Aging-controlled cryogenic TRIP kinetics in a Cu-added Fe-based medium-entropy alloy via Cu-rich precipitation and dislocation density reduction

The cryogenic mechanical properties of metastable Fe-based medium-entropy alloys (MEAs) are strongly governed by deformation-induced martensitic transformation. However, the interplay between matrix phase stability and defect-assisted transformation kinetics remains insufficiently understood. In this study, a Fe 64 Ni 15 Co 8 Mn 8 Cu 5 (at%) Fe-based MEA was cold-rolled and subsequently aged at 550 °C for 5 h and 72 h to investigate the effects of Cu-rich precipitation and dislocation density on transformation-induced plasticity (TRIP) behavior. Aging simultaneously induces Cu-rich precipitation and recovery of the cold-rolled dislocation structure, thereby modifying both the thermodynamic stability of the FCC matrix and the density of defects available as heterogeneous nucleation sites for martensitic transformation. The 5 h-aged sample shows the highest ultimate tensile strength of 1437 MPa due to accelerated TRIP kinetics, whereas prolonged aging for 72 h delays martensite transformation to higher strains and thereby improves ductility. Consequently, short-term aging provides an optimal balance between the thermodynamic driving force and kinetics of deformation-induced martensitic transformation. These results demonstrate that TRIP behavior in Cu-added Fe-based MEAs is governed by the competition between precipitation-induced matrix destabilization and recovery-induced reduction in martensite nucleation sites. Decoupling the thermodynamic driving force for martensitic transformation from the kinetic availability of nucleation sites therefore offers an effective strategy for optimizing cryogenic strength–ductility synergy in metastable Fe-based MEAs.