Atomic-scale Simulation Study of Creep Behavior in Nanocrystalline Mo

Molybdenum (Mo) alloys are widely employed in high-temperature environments, such as advanced nuclear systems, due to their excellent properties, including high melting point and thermal conductivity. However, Mo alloys suffer from poor ductility at room and intermediate temperatures, which can be improved by adding rhenium (Re) or dispersing second-phase particles. These particles refine the grain size by promoting nucleation and inhibiting grain growth, improving strength while reducing the concentration of harmful solutes. Nevertheless, creep behavior is a critical performance aspect for Mo-Re alloys during service, as it can limit their high-temperature applications. Creep refers to the time-dependent plastic deformation that occurs at high-temperatures under stress levels below the yield strength of a material. In polycrystalline Mo alloys, grain boundary sliding exacerbates creep at elevated temperatures, restricting their use. Previous studies on Mo creep behavior have indicated that subgrain formation and grain growth occur during high-temperature service, and nanocrystalline materials exhibit distinct creep mechanisms compared to polycrystalline counterparts. Recent findings suggest that nanocrystalline materials can experience significant creep even at lower temperatures, indicating the importance of investigating the effects of high grain boundary density on the creep behavior of Mo alloys. Given that experimental creep tests require long durations, molecular dynamics (MD) simulation offers an efficient alternative for studying atomic-scale processes at grain boundaries. In this study, MD simulation was employed to investigate the tensile creep behavior of nanocrystalline Mo with varying grain sizes under different temperature and stress conditions. The Voronoi method was used to generate nanocrystalline Mo structures with random grain orientations, and tensile creep simulations were conducted using the LAMMPS software. The results reveal that increasing temperature and applied stress accelerates the creep process, with smaller grain sizes exhibiting more pronounced creep behavior. Atomic-level visualization shows that dislocation density and grain structures remain largely unchanged, while local atomic environments change due to vacancy diffusion along grain boundaries. These changes are responsible for the observed deformation mechanisms, particularly Coble creep, which dominates under the simulated conditions. This study provides valuable insights into the mechanisms of creep in nanocrystalline Mo at 800-1 400 K, which is critical for its potential application in nuclear industry designs. The findings highlight the importance of understanding grain boundary diffusion and its role in controlling the overall creep behavior in nanocrystalline materials.