RH. Jones et al. I Journal of Nuclear Materials 307-311(2002)1057-1072 550 keV Si 190K o0g 200K 0.6 ▲ Si Disorder a Si Disorder 0.2 Model Prediction 0.3 Dose(dpa) Fig. 5. Relative disorder in Sic based on MD simulations [16- Fig. 4. Relative disorder on the si sublattice at the damage ak as a function H-SiC irradiated with 550 keV Sit 190K[2-14 agreement with experimental measurements [18]. Th the good agreement between MD simulations and ex- simulation study, 10 keV Si cascades, similar to those in perimental results provides atomic-level insights into Fig 3, were randomly overlapped at 200 K in an Md the interpretation of radiation damage processes in SiC simulation cell containing 40000 atoms until a fully Ongoing MD simulations on cascade annealing and disordered state was achieved after 140 cascades [16-18]. defect migration in Sic will yield new atomic-level un- t low doses, damage is dominated by single interstitials derstanding of the temperature dependence of radiation and small clusters consisting of interstitials and antisite damage processes in SiC. defects and their concentration increases with increas- ing dose. The coalescence of small and large clusters at rtant mechanism leading to 6. Recent advances in SiC /sic amorphization in SiC, and the homogeneous nucleation of small clusters at low doses is consistent with the 6. New materials homogeneous amorphization process that is observed experimentally by high-resolution TEM [19]. Under 6.1.I. Composites with advanced fiber these conditions, the primary driving force for irradia Composites produced with the advanced fibers, Hi- tion-induced amorphization is the accumulation of both Nicalon Type S and Tyranno-SA have been irradiated interstitials and antisite defects. The relative disorder and the test results are presented and compared to other from the MD simulations exhibits a sigmoidal depen- data in Fig. 6 [20-29]. Comparison is made to both dence on dose, as shown in Fig. 5, that is in good monolithic SiC and composites made with Ceramic agreement with the experimental measurements for 550 Grade-Nicalon and Hi-Nicalon fibers. The results cover keV Sit irradiation(Fig. 4). The interpretation of the a range of temperatures but the trend of the irradiated to MD results is consistent with the direct-impact/defect unirradiated ultimate strength, Surra / Sunirr, clearly stimulated model for amorphization, where the pro- shows that composites with the advanced fibers Hi- and antisite defects stimulates Nicalon Type S and Tyranno Sa showed no loss in morphous growth at crystalline-amorphous interfaces. strength up to a dose of 10 dpa. The results of Price The model fit shown in Fig. 5 is based on the average [28, 29 and Jones et al. [22] give some support to the relative cross sections determined previously for single possibility that the strength of irradiated advanced fiber 10 keV Si cascades [12]. High-resolution TEM image material could remain unchanged up to at least 10 dpa simulations of specific damage states in the MD simu- and perhaps higher. Further advances will likely require lation cell have been performed to reveal the change in ailoring the interface swelling characteristics to com- microstructural features with increasing dose from cas- pensate for differential swelling between the fiber and cade overlap [17]. The microstructural evolution in the matrix. Advanced interface developments that could MD simulations is very similar to that observed previ- provide this tailoring have been reported by Snead [27] ously in experimental HRTEM images obtained from and they include multilayer Sic/C interfaces and porou 19. Likewise, the swelling and or pseudo-porous Sic interfaces. An example of a stored energy determined as a function of dose from multilayer interface is shown in Fig. 7 and the resulting cascade overlap in the MD simulations are in good room temperature bend strengths are given in Fig. 8simulation study, 10 keV Si cascades, similar to those in Fig. 3, were randomly overlapped at 200 K in an MD simulation cell containing 40 000 atoms until a fully disordered state was achieved after 140 cascades [16–18]. At low doses, damage is dominated by single interstitials and small clusters consisting of interstitials and antisite defects, and their concentration increases with increas￾ing dose. The coalescence of small and large clusters at higher doses is an important mechanism leading to amorphization in SiC, and the homogeneous nucleation of small clusters at low doses is consistent with the homogeneous amorphization process that is observed experimentally by high-resolution TEM [19]. Under these conditions, the primary driving force for irradia￾tion-induced amorphization is the accumulation of both interstitials and antisite defects. The relative disorder from the MD simulations exhibits a sigmoidal depen￾dence on dose, as shown in Fig. 5, that is in good agreement with the experimental measurements for 550 keV Siþ irradiation (Fig. 4). The interpretation of the MD results is consistent with the direct-impact/defect stimulated model for amorphization, where the pro￾duction of interstitials and antisite defects stimulates amorphous growth at crystalline-amorphous interfaces. The model fit shown in Fig. 5 is based on the average relative cross sections determined previously for single 10 keV Si cascades [12]. High-resolution TEM image simulations of specific damage states in the MD simu￾lation cell have been performed to reveal the change in microstructural features with increasing dose from cas￾cade overlap [17]. The microstructural evolution in the MD simulations is very similar to that observed previ￾ously in experimental HRTEM images obtained from ion-irradiated 3C–SiC [19]. Likewise, the swelling and stored energy determined as a function of dose from cascade overlap in the MD simulations are in good agreement with experimental measurements [18]. Thus, the good agreement between MD simulations and ex￾perimental results provides atomic-level insights into the interpretation of radiation damage processes in SiC. Ongoing MD simulations on cascade annealing and defect migration in SiC will yield new atomic-level un￾derstanding of the temperature dependence of radiation damage processes in SiC. 6. Recent advances in SiCf/SiC performance 6.1. New materials 6.1.1. Composites with advanced fibers Composites produced with the advanced fibers, Hi￾Nicalon Type S and Tyranno-SA have been irradiated and the test results are presented and compared to other data in Fig. 6 [20–29]. Comparison is made to both monolithic SiC and composites made with Ceramic Grade-Nicalon and Hi-Nicalon fibers. The results cover a range of temperatures but the trend of the irradiated to unirradiataed ultimate strength, Sirrad u =Sunirr u , clearly shows that composites with the advanced fibers Hi￾Nicalon Type S and Tyranno SA showed no loss in strength up to a dose of 10 dpa. The results of Price [28,29] and Jones et al. [22] give some support to the possibility that the strength of irradiated advanced fiber material could remain unchanged up to at least 10 dpa and perhaps higher. Further advances will likely require tailoring the interface swelling characteristics to com￾pensate for differential swelling between the fiber and matrix. Advanced interface developments that could provide this tailoring have been reported by Snead [27] and they include multilayer SiC/C interfaces and porous or pseudo-porous SiC interfaces. An example of a multilayer interface is shown in Fig. 7 and the resulting room temperature bend strengths are given in Fig. 8. Fig. 5. Relative disorder in SiC based on MD simulations [16– Fig. 4. Relative disorder on the Si sublattice at the damage 18]. peak as a function H–SiC irradiated with 550 keV Siþ ions at 190 K [12–14]. 1062 R.H. Jones et al. / Journal of Nuclear Materials 307–311 (2002) 1057–1072