Unveiling grain size effect on shock-induced plasticity and its underlying mechanisms in nano-polycrystalline Ta
D Wu and KG Chen and YX Zhu and L Zhao and MS Huang and ZH Li, MECHANICS OF MATERIALS, 160, 103952 (2021).
To study macroscopic mechanical response and microscopic deformation mechanism of nano-polycrystalline (NPC) Ta under shock compression, massive-scale non-equilibrium molecular dynamics (NEMD) simulations were systematically performed. The effects of grain size, shock strength on the wave structure, plasticity mechanism and flow stress were unveiled. Under the weak shock compression, as the grain size increases, a transition of plastic wave structure from single to double and then to single one was found, driven by the transition of dominant deformation mechanism from grain boundary (GB)-mediated plasticity to its coexistence with twinning and slipping and then to twinning and slipping, respectively. A transition from twinning-to slipping-preferred plastic deformation as the grain size exceeds similar to 30 nm was captured in the simulations and explained by a simple theoretical model proposed in this work. Under the strong and ultra-strong shock compressions, twinning-detwinning and amorphization-recrystallization were revealed as the dominant deformation mechanisms, respectively, which show weak grain size dependences. The flow stresses at the Hugoniot state were calculated, which follow the Hall-Petch relation under the weak and strong shocks but show complexity under the ultra- strong shock. These results can help understand intrinsic grain size and extrinsic shock strength effects on the mechanical and microstructural responses of Ta, as a key structural and typical BCC metal.
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