In-situ laser ultrasonic grain size measurement in superalloy INCONEL 718

The nickel-base superalloy, Inconel 718, is widely used in aircraft engines. The control of its final microstructure requires detailed knowledge on recrystallization, grain growth and second phase evolution during thermomechanical processing. The present study focuses on the in-situ evaluation of heterogeneous grain growth in the super-δ-solvus domain (above 1050 °C) using laser ultrasonics. This technology is dedicated to the ultrasonic characterization of microstructure evolution during thermomechanical processing of metals and alloys. The real time measurement of the ultrasonic attenuation provides the evolution of a representative grain size in the material. A criterion is developed to determine the onset and completion of heterogeneous grain growth stages associated with the dissolution of precipitates. The laser ultrasonic methodology is validated with ex-situ metallographic observations and quantitative characterization of the grain size heterogeneity.