Microstructural characterization and hardness variation of pure Ti surface-treated by pulsed laser

Electron channeling contrast imaging and electron backscatter diffraction techniques were jointly employed to characterize microstructures in a high-purity Ti sheet surface-treated by pulsed laser. Hardness variation induced by the laser surface treatment (LST) was also examined and correlated with these microstructural characteristics. After the LST, two microstructurally distinct modification zones are identified: the phase transformation zone (PTZ) is comprised of fine α plates with interior nanotwins while the heat affected zone (HAZ) is featured by the presence of multiple sub-grains with irregular-shaped boundaries. The occurrence of β → α martensitic transformation in the PTZ during the LST-induced rapid cooling results in plate structures with scattered orientations. The sub-grains in the HAZ are formed by thermal recovery of stress-induced dislocation activities. The absence of a duplex-phase domain in the high-purity Ti leads to a sharp PTZ/HAZ interface, while the HAZ/matrix interface is obscure due to inhomogeneous high angle boundary migration. Remarkable hardness increase (by 130%) is noted in the PTZ, which can be ascribed to both greatly refined grains (α plates) and nanotwins; there is only mild hardening effect in the HAZ.