T Nozawa et aL/Journal of Nuclear Materials 384(2009)195-211 total thickness of the Pyc layers was applied. As mentioned later. rized as follows:(1)very minor change in Young,'s modulus by residual stresses at the interface for the multilayer composites irradiation, (2)enhanced quasi-ductility beyond the proportional are nearly compatible by taking the case of monolayer composites limit, i. e, no embrittlement by irradiation, resulting in apparent with a total thickness of multilayered Pyc. increase of total elongation and 3)reduction of PLS. The tensile ngth was nearly unchanged for monolayer composites at any 3. Results irradiation temperatures. Similarly no significant decrease of UTs was observed for multilayer composites when Tirr < 1000C, while 3. 1. Tensile properties minor reduction of UTS when Tirr >1000C was specified. Fig 4 plots tensile data as a function of neutron dose. In the fig Fig3 shows typical stress-strain curves of non-irradiated and ure, error bars indicate t1 standard deviation. The proportional irradiated Hi-Nicalon" M Type-s/CVI-Sic composites with either a limit tensile stress slightly decreases even at low neutron doses Pyc monolayer or a Pyc sic multilayer interphase and Table 3 lists of 0. 7 dpa and approaches a constant(-200 MPa)with increasing reduced tensile properties. Data from an irradiation experiment in neutron dose for both interphase types. Tensile strength and total the japan materials test reactor (MTR, Oarai, Japan) for the identi longation conversely tend to increase and approach a constant cal materials are included [12]. Key common findings are summa- with increasing neutron dose. The increment of UTS for monolayer s (30%)was greater than that of mult (20%) Similarly, the increment of total elongation for monolayer composites(160%)was almost double of multilayer composites (70%) Meanwhile, their Youngs moduli seem nearly unchanged 500 Monolayer by irradiation. Large data scatter is due primarily to the variance of porosity and fiber volume fraction. The data subsets in the fluence range of 0.7-4.2 dpa in Fig. 4 冒4005~+740°℃ were extracted and separately plotted as a function of irradiation temperature(Fig. 5). In Fig. 5, Young,s moduli for both interface 640°C 800° structure types appeared unchanged by irradiation accounting for 10G0c the wide data scatter(30%). In contrast, the pls slightly de- creased after irradiation at all temperatures, however, the temper 1080° ature-dependence seems very minor even at 1000C. This observed for both interphase types tested. Additionally, within 80° 800°C the limited data set provided, it appears that a further decrease of the pls has taken place when Tirr >1000C. Both UTS and total 380° elongation were nearly unchanged by irradiation for multilayer composites when Tirr 1000C, while they significantly increased by irradiation for monolayer composites. When Tirr>1000C,a clear difference between the monolayer and multilayer composite was obtained: decreases of UTS for multilayer composites vs no major deterioration for monolayer composites. Neutron Dose (dpa-SIc] 3. 2. Interfacial shear properties Fig 6 shows comparison of typical load-displacement during fiber push-out process between monolayer and multilayer composites As schematically shown in Fig. 2, the fiber push-out Monolayer process generally has the following four stages: (1)elastic defor mation with a tiny plastic deformation, (2)debond initiation at the interface from the loading surface, 3) progressive debond cou- 三c pled with interfacial friction on the debonded fiber surface and ( 4) 740°C complete fiber debonding and sliding. When using a sharp indenter tip. the initial segment of the load-displacement curve becomes ----=- non-linear due to the plastic deformation below the indenter. Be- yond debond initiation, the displacement rapidly increases and a load-displacement curve shows a secondary non-linear segment. In Fig. 6, a concave shape of the secondary non-linear segment 1030%C 800°C for multilayer composites is due to the inherently high interfacial 1080° friction resulting from the rough surface of the fiber. The limited load capacity of the test system does not allow the complete deb- °C 2800 onding and sliding of the composites with exceptionally high de- bond initiation stress. More than 80% of measurements of oma 2380° or multilayer composites were invalid when specimen thickness was >100 um. The slightly convex shape observed for monolayer composites is due to the comparably low friction stress. a plateau Neutron Dose [ dpa-SIC beyond complete debonding, i.e, sliding of the pushed-out fiber. was observed for the case of thick Py C monolayer composites. ress n adiatie tempera ue are nted ien ghe figure The further load increment after complete sliding indicates a phys- Irradiation-induced change of elastic properties of Sic and PyC was considered. No ical contact of the indenter sides with the Sic matrix when using a contribution from swelling was however considered Glassy carbon was assumed. conic indentertotal thickness