XIA and LANGDON: DEFORMATION OF AN ALUMINA COMPOSITE composites. For monolithic alumina, there are two SiC whiskers and this provides an opportunity for a distinct types of creep behavior with either a stress direct comparison with the present results exponent of unity when the stress is low and the Under conditions of high temperature creep, it is grain size is small or a higher stress exponent, gener- now established that the steady-state creep rate, ly close to 3, when the stress is high and the grain t, may be expressed by a relationship of the form size is large [10]. At high temperatures, similar to [10] those used in the present experiments, the grain size marking the transition between the two types of ADGb/b behavior is 20-30 um. Therefore, it may be antici- kr(a八G pated that in the present experiments, with a grain size of 1-2um, the alumina matrix should display where D is the appropriate diffusion coefficient,b a stress exponent of I in the absence of any silicon is the Burgers vector(4.75 x 10-10 m for A120,),k is Boltzmanns constant, d is the grain size, p is carbide whiskers. Reference to the deformation map the exponent of the inverse grain size and A is a for diffusion creep in unreinforced alumina at elev ated temperatures /22] indicates that the diffusional dimensionless constanyer-law creep, where n >3, creep process is Coble creep controlled by aluminum ion diffusion along the grain boundaries with there is no dependence on grain size so that activation energy of -419 kJ mol[23 Therefore, independent sets of creep data may be However, the composite used in this investigati compared by plotting the temperature compensated exhibits a higher stress exponent of 3. 8 and the strain rate, kT/ DGb, vs the normalized stress hated for the activation energy for creep a/g 820-830kJ mol-')is fairly close to the value In the present experiments, the measured acti- for oxygen diffusion in the alumina lattice vation energy is reasonably close to the value for (-600-800 kJ mol[24-28). Thus, the presence of the diffusion of the oxygen anions in the alumina diffusion creep is classified into Nabarro-Herring or D9=2×10exp(-63500R7m2s-1(5) Coble creep. In both cases, the diffusion creep is where R is the gas constant accompanied by Lifshitz grain boundary sliding [29]. Figure 10 shows the present data plotted in this In the present experiments, it is concluded that the normalized form together with the data reported silicon carbide whiskers, which are located preferen- earlier by Lin and Becher[17 for alumina reinforced tially at the grain boundaries, interfere with the with 10 vol% of SiC whiskers ability of the grains to move with respect to each All of the present datum points fall close to a single other. As a result, Lifshitz grain boundary sliding is line in this normalized plot. The results of Lin and pressed and the rate of diflusion creep is sig- Becher [17] exhibit a stress exponent of x 4 but, at nificantly lowered so that intragranular dislocation any selected stress level, the measured creep rate is processes become important. within the grains. First, the measured stress exponent of -3.8 is typical of an intragranular dislocation process, and the measured activation energ ent with a lattice diffusion process within the alumina matrix controlled by the slower-moving oxy gen anions. Second, there is microstructural evidence for dislocation activity within the grains, especially he samples deformed to strains above -10% 4.2. Comparison with other creep data on Al,O, with Although there are several reports of creep behav or in Al,O, strengthened with SiC whiskers, it is Lin and Becher(1991 difficult to make a direct comparison with most of these data because of significant differences in the However, Lin and Becher [17] reported creep data stress for the present data and for ain rate y volume percentages of the whisker reinforcement. Fig 10. Temperature for an alt lumina composite containing 10 vol % ofXIA and LANGDON: DEFORMATION OF AN ALUMINA COMPOSITE 1425 composites. For monolithic alumina, there are two distinct types of creep behavior with either a stress exponent of unity when the stress is low and the grain size is small or a higher stress exponent, gener￾ally close to 3, when the stress is high and the grain size is large [10]. At high temperatures, similar to those used