Composites Science and Technology 57( 1997)1363-1367 C 1997 Elsevier Scienc Printed in Northern Ireland. All rights reserved PII:s自266-3538(97)00063-8 0266353897/1700 OXIDE/OXIDE COMPOSITES PRODUCED BY THE INTERNAL CRYSTALLIZATION METHOD S.T. Mileiko v.I. Kazmin, *V.M. Kiiko &A m. rudnev Institute of Solid State Physics, Russian Academy of sciences, Chernogolovka, Moscow District 142 432, Russia (Received 2 December 1996; accepted 26 February 1997) Abstract can provide a greater variety of component prope The internal crystallization method for making fibrous and so amplifies the potential to develop tough composites, which consists of the infiltration of channels ceramic-fibre/ceramic-matrix made in the matrix with fibre material melt and The internal crystallization method for making subsequent crystallization of the melt to produce high fibrous composites, which consists of the infiltration trength fibre, is now abricate ceramic- of channels pre-made in the matrix with fibre material fibre/ceramic-matrix composites. Composites thus mclt and subscqucnt crystallization of the mclt to obtained were not optimized with respect to the produce high-strength fibres, is now applied to microstructure of the fibre, matrix and fibre/matrix fabricate ceramic-fibre/ceramic-matrix composites. The interface. However, the possibility of making oxide first stage of the work described in the present paper oxide composites by using the internal crystallization was aimed only at showing the possibility of making method has been demonstrated. Speculations about ceramic-matrix composites by the internal crystalliza- possible ways of improving mechanical properties of tion method. Hence, composite specimens obtained the composites are presented. C 1997 Elsevier Science were not optimized with respect to the microstructure Limited of the fibre matrix and fibre/matrix interface However, the real possibility of obtaining oxide/oxide Keywords: A ceramic-matrix composites; A oxides; B. composites by using the internal crystallization method fracture toughness; C. elastic properties; internal has been demonstrated. Possible ways of improving the mechanical properties of the composites are now clear: the corresponding speculations are also giv 1 INTRODUCTION 2 FABRICATION Ceramic-matrix composites are normally obtained either consolidation of pre-made fibre and matrix The method of internal crystallization includes the by growing fibres in the matrix. Methods of the first ollowing step kind include a variety of routes based on powder- 1. formation of continuous cylindrical channels in the metallurgy methods, liquid or chemical vapour infiltra matrIx, tion of the matrix material in the fibre preform, etc. 2. infiltration of the channels in the matrix with a The second group of methods includes either the well melted fibre material: ane known unidirectional solidification of eutectic mix- 3. crystallization of the fibres in the channels. tures or a less known'chemical mixing process, which In the case of ceramic-matrix composites, formation of is a modification of the powder-metallurgy scheme the channels in the matrix can the performed in a he formation of whiskers in situ. The latter process involves a stage of whiskerization reaction in a consisting of preliminary preparation of a molyb denum-wire/oxide-matrix composite and subsequent It is well known that mechanical properties. burning off of the molybdenum at a temperature of especially the fracture toughness, of ceramic-matrix sample of the matrix obtained in such a way is shown composites are very sensitive to the properties of the in Fig. 1 components and the interface. Enlarging the fabrica Easy infiltration of the matrix channels with tion possibilities to produce ceramic-matrix composites fibre material melt can be done if the matrix is wetted by the melt. In the present study of
