Evolution of nanoscale defects in Q235 power fitting steel during tensile deformation characterized by positron annihilation lifetime spectroscopy

Power fitting failure is one of the major causes of power grid accidents. Conventional non-destructive testing methods, including ultrasonic testing and X-ray radiography, can only detect defects at the micrometer scale, leaving a very short warning window before catastrophic failure. Since all micrometer-scale defects originate from nanoscale defects, identifying nanoscale defects can significantly extend the early warning timeframe. Positron annihilation lifetime spectroscopy (PALS) is a sensitive technique for detecting nanoscale defects in metallic materials. In this study, Q235 steel, the most commonly used material for power fittings, was subjected to tensile deformation at different elongation levels (0, 1, 1.5, 2.0, and 3.5 mm). The evolution of nanoscale defects was investigated using PALS. The stress-strain curve showed that the Q235 sample fractured at an elongation of 3.5 mm. Two positron lifetime components were used to fitted the positron lifetime spectrum. The short lifetime (~160-220 ps) corresponding to positron annihilation in the bulk lattice, and a long lifetime (~0.5-1.5 ns) attributed to positron trapping at defects. The short lifetime remained nearly constant throughout deformation, while the long lifetime increased progressively with elongation, indicating the growth and coalescence of vacancy clusters. The mean positron lifetime exhibited two distinct plateaus: the first plateau (0-1.5 mm elongation) corresponded to gradual defect propagation, and the second plateau (2.0-3.5 mm elongation) reflected a rapid increase in defect size approaching fracture. These findings demonstrate that PALS can detect vacancy cluster evolution during tensile deformation and provide a quantitative criterion for assessing the proximity to fracture failure in power fitting materials.