REFRACTORY METALS HARD MATERIALS ELSEVIER International Journal of Refractory Metals Hard Materials 16(1998)337-341 Fracture and creep of an Al2O3-SiC (whisker)-TiC (particle) posite A.R. de Arellano-Lopeza, B. I. Smirnov, JJ. Schuldies, E. T Park, K.C. Coretta J .L Routbort Departamento de Fisica de la Matena Condensada, Unn'erstdad de Sevilla, PO Bax 1065, 41080 Sevilla, Spain A F Ioffe Phystco-Techmcal Instttute, Russian Academy of Sciences, 194021 St Petersburg, Russia dustnal Ceramic Technology, Ann Arbor, MI 48103, USA Energy Technology Dunston, Argonne Natonal Laboratory, Argonne, IL 60439, USA Received 9 March 1998; accepted 14 July 1998 Abstract h-temperature fracture strength and compressive creep of an electrodischarge- machinable composite, AlO 30.9 vol %o SiC whiskers-23 vol TiC particles have been studied to 1200C and 1450.C, respectively, in Inert atmosphere. Microstructures of fractured and deformed specimens were examined using scanning and transmission electron microscopy. Fast fracture occurred at T 1350C at stresses <80 MPa, with the rate-controlling mechanism being partially unaccommodated grain-boundary sliding, with a stress exponent of x1 and an activation energy of x 470 kJ/mol. 1998 Elsevier Science Ltd. All rights reserved Keywords CeramIc-matrIx composites; Fracture, Creep, electrodischarge machInIng 1. Introduction SiC whiskers to produce an electrodischarge-machin able ceramic composite [7. These composites have Significant improvements in strength, toughness and high electrical conductivity [ 8] creep resistance of ceramic mate Laboratories in Spain, Russia and the uSa, under achieved in the past decade. This is particularly true the auspices of NAtO, have extensively characterized for ceramic-matrix composites [1, for which the microstructural and mechanical properties of this SiC-whisker-reinforced Al2Oj-based composites have new ceramic composite, with a goal of determining become a classic system and the object of extensive processing paths and microstructures, that will yield study [2-5. Although whisker-reinforced ceramic conducting composites with further improved mechan- composites are commercially available, high machining ical properties. Previous work on Al O3-SiC (whisker)- costs for complex parts have limited use of these Tic(particle) composites (AISiTi) dealt with composites and, therefore, they have not found microstructure and room-tempen echanica widespread application despite their uniquely favour- properties, such as fracture strength, fracture toug able properties, such as low density, chemical and ness, microhardness, elastic modulus and response to thermal stability, and mechanical durability [6]. A solid-particle erosion [9, 10]. Summarizing the earlier composite has recently been developed with sufficiently findings: AISiTi has an elastic modulus at room high electrical conductivity to take a step towards the temperature of 410 GPa, microhardness values that possible realization of the goal of wider commercial depend on whether the indentor is centred on a TiC usage. TiC particles are added to Al2O3 powder and particle or not and an indentation fracture toughness (Kic) of 9.6 MPa(m). As in other complex compo- of toughening mechanisms operates. In addition to debonding and bridging
