Effects of grain size and shape on mechanical properties of nanocrystalline copper investigated by molecular dynamics

Molecular dynamics (MD) simulation has been used to study effects of grain size and shape on mechanical properties of nanocrystalline copper with mean grain size varying from 2.6 to 53.1 nm. Three grain size regions are identified according to the plot of flow stress versus mean grain size d. In region I (d≈20-53 nm) flow stress increases with the decrease of d. Deformation twinning process and extended dislocation are observed in this region. In region II (d≈8-20 nm) detwinning process appears as a competitive deformation mechanism with the twinning process. The flow stress begins to decrease slightly in this region. In region III (d<8 nm) plastic deformation mainly occurs in grain boundaries and grain rotation is observed to accommodate the stress buildup induced by the grain boundary sliding. Deformation twin formed by sequent emission of partial dislocations at nearby sliding planes is not observed in this region. Young׳s modulus has a linear relation with d−1 in the simulated range of mean grain size, and it directly depends on the volume fraction of grain boundaries. The Young׳s modulus of the grain boundary component is found to be ~25% of that of the grain interior. MD simulations on samples with spherical and cylindrical grain shapes are also carried out. The influence of grain shape on flow stress is hardly observed, indicating that for different grain shapes the plastic deformation mechanism is the same. The grain shape has an obvious effect on Young׳s modulus, which is attributed to the difference of the volume fraction of grain boundaries for samples with the different grain shapes.