Composites Science and Technology 51(1997) 1363-1367 0 1997 Elsevier Science Limited Printed in Northern Ireland. AU rights reserved ELSEVIER PII: SO266-3538(97)00063-8 0266-3538/97/$17.00 OXIDE/OXIDE COMPOSITES PRODUCED BY THE INTERNAL CRYSTALLIZATION METHOD S. T. Mileiko, V. I. Kazmin,* V. M. Kiiko & A. M. Rudnev Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow District 142 432, Russia (Received 2 December 1996; accepted 26 February 1997) Abstract The internal crystallization method for making fibrous composites, which consists of the infiltration of channels made in the matrix with fibre material melt and subsequent crystallization of the melt to produce highstrength fibre, is now applied to fabricate ceramicfibre/ceramic-matrix composites. Composites thus obtained were not optimized with respect to the microstructure of the fibre, matrix and jibre/matrix interface. However, the possibility of making oxide- /oxide composites by using the internal crystallization method has been demonstrated. Speculations about possible ways of improving mechanical properties of the composites are presented. 0 1997 Elsevier Science Limited Keywords: A. ceramic-matrix composites; A. oxides; B. fracture toughness; C. elastic properties; internal crystallization 1 INTRODUCTION Ceramic-matrix composites are normally obtained by either consolidation of pre-made fibre and matrix or by growing fibres in the matrix. Methods of the first kind include a variety of routes based on powdermetallurgy methods, liquid or chemical vapour infiltration of the matrix material in the fibre preform, etc. The second group of methods includes either the wellknown unidirectional solidification of eutectic mixtures’ or a less known ‘chemical mixing process’, which is a modification of the powder-metallurgy scheme aimed at the formation of whiskers in situ. The latter process involves a stage of whiskerization reaction in a mixture of the powder of matrix material and necessary reagents to form the whiskers. It is well known that mechanical properties, especially the fracture toughness, of ceramic-matrix composites are very sensitive to the properties of the components and the interface.’ Enlarging the fabrication possibilities to produce ceramic-matrix composites * Deceased. can provide a greater variety of component properties and so amplifies the potential to develop tough ceramic-fibre/ceramic-matrix composites. The internal crystallization method for making fibrous composites,3*4 which consists of the infiltration of channels pre-made in the matrix with fibre material melt and subsequent crystallization of the melt to produce high-strength fibres, is now applied to fabricate ceramic-fibre/ceramic-matrix composites. The first stage of the work described in the present paper was aimed only at showing the possibility of making ceramic-matrix composites by the internal crystallization method. Hence, composite specimens obtained were not optimized with respect to the microstructure of the fibre, matrix and fibre/matrix interface. However, the real possibility of obtaining oxide/oxide composites by using the internal crystallization method has been demonstrated. Possible ways of improving the mechanical properties of the composites are now clear: the corresponding speculations are also given. 2 FABRICATION The method of internal crystallization includes the following steps:4*5 1. formation of continuous cylindrical channels in the matrix; 2. infiltration of the channels in the matrix with a melted fibre material; and 3. crystallization of the fibres in the channels. In the case of ceramic-matrix composites, formation of the channels in the matrix can the performed in a number of ways. For a sapphire matrix, a method consisting of preliminary preparation of a molybdenum-wire/oxide-matrix composite’ and subsequent burning off of the molybdenum at a temperature of about 1150-1350°C is suitable. A general view of a sample of the matrix obtained in such a way is shown in Fig. 1. Easy infiltration of the matrix channels with the fibre material melt can be done if the matrix is wetted by the melt. In the present study of a preliminary 1363
