WEB Molecular dynamics study of shock waves in iron: On the role of plasticity
Shock compression is widely used to investigate the mechanical responses of iron under dynamic loading. With shock waves, it is possible to investigate new, previously undiscovered regimes of material dynamics for the discovery of materials inaccessible at ambient conditions. It has been long known that alpha-iron transforms to epsilon-iron under high pressure. Recently, molecular dynamics simulations have shown that plasticity occurs just before the parent phase transforms into hcp iron. To provide insights into the interplay of elastic and plastic activities during shocks in iron, we performed atomistic simulations of shock compression of nanocrystalline iron with a mean grain size of 20 nm comprising a total number of 267.5 million atoms. We observed elastic and plastic deformations before the phase transformation takes place. The plastic state is metastable and highly depends on the deformation rate. After a relaxation process of a few picoseconds, the structure transforms into a quasi-3D compressed state in which the new phase is stable. We found that with increasing ramp time of the piston, plastic relaxation is stronger.
Analysis of the X-ray diffraction patterns calculated from the atomistic structure using the Debye equation revealed pronounced anisotropy of the line broadening in hcp Fe that is due to stacking faults and dislocations.