and 3. samples containing more than 10 vol.% of whiskers are characterized by n11 at lower stresses, and then n>3 at higher stress. The strain rates, corrected using dÿ3 [Fig. 1(b)] for the various compositions vary by approximately 100. 3. EFFECTIVE GRAIN SIZE 3.1. Development of the network of whiskers In this section, we describe the determination of the EGS for samples containing more than 10 vol.% whiskers. These types of composites are normally fabricated by uniaxial hot-pressing (HP), so there is a preferential orientation of the whiskers in planes perpendicular to the HP direction, within which the orientation of the whiskers can be considered random. A schematic is shown in Fig. 2. Table 1. Alumina nominal grain sizes (NGS) for ANL and ORNL composites Sample ANL0 ANL5 ANL15 ANL30 ORNL0 ORNL10 ORNL20 NGS (mm) 1.8 2.8 3.3 1.5 1.5 1.2 2.0 DE ARELLANO-LOÂ PEZ et al.: CREEP OF SiC-WHISKER-REINFORCED ALUMINA 6363

E ARELLANO. LOPEZ et al.: CREEP OF SIC-WHISKER-REINFORCED ALUMINA HP Axis Table 2. Average space available between the whiskers in sections with more than 10 vol% of whiskers, and equivalent three-dimen- sional diameter(defr) Sample ANLI5 ANL30 ORNLIO ORNL20 3.5 3. 2.4 measured using an image analyzer. The average values of d and di are listed in Table 2 The equivalent three-dimensional diameters of the ellipsoidal objects are calculated by means of the geometric average referential orientation of the whiskers, perpendicularly to the hot-pressing(HP)axis values are listed in the HP axis. Reflected polarized light was used to A comparison between the values in Table 2, and obtain a clear contrast of the whiskers (white) the reported NGS in Table 1, shows some corre- on the uniform gray background of the alumina lations, except for the ORNLIO sample, for which matrIX the EGs is as much as three times larger tha In order to estimate the egs for the different the NGS. Nevertheless the egs decreases as the samples, a metallographic study has been performed volume percentage of whiskers increases, consis on both types of sections for each sample. It was tent with intuition. An analytical expression for assumed that "regular-shaped"ellipsoidal objects defr=dend, ... )can be derived by a simple two- fit into the whisker network. The diameters of dimensional model that correlates d with the these"grains"will be called dhh. if it corresponds volume fraction of whiskers, o, and the whisker to the face parallel to the hP axis, and d, if it radius, r. orresponds to the face perpendicular to the hP Figure 4 shows the location of perfectly distri- axis. The objects were subsequently digitized and buted whiskers in square cells of side e. The perp ara Fig 3. Optic graphs of polished sections parallel (para)and perpedicular (perp) to the HP axis of samples (a) ORNLIO, and (b)ANL30. Reflected polarized light was used to contrast the whiskers (white) on the uniform background of the alumina matrix

Figure 3 presents a set of optical micrographs taken on polished sections parallel and perpendicular to the HP axis. Re¯ected polarized light was used to obtain a clear contrast of the whiskers (white) on the uniform gray background of the alumina matrix. In order to estimate the EGS for the dierent samples, a metallographic study has been performed on both types of sections for each sample. It was assumed that ``regular-shaped'' ellipsoidal objects ®t into the whisker network. The diameters of these ``grains'' will be called d6, if it corresponds to the face parallel to the HP axis, and d_, if it corresponds to the face perpendicular to the HP axis. The objects were subsequently digitized and measured using an image analyzer. The average values of d6and d_ are listed in Table 2. The equivalent three-dimensional diameters of the ellipsoidal objects are calculated by means of the geometric average: 4 3 p d? 2 2 dk 2 4 3 p deff 2 3 )deff d2 ?dk 3 q 2 The values are listed in Table 2. A comparison between the values in Table 2, and the reported NGS in Table 1, shows some correlations, except for the ORNL10 sample, for which the EGS is as much as three times larger than the NGS. Nevertheless, the EGS decreases as the volume percentage of whiskers increases, consistent with intuition. An analytical expression for de=de(f,...) can be derived by a simple twodimensional model that correlates d6 with the volume fraction of whiskers, f, and the whisker radius, r. Figure 4 shows the location of perfectly distributed whiskers in square cells of side `. The Fig. 2. Schematic of the preferential orientation of the whiskers, perpendicularly to the hot-pressing (HP) axis. Fig. 3. Optical micrographs of polished sections parallel (para) and perpedicular (perp) to the HP axis of samples (a) ORNL10, and (b) ANL30. Re¯ected polarized light was used to contrast the whiskers (white) on the uniform background of the alumina matrix. Table 2. Average space available between the whiskers in sections perpendicular (d_) and parallel (d6) to the HP direction of samples with more than 10 vol.% of whiskers, and equivalent three-dimensional diameter (de) Sample ANL15 ANL30 ORNL10 ORNL20 d_ (mm) 3.5 2.5 4.0 2.8 d6(mm) 2.4 1.6 2.9 1.8 de (mm) 3.1 2.2 3.6 2.4 6364 DE ARELLANO-LOÂ PEZ et al.: CREEP OF SiC-WHISKER-REINFORCED ALUMINA

alumina matrix ®lls the space between the whiskers. A comparison of the proposed distribution with the optical micrographs in Fig. 3 is satisfactory. It is obvious that the volume fraction, f, is: f pr2 `2 3 A measure of the space between the whiskers is the diameter of the circle that ®ts into this space, which has been denoted d6. We can then follow: ÿ dk 2r 2 2`2 4 and ®nally: dk r 2p f s ÿ 2 5 Table 3 shows a comparison between measured and calculated values of d6, using r = 0.5 mm. Good agreement is obtained for samples containing 10 and 20 vol.% whiskers. For ANL15 and ANL30, the measured values are slightly higher than the calculated ones, probably due to an imperfect distribution of whiskers. To a ®rst approximation, there is a direct proportionality between d_ and d6, at least over the composition range in this study, as shown in Fig. 5. Therefore, by using d_=ld6, l being a constant, and equation (2), the following is obtained: deff d2 ?dk 3 q l2=3 dk 6 where in this case l11.5, obtained by regression of the data in Fig. 5. Then using equation (5): deff l2=3 r 2p f s ÿ 2 7 This expression allows an estimate of the eective grain size knowing only the volume fraction of whiskers and the whisker radius. It should be valid over a range of compositions similar to the one used in this study. The value of l can be related to the homogeneity of the distribution of reinforcements, and to the whisker aspect ratio. When using the same type of whiskers and similar processing techniques, l is expected to vary a little from the value reported in this study. Fig. 5. Correlation between d6and d_, assuming direct proportionality through a constant l, that is calculated by linear regression to be 11.5, valid for the range 10±30 vol.% of whiskers. Fig. 4. Schematic of the space available between perfectly distributed whiskers of radius r, on a face that is parallel to the HP axis. Table 3. Measured and calculated [equation (5)] d6values for samples with more than 10 vol.% of whiskers using r = 0.5 mm Sample ANL15 ANL30 ORNL10 ORNL20 Calculated d6(mm) 2.2 1.3 3.0 1.8 Measured d6(mm) 2.4 1.6 2.9 1.8 DE ARELLANO-LOÂ PEZ et al.: CREEP OF SiC-WHISKER-REINFORCED ALUMINA 6365

《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_Al2O3-SiC蠕变-19

《复合材料 Composites》课程教学资源（学习资料）第五章陶瓷基复合材料_Al2O3-SiC蠕变-19