## 1 Introduction  
Probe springs provide compliant electrical contact in automated test platforms, lab-on-a-chip devices, and semiconductor packaging systems, demanding reliable performance under combined thermal and mechanical stress [1, 2]. As testing complexity increases, operational reliability becomes crucial to ensure signal integrity, minimize test errors, and reduce downtime and failures associated with thermal softening or contact resistance instability, which can directly impair production yield and equipment efficiency [3].  
Conventional probe springs, typically fabricated from gold-coated alloy (50 \(\upmu\)m diameter), exhibit limited wear resistance and mechanical stability under repeated contact [4, 5]. More critically, when subjected to high-frequency currents (current densities \(>\) 10\({}^{4}\) A/cm\({}^{2}\)), these wires undergo severe electrothermal degradation. This degradation is governed by four primary mechanisms [6, 7, 8]: (1) Joule heating, inducing local temperature increase and thermal fatigue; (2) residual stress relaxation, reducing elastic recovery; (3) electromigration, accumulating and voiding atomic vacancy; and (4) surface oxidation, impairing electrical conduction and introducing interfacial instability. These processes collectively undermine long-term reliability in high-cycle testing environments. Impulse-loading studies on MEMS metallic films further demonstrate that oxidation-assisted cracking can dominate failure even at submillisecond timescales, highlighting the need for oxidation-resistant conductors [9].  
Precipitation-hardened duplex stainless steels, particularly AISI 631 (17-7 PH), offer a promising pathway for enhanced spring probe performance. The 631 SS features thermally stable nanoscale B2-NiAl and Ni\({}_{3}\)Al precipitates, which provide high yield strength (~1.1 GPa), wear, corrosion, fatigue resistance, and oxidation tolerance at elevated temperatures [10; 11; 12; 13]. These precipitates act as barriers to dislocation motion, helping to suppress grain coarsening. While bulk properties of 631 SS are well documented, the electro-mechanical reliability of fine wires (\(\varnothing\approx 50~{}\upmu\)m) under electrical fatigue remains poorly understood. Early batch-mode, microelectro-discharge machining experiments revealed that microscale conductors suffer rapid surface degradation and conductivity loss when exposed to repetitive, high-density current pulses, yet a systematic link to duplex stainless steel wires is lacking [14]. Notably, the impact of such conditions on microstructure, oxide evolution, and failure behavior is insufficiently addressed in the current literature [15; 16; 17; 18].  
This study presents a systematic investigation of degradation mechanisms in cold-drawn 631 stainless steel fine wire under cyclic electrical loading, with emphasis on thermal fatigue cracking, electrothermal softening, and surface oxidation. Through combined microstructural and property-based analyses, this work aims to establish performance limits and design criteria for high-reliability, high-cycle probe spring applications. The findings provide both mechanistic insight and practical guidance for next-generation ATE and semiconductor test systems.