6368 E ARELLANO. LOPEZ et aL. CREEP OF SIC-WHISKER-REINFORCED ALUMINA superplastic deformation of monolithic Y-PSZ [25]. and the first two extrapolate to ao<6 MPa, while f o do. then the sliding of the grains is not poss- the third extrapolates to o<33 MPa. Additionally Ale and the creep mechanism has to change to ful- Fig 8 tends to substantiate that the creep mechan- this constraint. At high temperatures and low ism of anL5 is the same as monolithic alumina, stress, purely diffusional creep becomes the domi- difiusion-accommodated GBs, but with an nant controlling mechanism, for which classical increased threshold stress models predict [19] If diffusional kinetics are unchanged the slope of BPDonDelt a pd mechanism is about ten times smaller than kTdA ( 12) that of GBS. The PD slope has been represented by the solid line in Fig. 8, as a reference, and shows where BpD is a constant, which is about one order fair agreement with the EgS-corrected creep rates of magnitude smaller than BGBs. Additionally, in the low-stress regime for composites having more when the sliding of the grains is not possible, it has than 10 vol. whiskers. In the linear-linear plot of been shown [26 that monoliths as 3Y-TZP tend to Fig. 8, acceleration of creep rates is evident over a accommodate deformation forming large cavities certain value of the stress that corresponds to the (as large as five times the nominal grain size). This critical stress mentioned above. feature has not been found in the present micro- The critical stress for each of the compositions structural analysis. It is possible that the network of can be estimated by extrapolation of the high-stress whiskers also prevents the formation of cavities as regime of EGS-corrected creep rates to zero, using an accommodating mechanism, and the defor- the same slopes calculated for monolithic ation can be completely accommodated by trans- alumina and anl5. Such critical stresses must be port of matter. In this case a small contribution of distinguished from threshold stresses [24. Above the liding is needed but it seems to be slow enough to critical stress, creep rates do not correspond to true be born by the whisker network steady states because the production of damage is In light of equations(11) and (12), it is suggested significant. Critical and threshold stresses are hat the creep data in Fig. 6 can be represented in a plotted in Fig. 9. Although the estimate of the criti- linear-linear plot(Fig 8)to estimate the threshold cal stress is only approximate, the tendency for the stress as defined in equation(11). The slopes of the stress to increase as the whisker volume fraction plots for ANLO, ORNLO and ANL5 are similar, increases is evident. 3.0E-4 1400°C 2.0E4 O(MPa) Fig.8. Linear-linear plot of the data in Fig. 6, used for calculating the threshold stresses(filled sym- bols)and estimating the critical stresses(open symbols). The monoliths (ORNO and ANLO) creep by diffusion-accommodated GBS, characterized by a slope(dashed line) which is about one order of mag nitude higher than that of a purely diffusional(PD)mechanism, with the same diffusional kinetics (solid line, as reference)superplastic deformation of monolithic Y-PSZ [25]. If s < s0, then the sliding of the grains is not poss￾ible, and the creep mechanism has to change to ful- ®ll this constraint. At high temperatures and low stress, purely di€usional creep becomes the domi￾nant controlling mechanism, for which classical models predict [19]: e_PD ˆ BPDsODeff kTd2 …12† where BPD is a constant, which is about one order of magnitude smaller than BGBS. Additionally, when the sliding of the grains is not possible, it has been shown [26] that monoliths as 3Y-TZP tend to accommodate deformation forming large cavities (as large as ®ve times the nominal grain size). This feature has not been found in the present micro￾structural analysis. It is possible that the network of whiskers also prevents the formation of cavities as an accommodating mechanism, and the defor￾mation can be completely accommodated by trans￾port of matter. In this case a small contribution of sliding is needed, but it seems to be slow enough to be born by the whisker network. In light of equations (11) and (12), it is suggested that the creep data in Fig. 6 can be represented in a linear±linear plot (Fig. 8) to estimate the threshold stress as de®ned in equation (11). The slopes of the plots for ANL0, ORNL0 and ANL5 are similar, and the ®rst two extrapolate to s016 MPa, while the third extrapolates to s0133 MPa. Additionally, Fig. 8 tends to substantiate that the creep mechan￾ism of ANL5 is the same as monolithic alumina, di€usion-accommodated GBS, but with an increased threshold stress. If di€usional kinetics are unchanged, the slope of a PD mechanism is about ten times smaller than that of GBS. The PD slope has been represented by the solid line in Fig. 8, as a reference, and shows fair agreement with the EGS-corrected creep rates in the low-stress regime for composites having more than 10 vol.% whiskers. In the linear±linear plot of Fig. 8, acceleration of creep rates is evident over a certain value of the stress that corresponds to the critical stress mentioned above. The critical stress for each of the compositions can be estimated by extrapolation of the high-stress regime of EGS-corrected creep rates to zero, using the same slopes as calculated for monolithic alumina and ANL5. Such critical stresses must be distinguished from threshold stresses [24]. Above the critical stress, creep rates do not correspond to true steady states because the production of damage is signi®cant. Critical and threshold stresses are plotted in Fig. 9. Although the estimate of the criti￾cal stress is only approximate, the tendency for the stress to increase as the whisker volume fraction increases is evident. Fig. 8. Linear±linear plot of the data in Fig. 6, used for calculating the threshold stresses (®lled sym￾bols) and estimating the critical stresses (open symbols). The monoliths (ORNL0 and ANL0) creep by di€usion-accommodated GBS, characterized by a slope (dashed line) which is about one order of mag￾nitude higher than that of a purely di€usional (PD) mechanism, with the same di€usional kinetics (solid line, as reference). 6368 DE ARELLANO-LO PEZ et al.: CREEP OF SiC-WHISKER-REINFORCED ALUMINA