Role of α→ε→α phase transformation on the spall behavior of iron at atomic scales

Shock compression of iron microstructures above a threshold stress results in a α BCC → ε HCP transformation, and the propagation of the release wave brings the metal back to the α phase following the ε → α transformation. Predicting failure behavior under shock loading conditions (spallation) relies on understanding the evolution of defects in the microstructure as it undergoes the α → ε → α phase transformation. This study uses molecular dynamics (MD) simulations to investigate the role of defect evolution during the α → ε → α phase transformation on the spall strength values of single-crystal (sc) Fe microstructures. The MD simulations aim to characterize the ε phase fraction formed during shock compression and the defects during shock release for variations in loading orientations and shock stresses. The simulations are carried out for loading along the [100], [110], [111], and [112] orientations and for impact velocities ranging from 600 m/s to 1 km/s. The ε phase fractions during compression and defects (dislocations, twins) characterized during spall failure show an orientation dependence that affects the spall strength values. The lowest value for spall strength is observed for the 110 loading orientation that shows a high density of twins at the spall plane, whereas the highest value is observed for the 100 orientation and is associated with a α BCC → γ FCC transformation at the spall plane. The correlations of the spall strength values with the strain rates and with the ε phase fractions are discussed.