1422 XIA and LANGDON: DEFORMATION OF AN ALUMINA COMPOSITE cut from the bulk specimen using a diamond saw the slice was reduced to a disc of 3 mm dia using an ultrasonic cutter, and the disc was ground on diamond to a thickness of w 50 um and ion-milled to perforation. The TEM samples were coated with carbon to improve conductivity and avoid charging effects, and they were examined with a Philips EM420 electron microscope operating at 120 Creep tests were tures using a 4-point bending rig. Alumina was used for the base support, the support for the lower pivots, and the upper loading ram. The four loading pivots, of 3. 2 mm dia, were made of optically-polished sap- phire (single crystal alumina) so that they were stronger than the matrix material of the test speci- (a) ens (inner span) and lower two pivots(outer span)were 6.4 and 19 mm, respectively. All specimens were tested with the bars placed so that the dimension of 3 mm was vertical; the creep load was therefore applied in the hot pressing direction. Tests were performed in air under conditions of constant load and with the temperature controlled to +2 K. Each specimen was maintained at the testing temperature for at least 30 min prior to pplying the load. The vertical displacement of the loading ram was measured with a linear variable differential transducer and recorded with a strip-chart The outer fiber stress, o, was calculated from the applied load, P, through the expression 19 Fig. 1. Distribution of whiskers in Al, O,9.3 vol. SiC(w) at(a) low and(b)high magnifications: the hot-pressing where L is the outer span(19 mm), a is the inner span and in the plane of observation, respectively. The (6.4 mm), w is the width of the specimen (2 mm), h is higher magnifications in Figs 1(b)and 2(b)show that the height of the specimen(3 mm)and n is the stress the whiskers are reasonably sparsely spaced and exponent of creep. The flexure displacement, y, was randomly oriented but with a tendency to lie prefer- relationship [ 19] direction. Observations by TEM indicated that the whiskers were located primarily in the grain bound- ()(+25)y aries or at triple points, as shown in Fig. 3, although (2)a small percentage of the whiskers appeared to be isolated within the grains. There was no evidence for As documented in the Appendix, an iterative pro- any residual porosity at the interfaces between the cedure was adopted to determine n using equations matrix and the whiskers 1)and(2) It was difficult to clearly reveal the grain bound For examination of microstructures after defor- aries in the alumina matrix because of the presence of ation, TEM samples were prepared from the outer whiskers. However, it was estimated by etching and regions of the tensile parts of selected test specimens. use of SEM, and confirmed by TEM, that the aver- age grain size was within the range of 1-2 um 3. EXPERIMENTAL RESULTS Extensive examination by TEM showed the pres- ence of only a small number of dislocations in the 3. 1.Characterization of the material after fabrication as-fabricated material A careful examination, at both low and high mag- nifications, showed that the distribution of whiskers 3.2. Creep results in the matrix was essentially uniform. The results Figure 4 shows a typical plot of strain, f, vs time are shown in Figs I and 2, where the hot pressing 4, for an absolute testing temperature T, of 1773K direction is either perpendicular to or lying vertical with the various curves corresponding to different1422 XIA and LANGDON: DEFORMATION OF AN ALUMINA COMPOSITE cut from the bulk specimen using a diamond saw, the slice was reduced to a disc of 3 mm dia using an ultrasonic cutter, and the disc was ground on diamond to a thickness of ~50 #m and ion-milled to perforation. The TEM samples were coated with carbon to improve conductivity and avoid charging effects, and they were examined with a Philips EM420 electron microscope operating at 120 kV. Creep tests were conducted at elevated tempera￾tures using a 4-point bending rig. Alumina was used for the base support, the support for the lower pivots, and the upper loading ram. The four loading pivots, of 3.2 mm dia, were made of optically-polished sap￾phire (single crystal alumina) so that they were stronger than the matrix material of the test speci￾mens; the spacings between the upper two pivots (inner span) and lower two pivots (outer span) were 6.4 and 19mm, respectively. All specimens were tested with the bars placed so that the dimension of 3 mm was vertical; the creep load was therefore applied in the hot pressing direction. Tests were performed in air under conditions of constant load and with the temperature controlled to +2 K. Each specimen was maintained at the testing temperature for at least 30min prior to applying the load. The vertical displacement of the loading ram was measured with a linear variable differential transducer and recorded with a strip-chart recorder. The outer fiber stress, tr, was calculated from the applied load, P, through the expression [19] =( L-a ~f 2n + l tr \ wh2 //~ n )(P) (1) where L is the outer span (19 mm), a is the inner span (6.4 mm), w is the width of the specimen (2 ram), h is the height of the specimen (3 mm) and n is the stress exponent of creep. The flexure displacement, y, was converted to the outer fiber strain, E, through the relationship [19] E = L q-a (-n-+ 1) y" (2) As documented in the Appendix, an iterative pro￾cedure was adopted to determine n using equations (1) and (2). For examination of microstructures after defor￾mation, TEM samples were prepared from the outer regions of the tensile parts of selected test specimens. 3. EXPERIMENTAL RESULTS 3.1. Characterization of the material after fabrication A careful examination, at both low and high mag￾nifications, showed that the distribution of whiskers in the matrix was essentially uniform. The results are shown in Figs 1 and 2, where the hot pressing direction is either perpendicular to or lying vertical Fig. 1. Distribution of whiskers in A1203-9.3 vol.% SiC(w) at (a) low and (b) high magnifications: the hot-pressing direction is perpendicular to the plane of observation. and in the plane of observation, respectively. The higher magnifications in Figs 1 (b) and 2(b) show that the whiskers are reasonably sparsely spaced and randomly oriented but with a tendency to lie prefer￾entially in the plane perpendicular to the hot-pressing direction. Observations by TEM indicated that the whiskers were located primarily in the grain bound￾aries or at triple points, as shown in Fig. 3, although a small percentage of the whiskers appeared to be isolated within the grains. There was no evidence for any residual porosity at the interfaces between the matrix and the whiskers. It was difficult to clearly reveal the grain bound￾aries in the alumina matrix because of the presence of whiskers. However, it was estimated by etching and use of SEM, and confirmed by TEM, that the aver￾age grain size was within the range of ~ 1-2/tm. Extensive examination by TEM showed the pres￾ence of only a small number of dislocations in the as-fabricated material. 3.2. Creep results Figure 4 shows a typical plot of strain, ,, vs time, t, for an absolute testing temperature, T, of 1773 K with the various curves corresponding to different