Issue 19

L. Kunz et alii, Frattura ed Integrità Strutturale, 19 (2012) 61-75; DOI: 10.3221/IGF-ESIS.19.06 63 holds also for Cu, which was the simple f.c.c. model material used for pioneering studies on fatigue behaviour of UFG structures prepared by SPD [6,7]. ECAPed microstructure of this study UFG materials typically exhibit a grain size in the range of hundreds of nanometers. This is a transition region between the coarse grained (CG) materials and nanostructured metals, where the grain boundaries play a decisive role during plastic deformation. The first few ECAP passes result in an effective grain refinement taking place in successive stages: homogeneous dislocation distribution, formation of elongated sub-cells, formation of elongated subgrains and their following break-up into equiaxed units. On the contrary, the microstructure tends to be more equiaxed as the number of passes increases. Later on, the sharpening of grain boundaries and final equiaxed ultrafine grain structure develops. The microstructure of Cu prepared by ECAP can vary in many parameters. UFG Cu produced in different laboratories often slightly differs in the grain size distribution, particularly in grains shape and orientation, in dislocation structure and dislocation arrangement in grain boundaries, and in texture and misorientation between adjacent grains [8,9]. The mutual orientation of structural units cannot be satisfactorily described as high-angle random orientation, because there are regions where low angle boundaries are present, and also regions which can be described as regions of near-by oriented grains. That is why instead of the term “grain size” a term “dislocation cell size” is sometimes used. UFG structure of Cu of purity 99.9 % is shown in Fig. 2. Cylindrical billets of 20 mm in diameter and 120 mm in length were produced by eight ECAP passes by the route Bc (i.e. billet rotation by 90° in the same direction after each consecutive pressing). After the last pass through the die the samples of 16 mm in diameter and 100 mm in length were machined from the billets. The severe plastic deformation was conducted at room temperature. The microstructure as observed by transmission electron microscopy (TEM) in the middle of a longitudinal section of the cylindrical sample is shown in Fig. 2a. The structure in transversal direction is shown in Fig. 2b. The average grain size, determined on at least 10 electron micrographs, is 300 nm. This is in full correspondence with other reports on ECAPed pure Cu that indicate the most frequent grain size of 300 nm, irrespectively to the number of passes applied [10]. The size distribution, however, is getting narrower with increasing number of passes and both total and grain-to-grain misorientation tends to reach high-angle type. Structures with different morphological features can be distinguished according to [11]: the equiaxial structure, referred to as “A”, and elongated grain structure called “B”. In the course of the ECAP procedure, it is highly possible to obtain a mixture of the type A and B structures. The microstructure in Fig. 2a resembles the type B, and the structure in Fig. 2b represents the equiaxial type A. Figure 2a : Microstructure of Cu after ECAP as observed in TEM, longitudinal section. Figure 2b : Microstructure of Cu after ECAP as observed in TEM, transversal section. For characterisation of microstructure, electron back scattering diffraction (EBSD) has been recently used beyond the TEM [12]. EBSD analysis is predominantly focused on the experimental determination of misorientation of a crystallographic lattice between adjacent analysed points. This technique, in contrast to TEM of thin foils, enables one to investigate the changes of microstructure in the course of fatigue loading, because the same area of the specimen gauge length can be examined before and after the fatigue loading. An example of a microstructure as observed by EBSD and