1424 XIA and LANGDON: DEFORMATION OF AN ALUMINA COMPOSITE Fig. 8. a dislocation network bound to siC whiskers after deforming to a strain of 414% at 1773 K and 25.8 MPa: the foil plane is perpendicular to the hot-pressing direction Fig. 6. Steady-state strain rate vs stress. o"G-T, vs the reciprocal of the absolute tempera- activity. In general, higher dislocation densities were ture, 1/T, for strain rates of 1.0 x 10-3and 1.0 x observed in specimens deformed to larger strains 10-6s-l, respectively, where G is the shear modulus Contrary to some earlier reports [17, 18], there was of monolithic alumina; the value of G was estimated very little evidence of the development of internal for each testing temperature from the relationship cavities. An example of dislocation activity is shown in Go-(aG /aT)T (3)Fig 8 for a specimen tested to failure at a strain of 14% at 1773 K and under an applied stress of here Go is the extrapolated shear modulus of 25.8 MPa: the foil plane is perpendicular to the hot aG/oT is the variation of G per degree Kelvin pressing direction and the TEM sample was taken (23.4 MPa-[21D. From Fig. 7, the activation from the outer portion of the tensile side of the bend energy for creep, e, is nated as c830+30 specimen. Figure 8 shows a dislocation network nned by whiskers, and Fig. 9 shows, for the same and <820+ 10 kJ mol-1 for the two strain rates, specimen, an array of dislocations associated with the end of a whisk Inspection of several specimens after deformation evealed the occurrence of extensive dislocation 4.1. Deformation mechanism in the composite The creep behavior of monolithic alumina has been well documented and thus provides a reference when Al 03-9. 3 wolx Sicl) considering the creep properties of alumina matrix 101 oL0x10 Fig. 7. Temperature compensated stress vs the reciprocal of Fig. 9. An array of dislocations associated he end o he absolute temperature for strain rates of 1.0 d a whisker after deforming at 1773 K and 25. 8 MPa; the foil 1.0 x 10-65", respectively plane is perpendicular e hot-pressing directio1424 XIA and LANGDON: DEFORMATION OF AN ALUMINA COMPOSITE i0"3 10"~ ~w~ I0"~ i0"~ I i , i 'lH] AI203 - 9.3 volg SiC (w T(K) t,1823 01773 o 1723 '~ 1673 1 10 I I L L llltlll 0.5 10 ~ 10 z 58 ~(MPa) Fig. 6. Steady-state strain rate vs stress. Fig. 8. A dislocation network bound to SiC whiskers after deforming to a strain of ~ 14% at 1773 K and 25.8 MPa: the foil plane is perpendicular to the hot-pressing direction. a'/G'-IT~ vs the reciprocal of the absolute tempera￾ture, l/T, for strain rates of 1.0 x 10 -5 and 1.0 x 10 -6 S -l, respectively, where G is the shear modulus of monolithic alumina; the value of G was estimated for each testing temperature from the relationship G = Go - (aG/OT)T (3) where Go is the extrapolated shear modulus of alumina at absolute zero (1.71 x l05 MPa [21]) and dG/OT is the variation of G per degree Kelvin (23.4MPaK -] [21]). From Fig. 7, the activation energy for creep, Q, is estimated as ~830 + 30 and ~820+ lOkJmo1-1 for the two strain rates, respectively. 3.3. Microstructures after deformation Inspection of several specimens after deformation revealed the occurrence of extensive dislocation r (g) 7xi0-II 1823 1773 17~3 1673 AI2C) 3 - 9.3 vol % SiC(w) [ (s -I ) o1.0x10 "5 10 "11 O 1.0xl0-6 / / 10-13 10-14 7xl0 "15 I I 5.4 5.6 5.8 6.0 ~ xlO 4 (K "l} Fig. 7. Temperature compensated stress vs the reciprocal of the absolute temperature for strain rates of 1.0 x 10 -5 and 1.0 x 10 -6 s -~, respectively. activity. In general, higher dislocation densities were observed in specimens deformed to larger strains. Contrary to some earlier reports [17, 18], there was very little evidence of the development of internal cavities. An example of dislocation activity is shown in Fig. 8 for a specimen tested to failure at a strain of ~ 14% at 1773 K and under an applied stress of 25.8 MPa: the foil plane is perpendicular to the hot pressing direction and the TEM sample was taken from the outer portion of the tensile side of the bend specimen. Figure 8 shows a dislocation network pinned by whiskers, and Fig. 9 shows, for the same specimen, an array of dislocations associated with the end of a whisker. 4. DISCUSSION 4.1. Deformation mechanism in the composite The creep behavior of monolithic alumina has been well documented and thus provides a reference when considering the creep properties of alumina matrix Fig. 9. An array of dislocations associated with the end of a whisker after deforming at 1773 K and 25.8 MPa: the foil plane is perpendicular to the hot-pressing direction