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Helium-induced defect nucleation and accumulation in polycrystalline W and W0.5 wt%ZrC (W0.5ZrC) were studied in-situ using the transmission electron microscopy (TEM) combined with 40 keV He+irradiation at 800 and 1000°С at the maximum damage level of 1 dpa. Radiation-induced dislocation loops were not observed in the current study. W0.5ZrC was found to be less susceptible to irradiation damage in terms of helium bubble formation and growth, especially at lower temperature (800 °C) when vacancies were less mobile. The ZrC particles present in the W matrix pin the forming helium bubbles via interaction between C atom and neighbouring W atom at vacancies. This reduces the capability of helium to trap a vacancy which is required to form the bubble core and, as a consequence, delays, the bubble nucleation.

AbstractIron carbide (Fe3C), also known as cementite, is present in many steels and has also been seen as nanosized precipitates in steels. We examine the interaction of edge dislocations with nanosized cementite precipitates in Fe by molecular dynamics. The simulations are carried out with a Tersoff-like bond order interatomic potential by Henriksson et al. for Fe-C-Cr systems. Comparing the results obtained with this potential for a defect free Fe system with results from previously used potentials, we find that the potential by Henriksson et al. gives significantly higher values for the critical stress, at least at low temperatures. The explanation was found to be the difference in the core structure of the edge dislocation. The results show that edge dislocations can unpin from cementite precipitates of sizes 1 nm and 2 nm even at a temperature of 1 K, although the stresses needed for this are high. On the other hand, a 4 nm precipitate is not sheared by edge dislocations at low temperatures (≤100 K) on our simulation timescale.

Interaction of tungsten surface with plasma beam has tremendous importance for development of materials for fusion applications. Trapping and release of Helium (He) and Deuterium (D) in tungsten exposed to high flux plasma with varying He and D content are studied here. Recrystallized and plastically deformed tungsten samples were used to clarify the impact of the material microstructure on trapping and release of plasma components. Thermal desorption spectroscopy measurements were performed to reveal the release stages and quantify the integral desorption of both types of molecules. Comparison of reference and plastically deformed samples was used to clarify the impact of plastic deformation (expressed in the high dislocation density and dislocation networks) on the trapping and release processes. It has been demonstrated that in the mixed beam exposures, the integral He release weakly depends on the He/D ratio, but suddenly grows as the fraction of He/D increases from 80/20 to 100/0. The integral release of D is remarkably enhanced by He seeding in both reference and plastically deformed samples. It is concluded that under the mixed beam exposure conditions, the D release is primary controlled by the He seeding and not by the deformation-induced microstructure.

The structure of copper nanoclusters/precipitates formed under neutron irradiation in Fe-0.3 wt. % Cu-C alloy is studied by the internal friction and positron annihilation experiments of postirradiation annealed alloys. The appearance of a carbon-relaxation peak during the first recovery stage at about 723 K reveals the fact that complex carbon-vacancy-copper clusters have formed during the irradiation-mediated copper precipitation process. The stability of Cu-C-vacancy clusters is confirmed by ab initio calculations. The existence of a structural phase transition of copper precipitates is observed in the alloys that are annealed at temperatures between the first and second recovery stages, which indicates that the dissociation of vacancies and carbon atoms from the vacancy-copper-carbon clusters is accompanied by the crystallization of Cu precipitates. [ABSTRACT FROM AUTHOR]