MObERLYCHAN et al: ROLES OF AMORPHOUS GRAIN BOUNDARIES 1627 Hexoloy 5 um HP@1900°c/1hr x、5pm Fig. 1. SEM micrographs of crack path that emanated from the corner of a Vickers microhardness indentation made on the polished surface of a commercial SiC, Hexoloy-SA(a) and ABC-Sic (b) Dark features in the Hexoloy-SA were indicative of porosity in this material. The crack path deflected around elongated grains of ABC-SiC and did not deflect toward secondary phases(characterized as Alg B C7, Al CO4, Al2O3 and BC [17-19, 21]). vations of intergranular fracture around plate-like sistent with reported mechanisms for the B-to-z grains [Fig. 2(a). Much of the contrast observed transformation [28, 29]. The transformation of B-3C within the ABC-Sic grains was from stacking to a-6H has been the typical reported faults within the a-4H microstructure. This growth reaction [28, 29). however, a-4H was the major of hexagonal Sic with grains elongated along basal transformational product observed in the present ell as residual stacking defects, was con- ABC-SiC. Details of this transformation are thevations of intergranular fracture around plate-like grains [Fig. 2(a)]. Much of the contrast observed within the ABC±SiC grains was from stacking faults within the a-4H microstructure. This growth of hexagonal SiC with grains elongated along basal planes, as well as residual stacking defects, was con￾sistent with reported mechanisms for the b-to-a transformation [28, 29]. The transformation of b-3C to a-6H has been the typical reported reaction [28, 29], however, a-4H was the major transformational product observed in the present ABC±SiC. (Details of this transformation are the Fig. 1. SEM micrographs of crack path that emanated from the corner of a Vickers microhardness indentation made on the polished surface of a commercial SiC, Hexoloy±SA (a) and ABC±SiC (b). Dark features in the Hexoloy±SA were indicative of porosity in this material. The crack path de¯ected around elongated grains of ABC±SiC and did not de¯ect toward secondary phases (characterized as Al8B4C7, Al4CO4, Al2O3 and B4C [17±19, 21]). MOBERLYCHAN et al.: ROLES OF AMORPHOUS GRAIN BOUNDARIES 1627