An apparatus comprises a body assembly, a shaft, an acoustic waveguide, an articulation section, an end effector, and an articulation drive assembly. The shaft extends distally from the body assembly and defines a longitudinal axis. The acoustic waveguide comprises a flexible portion. The articulation section is coupled with the shaft. A portion of the articulation section encompasses the flexible portion of the waveguide. The articulation section comprises a plurality of body portions aligned along the longitudinal axis and a flexible locking member. The flexible locking member is operable to secure the body portions in relation to each other and in relation to the shaft. The end effector comprises an ultrasonic blade in acoustic communication with the waveguide. The articulation drive assembly is operable to drive articulation of the articulation section to thereby deflect the end effector from the longitudinal axis.