S.T. Mileiko Current Opinion in Solid State and Materials Science 9(2005)219-229 Table I creep properties that have been really observed for a fam- Constituents prop used in model calculations ily of ICM-oxide-matrix/nickel-based-matrix composites Interface property Fibre properties at 1150C. The upper line corresponds to an upper limit (o= l mm. of the he creep resistance of ICM-oxide-m d=0.1mm) metal-matrix composites at 1150C. It should be empha o(D(MPa) sized that the experimental point obtained earlier for interface, Vr>0.25 0.01 interface, Vr<0.15 0.5 33 alumina-YAG-eutectic-matrix/molybdenum-matrix com- posite at the same temperature and sapphire-fibre/TiAl- matrix composite at 900-1000C lie nearly on this line Matrix properties Note that both composites are characterised by an ideal 41.5 MPa Inter Note: The fibre strength at a= l corresponds to data presented in Fig. 7. The experiments and results of calculations based on the micro-mechanical model allow expecting a maximum use stronger the interface, the larger portion of the fibre sur- temperature for the oxide-fibre/Ni-based-matrix system face is in contact with the matrix. A continuous interface to be at least 1150C [16]. The criterion for the maximum was observed in the Mo-based composite. Therefore, the temperature is that specific creep resistance of a composite assumption made in modelling creep behaviour, which is equal to that of a superalloy with density of 8 g/cm at yields Eq. (6), namely introducing continuity factor a, 150 MPa has a physical meaning rather than produces just a phe- A further increase in the use temperature of composites nomenological parameter in the mechanical model. With in the oxide/Ni-system depends on the ability to appropri- increasing the value of a a larger portion of the surface ately control the defects are healed that yields an increase in the fibre effec- tive strength. This effect is added to a normal strength/ 4. Oxide-fibre/oxide-matrix composites scale effect It is informative to replot the calculated dependencies The fibre/ matrix interface in such composites made of presented in Fig. 13b together with that for an ideal inter- brittle continuances entirely determines their applicability face (a= 1), which corresponds also to an increase in the as structural materials. The problem is of importance since characteristic fibre strength to a value determined by test- such composites being reinforced with single crystalline ing oxide/molybdenum composites(Fig. 7). This is done fibres promised the use temperatures up to.C(see weak and strong (real) interfaces does actually represent the macrostructures of brittle-fibre/brittle-matrix compos- tes with enhanced fracture toughness. The first one is based on introducing a"weak"interphase between the fibre and matrix, which deviates a macrocrack and pro- vides the energy dissipation due to pull-out(see, for exam- 240 Eutectic-fibre/Mo-matrix Sapphire-fibre/TiAl-matrix, 900-1000"C ple [27]). The second one used mostly for oxide/oxide composites is actually nearly the same, just the weak inter- 200 face occurs as a result of using a porous matrix [28, 29 The ICM-fibres can obviously be used in both two ways The measurements of values of the critical stress intensity factor K" for sapphire/carbon-interphase/alumina compos- ites with fibre volume fraction of 0. 12-0. 28 yield values of critical stress intensity factor equal to 11.4+0.9 MPa m"/2 as compared with corresponding values for un-reinforced natrix o obtained under the same conditions of 5.9+1.1 MPa m/2[30]. Results of the experimental study of creep behaviour of such model composites at 1200C [31] show that their creep behaviour could be described by Eq(6). The calculated dependence of the creep resis- tance(stress to cause 1% creep strain for 100 h)on fibre volume fraction is presented in Fig. 15. The experimental Fibre volume fraction data for four specimens tested are given in Table 2, and typical microstructures of the composites are presented in Fig. 14. Creep resistance versus fibre volume fraction of ICM-oxide- matrix/nickel Fig. 16. Specimen A2053 is characterised by extremely iven in Table le l. Also expe perimental points for the alumina-YAG-eutectic- non-homogeneous fibre packing: it causes a decrease fibre/TiAl-matrix(see trix composite taken from Ref [8]and sapphire- the creep resistance. It should be noted that the effective Fig. 10)composit creep properties of the matrix occur to be quite moderatestronger the interface, the larger portion of the fibre sur￾face is in contact with the matrix. A continuous interface was observed