 | STABLE AND UNSTABLE GRAIN BOUNDARY STRUCTURES IN NANOCRYSTALLINE SiC
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| | | 1) Ceramics Superplasticity Project, ICORP, Japan Science and Technology Corporation |
| | Nanocrystalline silicon carbides (SiC) were selected as the candidates for studying superplasticity in covalent materials. Grain boundary analysis plays a key role in understanding the deformation mechanism that is largely depending on the properties of grain boundaries. To achieve this goal, high performance analytical electron microscope with an electron probe as small as 2.5Å, equipped with electron energy-loss spectroscopy (EELS) facility and EELS “spectrum imaging” function, and high-resolution electron microscopy (HREM), have been employed to investigate the local structure, segregation, composition and chemical bonding of grain boundaries in two material systems: the liquid-phase sintered (LPS) SiC and the hot-isostatic-pressed (HIPed) boron-doped SiC. The grain boundary characteristics of these two systems are quite different under deformation. In LPS-SiC, the alumina and yttria dopants segregate to grain boundaries to form amorphous films. In one case with relatively higher alumina content, deformation tests did not eliminate the grain boundary films, but the film composition has changed significantly following the crystallization of most of the dopants staying at intergranular pockets. In another case with less alumina, not only the crystallization of pockets occurred during the deformation, but severe vaporization of mainly alumina dopant (perhaps together with carbon) removed the film from grain boundary. This system indicates that although sintering by liquid phases significantly enhanced the diffusion, the grain boundary does not have a chemically stable structure. On the other hand, amorphous film was not observed in boron-doped SiC, either HIPed or hot-pressed. EELS has found that boron segregated to the grain boundary and carbon has an excess in respect to silicon. In the HIPed sample oxygen was also found at grain boundary. The energy loss near-edge structures (ELNES) analysis discovers that B-C and Si-O bonds were formed at these boundaries. The boron segregants are quite stable against the deformation as well as against the overall composition change. For HIPed SiC doped with boron in a range from 0.3wt% to 3.0wt%, boron segregation remained little changed at the same level, one monolayer, and the accompanying carbon excess has similar quantity. Upon annealing and deformation, boron stayed at the same segregation level while oxygen segregants disappeared rapidly. It is not until the final stage of deformation, more than 100% of elongation for 3 hours at 1800°C, that boron segregants dropped to one third of the initial level when significant amount of cavities already formed in the material. It is clear that boron and carbon formed a rather stable grain boundary structure, in contrast to the situation with alumina-based films. |
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