Nickel-induced transition of deformation mechanisms in high-Mn lightweight steels: Insights into dislocation behavior and planar fault evolution

Pursuing lightweight, high-strength alloys for sustainable automotive and aerospace applications has catalyzed significant interest in high-Mn steels, where alloying elements such as nickel (Ni) play a pivotal role in tailoring deformation mechanisms to optimize mechanical performance. In this study, we explore the influence of Ni content (0, 2.5, and 5 at.%) on the deformation mechanisms of high-Mn lightweight steels using a combination of X-ray line profile analysis (XLPA) and transmission electron microscopy (TEM). Despite a constant stacking fault energy (SFE) of approximately 90 mJ/m² across all compositions, our findings reveal a marked increase in dislocation density (2.75 to 9.56 × 10¹⁵ m⁻²) and stacking fault probability (3.3 to 6.3 × 10⁻⁴) with higher Ni concentrations, leading to a notable transition in deformation mechanism. From slip refinement-induced plasticity (SRIP) to microband-induced plasticity (MBIP) and ultimately to planar fault-mediated mechanisms, the role of Ni is demonstrated to enhance dislocation interactions and promote the nucleation of B2 precipitates, which significantly influence plasticity. At 5Ni, synergy among deformation twins, B2 precipitates, and correlated dislocations yields exceptional strength-ductility synergy (1126 MPa UTS, 49 % elongation). This study establishes a direct correlation between Ni content and deformation mechanism, offering a transformative perspective on the design of high-SFE lightweight alloys with tunable deformation mechanism, beyond conventional SFE-driven paradigms.