Atomistic simulation of the elongation response of a <011> oriented columnar nano-grain bcc Fe polycrystalline sample

The tensile elongation of an < 011 > oriented columnar nanocrystalline pure iron structure at a temperature of 300 K has been simulated by molecular dynamics (MD). The simulated sample contains 4.3 x 10(6) atoms and has been subject to free elongation along the < 011 > axis common to the grains. Periodic boundary conditions have been assumed. The grains are randomly oriented around their common < 011 > and the size of their cross section is about 10 nm. The stress-strain curve has been calculated up to 0.5 true strain. After elastic deformation and heterogeneous dislocation nucleation from the grain boundaries, it shows a peak stress of 8 GPa followed by a remarkably stable steady state with a flow stress of 5.15 GPa, where neither the crystallographic texture nor the grain structure show any important change despite the large plastic deformation imparted. Upon a strain reversal, a pronounced Bauschinger effect is then observed (-3.3 GPa compressive yield stress), followed by a hardening transient until the absolute level of the flow stress in compression reaches near the same value it had in tension when the unloading took place. The results of the MD simulation are discussed by comparison with experimental values of the strength and structural evolution of heavily drawn iron wires available in the bibliography.