Processing of metals by severe plastic deformation (SPD) – structure and mechanical properties respond

Severe plastic deformation (SPD) is one of the methods of obtaining very fine crystalline structure in different bulk metals and alloys, which possess different crystallographic structure. SPD causes the formation of micrometer and sub-micrometer sized subgrains in the initially coarse grain materials. As a result of that enhanced mechanical performance is observed. The mechanism responsible for this effect is still under investigation, however, it is believed that short and long-range intersecting shear bands produced by plastic deformation play a major role at grain subdivision and local dynamic recovery and recrystallization processes contribute to grain refinement 1,2 . Sufficiently large deformation leads to a distinct structure of dislocation-free and highly misoriented fine grains. When defining a submicron grain structure the important parameters, which are matter of concern, are average spacing of high angle grain boundaries (HAGB) and proportion of HAGB area 3,4 . The structural changes caused by SPD are reflected in improved mechanical properties of metals. The reported effects include increased hardness and yield stress, both featuring tendency to saturation. The drawback of ultrafine grained structure materials is their limited ductility 5,6 . Some other research revealed increased ductility and toughness as well as improved dumping and physical properties. The fine grained structure of UFG materials obtained by SPD leads to superplastic behaviour of these materials at lower temperatures and yet with higher deformation rates. Various aspects of structural changes caused by SPD have been the research goals in laboratories worldwide. Hundreds of papers are published each year in distinguished journals and conference proceedings (see proceedings of two TMS conferences on UFG structure development and properties evaluation 7,8 ). Today the most effort is paid to the study of the mechanics of material flow and grain subdivision when low strains ( ev < 3) and high strains ( ev > 3) are considering at the SPD. Usually, in dependence of the applied deformation methods (processing conditions) when different strain (von Mises) at deformation is developed the various structure can be found in deformed materials. At low strain the orientation splitting and microshear banding