J. Sha et al. / Materials Characterization 57(2006)6-1l (a) (11 (c)(11 (311) 160°cHT1=(200 222) 1400C HTT 1400°cHTT 1600°cHT As-received As-received As-received wwW WAANW/W Fig 3. XRD patterns for fibers heat treated at different temperatures:(a) HNL fibers;(b) HNLS fibers; (c) TySA fibers fibers, as listed in Table 2. From these results, we can see(20=35.7, d-0 251 nm),(220)(20=60.0, d=0. 154 the g values increase with an increase of the test nm)and(311)(20=720o, d=0.131 nm) planes. After temperature. The same change in apparent activation heat treatment at temperatures > 1600C, two other energy was also found in earlier studies. The large peaks, which are indexed as the(200) and(222)planes activation energies for the high temperature regions- of the B-SiC phase, become obvious i.e. low values of m-could be related to the concurrent Using Scherrer's formula, the apparent crystallite microstructure change during the BSr tests [9] size of B-SiC, Dsic, was calculated from the half-value The apparent activation energies for creep for heat width of the(111) peak reated fibers were also calculated and compared with Fig. 4 show the correlation between 1 h stress those of as-received fibers (Table 2). The o value for relaxation temperatures for m=0.5 and the crystallite HNL and hnls fibers increased with an increase in the heat treatment temperature. However, no obvious HTT -HNL fiber dependence of the apparent activation energy for TySA fibers was observed (Table 2) -TysA fiber The apparent activation energies obtained in our work are in acceptable agreement with those for carbon 9 and silicon self-diffusion in SiC [10, 11], which suggests 9 1400- ■-+」2 that the thermally activated diffusion mechanism plays an important role on the creep resistance of si materials The apparent activation energy is a crucial parameter 1200 -·---■ in the evaluation of servic times of sic materials [2] 3. 4. XRD characterization nitia|12001400160 18002000 Heat treatment temperature (c) Fig 3(a)-(c)show typical X-ray diffraction(XRD) pattens of Sic fibers after 1 h heat treatments in Ar. The Fig. 4. Correlation between I h relaxation XRD pattens of the as-received Sic-based fibers show m=0.5 and crystallite size of B-sic for fibers heat treated at elevated hree main peaks which were assigned to the (lll) temperatures in Ar for 1 hfibers, as listed in Table 2. From these results, we can see the Q values increase with an increase of the test temperature. The same change in apparent activation energy was also found in earlier studies. The large activation energies for the high temperature regions— i.e., low values of m—could be related to the concurrent microstructure change during the BSR tests [9]. The apparent activation energies for creep for heat treated fibers were also calculated and compared with those of as-received fibers (Table 2). The Q value for HNL and HNLS fibers increased with an increase in the heat treatment temperature. However, no obvious HTT dependence of the apparent activation energy for TySA fibers was observed (Table 2). The apparent activation energies obtained in our work are in acceptable agreement with those for carbon and silicon self-diffusion in SiC [10,11], which suggests that the thermally activated diffusion mechanism plays an important role on the creep resistance of SiC materials. The apparent activation energy is a crucial parameter in the evaluation of service lifetimes of SiC materials [12]. 3.4. XRD characterization Fig. 3 (a)–(c) show typical X-ray diffraction (XRD) patterns of SiC fibers after 1 h heat treatments in Ar. The XRD patterns of the as-received SiC-based fibers show three main peaks which were assigned to the (111) (2θ= 35.7°; d= 0.251 nm), (220) (2θ= 60.0°; d= 0.154 nm) and (311) (2θ= 72.0°; d= 0.131 nm) planes. After heat treatment at temperatures ≥1600 °C, two other peaks, which are indexed as the (200) and (222) planes of the β-SiC phase, become obvious. Using Scherrer's formula, the apparent crystallite size of β-SiC, DSiC, was calculated from the half-value width of the (111) peak. Fig. 4 show the correlation between 1 h stress relaxation temperatures for m= 0.5 and the crystallite Fig. 3. XRD patterns for fibers heat treated at different temperatures: (a) HNL fibers; (b) HNLS fibers; (c) TySA fibers. 1000 1100 1200 1300 1400 1500 1600 1200 1400 1600 1800 2000 0 5 10 15 20 25 30 35 40 HNL fiber HNLS fiber TySA fiber Initial Crystallite size of SiC (111), DSiC/nm Heat treatment temperature (°C) Stress relaxation temperature for m = 0.5 (°C) Fig. 4. Correlation between 1 h stress relaxation temperature for m= 0.5 and crystallite size of β-SiC for fibers heat treated at elevated temperatures in Ar for 1 h. J.J. Sha et al. / Materials Characterization 57 (2006) 6–11 9