ELSEVIER International Journal of Refractory Metals & Hard Materials 16 (1998) 337-341 International Journal of REFRACTORY METALS & HARD MATERIALS Fracture and creep of an A1203-SiC (whisker)-TiC (particle) composite A. R. de Arellano-L6pez a*, B. I. Smirnov b, J. J. Schuldies c, E. T. Park d, K. C. Goretta d, J. L. Routbort d aDepartamento de Ftstca de Ia Materta Condensada, Umverszdad de Sevtlla, PO Box 1065, 41080 Sevdla, Spam bA E Ioffe Phystco-Techmcal lnstttute, Russtan Academy of Sctences, 194021 St Petersburg, Russta Clndustnal Ceramtc Technology, Ann Arbor, 341 48103, USA aEnergy Technology Dtvtston, Argonne National Laboratot); Argonne, 1L 60439, USA Recewed 9 March 1998; accepted 14 July 1998 Abstract High-temperature fracture strength and compresswe creep of an electrodischarge-machinable composite, Al203-30.9 vol.% SiC whiskers-23 vol % TiC particles have been studied to 1200°C and 1450°C, respectively, in inert atmosphere. Microstructures of fractured and deformed specimens were examined using scanning and transmission electron microscopy. Fast fracture occurred at T_ 1350°C at stresses <80 MPa, with the rate-controlling mechanism being partially unaccommodated grain-boundary sliding, with a stress exponent of ~ 1 and an activation energy of ~470 kJ/mol. © 1998 Elsevier Science Ltd. All rights reserved. Keywords" Ceramic-matrix composites; Fracture, Creep, electrodischarge machining 1. Introduction Significant improvements in strength, toughness and creep resistance of ceramic materials have been achieved in the past decade. This is particularly true for ceramic-matrix composites [1], for which SiC-whisker-reinforced A1203-based composites have become a classic system and the object of extensive study [2-5]. Although whisker-reinforced ceramic composites are commercially available, high machining costs for complex parts have limited use of these composites and, therefore, they have not found widespread application despite their uniquely favourable properties, such as low density, chemical and thermal stability, and mechanical durability [6]. A composite has recently been developed with sufficiently high electrical conductivity to take a step towards the possible realization of the goal of wider commercial usage. TiC particles are added to A1203 powder and *Corresponding author SiC whiskers to produce an electrodischarge-machinable ceramic composite [7]. These composites have high electrical conductivity [8]. Laboratories in Spain, Russia and the USA, under the auspices of NATO, have extensively characterized the microstructural and mechanical properties of this new ceramic composite, with a goal of determining processing paths and microstructures, that will yield conducting composites with further improved mechanical properties. Previous work on A1203-SiC (whisker)- TiC (particle) composites (A1SiTi) dealt with microstructure and room-temperature mechanical properties, such as fracture strength, fracture toughness, microhardness, elastic modulus and response to solid-particle erosion [9,10]. Summarizing the earlier findings: A1SiTi has an elastic modulus at room temperature of 410 GPa, microhardness values that depend on whether the indentor is centred on a TiC particle or not and an indentation fracture toughness (Kic) of 9.6 MPa(m) °5. As in other complex composites, a combination of toughening mechanisms operates. In addition to debonding and bridging, 0263-4368/98/$ -- see front matter © 1998 Elsevier Science Ltd All rights reserved PII" S0263-4368(98)00037-7
A R. de Arellano-Lopez et al /Internatonal Jownal of Refractory Metals Hard Matenals 16(1998)337-341 microcracking can play a role as the thermal expansion Microstructural features of both undeformed and mismatch between the three phases is significant. deformed specimens were examined using X-ray ee-and four-point bending strengths of AISiTi were diffraction, scanning electron microscopy (SEM), and 825 and 680 MPa, respectively. Tensile surfaces were, transmission electron microscopy (TEM). SEM however,not polished. High-temperature compressive samples were prepared by polishing the composite to a creep of this composite has been investigated