S. i et al. / Journal of the European Ceramic Society 25(2005)301-311 (i)In a composite, the softer phase deforms at a greater strain rate(Eis)than the bulk strain rate of the composite, Ec. Then the in situ softer phase in the composite has a higher resistance to plastic flow(ois)than the same material in bulk form(oo). This effect can be evaluated by taking account of relative strain amounts partitioned between the Qtz and CAs layers using .gK19.B2.99 -potsdam J4-GA where n is the stress exponent. The hardening effect is more pronounced when n has a smaller value than a larger value (il) Flow of the soft phase in a composite is constrained by the hard phase. As a result, the in situ response of the soft phase is hardened relative to its single phase aggregate3I The compressive flow strength of the con strained weak layer(ow)is higher than the compressive flow strength without constraint or in the monolithic ag- (oo)at a given pl 小 where u is the friction coefficient of the interface be- tween the strong and weak layers, d and h are the di- ameter and thickness of the weak layer. For a given Fig. 13. TEM(bright field) micrographs showing typical substructures u, even though it is as small as 0.25, increasing the of dislocations in deformed CAs grains from sample J4-QA(particulate diameter-to-thickness ratio(d/h)of the layer easily leads composite deformed by axial compression at 300 MPa, 1373K, 10-s and 29% strain). The distribution of dislocations is heterogeneous. (a) to a significant increase in its resistance to compressive Two sets of dislocations that are tangled are observed in a plane nearly plastic deformation. Taking into account the above two parallel to(0 10).(b) Grain boundary migration toward highly strained effects described by Eqs. (1)(2), we can readily explain grains with very high(tangled) dislocation density, forming recrystallized the hardening of the Qtz-CAS layered composites with neograins(n). respect to the bulk strength of monolithic CAs 5.2. Constant load creep tests under uniaxial compression versus constant strain-rate deformation tests under axial introducing quartz grains in the CAs matrix as either par- compression ticulate or layered reinforcements. Ceramic composites of these sorts can be easily prepared by hot-pressing or sinter- Most of the previous uniaxial creep tests.- were per ing Quartz is inexpensive and also has the advantage that it formed under constant load and at ambient pressure, where is chemically stable and does not react with CAS up to the the CAs samples have a significant increase in porosity be eutectic melting point. Relative to the monolithic CAS ag- cause there is no sufficient confining pressure to hinder the gregates, the overall compressive flow strength for the partic- creep-induced cavitation. For example, Nair et al. showed ulate composites with equal volume fractions of quartz and 8% axial strain resulted in an increase in porosity to 24% in CAS increases more than fourfold, and that for the layered their CAS-lI specimens Furthermore, their microstructural composites of the same composition increases substantially analysis suggested no significant difference in grain mor- with decreasing the thickness of the layers. For instance, phology between deformed and undeformed samples. The the layered composites with d/h=9 have an overall flow cavitation at the grain boundaries could be the dominant de- strength 10 times higher than that of the monolithic CAs ag- formation mechanism for the previous creep tests conducted gregates. In both particulate and layered composites, quartz at ambient pressure. However, our experiments on the de- essentially rigid in the plastic CAS matrix. The above formation of the CAs aggregates and CAS-Qtz composites sults are the principal mechanical findings of this study were carried out at a confining pressure of 300 MPa, where The layering-induced hardening of Qtz-CAS multilayers cavitation could not occur. TEM observations and textu- can be interpreted by taking into account the following me- ral analysis strongly suggest that the plastic deformation of chanical processes. CAS under the experimental conditions is dominated by dis-S. Ji et al. / Journal of the European Ceramic Society 25 (2005) 301–311 309 Fig. 13. TEM (bright field) micrographs showing typical substructures of dislocations in deformed CAS grains from sample J4-QA (particulate composite deformed by axial compression at 300 MPa, 1373 K, 10−5 s−1 and 29% strain). The distribution of dislocations is heterogeneous. (a) Two sets of dislocations that are tangled are observed in a plane nearly parallel to (0 1 0). (b) Grain boundary migration toward highly strained grains with very high (tangled) dislocation density, forming recrystallized neograins (n). introducing quartz grains in the CAS matrix as either par￾ticulate or layered reinforcements. Ceramic composites of these sorts can be easily prepared by hot-pressing or sinter￾ing. Quartz is inexpensive and also has the advantage that it is chemically stable and does not react with CAS up to the eutectic melting point. Relative to the monolithic CAS ag￾gregates, the overall compressive flow strength for the partic￾ulate composites with equal volume fractions of quartz and CAS increases more than fourfold, and that for the layered composites of the same composition increases substantially with decreasing the thickness of the layers. For instance, the layered composites with d/h = 9 have an overall flow strength 10 times higher than that of the monolithic CAS ag￾gregates. In both particulate and layered composites, quartz is essentially rigid in the plastic CAS matrix. The above results are the principal mechanical findings of this study. The layering-induced hardening of Qtz–CAS multilayers can be interpreted by taking into account the following me￾chanical processes: (i) In a composite, the softer phase deforms at a greater strain rate (ε˙is) than the bulk strain rate of the composite, ε˙c. Then the in situ softer phase in the composite has a higher resistance to plastic flow (σis) than the same material in bulk form (σ0). This effect can be evaluated by taking account of relative strain amounts partitioned between the Qtz and CAS layers using σis = σ0
ε˙is ε˙c 1/n (1) where n is the stress exponent. The hardening effect is more pronounced when n has a smaller value than a larger value. (ii) Flow of the soft phase in a composite is constrained by the hard phase. As a result, the in situ response of the soft phase is hardened relative to its single phase aggregate.31 The compressive flow strength of the con￾strained weak layer (σw) is higher than the compressive flow strength without constraint or in the monolithic ag￾gregate (σ0) at a given plastic strain,32 σw = 2σ0
µ d h −2 exp
µ d h − µ d h − 1  (2) where µ is the friction coefficient of the interface be￾tween the strong and weak layers, d and h are the di￾ameter and thickness of the weak layer. For a given µ, even though it is as small as 0.25, increasing the diameter-to-thickness ratio (d/h) of the layer easily leads to a significant increase in its resistance to compressive plastic deformation. Taking into account the above two effects described by Eqs. (1)–(2), we can readily explain the hardening of the Qtz–CAS layered composites with respect to the bulk strength of monolithic CAS. 5.2. Constant load creep tests under uniaxial compression versus constant strain-rate deformation tests under axial compression Most of the previous uniaxial creep tests5,8–11 were per￾formed under constant load and at ambient pressure, where the CAS samples have a significant increase in porosity be￾cause there is no sufficient confining pressure to hinder the creep-induced cavitation. For example, Nair et al.11 showed 8% axial strain resulted in an increase in porosity to 24% in their CAS-II specimens. Furthermore, their microstructural analysis suggested no significant difference in grain mor￾phology between deformed and undeformed samples. The cavitation at the grain boundaries could be the dominant de￾formation mechanism for the previous creep tests conducted at ambient pressure. However, our experiments on the de￾formation of the CAS aggregates and CAS–Qtz composites were carried out at a confining pressure of 300 MPa, where cavitation could not occur. TEM observations and textu￾ral analysis strongly suggest that the plastic deformation of CAS under the experimental conditions is dominated by dis-