1628 MOBERLYCHAN et al: ROLES OF AMORPHOUS GRAIN BOUNDARIES subject of a separate paper. The elongated micro- structure of the ABC-Sic developed during inter locking growth of plate-like grains. These interlocking grains, similar to a-alumina [30], would be more ideal manner than a microstructure of elongated, fibrous grains, such as that reported for the transformation in toughened Si3N4 [9-11]. The Hexoloy-SA, although having a grain size similar to ABC-SiC, had a more equiaxed micro- HP@1900°c Fig. 4). Pores were commonly observed in Hexoloy-SA, as well as secondary phase regions of 5 um graphite(see black arrow). The diffraction contrast within grains in Fig 4 was determined to be due to variations in thickness and bend contours. Stacking faults and microtwins, commonly observed by tEM and hr-tem in ABC-Sic and other sic ma- terials, were not present in Hexoloy-SA Bright field imaging was also utilized to observe crack paths in thin TEM specimens of the ABC- Hexoloy a-6H SiC, The crack imaged in Fig s was propagated by aration. This particular ABC-SiC ceramic(Fig. 5) SEM fract of aBc-sic ressed at had been hot pressed at 1950C for I h, which 900° c for I h(a) Hexoloy-SA(b), from con- resulted in a larger grain size but reduced aspect trolled flaw bending tests. The tortuous surface mor- ratio as compared to material hot pressed at ind bridging of ated, plate-like a-4H grains. The sur- 1900 C(Fig. 3). Grain boundaries, which were not face morphology of the Hexoloy-SA indicated transgranu. easily resolved by optical metallography nor SEM ar fracture of the -6H grains [see Fig. 1(b) were easily distinguished in TEM a-4H HP 19009c/1hr um Fig 3. Bright field TEM image of the microstructure of ABC-Sic hot pressed at 1900C for I h Elongated late- like grains, with an interlocking microstructure developed during p to a phase transformation. Streaks within grains were determined to be stacking faults and microtwins in the a-4H structure. Black arrow denotes secondary phases at triple junction[36], which are also present in larger pockets [17, 21subject of a separate paper.) The elongated micro￾structure of the ABC±SiC developed during inter￾locking growth of plate-like grains. These interlocking grains, similar to a-alumina [30], would be expected to cause good creep resistance, in a more ideal manner than a microstructure of elongated, ®brous grains, such as that reported for the transformation in toughened Si3N4 [9±11]. The Hexoloy±SA, although having a grain size similar to ABC±SiC, had a more equiaxed micro￾structure, with numerous triple junctions exhibiting close-to-ideal 1208 angles (see white arrows in Fig. 4). Pores were commonly observed in Hexoloy±SA, as well as secondary phase regions of graphite (see black arrow). The di€raction contrast within grains in Fig. 4 was determined to be due to variations in thickness and bend contours. Stacking faults and microtwins, commonly observed by TEM and HR-TEM in ABC±SiC and other SiC ma￾terials, were not present in Hexoloy±SA. Bright ®eld imaging was also utilized to observe crack paths in thin TEM specimens of the ABC± SiC. The crack imaged in Fig. 5 was propagated by bending a doubly-dimpled TEM sample after prep￾aration. This particular ABC±SiC ceramic (Fig. 5) had been hot pressed at 19508C for 1 h, which resulted in a larger grain size but reduced aspect ratio as compared to material hot pressed at 19008C (Fig. 3). Grain boundaries, which were not easily resolved by optical metallography nor SEM [see Fig. 1(b)], were easily distinguished in TEM Fig. 2. SEM fractographs of ABC±SiC hot pressed at 19008C for 1 h (a) and of Hexoloy±SA (b), from con￾trolled ¯aw bending tests. The tortuous surface mor￾phology in ABC±SiC resulted from intergranular fracture and bridging of elongated, plate-like a-4H grains. The sur￾face morphology of the Hexoloy±SA indicated transgranu￾lar fracture of the a-6H grains. Fig. 3. Bright ®eld TEM image of the microstructure of ABC±SiC hot pressed at 19008C for 1 h. Elongated, plate-like grains, with an interlocking microstructure developed during b to a phase transformation. Streaks within grains were determined to be stacking faults and microtwins in the a-4H structure. Black arrow denotes secondary phases at triple junction [36], which are also present in larger pockets [17, 21]. 1628 MOBERLYCHAN et al.: ROLES OF AMORPHOUS GRAIN BOUNDARIES