J Sha et al. /Corrosion Science 50(2008)3132-3138 3133 herefore, for understanding the mechanical and thermal sta bilities and failure mechanism of Sic fibers over a wide range of 一A temperatures and varied environments, this work proceeded a complementary investigation on the microstructure features and high temperature properties of Hi-Nicalon fibers under annealing and creep in various oxygen partial pressures at elevated temper- atures, and attempted to clarify the correlation between the envi- ronment with mechanical and thermal stabilities with this result a further discussion was made on the environment-pertinent 502 Tk 2. Experimental The SiC fiber examined in this study is Hi-Nicalon(C/Si atomic ratio: 1.38, oxygen: 0.5 wt%, diameter: 14 um). This fiber was an- nealed and crept in air(O2: 20%, dew point: 3C), high-purity Ar 1000110012001300140015001600 (HP-Ar, 02: 2 ppm, dew point:-55C)and ultra high-purity Ar HP-Ar, 02: 0. 1 ppb: dew point:-5.5C)under flowing atmo- here with a pressure of 10 Pa and held for 1 h at desired tem- Fig 1. Mass change of Hi-Nicalon hiber under annealing at elevated temperatures in peratures ranging from 1000 to 1500C. Fibers' annealing was fferent atmospheres for 1h. performed on the 5 cm fragments which were positioned in the hot zone of furnace chamber. The mass change was measured by electronic balance(mass resolution: #0. 1 mg). After annealing at 3.3. Tensile properties and creep resistance 1500C, individual fiber was carefully separated and pulled out from the fiber bundle for single fiber tensile test by a technique Fig 3 shows the tensile stress-strain curve for fibers un as described in our previous studies[8-10 annealing in different environments at 1500C. The tensile stress The creep resistance was assessed by bend stress relaxation (BSR)method which was developed by Morscher [11]. The detailed ven that conversion of load to tensile stress would depend on the configuration of test jig can be found elsewhere 121 By means of specific diameter of each fiber. In this work, the individual fiber this configuration, the influence of environment on BSR creep diameter was measured by SEM image but not using the average resistance could be evaluated. The meter used to index the diameter. The true value of ultimate tensile strength can be read reep resistance is the bend stress relaxation parameter m, which from the stress-strain curve is defined as: m=1-Ro/Ra, where ro and Ra are, respectively the Fig 4 shows the dependence of mean strength on the testing curvature for the initially imposed bend strain and the residual environments. The fiber's strength decreased with decreasing the curvature after thermal exposure and strain removal, if m-1, no oxygen partial pressure It should be noted during the specimen relaxation has occurred: if m=0, complete relaxation has occurred preparation that fibers with low strength became very difficult to (Ro=Ra). Furthermore, the surface morphologies of fibers were set without breaking them. The mean strength we gave will conse- examined by the observation of field-emission scanning electron quently not take the weakest fibers into account(no strength could microscopy(FE-SEM): the phase in fibers was analyzed by X-ray be obtained ) Due to this shortcoming, overestimation of tensile diffraction(XRD). strength is likely. As observed for fibers after annealing in UH Ar at 1500C for 1 h, the fibers are too fragile to measure the 3. Results Furthermore, Fig. 4 shows the dependence of 1-h BSR 3.1. Mass chang resistance m on testing environments at 1300C. The BSr Fig. 1 shows the mass change for fibers under annealing in different oxygen partial pressure atmospheres for 1 h at elevated is observed for fibers under UHP-Ar in air, but the mass gain at 1400C is lower than that at 1300C. For the fibers under annealing in inert atmospher (HP-Ar and UHP-Ar), a mass loss is observed at temperature be- yond1300°C 3. 2. X-ray diffraction patten Fig 2 shows X-ray diffraction patterns for fibers under anneal ing in different oxygen partial pressures for 1 h at temperatures ranging from 1300 to 1500C. Obviously, the fibers under anneal ing in air result in the formation of silica peaks with different height at 20=22 which are identified as the cristobalite but the silica peak does not appear for fibers under annealing in inert atmospheres. Furthermore, it can be seen in Fig. 2 that B-Sic peaks 1520253035404550556065707580 become sharper for fibers under annealing at temperature over 2 Theta 1300C in comparison to as-received fibers, but it is more obvious Fig. 2. X-ray diffraction patterns for Hi-Nicalon fibers under annealing in different for fibers under annealing at 1500C. oxygen partial pressure atmospheres,◆:阝Sic:▲: cristobaliteTherefore, for understanding the mechanical and thermal sta￾bilities and failure mechanism of SiC fibers over a wide range of temperatures and varied environments, this work proceeded a complementary investigation on the microstructure features and high temperature properties of Hi-Nicalon fibers under annealing and creep in various oxygen partial pressures at elevated temper￾atures, and attempted to clarify the correlation between the envi￾ronment with mechanical and thermal stabilities. With this result, a further discussion was made on the environment-pertinent performance. 