Positron Annihilation Studies of the Near-Surface Regions of Niobium before and after Wear Treatment

Abstract

The positron annihilation studies of well-annealed niobium samples after dry sliding or sandblasting are reported. These treatments modify surface and subsurface regions introducing crystalline defects, which are extended beneath the surface of the sample at a depth of about 140 lm for the highest applied load of 50 N. These defects can be identified as dislocations with jogs or vacancies near dislocations. The obtained depth profile of mean positron lifetime reflects the defect depth distribution and exhibits the characteristic plateau near the worn surface, which disappears when lowering of the applied load. A similar depth profile was obtained also for niobium samples after sandblasting. Slow positron beam studies revealed that at the surface of the samples after dry sliding or other treatments some defects that localize positrons are present. Additionally, a large value of positron diffusion length of about 266(33) nm was found for well annealed niobium.

Positron Annihilation Study of RPV Steels Radiation Loaded by Hydrogen Ion Implantation

“…“Positrons can be trapped at the surface where the electron density is lower than in the bulk. Additionally, they can be trapped at surface defects or positronium, i.e., a bound state between positron and electron can be created” [ 96 ]. Another factor that might be contributing is residue defects from specimen preparation—ones that remained even after polishing.…”

“…Furthermore, the PALS data for the annealing at 500, 525, and 550 °C proved that higher temperatures affect the structure via thermal structural changes. The increase in positron lifetime above 475 °C corresponds to the nucleation and growth of metal carbide/nitride precipitates, with a majority of the precipitates being MC, M 7 C 3 , and M 23 C 6 carbides [ 96 , 112 , 113 , 114 ]. The carbide precipitation begins at 450 °C with vanadium-rich carbides (MC).…”

Application of Positron Annihilation Spectroscopy Studies of Bismuth and Subsurface Zone Induced by Sliding

“…The skin effect causes that electrons flow mainly near the surface of the conductor, but in this region, they can scatter at crystalline defects which were created during manufacturing of wires or other devices. Therefore, recently, we focused our interest on the subsurface zone in metals and alloys, which are used for the construction of the electronic devices in radio frequency techniques [9].…”