Issue 7

S. Bagheri Fard et alii, Frattura ed Integrità Strutturale, 7 (2009) 3-16; DOI: 10.3221/IGF-ESIS.07.01 5 Figure 2 : Typical cross-sectional optical micrographs close to the SMAT-treated surfaces under different treating conditions (annealed, commercially pure titanium specimens): (a)vibration amplitude50%, treatment time 10s; (b) vibration amplitude50%, treatment time 16s; (c) vibration amplitude100%, treatment time 30s; (d) vibration amplitude100%, treatment time 60s [57]. Bearing this fact in mind, it is natural to think to shot peening as a treatment effectual to obtain nanostructured materials. The concept of nanocrystallization by shot peening is that during the process with the hit of high energy particles, many pits and also extruded ridges around the edge of the pit are formed on the surface. When the ridge is hit by another particle, the contact area between sample and the particle can decrease significantly, therefore the strain and strain rate can be increased. Additionally, the collision mode also is changed from single direction to multiple directions due to the ridge, which is more favorable to the accumulation of dislocations. With the proceeding of collisions, some areas will approach the critical condition of nanocrystallization after several suitable hits [19]. Recent researches have successfully shown that different shot peening processes are able to introduce nanostructured layers with different characteristics, as concern their depth, the dimension of the crystals and microstructural properties. These methods are different both for the needed technological facilities and also for the mechanics of the treatment itself. Here is a very brief introduction about each of these processes: Shot blasting In this method, in order to obtain nanolayers the specimen’s surface is sandblasted repeatedly by high-speed sand particles and then subsequently anneale d [43-46] . In contrast to other SP methods, in shot blasting the size and also the geometry of shots are typically random and accidental and generally speaking shot sizes are smaller in comparison with the shots used in air blast shot peening. The sandblasted surface layer is heavily (plastically) deformed and consequently have high- density dislocations. After annealing, the initially formed dislocation network or fine “sub-grains” will change to nanosized grains with sharper grain boundaries. It is observed that the mechanical properties of the sandblasted surface are commonly inferior to those of the sandblast-annealed surface [44,45] and it can attributed to the different characteristics of nanograins after annealing. Air blast shot peening (ABSP) ABSP is a SP process through which the shots are projected by compressed air. The schematic of the equipment is illustrated in Fig.3. In air blast shot peening, NC is produced when higher shot speed and larger coverage than