Strain Rate Sensitivities Of FCC Metals Using Molecular Dynamics Simulation

Based on the theory of dislocation and the theory of molecular dynamics simulation,this study focuses on the mechanism of the strain rate effect of crystalline materials via dislocation dynamics simulation and molecular dynamics simulation.Strain rate effects are referred to as the variation in mechanical properties(flow stress,strength,etc.) of a material with strain(or stress) rate.An increase in flow stress with strain rate(or strain rate dependence) is a typical characteristic of the strain rate sensitivity of a material.Although many attempts have been made to study these phenomena,the actual physical cause of the strain rate effect or the strain rate sensitivity of crystalline material is still not clear.In this study,the strain rate effect of copper has been simulated by running the dislocation dynamics code Micro3d.The simulation result reveals that the number of dislocation sources activated increases with a strain rate.Through analyzing this finding with dislocation theory,a way to bridge a stress and a strain rate has proposed, namely,building their relationship by using the length of certain dislocation source.Based on the above work,Orowan' relationship has been considered as the bridge connecting strain rate effect with dislocation motion and the stress dependence of dislocation velocity has been identified as the dynamics relationship of dislocation motion.Consequently,it has been proposed that the dynamics relationship determines flow stress increasing with a strain rate.A qualitative interpretation of strain rate effect has been obtained through these two relationships although the kinematics relationship of dislocation motion is absent due to complicated styles of dislocation motion.Based on the qualitative interpretation,the effect of transition segment of a stress-strain curve has been emphasized;phenomenon relating to strain rate effect have been adequately discussed,such as the stress-strain curve of a given strain rate, the increase of strain rate sensitivity of a metal with temperature,the decomposition of strain rate effect and strain hardening and multi-step loading tests.The analyses agree with the existing experimental results and update the knowledge on strain rate effect.Through the analysis based dislocation theory and MD simulations,the connection between atom properties and macroscopic strain rate sensitivity of metals has been established by using Orowan's relationship as the bridge. The influences of interatomic potential and atomic mass on the stress dependence of dislocation velocity of FCC metals and strain rate sensitivity have been examined through MD simulations of a moderate scale.It is concluded that the stress dependence of dislocation velocity of FCC metals and strain rate sensitivity mainly depends on its atom mass of a metal rather than its interatomic potential.Strain rate sensitivities of five FCC metals have been ordered based on the results of MD simulation.If their initial dislocation configuration is the same,the metal of larger atom mass should be more strain rate sensitive,namely,the order of their strain rate sensitivity should be consistent with the order of their atom masses. This conclusion about strain rate sensitivity of the five FCC metals is coincident with empirical knowledge.