Study of the nature of the processes occurring in the recovery stages in the recovery stages III and IV of plastically deformed Ni of different purity by means of the positron annihilation method

By measurements of the peak value of the angular correlation curve of the positron annihilation on cold rolled Ni of purity 99.99, 99.95, and 99.84% during isothermal annealing at different temperatures information on the annealing of point defects are obtained. From these, Corbett's recovery model for f.c.c. metals modified with respect to plastic deformation can be confirmed for Ni. For pure Ni the recovery stage III is connected with the migration of vacancies, to a large extent in the form of divacancies. On this basis and taking into consideration a possible temperature dependence of the migration energy of monovacancies, EIVM, the different amounts of EIVM and EIII, the activation energy of stage III, can be explained. In impure Ni stage III consists of two over-lapping substages; the lower is attributed to the release of interstitials from impurities. In the upper substage free vacancies migrate and agglomerate at impurity sites. Stage IV, observed only in impure Ni, is explained by a release of vacancies from impurity traps. The migrating vacancies form small clusters at deeper traps. The process of formation and dissolution of the vacancy clusters is observed. It is shown that this recovery model can explain the characteristic features of the recovery even of the electrical resistivity.