Positron Lifetimes and Doppler Broadening in Polycrystalline Titanium

Positron lifetimes and Doppler broadening in polycrystalline Titanium have been investigated in the temperature range 286 K < T < 1773 K. The measurements were done in a narrow high-vacuum furnace equipped with tube-shaped graphite heating elements making use of a sealed-source specimen of nominally pure (99.99+%) Titanium produced by electron beam welding. Both mean and free lifetimes increase almost linearly over the entire temperature range yet a marked change in the slope of this relation is observed at the hcp- > bcc phase transition (1167 K). Positrons prove to be sensitive to defects generated during the phase transformation, however, no indications of vacancy trapping can be detected even at the highest temperatures investigated. Evaluation of the Doppler broadening measurements gives similar results. The S-Parameter exhibits a linear increase over the entire temperature range and likewise no indication of vacancy trapping is found. Model calculations based on density-functional theory are performed leading to good agreement with the free lifetimes obtained in the experiment. In order to contribute to the understanding of the absence of vacancy trapping the calculations are extended to both gaseous and metallic impurities on vacancy sites.