mObERLYChAN et al: ROLES OF AMORPHOUS GRAIN BOUNDARIES 2 OL-6H Hexoloy Fig 4. Bright field TEM image of the microstructure of Hexoloy-SA. Voids and regions of graphi black arrows) were commonly observed in this material. The white arrows indicated the predominan f - 120 triple junctions between equiaxed x-6H grains images acquired using diffraction contrast. The grain in Fig. 6(b) was oriented close to a (8 10 2 crack imaged in Fig. 5 propagated along grain 3)2-6H zone axis for imaging oundaries, producing a tortuous crack path similar In general, no specific crystallographic relation- to that observed in the SEM image of Fig. I(b). ship existed between the two zone axes orientations The crack path did not seek out voids nor weaker on either side of a grain boundary. However, the secondary phases. Diffraction contrast detailed hexagonal basal plane of the lower grain (of the merous stacking faults(and microtwins) within the ABC-SiC) also represented the surface of a plate a-4H grains of this SiC, even though it had been like grain, and this grain boundary facet had to hot pressed at1950°C fracture intergranularly to allow for the bridging which provided the improved toughness. The grains High resolution TEMf was used to determine the depicted in Fig. 6(a) and 6(b)were not oriented presence of amorphous phases at the grain bound- exactly along their respective zone axes, thereby aries of both the ABC-Sic and the Hexoloy-Sa sacrificing good high resolution imaging conditions [Fig. 6(a)and 6(b), respectively]. For both images, The probability was low that two randomly the lower grain was oriented to a (2110) zone axis, oriented grains had parallel, low-index axes,while with the [000l] direction normal to the grain also having a parallel grain boundary. Since the im- boundary layer. As has been noted, ABC-SiC pro- portant grain boundaries(for toughness due to cessed at the higher temperatures had been trans- bridging) involved a basal plane as the long facet formed to the a-4H structure with numerous for one grain, this grain boundary face was first stacking faults, whereas Hexoloy-SA exhibited the rotated to be imaged parallel to the TEM electron 2-6H structure. The ABC-SiC imaged in Fig. 6(a) beam. Subsequent tilting along the grain boundary had only been hot pressed at 1780.C for I h. and was conducted until a compromise image within 5o therefore retained substantial B phase, both as sep- of two, zone axes in the adjacent grains, was arate B grains and as dual-phase grains comprised obtained. As long as the basal plane in the lower grain was discretely presented without tilt in the of a-4H and B-3C. The upper grain in Fig. 6(a)was lattice image, the thickness of the amorphous grain tilted close to a(110)B zone axis for high resolution boundary layer could be measured. The amorphous imaging. On the other hand, all grain boundaries in grain boundary layer observed in the ABC-SiC was Hexoloy-SA separated two a-6H grains. The upper always <2 nm and usually <I nm thick.Most grain boundary layers observed in the Hexoloy-SA tA JEOL ARM1000 was operated at 800 kV, and a were also <2 nm thick; however, some amorphous Topcon ISI-002B was operated at 200 kV. egions were up to 5 nm thick [Fig. 6(b)]. Theimages acquired using di€raction contrast. The crack imaged in Fig. 5 propagated along grain boundaries, producing a tortuous crack path similar to that observed in the SEM image of Fig. 1(b). The crack path did not seek out voids nor weaker secondary phases. Di€raction contrast detailed nu￾merous stacking faults (and microtwins) within the a-4H grains of this SiC, even though it had been hot pressed at 19508C. High resolution TEM{ was used to determine the presence of amorphous phases at the grain bound￾aries of both the ABC±SiC and the Hexoloy±SA [Fig. 6(a) and 6(b), respectively]. For both images, the lower grain was oriented to a h2110i zone axis, with the [0001] direction normal to the grain boundary layer. As has been noted, ABC±SiC pro￾cessed at the higher temperatures had been trans￾formed to the a-4H structure with numerous stacking faults, whereas Hexoloy±SA exhibited the a-6H structure. The ABC±SiC imaged in Fig. 6(a) had only been hot pressed at 17808C for 1 h, and therefore retained substantial b phase, both as sep￾arate b grains and as dual-phase grains comprised of a-4H and b-3C. The upper grain in Fig. 6(a) was tilted close to a h110ib zone axis for high resolution imaging. On the other hand, all grain boundaries in Hexoloy±SA separated two a-6H grains. The upper grain in Fig. 6(b) was oriented close to a h8 10 2 3ia-6H zone axis for imaging. In general, no speci®c crystallographic relation￾ship existed between the two zone axes orientations on either side of a grain boundary. However, the hexagonal basal plane of the lower grain (of the ABC±SiC) also represented the surface of a plate￾like grain, and this grain boundary facet had to fracture intergranularly to allow for the bridging which provided the improved toughness. The grains depicted in Fig. 6(a) and 6(b) were not oriented exactly along their respective zone axes, thereby sacri®cing good high resolution imaging conditions. The probability was low that two randomly oriented grains had parallel, low-index axes, while also having a parallel grain boundary. Since the im￾portant grain boundaries (for toughness due to bridging) involved a basal plane as the long facet for one grain, this grain boundary face was ®rst rotated to be imaged parallel to the TEM electron beam. Subsequent tilting along the grain boundary was conducted until a compromise image within 58 of two zone axes in the adjacent grains was obtained. As long as the basal plane in the lower grain was discretely presented without tilt in the lattice image, the thickness of the amorphous grain boundary layer could be measured. The amorphous grain boundary layer observed in the ABC-SiC was always <2 nm and usually <1 nm thick. Most grain boundary layers observed in the Hexoloy±SA were also <2 nm thick; however, some amorphous regions were up to 5 nm thick [Fig. 6(b)]. The Fig. 4. Bright ®eld TEM image of the microstructure of Hexoloy±SA. Voids and regions of graphite (black arrows) were commonly observed in this material. The white arrows indicated the predominance of 01208 triple junctions between equiaxed a-6H grains. {A JEOL ARM1000 was operated at 800 kV, and a Topcon ISI-002B was operated at 200 kV. MOBERLYCHAN et al.: ROLES OF AMORPHOUS GRAIN BOUNDARIES 1629