Issue 7

S. Bagheri Fard et alii, Frattura ed Integrità Strutturale, 7 (2009) 3-16; DOI: 10.3221/IGF-ESIS.07.01 4 Among the mentioned techniques, shot peening is a very well known method used to increase the performance of components in different service conditions. Fatigue, rolling contact fatigue, fretting and stress corrosion are operative situations where shot peening can strongly improve the performance of mechanical parts and structural elements. The present study focuses on the application of shot peening processes to obtain NC surfaces as this technique is a popular process in industries widely used due to its flexibility, which makes it possible to be used for components of almost any shape. It can be performed on commercial scale to provide NC layers with a high productivity. The paper is written with the aim to describe the actual state of the art and review different properties of NC layered materials in order to describe the effect of Nanocrystallization on material behavior. On this basis some possible addresses for future research in this field are drawn and underlined. S URFACE NANOCRYSTALLIZATION s stated before, the key point of surface nanocrystallization process of a bulk material is to introduce a large amount of defects and/or interfaces into the surface layer so that its microstructure is transformed into nano- sized crystallites while the structure of the coarse-grained matrix remains unchanged without any associated changes in the cross sectional dimensions of the samples [56]. Observations of these processes have revealed a rearrangement of dislocations during the process in the way that they move from the grain interiors to the region near the grain boundaries [10]. A schematic illustration of the defect rearrangement is presented in Fig. 1. The figure emphasizes that the local density of dislocations at grain boundaries grows, thus increasing their non-equilibrium. Typical optical microscopic images of treated surfaces which can describe the initiation of nanocrystals are shown in Fig. 2. (a) (b) Figure 1 : Arrangement of grain boundaries in nanostructured layers: (a) the dislocation structure during SPD processing (b) the dislocation structure after SPD processing leading to formation of non-equilibrium grain boundaries. S HOT PEENING hot peening (SP) is a mechanical surface treatment in which small spherical peening media with sufficient hardness are accelerated in peening device of various kinds and impact with the surface of the treated work piece with a quantity of energy able to cause surface plastic deformation. The aim of the process is the creation of compressive residual stresses close to the surface and the work hardening of the same layer of material. These effects are very useful in order to totally prevent or greatly retard the failure of the part [58-62] . There are a lot of papers about the ability of SP to improve the mechanical behavior of materials [62-66]. And most of them affirm that the effect of shot peening is mainly related to the induced residual stresses. Miller also hypnotized that shot peening effect could be related to grain distortion and to the increased microstructural barriers. He proposed that due to the multiplication of structural defects and dislocations, a crack would propagate with more difficulty in work-hardened surfaces [67]. Recent results show that in some cases surface hardening can be considered as the main cause of modified behavior of shot peened material s [68, 69] . A S