Autoclaving involves high-pressure steam at high temperatures, which can damage the delicate piezoelectric crystals within the ultrasound transducer. These crystals are responsible for converting electrical energy into sound waves and vice versa. Exposure to the extreme conditions of an autoclave can cause thermal and mechanical damage to the crystals, rendering the transducer ineffective.
Reference:
ARDMS Sonography Principles and Instrumentation guidelines
Zagzebski, J. A. (1996). Essentials of Ultrasound Physics.