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-LO PEZ1 {, A. DOMI NGUEZ-RODRI 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 e€ective 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 fractions of whiskers, before the formation of a point-contact percolative limit is reached, deformation pro￾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 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 di€usion. In this regime, the most important microstructural feature is an e€ective 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 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-particle￾reinforced ceramic-matrix composites. These three regimes are separated by two critical volume frac￾tions. 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 facet￾to-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 per￾colation of high-aspect-ratio randomly oriented whiskers it has been shown that [2] fpcp ˆ 0:7 aspect ratio …1† The system SiC(whisker)/Al2O3 has been the subject of numerous studies [3±15]. Whisker volume frac￾tions 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 dicult to pack eciently, 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 com￾panies 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 exper￾iments tend to support the formation of a connect￾ing 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 di€usion￾Acta 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