Unusual work hardening rate of a 3D gradient high purity Ti fabricated by laser surface treatment

Three types of heterogeneously structured high purity Ti samples, containing two iso-strained types and one iso-stressed type, were fabricated by using linear-scanning of laser surface treatment. The distances between the scanning-lines were modified to alter the space between the treated-zone and the matrix, forming a 3D gradient structure. These samples were tensile-tested at ambient temperature to characterize the strength and ductility. Scanning electron microscope (SEM), electron backscatter diffraction (EBSD), and transmission electron microscope (TEM) were employed to characterize the microstructures. Results indicate that the laser-treated zone contains a melting zone (MZ) and a heat affected zone (HAZ). Among all the laser-treated samples, a variant of iso-stress type specimen exhibits the best combination of mechanical properties together with unusual work hardening rate. Microstructure investigation and DAMASK simulation results indicate that a variant of iso-stress type specimen might have lower stress and strain concentrations under unaxial tension than homogenous samples, which explains their unusual performance.