E ARELLANO. LOPEZ et al.: CREEP OF SIC-WHISKER-REINFORCED ALUMINA +210 MPa -115 MPa o-90 MPa 33 MPa Threshold Stress Critical Stress φ(% Fig. 9. Regression-calculated threshold stresses(filled symbols) and estimated critical stresses(open symbols) plotted vs the volume percentage of whiskers. 4.2. Discussion of creep mechanisms between the low- and high-stress regimes, as seen in A threshold/critical stress has been used to Fig. 6. The estimated critical stress appears to be inde- alumina in previous publications (23, 27). The same pendent of creep technique, as expected. Figure 10 ideas were also applied to SiC-whisker-reinforced presents the results of the ORNL20 composite ZTA [28]. Parthasarathy et al. [22] have used a tested in compression [13] and in four-point temperature-dependent"threshold stress"to ration- bending [loj alize a set of results on sic-whisker-reinforced com- The creep activation energies must be reexamined posites with high whisker volume fraction. Other considering a temperature-dependent threshold examples of the use of threshold stress are found in stress. Activation-energy studies are available for the study of SiC-whisker-reinforced aluminum alumina-matrix composites fabricated by ARCO, [29, 30]. Creep in these metal-matrix composites was containing 0, 6, 18 and 30 vol. of SiC whiskers [6] controlled by the movement of dislocations and the Temperatures ranged between 1300 and 1450oC threshold stresses were interpreted in terms of and all the tests were conducted on single samples Orowan, back, and detachment stresses using temperature changes, under compressive stres- of alumina col ses of 60 MPa for composites and 40 MPa for the of undeformable whiskers to the alumina monolithic matrix. These stresses correspond to the inhibits the sliding of the grains below a low-stress regime for ARCO18 and ARCO30, but value of the stress. For stresses over that value. slid.- are over the threshold stress for ARCOO(5 MPa) ing is possible, but as the transport of matter in the and ARCO (2I MPa). The results are included in plastic alumina is not fast enough to accommodate Table 4. Similar values of the activation energies the microstructural constraints of the whisker net- were reported by Nutt et al. [7 and Liu et al.[8] work, damage is subsequently formed. The network The values of 2 for ARCOO, ARCOI8 and of whiskers is apparently set forφ>中pwp.Fo volume fraction of whiskers below that limit, defor- mation under high stresses can proceed at a steady 1300-1450-C under 40 MPa(arco)and 60 MP: state. For larger whisker volume fractions, the criti om Ref [6 cal stress increases as the number of whiskers increases. However, the formation of damage is ARCOO ARCO6 ARCOI8 ARCO30 more significant as less plastic material is available Q(k/mol) 511 556 in the composite. This results in a sharper change4.2. Discussion of creep mechanisms A threshold/critical stress has been used to explain the behavior of SiC-whisker-reinforced alumina in previous publications [23, 27]. The same ideas were also applied to SiC-whisker-reinforced ZTA [28]. Parthasarathy et al. [22] have used a temperature-dependent ``threshold stress'' to ration￾alize a set of results on SiC-whisker-reinforced com￾posites with high whisker volume fraction. Other examples of the use of threshold stress are found in the study of SiC-whisker-reinforced aluminum [29, 30]. Creep in these metal-matrix composites was controlled by the movement of dislocations, and the threshold stresses were interpreted in terms of Orowan, back, and detachment stresses. In the case of alumina composites, the addition of undeformable whiskers to the alumina matrix inhibits the sliding of the grains below a certain value of the stress. For stresses over that value, slid￾ing is possible, but as the transport of matter in the plastic alumina is not fast enough to accommodate the microstructural constraints of the whisker net￾work, damage is subsequently formed. The network of whiskers is apparently set for f>fpcp. For a volume fraction of whiskers below that limit, defor￾mation under high stresses can proceed at a steady state. For larger whisker volume fractions, the criti￾cal stress increases as the number of whiskers increases. However, the formation of damage is more signi®cant as less plastic material is available in the composite. This results in a sharper change between the low- and high-stress regimes, as seen in Fig. 6. The estimated critical stress appears to be inde￾pendent of creep technique, as expected. Figure 10 presents the results of the ORNL20 composite tested in compression [13] and in four-point bending [10]. The creep activation energies must be reexamined considering a temperature-dependent threshold stress. Activation-energy studies are available for alumina-matrix composites fabricated by ARCO, containing 0, 6, 18 and 30 vol.% of SiC whiskers [6]. Temperatures ranged between 1300 and 14508C, and all the tests were conducted on single samples using temperature changes, under compressive stres￾ses of 60 MPa for composites and 40 MPa for the monolithic matrix. These stresses correspond to the low-stress regime for ARCO18 and ARCO30, but are over the threshold stress for ARCO0 (15 MPa) and ARCO6 (121 MPa). The results are included in Table 4. Similar values of the activation energies were reported by Nutt et al. [7] and Liu et al. [8]. The values of Q for ARCO0, ARCO18 and Fig. 9. Regression-calculated threshold stresses (®lled symbols) and estimated critical stresses (open symbols) plotted vs the volume percentage of whiskers. Table 4. Activation energies for ARCO composites in the range 1300±14508C, under 40 MPa (ARCO0) and 60 MPa (composites), from Ref. [6] ARCO0 ARCO6 ARCO18 ARCO30 Q (kJ/mol) 511 691 556 610 DE ARELLANO-LOÂ PEZ et al.: CREEP OF SiC-WHISKER-REINFORCED ALUMINA 6369