Characterization of Defect Microstructure in MgRE (re=ce, Nd) Alloys after Processing by High-Pressure Torsion Using Positron Annihilation Spectroscopy and a High Resolution X-ray Diffraction

Two MgRE (RE = Ce, Nd) alloys with ultrafine-grain (UFG) microstructures were prepared by high-pressure torsion (HPT) at room temperature. The in-depth distribution of defects was characterized by Doppler broadening -variable energy positron annihilation spectroscopy (DB-VEPAS). The characteristic S parameter increases in bulk after HPT processing relative to an as-received sample and shows a relative stability between 1/2 and 10 turns, which suggests a rise in the open volume defect density. However, a theoretical analysis of the S(E) depth profile reveals an increase in the positron diffusion length from-115 nm for the as-received state to-207 nm after 10 HPT turns. Almost all the open volume defect consisted of dislocations (positron lifetime of & tau; = 260 ps). The dislocation density deduced from high-resolution X-ray diffraction in the HPT disc radial direction was reasonably homogeneous (around 4-6 x 1014 m-2).