Formation mechanism of a plateau stress region during dynamic compression of porous iron: Interaction between oriented cylindrical pores and deformation twins

Dynamic and quasi-static compressions of porous copper and iron with cylindrical unidirectional pores were investigated at 298 and 77 K, with a focus on the effect of the interaction between the cylindrical pores and the deformation twins on the appearance of a plateau stress region in the stress-strain curves where the deformation proceeds at almost constant stress. The dynamic and quasi-static compressions were examined using the split Hopkinson pressure bar method and a universal testing machine, respectively. When porous copper undergoes dynamic (298 and 77 K) and quasi-static (298 K) compressions parallel to its cylindrical pores, the flow stresses increase monotonically with an increase in the strain. This is due to a homogeneous slip deformation originating from the high mobilities of the dislocations. However, when porous iron undergoes dynamic compression parallel to its pores at 298 K, macroscopically localized deformation bands analogous to bucking deformation occur, owing to the interaction between the oriented cylindrical pores and the deformation twins. This interaction acts as a trigger for the deformation band formation. As a result, the work hardening rate decreases drastically. Thus, in the case of dynamic parallel compression of porous iron at 298 K, a plateau stress region with a high stress (~300 MPa) and a wide strain range (of up to ~45%) appears. Because of this, the absorption of high-impact energy six times that can be absorbed by commercial aluminum foam with isotropic pores is achieved.