in the Mo-based composite. Therefore, the assumption made in modelling creep behaviour, which yields Eq. (6), namely introducing continuity factor a, has a physical meaning rather than produces just a phe￾nomenological parameter in the mechanical model. With increasing the value of a a larger portion of the surface defects are healed that yields an increase in the fibre effec￾tive strength. This effect is added to a normal strength/ scale effect. It is informative to replot the calculated dependencies presented in Fig. 13b together with that for an ideal inter￾face (a = 1), which corresponds also to an increase in the characteristic fibre strength to a value determined by test￾ing oxide/molybdenum composites (Fig. 7). This is done in Fig. 14. A field between the lines corresponding to weak and strong (real) interfaces does actually represent creep properties that have been really observed for a fam￾ily of ICM-oxide–matrix/nickel-based-matrix composites at 1150 C. The upper line corresponds to an upper limit of the creep resistance of ICM-oxide–matrix/ metal–matrix composites at 1150 C. It should be empha￾sized that the experimental point obtained earlier for alumina–YAG-eutectic–matrix/molybdenum–matrix com￾posite at the same temperature and sapphire–fibre/TiAl– matrix composite at 900–1000 C lie nearly on this line. Note that both composites are characterised by an ideal interface. The experiments and results of calculations based on the micro-mechanical model allow expecting a maximum use temperature for the oxide–fibre/Ni-based-matrix system to be at least 1150 C [16]. The criterion for the maximum temperature is that specific creep resistance of a composite is equal to that of a superalloy with density of 8 g/cm3 at 150 MPa. A further increase in the use temperature of composites in the oxide/Ni-system depends on the ability to appropri￾ately control the fibre/matrix interface. 4. Oxide–fibre/oxide–matrix composites The fibre/matrix interface in such composites made of brittle continuances entirely determines their applicability as structural materials. The problem is of importance since such composites being reinforced with single crystalline fibres promised the use temperatures up to 1600 C (see Fig. 8). There have been known two ways of organizing the macrostructures of brittle–fibre/brittle–matrix compos￾ites with enhanced fracture toughness. The first one is based on introducing a ‘‘weak’’ interphase between the fibre and matrix, which deviates a macrocrack and pro￾vides the energy dissipation due to pull-out (see, for exam￾ple [27]). The second one used mostly for oxide/oxide composites is actually nearly the same, just the weak inter￾face occurs as a result of using a porous matrix [28,29]. The ICM-fibres can obviously be used in both two ways. The measurements of values of the critical stress intensity factor K* for sapphire/carbon-interphase/alumina compos￾ites with fibre volume fraction of 0.12–0.28 yield values of critical stress intensity factor equal to 11.4 ± 0.9 MPa m1/2 as compared with corresponding values for un-reinforced matrix obtained under the same conditions of 5.9 ± 1.1 MPa m1/2 [30]. Results of the experimental study of creep behaviour of such model composites at 1200 C [31] show that their creep behaviour could be described by Eq. (6). The calculated dependence of the creep resis￾tance (stress to cause 1% creep strain for 100 h) on fibre volume fraction is presented in Fig. 15. The experimental data for four specimens tested are given in Table 2, and typical microstructures of the composites are presented in Fig. 16. Specimen A2053 is characterised by extremely non-homogeneous fibre packing; it causes a decrease in the creep resistance. It should be noted that the effective creep properties of the matrix occur to be quite moderate. Table 1 Constituents properties used in model calculations Interface property Fibre properties (lo = 1 mm, d = 0.1 mm) a brðfÞ 0 (MPa) Weak interface, Vf > 0.25 0.01 3 150 Strong interface, Vf < 0.15 0.5 3 450 Ideal interface 1 3 600 Matrix properties gm = 104 h1 , m = 2.8, rm = 41.5 MPa Note: The fibre strength at a = 1 corresponds to data presented in Fig. 7. Fig. 14. Creep resistance versus fibre volume fraction of ICM-oxide– matrix/nickel-based-matrix composites. Parameters in the creep model are given in Table 1. Also experimental points for the alumina–YAG-eutectic– matrix/molybdenum–matrix composite taken from Ref. [8] and sapphire– fibre/TiAl–matrix (see Fig. 10) composites are shown. S.T. Mileiko / Current Opinion in Solid State and Materials Science 9 (2005) 219–229 227