Volume 7, Issue 2 (February 2010)
Kinetic Monte Carlo Simulation of Helium-Bubble Evolution in ODS Steels
Oxide dispersion strengthened (ODS) ferritic/martensitic steels are being developed for high temperature applications for fission reactors and future fusion devices. ODS-Eurofer97 (Fe–9CrWVTa–0.3Y2O3) and ODS-MA957 (Fe–14CrTiMo–0.25Y2O3) have shown promising high temperature mechanical properties, such as tensile strength, toughness, fatigue, and creep rupture. Recent neutron irradiation experiments with simultaneous helium implantation indicate that helium transport is favorably impacted by the nanometer-sized oxide particles, small grain sizes, and high dislocation densities of ODS steels. Simulating helium transport in ODS steels requires a three-dimensional spatially resolved model, which takes into account discrete geometric and microstructural features of the steel. We have developed such a helium transport simulation model using an event kinetic Monte Carlo (EKMC) approach called Monte Carlo simulation of helium-bubble evolution and resolutions (McHEROS). First, a spatially resolved kinetic rate theory is used to establish helium-vacancy cluster and stable helium-bubble nuclei concentrations. The maximum helium-bubble density is then used as an initial condition for randomly distributed matrix bubbles for the EKMC simulation. Migration, coalescence, and trapping of helium bubbles by oxide particles are simulated. Matrix helium bubbles that come into contact with each other are assumed to undergo instantaneous coalescence, which leads to bubble growth. However, migrating bubbles that are intercepted by oxide particles are assumed trapped but can grow through coalescing with newly arriving bubbles. The oxide particles effectively reduce the growth rate of matrix bubbles. Helium-bubble size and spatial distributions of the EKMC simulation are compared with recent experimental measurements. As part of this study, the effectiveness of the ODS microstructure on reducing helium-bubble growth rates is presented by comparing EKMC simulations of steels with and without ODS particles. This first application of the McHEROS code has demonstrated the viability of the code as a tool in describing the behavior of helium-bubble transport in ODS alloys.