## 4 Discussion
### Electrical Fatigue Failure Mechanisms of Wire  
Visual evidence (Figure 7b,c) confirms pronounced red luminescence and surface morphologies after fatigue exhibit current-dependent features: (1) at extremely high current levels (0.47-0.45 A), ridge-like oxide structures form due to localized melting and rapid oxidation; (2) at high currents (0.41-0.39 A), granular oxide layers dominate; and (3) at low currents (\(\leq\)0.37 A), the surface retains fine metal flow lines with oxidation. The narrow 0.35-0.41 A window thus marks the transition from comparably thermally stable operation to oxidation-driven degradation and should be the focus of targeted fatigue-failure studies. Electrical Characteristic, Microstructure Evolution, and Tensile Fracture Mechanism After Low-Current Electrical Fatigue  
Under 0.35 A cycling, the electrothermal input is sufficient to activate uniform dynamic recovery and grain coarsening, leading to partial softening, while oxidation remains limited and non-detrimental. The dominant mechanism at the 0.35 A condition is therefore interpreted as a benign annealing effect, which stabilizes both mechanical and electrical properties without inducing critical surface degradation. In contrast, the 0.37-C200 subsur  
Figure 21: 631 SS wire current-assisted tensile test under different current: (**a**) Strength-elongation curve; (**b**) YS and UTS; (**c**) TE; and (**d**) tensile toughness. face EPMA mapping identified fine carbides and Al-Si-rich phases, indicative of localized overheating [26]. Subsurface carbide concentration further suggests thermally driven carbon migration consistent with electrothermal diffusion [35]. Although the (110) peak's FWHM narrows by about 10 %, compared to 5 % for 0.35-C200, the dominant contribution to resistance increase is the thick, insulating Fe\({}_{2}\)O\({}_{3}\)-rich oxide scale (\(\sim\)3.8 \(\upmu\)m). Optical microscopy further reveals heterogeneous grain coarsening, indicating non-uniform Joule heating that enhances, but does not exceed, the impact of surface oxidation on electrical degradation. The Cr\({}_{2}\)O\({}_{3}\)-rich surface oxide layer on the 0.35A sample indicates mechanical compatibility with the underlying matrix. This behavior supports the interpretation that oxidation remains superficial and does not compromise fracture integrity at this current level. The observation of 0.37A sample is consistent with the intrinsic brittleness of Fe\({}_{2}\)O\({}_{3}\) and its tendency to act as a crack initiation site under tensile loading [36]. These findings support the conclusion that oxidation-induced surface embrittlement, rather than microstructural recovery, dominates the fracture evolution under elevated current fatigue. Electrical Characteristic, Microstructure Evolution, and Tensile Fracture Mechanism After High-Current Electrical Fatigue  
A modest increase in current from 0.37 A to 0.41 A shifts the predominant mechanism from recovery-driven stabilization to oxidation-facilitated degradation. At 0.37 A, uniform recovery surpasses early oxidation, resulting in a marginal decrease in resistance. Elevated currents induce non-uniform scale thickening and interfacial instability; at 0.41 A, an abrupt increase in resistivity and oxide-induced damage indicates the initiation of surface-driven failure under heightened Joule heating. Under 0.37A current loading, strain delocalization was observed, suggesting improved deformation uniformity and enhanced ductility. This behavior suggests that moderate Joule heating can promote dislocation mobility or partial recovery, thereby reducing strain localization. At 0.39 A, the wire response appears transitional; although ductility stayed favorable, signs of interfacial degradation appeared through localized surface oxidation. This intermediate regime likely represents a threshold where thermal softening begins to counteract mechanical strengthening.