Effects of Shock-Induced Volume-reducing Phase Transitions on Spall Failure in Fe-Mn Alloys.

This work investigates the interactions of impact-generated shock waves on volume-reducing phase transitions and its influence on shock disruption and tension-induced spall failure. While pure Fe undergoes α-bcc to ε-hcp phase transformation at ~13 GPa resulting in volume reduction of ~0.453 cm³/mol, the threshold stress for the transformation decreases with Mn addition to 10.1 GPa for Fe-4Mn, 8.1 GPa for Fe-7Mn, and 5.8 GPa for Fe-11Mn [J. T. Lloyd, et al, Acta Materialia 234 (2022) 118042]. Plate-impact gas-gun experiments were performed on these alloys at various impact velocities generating pressures corresponding to thresholds for phase transitions in the respective alloys. High-fidelity multi-probe PDV interferometry was used to obtain free surface velocity profiles capturing signatures corresponding to phase transition and resulting differences in spall pullback signals (timing and magnitude) as a function of impact conditions related to the transformation pressure thresholds. Additionally, post-shock microstructural characterization using optical and electron microscopy were used to investigate transformation, deformation, and fracture characteristics in the soft recovered impacted samples. Results of the dissipative effects of phase transitions on shock wave propagation and subsequent spall failure will be presented.