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

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 432; no. 1; pp. 191 - 201
• Main Authors: Rittel, D., Ravichandran, G., Venkert, A.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 25-09-2006; Elsevier
• Subjects: Condensed matter: structure, mechanical and thermal properties; Deformation and plasticity (including yield, ductility, and superplasticity); Dominant shear; Dynamic recrystallization; Exact sciences and technology; High strain rate; Iron; Martensitic phase transformation; Mechanical and acoustical properties of condensed matter; Mechanical properties of solids; Physics; Dominant shear; Iron; Dynamic recrystallization; Martensitic phase transformation; High strain rate; Annealing; Shear; Phase transformations; Strain softening; Twinning; Dynamical recrystallization; Dislocations; Transmission electron microscopy; BCC lattices; HCP lattices; Microstructure; Strain rate
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2006.05.154

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.