Zhiyuan Liang

Discontinuous yielding and Lüders banding are critical plastic instability phenomena that govern the mechanical reliability of medium Mn steels (MMSs) under crash-relevant loading. Nevertheless, whether the severe strain localization observed under quasi-static conditions persists at high strain rates, and how it interacts with rate dependent constitutive behavior and microstructural evolution, remain unresolved. Here, the Lüders banding behavior of a ferrite–austenite duplex MMS was systematically investigated under quasi-static-strain-rate (QSR, 5×10-4 s-1) and high-strain-rate (HSR, 103 s-1) deformation using high-speed digital image correlation. Compared with classical Lüders banding at QSR, HSR deformation produces a diffuse Lüders band that nucleates near the specimen center, with both internal strain and spatial extent increasing simultaneously. Correspondingly, the engineering stress–strain curve exhibits a gradual yield drop followed by progressive stress recovery, rather than a sharp yield drop and extended stress plateau. Nanoindentation reveals that HSR deformation significantly increases both yield strength and intrinsic yield drop, whereas X-ray diffraction reveals suppressed martensitic transformation and dislocation multiplication, resulting in reduced strain hardening. Although the larger yield drop and weaker strain hardening should promote strain localization, HSR deformation produces a smaller strain gradient and weaker local strain rate amplification at the Lüders front. Finite element simulations show that this apparent contradiction originates from the enhanced strain rate sensitivity of flow stress associated with the phonon drag effect, which suppresses strain rate amplification at the Lüders front, forcing the applied deformation to be accommodated over a broader region. These findings provide new insight into discontinuous yielding under dynamic loading and establish a mechanistic framework for understanding Lüders band evolution in MMSs subjected to crash-relevant deformation rates.

The large-scale superconducting magnets used in the International Thermonuclear Experimental Reactor (ITER), demand structural materials that can endure long-term cyclic electromagnetic loading at 4.2 K. In this study, the microstructural evolution and plasticity mechanisms of a 316LN stainless steel welded joint subjected to fatigue at 4.2 K were systematically investigated for the first time. A combination of nanoindentation, electron backscatter diffraction, energy dispersive spectroscopy, electron channeling contrast imaging and transmission electron microscopy was employed for comprehensive characterization. The results reveal distinct fatigue mechanisms between the heat-affected zone (HAZ) and the fusion zone (FZ). The HAZ primarily accommodates deformation through wavy dislocation slip, while the FZ is dominated by planar slip. Notably, the FZ exhibits enhanced formation of stacking faults and nanotwins. These features are attributed to the elemental segregation in the interdendritic regions, which locally reduces the stacking fault energy and lowers the critical stress required for stacking fault formation via dislocation dissociation. Besides, a strong solidification-induced texture is formed in the FZ, which increases the local Schmid factor and thereby the resolved shear stress for promoting the nucleation of stacking faults and nanotwins. Additionally, Lomer-Cottrell locks are observed in the FZ as a result of dominant planar slip, contributing to dislocation multiplication. These findings offer important insights into the cryogenic fatigue behavior of austenitic stainless steel welds and contribute to the design of reliable structural components for fusion energy applications.