Microstructure and fracture of helium implanted pure SiC and SiC/C composite
Microstructure and fracture of helium implanted pure SiC and SiC/C composite
In this thesis a detailed study of effects of He-implantation on strain and strain recovery, on cracking and fracture stress, and on microstructural changes of a SiC/C composite and pure SiC is presented by means of profilometry, 3-point bending, SEM and TEM. Strains from He-implantation were derive...
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Personal Name(s): | Chen, J. (Corresponding author) |
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Contributing Institute: |
Publikationen vor 2000; PRE-2000; Retrocat |
Imprint: |
Jülich
Forschungszentrum Jülich, Zentralbibliothek, Verlag
1998
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Physical Description: |
III, 115 p. |
Document Type: |
Report Book |
Research Program: |
Addenda |
Series Title: |
Berichte des Forschungszentrums Jülich
3585 |
Link: |
OpenAccess OpenAccess |
Publikationsportal JuSER |
In this thesis a detailed study of effects of He-implantation on strain and strain recovery, on cracking and fracture stress, and on microstructural changes of a SiC/C composite and pure SiC is presented by means of profilometry, 3-point bending, SEM and TEM. Strains from He-implantation were derived with very high sensitivity (ppm range) from a new method based on the bending of inhomogenously implanted thin ($\approx$ 0.3 mm) specimens. Stress distributions in this case are calculated by the finite element method and limitations of analytical stress calculations are given. Changes of volume of thin silicon carbide specimens and a SiC/C composite were measured under helium implantation and during subsequent annealing. The results indicate that mainly displacement defects are responsible for damage, while implanted helium plays a minor role in the case of He-concentration below about 0.2 at%. Most displacement defects are annealed below 1050°C. Comparison of the annealing behaviour of dilatation after implantation to that of specimens deformed by polishing indicated that essentially the same defects are produced in both cases. Helium was also implanted at room temperature and 1000°C into one surface of 3x3 mm bars of a SiC/C composite uniformly to depths of 117 and 254 µm, respectively. Profilometry showed a large helium-induced volume expansion of the implanted layer which caused bending of the bars. For helium concentrations above $\approx$ 500 atppm in layers of 254 $\mu$m thickness, SEM revealed spontaneous cracking just below the implanted region where the tensile stress is maximum. No spontaneous cracking was observed below the 117 µm thick layers for concentrations up to $\approx$ 1400 atppm. For investigation of the influence of helium on the strength of the material, a computerized 3-point bending tester, operating at temperatures from RT to 1000°C was installed on a SEM specimen holder. Below the critical concentrations for cracking, fracture stresses were studied by this tester. The observed dependencies of cracking on parameters such as specimen geometry, volume expansion, and applied stress are in good agreement with analytical and finite element calenlations. Apparent fracture stress in 3-point bending of SiC/C decreases with helium implantation. If irradiation induced internal stresses are taken into account, an increase in fracture strength is obtained, at least for helium concentrations up to 200 atppm. Most effort focused on He-clustering in 4H- and 6H-SiC grains by microstructural investigation. Helium has a decisive effect on the microstrutural changes in SiC after annealing. A dense population not only of bubbles but also of dislocation loops is observed after annealing of helium implanted SiC, while no damage was seen under the saure conditions after irradiation without implantation. Damage in He-implanted SiC after annealing show three clearly separated regions: grainboundaries (GB), depleted region (or defect free zone) along GBs (DR) and matrix. The DRs give a good chance to keep thin fibre free of defects which is important for applications of SiCf/SiC composites in future fusion reactors. He-platelets were observed after He-implantation at room temperature and calculations give pressures of about 20 GPa in these platelets. This would be the first indication of solid helium at ambient temperature. Bubble-loop complexes including faceted bubbles, simple loops, multiple loops and loop-cluster, were formed after anneaiing. The habit plane of these complexes and interstitial loops, and the facets of the bubbles were determined. The number of atoms in the loops equals within measuring accuracy those missing in the associated bubbles. This is first quantitative evidence for bubble growth by kicking out of matrix atoms, (also called trag mutation). |