Bidirectional transformation enabled improvement in strength and ductility of metastable Fe50Mn30Co10Cr10 complex concentrated alloy under dynamic deformation

High strain rate compression experiments performed on a non-equiatomic metastable fcc + hcp Fe50Mn30Co10Cr10 high entropy alloy using split Hopkinson pressure bar setup shows improved flow stress and compression ductility compared to quasistatic deformed samples. The deformation response was characterised by the occurrence of hardening and softening stages compared to sustained strain hardening for quasistatic deformation. Detailed EBSD, BSE imaging analysis coupled with TEM shows significant bi-directional transformation (B-TRIP) and increased fcc gamma phase stability in high strain rate regime while only forward (fcc to hcp) transformation dominates with increasing fraction of hcp epsilon phase in the quasistatic regime of deformation. Bidirectional transformation aided by adiabatic heating and heterogeneous deformation in the high strain rate regime leads to optimal stress and strain partitioning between the two phases and delays the initiation of damage at the interface. The presence of concomitant strain rate hardening and improvement in ductility in the dynamic deformation regime opens up avenues for microstructural tunability to achieve simultaneous improvement in strength and ductility using the metastability paradigm in complex concentrated alloys.