Cavitation erosion of materials is mainly caused by the impact of strong pressure pulses and high speed micro--jets, which is formed at the end of bubble collapse. Its mechanisms have been intensively studied based on cavitation tests of heterogeneous materials. Generally speaking, the metallic phase, which is softer and has lower yield limit than the other in a material, is damaged first. However, a metallic phase may perform distinctively in different materials. Satisfactory explanation of this phenomenon is far from being achieved and further studies are necessary. 
        The present work is devoted to the mechanical effect of the impact of the cavitation erosion on duplex steels. Vibration cavitation tests were conducted to mild, medium and high carbon steels which are composed of ferrite and cementite. Mass losses of materials during tests were recorded. Investigations were made on the variation of the microstructure and morphology of specimen surfaces by scanning electronic microscope (SEM) and surface profilometer (SP), as well as the peak strength of binding energy of elements by X--ray photo electron spectroscopy (XPS). It was found that the cavitation damage of all the materials tested is characterized by the serious deformation and fracture of the ferrite phase in them. But the appearance of cavitation surfaces differs from each other because the volume (or mass) fraction and distribution of ferrite are different. Mild carbon steel with dominating integral phase of ferrite deforms uniformly. Medium (hypoeutectoid) steel contains approximately the same fraction of reticulated ferrite as that of pearlite (composed of alternating lamellae of ferrite and cementite). Its deformation mainly occurring in ferrite phase makes the reticulated ferrite bulged up and spalled off the surface gradually. High carbon steel mainly composed of pearlite is damaged because the lamellar ferrite is expelled out and splitting occurrs at the phase boundaries between ferrite and cementite. Analysis with the application of stress wave theory shows that the particular patters of damage of these materials are contributed to the yield of lower strength phase caused by compression stress waves produced by the impact of micro--jets.