at 1 um finish and coating with carbon. TEM foils were 1350-1450oC in inert atmospheres [11]. The creep prepared by grinding, dimpling and ion-milling resistance is good and, at lower stresses, deformation Preparation of TEM foils was complicated because the occurs by partially unaccommodated grain-boundary iC particles proved to be very difficult to thin by ion-milling. Information about the micromorphology of High-temperature mechanical properties play an the phases has been published previously [11]:alt important role in development and implementation of actical applications of ceramic composites because many of the potential applications are at elevated temperatures. Therefore this work is aimed at measuring the high-temperature fracture strength and CIO\ creep response of an AISiTi composite. Additionally, the elastic modulus was measured to 1000C 2. Experiments A commercial composite(CRYSTALOY 231IEDX fabricated by hot-pressing at 1700-1800'C a mixture of 30.9 vol. SiC whiskers, 23.0 vol. TiC powder and balance AlO, was examined 7]. Optical micrographs of the Sic whiskers and a surface polished perpen dicular to the hot-pressing direction are shown in Fig 1. The material is x% dense. Comparison of the O) pun initial(Fig. 1(a))and final whisker lengths(Fig. 1(b)) indicated that considerable damage to the Sic whisker occurred during processing. X-ray diffraction showed strong TiC and Al2O3 peaks and weaker SiC peak Bend-bar samples25×3.8×38mmor2.5×3.0×38 mm were cut with a slow-speed diamond saw. Bar dges were chamfered and each tensile surface was polished with 1 uam diamond paste. Bending strength tests were performed at a crosshead velocity of a1.3 mm/min in an Instron Model 1125 [12]. Room temperature tests were conducted in air with steel tooling, inner load span of 9.5 mm, outer load span of 23.8 mm. High-temperature tests were conducted in Ar with AlO3-SiC whisker tooling [5], inner load span of 9.9 mm, outer load span 17.6 mm. The elastic modulus was measured by a resonance frequency method [13] For creep, parallelepipeds≈5×2×2 mm were cut and the compression surfaces were polished to be flat and parallel. Specimen do m 1350-1450C under uniaxial compression in the direc- tion of the longer axis. The low-stress range was studied in Ar with a constant-load (CL)cree apparatus [14; higher stresses and strain rates were omposIte, where the bright grains are TiC particles, the light-grey studied in high-purity N2 at approximately constant filaments are the SiC whiskers, and the dark-grey background is the strain rate(CSR)[12 alumina matrix
338 A R. de AlelIano-L6pez et al /International Jownal of Refractory Metals & Hard Matertals 16 (1998) 337-341 microcracking can play a role as the thermal expansion mismatch between the three phases is significant. Three- and four-point bending strengths of A1SiTi were 825 and 680 MPa, respectively. Tensile surfaces were, however, not polished. High-temperature compressive creep of this composite has been investigated at 1350-1450°C in inert atmospheres [11]. The creep resistance is good and, at lower stresses, deformation occurs by partially unaccommodated grain-boundary sliding. High-temperature mechanical properties play an important role in development and implementation of practical applications of ceramic composites because many of the potential applications are at elevated temperatures. Therefore, this work is aimed at measuring the high-temperature fracture strength and creep response of an A1SiTi composite. Additionally, the elastic modulus was measured to 1000°C. 2. Experiments A commercial composite (CRYSTALOY 2311EDX) fabricated by hot-pressing at 1700-1800°C a mixture of 30.9 vol.% SiC whiskers, 23.0 vol.