Hydrostatic pressure effects on deformation mechanisms of nanocrystalline fcc metals

A series of large-scale molecular dynamics (MD) simulations have been performed to investigate hydrostatic pressure effects, and the interplay between pressure and grain size, on the flow stress and the related atomic-level deformation mechanisms in nanocrystalline (NC) Cu. The strength of NC Cu increases with increasing hydrostatic pressures for all grain sizes studies in the present paper (3-15 nm). The critical grain size for maximum strength first shifts towards lower values with increasing hydrostatic pressure (0-5 GPa), and then shifts towards higher values as the hydrostatic pressure becomes even higher (5-80 GPa). Below the critical hydrostatic pressure, the dislocation behaviors increase with increasing hydrostatic pressure for all grain sizes and the dependency of effective modulus as a function of hydrostatic pressure is almost the same for all grain sizes, which should lead to the position shifting of maximum strength towards lower grain sizes. Above the critical hydrostatic pressure, the dislocation behaviors start to decrease with increasing hydrostatic pressure for small grain sizes, and continue to increase with increasing hydrostatic pressure for large grain sizes. The slopes of effective modulus as a function of hydrostatic pressure increase slightly with increasing grain size above the critical hydrostatic pressure. The position shifting of maximum strength towards larger grain sizes at large hydrostatic pressure should be attributed to these two observations. Moreover, GB thickening is observed to increase monotonically with increasing pressure for all grain sizes, and the NC Cu with 3 nm grain size has the trend to become amorphous state under hydrostatic pressure of 80 GPa, which gives a new way to produce crystalline-to-amorphous transition. The findings in the present study should provide insights to the potential applications of NC metals under extreme environments.