E. Dryzek

Positron annihilation studies of subsurface zones incopper

This paper presents the results of positron annihilation studiesof defect profiles in well-annealed copper samples whosesurfaces were exposed to wear and friction processes. The defectprofiles exhibited exponential decay. The dependence of thedefect distribution behavior on the load, duration of the pin-on-disc test, the temperature of the sample and the velocity ofthe ball were studied. The defect profile induced bytribotesting was compared with its analogues in samplessubjected to cutting or compression.

Migration of vacancies in stainless steel measured by positron annihilation

Abstract The results of Doppler broadening studies of annealing processes in the commercial stainless steel are presented. Two stages were established in the temperature dependence of the S -parameter. The first one corresponds to the steep decrease of the S parameter in the temperature range from room temperature up to 500°C and reflects annealing of defects introduced during the manufacture process. Three models of vacancy sinking and migration were considered. Taking into account diffusion of vacancies to the surface as well as to the grain boundaries gave the activation energy of vacancy diffusion equal to 1.215 ± 0.016 eV. The increase and consequent decrease of the S -parameter in the temperature range from 600°C up to 1060°C in the second stage is considered as caused by the precipitation of the carbide M 23 C 6 .

Subsurface Zones Created Under Lubrication Conditions Studied by Positron Annihilation

We present positron annihilation studies of the subsurface zones induced in the sliding contact of two metals under lubrication conditions. The surfaces of copper samples were exposed to the sliding of a small steel ball and the open volume defects induced under the surface were detected using both a positron beam experiment and a conventional positron experiment. From our studies we deduced that the total range of subsurface zones was almost independent of the lubrication conditions. For the highest load of the ball we observed that the subsurface zone created under lubrication condition extends deeper than for the case where the zone was induced in the dry sliding contact. We found that sliding with lubricant induces a layer on the surface whose properties, as detected by positrons, were different than those of other samples. It seems that this layer could contain open volume defects that are larger than those in deeper layers. Our studies have shown that the positron beam technique is a suitable tool for defect characterization in tribotested samples.

Study of the Mg-Cd system by positron annihilation methods

The problem of preferential positron annihilation in binary alloys has not been satisfactorily solved in the past. We examine this effect experimentally in Mg-Cd alloys using the new technique of Doppler broadening measurements with background reduction which allows us to observe positron annihilation with core electrons. Conventional positron lifetime spectroscopy is applied as well. We measure the Doppler spectra and positron lifetimes for selected alloys in the whole concentration range of the Mg-Cd system. The analysis of experimental results is supported by theoretical calculations. The conclusion is given that a small positron preferential annihilation occurs at Mg sites. Besides, the temperature dependencies of the peak counting rate of the angular correlation curve are measured for several samples and the vacancy formation energies are established. In the case of Mg-20 at.% Cd alloy, the increase of the peak counting rate characteristic for generation of thermal vacancies shows a discontinuity close to the order-disorder transition temperature.

Study of subsurface zones induced by friction and wear using positron annihilation method

Abstract The positron annihilation studies of defect profiles in well annealed gold and copper samples whose surfaces were exposed to the friction and wear processes are presented. In the case of copper samples the defect profile was obtained by etching the layers and measuring the lineshape parameter of annihilation radiation. The defect profile detected in such a case exhibited the exponential decay dependence. For gold samples the concentration of defects in the near surface laying region were obtained using the monoenergetic positrons. Close to the surface the defect concentration is an order 10−4 and the type of the defects as positron traps did not change with the depth.

Positron Absorption Studies in Electrodeposited Copper Layers

Positron annihilation studies within the standard methods in copper layers electrodeposited on the copper substrate are presented. The main interest was focused on the studies of the S-parameter evaluated from the annihilation line as a function of the thickness of the covered layer. The thickness of the layer ranged from 5 to 250 μm and we were interested in the fraction of positrons absorbed in it. From that we tested two relations linking the local and the measured positron annihilation characteristics, and the positron implantation profile. The reasonable description of the experimental data was obtained when the fraction of positrons implanted in the layer was deduced from the exponential positrons profile averaged over a solid angle. The experimental studies have been performed with 22 Na and 68 Ge positron sources. The present considerations could be dedicated to positron studies of inhomogeneous solids.

Subsurface Zones in Steel Samples Studied by Means of Positron Annihilation

This paper presents the results of positron beam experiments performed on raw steel samples whose surfaces have been damaged during sliding contact in the conventional pin-on-disc tribotest. It was important to use raw samples, because we were examining the usefulness of the positron beam in technical applications where such samples could be available. We were able to detect a change of experimental parameters, i.e., the positron diffusion length related to the type of the lubricant used in the tribotest. The analysis of the S parameter on the surface did not suggest any characteristic changes which would prove the creation of the top layer of new properties induced by the friction process. To complete our studies, we also carried out studies by measurement of the positron annihilation characteristics in the conventional experiment.

The Positron Probe Microanalyser Studies of Defect Distribution Induced by Machining of Copper, Iron and Titanium

Abstract We demonstrate the application of the new experimental technique: positron probe microanalyser for studies of the defect profiles induced by milling. Metal foils of Cu, Fe and Ti were subjected to machining in a milling machine with different cutting speed. This technique revealed well-defined profiles of defects below the machined surface which was extended up to hundreds of micrometres. The extent of the profile depended on the metal and the cutting speed. The positron probe microanalyser allowed us to trace the evolution of the profile after annealing in different temperatures. Surprisingly, the high thermal stability of a few tens of micrometres thick layer adjacent to the machined surface was detected.

Positron annihilation studies of high-manganese steel deformed by rolling

Abstract Positron annihilation spectroscopy (PAS) has been used to study the annealing behavior of cold rolled Fe – 21 wt% Mn steel with 0.05 wt% C. After the initial annealing of defects shown by Doppler broadening of the annihilation line, a slight increase in the annihilation line shape parameter, i.e., the so-called S parameter and then its decrease in the temperature range between 225°C and 450°C indicates generation of new defects and their subsequent annealing. This temperature range coincides with X-ray diffraction measurements, which indicate reversion of deformation-induced ε-martensite. However, for annealing in this temperature range with slow cooling of the sample, the formation of ferrite already starts. The results are compared with our previous results for deformed austenitic stainless steel 1.4301 (EN) where only reversion of deformation-induced α′-martensite was detected.

Positron annihilation studies in pyrophyllite

Results of positron annihilation studies in thermally treated natural pyrophyllite (PP) are presented. In this material we have found the positron lifetime component to range between 0.5 and 1 ns. This can be associated with voids whose evolution follows the dehydroxylation process induced by thermal treatment. The annealing of the PP at 1100oC caused the transition of the voids to micro-pores whose estimated radius was 0.23 ± 0.01 nm where the positronium state was present. This was confirmed using the magnetic quenching phenomenon. The values of the positron lifetime found and the measured Doppler broadening of the annihilation line indicated unexpected behaviour of positrons in this mineral.