% TiC powder and balance A1,O3 was examined [7]. Optical micrographs of the SiC whiskers and a surface polished perpendicular to the hot-pressing direction are shown in Fig. 1. The material is ~99% dense. Comparison of the initial (Fig. l(a)) and final whisker lengths (Fig. l(b)) indicated that considerable damage to the SiC whisker occurred during processing. X-ray diffraction showed strong TiC and A1203 peaks and weaker SiC peaks. Bend-bar samples 2.5 x 3.8 x 38 mm or 2.5 x 3.0 x 38 mm were cut with a slow-speed diamond saw. Bar edges were chamfered and each tensile surface was polished with 1 #m diamond paste. Bending strength tests were performed at a crosshead velocity of ~ 1.3 ram/rain in an Instron Model 1125 [12]. Roomtemperature tests were conducted in air with steel tooling, inner load span of 9.5 ram, outer load span of 23.8 ram. High-temperature tests were conducted in Ar with A1203-SiC whisker tooling [5], inner load span of 9.9 mm, outer load span 17.6 mm. The elastic modulus was measured by a resonance frequency method [13]. For creep, parallelepipeds ~5 x 2 x 2 mm were cut and the compression surfaces were polished to be flat and parallel. Specimens were deformed at 1350-1450°C under uniaxial compression in the &rection of the longer axis. The low-stress range was studied in Ar with a constant-load (CL) creep apparatus [14]; higher stresses and strain rates were studied in high-purity N2 at approximately constant strain rate (CSR) [12]. Microstructural features of both undeformed and deformed specimens were examined using X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). SEM samples were prepared by polishing the composite to a 1 ltm finish and coating with carbon. TEM foils were prepared by grinding, dimpling and ion-milling. Preparation of TEM foils was complicated because the TiC particles proved to be very difficult to thin by ion-milling. Information about the micromorphology of the phases has been published previously [11]: alumina Fig. 1 Optical photomicrographs of (a) SIC whiskers and (b) A1SIT~ composite, where the bright grams are TIC parUcles, the hght-grey filaments are the SiC whiskers, and the dark-grey background is the alumina matrLx
A.r de Arellano-Lopez ef al /Internatonal Journal of efractory Metals& Hard Matenals 16(1998)337-341 339 grain size Is≈ I um and TiC particle size is≈5um.In situ whisker lengths are typically below 10 un Results and discussion 500 3. 1. Elastic modulus Variation of Youngs modulus(E)with temperature shown in Fig. 2. The value of E at room temperature for the composite was approximately 2.5% higher than 300 E for pure AlO3 [15]. The value of 1/E(dEldT) fror 25to1000cwas8.5×103, almost equal to the让 value of 8. x 10-C- found for Al O3 in the same 200400600800100012001400 temperature range [15]. The SiC and TiC. additions, therefore seem to have had little effect on e T 3. 2. Fracture strength temperature fracture strength is plotted as a function of tempera 1200C(Fig. 4(b)) indicate that fracture was a combination of intergranular and transgranular. Thus, ture in Fig. 3. The strength of 440 MPa measured at the dominant fracture mechanism was independent of room temperature is considerably lower than that temperature reported previously [10]. Fractures originated at processing flaws. The discrepancy between these measurements performed on polished samples and our previous measurements must reflect variability in processing or sample preparation and will require further investigation. Strength was virtually indepen dent of temperature to 1000C, but decreased slightly at 1200'C, which is probably the limit of practical use of this material because creep could be excessive at higher temperatures. This result is consistent with finding for Al O,/TiC composites [16]. Surfaces of samples fractured at room temperature(Fig. 4(a)and 380 370 200 400 600 8001000 Fig 4 SEM photomIcrographs of fracture surfaces at(a)room Fig 2 Vanation of E as a function of temperature mperature and(b)1200.C