in the present experiments, the grain size marking the transition between the two types of behavior is ~ 20-30 #m. Therefore, it may be antici￾pated that in the present experiments, with a grain size of ~ 1-2 #m, the alumina matrix should display a stress exponent of 1 in the absence of any silicon carbide whiskers. Reference to the deformation map for diffusion creep in unreinforced alumina at elev￾ated temperatures [22] indicates that the diffusional creep process is Coble creep controlled by aluminum ion diffusion along the grain boundaries with an activation energy of ~419 kJ mol ~ [23]. However, the composite used in this investigation exhibits a higher stress exponent of ~3.8 and the value estimated for the activation energy for creep (~820-830kJmol -~) is fairly close to the value for oxygen diffusion in the alumina lattice (~ 600-800 kJ mol t [24-28]). Thus, the presence of whiskers markedly affects the creep behavior of the alumina matrix. Depending upon the path for diffusion, whether through the lattice or along the grain boundaries, diffusion creep is classified into Nabarro Herring or Coble creep. In both cases, the diffusion creep is accompanied by Lifshitz grain boundary sliding [29]. In the present experiments, it is concluded that the silicon carbide whiskers, which are located preferen￾tially at the grain boundaries, interfere with the ability of the grains to move with respect to each other. As a result, Lifshitz grain boundary sliding is suppressed and the rate of diffusion creep is sig￾nificantly lowered so that intragranular dislocation processes become important. There are two experimental observations support￾ing the occurrence of extensive dislocation activity within the grains. First, the measured stress exponent of ~3.8 is typical of an intragranular dislocation process, and the measured activation energy is con￾sistent with a lattice diffusion process within the alumina matrix controlled by the slower-moving oxy￾gen anions. Second, there is microstructural evidence for dislocation activity within the grains, especially in the samples deformed to strains above ~ 10%. 4.2. Cornpar&on with other creep data on Ale0 J with SiC whiskers Although there are several reports of creep behav￾ior in A1203 strengthened with SiC whiskers, it is difficult to make a direct comparison with most of these data because of significant differences in the volume percentages of the whisker reinforcement. However, Lin and Becher [17] reported creep data for an alumina composite containing 10vol.% of SiC whiskers and this provides an opportunity for a direct comparison with the present results. Under conditions of high temperature creep, it is now established that the steady-state creep rate, ~, may be expressed by a relationship of the form [10] ADGb g - (4) ,, where D is the appropriate diffusion coefficient, b is the Burgers vector (4.75 × 10-ram for A1203), k is Boltzmann's constant, d is the grain size, p is the exponent of the inverse grain size and A is a dimensionless constant. In intragranular power-law creep, where n >/3, there is no dependence on grain size so that p = 0. Therefore, independent sets of creep data may be compared by plotting the temperature compensated strain rate, ~kT/DGb, vs the normalized stress, tr/G. In the present experiments, the measured acti￾vation energy is reasonably close to the value for the diffusion of the oxygen anions in the alumina lattice. Thus, the diffusion coefficient was taken as the value for lattice diffusion of oxygen, D °2- t , given by [24] D °2 =2x lO-lexp(-635,000/RT)m2s J (5) where R is the gas constant. Figure 10 shows the present data plotted in this normalized form together with the data reported earlier by Lin and Becher [17] for alumina reinforced with 10 vol.% of SiC whiskers. All of the present datum points fall close to a single line in this normalized plot. The results of Lin and Becher [17] exhibit a stress exponent of ~4 but, at any selected stress level, the measured creep rate is 10-5 i i J iiiii] AI203-9.3vol % SiC w A 1~-3 Z' , lo -~ 0 1773 z~' / [] 1723 [~ el 10-8 tJ AI20 g -10 vol % SIC(, / OT:1573 K,d=SFm Lin and Becher (1991) 10"~0. 5 1/.4' ''''"~)_3 5x10- 3 o-/G Fig. 10. Temperature compensated strain rate vs normalized stress for the present data and for the results of Lin and Becher [17] on A1203-10 vol.% SiC(w)