Ductility of ultrafine-grained copper processed by equal-channel angular pressing

In a previous study (R.Z. Valiev et al.: J. Mater. Res. 17 (2002) 5), unusual combinations of yield strength and ductility in ultrafine grained copper processed by equal-channel angular pressing of 360 MPa x 23 % (by Bc route two passes) and 380 MPa x 55 % (by Be route 16 passes) were reported. However, results from recent work suggest that the tensile strain. when measured using a non-standard method and miniature dog-bone tensile specimens, is likely to contain significant errors. In this study, by implementing a standard strain measurement and tensile specimens with geometries that meet the ASTM requirements, we report yield strength and ductility combinations of 370 MPa x 9 % (Be 2 passes) and 370 MPa x 16 % (Bc 16 passes) in the ultrafine grained copper. The higher ductility of the ultrafine grained copper processed by route Bc 16 passes was rationalized on the basis of several factors including the presence of equiaxed grains, lower dislocation density and higher fraction of high-angle grain boundaries (which result in larger strain rate sensitivity and strain hardening).