Positron annihilation studies of recrystallization

Authors: Jerzy Dryzek

Publish Date: 2013/03/29

Volume: 114, Issue: 2, Pages: 465-475

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Abstract

The discussion of the positron annihilation studies of crystal structure defects like vacancies dislocations grain boundaries and the defect depth profile is presented The role of the positron implantation depth and positron diffusion in such studies has been considered in detail For description of the measured annihilation characteristics the proposed theoretical models take into account both effects The annealing studies of defects created in pure magnesium by compression or dry slidingwear were used for demonstration of the discussed thesis The positron lifetime measurements were applied for monitoring open volume defects behavior It was demonstrated that annealing at the temperature of about 300 °C removes the defects created by compression Application of the proposed model to description of the data obtained allows to determine the activation energy of the grain boundary mobility in pure magnesium equal to Q=056±018 eV However defects created by the dry sliding are not completely annealed up to the temperature of 500 °C The defect depth profile induced by dry sliding evolves with the annealing temperature in such a way that at the worn surface concentration of defects gradually decreases but at the depth between 60 and 100 μm the generation of new defects takes place at temperature of 150 and 225 °C Above 300 °C the defects still are extended up to the depth of about 80 μmApplication of the positron annihilation spectroscopy to studies of matter is based on the unique properties of a positron as a probe at the atomic level The positron annihilates with an electron and mainly two energetic quanta are emitted almost in opposite directions which take over the total energy and momentum of the pair Because thermalized positron occupies the lowest energy level in its band the measurement of the total quanta momentum ie the angular correlation of these quanta is determined by the electron momentum in the annihilation place Due to the positive charge the positron is also sensitive to the local distortion of electron density which is when the open volume defects vacancies and their clusters are present in the structure The angular correlation of annihilation quanta ACAR and the complementary Doppler broadening DB of the annihilation line are the main measured positron annihilation characteristics which are related to the local electron states in a matter Another characteristic the positron lifetime PALS depends mainly on the local electron density Nevertheless before annihilation the positron as a mobile particle scans a certain volume of the implanted sample first as an energetic particle in the implantation and thermalization process and then during a random walk as a thermalized one Thus the annihilation characteristics reflect the local properties of the sample but they are averaged over the volume This is not taken into account in the simple positron trapping model proposed by Brandt 1 Bergersen and Stott 2 and Connors and West 3 which is commonly used in the analysis of the PALS or DB spectra In this model it is assumed that the defects which trap positrons with a certain rate are distributed homogeneously Additionally they localize only thermalized positrons The extension of this model to the diffusion trapping model allows taking into account the fact that defects are distributed in a certain way in the volume This problem was attacked theoretically and experimentally by many authors some of them are listed in Refs 4 5 6 7 8 The size of the scanned volume depends on the positron diffusion length which is equal about 01 μm However positrons used for PALS or DB measurements are mainly emitted from the radioactive sources into matter and distributed over the implantation profile Its total depth is about hundreds of micrometers and depends on the positron energy and density of the matter 9 10 If the studied sample exhibits a nonhomogeneous defect distribution across the implantation profile and/or the path of random walk then the measured annihilation characteristics must be sensitive to itThe aim of the paper is to illustrate both effects in experimental studies They concern a socalled subsurface zone SZ in the pure magnesium which is formed by dry sliding against another body The SZ occurs below the surface exposed to the different technological processes and can extend up to hundreds of micrometers Properties of the SZ are interesting because it is created directly by the processes at the worn surface which are difficult to characterize and because it is then an entering surface for positrons during measurements We intended to find out the thermal stability of the SZ and to show how it is affected by the annealing process In the first part of the paper we discuss the theoretical considerations of influence of the defect distribution on the positron annihilation characteristics In the second part the experimental results concerning our studies of the SZ in pure magnesium are presentedIt is well established that the Sparameter is sensitive to the open volume type of defects then both relations 8 and 9 can be useful for the study of the defect distribution generated in the process like cold working thermal shock quenching flexfatigue or irradiation with heavy ions or Xrays sliding and friction machining or sandblasting processes In those cases the processes generates a certain defect distribution in the sample interior which can be studied using the positron techniques As mentioned above one can use the energetic positrons to send them to the desired depth from the entering surface instead of the sequenced etching measurement or DSIP technique Equations 5 and 6 are valid also for that case however the appropriate implantation profile should be taken into considerations instead of the relation 7where Lz=cothz−1/z is the Langevin function L + is the positron diffusion length expressed as L + = sqrtD + tau mathrmp with D + the positron bulk diffusion coefficient and τ p the positron lifetime in the particle τ s is the positron lifetime in the surrounding region It is worth noticing that the measured S m parameter and bartau m are the functions of the particle of radius R and the properties of the boundary between the particle and its surrounding represented by the αparameter which is the transition rate of positrons from the particle to the surrounding For α→∞ all the positrons that reach the boundary go into the surroundings When α→0 the boundary reflects the positrons and the relation 11a tends to 10a–10bA simple model can be useful also for another case when eg cold worked metals and alloys start recovering their properties towards the initial one in the annealing process After deformation the microstructure is highly destroyed and can be approximated as statistically uniform region full of dislocations and other defects which can localize the positrons However the restoration process involved by subsequent annealing induces occurrence of new dislocationfree grains formed within the deformed or recovered regions This is the partial recrystallization process The new grains can be treated each as the particle in the model above When the sample is fully recrystallized ie the dislocations introduced during deformation were removed it still contains grain boundaries which are thermodynamically unstable The annealing process results in a grain growth in which the smaller grains are consumed by larger ones For such a case we propose another modelwhere S v τ v S b τ b S f and τ f denote the Sparameter and the positron lifetime for positrons annihilated only being trapped at a vacancy at the grain boundary and in bulk respectively Note that the S m parameter in Eqs 12a and 13a is an identical decreasing function of the grain radius We believe that these equations can be applied to the study of processes where the grain size of a polycrystalline sample alters under certain conditions

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