1364 s.t Mileiko et al Fig. 2. Longitudinal section of an AlO3+ZrO2(+Y2O3) Al2O3 composite(X 100) Fig. 1. Sapphire matrix specimen with cylindrical channels left after burning out molybdenum wires. effect as a result of the phase transformation in the zirconia. At the same time, microcracking of the nature, wetting was provided by careful selection of matrix yiclds some improvemcnt in crack resistance the fibre/matrix combination. In particular, if the (Table 1) matrix material is A(or an M+N eutectic) then the When the matrix is obtained by crystallization of fibre material is an A+B eutectic (or an M+N+P eutectic mixture of two oxides, its microstructure is of eutectic). Obviously, to prevent severe dissolution of a composite nature(Fig. 6(b)). The colonies of nearly the matrix, it is necessary to keep the infiltration constant orientations of the structure which are clearly temperature just above the eutectic point. Also it seen in Fig. 6(c), together with smaller formations of should be noted that with such a choice of the constant orientations that could be called subcolonies components, the fibre/matrix interface is expected to Fig 6(b)), can have various orientations of the fibrous be strong and effects such as delamination, fibre pull phase. The same type of microstructure is character out, etc, are not to be expected to play an essential istic for the fibre/matrix interface(Fig. 6(d)which is role in enhancing the composite fracture toughness. certainly always a eutectic composed of the oxides Obviously, a procedure can be included in the contained in re an fabrication route to control the microstructure and structure of the interface yields a high interface properties of the interface and thus optimize com strength, so a macrocrack in the composite does not osite behaviour trigger specific mechanisms of toughening in ceramic The crystallization rate of the fibre is chosen do be matrix composites (fibre pull-outs, crack bridging as high as possible and the channels must he fed with the melt to compensate the volume effect at stallization. In the case under consideration withdrawal rate of about 30 cm min appeared to atisfy that condition 3 MICROSTRUCTURE AND PROPERTIES Micrographs presented in Figs 2-6 show characteristic microstructures of the composites. First, we note that the fibres are not perfectly aligned in a longitudinal section. Second, rather a high degree of interaction the fibre/matrix interface takes place in all cases. Then one can see that the partial stabilization of zirconia, as a component of the eutectics to form the fibre material,leads to a regular microstructure without any microcracking(Fig. 2). When zirconia in the fibre is not stabilized the sapphire matrix undergoes micro- Fig 3. Longitudinal section of an ALO,+ zrO,+Y,a1,Ou/ cracking (Fig. 5), almost certainly owing to a volumc 12O3+ ZrO2 composite(X 100
1364 S. T. Mileiko et al. Fig. 1. Sapphire matrix specimen with cylindrical channels left after burning out molybdenum wires. nature, wetting was provided by careful selection of the fibre/matrix combination. In particular, if the matrix material is A (or an M+N eutectic) then the fibre material is an A+B eutectic (or an M+N+P eutectic). Obviously, to prevent severe dissolution of the matrix, it is necessary to keep the infiltration temperature just above the eutectic point. Also it should be noted that with such a choice of the components, the fibre/matrix interface is expected to be strong and effects such as delamination, fibre pullout, etc., are not to be expected to play an essential role in enhancing the composite fracture toughness. Obviously, a procedure can be included in the fabrication route to control the microstructure and properties of the interface and thus optimize composite behaviour. The crystallization rate of the fibre is chosen do be as high as possible and the channels must be fed with the melt to compensate the volume effect at crystallization. In the case under consideration, a withdrawal rate of about 30 cm min-’ appeared to satisfy that condition. 