Evaluation of creep damage in Ni base superalloy by ultrasonic velocity measurement

Microstructural changes in the artificially creep damaged Ni base superalloy, PM1000, were nondestructively evaluated by ultrasonic velocity measurement. The ultrasonic velocity decreased as the volume fraction of cavity in the crept specimen increased, which was caused mainly by the decrease of Young's modulus. The change in theoretical ultrasonic velocity with amount of cavity could be calculated by using the composite material model assuming that cavities were dispersed in the matrix as second phases with zero elastic stiffness. The increase of dislocation density also resulted in the decrease of ultrasonic velocity. The discrepancy between experimental ultrasonic velocity and theoretical one was rationalized in terms of dislocation substructure