Search results for "positron"
showing 10 items of 1346 documents
Positron Microscopy Studies of Wear Tracks on a Copper Surface
2014
Positron microscopy was used for studies of defects induced by ball indentation or sliding on the surface of well-annealed copper samples. A focused positron beam of 24.5-keV energy was used to scan the deformed region of the indenter impression, and measurements of the annihilation line parameter were performed. In the case of ball indentation, the measured defect distribution correlates well with the von Mises stress distribution of the Hertz contact and the von Mises yield criterion. For the wear track produced by pin on disc dry sliding, an asymmetric defect distribution near the wear track is observed. It indicates the presence of a tangential force that additionally deforms the sample.
Recrystallization in subsurface zone seen by positron annihilation
2010
Abstract The paper presents studies of the defect distribution, detected by the positron annihilation method in the subsurface zone (SZ) of copper samples after dry sliding wear. It takes advantage of a new experimental technique based on scanning of the positron implantation profile. It allowed us to detect the defect profile in copper to the depth of 200 μm in a nondestructive way and study isochronal annealing behavior of this profile. We determined that annealing induces a recrystallization process which runs faster close to the surface than in deeper regions. After sliding, the complete recrystallization of the substructure takes place at temperature c.a. 600 °C. Some changes in the de…
Melting Process of the Sn-45 wt.% In and Sn-77 wt.% In Alloys as Viewed by Positron Annihilation
2006
The temperature dependences of the positron annihilation parameters F and R were determined for Sn–45 wt.% In and Sn–77 wt.% In alloys. The obtained results enabled to verify the vacancy and the quasicrystalline models of the melting process. It was found that for metals transforming into the liquid state from the structures different than the close packed structure, the melting transition are hardly explainable in terms of the vacancy model of melting. The strong trapping of positrons in metallic liquids is an indication of the presence of microvoids (vacancy clusters) or microcrystals containing defects more immense than vacancies. The share of these positron trapping centers in the volum…
Positron Annihilation Study of Defects Induced by Various Cutting Methods in Stainless Steel Grade 304
2011
We report in this article the comparison of the subsurface zones (SZs) in austenitic stainless steel 304 samples created by three cutting techniques, i.e., the laser cutting (LC), abrasive water jet (AWJ), and traditional milling cutting (MC). The crystal lattice defects distribution in this zone were investigated using the positron annihilation method. It was shown that the MC creates the great number mainly edge dislocations decorated by vacancies. Their concentration decreases with the increase of the depth from the cut surface. The total depth of the SZ is extended up to 150 μm from the cut surface. Similar dependency was observed for the AWJ machining but the total depth is much lower,…
Application of Positron Annihilation Spectroscopy to Studies of Subsurface Zones Induced by Wear in Magnesium and Its Alloy AZ31
2011
Interaction of sliding bodies is an important aspect of numerous applications and subject of many studies (Solecki, 1989). Generally, when two surfaces are loaded together the true contact area is much smaller than the apparent one. The true contact is only at high points or asperities of the surfaces where the interactions in the atomic scale take place. Relative movement between the surfaces leads to friction and wear processes. The rate of wear is controlled by the load, the relative velocity and the behaviour of the material near asperities. The region of asperities can be plastically deformed and the stress is transported to the deeper laying region that becomes elastically deformed (F…
The Annealing Behavior of the Subsurface Zone Induced by Friction in Bismuth Detected by Positron Lifetime Technique
2013
The annealing behavior of the subsurface zone (SZ) in pure bismuth induced by dry sliding was studied using the positron lifetime measurement. This measurement allows us to detect the SZ and its recovery, and recrystallization processes. The comparative measurements of the sample exposed to compression revealed the thermal stability of the SZ. The compressed sample rebuilt its structure due to the recovery and recrystallization processes at the temperature of 60 °C, whereas the sample exposed to dry sliding does it at higher temperature of 260 °C, which is close to the melting point. The isothermal annealing at the temperature of 100 °C confirmed these results. The defect depth profile indu…
Cooperativity Scaling and Free Volume in Plasticized Polylactide
2019
The authors would like to thank the region Haute Normandie for their financial support and the acquisition of the Broadband Dielectric Spectrometer. R.G. acknowledges U.S. National Science Foundation (Grant no. DMR-1725188) for the acquisition of PAL spectrometer. The experimental evidence of the increase of activation energy associated with the super Arrhenius behavior governing amorphous polylactide by free volume variations has been obtained through a combination of calorimetric, dielectric, and positron annihilation lifetime measurements. The amount of free volume in polylactide was controlled by the amount of acetyltributylcitrate plasticizer in the composition. Plasticization is shown…
Study of the e<sup>+</sup> Distribution in a Layered Stack Sample Using Positron Lifetime Spectroscopy
2010
The results of the Monte Carlo (MC) simulations, using GEANT4 codes, indicate the enhancement of the fraction of the e+ in the denser regions of the sample embedded by the less dense environment. The positron lifetime measurements, performed for two stacks of layers consisting of pure aluminum and silver do not seem to reveal this effect. Some features of the e+ distribution detected in the experiments are not reproduced by the MC simulations.
'Cold' crystallization in nanostructurized 80GeSe2-20Ga2Se3 glass
2015
International audience; 'Cold' crystallization in 80GeSe 2-20Ga 2 Se 3 chalcogenide glass nanostructurized due to thermal annealing at 380°C for 10, 25, 50, 80, and 100 h are probed with X-ray diffraction, atomic force, and scanning electron microscopy, as well as positron annihilation spectroscopy performed in positron annihilation lifetime and Doppler broadening of annihilation line modes. It is shown that changes in defect-related component in the fit of experimental positron lifetime spectra for nanocrystallized glasses testify in favor of structural fragmentation of larger free-volume entities into smaller ones. Nanocrystallites of Ga 2 Se 3 and/or GeGa 4 Se 8 phases and prevalent GeSe…
Measurements of Positrons Lifetimes in Corroded Nickel
1999
It is over twenty years now that the nuclear method of conducting investigations into defects in the volume and on the surface of solids, based on positron annihilation phenomenon, has been developing on a quite intensive scale [1-3]. The present work is an attempt at adopting positron annihilation to investigations of the phenomenon of pitting corrosion in metals. Pitting occurs mainly in environments containing chlorides and affects metals and alloys covered with passivating oxide layers. Research carried out for many years by the use of various methods . has proved that pittings, which could — after some time — take sizes as large as the tenth of the millimeter order, are formed in the p…