3 MICROSTRUCTURE AND PROPERTIES Micrographs presented in Figs 2-6 show characteristic microstructures of the composites. First, we note that the fibres are not perfectly aligned in a longitudinal section. Second, rather a high degree of interaction at the fibreimatrix interface takes place in all cases. Then one can see that the partial stabilization of zirconia, as a component of the eutectics to form the fibre material, leads to a regular microstructure without any microcracking (Fig. 2). When zirconia in the fibre is not stabilized the sapphire matrix undergoes microcracking (Fig. 5), almost certainly owing to a volume Fig. 2. Longitudinal section of an Al,O, + ZrO,( + Y,O$ A1203 composite ( X 100). effect as a result of the phase transformation in the zirconia. At the same time, microcracking of the matrix yields some improvement in crack resistance (Table 1). When the matrix is obtained by crystallization of a eutectic mixture of two oxides, its microstructure is of a composite nature (Fig. 6(b)). The colonies of nearly constant orientations of the structure which are clearly seen in Fig. 6(c), together with smaller formations of constant orientations that could be called subcolonies (Fig. 6(b)), can have various orientations of the fibrous phase. The same type of microstructure is characteristic for the fibre/matrix interface (Fig. 6(d)) which is certainly always a eutectic composed of the oxides contained in the fibre and matrix. No doubt, such a structure of the interface yields a high interface strength, so a macrocrack in the composite does not trigger specific mechanisms of toughening in ceramicmatrix composites (fibre pull-outs, crack bridging, Fig. 3. Longitudinal section of an A&O3 + ZrO, + Y3A15012/ A&O, + ZrOZ composite ( X 100)
Oxide/oxide ceramics by internal crystallization 1365 When an oxide with a low thermal expansion coefficient. such as mullite. is introduced in the fibre material then neither fibre nor matrix undergoes microcracking(Fig 4) 4 FIBRE CONTAINING TIAL O The complex oxide T1O2 A2Os appears to be attractive candidate for use as a component of an oxide/oxide composite because of its low thermal expansion coefficient (about 2X10 K ) On the other hand, its Youngs modulus is low (about 50 GPa). An even more essential drawback of this oxide is its instability in the temperature interval between 700 and 1300C. Moreover, the stoichiometry of Fig. 4. Longitudinal section of an Al O3+ Zroz+ mullite, titanium-containing oxides is known to be easily lost Al2O3+ZrO2 composite(X 100) on heating in vacuum. Hence, dealing with TiAlO, in both fabrication and service stages requires extreme interface delamination, etc. ) Some evidence of such behaviour of the composites can be seen in Fig. 7 If an oxide which is known to stabilize zirconia is present in the fibre, then a zirconia-containing matrix appears to remain uncracked(Fig. 3): however, the mechanical properties of such composites are low (c) Fig. 6. Scanning electron micrographs of the cross-section of an Al,Oa+ ZrO,+Y3AlsO1Al,O )3+Zro + mgo)com- Fig. 5. Longitudinal section of an AlO3+ ZrO2Al2O3 posite:(a) general view,(b) the ix, (c)a fibre and composite(×100 surroundings, (d)the fibre/matrix interface Fibre material Matrix material Fibre volume Bending strength Critical stress (MPa) intensity factor(MPa vm) Al,O3+ Zro Y2O3+Al2O3 1,O3+ZrO2+Y3AlsO1 Al,O3+ZrO2+mullite AlO3+Zro
Oxide/oxide ceramics by internal crystallization 1365 Fig. 4. Longitudinal section of an AlzO, + ZrO, + mullitel A&O3 + ZrO, composite ( X 100). interface delamination, etc.). Some evidence of such behaviour of the composites can be seen in Fig. 7. If an oxide which is known to stabilize zirconia is present in the fibre, then a zirconia-containing matrix appears to remain untracked (Fig. 3): however, the mechanical properties of such composites are low (Table 1). Fig. 5. Longitudinal section of an A1,0,+ZrOZIA1203 composite ( X 100). When an oxide with a low thermal expansion coefficient, such as mullite, is introduced in the fibre material then neither fibre nor matrix undergoes microcracking (Fig. 4). 4 FIBRE CONTAINING TIAL205 The complex oxide Ti02*A1,05 appears to be an attractive candidate for use as a component of an oxide/oxide composite because of its low thermal expansion coefficient (about 2 X 10F6 K-‘). On the other hand, its Young’s modulus is low (about 50 GPa). An even more essential drawback of this oxide is its instability in the temperature interval between 700 and 1300”C6 Moreover, the stoichiometry of titanium-containing oxides is known to be easily lost on heating in vacuum. Hence, dealing with TiA1205 in both fabrication and service stages requires extreme Fig. 6. Scanning electron mlcrographs of the cross-section of an A1203 + ZrOz + Y3A1501JA120s + ZrOz( + MgO) composite: (a) general view, (b) the matrix, (c) a fibre and surroundings, (d) the fibrelmatrix interface. Table 1. Strength and fracture toughness of composites Fibre material A&O3 + ZrO, A1203 + ZrOz A1203 + ZrO, + Y3A15012 A1,03 + ZrO, + mullite Matrix material Y203 + A&O3 A&O, A1203 + ZrO, A1203 + ZrO, Fibre volume Bending strength fraction (MPa) 0.40 l@-300 0.40 160 0.40 150 0.40 220 Critical stress intensity factor (MPa V/m) 0.9 5-6 3-4 -
