Microscopic investigation of sensitized Ni-base alloy 600 after laser surface melting

A study using a transmission electron microscope (TEM) equipped with an energy-dispersive X-ray spectrometer was carried out to investigate microstructural and compositional changes in sensitized Ni-base Alloy 600, the surface of which had been melted by a CO2 laser beam. Cr-rich carbides, having formed along grain boundaries during sensitization treatment, were dissolved completely or partially in the heat-affected zone (HAZ) close to or far from the laser-melted zone (LMZ), respectively, accompanied by the disappearance of Cr depletion at the boundaries. The microstructure of the LMZ consisted of fine cells in the grains having epitaxially solidified from the HAZ, and Cr concentration was observed to increase along the cell and grain boundaries in the LMZ. In addition, the cell walls in the LMZ were decorated with dislocations, and very fine particles, found to be a TiN type, were distributed randomly along the cell walls with tangled dislocations around them. The high dislocation density in the cell walls is attributed to the residual tensile stress developed during the rapid solidification.