Issue 19

L. Kunz et alii, Frattura ed Integrità Strutturale, 19 (2012) 61-75; DOI: 10.3221/IGF-ESIS.19.06 62 U LTRAFINE - GRAINED COPPER evere plastic deformation of metallic materials has attracted considerable attention of researchers in material science within the last two decades. The main reason and expectation was to improve the mechanical properties of metals and alloys by substantial grain refinement, which is unavailable by conventional methods. It has been well known for a long time that a fine-grained material exhibits better strength and hardness than that one which is coarse-grained. Reduction of the grain size usually also improves fracture toughness. The physical reason of improved mechanical properties lies in the higher grain boundary volume in fine-grained structures, which makes the dislocation motion and resulting plastic deformation more difficult. For many materials the yield stress follows the Hall-Petch equation in very a broad range of grain size between 1  m and 1 mm [2]. Deviations from this law are observed only for very coarse grained and for nano-grained structures. Equal Channel Angular Pressing Equal Channel Angular Pressing is the most popular SPD technique. The fundamentals of this procedure, the details of the process and conditions of material flow during pressing can be found in many specialized papers, e.g. [3,4]. The principle of the method is simple. It consists in pressing of a rod-shaped billet through a die with an angular channel having an angle  , often equal to 90°, Fig. 1. Figure 1 : Principle of ECAP procedure When the billet is pressed by a plunger through the knee of the channel the material of the billet is severely shear strained. Since the cross-sectional dimensions of the billet remain constant after passing the channel, the procedure can be repeated. The final result is the imposition of a very high plastic strain to the processed material. The majority of laboratory ECAP dies has a channel with a quadratic cross-section. The billets can be rotated by increments of 90 degree between particular passes. Indeed, the rotating procedure is feasible also for dies and samples with circular cross-section. Four different ECAP routes are distinguished (Route A, route Ba, route Bc and route C). The equivalent strain,  , reached by pressing through a die characterised by outer arc of curvature  of the cannels inclined mutually at an angle  , is given by the relationship [4]:             3 2cot 2 2 cosec 2 2 N                   (1) Processing by SPD methods, investigation of the resulting UFG structures and their properties, is a matter of rapidly increasing number of research papers. The current results are regularly presented at the NanoSPD conference series; the last was held in 2011 [5]. A plenty of improvements of ECAP procedure has been proposed in the past. Even though the requirements of process improvements and economically feasible production of UFG materials in sufficient volumes activates development of plenty of SPD methods (like accumulative roll bonding, multiaxial forging or twist extrusions), the majority of basic knowledge on UFG materials is based just on research on materials processed by simple ECAP. This S