S Ji et al. /Journal of the European Ceramic Society 25(2005)301-317 J15(1273K).(ii) The bulk flow strength of the layered com- 113(1173 K, 10/s) Particulate composites (50%Qtz,50%CAS) posites increases with increasing d/h ratio even though the P=300 MPa volume fractions of Qtz and CAs are constant(Fig 9). For 261373K,10/s) example, the samples with d/h=9 are significantly stronger than those with d/h=3. and the latter are also stronger than the samples with d/h= 1. It is important to note that J25(1373K2510/8) the layered composites with d/h=3 have a flow strength 4(1373K10 similar to the particle composites deformed under the same J9(1473K,10-s conditions. Furthermore, all the layered composites are con- sistently stronger than the pure CAs aggregates although the J4(1373K10°ls) CAS forms continuous layers normal to o1. The difference 000010020030040050 in the bulk flow strength between samples J22 and J40X Axial strain is due to the following fact: j22 consists of a 10 mm thick layer of CAs between two 5 mm thick layers of Qtz while volume fractions of quartz and CAS, deformed under axial compression J40X is composed of a 10 mm thick layer of Qtz between at a confining pressure of 300 MPa, strain rates of 10-, 2.5 x 10-and two 5 mm thick layers of CAs. This indicates that the con- 10-45-I, and temperatures of 1173, 1273, 1373 1473K centration of soft material such as Cas into a single thicker layer results in remarkable softening at higher strains strength of the Qtz-CAs particle composite reduces about 40%. The coefficient of strain softening, do/de, at lower temperature or higher strain rate is larger than at higher 4. Microstructures temperature or lower strain rate. Comparison of Fig. 8 with Fig 6 reveals that the PC samples are much stronger than the The CAs aggregates and particularly the Cas layers in the pure CAS aggregates. At a constant strain rate of 10-s deformed Qtz-CAs layered composites are intensively de the maximum flow strength of PC samples is about two formed with well-developed foliation(Fig. 10a), the latter is three and six times higher than that of CAs aggregate at defined by preferential alignment of CAs lath-shaped crys 1273, 1373 and 1473K, respectively. Therefore, quartz is an tals and polycrystalline ribbons or fibers. The spherulites, effective reinforcement to the CAS matrix particularly when which initially had a spherical shape in HIPed CAS aggre the composite material used at high temperatures gates or layers within the layered composites(Fig. 10a), be- Representative compressive stress-strain curves for LC come oblate-ellipsoidal in the deformed specimens with their samples deformed by layering-normal compression at a con- major axes in the foliation plane and minor axes parallel to fining pressure of 300 MPa and a strain rate of 10-5s-I are the compressive direction(o1). The variation in spherulite shown in Fig 9. Inspection of these curves reveals the fol- ape displays a spatial variation of strain in each individual lowing features: () Flow strength of the layered compos- CAS layer of deformed LC samples: lower strain near the ites decreases with increasing temperature, as shown by the upper and lower boundaries of the layer and higher strain comparison among samples J1(1473 K), J2(1373 K)and in the central part of the layer(Fig. 10a). At large shorten- strains(x20%), the spherulites were recrystallized into ery fine neograins and the recrystallization generally started first from the center of spherulites and from the spherulite 50%at.50%cAs) boundaries at high angles to I Strain rate=10/s The texture of cas from a cas lay red composite(sample J22) was measured using the eBsd technique 21-23 This CAS layer achieved an average short J(1473K.dh=9 J2(1373K,dh·9 ening strain of 52%. Grid measurements with 40 um spac X(1373K,dh=1) ing were made across the central part of the CAs layer J22(1373Kdh=1) (Fig. 10a), where the accumulated strain is maximum. As shown in Fig. Il, the(0 10) poles display a strong concentra- tion around the oi direction while the [ 100] directions are preferentially aligned in the foliation plane perpendicular to 0.000.050.100.150.200.250.300.350.40 o1. Both [00 1] directions and(00 1) poles are scattered. The Axial strain CPO pattern can be interpreted as a result of dislocation slip on the(010)[100] system. If anisotropic growth of CAs Fig. 9. Stress-strain ed composites containing equal vol- ume fractions of quartz and CAs, deformed under axial compression at under the axially compressive strain field were the me temperatures of 1273, 1373 and 1473K; d and h are the diameter and aligned in the foliation plane perpendicll ection should be of 300 MPa, a constant strain rate of 10-5s-I,and nism for the CPO formation, the [001] g because thickness of material layer, respectively 00 1] is the fastest growth direction for CAS. 28,29306 S. Ji et al. / Journal of the European Ceramic Society 25 (2005) 301–311 Fig. 8. Stress–strain