A. R de Arellano-L6pez et al /Internatzonal Journal of Refractory Metals& Hard Materials 16 (1998) 337-341 339 grain size is ~ 1 #m and TiC particle size is ~ 5 #m. In situ whisker lengths are typically below 10/~m. 3. Results and discussion 3.1. Elastic modulus Variation of Young's modulus (E) with temperature is shown in Fig. 2. The value of E at room temperature for the composite was approximately 2.5% higher than E for pure A1203 [15]. The value of 1/E (dE/dT) from 25 to 1000°C was 8.5 x 10 5 o C 1, almost equal to the value of 8.3 x 10 .5 °C -1 found for AlzO3 in the same temperature range [15]. The SiC and TiC additions, therefore, seem to have had little effect on E. 3.2. Fracture strength Fracture strength is plotted as a function of temperature in Fig. 3. The strength of 440 MPa measured at room temperature is considerably lower than that reported previously [10]. Fractures originated at processing flaws. The discrepancy between these measurements performed on polished samples and our previous measurements must reflect variability in processing or sample preparation and will require further investigation. Strength was virtually independent of temperature to 1000°C, but decreased slightly at 1200°C, which is probably the limit of practical use of this material because creep could be excessive at higher temperatures. This result is consistent with finding for A1203/TiC composites [16]. Surfaces of samples fractured at room temperature (Fig. 4(a)) and D. v t-- e" .i..a .i..a o u.. Fig 600 500 400 300 ''1'''1'''1'''1'''1'''1''' 200 , , I , , , I , , , I , , , I , , , I , , , I , , , 0 200 400 600 800 1000 1200 1400 T (°C) 3 Four-point-bend fracture strength as a function of temperature 1200°C (Fig. 4(b)) indicate that fracture was a combination of intergranular and transgranular. Thus, the dominant fracture mechanism was independent of temperature. Q.. UJ 410 ' , i I i i i I i i i I i i ~ I ' ' ' [ 400 390 380 370 0 200 400 600 800 1000 T (°C) Fig 2 Variation of E as a function of temperature Fig 4 SEM photomacrographs of fracture surfaces at (a) room temperature and (b) 1200°C
a r de arellano-lLopez et al /Internattonal Journal of Refractory Metals Hard Maternals 16(1998)337-341 3.3. Creep plastic defc remore considered as rigid inclusions A standard creep equation(17 was used to analyze A stress exponent of x1 for lower stresses, the results structural observations that cavities formed plastic deformation and absence of dislocation activity Q can be related to a mechanism that involves grain 8=Ad∞-Rr boundary sliding, but for which the sliding was not fully accommodated by diffusion [191. Creep resistance for where g is the strain rate, g is the stress r and t have the AISiTi composite might be improved by inhibiting their usual meanings and A is constant. The param grain-boundary sliding, which could be achieved by eters the stress exponent and @, the activation adding more of a rigid reinforcing phase or by energy,are related to plastic deformation mechanisms preserving more of the initial SiC whisker length through various models (14, 171 The creep resistance of the AlsiTi composite is Figure 5 shows a log-log plot of strain rate vs stress comparable to that of other whisker-reinforced ceramic for five different samples. At 1400.C, the maximum composites with similar grain size(Al2O3 grain size 1 stress in the CL tests was a80 MPa; stresses in the um). A comparison with creep of Al, O3-30 vol% SiC CSR tests were 