1366 s.T. Mileiko et al accuracy. It seems interesting, nevertheless, to study 10 h revealed Al2O3 and TiO2, and did not reveal composites with a TiAlOs conponent TiAl2Os. Hence, after the second annealing, the fibre Composite specimens with an Al2O3 matrix were consists of a mixture of two simple oxides. The Laue obtaincd by infiltration of the matrix channels with an patterns showed that the fibre contains large volumes oxide mixture corresponding to a eutectic composition of single crystalline materials. Optical micrographs of in the Al,O,TiAl, O, system. After infiltration in specimens after the first and the second annealing are vacuum and crystallization of the fibre, the composite similar to that shown in Fig. 5, with an intensive matrix as annealed at 1400-1500 C for 5 h in air to restore microcracking he stoichiometry of titanium-containing oxides. X-ray The phase composition and microstructure of the phase analysis revealed both Al,O, and TiAl2Os in composites are also indicated by the elastic character pecimens after annealing. To observe decomposition istics determined on cubic specimens by measurements of TiAl,Os into Al2O3 and TiO2, additional annealing of ultrasonic wave velocities. The cubic specimens with at temperatures between 1100 and 1300oC was an edge size of 1 cm were prepared in such a way as to performed. X-ray phase analysis of a specimen which make one side to be normal to the fibre direction (i.e had undergone the additional annealing at 1200C for the xi direction). The wave dispersion within the frequency interval used(0.5-5 MHz) was marked, so the characteristic wave velocities were obtained by extrapolating the velocity/frequency dependences to zero frequency. Results of the calculation of the components of the elastic tensor, Cll, C2 and C33, according to the well-known formulae, are presented in Table 2 We see that. first, the decomposition of TiAl2Os into Al2O, and TiO, after low-temperature annealing yields a large increase in all of the elastic constants The difference between the values of Cz and C33 is certainly caused by the non-hexagonal arrangement of the fibres in the plane normal to the fibre direction The irregular microcracking of the matrix makes it difficult to compare possible theoretical evaluations of the elastic characteristics with the measured values. The dependences of bending strength of the composites on annealing regimes are presented in Figs 8 and 9. The dependences can be considered to be a result of the kinetics of two processes, the first being changes in the composition of oxides approaching stoichiometry during annealing in air, and the second being the formation of alumina and rutile as a result of of TiAl,Os. Com of data presented in Fig. 8, we may suppose that annealing of the as-produced composites at 1400oC for 5 h is in sufficient to restore the stoichiometry as annealing at 1500C yields higher strength composites It is then obvious(see also data on the elastic moduli and phase composition) that the strength of the TiAl,Os fibre is very low: the second annealing at 1300.C leads to a drastic drop in the strength(Fig 8) The more complete the decomposition of TiAl,O, the gth of the comp should still be a third process yielding strength crease during the second annealing ng. In fact. the x ray phase analysis does not reveal the presence TiAl,Os in the specimens of highest strength achieved but looking at the temperature/strength and time- /strength dependences shown we see that a maximum Fig. 7. Failure surface of an Al203+ZrO2(+Y2O, 0, in strength has not been reached. It is impossible to composite identify the third process (or processes) without
