ULTRA-FINE GRAINED COPPER PREPARED BY HIGH-PRESSURE TORSION: A POSITRON ANNIHILATION STUDY OF MICROSTRUCTURE EVOLUTION AND LATERAL DISTRIBUTION OF DEFECTS
Abstract
A defect study of ultra-fine grained (UFG) Cu prepared by high-pressure torsion (HPT) will be reported. HPT introduces a huge amount of defects (vacancies, dislocations) what makes HPT-made materials a challenge for positron annihilation spectroscopy (PAS) because of PAS non-destructivity and excellent sensitivity to open-volume defects.
Thus, conventional PAS including positron lifetime (PLT) and Doppler broadening (DB) techniques was the main experimental tool. PAS was combined with transmission electron microscopy, X-ray diffraction and Vickers microhardness (HV) measurements.
Lattice defects introduced by HPT were characterized first. A very high concentration of defects created during HPT deformation was observed and the two kinds of defects could be identified: dislocations and small vacancy clusters (microvoids).
Because of a significant increase in imposed shear strain with radial distance, microstructure of a HPT-deformed sample at the centre is expected to differ from that at the sample periphery. Therefore, further investigations focused on (i) development of microstructure with HPT turns and (ii) radial distributions of defects.
The results are consistent with torsion-induced strain increase from the sample centre toward its edge and predictions of strain gradient model. Extended lateral mapping of microstructure was performed using HV and DB techniques.
The latter one could reveal significant non-uniformity of defect distribution which was less pronounced in the HV measurements.