150-540 MPa. The low-stress regime whisker [20] and a Al2O3-55 vol %o ZrO2 particle-28 vol. SiC whisker [21 composites is shown in Fig (<80 MPa)can be characterized by a stress exponent At lower stresses, in the ns1 region, all composites N1.0+0.3, which is typical for diffusional creep of monolithic fine-grained polycrystalline ceramics [18. Creep at about the same rate, whereas the AISiTi shows somewhat more creep resistance and damage tolerance On the other hand, the high-stress regime showed ar important degree of sample to sample variability, due at the higher stresses to the formation of macroscopic damage [10]. The activation energy was determined to be a470 kJ/mole at 23 MPa and 1350-1450.C. The values of the creep parameters, n and Q, in the steady-state region are consistent with that of fine-grained polycrystalline Al2O3, which is the only plastic phase in this system s The strength of an Al2O.9 vol. SiC(whiskers)- vol. TiC(particles) was independent of tempera- under the test conditions ture to 1000.C, but decreased slightly at 1200.C. The fracture mode, a combination of transgranular and especially at the higher stresses. Little evidence of intergranular, was unaffected by temperature. At samples before or after deformation 10]. The TiC chieved. At stresses below 80 MPa, creep occurred by rticles appeared to remain intact throughout the partially unaccommodated grain-boundary sliding, with Al O 10 5.5 voL% Zro 28 vol. sic AlO 30 vol, % SIC 010 1000 10 1000 Stress(MPa) Stress(MPa) Fig 6 Creep data at 1400 C from this work(triangles)compared wIth creep of Al_O330 vol SIC whisker (dashed line)and sents a dnfe. p results from five AISITi samples, each symbol repre- Al2O3 55 vol %o ZrO, particle-28 vol %o SiC whisker (sold lne)
340 A R de Arellano-L@ez et al /Internattonal Journal of Refractory Metals & Hard Materials 16 (1998) 337-341 3.3. Creep A standard creep equation [17] was used to analyze the results: where + is the strain rate, a is the stress, R and T have their usual meanings and A is constant. The parameters n, the stress exponent and Q, the activation energy, are related to plastic deformation mechanisms through various models [14,17]. Figure 5 shows a log-log plot of strain rate vs stress for five different samples. At 1400°C, the maximum stress in the CL tests was ~80 MPa; stresses in the CSR tests were 150-540 MPa. The low-stress regime ( 1350°C, steady-state creep was achieved. At stresses below 80 MPa, creep occurred by partially unaccommodated grain-boundary sliding, with 10 "5 V ¢) 10 .0 ¢- .m 10-7 A ~7 V V / I0 -8 , , ~ , , ,,,I , L ~ , r ,, 10 100 1000 Stress (MPa) Fig. 5. Creep results from five A1S~TI samples, each symbol represents a different sample , , , , , ''4 I ' ' ' , , AlaO 3- ," - // 5.5 vol.% Zr_ aO ,' A /x,, 10 s 28 vol.% SiCk/ ,z~ A '~ A ,' A 10-6 ,' "~ /t I .~- .., AlaO £ ~ - " 30 vol.°/o SiC 10 .7 10 .8 ....... ,~ ....... I0 I00 I ~00 Stress (MPa) Fig 6 Creep data at 1400°C from this work (triangles) compared with creep of A1203-30 vol.% S1C wh]sker (dashed line) and A1203-5 5 vol % ZrO2 particle-28 vol % SIC whisker (sohd hne) composites
A.R. de arellano-Lopez et al nternatonal Journal of Refractory Metals& Hard Matenals 16(1998)337-341 341 a stress exponent of s 1 and an activation energy of 6 Smith V, Deckman B, Brueck D. Am Ceram Soc Bull ≈470kJ/mol 1994:73(12)49. [7 Schuldes JJ EDM Today 1992, Nov/ Dec 16 [8 Smirnov BI, Nikolaev VI, Burenkov YuA, Routbort JL, Goretta KC J Tech Phys Lett 1997, 23.923 Acknowledgements L Schuldes丁 Ceram Eng sci proc 1997. 