1366 S. T. Mileiko et al. accuracy. It seems interesting, nevertheless, to study composites with a TiA1,OS component. Composite specimens with an A1201 matrix were obtained by infiltration of the matrix channels with an oxide mixture corresponding to a eutectic composition in the Al,O,iTiA1,OS system. After infiltration in vacuum and crystallization of the fibre, the composite was annealed at 1400-1500°C for 5 h in air to restore the stoichiometry of titanium-containing oxides. X-ray phase analysis revealed both A1,03 and TiAl*O, in specimens after annealing. To observe decomposition of TiAl*O, into A&O, and Ti02, additional annealing at temperatures between 1100 and 1300°C was performed. X-ray phase analysis of a specimen which had undergone the additional annealing at 1200°C for Fig. 7. Failure surface of an A&O, +ZrO,( +Y,O,)/AI,O, composite. 10 h revealed A1,03 and TiOz, and did not reveal TiA120s. Hence, after the second annealing, the fibre consists of a mixture of two simple oxides. The Laue patterns showed that the fibre contains large volumes of single crystalline materials. Optical micrographs of specimens after the first and the second annealing are similar to that shown in Fig. 5, with an intensive matrix microcracking. The phase composition and microstructure of the composites are also indicated by the elastic characteristics determined on cubic specimens by measurements of ultrasonic wave velocities. The cubic specimens with an edge size of 1 cm were prepared in such a way as to make one side to be normal to the fibre direction (i.e. the x1 direction). The wave dispersion within the frequency interval used (0.5-5 MHz) was marked, so the characteristic wave velocities were obtained by extrapolating the velocity/frequency dependences to zero frequency. Results of the calculation of the components of the elastic tensor, C,,, C,, and &, according to the well-known formulae, are presented in Table 2. We see that, first, the decomposition of TiAl,OS into A120, and TiOz after low-temperature annealing yields a large increase in all of the elastic constants. The difference between the values of C,, and C,, is certainly caused by the non-hexagonal arrangement of the fibres in the plane normal to the fibre direction. The irregular microcracking of the matrix makes it difficult to compare possible theoretical evaluations of the elastic characteristics with the measured values. The dependences of bending strength of the composites on annealing regimes are presented in Figs 8 and 9. The dependences can be considered to be a result of the kinetics of two processes, the first being changes in the composition of oxides approaching stoichiometry during annealing in air, and the second being the formation of alumina and rutile as a result of the decomposition of TiA120,. Comparing the two sets of data presented in Fig. 8, we may suppose that annealing of the as-produced composites at 1400°C for 5 h is in sufficient to restore the stoichiometry as annealing at 1500°C yields higher strength composites. It is then obvious (see also data on the elastic moduli and phase composition) that the strength of the TiA1,OS fibre is very low: the second annealing at 1300°C leads to a drastic drop in the strength (Fig. 8). The more complete the decomposition of TiA120s, the higher the strength of the composite. However, there should still be a third process yielding strength increase during the second annealing. In fact, the Xray phase analysis does not reveal the presence of TiAl*O, in the specimens of highest strength achieved, but looking at the temperature/strength and time- /strength dependences shown we see that a maximum in strength has not been reached. It is impossible to identify the third process (or processes) without
