Vacancy clusters in ultra fine grained metals prepared by severe plastic deformation

Severe plastic deformation of metals introduces not only dislocations, but also a high concentration of vacancies. In the present work we employed positron lifetime spectroscopy for investigation of deformation-induced vacancies in ultra fine grained metals prepared by high pressure torsion. It was found that in all metals studied deformation-induced vacancies agglomerate into small vacancy clusters. Experimental positron lifetime results were combined with ab-initio theoretical calculations of positron parameters for vacancy clusters of various sizes. This new approach described in this paper enables to determine the size distribution of vacancy clusters.