The controlled incident dislocation boundaries and cell arrangements promoted multi-variant transformation and enhanced strain-hardening in a metastable ferrous medium entropy alloy

Abstract

In this work, we introduced and regulated incident dislocation boundaries (IDBs) to tailor cellular structures in a metastable ferrous Fe₅₀Mn₃₀Co₁₀Cr₁₀ medium-entropy alloy (MEA) through successive cold-warm rolling (CWR). This approach aimed to enhance yield strength (YS) without compromising ductility. Compared to one-step warm rolling (WR), the prior cold deformation introduced a higher density of mobile dislocations and intensified dislocation-dislocation interactions, promoting the formation of finer and more numerous dislocation cells. Both rolled samples exhibited higher YS while maintaining uniform elongation (UEL) levels comparable to those of the dislocation-free as-annealed reference. Notably, the CWR samples demonstrated simultaneous improvements in YS and strain hardening rate (SHR), and reduced mechanical anisotropy, particularly under liquid nitrogen temperature (LNT) deformation. The enhanced YS primarily stems from grain refinement via densely distributed dislocation cells, while the reduced mechanical anisotropy arises from a weakened {001}<111> texture due to dislocation-assisted recrystallization. Although IDBs initially decelerate phase transformation kinetics during early deformation, the refined cell structure in CWR samples facilitates multi-variant nucleation of nano-lamellar ε-laths and microbands, thereby generating dynamic Hall-Petch barriers for strain hardening. Additionally, the elevated flow stress promotes the proliferation of nano-lamellar ε-laths within microbands and enables reversible γ-domain formation at the shear intersection zones of multi-variant ε-laths. Consequently, the CWR-processed MEA achieves a high YS of ∼985 MPa and sustains an exceptional SHR of ∼3.5 GPa at LNT. This study establishes a "dislocation engineering" strategy to circumvent the traditional strength-ductility and YS-SHR trade-offs in metastable ferrous MEAs.