2. Experimental The SiC fiber examined in this study is Hi-Nicalon (C/Si atomic ratio: 1.38, oxygen: 0.5 wt%, diameter: 14 lm). This fiber was an￾nealed and crept in air (O2: 20%, dew point: 3 C), high-purity Ar (HP-Ar, O2: 2 ppm, dew point: 5.5 C) and ultra high-purity Ar (UHP-Ar, O2: 0.1 ppb: dew point: 5.5 C) under flowing atmo￾sphere with a pressure of 105 Pa and held for 1 h at desired tem￾peratures ranging from 1000 to 1500 C. Fibers’ annealing was performed on the 5 cm fragments which were positioned in the hot zone of furnace chamber. The mass change was measured by electronic balance (mass resolution: ±0.1 mg). After annealing at 1500 C, individual fiber was carefully separated and pulled out from the fiber bundle for single fiber tensile test by a technique as described in our previous studies [8–10]. The creep resistance was assessed by bend stress relaxation (BSR) method which was developed by Morscher [11]. The detailed configuration of test jig can be found elsewhere [12]. By means of this configuration, the influence of environment on BSR creep resistance could be evaluated. The parameter used to index the creep resistance is the bend stress relaxation parameter m, which is defined as: m = 1 R0/Ra, where R0 and Ra are, respectively, the curvature for the initially imposed bend strain and the residual curvature after thermal exposure and strain removal, if m = 1, no relaxation has occurred; if m = 0, complete relaxation has occurred (R0 = Ra). Furthermore, the surface morphologies of fibers were examined by the observation of field-emission scanning electron microscopy (FE-SEM); the phase in fibers was analyzed by X-ray diffraction (XRD). 3. Results 3.1. Mass change Fig. 1 shows the mass change for fibers under annealing in different oxygen partial pressure atmospheres for 1 h at elevated temperatures. A mass gain is observed for fibers under annealing in air, but the mass gain at 1400 C is lower than that at 1300 C. For the fibers under annealing in inert atmosphere (HP-Ar and UHP-Ar), a mass loss is observed at temperature be￾yond 1300 C. 3.2. X-ray diffraction pattern Fig. 2 shows X-ray diffraction patterns for fibers under anneal￾ing in different oxygen partial pressures for 1 h at temperatures ranging from 1300 to 1500 C. Obviously, the fibers under anneal￾ing in air result in the formation of silica peaks with different height at 2h = 22 which are identified as the cristobalite, but the silica peak does not appear for fibers under annealing in inert atmospheres. Furthermore, it can be seen in Fig. 2 that b-SiC peaks become sharper for fibers under annealing at temperature over 1300 C in comparison to as-received fibers, but it is more obvious for fibers under annealing at 1500 C. 3.3. Tensile properties and creep resistance Fig. 3 shows the tensile stress–strain curve for fibers under annealing in different environments at 1500 C. The tensile stress was calculated from the load acquired during the tensile test. Gi￾ven that conversion of load to tensile stress would depend on the specific diameter of each fiber. In this work, the individual fiber diameter was measured by SEM image but not using the average diameter. The true value of ultimate tensile strength can be read from the stress–strain curve. Fig. 4 shows the dependence of mean strength on the testing environments. The fiber’s strength decreased with decreasing the oxygen partial pressure. It should be noted during the specimen preparation that fibers with low strength became very difficult to set without breaking them. The mean strength we gave will conse￾quently not take the weakest fibers into account (no strength could be obtained). Due to this shortcoming, overestimation of tensile strength is likely. As observed for fibers after annealing in UHP￾Ar at 1500 C for 1 h, the fibers are too fragile to measure the strength. Furthermore, Fig. 4 shows the dependence of 1-h BSR creep resistance m on testing environments at 1300 C. The BSR creep -2 -1 0 1 2 3 4 1000 1100 1200 1300 1400 1500 1600 Air HP-Ar UHP-Ar Mass change (%) Temperature(°C) Fig. 1. Mass change of Hi-Nicalon fiber under annealing at elevated temperatures in different atmospheres for 1 h. Intensity 15 20 25 30 35 40 45 50 55 60 65 70 75 80 2 Theta Air HP-Ar UHP-Ar As-received 1500 C 1400 C 1300 C 1500 C 1400 C 1300 C 1500 C 1400 C 1300 C Fig. 2. X-ray diffraction patterns for Hi-Nicalon fibers under annealing in different oxygen partial pressure atmospheres, : b SiC; N: cristobalite. J.J. Sha et al. / Corrosion Science 50 (2008) 3132–3138 3133