Investigation of microstructures of laser surface-treated Zr702 sheet using electron channeling contrast imaging and electron backscatter diffraction techniques

• Microstructures of laser surface-treated Zr702 at different powers are investigated.
• Various modified zones by the laser treatments are clearly distinguished.
• Cooling rate for the 200 W specimen is relatively low and allows significant diffusion.
• Twin martensite is produced in the 50 W specimen due to ultra fast cooling.
• The modification depth in the 50 W specimen is about 1/3 of that in the 200 W specimen.

In this paper, specimens cut from a fully recrystallized Zr702 sheet were surface-treated by pulsed laser at two different powers (200 and 50 W) and their microstructures were investigated by electron channeling contrast imaging and electron backscatter diffraction techniques. For both the laser-treated specimens, three modified zones with distinctly different microstructural features are presented from the surface to the substrate: melting zone (MZ), solid state phase transformation zone (SSPTZ) and essentially unaffected base metal zone. The MZ is completely comprised of α plates while the SSPTZ could be further divided into the SSPTZ-1 and the SSPTZ-2, according to cooling mainly from β and α + β domains, respectively. With decreasing the laser power, the modification depth and the mean width of α plates decrease. Detailed characterization on microstructural features suggests that the plate structures (MZ and SSPTZ-1) in the 200 W and the 50 W specimens are produced by diffusion and diffusionless transformation, respectively. For those untransformed α grains in the SSPTZ-2 of both specimens, in spite of maintained bulk morphologies, slight subdivisions by subgrains are found inside them due to extensive dislocation activities induced by transformation stresses and/or their recovery.