Issue 19

L. Kunz et alii, Frattura ed Integrità Strutturale, 19 (2012) 61-75; DOI: 10.3221/IGF-ESIS.19.06 64 the analysis of EBSD data is shown in Fig. 3. A grain map is shown in Fig. 3a. Grains, defined as areas having the mutual misorientation higher than 1 degree (a threshold angle value, which can be obviously adjusted), are marked by particular colours. Fig. 3b shows the grain size distribution. It can be seen that nearly 50 % of area fraction is composed of grains having an area smaller than 0.5  m. Indeed, the direct comparison with the structure displayed by TEM is not possible, because the evaluation procedure of EBSD images is primarily dependent on the adjusted threshold angle. However, for the constant threshold angle the method is very suitable for detection of changes of microstructure and grain orientation due to fatigue loading or temperature exposition. The UFG structure produced by ECAP is sensitive to the technological details of the process, lubrication, deformation rate, dimensions of the die, etc. With no doubt, these factors influence the microstructure and finally the properties of UFG material. So, the diversity in behaviour of “nominally” identical UFG structures produced in different laboratories, makes the comparison of results published in literature troublesome. A variety of possible structures give rise to significant scattering of experimental data on fatigue behaviour published to date [11]. Figure 3a : Grain map of UFG Cu as displayed by EBSD. Figure 3b : Analysis of the grain size and grain size distribution. T ENSILE PROPERTIES he stress-strain diagrams of UFG Cu prepared by eight passes by route Bc are shown in Fig. 4. The diameter of the gauge length of specimens was 5 mm. Results for two values of the loading rate, 1 and 100 mm/s, are presented. Surprisingly, there is no strain rate influence in the range of rates applied: both curves are located close each other. The shape of the curves is identical with that observed for UFG Cu prepared by 10 passes by the route C [13]. The curves exhibit a long elastic part at the beginning. The basic tensile properties and modulus of elasticity determined as an average of four measurements are given in Tab.1. Ultimate tensile strength  UTS Yield stress  0.1 Yield stress  0.2 Modulus of elasticity E [MPa] [MPa] [MPa] [GPa] 387 ± 5 349 ± 4 375 ± 4 115 ± 11 Table 1 : Tensile properties of UFG Cu prepared by ECAP, route Bc, 8 passes. The ultimate tensile strength after 8 passes is 387 MPa. The yield strength  0.2 = 375 MPa is very close to the  UTS and makes 97 % of it. The scatter of the data determined on particular specimens is quite small. The basic tensile data reported in literature for Cu processed by ECAP in different laboratories differs considerably, though the microstructure seems to be very similar. For instance, [13] reports  UTS values ranging from 410 to 470 MPa, T