Ultrasonic impact inducing surface magnetic neutrality and mechanical enhancement in medium-Mn steels

Non-magnetic steels are essential for applications requiring both high mechanical strength and strict magnetic neutrality, such as superconducting systems and nuclear engineering. However, conventional strengthening methods often deteriorate magnetic stability, leading to a persistent trade-off between strength and magnetism. Here, we show that ultrasonic impact treatment (UIT) effectively overcomes this limitation by inducing a gradient dislocation structure that enhances mechanical properties while triggering a near-complete body-centered cubic (BCC) to face-centered cubic (FCC) reverse martensitic transformation in medium-Mn steel, forming a fully austenitic surface layer that suppresses ferromagnetism. Micropillar compression tests show simultaneous increases of ∼138% in strength and ∼96% in plasticity compared with the untreated counterparts, attributed to the gradient dislocation structure. The gradient layer also exhibits nearly doubled wear resistance relative to commercial non-magnetic steels. Atomic-scale analysis further reveals a shear-mediated BCC-to-FCC reverse transformation pathway that challenges the classical Bogers-Burgers model, where the BCC lattice first distorts into an intermediate structure and then evolves into FCC through sequential shear. These findings establish UIT as a promising route to design non-magnetic steels with enhanced mechanical performance and magnetic stability.