Laser ultrasonic measurement of recrystallization and grain growth in an L605 cobalt superalloy

Recrystallization and grain growth following hot deformation of an L605 cobalt superalloy was measured in real time using laser ultrasonics. Ex-situ microstructure characterization of quenched samples in combination with double hit tests was used to evaluate the potentials and limitations of the laser ultrasonic method for these in-situ measurements. Significant grain refinement occurs during recrystallization and was recorded with the ultrasonic attenuation using a calibration previously developed for Co superalloys including L605. At higher deformation temperatures complete recrystallization takes place and the recrystallized grain size is independent of temperature. At lower deformation temperatures, recrystallization remains incomplete which can be deduced from the apparently larger grain size obtained from attenuation measurements than in the cases of complete recrystallization. The present approach has the potential to accelerate the development of thermomechanical processing paths through real time measurements of microstructure evolution in Co superalloys as well as other metals and alloys.