Low Cycle Fatigue Properties Of .316 L Stainless Steel Surface Nanocrystallization

Nanocrystalline (nc) materials, as key materials of this century, have been developed more rapidly in the past decades. These materials exhibit unique properties over their coarse-grained polycrystalline counterparts for their ultra-fine grains and high density of interface, therefore they are expected to provide a practical approach for developing new metal materials. In order to explore the unusual structure-property relationship in nanocrystalline materials and to make use of nanocrystallization to improve the properties and behavior of engineering materials, more and more research activities have been concentrated on bulk nanocrystalline materials. Up to now, a variety of synthesis techniques have been developed for producing bulk nanocrystalline materials. However, difficulties still exist in forming 'ideal' samples (i.e. free of porosity and contamination, large dimensions, etc.), that are obstacles for the investigations and applications of nc materials.Actually, the majority of failures of engineering materials are very sensitive to the structure and properties of the material surface, and in most cases material failures occur on the surface. Therefore, optimization of the surface structures and properties may effectively enhance the global behavior of materials. With increasing evidence of novel properties in nc materials, it is reasonable to speculate to significantly improve overall properties and behaviors of materials through the surface modification by generation of a nanostructured surface layer, i.e. surface nanocrystallization (SNC). SNC is expected to greatly enhance the surface property without changing the chemical compositions and shape of materials. It is also a flexible approach to realize specific structure property requirements localized at the surface of materials.Most of conventional mechanical surface treatments might be potentially used for the SNC, among them ultrasonic shot peening (USSP) is considered to be a promising process to rapidly and effectively realize the SNC. Up to now, the USP has been utilized as a prestress surface treatment for mechanical components. However, detailed investigations on its effects on the fatigue properties remain open.In this work, a 316L stainless steel treated by USSP was selected to be studied. The microstructural evolution was characterized by means of different techniques, and the grain refinement and the deformation behaviors were analyzed. In addition, effect of surface nanocrystallization on the fatigue properties of 316L stainless steel induced by USSP was also researched.The results also indicate that the nano-structured layer on the surface has high resistance to the...