Nano Grained Structure in KOBO Extruded Bulk Products
There are several known severe plastic deformation (SPD) methods, which allow receiving nano-grained structure of materials. One of the methods is called the KOBO method (the name is composed of the first two letters of the surnames of the inventors: KOrbel and BOchniak). The method was used for extrusion of copper alloys at room temperature to manufacture thin-wall tubes. During the KOBO extrusion, reversible cyclic torsion of the material results in multiple changing the deformation path. Hence, a heavy localized plastic flow leads to shear bands development that reduces the total strain hardening effect and results in effective structure refinement. In particular, thin-wall tubes produced from single-phase copper alloys, intensively cooled close to the die outlet, had homogeneous structure consisting of fine grains/subgrains of a few hundreds nanometers in size. The final structure of the material is practically independent on the initial structure of processed billets. If the accelerated cooling of extruded material was omitted, grain coarsening to the size of dozens of micrometers was observed. In multi-phase copper alloys, the grain refinement was limited to about 1 μm, both for intensively cooled and air-cooled products.
W. Bochniak et al., "Nano Grained Structure in KOBO Extruded Bulk Products", Journal of Nano Research, Vol. 10, pp. 11-17, 2010