18 239 This work was supported by the North Atlantic Arellano-Lopez AR, Coretta KC, Singh D. T TECHNOLOGY CRG/NO. 960793: the Ministerio de [11] de Arellano-Lopez AR, Smirnov Bl, Goretta KC. Routbort JL. Mater Sci en Educacion y Ciencias of Spain, under CICYT Project [12] Routbort JL Acta Metall 1982, 30.663 MAT97-0562-C02-01; and the U.S. Department of [13] McSkimin HJ In. Mason WP, edtor Physical acoustics, vol 1, Energy, Energy Research Laboratory Technology art A New York Academic Press, 1964. 271 Transfer Program, under Contract W-31-109-Eng-38 [14] GervaIs H, Pellisier B, Castaing J Rev Int Hautes Temp fract1978,1543 115 Fukuhara M, Yamauchi I J Mater Sci 1993; 28: 4681 [16 Lee M, Borom MT Adv Ceram Mater 1988, 3: 38 References [17] Cannon WR, Langdon TG. J Mater Sci 1988: 23 1 [18] Frost HJ, Ashby MF Deformation mechanIsm maps. New York Pergamon Press, 1982 [1 Evans AG, Marshall DB Acta Metall 1989,37 2567 [19 de arellano-Lopez AR, Dominguez-Rodriguez A, Go [2 Lin H-T, Becher PF J Am Ceram Soc 1990: 73 1378 Routbort JL In Bradt RC. Brookes CA, Routbort 3 Singh JP, Coretta KC, Routbort JL, Kupperman DS, Rhodes Plastic deformation of ceramics New York: Plenum Pr JF Adv ceram Mater 1988,3 357. [4] Fisher ES, ManghnanI MH, Wang J-F, Routbort JL J Am [20] de Arellano-Lopez AR, Cumbrera FL, Dominguez-Rodriguez Ceram Soc 1992, 75 908. A, Coretta KC, Routbort JL J Am Ceram Soc 1993, 73 1297 5 de Arellano-Lopez AR, Dominguez-Rodriguez A, Goretta KC, [21 Calderon-Moreno JM. de Arellano-LOpez AR, Dominguez Routbort jl j Am Ceram Soc 1993: 76 1425 Rodriguez A, Routbort JL Mater Sci Eng 1996; A209 111
A. R. de Arellano-L6pez et al./Intemattonal Journal of Refiactory Metals& Hard Matenals 16 (1998) 337-341 341 a stress exponent of ~ 1 and an activation energy of 470 kJ/mol. Acknowledgements This work was supported by the North Atlantic Treaty Organization, under Grant HIGH TECHNOLOGY CRG/No. 960793; the Ministerio de Educaci6n y Ciencias of Spain, under CICYT Project MAT97-0562-C02-01; and the U.S. Department of Energy, Energy Research Laboratory Technology Transfer Program, under Contract W-31-109-Eng-38. References [1] Evans AG, Marshall DB Acta Metall 1989,37 2567 [2] Lin H-T, Becher PF J Am Ceram Soc 1990;73 1378 [3] Slngh JP, Goretta KC, Routbort JL, Kupperman DS, Rhodes JF Adv Ceram Mater 1988,3 357. [4] Fisher ES, Manghnanl MH, Wang J-F, Routbort JL J Am Ceram Soc 1992;75 908. [5] de Arellano-L6pez AR, Dominguez-Rodriguez A, Goretta KC, Routbort JL J Am Ceram Soc 1993;76'1425 [6] Smith V, Deckman B, Brueck D. Am Ceram Soc Bull 1994;73(12) 49. [7] Schuldles JJ EDM Today 1992,Nov/Dec 16 [8] Smlrnov BI, Nikolaev VI, Burenkov YuA, Routbort JL, Goretta KC J Tech Phys Lett 1997,23.923. [9] J1ang M. Goretta KC, Singh D, Routbort JL, Schuldles JJ Ceram Eng Scl Proc 1997,18 239. [10] Smlrnov BI, Nikolaev VI, Orlova TS, Shpelzman VV, de Arellano-L6pez AR, Goretta KC, Smgh D, Routbort JL. Mater Scl Eng A, in press. [11] de Arellano-L6pez AR, Smirnov BI, Goretta KC, Routbort JL. Mater Sci Eng A, in press [12] Routbort JL Acta Metall 1982,30.663. [13] McSkimin HJ In. Mason WP, editor Physical acoustics, vol 1, part A New York' Academic Press, 1964'271 [14] Gervals H, Pelhsier B, Castaing J Rev Int Hautes Temp Refract 1978,15 43 [15] Fukuhara M, Yamauchi I. J Mater Scl 1993;28:4681. [16] Lee M, Borom MT. Adv Ceram Mater 1988,3:38 [17] Cannon WR, Langdon TG. J Mater Scl 1988;23 1 [18] Frost HJ, Ashby MF Deformation mechanism maps. New York Pergamon Press, 1982. [19] de Arellano-L6pez AR, Dominguez-Rodrfguez A, Goretta KC, Routbort JL. In. Bradt RC, Brookes CA, Routbort JL, edRors. Plastic deformation of ceramics New York: Plenum Press, 1995, p 533 [20] de Arellano-L6pez AR, Cumbrera FL, Domlnguez-Rodriguez A, Goretta KC, Routbort JL J Am Ceram Soc 1993,73 1297 [21] Calderon-Moreno JM, de Arellano-L6pez AR, DominguezRodrfguez A, Routbort JL Mater Sca Eng 1996;A209'111