curves for particulate composites containing equal volume fractions of quartz and CAS, deformed under axial compression at a confining pressure of 300 MPa, strain rates of 10−5, 2.5 × 10−5 and 10−4 s−1, and temperatures of 1173, 1273, 1373 and 1473 K. strength of the Qtz–CAS particle composite reduces about 40%. The coefficient of strain softening, dσ/dε, at lower temperature or higher strain rate is larger than at higher temperature or lower strain rate. Comparison of Fig. 8 with Fig. 6 reveals that the PC samples are much stronger than the pure CAS aggregates. At a constant strain rate of 10−5 s−1, the maximum flow strength of PC samples is about two, three and six times higher than that of CAS aggregate at 1273, 1373 and 1473 K, respectively. Therefore, quartz is an effective reinforcement to the CAS matrix particularly when the composite material used at high temperatures. Representative compressive stress–strain curves for LC samples deformed by layering-normal compression at a con- fining pressure of 300 MPa and a strain rate of 10−5 s−1 are shown in Fig. 9. Inspection of these curves reveals the fol￾lowing features: (i) Flow strength of the layered compos￾ites decreases with increasing temperature, as shown by the comparison among samples J1 (1473 K), J2 (1373 K) and 0 100 200 300 400 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 Axial strain Differential stress, MPa Layered composites (50% Qtz, 50% CAS) Strain rate = 10-5 /s P = 300 MPa J22 (1373 K, d/h=1) J36 (1373 K, d/h=3) J1 (1473 K, d/h=9) J15 (1273 K, d/h=9) J2 (1373 K, d/h=9) J40X (1373 K, d/h=1) Fig. 9. Stress–strain curves for layered composites containing equal vol￾ume fractions of quartz and CAS, deformed under axial compression at a confining pressure of 300 MPa, a constant strain rate of 10−5 s−1, and temperatures of 1273, 1373 and 1473 K; d and h are the diameter and thickness of material layer, respectively. J15 (1273 K). (ii) The bulk flow strength of the layered com￾posites increases with increasing d/h ratio even though the volume fractions of Qtz and CAS are constant (Fig. 9). For example, the samples with d/h = 9 are significantly stronger than those with d/h = 3, and the latter are also stronger than the samples with d/h = 1. It is important to note that the layered composites with d/h = 3 have a flow strength similar to the particle composites deformed under the same conditions. Furthermore, all the layered composites are con￾sistently stronger than the pure CAS aggregates although the CAS forms continuous layers normal to σ1. The difference in the bulk flow strength between samples J22 and J40X is due to the following fact: J22 consists of a 10 mm thick layer of CAS between two 5 mm thick layers of Qtz while J40X is composed of a 10 mm thick layer of Qtz between two 5 mm thick layers of CAS. This indicates that the con￾centration of soft material such as CAS into a single thicker layer results in remarkable softening at higher strains. 4. Microstructures The CAS aggregates and particularly the CAS layers in the deformed Qtz–CAS layered composites are intensively de￾formed with well-developed foliation (Fig. 10a), the latter is defined by preferential alignment of CAS lath-shaped crys￾tals and polycrystalline ribbons or fibers. The spherulites, which initially had a spherical shape in HIPed CAS aggre￾gates or layers within the layered composites (Fig. 10a), be￾come oblate-ellipsoidal in the deformed specimens with their major axes in the foliation plane and minor axes parallel to the compressive direction (σ1). The variation in spherulite shape displays a spatial variation of strain in each individual CAS layer of deformed LC samples: lower strain near the upper and lower boundaries of the layer and higher strain in the central part of the layer (Fig. 10a). At large shorten￾ing strains (>∼20%), the spherulites were recrystallized into very fine neograins and the recrystallization generally started first from the center of spherulites and from the spherulite boundaries at high angles to σ1. The texture of CAS from a CAS layer within a lay￾ered composite (sample J22) was measured using the EBSD technique.21–23 This CAS layer achieved an average short￾ening strain of 52%. Grid measurements with 40
m spac￾ing were made across the central part of the CAS layer (Fig. 10a), where the accumulated strain is maximum. As shown in Fig. 11, the (0 1 0) poles display a strong concentra￾tion around the σ1 direction while the [1 0 0] directions are preferentially aligned in the foliation plane perpendicular to σ1. Both [0 0 1] directions and (0 0 1) poles are scattered. The CPO pattern can be interpreted as a result of dislocation slip on the (0 1 0)[1 0 0] system. If anisotropic growth of CAS under the axially compressive strain field were the mecha￾nism for the CPO formation, the [0 0 1] direction should be aligned in the foliation plane perpendicular to σ1 because [0 0 1] is the fastest growth direction for CAS.28,29