Enhanced strength with retained ductility in SLM-processed high-entropy alloys via dislocation regulation in L2₁-BCC co-precipitate

Abstract

L2₁-BCC co-precipitates were formed in a Fe-Co-Ni-Cr-Al-Ti high-entropy alloy fabricated via selective laser melting, followed by a specific heat treatment process. Two types of co-precipitates were identified based on the scale of the BCC phase, with both exhibiting fully coherent interfaces. For the first time, the dynamic interaction mechanism between co-precipitates and dislocations was revealed through in-situ transmission electron microscope. First, the cross-slip of dislocations occurred, promoting uniform deformation within the co-precipitates. Additionally, back stress exerted by the BCC phase facilitated the activation of slip systems in the L2₁ phase. The dislocation interaction with the L2₁ phase shifted from conventional bypassing to cutting, mitigating stress concentration at the FCC/L2₁ phase boundary. Consequently, a high-density dislocation zone formed in the L2₁ phase near the interface, which not only prevented dislocation pile-up but also promoted cross-slip. Finally, a cracking prevention mechanism associated with the gradient precipitation phase was identified. The co-precipitate structure achieved a remarkable 73.6 % enhancement (813 MPa to 1411 MPa) in ultimate tensile strength compared to the single L2₁ precipitate system while maintaining considerable ductility. These findings provide a foundation for the development of multiphase structural designs in high-entropy alloys and a breakthrough in the strength-ductility trade-off.