During the past century studies on cavitation impacts were mainly based on negative effects in hydraulic machinery [1-4]. Recently we have developed a new surface enhancing technology, cavitation shotless peening (CSP), which reverses these effects to wider industrial applications. In CSP, the cavitation impact induced by the collapse of cavitation bubble produces compressive residual stress and work hardening on the material [5-10]. The surface enhancing process is similar to shot peening except that shots are not required and that is why it is referred to as CSP. Soyama et al. [6,8-10] have already demonstrated that CSP can improve the fatigue strength of crystalline materials. However, most of the engineers are still skeptical about peening materials by cavitation impacts. Thus, there is need to reveal that CSP can produce plastic deformation on materials which in turn results in a peening effect. In order to investigate plastic deformation caused by CSP, the recrystallization-etch technique [11] was used.
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