The mechanical response of pure iron at high strain rates under dominant shear

The mechanical behavior and microstructure of pure iron subjected to dominant shear loading has been characterized over a wide range of strain rates. Pure iron is found to be highly strain-rate sensitive. Iron exhibits marked strain softening at ε˙≈8000 s−1−σ≈850 MPa that is unexpected for the annealed material, as characterized by TEM, but is identical to that of iron preshocked at 40 GPa [G.M. Weston, J., Mater. Sc. Lett. 11 (1992) 1361]. The microstructure is found to undergo significant refinement with increasing strain rate, from large initial grains (50 μm), through dislocation cells and large twinning, and finally micro-twins and dynamically recrystallized 200 nm grains at the higher strain rates. In situ temperature measurements indicate the release of an external heat source, other that the thermomechanical conversion of plastic work, which is identified as dynamic recrystallization. The present results suggest the operation of the α (BCC) ⇔ ɛ (HCP) phase transition that is known to occur during hydrostatic or shock loading at 13 GPa. The combination of the high strain-rate sensitivity and dominant shear loading conditions seem to trigger this phase transition, thus supporting recent work [K.J. Caspersen, A. Lew, M. Ortiz, M., E.A. Carter, Phys. Rev. Lett. 10 (2004) 115501] emphasizing the role of shear.