Phase transformation of iron under shock compression: Effects of voids and shear stress

Abstract

Unlike the perfect single crystals, most engineering materials contain a large number of defects that would affect their properties critically. By introducing a nanovoid into the single-crystal iron, the shock-induced phase transformation of body-centered cubic $\ensuremath{\alpha}$ phase to hexagonal close-packed $\ensuremath{\epsilon}$ phase has been investigated by means of molecular-dynamics (MD) simulation. Results have revealed that the threshold pressure and the nucleation velocity of $\ensuremath{\alpha}\ensuremath{\rightarrow}\ensuremath{\epsilon}$ phase transformation are significantly different from that of the perfect single-crystal iron. The nanovoid has reduced the threshold pressure and accelerated the nucleation speed, which induces the new phase to generate and grow much easier. The effects of the void size on the phase transformation are also illustrated. Furthermore, the nanovoid also has affected the phase transformation district. In the case of perfect single-crystal iron, the homogeneous transformation is observed beginning from the leading front of the shock wave, but in the defective crystal iron it first occurs around the edge of the nanovoid, and then forms a butterfly-shaped phase transformation zone due to the slippage of the atoms in the (011) and $(01\ensuremath{-}1)$ planes along the $[01\ensuremath{-}1]$ and [011] directions on the surface of the void. The effect of shear stress on the phase transformation of iron has been analyzed in detail as well. By calculating the distribution of the resolved shear stress along the slip plane of the shocked crystal iron, the relationship between the shear stress and the slip plane has been discussed.