Browsing by Subject "NEUTRON-IRRADIATION"

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  • Lehtinen, Arttu; Laurson, Lasse; Granberg, Fredric; Nordlund, Kai; Alava, Mikko J. (2018)
    Plastic deformation of crystalline materials is governed by the features of stress-driven motion of dislocations. In the case of irradiated steels subject to applied stresses, small dislocation loops as well as precipitates are known to interfere with the dislocation motion, leading to an increased yield stress as compared to pure crystals. We study the combined effect of precipitates and interstitial glissile 1/2 <111 > 2 dislocation loops on the yield stress of iron, using large-scale three-dimensional discrete dislocation dynamics simulations. Precipitates are included in the simulations using our recent multi-scale implementation [A. Lehtinen et al., Phys. Rev. E 93 ( 2016) 013309], where the strengths and pinning mechanisms of the precipitates are determined from molecular dynamics simulations. In the simulations we observe dislocations overcoming precipitates with an atypical Orowan mechanism which results from pencil-glide of screw segments in iron. Even if the interaction mechanisms with dislocations are quite different, our results suggest that in relative terms, precipitates and loops of similar sizes contribute equally to the yield stress in multi-slip conditions.
  • Fellman, A.; Sand, A. E.; Byggmästar, Jesper; Nordlund, Kai (2019)
    We have performed a systematic molecular dynamics investigation of the effects of overlap of collision cascades in tungsten with pre-existing vacancy-type defects. In particular, we focus on the implications for fusion neutron irradiated tungsten in relation to comparisons with damage production under ion irradiation conditions. We find that overlap of a cascade with a vacancy-type defect decreases the number of new defects with roughly the same functional dependence as previously shown for interstitial clusters. We further find that different mechanisms govern the formation of dislocation loops, resulting in different Burgers vectors, depending on the degree of overlap between the cascade and the defect. Furthermore, we show that overlapping cascades consistently decrease the size of the pre-existing defect. We also observe void-induced cascade splitting at energies far below the subcascade splitting threshold in tungsten. The impact of these mechanisms on radiation damage accumulation and dose rate effects are discussed.
  • Castin, N.; Dubinko, A.; Bonny, G.; Bakaev, A.; Likonen, J.; De Backer, A.; Sand, A.E.; Heinola, K.; Terentyev, D. (2019)
    The microstructure changes taking place in W under irradiation are governed by many factors, amongst which C impurities and their interactions with self-interstitial atoms (SIA). In this work, we specifically study this effect by conducting a dedicated 2-MeV self-ions irradiation experiment, at room temperature. Samples were irradiated up to 0.02, 0.15 and 1.2 dpa. Transmission electron microscopy (TEM) expectedly revealed a large density of SIA loops at all these doses. Surprisingly, however, the loop number density increased in a non-monotonous manner with the received dose. Performing chemical analysis with secondary ion spectroscopy measurements (SIMS), we find that our samples were likely contaminated by C injection during the irradiation. Employing an object kinetic Monte Carlo (OKMC) model for microstructure evolution, we demonstrate that the C injection is the likely factor explaining the evolution of loops number density. Our findings highlight the importance of the well-known issue of C injection during ion irradiation experiments, and demonstrate how OKMC models can help to rationalize this effect.