Oxide/oxide ceramics by internal crystallization 1367 Table 2. Elastic characteristics of the composites containing TiAL,Os Specimen characterization Cu(GPa) C2(GPa) C33(GPa) at 1500 C for 5 h in air, specimen 1 7285 19508 8633 at 1500C for 5 h in air plus annealing at 324 63 40632 for 10 h in air, specimen 2 experiments. We may simply specula 5 CONCLUSION ole recrystallization of alumina andyo s leading to their strengthening The internal crystallization method, which was initially developed to produce metal-matrix composites, can now be used to obtain oxide/oxide fibrous composites. The complicated structure of such materials calls for an optimization procedure, which remains to be done, properties of the composite ACKNOWLEDGEMENTS The work was partially supported by the russian Foundation for Basic Research under grant First annealing 1400-sh 93-013-16742. The authors are thankful to dr V. F 110011501200,12501300 Degtereva for the X-ray phase analysis of composites and Dr N. v. Kondrashova for scanning electron Fig. 8. Bending strength of AlO3-TiAlOSALO, compos Ites versus the temperature of the second annealing stage The annealing time is 10 h. REFERENCES 1. Rabinovich, M, Stohr, J. F, Khan. T. and Bibring, H In Fabrication of Composites(Handbook of Composites, Vol. 4), ed. A. Kelly and S. T. Mileiko. North Holland, Amsterdam, 1983, pp. 295-372 2. Evans, A G. and Zok, F. w, The physics and mechanics of fibre-reinforced brittle matrix composites. J. Mater. Sci. 1994,29,3857-38 3. Mileiko, S. T. and Kazmin, V.l., Crystallization of fibres inside a matrix: a new way of fabrication of composites. J Mater..sci1992,27,2165-217 20 4. Mileiko, S.T. and Kazmin. v.l. structure and mechanical roperties of oxide fibre reinforced metal matrix ation 5. Kazmin. V. I. Mileiko, S. T vardo Fig. 9. Bending strength of AlO3-TiALOS /Al2O3 compos Strength of ceramic matrix fibre composites. ites versus the time of the second annealing stage. The mpos. Sci. Technol 1990, 38, annealing temperature is 1200C, 6. Hennicke, H M, J de Phys. 1986, 47, C1-533
Oxide/oxide ceramics by internal crystallization Table 2. Elastic characteristics of the composites containing ‘II&O5 Specimen characterization Cl1 (GRa) C,, (GRa) C,, (GPa) 1367 Annealing at 1500°C for 5 h in air, specimen 1 72.85 19508 158.99 The same, specimen 2 86.33 184.39 184.39 Annealing at 1500°C for 5 h in air plus annealing at 32463 406.32 479.27 1200°C for 10 h in air, specimen 2 special experiments. We may simply speculate on possible recrystallization of alumina and/or rutile phases leading to their strengthening. 100 - 0 < 80- % “a 60 - 40 - 1100 1150 1200 1 C? 50 T/O 1300 Fig. 8. Bending strength of A120s-TiA1,05/A1203 composites versus the temperature of the second annealing stage. The annealing time is 10 h. I I 1 I 1 I 100 - ilk s u 8o . 0 1 ;: 60- D . 40 - 2 4 6t/h 8 10 Fig. 9. Bending strength of Al@-TiA1205/Alz03 composites versus the time of the second annealing stage. The annealing temperature is 1200°C. 5 CONCLUSION The internal crystallization method, which was initially developed to produce metal-matrix composites, can now be used to obtain oxide/oxide fibrous composites. The complicated structure of such materials calls for an optimization procedure, which remains to be done, to achieve a satisfactory combination of mechanical properties of the composite. ACKNOWLEDGEMENTS The work was partially supported by the Russian Foundation for Basic Research under grant 93-013-16742. The authors are thankful to Dr V. F. Degtereva for the X-ray phase analysis of composites and Dr N. V. Kondrashova for scanning electron microscopy. REFERENCES 1. 2. 3. 4. 5. 6. Rabinovich, M., Stohr, J. F., Khan. T. and Bibring, H., Directionally solidified composites for application at high temperature. In Fabrication of Composites (Handbook of Composites, Vol. 4) ed. A. Kelly and S. T. Mileiko. NorthHolland, Amsterdam, 1983, pp. 295-372. Evans, A. G. and Zok, E W., The physics and mechanics of fibre-reinforced brittle matrix composites. J. Muter. Sci. 1994,29,3857-3896. Mileiko, S. T. and Kazmin, V. I., Crystallization of fibres inside a matrix: a new way of fabrication of composites. J. Mater. Sci. 1992,27,2165-2172. Mileiko, S. T. and Kazmin, V. I., Structure and mechanical properties of oxide fibre reinforced metal matrix composites produced by the internal crystallization method. Compos. Sci. Technol. 1992,45,209-220. Kazmin, V. I., Mileiko, S. T. and Tvardovsky, V. V., Strength of ceramic matrix-metal fibre composites. Compos. Sci. Technol. 1990,38,69-84. Hennicke, H. M., J. de Phys. 1986,47, Cl-533
