Near-in-situ investigation of surface crack healing and strengthening mechanism of Ti-43.5Al-4Nb-1Mo-0.1B alloy by novel electroshock treatment

Abstract

The Ti-43.5Al-4Nb-1Mo-0.1B (TNM) alloy is prone to emerge surface cracks and other defects during processing and service, which will deteriorate the mechanical properties and fatigue life of the materials. In this work, a novel electroshock treatment (EST) for the surface crack repair and strengthening process has been proposed. Combined with near-in-situ experiments and theoretical analysis, the influence law and healing mechanism of different current densities and EST modes on the healing of surface cracks in the alloy were investigated. The scanning electron microscopy (SEM) near-in-situ observation reveals that the main factors influencing the healing of surface cracks relate to the time of energization, the current density, and the cracking angle. Meanwhile, the crack tip and locally bulging deflected positions are more likely to be healed in a short time under continuous EST mode and medium current density (75 A/mm2). The surface crack width at the calibration position decreases from 0.47 μm and 2.23 μm to 0 μm and 0.03 μm, respectively. The electron back scattered diffraction (EBSD) results show that the dislocation density increases around the surface crack healing area, and the dislocation density at the crack tip and away from the healing region is decreased and uniformly distributed after EST, which can further suppress the generation and propagation of the crack, resulting in a 20.1 % increase in tensile strength and a 40.3 % increase in fracture strain. Considering the coupling influence of the thermal and non-thermal effects of EST, a two-stage progressive atomic diffusion of thermal compressive stresses and crack bonding interfaces has been proposed to describe the evolution process of surface crack healing during EST.