l174 T. Nozawa et al. /Journal of Nuclear Materials 307-311(2002)1173-1177 degradation of stoichiometric fibers themselves does not a100 occur after neutron irradiation up to 10 dpa [8, 9]. 8 Consequently, the composite structure and strength be- E came more stable against neutron exposure. However, there are few studies on the stability of Sic/Sic against neutron exposure, especially far fuences <l dpa. This will be important to give us useful knowledge about initial behavior of sic/sic for neutron irradiate Also, it is considered to be more important to iden tify neutron effects for SiC/Sic derived by recently de veloped alternate processes such as RS or PlP processes promising for commercial applications. However, radi- ation stability of SiC/SiC fabricated by advanced pro- cesses still remains unrevealed 020406080100120 The objective of this study is to evaluate effects of Distance from Fiber Surface(nm) neutron irradiation <l dpa, on structural and mechanical stability of SiC/ Fig. 1. Compositionally gradient SiC-C interphase Sic composites from the viewpoints of the influences of crystallinity and impurities of reinforced fibers, and Mechanical stabilities against neutron irradiation micro-structural changes of matrix and F/M interface were evaluated by 3-point flexural tests Support span of formed by each fabrication process. test beam was 18.0 mm. All the tests were conducted by using an electromechanical testing machine at room temperature with the crosshead control of 3. x 10-3 2. Experimental m/s, on the basis of ASTM C1341 [12]. Micro-structural observations were also conducted by using scanning Several kinds of Sic/SiC composites were fabricated electron microscopy(SEM) after mechanical tests by F-CVI, RS and Plp processes, respectively (Table 1) Highly crystalline, low oxygen content fibers such as Hi NicalonM Type-S and Tyranno TM SA were used as re- 3. Results and discussion inforcement For comparison, Hi-NicalonM fiber, with lower crystalline microstructure, reinforced Sic matrix 3. 1. Neutron effect on F-Cv Sic/sic composites were also prepared. F-CVI, RS and PIP SiC/ Sic composites had a thin PyC, boron nitride(Bn and Significant reduction of flexural strength did not gradient composition carbon as F/M interface, respec ccur and flexural strength was quite stable for all F tively. The last one was modified by specific thermo- CVi derived composites regardless of the fiber types as chemical treatment called S6 treatment(Fig. 1)[11] shown in Figs. 2 and 3. This is because dimensional All the test(25 mm x 4 mm x 2 mm)pieces cut into changes in fiber and matrix might be quite small up to from these composites respectively were irradiated in the 0.4 dpa in neutron dose and also the strength of each Japan materials testing reactor (JMTR). They were ir- omponent was still stable under such a low fluence adiated to a fluence of 0.4 x 10n/m(E>0.1 Mev) at irradiation [13, 14. In this experiment, volume changes 1073 K. In this study, this fluence corresponds to 0. 4 dpa of each F-CVI SiC/SiC were less than 0.6%. High crys- (1dpa=1×1035n/m2) talline SiC fibers like Hi-NicalonM Type-s, Tyranno SA Table I Test materials CS02 CHOI POL Matrix F-CVI SIC FCVI SIC FCVI SiC RS SIC PIP SIC Fiber Hi-Nicalon Type-s Tyranno-SA Hi-Nicalon Hi-Nicalon Crystal ure of fiber Crystal ro crystal Micro crystal P/W P/W P/W Pyc Py Gradient C Thickness of interphase(nm) 150 150 l50 Density (Mg/m) 2.82 2.11 ORNL ORN ORNL Toshiba Ubedegradation of stoichiometric fibers themselves does not occur after neutron irradiation up to 10 dpa [8,9]. Consequently, the composite structure and strength be￾came more stable against neutron exposure. However, there are few studies on the stability of SiC/SiC against neutron exposure, especially far fluences <1 dpa. This will be important to give us useful knowledge about initial behavior of SiC/SiC for neutron irradiation. Also, it is considered to be more important to iden￾tify neutron effects for SiC/SiC derived by recently de￾veloped alternate processes such as RS or PIP processes promising for commercial applications. However, radi￾ation stability of SiC/SiC fabricated by advanced pro￾cesses still remains unrevealed. The objective of this study is to evaluate effects of neutron irradiation, especially low fluence irradiation <1 dpa, on structural and mechanical stability of SiC/ SiC composites from the viewpoints of the influences of crystallinity and impurities of reinforced fibers, and micro-structural changes of matrix and F/M interface formed by each fabrication process. 2. Experimental Several kinds of SiC/SiC composites were fabricated by F-CVI, RS and PIP processes, respectively (Table 1). Highly crystalline, low oxygen content fibers such as Hi￾NicalonTM Type-S and TyrannoTM SA were used as re￾inforcements. For comparison, Hi-NicalonTM fiber, with lower crystalline microstructure, reinforced SiC matrix composites were also prepared. F-CVI, RS and PIP SiC/ SiC composites had a thin PyC, boron nitride (BN) and gradient composition carbon as F/M interface, respec￾tively. The last one was modified by specific thermo￾chemical treatment called S6 treatment (Fig. 1) [11]. All the test (25 mm
4 mm
2 mm) pieces cut into from these composites respectively were irradiated in the Japan materials testing reactor (JMTR). They were ir￾radiated to a fluence of 0:4
1025 n/m2 (E > 0:1 MeV) at 1073 K. In this study, this fluence corresponds to 0.4 dpa (1 dpa ¼ 1
1025 n/m2). Mechanical stabilities against neutron irradiation were evaluated by 3-point flexural tests. Support span of test beam was 18.0 mm. All the tests were conducted by using an electromechanical testing machine at room temperature with the crosshead control of 3:0
105 m/s, on the basis of ASTM C1341 [12]. Micro-structural observations were also conducted by using scanning electron microscopy (SEM) after mechanical tests. 3. Results and discussion 3.1. Neutron effect on F-CVI SiC/SiC Significant reduction of flexural strength did not occur and flexural strength was quite stable for all F￾CVI derived composites regardless of the fiber types as shown in Figs. 2 and 3. This is because dimensional changes in fiber and matrix might be quite small up to 0.4 dpa in neutron dose and also the strength of each component was still stable under such a low fluence irradiation [13,14]. In this experiment, volume changes of each F-CVI SiC/SiC were less than 0.6%. High crys￾talline SiC fibers like Hi-NicalonTM Type-S, Tyranno SA Fig. 1. Compositionally gradient SiC–C interphase. Table 1 Test materials CS01 CS02 CH01 RH01 PN01 Matrix F-CVI SiC F-CVI SiC F-CVI SiC RS SiC PIP SiC Fiber Hi-Nicalon Type-S Tyranno-SA Hi-Nicalon Hi-Nicalon Tyranno-TE Crystal structure of fiber Crystal Crystal Micro crystal Micro crystal Amorphous Architecture S/W P/W P/W UD P/W Interphase PyC PyC PyC BN Gradient C Thickness of interphase (nm) 150 150 150 – 200 Density (Mg/m3) 2.22 2.26 2.38 2.82 2.11 Supplier ORNL ORNL ORNL Toshiba Ube 1174 T. Nozawa et al. / Journal of Nuclear Materials 307–311 (2002) 1173–1177