Yield stress, polymorphic transformation, and spall fracture of shock-loaded iron in various structural states and at various temperatures

The response of polycrystalline 99.5% pure iron was studied in a series of planar impact experiments, with samples of different thickness having an initial temperature that ranged between 300 and 1233K. The free surface velocity histories of the shocked samples were recorded in the course of the experiments. Almost all recorded histories are characterized by a three-wave structure containing an elastic precursor P e l and two plastic, P 1 and P 2, waves. It was found that at 300, 900, and 1039K (some 5K away from iron's Curie point), the decay of the P e l wave with propagation distance is characterized by two different regimes; a fast one that corresponds to plastic strain rates above ∼10⁵s^-1 and a slower one at lower strain rates. Since the shear stress at which the change-over takes place is very close to the Peierls stress of iron, we assume that above this stress the decay is governed by the phonon-damped over-barrier dislocation motion, while below it, the thermally activated generation and motion of the dislocation kinks is the governing mechanism. Based on the parameters of the P 1 and P 2 waves, both the initial and the maximum rates of the α → ε transformation in iron have been estimated. The results indicate that the approach to the Curie point is associated with a substantial, by 2-3 times, increase of the transformation rate.