of the PyC layers was applied. As mentioned later, residual stresses at the interface for the multilayer composites are nearly compatible by taking the case of monolayer composites with a total thickness of multilayered PyC. 3. Results 3.1. Tensile properties Fig. 3 shows typical stress–strain curves of non-irradiated and irradiated Hi-NicalonTM Type-S/CVI-SiC composites with either a PyC monolayer or a PyC/SiC multilayer interphase and Table 3 lists reduced tensile properties. Data from an irradiation experiment in the Japan materials test reactor (JMTR, Oarai, Japan) for the identi￾cal materials are included [12]. Key common findings are summa￾rized as follows: (1) very minor change in Young’s modulus by irradiation, (2) enhanced quasi-ductility beyond the proportional limit, i.e., no embrittlement by irradiation, resulting in apparent increase of total elongation and (3) reduction of PLS. The tensile strength was nearly unchanged for monolayer composites at any irradiation temperatures. Similarly no significant decrease of UTS was observed for multilayer composites when Tirr < 1000 C, while minor reduction of UTS when Tirr > 1000 C was specified. Fig. 4 plots tensile data as a function of neutron dose. In the fig￾ure, error bars indicate ±1 standard deviation. The proportional limit tensile stress slightly decreases even at low neutron doses of 0.7 dpa and approaches a constant (200 MPa) with increasing neutron dose for both interphase types. Tensile strength and total elongation conversely tend to increase and approach a constant with increasing neutron dose. The increment of UTS for monolayer composites (30%) was greater than that of multilayer composites (20%). Similarly, the increment of total elongation for monolayer composites (160%) was almost double of multilayer composites (70%). Meanwhile, their Young’s moduli seem nearly unchanged by irradiation. Large data scatter is due primarily to the variance of porosity and fiber volume fraction. The data subsets in the fluence range of 0.7–4.2 dpa in Fig. 4 were extracted and separately plotted as a function of irradiation temperature (Fig. 5). In Fig. 5, Young’s moduli for both interface structure types appeared unchanged by irradiation accounting for the wide data scatter (30%). In contrast, the PLS slightly de￾creased after irradiation at all temperatures, however, the temper￾ature-dependence seems very minor even at 1000 C. This is observed for both interphase types tested. Additionally, within the limited data set provided, it appears that a further decrease of the PLS has taken place when Tirr > 1000 C. Both UTS and total elongation were nearly unchanged by irradiation for multilayer composites when Tirr < 1000 C, while they significantly increased by irradiation for monolayer composites. When Tirr > 1000 C, a clear difference between the monolayer and multilayer composites was obtained: decreases of UTS for multilayer composites vs. no major deterioration for monolayer composites. 3.2. Interfacial shear properties Fig. 6 shows comparison of typical load-displacement curves during fiber push-out process between monolayer and multilayer composites. As schematically shown in Fig. 2, the fiber push-out process generally has the following four stages: (1) elastic defor￾mation with a tiny plastic deformation, (2) debond initiation at the interface from the loading surface, (3) progressive debond cou￾pled with interfacial friction on the debonded fiber surface and (4) complete fiber debonding and sliding. When using a sharp indenter tip, the initial segment of the load-displacement curve becomes non-linear due to the plastic deformation below the indenter. Be￾yond debond initiation, the displacement rapidly increases and a load-displacement curve shows a secondary non-linear segment. In Fig. 6, a concave shape of the secondary non-linear segment for multilayer composites is due to the inherently high interfacial friction resulting from the rough surface of the fiber. The limited load capacity of the test system does not allow the complete deb￾onding and sliding of the composites with exceptionally high de￾bond initiation stress. More than 80% of measurements of rmax for multilayer composites were invalid when specimen thickness was >100 lm. The slightly convex shape observed for monolayer composites is due to the comparably low friction stress. A plateau beyond complete debonding, i.e., sliding of the pushed-out fiber, was observed for the case of thick PyC monolayer composites. The further load increment after complete sliding indicates a phys￾ical contact of the indenter sides with the SiC matrix when using a conic indenter. Fig. 7. Effects on neutron dose on (a) the interfacial debond shear strength and (b) the interfacial friction stress. Irradiation temperatures are noted in the figure. Irradiation-induced change of elastic properties of SiC and PyC was considered. No contribution from swelling was however considered. Glassy carbon was assumed. T. Nozawa et al. / Journal of Nuclear Materials 384 (2009) 195–211 203