On the grain size effects of the spallation in Pb by quasi-coarse-grained molecular dynamics

Abstract

FCC-HCP phase transition plays a pivotal role in many intelligent materials, which also occurs in Pb under high pressures. However, its impacts on the spallation of polycrystalline, as well as the effects related to grain size, are still unclear. In this work, spallation behaviors of Pb polycrystals with different grain sizes under various shock loadings are investigated using the quasi-coarse-grained molecular dynamics (QCGD) method based on our recently developed response embedding atom model potential. The QCGD method is rigorously validated for applications in the metals exhibiting solid–solid phase transitions. Due to the restriction of the critical size for the phase transition nucleus, the coarsening level of the QCGD method cannot exceed two times the lattice parameter. Nevertheless, such a method enables us to explore the whole rule of the grain-size-dependence incipient spall strength. Our results suggest that the incipient spall strength exhibits a transition from the Hall–Petch to the inverse Hall–Petch relationship at about 13 nm and the spallation strength converging to that of a single crystal for grain sizes larger than 60 nm. As the grain size decreases, void nucleation becomes more prevalent than void growth, making the material better equipped to prevent the progression of damage into fractures. When the grain size is sufficiently large, voids nucleate and grow in the grain interior, making the spallation behave like in a single crystal. Interestingly, the phase transition from HCP to FCC phase enhances dislocation entanglement, leading to heterogeneous nucleation of voids in the grain interior.