Issue 7

S. Bagheri Fard et alii, Frattura ed Integrità Strutturale, 7 (2009) 3-16; DOI: 10.3221/IGF-ESIS.07.01 3 Effects of surfaces nanocrystallization induced by shot peening on material properties : a Review Sara Bagheri Fard, Mario Guagliano Dipartimento di Meccanica, Politecnico di Milano, Via La Masa 1, 20156 Milan, Italy R IASSUNTO . Si presenta una breve descrizione del processo di nanocristalizzazione delle superfici per mezzo di deformazioni plastiche severe. In particolare, si concentra l’attenzione sui processi di pallinatura che fino ad oggi si sono dimostrati efficaci per ottenere superfici nano strutturate e se ne descrive lo stato dell’arte. L’influenza del processo utilizzato sul comportamento del materiale in relazione alle differenti proprietà di interesse è, poi, analizzata criticamente. Infine, sulla base delle attuali conoscenze si tracciano e sottolineano alcuni possibili sviluppi di ricerca in questo settore. A BSTRACT . A brief description of surface nanosrystallization process via severe plastic deformation is presented. To come to the point different shot peening methods which have proved to be able to create nanocrystalline layers are demonstrated clarifying the actual state of the art. Then the influence of the process is reviewed on material behavior and a wide range of affected properties are investigated. On this basis some possible addresses for future research in this field are drawn and underlined. K EYWORDS . Shot peening, Nanocrystallization, High energy, Surface treatments, Severe plastic deformation (SPD). I NTRODUCTION n the last few decades, ultrafine-grained materials, especially nanocrystalline (NC) characterized by crystal grains with dimensions up to 100 nm, have attracted considerable scientific interest, since nanostructured materials are expected to possess superior mechanical properties in simple chemical compositions fundamentally different from their conventional coarse-grained polycrystalline counterparts [1-10]. Actually, the majority of failures of engineering materials such as fatigue fracture, fretting fatigue, wear and corrosion, etc are very sensitive to the structure and properties of the material surface, and in most cases material failures originate from the surface. Accordingly, it is recommended that the entire components made of NC materials may not be necessary in many applications, particularly in components subjected to fatigue and merely optimization of the surface structure and properties may effectively enhance the global behavior and the service lifetime of materials. Thus surface nanocrystallization is expected to greatly enhance the surface property without changing the chemical compositions and shape of materials [11]. Among various methods proposed to produce ultrafine-grained materials, severe plastic deformation (SPD) technique has received the greatest attention due to its simplicity and applicability for all classes of materials. Although submicron grained materials can be successfully produced by most SPD processes, NC materials are obtained only by non- homogeneous deformation processes with large strain gradient s[12-19]. The basis of the SPD method is to increase the free energy of the polycrystals and generate much more defects and interfaces (grain boundaries) in various nonequilibrium processes such as high-pressure torsion [6, 8, 20-23] , ball milling [24-35], sliding wear [36, 37], drilling [38-42], shot blasting and annealing [43-46] , ultrasonic shot peening [15, 47-52] and air blast shot peening [16, 17, 39, 50, 53-55]. I