Abstract
(Englisch)
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Experimental contribution
The target structural materials studies at the spallation source division (ASQ) PSI are partly supported by ESS TMR funding for one PhD student, Mr. Xuejun Jia. This support will be ended by 31.07.2002. In the period 01.07.2001 - 30.06.2002 following work has been done mainly by Mr. Xuejun Jia, which is related the TMR program.
1. Investigations on the microstructure of martensitic steels F82H and T91 irradiated in SINQ Target-3 to 12 dpa.
The main results of F82H were reported last year. The observation on T91 samples shows that the microstructure (namely irradiation induced dislocation loops, high-density small helium bubbles, and changes of the precipitate structure) of T91 steel is similar to that of F82H.
2. Mechanical tests on martensitic steels T91, F82H and OPTIMAX irradiated in SINQ Target-3 to 9.4 dpa.
Small punch tests have been performed on more than 80 discs of 3 mm diameter and 0.25 thick in the temperature range from -185° to 25°C. The ductile-brittle transition temperature (DBTTSP) has been evaluated for the three kinds of steels for conditions of 'as-received' and at different doses. The results demonstrate that the DBTTSP has increased significantly after irradiation. For T91 steel, the DBTTSP increases by 128K after irradiation to 9.4 dpa, which is equivalent to about 320K obtained from normal Charpy impact tests. Among the three steels T91 has largest and F82H has the lowest increase of DBTT after irradiation. This observation is similar to that of Charpy impact tests.
Publications:
1. Proceedings of the 4th International Workshop on Spallation Materials Technology (IWSMT-4), ed. Y. Dai, L.K. Mansur, G.S. Bauer, H. Ullmaier, S. Maloy, Journal of Nuclear Materials, volume 296 (2001).
2. Microstructure of both as-irradiated and deformed 304L stainless steel irradiated with 800 MeV protons, Y. Dai, X. Jia, J.C. Chen, W.F. Sommer, M. Victoria, G.S. Bauer, J. Nucl. Mater., 296 (2001) 174.
3. The impact of irradiation temperature on the microstructure of F82H martensitic/ferritic steel irradiated in a proton and neutron mixed spectrum, X. Jia, Y. Dai, M. Victoria, to be published in J. Nucl. Mater.
4. Mechanical properties of F82H and T91 irradiated at £350°C in a proton and neutron mixed spectrum, Y. Dai, X. Jia, K. Farrell, reported at IWSMT-5, Charleston, USA, May 19-24, 2002, to be published in J. Nucl. Mater.
5. Microstructure in martensitic steels T91 and F82H after irradiation in SINQ Target-3, X. Jia, Y. Dai, reported at IWSMT-5, Charleston, USA, May 19-24, 2002, to be published in J. Nucl. Mater.
Progress report for the period of 15.05.2001-30.6.02: Modelling (M. Samaras, H. Van Swygenhoven)
Large scale molecular dynamics computer simulation of radiation effects on materials provide unprecedented information on the mechanisms and defect states of irradiated materials in the collisional time frame of a cascade. The work performed here focuses specifically on nanocrystalline metals and the effect of the introduction of a grain boundary near a cascade.
Investigation of nanocrystalline Ni
1. Samples of 1.2 million atoms of 5nm, 12nm grain sizes were irradiated at room temperature with 5-30keV primary knock-on atoms (PKAs) and the simulation was continued till the sample back to room temperature. Analysis of the sample during the simulation reveal that the grain boundaries act as sinks to interstitials (as seen in experiment by Rose et al. 1991, Chimi et al. 2001) which move via replacement collision sequences and via 1D/3D motion to annihilate with free volume within grain boundaries. The interstitials are attracted to grain boundary dislocations and triple junctions. The remaining grain structure is vacancy dominated and at 10keV larger truncated stacking fault tetrahedra (SFTs) of 1- 2nm form (c.f. 2nm SFTs seen in experimental irradiation of Ni by Zinkle and Snead 1995). At even larger PKA energies a complex stacking fault network made up of truncated SFTs forms. No significant change in the grain boundary network was observed .
2. Simulations are currently underway to investigate the size effect of the nanocrystalline structure with the irradiation of 5nm, 12nm and 20nm grain size samples with 5keV PKAs.
3. Cascade overlap was performed at 20keV on the 12nm grain size sample at 30ps time intervals. Pre-existing vacancy defects act as sinks, absorbing interstitials leaving an fcc lattice structure. A complex stacking fault network made up of truncated SFTs forms with local migration of the grain boundary, forming a kink inwards of the irradiated grain.
Investigation of single crystal fcc Ni
4. Samples of 1.2million atoms of single crystal Ni were irradiated with 5-30keV PKAs to make direct comparison with the nanocrystalline sample. The lack of sinks led to a vacancy dominated core region surrounded by interstitial clusters formed at the periphery of the cascade.
5. Cascade overlap was performed at 20keV at 30ps time intervals and it was found that a truncated SFT formed only at the end of the fourth PKA, in the vicinity of a pre-existing large vacancy cluster. The 1D/ 3D behaviour of the interstitial clusters is currently being studied.
Publications:
Computer simulation of displacement cascades in nanocrystalline Ni,
M. Samaras, P. M. Derlet, H. Van Swygenhoven, M. Victoria,
Physical Review Letters, 88, 125505 (2002).
Radiation damage near grain boundaries
Proceedings of Multiscale Materials Modeling, London, June 2002; To be published in Phil. Mag. A,
M. Samaras, P. M. Derlet, H. Van Swygenhoven, M. Victoria,
submitted.
SFT formation in the neighbourhood of grain boundaries
Proceedings of the 6th International Conference on Computer Simulation of Radiation Effects in Solids, COSIRES 2002, Dresden, June 2002; to be published in Nuclear Instruments and methods B
M. Samaras, P. M. Derlet, H. Van Swygenhoven, M. Victoria
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