Influence of temperature, stress, and grain size on behavior of nano-polycrystalline niobium

Abstract

Atomic simulations were executed to investigate the creep responses of nano-polycrystalline (NC) niobium established by employing the Voronoi algorithm. The effects of varying temperature, applied stress, and grain size (GS) on creep properties and mechanisms were investigated. Notably, the occurrence of tertiary creep was exclusively observed under conditions where the applied stress exceeded 4.5 GPa and the temperature surpassed 1100 K. This phenomenon can be attributed to the significant acceleration of grain boundary and lattice diffusion, driven by the elevated temperature and stress levels. It was found that the strain rate increases with an increase in both temperature and stress. However, an interesting trend was observed where the strain rate decreases as the grain size increases. The stress and temperature are crucial parameters governing the creep behavior. As these factors intensify, the creep mechanism undergoes sequential transitions: initially from lattice diffusion under low stress and temperature conditions, subsequently evolving to a mixed mode incorporating both grain boundaries (GBs) and lattice diffusion at moderate stress and temperature levels, and ultimately culminating in a disruption of the power-law controlled creep behavior, inclusive of grain boundary recrystallization for high stress and temperature conditions. This comprehensive analysis provides a deeper understanding of the intricate creep behavior of nano-polycrystalline niobium and its dependence on various physical parameters.