Classical thermodynamic approach to void nucleation in irradiated materials
Abstract One of the major problems in building a future fusion power station is the development of suitable structural materials. These materials will be exposed to high energy neutron bombardment, with consequent changes in their mechanical properties – embrittlement, hardening and swelling, for example. A missing link in modelling these effects is an effective treatment of the nucleation of voids under irradiation. These voids are initially stabilised by transmutation helium but, once formed, grow by vacancy accumulation. In this paper, a quasi-chemical model is developed to calculate the entropy of a steel/helium system. Although a substantial contribution from quasi-chemical effects might be expected, the steady state concentrations of dissolved helium are found to be too low for such effects to manifest. The steady state concentration of dissolved helium is low in absolute terms, but the resulting supersaturation is very high, making it reasonable to assume that all available nucleation sites are instantaneously activated.
Grain size effects on He bubbles distribution and evolution
Abstract Grain boundary and grain size effects on He bubble distribution and evolution were investigated by He implantation into nanometer-size-grained T91 obtained by Surface Mechanical Attrition Treatment (SMAT) and the conventional coarse-grained T91. It was found that bubbles in the nanometer-size-grained T91 were smaller than those in the conventional coarse-grained T91 in as-implanted case, and bubbles in the matrix of nanograins were undetectable while those at nanograin boundaries (GBs) coalesced and filled in GBs after heat treatment. These results suggested that the grain size of structural material should be larger than the mean free path of bubble’s Brownian motion and/or denuded zone around GBs in order to prevent bubbles accumulation at GBs, and multiple instead of one type of defects should be introduced into structural materials to effectively reduce the susceptibility of materials to He embrittlement and improve the irradiation tolerance of structural materials.
Helium implantation of an Fe-Ni-Mo-B amorphous alloy: I. Experimental data on bubble growth and blistering during 5 keV He+ implantation in the temperature range 200–600 K
Abstract The formation and growth of helium bubbles and blisters under 5 keV He + implantation has been studied for the amorphous alloy Metglas 2826 MB in the implantation temperature range 200–600 K and dose range 5 × 10 20 He + / m 2 −10 22 He + / m 2 . For all implantation temperatures bubbles are observed in the above-mentioned dose range and they grow with increasing dose. At a dose lower than the critical dose for blistering (~ 8 × 10 21 He + / m 2 ) a second population of bubbles starts to grow, resulting in a class of larger bubbles. There is good evidence that these larger bubbles are the intermediate stages between the smaller-sized bubbles and the final blisters. It is suggested that the mechanism for blister formation is different for implantations below room temperature from that for implantations at or above room temperature. This suggestion is based on the variation of mechanical properties of the amorphous alloy with temperature.