I.. Davies et al. Composites Science and Technolog y 59(1999)801-811 Table 2 In situ fibre fracture surface characteristics for Tyranno* Si-THC-O fibres Test condition Fracture mirror Fracture mirror Flat fracture Undetermined Total (surface)%(N (internal)%(M surface %( %(N 75(114) 13(19) 100(151) 200°C/ vacuum 10(11) l(1 00(112) 1300° C/vacuum 3(5) 1200°C/air 100(788) a 3 um 4 um (b) (b) lm 2 um Fig. 5. Scanning electron micrographs illustrating the fracture surface Tyranno"ShTHC-O fibre tested in situ at 1200C in vacuum tha Fig. 6. Scanning electron micrographs illustrating the fracture surface failed due to an internal flaw:(a) general view, and (b) detailed view of f a Tyranno"Si-Ti-C-O fibre tested in situ at 1300.C in vacuum that fracture mirror failed due to a surface flaw:(a) general view, and (h) detailed view of fracture mirror of fibres that"shattered"upon failure. Likewise, fibres evolution of Co, with associated decreased tensile characterised as"flat fracture surface"in Table 2 for strength [21]. That this phenomenon was not observed the room temperature test condition possessed smooth in current specimens until 1300C was believed, in part features that suggested them to be the weakest group of to be due to the relatively short time above 1000C during testing compared to previous resea observed in Fig. 6 is a fracture mirror that was typical 3..2.l300° c in vacu of those seen in 84% of fibres at 1300C in vacuum Whereas fibres tested at 1200'C in vacuum indicated (Table 2). The number of fibres that failed due to inter- no obvious grain growth compared to room tempera- nal flaws was about 12% and only slightly larger than ture fibres, those tested at 1300@C exhibited significant that at 1200C (10%)indicating that the voids observed grain growth and finely distributed voids(Fig. 6). Such in Fig. 6 were not large enough to significantly further a phenomenon is known to occur in SiC-based fibres challenge the surface flaw-induced failure mode follow- held in an inert atmosphere above 1000.C[20] and ing the initial increase between room temperature and attributed to chemical decomposition of the fibre and 1200oCof ®bres that ``shattered'' upon failure. Likewise, ®bres characterised as ``¯at fracture surface'' in Table 2 for the room temperature test condition possessed smooth features that suggested them to be the weakest group of ®bres. 3.1.2. 1300C in vacuum Whereas ®bres tested at 1200C in vacuum indicated no obvious grain growth compared to room tempera￾ture ®bres, those tested at 1300C exhibited signi®cant grain growth and ®nely distributed voids (Fig. 6). Such a phenomenon is known to occur in SiC-based ®bres held in an inert atmosphere above 1000C [20] and attributed to chemical decomposition of the ®bre and evolution of CO, with associated decreased tensile strength [21]. That this phenomenon was not observed in current specimens until 1300C was believed, in part, to be due to the relatively short time above 1000C during testing compared to previous researchers. Also observed in Fig. 6 is a fracture mirror that was typical of those seen in 84% of ®bres at 1300C in vacuum (Table 2). The number of ®bres that failed due to inter￾nal ¯aws was about 12% and only slightly larger than that at 1200C (10%) indicating that the voids observed in Fig. 6 were not large enough to signi®cantly further challenge the surface ¯aw-induced failure mode follow￾ing the initial increase between room temperature and 1200C. Table 2 In situ ®bre fracture surface characteristics for Tyranno1 Si±Ti±C±O ®bres Test condition Fracture mirror (surface) % (N) Fracture mirror (internal) % (N) Flat fracture surface % (N) Undetermined % (N) Total % (N) Room temperature 75 (114) 1 (2) 11 (16) 13 (19) 100 (151) 1200C/vacuum 89 (100) 10 (11) 0 (0) 1 (1) 100 (112) 1300C/vacuum 84 (132) 12 (19) 1 (1) 3 (5) 100 (157) 1100C/air 18 (128) 0 (0) 82 (570) 0 (0) 100 (698) 1200C/air 18 (64) 0 (0) 92 (724) 0 (0) 100 (788) Fig. 6. Scanning electron micrographs illustrating the fracture surface of a Tyranno1 Si±Ti±C±O ®bre tested in situ at 1300C in vacuum that failed due to a surface ¯aw: (a) general view, and (h) detailed view of fracture mirror. Fig. 5. Scanning electron micrographs illustrating the fracture surface of a Tyranno1 Si±Ti±C±O ®bre tested in situ at 1200C in vacuum that failed due to an internal ¯aw: (a) general view, and (b) detailed view of fracture mirror. I.J. Davies et al. / Composites Science and Technology 59 (1999) 801±811 805