Characterization of microstructure and hardness of a Zr-2.5Nb alloy surface-treated by pulsed laser

A duplex-phase Zr alloy (Zr-2.5Nb) was surface-treated by pulsed laser with modified microstructures characterized by electron channeling contrast imaging (ECC) and electron backscatter diffraction (EBSD) techniques. Hardness variation induced by the laser treatment was also examined and tried to be correlated with various microstructural characteristics. After the laser surface treatment, two modification zones are identified: the microstructure in zone I is fully composed of martensitic α plates with a few hundred nanometers in width and abundant nanotwins inside them; zone II possesses a mixed microstructure of transformed α plates, prior bulk α grains and β films. Comparative analyses with the case in single-phase Zr alloys suggest that the β films in the Zr-2.5Nb could act as stress absorber to suppress dislocation activity in α grains in zone II. Hardness measurements show that surface regions near the laser beam center can be remarkably hardened, which could be attributed to combined contribution from grain refinement, dense nanotwins and solid solution of Nb. For locations with the mixed microstructures in zone II, only compromised hardness is obtained due to less effective hardening contributor.