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Novel silicon carbide joining for new generation of accident-tolerant nuclear fuels stable brazing interlayer that does not areas are aluminum (with some iron), see how the SiC–SiC assembly would require extensive heating times or pres- and the black areas are SiC. perform in an environment close to sures that would make the manufacture Energy-dispersive X-ray spectroscopy that which would be encountered in a of fuel rod cladding assemblies—which mapping and cross sectional images nuclear reactor application. As a first are only a few millimeters thick—dif- showed that fracture occurs within the step, irradiation testing was completed ficult or impossible. silicon bond layer and at the SiC–braze at the pressurized water research reactor The EWI approach employs a multi- interface in the aluminum-rich areas. (PWR) at the Massachusetts Institute phase braze alloy interlayer consisting of Also, knowing that aluminum rapidly of Technology. The test was conducted silicon and aluminum with a two-phase forms tenacious aluminum carbides on with the typical PWR primary water joined microstructure. The proprietary, contact with SiC, leads to the conclu- conditions of 300°C, 1,000 parts per patent-pending technology has the sion that a composite, three-dimensional million of boron, and 7 parts per mil- potential to meet all of the in-service fracture occurred. It is apparent that lion of lithium at saturation pressure. and manufacturing requirements.11,12 the silicon has fractured in the “gauge Researchers did a preliminary exami- The novel aspect of this approach is center” of the braze, roughly equidistant nation of the samples after they were in the use of a hypereutectic mixture of alu- from the two SiC substrates. In the alu- the MIT reactor for a little more than minum and silicon. The initial joining minum-rich areas, either the top or the six months. Figure 5 shows an opti- interlayer, however, is not a mixed alloy bottom face of the SiC has failed, and cal image of the samples loaded in the but rather a two-phase mixture of nearly the fracture is to the positive or negative MIT bonding sample test capsule before pure silicon with nearly pure aluminum z direction relative to the silicon area. irradiation. Figure 1 shows an image of and a small amount of alloying elements. The fracture of the braze in many differ- a sample after irradiation, and, as is evi- By heating the braze mixture above the ent planes has positive implications for dent, the sample did not fail. melting point of aluminum, but below damage tolerance. Indeed, investigators During the time in the reactor, MIT the melting point of silicon, a distinctive observed this experimentally during a estimates that the samples experienced microstructure is formed consisting of three-point bend test, when an initial about 11,200 megawatt-hours of energy. plates of silicon with areas of aluminum- crack formed: The sample relieved the Based on typical flux numbers for the rich silicon-containing phases. This two- stress, and catastrophic failure did not facility, that would correspond to a flu- phase joined microstructure provides appear until they imposed a higher cross- ence rate of about 3.7 × 1020 neutrons/ crack-arresting paths that enable high head displacement. (square centimeter per second) (E > 0.1 toughness of the joined assembly. megaelectronvolts) or about 0.4 displace- To test this novel joining technique, Initial radiation testing of joined ments per atom (dpa) based on a rule-of- EWI researchers began by joining small assembly thumb dose/dpa conversion. To put these monolithic samples of SiC. Engineers In addition to three-point bend tests, numbers into context, SiC has been accomplished this by joining two 1-inch EWI conducted several temperature- shown to volumetrically swell under × 0.5-inch × 0.5-inch blocks of α-phase cycling tests. Investigators cycled joined irradiation on the order of a few percent Hexoloy SiC manufactured by Saint- assemblies 25 times in air to 350°C, when irradiated at 1–5 dpa level, corre- Gobain Ceramics with a thin, approxi- and then to 1,200°C for mately 10-micrometer-thick, interlayer one cycle. Subsequent of the silicon braze (Figure 1). mechanical testing and Some of the joined samples were microstructural analysis prepared for initial SEM analysis. SEM showed no postther- images of the silicon-only regions of the mal cycling changes in bond indicated that the braze wetted the braze joint. Also, the interface well and that the bond one sample assembly was pore- and crack-free (Figure 2). was quenched in water SEM images also showed that the inter- after heating to 700°C. layer contained aluminum-rich phases Although the braze did interspersed through the bond layer crack, the crack-arresting (Figure 3). properties of the two- samples to mechanical testing. They braze held the SiC–SiC (Credit: EWI.)phase structure of theNext, researchers subjected the fractured joined assemblies using a three- assembly joined macro- Figure 4. Back-scattered SEM image of the braze fracture point bend test, and the braze fracture scopically (as opposed to areas are aluminum (with some iron), and the blackface. The gray areas are pure silicon, the white interface was characterized. complete debonding). areas are SiC. By combining the information embodied Figure 4 is a back-scattered SEM In addition to the there with information from EDS mapping and cross sec- image of the braze fracture face. The above tests, the EWI tional images, EWI investigators could deduce the frac- gray areas are pure silicon, the white group was anxious to ture characteristics. 34 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No. 1


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